Telecommunication Systems The telecommunication system is the largest and most complex electronic communication system in the world. It includes not only voice, but facsimile and computer data transmission, as well as, wireless transmission used in cellular and paging telephone systems.
Topics Covered in Chapter 17 Telephones The Telephone System Facsimile Paging Systems Integrated Services Digital Network
Telephones The telephone system was designed for full-duplex analog communication of voice signals. Today, this system is still primarily used for voice, but it employs mostly digital techniques, not only in signal transmission but also in control operations. The telephone system permits any telephone to connect with any other telephone in the world.
Local Loop Standard telephones are connected to the telephone system by way of a two-wire, twisted-pair cable that terminates at the local exchange or central office. As many as 10,000 telephone lines can be connected to a single central office. The two-wire, twisted-pair connection between the telephone and central office is referred to as the local loop or subscriber loop. The circuits in the telephone and at the central office form a complete electric circuit, or loop.
Basic Telephone System
Telephone Set A basic telephone or telephone set is an analog baseband transceiver. It consists of the following: The ringer is either a bell or electronic oscillator connected to a speaker. A switch hook is a double-pole mechanical switch that is usually controlled by a mechanism actuated by the telephone handset.
Telephone Set (Continued) The dialing circuits provide a way for entering the telephone number to be called. Most telephones use the dual-tone multifrequency (DTMF) system. The handset contains a microphone for the transmitter and a speaker or receiver. The hybrid is a special transformer used to convert signals from the four wires from the transmitter and receiver into a signal suitable for a single two-line pair to the local loop.
Basic Telephone Set
Standard Telephone and Local Loop The central office applies a -48 VDC over the twisted-pair line to the telephone. When a subscriber picks up the telephone, the switch hook closes, connecting the circuitry to the telephone line. The frequency response of the local loop is approximately 300 to 3400 Hz. Telephone wires are usually color coded where the tip wire is green and usually connected to ground and the ring wire is red.
Ringer The ringer in most older telephones is an electromechanical bell. The ringing voltage supplied by the central office is a sine wave of approximately 90 V at a frequency of about 20 Hz. In US telephones, the ringing voltage occurs for 1 s followed by a 3-s interval.
Transmitter The transmitter is the microphone into which you speak during a telephone call. In a standard telephone, this microphone uses a carbon element that effectively translates acoustical vibrations into resistance changes. The transmitter element is in series with the telephone circuit, which includes the 48-V central office battery and the speaker in the remote handset.
Transmitter and Receiver in a Telephone
Receiver The receiver, or earpiece, is basically a small permanent magnet speaker. A diaphragm is physically attached to a coil which rests inside a permanent magnet. Whenever a voice signal comes down a telephone line, it develops a current in the receiver coil. The coil produces a magnetic field that interacts with the permanent magnetic field. The result is vibration of the diaphragm which converts electrical energy into acoustic energy.
By Definition… The term dialing is used to describe the process of entering a telephone number to be called. The use of a rotary dialing mechanism produces what is known as pulse dialing. A dialing system called TouchTone uses pairs of audio tones to create signals representing the numbers to be dialed.
Electronic Telephones In the late 1950s, electronic telephones became practical and today most telephones use integrated circuits. Most multiple-line and full-featured telephones contain microprocessors. A built-in microprocessor permits automatic control of the telephones functions and provides features such as telephone number storage and automatic dialing and redialing that are not possible in conventional telephones.
IC Electronic Telephone Most functions of an IC electronic telephone are implemented with circuits contained within a single IC. A TouchTone keypad drives a DTMF tone generator circuit. An external crystal or ceramic resonator provides an accurate frequency reference for generating the dual dialing tones.
IC Electronic Telephone (Continued) A tone ringer is driven by the 20-Hz ringing signal from the phone line and drives a piezoelectric sound element. The IC contains a built-in line voltage regulator. An internal speech network contains a number of amplifiers and related circuits that fully duplicate the function of a hybrid in a standard telephone. A bridge rectifier provides a pulsating DC voltage. When the hook switch closes, the DC voltage is applied around an RC circuit.
Microprocessor Control All modern electronic telephones contain a built-in microcontroller. This microcontroller contains the CPU, a ROM in which a control program is stored, a small amount of random access read-write memory, and I/O circuits. The microcontroller, usually a single-chip IC, may be directly connected to the telephone IC, or some type of intermediate interface circuit may be used.
Microprocessor Control Functions performed by the microcomputer include: Operate keyboard Operate display (if included) Store telephone numbers Provide automatic redialing Store commonly called numbers Caller ID feature Answering machine feature
Answering Machine Answering machine which is more often called voice mail is sometimes implemented on more expensive electronic phones. The microcontroller automatically answers a call after a preprogrammed number of rings and saves the voice message. In modern phones, the voice message is digitized and compressed and then stored in a small flash ROM ready for replay.
Caller ID Caller ID, also known as the calling line identification service, is a feature that is now widely implemented on most electronic telephones. To make use of this service, you must sign up and pay for it on a monthly basis. With this feature, any calling number will be displayed on an LCD readout when the phone is ringing.
Line Interface Most telephones are connected by way of a thin multi-wire cable to a wall jack. A special connector on the cable, called an RJ-11 modular connector plugs into the matching wall jack. The wall jack is connected by way of wiring inside the walls to a central wiring point called the subscriber interface. This is also known as the wiring block or modular interface.
Cordless Telephone Concepts A cordless telephone is a full-duplex, two-way radio system made up of two units, the portable unit or handset and the base unit. The base unit is wired to the telephone line by way of a modular connector. It receives its power from the AC line. The base unit is a complete transceiver in that it contains a transmitter that sends the received audio signal to the portable unit and receives signals transmitted and retransmits on the telephone line.
Cordless Telephone Concepts (Continued) The base unit contains a battery charger that rejuvenates the battery in the handheld unit. The portable unit is also a battery-powered transceiver. Both units have an antenna. The transceivers in both the portable and the base units use full-duplex operation.
Cordless Telephone System
Cordless Phone Types The frequency range defines the three basic classes of cordless phones. They are: The simplest and least expensive cordless phones use the 43- to 50-MHz range. If higher quality and longer range are desired, phones in the 900-MHz or 2.4-GHz range can be used. The newer and perhaps the best cordless phones use DSSS in the 2.4-GHz band.
Telephone System The telephone system includes organizations and facilities involved in connecting a telephone to the called telephone regardless of where it might be in the United States or anywhere else in the world.
Subscriber Interface The subscriber interface or the subscriber line interface circuit (SLIC) is comprised of a group of basic circuits that power the telephone and provide all the basic functions such as ringing, dial tone, and dialing supervision. Most functions of the SLIC are implemented by one or two ICs plus supporting equipment. The SLIC provides seven basic functions referred to as BORSCHT (i.e. battery, overvoltage protection, ringing, supervision, coding, hybrid, and test.)
Telephone Hierarchy Whenever you make a telephone call, your voice is connected through your local exchange to the telephone system. From there it passes through at least one other local exchange, which is connected to the telephone you are calling. Several other facilities may provide switching, multiplexing, and other services required to transmit your voice.
By Definition… The central office or local exchange is the facility to which your telephone is directly connected by a twisted-pair cable. Regional operating companies (RBOCs) also called local exchange carriers (LECs) provide local telephone service. The LECs provide telephone services to designated geographical areas referred to as local access and transport areas (LATAs).
Private Telephone System Private telephone systems implement telephone service among the telephones in the organization and provide one or more local loop connections to the central office. The two basic types of private telephone systems are known as: Key systems Private branch exchanges
Key Systems Key systems are small telephone systems designed to serve from 2 to 50 user telephones within an organization. Simple key telephone systems are made up of the individual telephone units generally referred to as stations, all of which are connected to a central answering station. The telephone sets in a key system typically have a group of pushbuttons that allow each phone to select two or more outgoing trunking lines.
Private Branch Exchange A private branch exchange, or PBX is a private telephone system for larger organizations. Most PBXs are set up to handle 50 or more telephone interconnections. They can handle thousands of individual telephones within an organization. These systems may also be referred to as private automatic branch exchanges (PABXs) or computer branch exchanges (CPXs).
Private Telephone System (Continued) A PBX provides baseband interconnections to all the telephones in an organization. The PBX offers the advantage of efficiency and cost reduction when many telephones are required. The modern PBX is usually fully automated by computer control. An alternative to PBX is Centex. This service performs the function of a PBX but uses special equipment and special trunk lines.
Facsimile Facsimile, or fax, is an electronic system for transmitting graphic information by wire or radio. Facsimile is used to send printed material by scanning it and converting it into electronic signals that modulate a carrier to be transmitted over the telephone lines. Since modulation is involved, fax transmission can also take place by radio.
Facsimile (Continued) With facsimile, documents such as letters, photographs, line drawings, or any printed information can be converted into an electrical signal and transmitted. Facsimile uses scanning techniques that are generally similar to those used in TV. A scanning process is used to break a printed document up into many horizontal scan lines which can be transmitted and reproduced serially.
Facsimile Machine Todays modern fax machine is a high-tech electro- optical machine. Scanning is done electronically and the scanned signal is converted into a binary signal. Digital transmission with standard modem techniques is used.
Facsimile Operation The process begins with an image scanner that converts the document into hundreds of horizontal scan lines. Many techniques are used, but they all incorporate a photo-(light) sensitive device to convert light variations along one scanned line into an electric voltage. The resulting signal is then processed in various ways to make the data smaller and faster to transmit.
Facsimile Operation (Continued) The resulting signal is sent to a modem where it modulates a carrier set to the middle of the telephone voice spectrum bandwidth. The signal is then transmitted to the receiving fax machine over the public-switched telephone network. The receiving machines modem demodulates the signal that is then processed to recover the original data.
Modem Fax Machine
By Definition… Most fax machines use charged coupled devices (CCDs) for scanning. A CCD is a light-sensitive semiconductor device that converts varying light amplitudes into an electrical signal. Data compression is a digital data processing technique that looks for redundancy in the transmitted signal. Every fax machine contains a built-in modem that is similar to a conventional data modem for computers.
Paging Systems Paging is a radio communication system designed to signal individuals wherever they may be. Paging systems operate in the simplex mode, for they broadcast signals or messages to individuals who carry small battery-operated receivers. To contact an individual with a pager, all you need to do is make a telephone call. A paging company will send a radio signal that will be received by the pager. The paging receiver has a built-in audible signaling device or silent vibrator that inform the person that he or she is being paged.
Paging System Operation To contact a person via a pager, an individual dials the telephone number assigned to that person. The call is received at the office of the paging company. The paging company responds with one or more signaling tones that tell the caller to enter the telephone number the paged person should call. Once the number is entered, the caller presses the pound sign key to signal the end of the telephone entry.
Paging System Operation (Continued) The paging system records the telephone number in a computer and translates this number into a serial binary-coded message. The message is transmitted as a data bit stream to the paging receiver. Paging systems usually operate in the VHF and UHF frequency ranges. Most paging systems can locate an individual within a 30-mi radius.
The Paging Process
By Definition… Each paging receiver is assigned a special code called a cap code, which is a sequence of numbers or a combination of letters and numbers. The cap code is broadcast over a paging region and if the pager is in the region, it will pick up and recognize its unique code. Popular pager technologies include: FLEX, REFLEX, and INFLEXION. REFLEX and INFLEXION are two-way pagers.
Paging Receiver A paging receiver is a small battery-powered superheterodyne receiver. Most pagers use a single-chip IC receiver. Single- and double-conversion models are available. Direct Conversion receivers (ZIF) are also used. Most basic paging systems use some form of frequency modulation.
Integrated Services Digital Network Integrated services digital network (ISDN) is a digital communication interface designed to replace the local analog loop now used in the public switched network. It is designed to support digital voice telephones as well as fax machines, computers, video, and other digital sources. It allows computers to access online services directly without a modem.
ISDN (Continued) ISDN permits remote LANs to be easily interconnected without a modem or other expensive interface. ISDN is a telephone company service that anyone can order and use just like standard telephone services. ISDN is still available but is rarely used because of the rapid development of faster and cheaper cable TV modems and DSL.