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Page 1 ©1996-2006, R.Levine Digital Telecommunications Technology - EETS8320 Fall 2006 Lecture 2 Analog and Digital Telephone and Wireless Sets (Slides with Notes)
Page 2 ©1996-2006, R.Levine 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.
Page 3 ©1996-2006, R.Levine 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.
Page 4 ©1996-2006, R.Levine 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.
Page 5 ©1996-2006, R.Levine Direct Acoustic Communication Small variations in air pressure at audio frequencies, produced by the mouth and throat, propagate through the air as an acoustic wave. Sound pressure variations at eardrum ultimately cause nerve signals to the brain, perceived as sound.
Page 6 ©1996-2006, R.Levine Telephonic Communication Telephone System 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. A real telephone system only imperfectly reproduces the speech (high frequency components are attenuated, some distortion and delay are introduced as well).
Page 7 ©1996-2006, R.Levine 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 (1/1000 th ) voice power level
Page 8 ©1996-2006, R.Levine 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.
Page 9 ©1996-2006, R.Levine 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
Page 10 ©1996-2006, R.Levine 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) LowAnalog telephone set.highWestern Union used this to negotiate with Bell in ca. 1879 Liquid TransmitterMedium/ highHistorically one of the abandoned Bell investigations. HighImpractical because liquid evaporates and is corrosive. Variable resistance method probably inspired Edison Flexible iron disk near coil on permanent magnet. mediumSeldom used in telephone. Used in some wireless or intercom units. LowFirst 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) Medium/ highRadio 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 Medium/ highLandline 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/ highMedium quality recording or public address systems. Some wireless systems. Low. Requires amplifier Many natural crystals – quartz, rochelle salts, even table salt – have piezoelectric properties. Condenser mike. Flexible metal disk is one plate of a capacitor. Medium/ highHigh quality recording systems. Low. Requires amplifier Requires high-voltage power supply.
Page 11 ©1996-2006, R.Levine Earphone (Receiver) Electromagnetic transducer used almost universally ever since Bells 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 users 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.
Page 12 ©1996-2006, R.Levine 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
Page 13 ©1996-2006, R.Levine BORSCHT Acronym Functions in the Tel Set and Switch B attery: dc electric power O ver-voltage protection: not in the telephone set itself R inger: pre-answer alerting in general. May include caller ID feature signal between rings. S upervision: that aspect of signaling which conveys busy/idle status C odec: Analog-digital COder/DECoder in a digital telephone system. Not in analog telephone set itself. H ybrid: directional coupler, 2-wire to 4-wire conversion T est: modern telephone switches have built-in test capabilities. Simple analog telephone sets have little or no internal test-related equipment.
Page 14 ©1996-2006, R.Levine 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.
Page 15 ©1996-2006, R.Levine 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
Page 16 ©1996-2006, R.Levine Subscriber Loop Jargon Analog Subscriber Wire Pair A third wire called Sleeve (C) was used in electro- mechanical switches, but not today in digital switches.
Page 17 ©1996-2006, R.Levine 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 100-200 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 providers schedule, delay is typically 5 to 20 seconds.
Page 18 ©1996-2006, R.Levine 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.
Page 19 ©1996-2006, R.Levine 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.
Page 20 ©1996-2006, R.Levine 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
Page 21 ©1996-2006, R.Levine 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 switchs 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.
Page 22 ©1996-2006, R.Levine 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).
Page 23 ©1996-2006, R.Levine 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 originators 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 callers 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 originators 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.
Page 24 ©1996-2006, R.Levine Supervision S upervision 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).
Page 25 ©1996-2006, R.Levine 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.
Page 26 ©1996-2006, R.Levine 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.
Page 27 ©1996-2006, R.Levine 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.
Page 28 ©1996-2006, R.Levine Hybrid Coils H ybrid 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
Page 29 ©1996-2006, R.Levine Background about Transformers*-1 Prolific American inventor William Stanley made the first transformer in 1886. 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 voltsec/meter 2 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 = -d B /dt A rea 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 childrens toys (of the 1980s to the present) with parts that can be rearranged to make a robot, a truck (lorry) etc.
Page 30 ©1996-2006, R.Levine Magnetic Induction We can stack up such loops to form a helical coil of wire. Each added turn adds another v m volts A voltage V m will occur here if B is changing with time 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. Arrows represent magnetic B field. Loop area A is about ·(D/2) 2, where D is diameter of loop. v m = -d B /dt A
Page 31 ©1996-2006, R.Levine Background about Transformers-2 Electrical inductance measured via a unit called a henry = voltsec/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 n 2 A/g »Where µ is the magnetic permeability of the core material. For air or vacuum µ is 4 10 -7 henry/meter. If iron is used in the core instead of air, typical µ iron 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= (d/2) 2 »g is the length *For most inductors, the unit millihenry (mHy), 1/1000 of a Hy, is used. Incidentally, 4 = 12.56636 d g Graphic schematic symbol
Page 32 ©1996-2006, R.Levine 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.
Page 33 ©1996-2006, R.Levine 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.
Page 34 ©1996-2006, R.Levine Step-Up or Step-Down 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 primarycoil 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: R=10 V 2 =2v 1 and I 2 =i 1 /2, So V 2 /i 2 =4R or 40 i2i2 i1i1 v2v2 v1v1 + - + - Schematic transformer symbol Left coil has 2 times the number of turns on right coil.
Page 35 ©1996-2006, R.Levine Lowest Frequency for Transformer Transformers dont work at dc. What is the lowest useful frequency? In this ideal model of a transformer, used with driving current source I s, and self inductance L, the high frequency power in load resistor R is (Ni s ) 2R. (N is the coils turns ratio n 1 /n 2.) At dc (zero frequency), the power in resistor R is zero since all current is diverted by the inductor L. At sine wave frequency f c =R/(2 L), resistor power is ½ of its high frequency value. Half Power Frequency is convenient to measure. In telephone transformers, f c is typically 300 Hz by design. This is low enough so speech intelligibility is adequate. IsIs + - frequency Power/(Ni s ) 2R 1 0.5 fcfc R L Nv 2 Ni 1 i1i1 i2i2 v1v1 v2v2 + - + - Ideal transformer model
Page 36 ©1996-2006, R.Levine 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.
Page 37 ©1996-2006, R.Levine Current and Power Flow 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. ABCD + v - v + Power flow Power flow
Page 38 ©1996-2006, R.Levine 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!
Page 39 ©1996-2006, R.Levine 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.
Page 40 ©1996-2006, R.Levine 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.
Page 41 ©1996-2006, R.Levine 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 xs 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
Page 42 ©1996-2006, R.Levine Historical Telephone Schematic Microphone (transmitter) Earphone (receiver) + _ 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.
Page 43 ©1996-2006, R.Levine 4-Wire Circuit + _ + _ Battery Simplified physical 4-wire circuit, as used in some military telephone systems Simplified diagram dies not show details of battery feed, dial, ringer, transformer coupling of voice signals, etc.
Page 44 ©1996-2006, R.Levine Historical 2-wire Carbon Mike Circuit + _ Simple, but inefficient and causes excessive sidetone in earphones. Audio frequency power from this microphone is wasted in the local earphone and the other mike. Audio frequency sidetone appears at this earphone. Desired Destination Earphone. Audio frequency power wasted here Audio input here. Simplified diagram dies not show all details of battery feed, ringing, transformer coupling of voice signals, etc. Common battery Installed at Central Office. Switching aspects not shown.
Page 45 ©1996-2006, R.Levine 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 Microphone signal current (red arrows) divides, produces canceling effects on secondary winding - Secondary winding - Iron core - Split primary winding Line matching circuit Current from distant telephone (green arrows) produces same sense (direction) voltage in secondary, increases audio level. Two wires to CO switch. Simplified diagram of induction coil in telephone; many actual details set omitted.
Page 46 ©1996-2006, R.Levine Capacitor (Condenser) Electrical capacitance measured via a unit called a farad = amperesec/volt (abbreviated F) Capacitance C (in farads)* of two metal plates separated by an insulating dielectric is approximately: »C = A/d »Where is the dielectric permittivity of the core material. For air or vacuum is 8.85 10 -12 farad/meter. If plastic is used instead of air, typical plastic is 50 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 d Plate area A sq. meters Graphic symbol. Curved line is the outer plate in a rolled up capacitor made of flexible metal foil and plastic sheet dielectric.
Page 47 ©1996-2006, R.Levine 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.
Page 48 ©1996-2006, R.Levine Telephone Connection with CO Hybrid Coils Hybrid or induction coiland matching network Hybrid and matching network _ + Telephone set (dial, ringer, cradle switch circuits for loop length level compensation not shown) Wire loop, up to ~10 km 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. Amplifier and A/D converter Amplifier and D/A converter Transmit signal Receive signal CO part telephone set and subscriber loop Common battery feed and voice coupling 48 V battery
Page 49 ©1996-2006, R.Levine 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 (Ohms law: v= Ri, 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= Kv 2, 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= Ksignum (v)v 2. Signum (v) is +1 for positive v, and -1 for negative v.
Page 50 ©1996-2006, R.Levine Varistor Symbol & Graph voltage current V I vovo Incremental or small signal resistance is r e = V/ I. Varies with operating point voltage v o or current i o. Larger i o gives smaller r e. ioio Schematic symbol 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 i o is large) so high microphone audio level is not required at the CO.
Page 51 ©1996-2006, R.Levine 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 1948. Its relatively bulky by todays 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
Page 52 ©1996-2006, R.Levine 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
Page 53 ©1996-2006, R.Levine 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 participants actions)
Page 54 ©1996-2006, R.Levine 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
Page 55 ©1996-2006, R.Levine 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
Page 56 ©1996-2006, R.Levine Switchboard Plug Same diameter used today for 1/4 in (6.35 mm) stereo headset plug Tip (green positive wire) Ring (red negative wire) Sleeve (only in electro- mechanical switches, no standard outside-plant color) Insulators Tip Ring Sleeve Ring Plug Assembly Graphic Symbol Socket Assembly Graphic Symbol 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.
Page 57 ©1996-2006, R.Levine 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 subscribers 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 1890s; operator corps (except in military settings) was exclusively female until 1960s.
Page 58 ©1996-2006, R.Levine 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 operators 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.
Page 59 ©1996-2006, R.Levine 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.
Page 60 ©1996-2006, R.Levine 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 todays (2005) level of public telephone traffic using operator cord board switching! *The GSM cellular system can optionally be configured to do this.
Page 61 ©1996-2006, R.Levine 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 Strowgers 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 1930 - 1960 decades
Page 62 ©1996-2006, R.Levine Schematic Stepper Diagram Many details omitted here Vertical Motion Rotary Motion Rank 1 Rank 0 1 2 3 4 5 6 7 8 9 0 Tip, Ring, Sleeve wires from Rank 8, column 7. Electromagnets and springs activate the motions of the wiper arm in response to dial impulses. Ten places on each circular rank where a 3-contact assembly is located -- not illustrated in detail. Rank 9 Axle Arm
Page 63 ©1996-2006, R.Levine 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…)
Page 64 ©1996-2006, R.Levine 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
Page 65 ©1996-2006, R.Levine 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
Page 66 ©1996-2006, R.Levine 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
Page 67 ©1996-2006, R.Levine 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 SO 2 in the air, etc.) Slow mechanical operation –Even when tone-to-pulse converters support Touch-tone dialing Slow signaling –Cant 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.
Page 68 ©1996-2006, R.Levine 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.
Page 69 ©1996-2006, R.Levine 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)
Page 70 ©1996-2006, R.Levine 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 Swedens 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 Б ) 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 1-800- FLOWERS, or 1-800-NORSTAR, 1-800-AMERICAn, etc.
Page 71 ©1996-2006, R.Levine De Facto Modern Alpha Dial Labels Emerged as cellular radio de facto standard Alternatively used for composing short alphabetic text messages (SMS) 1 2 3 abc def 4 5 6 ghi jkl mno 7 8 9 pqrs tuv wxyz * 0 # Note restored letters q and z. Otherwise backward compatible with North American & British alpha dial labels.
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