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Digital Telecommunications Technology - EETS8320 Fall 2006

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1 Digital Telecommunications Technology - EETS8320 Fall 2006
Lecture 2 Analog and Digital Telephone and Wireless Sets (Slides with Notes) Print in Power Point “notes” format to see notes on most pages. 1

2 Topics of Lecture What are the major parts (or modules) of a wired landline analog telephone set? What major parts for a digital wireless handset? What functions do these parts perform? If time permits, we will open and view a wired analog telephone set on camera. Please don’t open up a working telephone. If you want to open and examine a telephone or a handset, disconnect it from power or turn off power first.

3 Analog Wired Telephone Set
Basic parts/functions of an analog telephone set: Microphone: converts acoustic waveform (instantaneous incremental air pressure) to electrical audio frequency waveform Earphone: converts electrical to acoustic waveform Transmission via wire pair (loop). Directional coupler (hybrid or induction coils) used toaid in separation odf incoming/ outgoing electrical power flow to/from the earphone/microphone respectively. (2-wire/4-wire conversion) Signaling: Dialing via rotary current impulse count or via DTMF (touch tone) alerting via ringer or special sound source Analog/Digital conversion: Analog telephone set (digital conversion at central office switch) A/D conversion in the telephone set for ISDN Power from central office battery. Extra Optional Features: Caller ID, stored number dialing, etc. The basic functions of a telephone system include: Conversion of acoustic sound waveforms into an analogous electrical signal, via a microphone. From about 1880 until 1980, telephones have mostly used the Edison/Berliner carbon microphone. It operates as a variable electric resistance in response to acoustic changes in air pressure. Small granules of carbon derived from ordinary anthracite (hard) coal are in relatively loose contact in a capsule behind a flexible metal diaphragm. The diaphragm is flexed as a result of changing air pressure in a sound wave. This changing pressure compresses or releases pressure on the granules so that they increase or decrease their mutual contact area, and therefore the resistance to electrical current flow through the capsule varies. Instantaneously higher air pressure produces instantaneously higher current, and vice versa. An electric battery located at the central office telephone switch location provides the source of electricity. Instead of the Edison/Berliner carbon capsule, some modern electronic telephone sets use an electret (permanent dielectric charge) or dynamic (electromagnetic) microphone, each with an associated electronic amplifier. An inverse device, the earphone, converts a varying electric current into corresponding air pressure and thus sound waveform. A coil of wire produces a magnetic force on a flexible magnetic (usually steel) diaphragm, which flexes and causes a sound waveform. 2

4 Digital Wireless (2G) Handset
Basic parts/functions of a digital wireless telephone set: Microphone: converts analog acoustic waveform to analog electrical audio frequency waveform Earphone: converts analog electrical to acoustic waveform Analog/Digital conversion: A/D conversion in the digital cellular handset Digital signal sent via base-mobile radio link typically comprises 50% digitally coded speech, 50% error protection codes. Transmission via radio for cellular. Typically separate radio frequencies are used for earphone/ microphone signals (FDD). Signaling: Pushbutton dialing via binary coded messages using a separate logical radio channel than the voice. alerting via special sound source (ring tone generator) activated by message Power from internal rechargeable battery. Extra Optional Features: Caller ID, stored number dialing, etc. more so than most landline tel sets. This slide describes a second generation digital wireless handset of the GSM or TDMA type. CDMA handsets do many other things as well, not shown here, but disclussed later in the course. Older telephone terminology called a microphone a transmitter, and called an earphone a receiver. There was a conflict with the way these words were already used in radio systems, and the radio usage has mostly prevailed. The A/D conversion used in the landline PSTN (including ISDN) is a waveform coding process. The A/D conversion processes used in digital wireless systems does not reproduce the waveform, but reproduces the same audio power spectrum and thus “fools” the ear. Compare this and previous slide to the so-called BORSCHT list presented later.

5 Direct Acoustic Communication
Sound pressure variations at eardrum ultimately cause nerve signals to the brain, perceived as sound. Luckily for Alexander Graham Bell and the modern telephone engineer, a highly developed process of speech (acoustic sound sequences with meaning known to a large number of users) and written communication already existed before the possibility of extending such communication by electrical means was even proposed. (The major shortcomings of the traditional spoken language communication are: 1) the world has a number of mutually incomprehensible traditional languages; 2) some people are deaf and cannot use an ordinary telephone.) Telephone engineers have found, by empirical testing, just what level of audio fidelity and quality is necessary to permit voice communication between two people who know the same language. Curiously enough, the standard audio quality of public telephone systems is not adequate for communication between people who do not already know the same language well! Hissing sounds, such as the letters “f” and “s,” and some mutually similar sound groups such as “t” and “b” are easily confused in a telephone quality conversation. This is apparent when you try to spell a name, and must resort to identifying words like “f for foxtrot” to prevent confusion. We will not review the process of speech production in this session, but the action of the mouth and throat produces an oscillatory variation in instantaneous air pressure which is about 0.1 pascal (=newton/m2) or much less (corresponding to about a microwatt per square centimeter), compared to normal static air pressure which is about 100,000 pascal. Speech waveforms contain many different frequency components in the range of about 30 Hz (cycles/s) to 20,000 Hz, depending on the age and sex of the speaker. Small variations in air pressure at audio frequencies, produced by the mouth and throat, propagate through the air as an acoustic wave. 3

6 Telephonic Communication
An ideal telephone system (sometimes called an ortho- telephonic system) reproduces precisely the same acoustic waveform that the listener would hear in a face-to-face conversation. The purpose of a telephone system is to produce an adequate replica of the speech sound at the ear of the listener. What is adequate in quality is determined by a combination of requirements of intelligibility and costs. Life is easier for the telephone engineer and system designer because the human ear can perceive and recognize speech sounds rather well despite considerable distortion and modification of the speech waveform in a telephone system. Aside from removing most of the higher frequency audio power above 3.5 kHz, a certain amount of added undesired audio signal (so-called noise) is present with the voice signal, and there is usually some distortion of the voice waveform. In a modern digital telephone system the effects of all these undesired modifications of the signal are equivalent to adding a small equivalent noise signal to the “perfect” (but low frequency components only) voice waveform. That equivalent noise is typically only about 1/1000th of the power level of the voice itself. We express that in telephone jargon using the logarithmic unit called the decibel, by saying that the signal to noise ratio is 1000/1 or 30 decibels. Although most telephone users recognize that the overall quality of telephone speech is not “high fidelity” (primarily due to the low pass filtering noted above), the speech is understandable and we can usually recognize the “timbre” of each particular speaker’s voice. In some special military or other low voice quality systems, intelligibility is still adequate, but the speaker’s voice is so distorted that speaker recognition must be achieved by means of a password of the day or other special identification methods. Telephone System A real telephone system only imperfectly reproduces the speech (high frequency components are attenuated, some distortion and delay are introduced as well). 4

7 Imperfect Telephone Speech
Telephone speech quality is intentionally sub-optimal But it does what is needed economically. Audio Spectrum is intentionally incomplete Typically 300 Hz to 3500 Hz audio spectrum is adequate for known-language voice communication Improved audio bandwidth is nice for music, entertainment, but providing it is costly and it adds little to voice intelligibility Time Delay Partly due to physical transmission time; partly due to low bit rate coding Typically 100 to 200 milliseconds is perceptible Over 200 ms (from geostationary satellite delays) is disturbing to many users Less disturbing for one-way broadcasting Small amount of “noise” and distortion is tolerated Ideally noise is 30 dB “below” (1/1000th) voice power level Some engineers describe the method of designing for the lowest cost by not including capabilities and/or quality and features beyond what is absolutely necessary with scorn. They call this method “barely adequate” design. Others take the opposite position, and deride designers who include many extra features and quality beyond what is asked for in the specification for the product. This approach is sometimes derisively called “hyper-elegant” design. There is something positive to be said for each of these views. The extensive control and processing via software in modern products, particularly computer and telecommunications products, opens up a new synthesis of these different design approaches. In some cases a line or range of similar products is sold wherein all the versions sold at different prices have tehe same hardware but the high price version has software that enables all the features and capabilities, while the lower price versions have software that does not activate them. In that case, there is an interesting combination of the two points of view: sufficient design resources have been expended to develop the fancy version, but the cost of this “elegant” development is spread over products at all price levels in the product line.

8 Microphone (“Transmitter”) -1
Carbon microphone is most widely used in analog wire telephone sets Invented by Thomas Edison; Improved by Emile Berliner Historically, the Bell “liquid transmitter” was also variable resistance, but impractical due to use of a liquid. Original Bell commercial telephone used electromagnetic microphone and earphone Some early telephone sets used two identical devices, some used only one device that the user moved from ear to mouth during the conversation. Electromagnetic “mike” output was weak. Carbon “mike” is sensitive but low fidelity. Carbon grain packing is a minor problem. Since about 1880, most telephones have used an Edison-Berliner carbon microphone. It operates as a variable electric resistance in response to changes in acoustic air pressure. Small granules of carbon made by pulverizing ordinary anthracite (hard) coal are in contact in a capsule behind a flexible metal diaphragm. The diaphragm flexes as a result of changing acoustic air pressure. An increase in air pressure compresses the granules so that they increase their mutual contact area, and therefore the resistance to electrical current flow through the capsule decreases. The resistance is (very approximately) inversely proportional to the instantaneous air pressure. An electric battery located at the central office (CO) location provides the source of electricity. The carbon microphone is very sensitive to small changes in air pressure. It actually produces a higher level of signal power electrically than the amount of acoustic power falling on it from the voice! (Some people call it the first solid-state amplifier!) This is feasible because the carbon microphone is not a transducer of physical power, converting power from acoustic to electrical form. Instead, it is analogous to a valve which controls the instantaneous flow of electric power from a source (the CO battery) in response to the instantaneous acoustic pressure. Many modern electronic telephone sets use an electret (variable capacitance) or dynamic (electromagnetic) microphone with an associated electronic amplifier. Thomas Edison invented the carbon microphone for the Western Union Telegraph Company in a business move designed to apply competitive business pressure on the Bell Telephone Company, which is a fascinating historical footnote described in the article by Wolff given in the course bibliography. Emile Berliner made significant improvements in the carbon microphone as well. 5

9 Microphone (“Transmitter”) -2
Technical alternatives for modern telephones: Electro-magnetic microphone Coil of insulated wire carries varying current due to motion of iron disk (“diaphragm”) near it. Can use either dc coil current or a permanent magnet inside the coil to establish basic magnetic field. Used in early production (1876) Bell telephones. Revived (1960s), with transistor amplification correcting the low electrical power level of the signal Electret microphone Used in some modern electronic telephone sets, with amplifier An electret is a permanently electrically polarized solid (analogous to a permanent magnet). Conductive diaphragm near an electrically charged electret surface has varying voltage, responsive to motion caused by air pressure Since about 1880, most telephones have used an Edison-Berliner carbon microphone. It operates as a variable electric resistance in response to changes in acoustic air pressure. Small granules of carbon made by pulverizing ordinary anthracite (hard) coal are in contact in a capsule behind a flexible metal diaphragm. The diaphragm flexes as a result of changing acoustic air pressure. An increase in air pressure compresses the granules so that they increase their mutual contact area, and therefore the resistance to electrical current flow through the capsule decreases. The resistance is (very approximately) inversely proportional to the instantaneous air pressure. An electric battery located at the central office (CO) location provides the source of electricity. The carbon microphone is very sensitive to small changes in air pressure. It actually produces a higher level of signal power electrically than the amount of acoustic power falling on it from the voice! (Some people call it the first solid-state amplifier!) This is feasible because the carbon microphone is not a transducer of physical power, converting power from acoustic to electrical form. Instead, it is analogous to a valve which controls the instantaneous flow of electric power from a source (the CO battery) in response to the instantaneous acoustic pressure. Many modern electronic telephone sets use an electret (variable capacitance) or dynamic (electromagnetic) microphone with an associated electronic amplifier. Thomas Edison invented the carbon microphone for the Western Union Telegraph Company in a business move designed to apply competitive business pressure on the Bell Telephone Company, which is a fascinating historical footnote described in the article by Wolff given in the course bibliography. Emile Berliner made significant improvements in the carbon microphone as well. 5

10 Some Microphone Types Category Variable Resistance Electro-magnetic Electro-static Type, (Name, Structure Operation) Fidelity/ Quality Application(s) Audio electrical power output Historical notes Carbon (Edison-Berliner) Low Analog telephone set. high Western Union used this to negotiate with Bell in ca. 1879 Liquid Transmitter Medium/ high Historically one of the abandoned Bell investigations. High Impractical because liquid evaporates and is corrosive. Variable resistance method probably inspired Edison Flexible iron disk near coil on permanent magnet. medium Seldom used in telephone. Used in some wireless or intercom units. First commercial Bell telephones used this. Some used the same device as a mike or earphone. Moving coil or velocity microphone. (Movable current-carrying element in magnetic field) Radio broadcast, sound recording. Low. Requires amplifier. Fragile. Used where it will not be dropped or shaken. Most loudspeakers use a moving-col in a magnetic field as well. Electret: Metal disk near end of a permanently polarized rod Landline telephones, primarily proprietary ones, and many wireless sets. Low. Requires amplifier Polarized rod made of plastic or wax.Overall mike is very low cost. Piezoelectric, “crystal” mike. Crystal is variably compressed by flexible metal disk. Medium quality recording or public address systems. Some wireless systems. Many natural crystals – quartz, rochelle salts, even table salt – have piezoelectric properties. Condenser mike. Flexible metal disk is one “plate” of a capacitor. High quality recording systems. Requires high-voltage power supply. For example, if your GSM wireless handset is equipped with appropriate connector, you can use a FAX machine over a GSM wireless channel, but perhaps at only 1/4 of the digital bit rate of a wired telephone connection (1.4 kb/s rather than 9.6 kb/s). In other words, a page you can send via FAX over a wired connection in one minute will take 8 minutes via a GSM channel. Why is this so? (Explanation coming in Lecture NN) This is not a fundamental limitation of all GSM-related technologies. If you have a GSM handset or data-adapter card that supports the related data services GPRS, you can send that same FAX just as quickly as you send it via a wired telephone connection, or 17.8 times as fast. In other words, if you can send that FAX page in a minute via wired telephone service, you can send it in a minute via GPRS as well, or in 1/17 of a minute (3.4 seconds) via GPRS. Why is that? (Again, see future lecture NN for explanation.)

11 Earphone (“Receiver”)
Electromagnetic transducer used almost universally ever since Bell’s original invention. Magnetically induced force from a current carrying coil of wire acts to flex an iron disk producing sound. Similar to mechanism of loudspeakers and radio earphones. Loudspeakers typically use a very large moving cone of stiffened paper, mechanically attached to the coil of wire fidtted into a groove near a permanent magnet, to obtain louder sound waves in air Fidelity is relatively good Use of same device for earphone and alerting or hands-free loudspeaker may present hazard of ear injury due to loud ringing sound if near the ear when ringing. Latest gimmick to prevent this is an infra-red beam “proximity detector” in some Nortel handsets. Automatically lowers earphone volume when user’s head is nearby. A blue grommet where the cord enters the handset on a public telephone indicates “hearing-aid compatible” Intentional external audio-frequency magnetic field. The earphone of today is very similar in principle to the original earphone used by Alexander G. Bell in The earphone produces varying air pressure corresponding to a varying electric current. A coil of wire carrying the voice current produces a varying magnetic force on a flexible magnetic (usually steel) diaphragm, which flexes and causes a sound waveform. An electro-magnetic earphone is a transducer of physical power, converting power from electrical to acoustic form. The acoustic power output cannot exceed the electric power input. In fact, it is less than 100% efficient as a power conversion device. In the 1960s a major improvement in power efficiency was achieved by an AT&T redesign of the earphone, which was cross-licensed by most manufacturers of telephone equipment. One source of the improved efficiency was a concentration of the magnetic field almost entirely in the region of the flexible metal diaphragm, with almost no “fringe” field in the surrounding air. This made the audio output somewhat stronger but had the unexpected (to the telephone industry) effect of impairing the operation of hearing aids. Unknown or little known to the telephone industry, most hearing aids were designed with both a microphone (to pick up sound near the ear) and also a magnetic sensor loop to pick up the “fringe” magnetic field from telephone earphones. In more recent designs of earphones a separate coil is provided specifically to provide external magnetic field for hearing aids. A blue grommet on the telephone cord at the handset in public telephones indicates that handset is equipped with this hearing aid “fringe” field coil. 6

12 Loudspeaker- “Hands Free”
Amplification (sometimes with separate loudspeaker) used for “hands-free” or “speaker-phone” Continuous amplification may allow audio feedback problems Hollow, reverberating or echoing sounds due to in-room audio reflections from walls, etc. Self-oscillation or squealing audio when reflections are too strong Hands-free sets have some type of echo canceling True echo cancellation (generation of a delayed inverse polarity waveform to cancel the echo) may be accomplished via DSP* or alternatively in the transmission system in the central office switch. … or automatic audio switching Mute the loudspeaker when there is local microphone audio Mute the local microphone when there is audio from distant end. Local microphone audio can take priority over distant audio. *DSP=Digital Signal Processing The earphone of today is very similar in principle to the original earphone used by Alexander G. Bell in The earphone produces varying air pressure corresponding to a varying electric current. A coil of wire carrying the voice current produces a varying magnetic force on a flexible magnetic (usually steel) diaphragm, which flexes and causes a sound waveform. An electro-magnetic earphone is a transducer of physical power, converting power from electrical to acoustic form. The acoustic power output cannot exceed the electric power input. In fact, it is less than 100% efficient as a power conversion device. In the 1960s a major improvement in power efficiency was achieved by an AT&T redesign of the earphone, which was cross-licensed by most manufacturers of telephone equipment. One source of the improved efficiency was a concentration of the magnetic field almost entirely in the region of the flexible metal diaphragm, with almost no “fringe” field in the surrounding air. This made the audio output somewhat stronger but had the unexpected (to the telephone industry) effect of impairing the operation of hearing aids. Unknown or little known to the telephone industry, most hearing aids were designed with both a microphone (to pick up sound near the ear) and also a magnetic sensor loop to pick up the “fringe” magnetic field from telephone earphones. In more recent designs of earphones a separate coil is provided specifically to provide external magnetic field for hearing aids. A blue grommet on the telephone cord at the handset in public telephones indicates that handset is equipped with this hearing aid “fringe” field coil. 6

13 BORSCHT Acronym Functions in the Tel Set and Switch
Battery: dc electric power Over-voltage protection: not in the telephone set itself Ringer: pre-answer alerting in general. May include caller ID feature signal between rings. Supervision: that aspect of signaling which conveys busy/idle status Codec: Analog-digital COder/DECoder in a digital telephone system. Not in analog telephone set itself. Hybrid: directional coupler, 2-wire to 4-wire conversion Test: modern telephone switches have built-in test capabilities. Simple analog telephone sets have little or no internal test-related equipment. The BORSCHT acronym is credited to John W. Iwerson of AT&T Bell Laboratories. Legend has it that he was listing the design functions and noticed that they almost spelled BORSCHT (the name of a Russian beet soup recipe), so he changed the word “surge” to “over-voltage.” This list describes a number of functions which are associated with the telephone set, the subscriber loop and the switch. Usually most of these functions (except over-voltage protection) must be provided by electronic circuits which are usually packaged in a so-called “subscriber loop interface card” or SLIC (not to be confused with SLC - subscriber loop concentrator, a different device but with a homophonic acronym). The circuits associated with just one (or sometimes 4 or 8 in some digital switch designs) subscriber loop(s) are packaged on a replaceable printed wiring card or pack. If any component associated with that one (or 4 or 8) subscriber lines fails, the associated SLIC card can be quickly and easily replaced, with minimum “down time” on the affected loops. Some of the BORSCHT functions are also partially due to components in the telephone set itself. It is also significant to examine which components in the telephone set make use of or are the reason for the existence of certain BORSCH functions. Some have noted that the presence of a dial tone is a test, although a very simple test. Digital switches like the DMS-100 or the 5ESS have more elaborate built-in test capability. 8

14 Landline Central Office Battery
Lead-acid rechargeable batteries in the CO building provide -48 V dc for subscriber loops and also to power almost all the electronic equipment. In telephone practice the + battery terminal is ultimately connected to the earth/ground. (opposite of vehicle power and most other dc power systems) This can cause surface corrosion (deposition of copper carbonate or “verdigris”) on the wire but will not “eat away” the copper wire “Float” charging circuits rectify commercial ac power (110 or 220/208 V ac) and produce dc Battery is main continuous power source, not just as a back up source. Backup (if used) comprises Diesel engine, electric power generator (on truck in some cases) and fuel. Some 19th century telephone circuits used a battery at the customer premises, which was difficult to maintain. Late in the 19th c. a circuit was developed which allowed a shared battery at the CO (called a “common battery”) to provide current for all the telephones on a switch. Today the subscriber lines in a switch are fed in sub-groups by separate secondary (rechargeable or storage) batteries of the lead-acid type. Although the same lead-acid battery type is used in automobiles, the batteries used in a telephone central office are made with extremely pure lead electrodes and therefore they last for many years and can be recharged many times. They provide power for hours even if local electric power fails. Lead-acid batteries have a nominal (no-current or open-circuit) voltage of 2.2 V, and about 2 V when drawing rated current. Standard public telephone systems use a series connection of 24 cells to produce a nominal voltage of 48 V. Some PBX systems use only 24 V since they have only short telephone wires with low resistance leading to each telephone set. In a few cases of very long wires (rural telephone service) a 96 V battery was used in the past, but this has almost disappeared from the North American telephone plant, because new technology such as remote line concentrators has permitted the telephone operating companies to almost always use less than 5 mi (8 km) of wire. 9

15 Landline Battery Functions
Provides power for loop current supervision supervision works via cradle switch (“switch hook”) Provides power for dial signals Rotary (decadic) dial pulsing Touch-tone (dual tone multi-frequency – DTMF – oscillator) Provides power for carbon microphone Or for amplified electret or electromagnetic microphone. Allows basic POTS* telephone service in case of municipal electric power failure Many PBXs have some designated telephone stations which automatically connect to pre-designated outside analog lines via relays actuated when local electric power fails. Best solution for digital T-1 type PBX trunks or ISDN is overall customer premises telecom power backup systems (UPS, lead-acid gel-cells, etc.) with sufficient reserve power to operate for the anticipated duration of outside power failure *POTS=Plain old Telephone Service Electric current flow not only indicates that the telephone set is “off hook” (supervision) but also powers several devices in the telephone. When the handset is lifted from the cradle, a spring-restrained switch contact closes, thus drawing current from the CO battery. This current flow is detected by the appropriate equipment at the CO. If there was prior ringing, the call is then considered to be answered and the voice circuits are connected for a conversation. If there was no prior ringing, the line is connected to a dial tone generator and appropriate devices for detecting and interpreting the significance of rotary dial pulsing or touch-tone dialing. The cradle switch is also called a “hook switch” because early telephone sets used a spring-operated switch on a pivoting metal hook. The handset (or earphone in some sets) was made with a loop to hang on the hook. This leads to the jargon terms “on hook” indicating idle or nominally zero loop current, and “off hook” indicating busy or in use, indicated by full dc loop current value. Actual operating loop current depends on wire resistance and other factors, but is typically about 20 milliamperes (20 mA). At the end of a call, replacing the handset turns off the cradle switch. The current nominally drops to zero (actually typically a few microamperes due to imperfect insulation or moisture permeating the insulation), and the CO can detect that the call has ended. Most circuits which detect loop current require 2 mA or more to operate. If the “leakage” current in the on-hook condition is greater, the line will appear to be permanently busy (also called a permanent receiver off hook -- ROH -- in telephone jargon). 10

16 Subscriber Loop Jargon Analog Subscriber Wire Pair
There is a memory aid phrase used by telephone technicians: “Ring, Red, Right.” The so-called ring wire has red insulation and all the connector blocks are arranged with the ring wires in a column on the right side (the tip wire being on the left side). The North American names Tip, Ring (and Sleeve) arise historically from three parts used in the plug and jack connector on an old manual cord telephone switchboard. A modern full size (1/4 inch or 6.35 mm diameter) stereo headphone plug and jack uses the same physical arrangement for 3 conductors. The tip of the plug is connected to the “tip” wire. A ring of metal behind the tip and separated from it by means of an insulating washer, is connected to the “ring” wire, and the last part of the plug, a cylindrical sleeve, is connected to the third or “sleeve” wire and insulated from the ring by means of another insulating washer. In some North American documents, the tip and ring wires are called L1 and L2 (for line 1 and line 2), respectively. The sleeve was used to indicate busy/idle status via a separate current flow in electromechanical switches, but there is no separate wire for this purpose in modern digital switches. Multi-pair telephone cables in North America are made with internal bundles of wire pairs. Each pair of wires is twisted together to form two interlinking helices. The direction and length of the twist repeat is designed to minimize the amount of magnetic coupling between the pairs, as well as to hold the two wires of a pair together physically. Each group of 5 pairs has a distinct body or primary color. White is the primary color for the first group. Each of the 5 pairs in that group has a distinct stripe color. The tip wire body color is the primary color of that group, and the stripe color follows a fixed sequence of 5 special colors. These 5 colors are blue, orange, green, brown, and slate (grey). The ring wire has the opposite body and stripe colors to the corresponding tip wire. That is, the first pair comprises a tip wire with a white body and blue stripe, together with a ring wire having a blue body and a white stripe. The tip wire of the second pair has a white body with an orange stripe, while the ring wire of the second pair has an orange body with a white stripe. A third wire called Sleeve (C) was used in electro-mechanical switches, but not today in digital switches. 11

17 Wireless Battery Batteries in wireless handsets are mostly secondary (rechargeable) dry cells After many years of living with batteries designed primarily for flashlights (electric torches) and toys, in the 1990s the wireless market for rechargeable cells got the battery industry to make greatly improved and smaller cells. Electrode choices of exotic metals such as nickel, cadmium, lithium, etc. produce a light weight repeatably rechargeable (typically up to times) battery. Battery “life” (time between needed recharges) is achieved partly by good system design Base wireless system broadcasts a sleep-wake time schedule for various ranges of e.g. telephone numbers. Handset can then be automatically internally turned almost completely off (except for a timer and power control device) for up to 90% of the time when not in use, and “awake” only 10% of the time. All “paging” messages indicative of incoming calls are delayed until the next “awake” time window for that particular group of handsets. Alerting delay depends on service provider’s schedule, delay is typically 5 to 20 seconds. Strictly speaking, a battery is a combination of individual cells. When you buy a so-called size AAA, AA, A, C or D size “battery” for a toy or flashlight you are buying a single cell. The small recdtangular block shaped 9 volt battery is really a battery. It is an assembly of 6 internal cells in series, each cell individually producing about 1.5 volts (1.5*6=9). Primary (disposable) cells (example: AA) are usually constructed with a carbon rod core (+ terminal) and a zinc (or other metal) cylindrical housing. An electrolyte (highly ionized acid or base/alkali) in solution with water is held in a spongy material in the cylindrical space between the two electrodes. The amount of liquid is sufficiently small so that the spongy material is not dripping wet, and the cell housing is water-tight, hence the name “dry cell.” The voltage produced by the cell is the difference between two voltages, each of which is characteristic of one of the electrodes. These characteristic voltages represent the so-called “ionization potential” of each material. It is the ratio of the energy required to expel one internal electron, which has the highest ordinary internal energy, in that material, divided by the electric charge of one electron. An electron can be expelled by chemical means as in a battery, by means of an appropriate “color” light beam – the photoelectric effecd’-- for some metals, or by heating for some materials such as the filament of a vacuum tube, etc. For most useful choices of material ford the two electrodes, the difference between the ionization potential of the two materials is typically in the range of 1 to 3 volts. When a cell delivers power, typically metal is going into solution from one of the electrodes -- the reverse operation vis-à-vis electroplating – thus withering away that electrode and ending output from that cell.. In many types of cells, tiny gas bubbles or non-conducting layers form as well and cover the electrodes, resulting in the end of the useful “life” of that cell. Certain choices of electrode material, allow this reaction to be reversed: the dissolved electrode substance is re-plated back onto the remains of the electrode by running an electric current through the cell in the opposite direction than the current flow occurring when the battery delivers power. Such a cell is called a secondary cell or rechargeable cell. Most wireless handsets use rechargeable cells, although some made for emergency use allow disposable cells (like AA cells) to be used.

18 Over-voltage Protection
Protect against lightning or “line crossing” with power (mains) wires Lightning arrestors installed at the point where the outside wire enters the customer or CO premises, limits over-voltage to 300 volts Most arrestors consist of a simple spark gap with sufficient space between the electrodes so gas between will spark-over (ionize) at ~300 V. Ionization voltage of enclosed gap sealed in dry nitrogen is more uniform and not affected by atmospheric pressure or humidity changes. In an ionized gas many molecules have one ore more electrons removed, thus leaving a net positive electric charged “Ion.” Moving ions and electrons carry electric current across the gap to make the spark. The insulating (usually ABS plastic) housing of the telephone set is designed to withstand far more than 300 V Despite all of this protection, telephone operating companies urge subscribers not to make telephone calls during a lightning storm unless absolutely necessary. Over-voltage from a lightning strike can be thousands of volts, but lasts only microseconds to milliseconds. Over-voltage from an electric power line is typically 110 or 220 volts, but can persist indefinitely. These two causes are different from a low-voltage (typically only 1 or 2 V) ac signal which is sometimes induced magnetically into the telephone lines from the power lines and causes a “hum.” There is probable danger to the user from over-voltage, although there is no documented case of lightning causing a fatality to a telephone subscriber. None the less, telephone operating companies recommend that subscribers not use a telephone during a lightning storm. There are some cases of power line crossing causing fatalities. Both lightning and power line crossings can cause damage to electronic components used in telephone equipment. 12

19 Analog Ringer Parameters
Early buzzers or chimes were replaced by low frequency ac ringing signal in late 19th century. Ringing frequency and voltage used today mimic the early hand-cranked magneto generator, originally used for both subscriber-to-CO and CO-to-subscriber ringing Ringing ~90 V ac RMS (about 127 V peak for sine wave) 20 Hz frequency (although other frequencies used for selective ringing on older multi-party lines, etc.) Occasional problem: Some PBX or key telephone equipment uses square (not sine) waveform with same RMS voltage but lower peak voltage. This waveform will not be detected by some voltage-sensitive electronic ringer devices. Today many telephone sets use a local audio oscillator triggered by ringing voltage, and a loudspeaker. Local oscillator typically produces a ~1-2 kHz waveform with other higher frequency components as well. Alternating current (ac) ringers have no interrupter contacts to spark or corrode, unlike earlier (“doorbell” type) buzzer devices, and thus have no maintenance requirements. Originally the ac ringing voltage was generated by a small rotary electric generator called a “magneto,” mechanically driven by a dc electric motor powered by battery power at the CO. Today most CO installations use a high power electronic oscillator (still called a ringing generator), which is more reliable since it has no moving parts. Different portions of the CO switch have their time cadences for ringing preset so that they will have non-simultaneous cadence bursts of ringing current to reduce the instantaneous peak current demand on the ringing generator. RMS is an abbreviation for “root of the mean (average) of the square.” If two different electric waveforms, such as for example, a constant dc voltage and a sine wave of higher peak amplitude, have the same RMS value, then they will deliver the same time average electric power to an electrical resistor. The ratio of the peak to RMS value of a sine wave is  2 or A 90 volt RMS ringing current has a peak voltage of 90*1.414= 127 volts. The 110 V RMS voltage available from a typical North American power outlet has 155 V peak. Some PBXs generate a non-sine waveform electronically. One of the most widely used non-sinusoidal waveforms is a square wave, because the type of electronic oscillator which generates this waveform is electrically very efficient. However, the peak voltage of a 90 V RMS square wave is also 90 volts. Although both a square wave and a sine wave of equal 90 V RMS values will deliver approximately the same power to a resistor, an electronic circuit which is used to detect ringing by detecting a peak voltage over 90 V will not detect square wave ringing! 13

20 Alerting Audio Requirements
Alerting audio typically contains power at ~1-2 kHz for maximum ear sensitivity Based on Fletcher-Munson measurements (coming in later lecture) describing relative ear sensitivity at different audio frequencies Also must contain some higher audio frequencies to permit listener to localize the sound source Low frequency audio does not allow listener to perceive the direction of the audio source accurately. Electromechanical metal chime ringer does all of this naturally A two-tone component “warbling” audio signal is frequently used for non-chime sound. Ringer current drawn is described by a Ringer Equivalent Number (REN) according to US FCC Rules Part 68. Example: REN 2.0 ringer draws twice the ringing current vis-à-vis a standard electromechanical ringer.l Electro-mechanical ringers use a coil of wire to produce an alternating magnetic field to vibrate a spring-restrained steel armature. A permanent magnet is also incorporated into the design to hold the armature in a steady position when there is no ringing current. The range of motion, and thus the ringer loudness, of the steel armature is adjustable, including a clamped position which produces no sound (silent ringer). FCC rules permit up to 8 ringers in parallel on the same line. More telephones may be used, but the CO is only required to deliver enough ringing waveform power to operate 8 ringers total. Part 68 of the FCC rules require a label on each telephone, answering machine or other device with the “ringer equivalent number” (REN). The REN indicates the relative amount of ringer current drawn by that device compared to a standard electro-mechanical ringer. The sum of all REN values for all the ringer devices connected (in parallel) to the same subscriber loop should thus not exceed 8. Some devices, like answering machines, that get their operating power from wall outlet electric power, have a REN value of zero (they sense the presence of the ringing voltage but draw no ringing current from the CO at all). 14

21 Other Ringing Topics Ringing cadence
North American public telephone systems standardize on a 6 sec cycle: 2 sec ringing and 4 sec silence. European systems vary widely. Example: UK uses 4 sec cycle with two ring bursts in one sec, then 3 sec silence. Most public telephone systems do not produce instantaneous ringing burst(s) at the beginning of a call Delayed ringing bursts are synchronized to the cadence for that portion of the switch’s telephone lines. Connect-before-ringing could cause “glare”* and false connections “Bell tap” is a jargon term for any false alerting signal (with an electro-mechanical or an electronic ringer) due to undesired causes: Transient changes in loop voltage due to decadic dialing, hanging up handset, etc. Lightning pulses or other “foreign” electrical signals *Glare is a condition due to seizure of both ends of a two-way loop or trunk due to time delay of the test used beforehand by the seizing equipment to determine that loop/trunk is idle vs. busy. Ringing cadence differences sometimes affect the proper operation of electronic ring detector circuits in answering machines, etc. To prevent false bell taps, many ring detector circuits are designed to expect a certain cadence and will not indicate a true ringing condition unless they “see” just that cadence. Therefore they will not always work in other countries whose public telephone systems have a different cadence, or in a PBX with a non-standard ringing cadence. Of course, if properly designed, they are not fooled by a lightning pulse or other non-ringing signal as well. The lack of instantaneous initial ringing produces a problem with some customer premises equipment (CPE), such as a PBX, which seizes a line for an outgoing call automatically when an inside user dials an outgoing line access digit such as ‘9.’ A line may be first selected at the CO end for an incoming call, and there could be a delay of up to 4 seconds (North American cadence) before there is a ringing burst. If the PBX also seizes the same line for an outgoing call then, due to this lack of proper synchronization of signals at the two ends, there will be an unintended connection. This problem of improper approximately simultaneous seizure at both ends of a line or trunk is called “glare” in telecommunications jargon (based on the metaphor of two signaling devices glaring at each other as they both attempt to seize opposite ends of the same line). An alternative method of signaling called “ground start” (as opposed to the normal “loop start”) is used for some PBX installations to prevent this problem. In an electro-mechanical ringer, bell taps are avoided by proper mechanical adjustment of the spring tension of the armature in the ringer to prevent it from moving sufficiently in response to a single false impulse so that it will not strike the chime(s) of the ringer assembly. 15

22 Wireless Ringing/Alerting
When a wireless handset is “on” but idle, its receiver scans the assigned range of radio frequencies, seeking an adequately powerful radio signal having the special signal characteristics that identify a so-called “paging” channel The exact format of the paging channel is different for GSM, TDMA and CDMA wireless systems, and will be described in a future lecture. If/when the radio signal strength of that paging channel fades – usually due to the handset moving out of the “cell” -- the handset receiver scans again to find the paging channel of the nearest base antenna cell. When an incoming call for that handset occurs, a paging message is transmitted (subject to the sleep/wake schedule previously mentioned) on the paging channels in all the cells where the base system “suspects” that the handset may be located. This is in some cases all the cells in the city. When a handset receives a paging message for itself, it responds with a “here I am” message, and then is commanded to exchange furhter messages, typically on a separate radio channel. One of these is an alerting message, which automatically causes the handset to “ring” (play a pre-recodrded sound or ring tone).

23 Caller ID A very popular optional service, which helped to “pay” for Common Channel No. 7 signaling upgrades in the public telephone network. The originating telephone switch sends a digital call setup message in SS7 format, called the “Initial Address Message” (IAM), containing both the dialed number and the originator’s number. This message is sent via a “common” (shared) call processing data channel, ultimately to the destination switch. If the originator has specified “private” option, a code is also sent indicating not to display the number to ordinary destination subscribers. If the destination subscriber has subscribed to Caller ID service, and the originator did not forbid it, the caller’s telephone number is transmitted via a modem signal between the first two ringing bursts. A Caller-ID modem* and display at the destination telephone displays the caller number. If the destination subscriber has also subscribed to caller name ID, the destination switch also obtains the originator’s directory listing name from a separate data base called the Line Information Data Base (LIDB). Each RBOC has its own LIDB. If the originator is outside the area of the destination RBOC, the number will display but the name is typically not available in the destination LIDB. *Actually just the receive part of a modem (a “DEM”). More info later. In the 1980s an upgrade of the call processing message system in North America was begun, introducing the so-called Integrated Services User Part (ISUP) of Common Channel Number 7 (SS7) signaling. This upgrade is almost complete. This signaling system transmits the originator’s telephone number as well as the dialed destination number in a digital message. Provided that all the “legal” requirements are met, the destination telephone switch temporarily switches one of a bank of provided modems to connect to the destination telephone line for the 4 second time interval between the first and second ringing burst. The caller’s telephone number (and time of day and other information) is then transmitted via this modem. The modem uses a so-called Bell 202 (frequency shift keying) modem protocol, that was historically used for medium bit-rate (1200 bits/second) data modems. Some modern analog telephones incorporate a modem receiver and a display to show the Caller ID information. Accessory Caller ID boxes are also available for use with a telephone set. Many of these were distributed free of charge to subscribers to induce them to subscribe to the Caller ID option, which is a very profitable service for the local telephone service provider. A subscriber to Integrated Services Digital Network (ISDN), digital cellular handsets, or certain types of proprietary digital telephones used with PBX equipment, can re4ceive the caller’s telephone number directly in a digital form (no FSK modem used). Caller ID is always displayed at the operator’s work position, and for Police, Fire Department or 911 public service answering points, regardless of the originator’s desire to block display to ordinary destination subscribers.

24 Supervision Supervision is traditionally that part of signaling which conveys busy/idle status In some systems, the signals for dialed digits etc. are considered distinct from supervision signals. In new fields of telecommunication, such as wireless, “supervision” is often used to describe all forms of signaling (rather than a subset of all types of signaling), thus leading to jargon confusion when a traditional telephone person discusses technology with a wireless person. In the analog subscriber loop, dc current flow, controlled by the cradle switch, indicates supervision status In digital transmission systems, this status may be indicated by digital messages or by means of periodic status bit values (1 vs. 0) that occur in certain digital time division multiplexing bit streams in switching or multiplexing equipment, at predetermined bit locations (like the least significant bit position in one of each 6 consecutive digital frames). In many telephone switches, the detection and interpretation of Touch-tone (dual tone multi-frequency or DTMF) dialed digits is not a part of the subscriber loop interface circuit. Instead, the telephone is temporarily connected during dialing to a separate dial tone generator and tone detector device which is sometimes called a register or a tone receiver. Consequently, dialed digit detection and interpretation is not a function included in the BORSCHT list, since that was intended only to describe those operations which require hardware in the subscriber loop circuit. However, for rotary dialing, the brief interruptions in loop current caused by the dial contacts are detected by the same hardware that also detects loop current on/off status for the purpose of supervision. This hardware can be a small electro-mechanical relay (a coil in series with the loop which magnetically actuates a pivoting metal armature that in turn closes or opens electric contacts in another electric circuit) or some type of electronic device may be used. Of course, the time interval for the on/off status must be checked (today this is mostly done by software!) to distinguish between a dial pulse (typically 60 milliseconds), a cradle-switch “flash”(typically 0.75 to 1.5 seconds), and a hang-up (longer time duration than the time for a flash). 16

25 Wireless Supervision The base system of a wireless call determines a call is still in progress by means of the successful reception of digital messages and digitally coded speech at an adequate power level. Error-protection coding used in the data stream allows evaluation of the amount of erroneous data bits. An intentional disconnection is the result of pressing the END button on the handset. This produces a repeated and acknowledged disconnect message. A similar sequence of disconnect messages is used when the other party ends the call. An unintentional disconnect could occur due to a weak signal or continual excessive data errors for 5 seconds. This slide describes GSM methods. Other technologies differ in certain ways. GSM service providers can optionally configure their system to automatically reconnect an unintentionally disconnected call, although this requires some processing time. Analog cellular systems used a super-audible (approx 6 kHz audio frequency, above the bandwidth of the 3.5 kHz analog voice channel) to communicate that the call was still “up.” Absence of the codrrectd tone freqwuency for 5 seconds caused an automatic disconnect. A special super-super-audible tone burst was used to indicate that the mobile user was hanging up.

26 CODEC (Coder-Decoder)
In most public (analog) telephone installations, the CODEC or analog-digital converter is in the CO equipment (on a “subscriber loop card”). The external loop and customer telephone equipment are all analog The details of the CODEC will be discussed later in the course Certain integrated services digital network (ISDN) or proprietary PBX telephone sets have a CODEC in the telephone set, and transmit digital signals to the CO or PBX over the subscriber loop. Digital cell phones have a CODEC in the handset. The cost of a CODEC was an important factor in the initial introduction of digital end office and PBX switches. Earlier digital multiplexers (channel banks) used a shared CODEC for 24 conversations. Lower cost due to use of large scale integration allowed the use of a dedicated CODEC microchip for each subscriber loop in an economically feasible design. In the last 30 years, the public telephone networks in most industrialized nations have almost completely converted from analog to digital transmission of speech because of its improved quality and lower system cost. Likewise, connections are almost 100% controlled by the end subscriber dialing of the call (in contrast with historical human operator call setup), which requires automatic supervision and signaling equipment. Digital transmission and switching of voice and data signals requires that inherently analog signals be converted by a digital coder. As we will study in greater detail later in the course, the electrical waveform of a voice signal is first filtered by electrical circuits which attenuate most of the signal power at audio frequencies above about 3500 Hz (3.5 kHz). Transformers in the voice circuit also attenuate audio power below about 300 Hz as well, so the signal is band limited. The resulting signal is then measured or sampled 8000 instances per second. In the public wire telephone network,each measured voltage sample is then digitally coded using an 8-bit binary digital code. The result of this process is a binary bit stream at a data rate of bits per second (64000 bit/s or 64 kbit/s or 64 kb/s). This digital bit stream is usually combined with the bit streams of other voice conversations via a process called time division multiplexing, so they can be economically transmitted together in a shared transmission link. Digitally coded voice signals can also be switched by storing each 8-bit sample in a temporary memory storage device and then organizing the samples from various input channels in a pre-specified time order on a designated output link leading to the desired destination. The output links and the time order can be changed from one call to another to provide flexible routing of the connections. At the destination the digital samples are converted back to an analog waveform via a digital-to-analog converter or decoder. 17

27 CODECs for Wireless Wireless systems use several different types of CODECs, all presently not waveform coders. Internal details of various wireless CODECs will be described in a later lecture. Typical net bit rates for these CODECs is from 6 kb/s to 13 kb/s. Although significantly less than the 64 kb/s used for standard PSTN waveform coding, the quality of most wireless CODECs is very close to the PSTN. Most parts of a wireless system are designed to allow new CODECs to be easily introduced into service.

28 Hybrid Coils “Hybrid coil” is telephone industry jargon for a particular “transformer” type of directional coupler. The version in a telephone set is also historically called an “induction coil” confusing, since any single coil -- not a multiple winding transformer -- is also called induction coil in general electrical jargon. Also called 2-wire to 4-wire converter Permits simultaneous two-way signal power transmission on subscriber loop, … yet separates microphone and earphone signals at the ends of the 2-wire loop Uses a multi-winding structure with a “matching circuit” that has approximately the same electrical impedance as the subscriber loop and CO equipment Electric waveforms can easily flow or propagate in both directions (microphone and earphone signals) at the same time via a single pair of wires. It is more economical to use only a single pair of wires between the CO and each subscriber, rather than two pairs. It is beneficial to separate these two oppositely flowing signals at the telephone set end and at the CO end. It is very difficult to amplify electrical signals in both directions simultaneously on one pair of wires, without inadvertently producing self-oscillation and undesired positive feedback. This causes a squealing sound (like the effect of audio feedback in a badly-adjusted public address system, where the loudspeaker sound gets back into the microphone) or at least a hollow echoing sound. At the CO end, the process of digitally coding and transmitting the signal through the switch and via digitally multiplexed transmission trunk links to other switches is also unidirectional. This requires two simultaneously operating separate unidirectional links for trunk connections between one switch and another. For the widely used T-1 digital multiplex system, this is actually implemented via two separate pairs of wire. Directional couplers provide a way to extract a signal that flows in one direction only from a single pair of wires that is also carrying another signal in the opposite direction simultaneously, without getting any of the other opposite flow signal in the output. There are several ways to do this, but the use of multi-winding transformers and matching networks has been used since the second decade of the 20th c. It is generally attributed to the AT&T engineer George A. Campbell. 18

29 Background about Transformers*-1
Prolific American inventor William Stanley made the first transformer in Transformers have both power and communication uses. Electric current (moving electrons) produces a magnetic field in space surrounding the current flow. Intensity and direction of that field mathematically described by a 3-component vector B, measured in volt•sec/meter2 When an almost-closed piece of conductive wire is placed in that region of space, and the magnetic field changes inside that wire, a voltage appears at the wire ends. This induced voltage is proportional to the time rate of change of the enclosed magnetic field. For a small area wire “loop” all in one plane, v = -dB/dt • Area enclosed by wire This is one of the ways to determine the presence of the magnetic field and to measure its rate of change The induced voltage can be 2, 3 or more times larger, by wrapping the wire around the same area 2, 3 or more times. A coil of insulated wire can be both the source and the detector for the magnetic field. Such a coil is usually called an inductor. *Not to be confused with children’s toys (of the 1980s to the present) with parts that can be rearranged to make a robot, a truck (lorry) etc. Stanley worked with both Hiram Maxim and George Westinghouse before starting his own company, Stanley Electric Co. which became the General Electric Company’s power equipment division in Pittsfield, Massachusetts. Inductors (also called “coils”) and transformers are widely used in electrical and electronic systems. These few slides and pages give some essential information about how they work. These devices utilize the principle of “electromagnetic induction,” namely the production of a voltage (or in more general terms, an electric field) as the result of a change of a magnetic field in time. Electric and magnetic fields are theoretical entities that apparently exist in space surrounding electric charges. Electric fields can be detected and measured because they exert a force on stationary electric charges. Magnetic fields can be detected and measured because they exert a distinct force on moving electric charges. In general, when both an electric and magnetic field are present, the total force on a moving electric charge can be mathematically analyzed into a component due to the electric field and another component due to the magnetic field. The magnetic field enclosed in a small loop of wire can change because of any of the following causes: The electric current that produces the magnetic field changes in magnitude or direction. The area of the loop is changed (enlarged or decreased). The orientation in space of the loop is changed (it is tilted in a different direction). Any of these things will, in most cases, produce a voltage at the terminals or ends of the loop of conductive wire.

30 Magnetic Induction vm = -dB/dt • A A voltage Vm will occur here
Arrows represent magnetic B field. Loop area A is about ·(D/2)2, where D is diameter of loop. Loop of wire, with small gap, penetrated by time-varying magnetic field. Field can be caused by current in the loop itself (self-inductance) or due to current in other wires (transformer) or due to a permanent magnet. A voltage Vm will occur here if B is changing with time vm = -dB/dt • A We can “stack up” such loops to form a helical coil of wire. Each added “turn” adds another vm volts

31 Background about Transformers-2
Graphic schematic symbol d Electrical inductance measured via a unit called a henry = volt•sec/ampere (abbreviated H) (Self-) inductance L (in henrys)* of the tightly wound helical insulated coil shown, in terms of its dimensions (meters) and material properties is approximately: L = µ • n2 • A/g Where µ is the magnetic permeability of the core material. For air or vacuum µ is 4•p•10-7 henry/meter. If iron is used in the core instead of air, typical µiron is 12000•10-7 henry/meter n is the total number of turns of wire (n=5 here) The cross section area of the core A=p•(d/2)2 g is the length *For most inductors, the unit millihenry (mHy), 1/1000 of a Hy, is used. Incidentally, 4•p= An ideal inductor has a voltage that is proportional to the time derivative (the time rate of change) of the current flowing through the wire that forms the inductor. v = L•(di/dt).(A non-ideal inductor has resistance in its wires, and therefore a real inductor has a small component of voltage proportional to the current, rather than proportional to the time derivative of the current.) This mathematically implies that a sine wave current produces a cosine wave voltage. The magnitude of this voltage is higher at higher sine wave frequencies. The cosine voltage wave is always 90 degrees (or one quarter of a cycle or p/2 radians of angle) advanced in phase or leading the phase compared to the sine current wave. That means the voltage reaches a positive peak one quarter of a cycle before the current reaches a positive peak. In casual conversation (among technical people) we sometimes use the term “sine wave” to describe both sine and cosine waves, and any phase angle in general. The slide above shows a helical winding on the surface of a cylinder, with two definite ends. (Please pardon the crude artwork.) Many inductors (and transformers) are made in other shapes as well. One widely used form is a toroid (like a doughnut or bagel). A toroid is a useful shape because the magnetic field intensity is stronger in the interior of the winding, compared to a straight helix with open ends as shown above. We say that the magnetic field that spreads out into space at the end of the straight helix inductor is instead confined to the interior of the winding on the toroid.

32 Inductor Electric Properties
Relationship between voltage and current is v= L•(di/dt) When the current does not change with time, there is zero voltage. The ideal inductor has effectively zero resistance for dc. Real inductors are typically represented for analysis by a series resistor with an ideal resistance-less inductor. Following a short voltage pulse, current continues to circulate indefinitely in a closed circuit zero resistance inductor (for example, a “super-conducting” wire inductor) An appropriate size and duration negative voltage pulse can restore the current to zero, or reverse the current direction if the pulse lasts longer. A sequence of positive and negative voltage pulses produces an alternating positive and negative current. When a sine voltage waveform is used, a negative cosine current waveform results. The sine wave voltage and current are “out of phase” by 90 deg (1/4 cycle). Voltage positive peak occurs ¼ cycle before current peak. The ratio of the magnitude of the voltage to the magnitude of the current is proportional to the frequency. That is, an inductor “passes” more current (has lower impedance) at lower sine wave frequencies. Intentionally no notes on this page. Compare page 36 describing the analogous properties of a capacitor.

33 Background about Transformers
A transformer comprises two insulated coils (typically multi-turn coils) surrounding the same interior space (typically one coil inside the other) A time-varying current in one coil will produce a voltage of the same waveform (proportional to time derivative of the current) in both coils The voltages appearing at the two coils will be proportional to their respective n (number of turns of wire) Transformer with equal number of turns are typically used to couple electrical non-dc waveforms at same voltage, but to isolate or separate the dc current flow in the primary and secondary winding. Instantaneous polarity of voltage is fixed by the relative direction of the two windings. A transformer can be used to produce a signal with same voltage waveform on the secondary coil as on the primary, but opposite polarity. Transformers are used extensively in electric power systems so that power can be generated (or stepped up from the generator voltage level) and transmitted at high voltage levels (and low current levels) over long distances via so-called “high tension” (high voltage) transmission lines, such as volts. Then, close to the end user’s home or business, step down transformers produce 110 volts (or for some users, 220 volts). This is, in fact, the reason why alternating current is used in power systems – just to permit the use of step up and step down transformers. The earliest electric power systems developed by Thomas Edison used direct current, and had to generate dc power at a voltage somewhat higher than the voltage appearing at the consumer’s home. The use of 110 volts was chosen so that, after the current passed through an average length of power wiring, 100 volts would appear at the customer’s home. When longer transmission wires were used, dc power proved to be unsatisfactory. (Incidentally, Edison fought the use of ac power – championed by George Westinghouse -- as long as he could. Edison’s patents did not cover alternating current!) Power losses occur in transmission wire for two reasons: 1) power losses proportional to I2•R, where I is the current and R is the “series”resistance of the wire, and 2) power losses proportional to V2/r, where V is the voltage, and small r is the resistance of the insulation that prevents leakage or bypass current from passing from one wire of the pair to the other. Because we have many excellent insulators, in most cases r is a VERY large number, and the main cause of power loss is type 1. By transmitting electric power over long distances at high voltage and low current, we can minimize type 1 losses.

34 Step-Up or Step-Down i1 V2=2•v1 and I2=i1/2, i2 So V2/i2=4•R or 40 
Transformers with unequal number of turns on primary and secondary coil are used to “step up” or “step down” voltage – typically power voltages Example: in power cords for portable equipment 110 volt ac “primary”coil produces, for example, 6 volts on “secondary” coil for use by low voltage device. Ratio of turns N is 110/6= 18.3 in this example. Because of change in voltage/current ratio seen via the coupled coils of a transformer, the apparent resistance (in general the “impedance”) of a circuit device is modified per the square of the turns ratio: Transformers are used extensively in electric power systems so that power can be generated (or stepped up from the generator voltage level) and transmitted at high voltage levels (and low current levels) over long distances via so-called “high tension” (high voltage) transmission lines, such as volts. Then, close to the end user’s home or business, step down transformers produce 110 volts (or for some users, 220 volts). This is, in fact, the reason why alternating current is used in power systems – just to permit the use of step up and step down transformers. The earliest electric power systems developed by Thomas Edison used direct current, and had to generate dc power at a voltage somewhat higher than the voltage appearing at the consumer’s home. The use of 110 volts was chosen so that, after the current passed through an average length of power wiring, 100 volts would appear at the customer’s home. When longer transmission wires were used, dc power proved to be unsatisfactory. (Incidentally, Edison fought the use of ac power – championed by George Westinghouse -- as long as he could. Edison’s patents did not cover alternating current!) Power losses occur in transmission wire for two reasons: 1) power losses proportional to I2•R, where I is the current and R is the “series”resistance of the wire, and 2) power losses proportional to V2/r, where V is the voltage, and small r is the resistance of the insulation that prevents leakage or bypass current from passing from one wire of the pair to the other. Because we have many excellent insulators, in most cases r is a VERY large number, and the main cause of power loss is type 1. By transmitting electric power over long distances at high voltage and low current, we can minimize type 1 losses. Schematic transformer symbol i1 Left coil has 2 times the number of turns on right coil. V2=2•v1 and I2=i1/2, So V2/i2=4•R or 40  i2 + - + - v2 v1 R=10

35 Lowest Frequency for Transformer
1 i2 Power/(N•is)2•R Ideal transformer model i1 Is + - + - 0.5 v1 + - v2 L R N•v2 N•i1 frequency fc Transformers don’t “work” at dc. What is the lowest useful frequency? In this ideal model of a transformer, used with driving current source Is, and self inductance L, the high frequency power in “load” resistor R is (N•is)2•R. (N is the coils turns ratio n1/n2.) At dc (zero frequency), the power in resistor R is zero since all current is diverted by the inductor L. At sine wave frequency fc=R/(2pL), resistor power is ½ of its high frequency value. “Half Power Frequency” is convenient to measure. In telephone transformers, fc is typically 300 Hz by design. This is low enough so speech intelligibility is adequate. A transformer only ‘works’ for alternating current (ac), but not for direct current (dc). What is the lowest ‘workable’ frequency for a transformer? Here is a simplified analysis. The result depends on the resistance R of the “load” or power destination, as well as on the properties of the transformer. The diagram contains a dotted line box which is a ideal model or mathematical representation of a transformer with turns ratio N=n1/n2. Side 1 is the left and 2 is the right. The model for the ideal transformer consists of a voltage-controlled voltage source and a current-controlled current source, with an inductor L representing the self inductance of coil winding 2 at the right (power output) terminals of the transformer. Here we assume “complete” coupling of the primary-secondary magnetic field. A model representing a transformer with imperfect coupling would also include a series inductor on side 1. The “load,” a resistor of R ohms, represents the device that ultimately consumes the power passed through the transformer from left to right. The current source is is the ultimate power source of the waveform passed through the transformer. We can find, via measurement or via theoretical analysis, the frequency at which the power diverted to the inductor is ½ of the power in the resistor at high frequencies. This “half-power frequency” is often used in the industry as the nominal low end of the useful frequency range of the transformer for this particular load resistance R, just because it is easy to measure. In order to make a transformer that has a lower half-power frequency (for use with the same load resistance R) we need to make a larger, heavier transformer with more turns of wire, larger core area A, or higher magnetic permeability material in the core, so the windings will have higher self inductance. For power applications, another way to effectively utilize a smaller transformer with low self inductance is to use a higher ac power frequency. That is the reason that 400 Hz electric power is used some aircraft, in contrast to the 60 Hz (in North America) or 50 Hz electric power used in most power systems.

36 Implications of Large L value
Inductor value L in previous figure is a representation of the combination of the primary and secondary coil self inductance values In order to design a transformer that works well at low electrical signal frequencies, its coils must have a large inductance. Requires many turns of wire, core material with high magnetic permeability (iron or ferrite ceramic, etc.), large area A, etc. Good power efficiency also requires low wire resistance (not explicitly analyzed here) Requires thicker (larger wire diameter) wires, use of lower resistance metals (silver, copper, etc.) These things make the transformer physically larger, heavier and costlier Every design is a compromise between high efficiency (100 % coupling of electric power from one coil to another) and low size/weight. For most power applications, when portability is not needed, large size and weight are not problem issues for a transformer. Also, copper is the second best conductor (after silver) and is already relatively plentiful and low in cost for making wire conductors. Many excellent magnetic materials are available, including iron (often alloyed with various exotic elements such as samarium, nickel, or cobalt) which have high magnetic permeability. Iron is abundant and low in cost. Its interesting magnetic properties arise from the configuration of the self-spin or inherent magnetism of some of its electrons, which are not all arranged in pairs of opposite magnetic direction like most other elements. Metal conductive materials used in the core of a transformer have an undesirable property of supporting electric current circulation in the magnetic core, known as “eddy current.” For certain applications, an electrically insulating magnetic core material is used to prevent eddy current. One of the most popular insulating magnetic core materials is ferrite (iron silicate) a ceramic material containing iron atoms in its chemical structure in place of the aluminum atoms in ordinary clay (aluminum silicate). For most telecommunications applications, the most important aspects of a transformer design are cost, efficiency of power coupling, size and weight, in about that order. Transformers can do things (like voltage step-up or step-down, or separating the dc current component from a composite dc-ac current) that cannot be achieved using other combinations of components. But transformers cannot be manufactured as a “flat” device on a plane surface because they require an inherently three-dimensional structure.

37 Current and Power Flow A B C D + v - - v +
Power flow depends on the polarity of both voltage and current. In the two examples above, current flows from box A to B in the upper wire and returns from B to A in the lower wire. The same directions of current flow exist between boxes C and D. The boxes contain power sources and other circuit elements. Due to the opposite polarity of the voltage on the wire pairs in the AB vs. the CD case, power flow is toward box B but away from box D. For your own education, examine two other cases where the voltage is the same as the two cases above, but the current flow is to the left in the top wire and to the right in the lower wire. In the two cases above, the boxes contain power sources (such as batteries) and also other components like resistors or perhaps telephone subscriber sets. In these two examples, current flow circulates in a clockwise direction, flowing to the right in the upper wire and to the left in the lower (return) wire. The direction of electric power flow can be found only by examining the polarity of the voltage between these two wires as well. Given this current flow, when the upper wire has a positive voltage compared to the lower wire, the power flow is to the right (into box B). The other illustration shows the same current flow pattern but the opposite voltage, leading to a power flow toward the left into box C. The numerical calculation of the power can be done by multiplying the current (in amperes) by the voltage (in volts). The unit of electrical power is the watt. Since we show only the direction but not the magnitude of the current and voltage we cannot compute the numerical power in these examples. We can conclude from the power flow direction in the AB case that box A must contain a power source. This source could be an electric generator that produces electric power from a mechanical “prime mover” like a waterfall, or an electrical energy source like a (primary) battery, or we could have previously stored energy in a (secondary) storage battery, or we could have previously stored energy in an inductor or capacitor in box A. Knowing that power flows into box B, we can conclude that it either contains resistors (that convert the power into heat) or perhaps an earphone or loudspeaker that converts the electrical power into acoustic power, or perhaps some energy storage device (like a storage battery, an inductor or a capacitor) to temporarily store the energy for later use.

38 Transformer Power Flow
Even though a transformer with unequal number of turns on the secondary vs. primary can produce increased voltage, it does not produce increased power The current flow in the winding with the larger number of turns is inversely proportional to the turns ratio. Thus the power flow into the primary (product of primary input voltage and current) will ideally be the same as the output power flow from the secondary winding (product of output voltage and current) Real transformers are slightly less than 100% efficient in transferring power due to the fact that both coils do not always enclose the same total magnetic field area, and due to power loss in the resistance of the wires, certain power loss due to cyclic magnetization and de-magnetization (hysteresis) of the iron or other core material, etc. A transformer is analogous to a lever: The short end of a lever has high force and small movement, while the long end has low force and large movement. The work (energy) transferred (product of force and distance moved) is the same in at one end of the lever as it is out at the other end! A transformer is one of a class of electrical devices that we call “passive.” This name implies that the device cannot generate or produce electric power. During a cycle of alternating current, an inductor or a capacitor stores electric energy coming from other parts of the system, and then returns it to the other parts of the system. It does not generate electric power. Similarly, a transformer does not generate electric power and the output power must always be equal to or (in almost all cases of practical interest) less than the input power. Some devices such as transistors or vacuum tubes (electric “valves”) used in amplifiers are loosely described as “active” devices. This means that, for certain types of analysis (called small signal analysis or incremental signal analysis), we treat these devices as if they could generate electric power. In fact, neither a transistor nor a vacuum tube can generate electric power. However, they are usually used in a system with a distinct power source such as a generator or a battery. They control the instantaneous flow of power from that battery or generator to a device such as a loudspeaker, so that the overall output power (in the form of audio frequency or radio frequency electric power) is greater than the audio or radio signal power input. For example, the power input to a battery operated radio receiver from the receiving antenna is miniscule (typically picowatts), but the power input from the battery power supply is a watt or more. If we carefully examine the overall power flow in such devices (amplifiers, oscillators, etc.) we find that the output power is never greater than the input power coming from the wall outlet or from the batteries!

39 Transformer Uses in Telephones
Multi-winding transformer in telephone set (“hybrid coil” or “induction coil” together with other components acts as a directional coupler Directs most of the audio frequency power from the microphone to the CO, rather than to the earphone. Directs most of the audio frequency power from the CO to the earphone, rather than to the microphone Simple transformer at CO couples ac speech waveform between subscriber and switching/ transmission equipment, without connecting through the dc loop current “Hybrid coils” multi-winding transformer at CO separates earphone and microphone audio power into two separate unidirectional signals. Known as 2-wire to 4-wire conversion. Many other uses in T-1 transmission lines, ISDN systems, etc. not described here. Transformers are used extensively in telephone systems, but not usually where they are visible to the end user.

40 Telephone Test Capabilities
Many modern telephone switches have built-in test capabilities Late at night the subscriber loop is switched over (via relay* contacts on the line card) to a loop tester Tests are done for on-hook resistance between wires and from each wire to ground Excessive test current flow (low resistance) indicates problems - usually due to moisture in cables, damaged insulation, etc. Some trunks can also be tested for idle circuit noise (clicks and pops) Problems are often caused by moisture in cables. “Wet” cables must be dried or replaced. Drying is often accomplished via infusion of dry nitrogen gas. Automatic testing anticipates problems, and levels the work load for repair personnel Built-in test equipment (BITE) is one of the most important features of modern telecom systems * A relay comprises electromechanical switch contact(s) actuated (on/off) by the magnetic field produced by a separate control current. There are a number of reasons why telephone operating companies have changed almost all their plant and equipment from analog electro-mechanical switching to digital switching. Some important reasons include their smaller size, lower electric power consumption, greater internal reliability, and greater flexibility (since the switching system is controlled by a software stored program which can readily be updated to include new services and features). However, one of the most important features of digital switches which is cited by many telephone personnel is the benefit of built-in test capabilities. Not only does the switch monitor its own operation, but it periodically tests all the subscriber loops and trunks connected to itself. By detecting the beginning of a problem (such as moisture in a cable) before it becomes serious enough to be noticed by the customer, the problem can be fixed on a scheduled maintenance basis. In the past, without automatic test equipment, a number of wire pairs might fail simultaneously due to moisture in the same cable, and this created a need for extra repair technicians on staff continually to be prepared for just such an unscheduled emergency. As a regulated monopoly, a telephone operating company is usually required to keep all services operating continuously, and to give the customer a partial refund if outages cannot be restored in 24 hours. Today, with automatic test equipment, an incipient faulty cable can be repaired by either installing an new cable, or by forcing out the moisture with pressurized dry nitrogen gas. All of this happens before the customer is aware of a problem, with no outages, and with lower total staff counts. 19

41 Manual and Automatic Tests
Craftsperson can dial test numbers Ringback numbers in the CO switch allow test of the ringer (historic example: 550-xxxx where x’s represent “your own” last 4 digits) “Quiet line” allows human audible assessment of line noise Above tests are due to the switching system, not to the analog telephone set. In PBX and special CENTREX telephone sets, automatic test of each indicator light and button may be performed Manual testing procedures are partially automated to reduce the total number of test technicians required to install and test equipment. In the past, for example, a second technician was required to help the technician in the field by placing test calls, listening for noise on the line, etc. 20

42 Historical Telephone Schematic
Earphone (receiver) Microphone (transmitter) _ + Battery In this simple two-wire circuit, the battery provides dc current to generate a static magnetic field in the earphone. In the original 1876 Bell installations, the microphone had the same structure as the earphone (magnetic coil and flexible iron diaphragm) so the talk direction through it was reversible (microphone/earphone). After the 1880s, a permanent magnet was used in the earphone and the more sensitive Edison-Berliner carbon microphone was used. This simple circuit with carbon microphone is now definitely one-way. The battery provides current for the carbon microphone. The original telephone of A.G. Bell transmitted speech beyond the geographical range of normal acoustic attenuation (that is, further than you can shout!!) by producing an electrical waveform signal which is instantaneously approximately proportional to the air pressure of the sound waveform. The electrical signal is conveyed via wires. The earphone produces an air pressure waveform like the electrical waveform. The two waveforms are not precisely proportional to each other, due to limitations of the microphone and earphone. These devices are slightly non-linear (that is, the electric current is not precisely proportional to the related air pressure), and they do not respond equally to all frequency components of the signal (restricted bandwidth). The equipment in this illustration, which is similar to Bell’s original experimental telephone, can transmit signals from the microphone to the earphone. Because Bell used an electromagnetic microphone having almost the same physical structure as the earphone, the two people in the conversation can reverse the rôle played by each of these devices. The person on the right can speak into the electromagnetic earphone, using it as a microphone, and the person on the left can hold the nominal microphone to his ear, thus using it as an earphone. The two people must co-ordinate their activities by means of a word, like “over!” to indicate when to change from listening to speaking. In modern terms, we can call this a “half duplex” or “alternate direction simplex” type of connection. The carbon microphone, which soon replaced the electromagnetic microphone, due to its greater electrical power output and consequent longer wire connection capability, is not reversible in its function. A carbon microphone in this situation produces a “simplex” 2-wire system, useful only for transmission in one direction. 21

43 4-Wire Circuit _ + _ + Battery
Simplified physical 4-wire circuit, as used in some military telephone systems _ + Simultaneous transmission of voice in both directions is possible (but seldom done end-to-end) by using two separate “simplex” circuits, even with a carbon microphone or other modern non-reversible microphones (amplified electret or magnetic microphones). This 4-wire circuit is almost never used in standard public telephone systems, because similar results can be achieved at much lower cost by using only two wires between the subscriber and the central telephone switch. Four-wire systems have little or no transmission echo or side tone, except that due to acoustic feedback from the far end of the connection. This is beneficial where long transmission delay (via satellite) or low bit-rate digital coding is used. However, 4-wire switching with 4 actual separate wires is used in some military private branch exchange (PBX) and field telephone systems. In some systems where the voice signals are conveyed by a mechanism which completely separates the two directions of transmission, the name “4-wire” is used even when 4 wires are not present. In data communication, today we use the similar terms “full duplex” for the corresponding simultaneous transmission of data signals in both directions without mutual interaction. As we will see, the transmission of voice via trunks between CO switches in the public telephone network is done almost completely by means of 4-wire links, even though the transmission over the wires from the switch to the end subscriber (the “subscriber loop”) is only two wires. The radio link from mobile to base station is generally described as 4-wire, but the mechanism may be to use two separate radio frequencies for simultaneous communication in two directions (as in most cellular systems, GSM, CDMA, etc.), or by means of very brief bursts of signal in alternating directions on the same radio frequency (as in CT-2 and DECT PCS systems). _ + Battery Simplified diagram dies not show details of battery feed, dial, ringer, transformer coupling of voice signals, etc. 22

44 Historical 2-wire Carbon Mike Circuit
Simple, but inefficient and causes excessive “sidetone” in earphones. Desired Destination Earphone. Audio frequency sidetone appears at this earphone. Common battery Installed at Central Office. Switching aspects not shown. Audio input here. _ + Historically, the simplest 2-wire telephone system used a simple series connection of the earphone and microphone at each end. There is no problem with the electrical waveforms from the two microphones travelling simultaneously in opposite directions on the two wire connection between the two subscribers. However, there are several other problems with this simple circuit: Part (more than half) of the speech power from one subscriber’s microphone end is converted back into sound in the earphone at the same end. This audio feedback at the same end of the conversation, called “side tone,” causes most speakers to lower their voice. Furthermore, part of the signal power from the distant end of the conversation is dissipated as heat in the electrical resistance of the microphone. These things together cause the speech power level heard at the other end to be quite low or possibly even inaudible on long loops. An improvement in the transmission level at the earphone can be achieved by use of a directional coupler fabricated with an electrical transformer having multiple wire coil windings on a common core of magnetic material (steel or ferrite ceramic). Because of the combination of several windings, this directional coupler is known historically in telephone jargon as “hybrid coils” (in the CO, or a generic term for both types) or an “induction coil” (in the telephone set). The term “induction coil” has a more general meaning outside of the telephone industry, describing any single (non-transformer) coil or inductor. This type of directional coupler has been used in telephone sets since the 1920s. Audio frequency power wasted here Audio frequency power from this microphone is “wasted” in the local earphone and the other mike. Simplified diagram dies not show all details of battery feed, ringing, transformer coupling of voice signals, etc. 23

45 Hybrid/“Induction” Coil Directional Coupler
More efficient, less (not zero) side tone, uses only two wires to CO. Earphone having permanent magnet does not need dc - Secondary winding - Iron core - Split primary winding Line matching circuit Microphone signal current (red arrows) divides, produces canceling effects on secondary winding Two wires to CO switch. Hybrid coils are used in conjunction with a “line matching network” to (ideally) couple the electrical waveform from the central office only to the earphone and not to the microphone. Similarly, the audio waveform from the microphone is (ideally) coupled only to the central office and not to the local earphone. These ideal objectives are not fully met in real equipment. The hybrid coils are represented by the coil winding symbols consisting of linked semi-circles. The common magnetic iron core is represented by the long parallel lines between the windings. When audio waveform current flows from the central office through the split lower coil, both parts induce currents of the same co-operating polarity into the secondary coil, which is connected to the earphone. When audio waveform current flows from the microphone to the junction point in the split lower coil, it divides into two approximately equal parts if the impedance of the matching network (ideally) is exactly equal to the impedance of the subscriber loop. (Impedance is the ratio of voltage to current, to give a simplified description.) These currents flow in opposite directions through the two coils, thus producing two opposing current components, which cancel each other in the secondary coil. In both cases, the directional coupling is not ideal or 100% effective because the electrical characteristics (impedance or ratio of voltage to current) of the matching network are not precisely equal to the impedance of the subscriber loop and central office equipment, but are an approximation based on use of a limited number of electrical components (a resistor and two capacitors) and a compromise design based on the characteristics of the average length and wire size of a subscriber loop, rather than equipment specifically calibrated for each installation. Current from distant telephone (green arrows) produces same sense (direction) voltage in secondary, increases audio level. Simplified diagram of “induction coil” in telephone; many actual details set omitted. 24

46 Capacitor (“Condenser”)
Plate area A sq. meters d Graphic symbol. Curved line is the outer plate in a “rolled up” capacitor made of flexible metal foil and plastic sheet dielectric. Electrical capacitance measured via a unit called a farad = ampere•sec/volt (abbreviated F) Capacitance C (in farads)* of two metal “plates” separated by an insulating “dielectric” is approximately: C = e•A/d Where e is the “dielectric permittivity” of the core material. For air or vacuum e is 8.85•10-12 farad/meter. If plastic is used instead of air, typical e plastic is •10-12 farad/meter A=is the area of each plate d is the dielectric thickness *For most capacitors, the units microfarad or picofarad (µF or pF) are used A capacitor will be shown in the next part of the lecture when we describe the circuits at the central office. We actually already used a capacitor when discussing the hybrid coil directional coupler as well. It is therefore useful to give a brief description of what a capacitor is and what it does.

47 Capacitor Electric Properties
Relationship between voltage and current is (for ideal non-resistive “plates”) i = C•(dv/dt) When the voltage does not change with time, there is zero current. The capacitor “does not pass dc.” Following a short current pulse, positive charge remains on one plate and equal negative charge remains on the other plate Electrons have moved from the positive plate to the negative plate. An appropriate size and duration negative current pulse can restore the electrically neutral status of the plates, or reverse the charge polarity if the pulse last longer. A sequence of positive and negative current pulses produces an alternating positive and negative voltage. When a sine voltage waveform is used, a cosine current waveform results. The voltage and current are “out of phase” by 90 deg (1/4 cycle). Voltage positive peak occurs ¼ cycle after current peak. The ratio of the magnitude of the current to the magnitude of the voltage is proportional to the frequency. That is, a capacitor “passes” more current (has lower impedance) at higher sine wave frequencies. In the application described on the next page, we loosely say that the capacitor placed in the center between the two primary windings on the audio transformer at the central office “passes the audio frequency waveforms but blocks the dc loop current.”

48 Telephone Connection with CO Hybrid Coils
telephone set and subscriber loop Amplifier and A/D converter CO part Common battery feed and voice coupling Transmit signal Hybrid or “induction coil”and matching network _ Hybrid and matching network + Receive signal This schematic illustration shows the voice transmission essentials of the modern telephone. The internal functionality of the telephone set is essential “4-wire,” while the electrical subscriber loop is “2-wire.” This is economical, since only two wires are used for the many installed subscriber loops, while achieving most of the advantages of 4-wire operation (low side tone level and efficient delivery of audio waveform power from the far end to the earphone, with only a small amount of audio power lost in other network elements). Because of the imperfections in the matching network, a small amount of side tone is heard in most telephones. In fact, a telephone system with perfect suppression of sidetone sounds “dead” to most telephone subscribers, who are used to normal public telephone systems. They remark on the absence of any sidetone from the earphone or ask, “Is this ‘phone working?” A cellular or other 4-wire equivalent radio connection to the normal public telephone network is part of a system with pre-existing sidetone, but a mobile-to-mobile call is completely “4-wire” equivalent, and may sound unusually “quiet” to the subscriber. The imperfections in the hybrid coils and matching network or joints between different types of transmission wires also produce some reflected or “echo” audio signal. If this reflected audio signal, caused by the distant telephone or other hybrid coils in the telephone transmission path, arrives back at the originating end with a delay exceeding a few hundred milliseconds, it will be perceived by the subscriber as a distinct delayed echo. If it arrives back in less than approximately 100 milliseconds, most people will not perceive it as delayed, and it merely appears as a part of the overall sidetone. 48 V battery Amplifier and D/A converter Wire loop, up to ~10 km Telephone set (dial, ringer, cradle switch circuits for loop length level compensation not shown) Central office switch equipment. Actual switching is not shown. Positive battery terminal grounded to minimize electrolytic corrosion. Audio frequency voice signals coupled via transformer. Ringing power, loop current detection not shown. 25

49 Varistors and their Uses
A varistor is a simple non-linear silicon electrical device used in Type 500, 2500 and related telephone sets for several purposes. Varistors are made by binding together small grains of impure silicon with a conductive “glue” and fastening on two wires as “terminals,” then coating with plastic. Typically made from “scrap silicon” discarded during the zone refining process. Unlike a linear electrical resistor, in which current is proportional to voltage (Ohm’s law: v= R•i, where v is voltage, R is constant resistance, and i is current), the current in a varistor increases more than proportionately An empirical approximate formula for the varistor is i= K•v2, where K is a constant depending on varistor material and size Sign correction required in this formula since current has same polarity as voltage (current is negative when voltage is negative). Using signum function symbol: i= K•signum (v)•v2. Signum (v) is +1 for positive v, and -1 for negative v. A device which is described by a linear algebraic or differential equation is called a linear device. This is an equation of the form y=A•x, where y and x are variables and A is a constant. Any other relationship is called non-linear. Non-linear systems may be described mathematically via a polynomial equation in which the variables are squared, cubed, etc., or by means of other symbolic representations as well. In general, a non-linear electrical device will cause a change in the waveform of an electrical signal. In contrast, a linear device will generally increase or decrease the amplitude of an electrical waveform (equivalent to multiplying it by a scale factor) but will not change the waveform. High fidelity in an audio amplifier or other music and sound systems is associated with linearity. Please do not be confused by the current distinct use of “linear” as a synonym for “sequential,” as in the statement “he does everything by linear thinking,” meaning he considers one thing at a time in sequential order. When an electric waveform of small amplitude is added to a constant current (or voltage), the small changes in the voltage and current can be approximately analyzed as though the non-linear device was actually a linear device of an appropriate parametric value. Thus, the varistor, which is used in a telephone system having a constant dc loop current of perhaps 20 mA, and a smaller time-varying voice waveform current of only 1 mA, can be analyzed by using an appropriate equivalent resistor to represent the small-signal resistance. Since the slope of the voltage vs. current relationship curve is different for different values of constant loop current (or voltage), the equivalent resistance value is dependent on this “operating point” current or voltage. 26

50 Varistor Symbol & Graph
current I Schematic symbol io V voltage vo “Incremental” or small signal resistance is re= V/ I. Varies with operating point voltage vo or current io. Larger io gives smaller re. Varistors are made by compressing granules of a semiconductor material such as germanium or silicon and binding them together with an electrically conductive binder and two terminal wires. The resulting bead of material is covered with insulating varnish. Varistors are made from the scrap silicon material which is not pure enough to make transistors and integrated circuits. The large number of internal electrical junctions (which behave electrically like a diode) between the semiconductor granules and the conductive material produces a very non-linear current-voltage relationship. The varistor is electrically bilateral, meaning that it has the same electrical properties for both positive and negative voltage (or current) polarities. One way to understand the operation of the varistor is to visualize it as a large number of microscopic junction diodes in parallel, with both polarities of diodes present in about equal quantities. This visualization is the basis of the schematic symbol. Varistors were invented during World War II as an early result of semiconductor research. Their primary application is in telephone sets, although new integrated circuits do not make much use of varistors. They are also used to some extent to prevent sparks at electrical contacts in some relays and to regulate current in certain electrical devices. Today most electronic telephones achieve the two objectives previously obtained with varistors by using the desired related non-linear properties of transistor amplifiers. The microphone amplification can be adjusted in response to loop current, and the earphone loudness can be electronically regulated to avoid overly loud audio sound in the ear. Three varistors are used in a type 500 telephone set: One parallel with earphone to bypass high peak voltage audio (from power crossing or manual switchboard clicks) Two in parallel with microphone and matching network, to bypass more microphone audio on short loops (where loop current io is large) so high microphone audio level is not required at the CO. 27

51 Traditional vs. Modern Telephone Sets
The previous explanations mostly show traditional telephone set structure, based mainly on the type 500 design by AT&T Bell Laboratories in It’s relatively bulky by today’s standards for several reasons: Discrete electrical components were used, since integrated circuits were not available in 1948 Numerous wiring variations (e.g. multi-party ringer connected from one wire to ground, etc.) were provided via re-arrangeable spade-lug tipped wires and brass screw terminals. (Multi-party service has today almost disappeared in North America.) Press-in or machine-screw terminals were used because of union work rules, which prohibited any tool or instrument more sophisticated than a screwdriver for an ordinary telephone craftsperson Any adjustment to telephone company apparatus using, for example, slide or rocker switches of the type now used extensively today to set connections on certain computer equipment, required a technician of higher pay level than an ordinary telephone craftsperson. Incidentally, multi-party service has almost disappeared in North America. It was implemented by connecting one subscriber line to several homes, with all the telephone sets electrically in parallel. Some type of selective ringing was used (different cadence such as “one ring” versus “two rings” or other methods) and the subscribers had to politely share the line and not interfere with each other’s calls. Despite all of these restrictions, the Type 500 set was very economical to manufacture, particularly due to the very large quantity made each year (so-called “economy of scale”). Due to cross-licensing of the patents and design details, most of the telephone manufacturing companies in the entire world produced essentially similar telephones. In some cases, the precise design was cross licensed and completely identical part-interchangeable telephone sets were produced by AT&T (Western Electric) in the USA, Northern Electric (later Nortel Networks) in Canada, ITT in the USA, Italy, etc. Many other manufacturers made sets which were electrically equivalent, but did not use mechanically interchangeable parts. When the touch tone pushbutton dial was introduced in the 1960s by Larned A. Meacham, a Bell Laboratories engineer, a modified housing with a flat approximately square face plate for the 12 button dial pad was designed, and designated Type (There was also a ‘type 1500’ made briefly with only 10 pushbuttons.) 28

52 Integrated Circuit Telephone Sets
Today most inexpensive one-piece telephone sets use an integrated subscriber line circuit (SLC), which performs the 2-to-4 wire functions of the telephone by means of unidirectional transistor amplifiers. A variable gain amplifier controlled by loop current is used to compensate the microphone signal level for different loop lengths (no varistors needed) The earphone signal level is automatically controlled via an adjustable amplifier to prevent overly loud audio (no earphone varistor used) The pushbutton dial can produce either rotary-impulse or tone signals at will (controlled by an auxiliary switch), using a digital waveform generator for the tone dialing signals Many inexpensive modern telephone sets have all the electronics in one or two silicon chips. With the addition of a microphone and earphone, the set is almost complete, needing only a housing and dial push buttons. The electronic amplifiers in the chip are powered by the dc loop current. The earphone or microphone unit also acts as an alerting loudspeaker. The dial can be switched between tone and pulse mode of operation. Typically, a basic telephone set can be obtained at retail at a price below US$10. A set with digital memory to store several directory numbers for speed-dialing is only slightly more expensive. The original design of touch-tone dials used an analog audio oscillator with filters in the feedback loop to produce two simultaneous audio tones to produce the desired dialing signal. The frequencies of the various tone pairs was chosen because they are very unlikely to occur in speech or background noise picked up by the microphone. The dialing tones are also much louder than any normal speech levels. Because analog oscillators require relatively bulky and heavy electrical inductance coils, improved designs using only capacitors were developed for integrated circuit implementation. Inductors and large value capacitors are difficult if not impossible to make as part of an integrated circuit. In a later development, a waveform generator which is even more compact was developed by using a digital waveform sequence generator. This device produces a sequence of binary numbers corresponding to samples of the desired waveform, typically at the rate of 8000 samples per second, which is a de facto standard in the telephone industry. These digital values are then processed via a digital-to-analog converter to produce the desired analog waveform. This is the most widely used method in small telephones today. 29

53 Telephone Switching Modern Electronic Digital Switching software is real-time event-driven: The driving events are end-user actions such as dialing digits, lifting or replacing handset, etc. Circuit-switched voice telephone software mimics the human interface behavior of historical electro-mechanical switches Including incidental items like intentional post-dialing delay and non-symmetrical treatment of origin/destination vis-à-vis disconnect (for landline switches only – modern cellular switches disconnect immediately due to either participant’s actions)

54 Historical Switching Original 1876 A.G.Bell installations were point-to-point hard wired. Examples: Office to warehouse of same firm (like a modern intercom circuit) Palace to beach-house of the King of Hawaii Manual cord-board switching introduced in Hartford, CT in 1880s. Teen-age boys pulled electric wires across the room and temporarily connected them in response to verbal instructions from subscribers Later developments led to standard cord-board: a desk-like panel with a retractable cord from each voice connection unit, and a wall panel in front of the human operator with a socket for each subscriber (and historically later, a socket for each trunk line to another switching center) Parallel historical development of common battery power and supervision technology also facilitated the cord switchboard

55 Other 19th Century Improvements
Carbon Microphone (Edison and Berliner) Permitted loops of up to ~5 mi (8 km) due to greater transmitted electrical audio power level 2-wire “loop,” instead of single wire using earth conductivity for current return path Earth return was previous standard in telegraph systems, but produced tremendous “cross-talk” for telephones Loop greatly improved voice quality and reduced audio noise “Invented” by J.J.Carty, later chief engineer of AT&T Alternating current ringer (low maintenance) instead of previous buzzer devices with vibrating electric contacts subject to sparking, corrosion and deterioration Common (central office) battery for dc loop current using transformer to couple audio voice signal between two telephones in a conversation

56 Switchboard Plug Same diameter used today for 1/4 in (6.35 mm) stereo headset plug Insulators Tip (green positive wire) Sleeve (only in electro- mechanical switches, no standard outside-plant color) Note: use of red insulation for neg- ative polarity is unique to the telephone industry. Other electrical standards (power, electronics, auto- motive) use red for positive. Ring (red negative wire) Plug Assembly Graphic Symbol Ring Tip Sleeve Tip Ring Sleeve Socket Assembly Graphic Symbol

57 Supervision Methods In traditional telephone jargon, “supervision” describes only the aspects of signaling which relate to busy/idle status Dialed digit information was historically distinct (called “signaling”) In modern cellular/PCS software both things are often described by the word “supervision” therefore, be careful about jargon! Historical method to get attention of the operator was a small hand-cranked AC generator or “magneto” at subscriber end Resembled a hand-operated pencil sharpener… Produced about 90 V ac, at 20 Hz frequency. Still standard ringing waveform for North America today Then the common-battery circuit was introduced Subscriber “switch-hook” closed a current loop and operated a light and/or buzzer near that subscriber’s socket on the switchboard panel, in response to lifting the handset. Operator lifted a retractable plug cord from the desk-top, connecting her* headset to the subscriber via a voice-frequency transformer Operator then asked, “Number Please?” * Boys were replaced by more polite ladies in 1890’s; operator corps (except in military settings) was exclusively female until 1960s.

58 Call Connection Operator plugged other end of cord circuit into callèd subscriber socket. (The second syllable of callèd is artificially stressed in telephone jargon to emphasize the spoken distinction with “call”) Outer part of socket and “sleeve” (called “C” wire in European jargon) of plug carried a voltage when that line was busy. (No C wire in modern electronic switches.) Voltage (if present) on sleeve produced an audible click in operator earphone, indicating busy line. If so, operator would advise caller and abandon the process. If callèd line is idle, destination cord circuit plug is pressed in, connecting voice circuit of both telephones … and temporarily connecting the operator as well Operator presses momentary contact switch to apply 20 Hz, 90 V ac ringing to the callèd loop. Note that human operator controls ringing cadence. When callèd person answers, operator presses a latching switch on desk near the cords to disconnect operator’s headphone from the cord circuit When either participant hangs up, dc loop current from common central office battery stops, indirectly operating a distinct buzzer and light on the cord board via a relay. Operator then “tears down” the connection by pulling both retractable cord plugs from the callèd and calling part circuit sockets. Cords fall back into desk surface due to weights installed under the desk.

59 Cord Switchboard Capacity
The number of simultaneous conversations is limited to the number of cord circuits installed in a cord switchboard Each cord circuit is similar to a storage address (byte) in an electronic switch vis-à-vis capacity The BHCA* (call processing) capacity is limited by the attention and operational speed available from the human operator Both were improved by providing more operator positions (and thus more cord circuits) Each subscriber loop appeared at multiple wall sockets, each one within reach of an individual operator position Thus a historical need for busy status signal (sleeve or C wire) Early example of switch “concentration” Operator-handled calls were controlled by human intelligence Computer controlled (stored program controlled - SPC) switches merely strive to put back into automatic service many of the clever things human operators did historically (example, ring back to originator when initially busy destination finally becomes available) *BHCA=Busy Hour Call Attempts, a measure of how many call attempts per hour a switch can handle.

60 Some Human Operator Features
Call by name (no telephone number required) Response to: “Please call the Smith home.” Wake up calls (at pre-determined time) Re-connect calls accidentally disconnected* Notify busy line of incoming call waiting Set up 3-way (or more) conference call Connect call to alternate line when subscriber is away from home (call forwarding) Note that modern “feature-rich” PBX, small business key systems, and some PSTN switches now do these things via computer control Several experts have calculated that there are not enough people on earth to support the today’s (2005) level of public telephone traffic using operator cord board switching! *The GSM cellular system can optionally be configured to do this.

61 Strowger Step-by-step Switch
Almon B. Strowger, a mortician (undertaker) in Kansas City, KS, invented the first practical automatic dialing system Famous story: fearing that the human operator was directing calls for a mortician to his competitor, he invented an automatic user-controlled switch First version (installed in LaPorte, IN, circa 1895) used extra wires and push buttons on each subscriber set Rotary dial with impulsive current on the voice wire pair was a later development Strowger’s manufacturing firm, Automatic Electric, moved to suburban Chicago, IL. Later absorbed by GTE, later moved to Phoenix AZ, now AG Communication Systems (partly owned by Lucent) “Stepper” progressive control switches were manufactured world wide for many decades as exact replicas Electromechanical common-control switches developed by other manufacturers, such as “panel” and “crossbar” types partially succeeded steppers in the decades

62 Schematic Stepper Diagram
Tip, Ring, Sleeve wires from Rank 8, column 7. Electromagnets and springs activate the motions of the wiper arm in response to dial impulses. 4 5 6 7 3 8 Rank 0 9 2 Rank 9 1 Arm Ten places on each circular rank where a 3-contact assembly is located -- not illustrated in detail. Axle Vertical Motion Many details omitted here Rank 1 Rotary Motion

63 Stepper Switching Strowger switches evolved into an assembly with a movable wiper switch “inlet” and 100 “outlets” (wire pairs with “sleeve” wire) 10 contact pairs are arranged in a horizontal arc, selected by rotating the wiper switch arm. (Also a third “sleeve” wire in addition) 10 such horizontal arc sub-assemblies are stacked and selected via vertical motion of the axle (actually the first motion is vertical) Single-motion (rotation only) switch assemblies were also used “Line Finder” switch (mostly single motion) acts as input concentrator (“inverse” of selector action) Wiper arm contacts act as the single outlet Each line finder single-motion stepper is typically wired to 10 subscriber lines, and selects a line when that line goes off-hook Stepper starts stepping from line to line when any of the 10 lines go off hook, then stops when correct “off-hook” line is “found” analogous to operator responding to buzzer and light Multiple line finders are wired in parallel to the same 10 telephone sets analogous to multiple operator stations with each having access to the same subscriber sockets. Number of simultaneous originating conversations for that particular group of 10 subscribers is limited to the number of line finder switches connected to those lines. Ten line finders wired to ten subscribers is “non-blocking” with regard to line finders. (Overall system may still block at later stages…)

64 Selector Switches Line finder outlet goes through a transformer “cord circuit” Connected to dial-tone generator until the first dialed digit. Then the circuit is switched through a chain of two-motion selector stepper switches, with a “motion” for each digit. Each burst of impulses (dialed digit) produces a rotary or vertical motion constituting the next stage of the wiper arm selection process Dial pulses from rotary dial (typically 10 impulses per second, each one approximately 60 millisecond current OFF and 40 ms current ON) are passed around the cord circuit by special electro-mechanical relays A relay employs magnetically operated switch contacts, so that current ON/OFF status in the contacts mimics the current ON/OFF status in the wire coil causing the magnetic relay action. Special “slow release” relays hold the line finder so the 60 ms OFF intervals do not cause a disconnection Rotary Dialing: The subscriber turns the dial to an angle corresponding to one of the 10 digits, and then releases it. A spring wound by this action then rotates the dial back to normal position at uniform speed, producing 1 to 10 brief current interruptions (impulses) Simultaneously, an “Off-normal” switch contact in the telephone set temporarily short-circuits earphone so clicking is not heard Following a stage of selection motion, a slow release relay is automatically connected into that line to prevent further disturbance of that particular selection due to the succeeding bursts of dialing impulses

65 Significant Properties of Stepper Switches
To add more traffic capacity, install more line finders and more paralleled selector switches This increases parallel path (traffic) capacity through the switch, since multiple last stage selectors lead to the same destination lines. Only one last stage selector can connect at a given time. The sleeve wire is also connected to each corresponding position on the selectors and is used to divert the call to a busy signal generator if the sleeve voltage is ON for that destination line and a call is attempted while destination line is busy. A non-blocking Strowger step switch assembly would require 100 last stage selector switches connected to 100 destination telephone lines, and similar replication of parallel paths all the way to the originating lines (line finders, earlier stepper stages, etc.). This automatically increases the call processing capacity (BHCA) of the switch as well Each selector is both a traffic path and a part of the digit processing hardware When there is a traffic path available to the destination, there is also the hardware to respond to the succeeding dialed digits. A stepper switch assembly “automatically” has enough call processing capability if it was provisioned with adequate traffic path capacity

66 Stepper Properties Stepper switches are extremely reliable overall, when maintained Because of parallel path capability through a large stepper switch, the failure rate of these switches (when properly maintained) is very good Failures affecting only one user amount to only about 1 hour cumulative in 20 years Failure of the entire switch is only 1 or 2 minutes in 20 years, and when this occurs it is mostly due to human error or power supply aspects of the system Steppers can be adapted to many improvements Touch-tone dialing (by means of a tone-to-pulse converter) Computer control has been adapted to steppers to make advanced features available (such as call waiting, 3-way conference, etc.) But speed of connections, basic reliability, power consumption and size are not improved! Inter-switch signaling between stepper switches requires electrical transmission of dialing impulses conversion between modern digital signaling (common channel 7) and impulse switching is feasible, but slow acting European version of SS7 signaling allows transmission of one dialed digit at a time, but North American (ANSI) version does not send dialed number onward until the “last” digit is dialed. several earlier “electronic” but non-digital switching systems still used electromechanical switching (small relays) and analog transmission (example: No. 1 ESS), but digital computer central control or stored program control

67 Undesirable Stepper Properties
Relatively High maintenance ‘“Gross Motion” or “Large Motion” wiping contacts Require lubrication, cleaning, adjustment, etc. Susceptible to corrosion from sparking, air pollution (such as SO2 in the air, etc.) Slow mechanical operation Even when tone-to-pulse converters support Touch-tone dialing Slow signaling Can’t take full advantage of SS7 and other electronic signaling systems Big and bulky Digital switches use ~1/50th the floor area of steppers; ~1/10th the floor space of crossbar switches.

68 Some Other Historical Electro-Mechanical Switches
Panel (AT&T 1930s through 1950s) A huge mechanical “monster” switch using continuously running electric motors and electrically operated clutches to move wipers vertically and horizontally on a rectangular wall panel of contacts. High maintenance was a serious problem. Not widely used. Crossbar (Ericsson and AT&T, 1930s through 1980s) An assembly of rocking contacts attached to vertical and horizontal rotating actuator axles. Because of relatively small motion and compact size, this was the heir apparent to the stepper switch in both North America and Europe until electronic switching appeared. X-Y (Stromberg-Carlson, 1930s through 1970s) A horizontally platform with rows and columns of contacts with wipers actuated by magnetic coils. Gross motion problems, but more compact than Strowger design. Used only in relatively small switches. Rotary Similar to X-Y switch, but platforms had contacts arranged in semi-circles of increasing radius. More compact than Stepper, but same gross motion problems. Multi-relay Rocking contact motion, but still rather complex and difficult to maintain. The last 3 were mainly used by “independent” telcos in North America. All here except Crossbar and Multi-relay were “gross motion” switches.

69 Common Control Many of these electro-mechanical designs, particularly crossbar, had separate relay assemblies to count (“decode”) the dial impulses, completely separate from the switching portion of the system. These so-called “common control” portions were analogous to the computer control in a digital switch. Once the desired destination directory number was decoded, it was “translated” by special purpose wired logic devices One method for this was to use magnetic core memory of a special wired type (not addressable RAM like modern computer memory) The equipment numbers resulting from the translation were used to select a path through the switching part of the system. The result of the “translation” was a code designating the proper bay, shelf, and switch outlet wire for the internal destination calls, or the proper outgoing trunk group for outgoing (other switch) calls. The first non-busy channel in a trunk group was selected by an appropriate special outgoing trunk switch. These systems first demonstrated the need for provisioning separately both sufficient call processing capacity (BHCA) and also sufficient switching capacity (Erlangs)

70 Incidental Facts Rotary dial label “0” represents 10 impulses everywhere in the world (except Sweden, where the dial is labeled 0, 1, 2…9) However, touch-tone dials in Sweden use the same digit labels for DTMF tones as the world standard. Impulsive signaling must be converted at Sweden’s international boundaries. But symbolic signaling (binary digit codes used in SS7, etc.) is the same everywhere. Alphabetic dial labels (2=“ABC”, 3=“DEF”, etc.) were introduced in New York City in ~1923 when subscribers complained about “long” 5 digit directory numbers. Alphabetic dial labels were introduced in US, Canada, UK, France, Scandinavia and USSR (three cities only) but not all the same: Examples: Q on French dial, Russian (Cyrillic A Б B... G ...F) letters in Moscow, Leningrad, Odessa, Considered an obstacle to direct international dialing, alphabetic exchange names were purged from telephone directories in 1960s by international agreement. The “anti-digit dialing league” and other grass roots groups in the US opposed all-digit directories in the 1960s. Letter labels still appear on the dial in most of these named countries. Business users highly value so-called “Anagram” numbers such as FLOWERS, or NORSTAR, AMERICAn, etc.

71 De Facto Modern Alpha Dial Labels
abc def ghi jkl mno pqrs tuv wxyz * # Note restored letters q and z. Otherwise backward compatible with North American & British alpha dial labels. Emerged as cellular radio de facto standard Alternatively used for composing short alphabetic text messages (SMS)


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