Principles of Electronic Communication Systems Third Edition Louis E. Frenzel, Jr.

Telecommunication Systems Chapter 18 Telecommunication Systems

Topics Covered in Chapter 18 18-1: Telephones 18-2: Telephone System 18-3: Facsimile 18-4: Paging Systems 18-5: Internet Telephony

18-1: Telephones The telephone system is the largest and most complex electronic communication system in the world. The primary purpose of the telephone system is to provide voice communication. It is also widely used for many other purposes including facsimile transmission and computer data transmission.

18-1: Telephones The original 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. Each telephone must have a unique identification code—the 10-digit telephone number assigned to each telephone.

18-1: Telephones The 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.

18-1: Telephones Figure 18-1: The basic telephone system.

18-1: Telephones Telephone Set A basic telephone or telephone set is an analog baseband transceiver. It has a handset which contains a microphone and a speaker, better known as a transmitter and a receiver. It also contains a ringer and a dialing mechanism. The ringer is either a bell or an electronic oscillator connected to a speaker.

18-1: Telephones Telephone Set A switch hook is a double-pole mechanical switch that is usually controlled by a mechanism actuated by the telephone handset. When the handset is “on the hook,” the hook switch is open, thereby isolating all the telephone circuitry from the central office local loop. When a call is to be made or to be received, the handset is taken off the hook, closing the switch and connecting the telephone circuitry to the local loop.

18-1: Telephones Telephone Set 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 circuit 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.

18-1: Telephones Figure 18-2: Basic telephone set.

18-1: Telephones Standard Telephone and Local Loop The central office applies a −48 V dc 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: The tip wire is green and usually connected to ground, and the ring wire is red.

18-1: Telephones Figure 18-4: Tip and ring designation on an old plug and jack.

18-1: Telephones 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 Vrms at a frequency of about 20 Hz. In US telephones, the ringing voltage occurs for 1 s followed by a 3-s interval.

18-1: Telephones 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.

18-1: Telephones Figure 18-6: The transmitter and receiver in a telephone.

18-1: Telephones 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-magnet field. The result is vibration of the diaphragm which converts electrical energy into acoustic energy that supplies the voice to the ear.

18-1: Telephones Hybrid The hybrid, sometimes called an induction coil, is a device composed of several transformers that is used to simultaneously transmit and receive on a single pair of wires. The windings on the transformers are connected so that signals produced by the transmitter are put on the two-wire local loop but do not occur in the receiver and vice-versa.

18-1: Telephones Hybrid In practice, the hybrid windings permit a small amount of the voice signal to occur in the receiver. This provides feedback, called side tone, to the speaker so that she or he may speak with normal volume.

18-1: Telephones All telephones contain some type of component or circuit that provides automatic voice level adjustment so that the signal levels are approximately the same regardless of the loop lengths of the two telephones connected to each other. 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.

18-1: Telephones 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 telephone’s functions and provides features such as telephone number storage and automatic dialing and redialing that are not possible in conventional telephones.

18-1: Telephones Electronic Telephones: Typical IC Electronic Telephone Most functions of an 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.

18-1: Telephones Electronic Telephones: Typical IC Electronic Telephone 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.

18-1: Telephones Electronic Telephones: 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.

18-1: Telephones Electronic Telephones: Microprocessor Control Functions performed by the microcomputer include: Operating keyboard Operating display (if included) Storing telephone numbers Automatic redialing Storing commonly called numbers Voice mail Caller ID feature

18-1: Telephones Electronic Telephones: Voice Mail Voice mail, a feature previously called an answering machine, is implemented on most 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.

18-1: Telephones Electronic Telephones: 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.

18-1: Telephones Electronic Telephones: Line Interface Most telephones are connected by way of a thin multiwire cable to a wall jack. An RJ-11 modular connector plugs into the matching wall jack. The wall jack is connected by wiring inside the walls to a central wiring point called the subscriber interface. It is also known as the wiring block or modular interface. The subscriber interface is a small plastic housing containing all the wiring that connects the line from the telephone company to all the telephone wires in the house.

18-1: Telephones Figure 18-14: Subscriber interface.

18-1: Telephones Cordless Telephones: 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. It contains a transmitter that sends the received audio signal to the portable unit, and receives signals transmitted by the portable unit and retransmits them on the telephone line.

18-1: Telephones Cordless Telephones: Cordless Telephone Concepts 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.

18-1: Telephones Figure 18-16: General block diagram of a cordless telephone.

18-1: Telephones Cordless Telephones: Frequency Allocations. The FCC has set aside four primary frequency bands for cordless telephones: 43 to 50 MHz 902 to 928 MHz 2.4 to 2.45 GHz 5.8 GHz. Most of the newer phones use the 900- or 2.4- GHz bands. The phones are programmed to automatically seek a channel pair with no activity and minimum noise.

18-2: Telephone System The telephone system refers to the 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.

18-2: Telephone System 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.)

18-2: Telephone System Figure 18-17: BORSCHT functions in the subscriber line interface at the central office.

18-2: Telephone System 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. The telephone system is referred to as the public switched telephone network (PSTN).

18-2: Telephone System Telephone Hierarchy The central office or local exchange is the facility to which your telephone is directly connected by a twisted-pair cable. Regional Bell operating companies (RBOCs), also called local exchange carriers (LECs), provide local telephone service. Independent phone companies provide local service in rural areas not served by RBOCs. The LECs provide telephone services to designated geographical areas referred to as local access and transport areas (LATAs).

18-2: Telephone System Telephone Hierarchy Long-distance service is provided by long-distance carriers known as interexchange carriers (IXCs). The IXCs are the familiar long-distance carriers such as AT&T (now SBC), WorldCom (now Verizon), and US Sprint. Long-distance carriers must be used for the interconnection for any inter-LATA connections.

18-2: Telephone System Figure 18-18: Organization of the telephone system in the United States.

18-2: Telephone System Private Telephone System Private telephone systems implement telephone service among the telephones in a company or 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

18-2: Telephone System Private Telephone System: Key Systems Key systems are small telephone systems designed to serve 2–50 user telephones within an organization. Simple key telephone systems are made up of the individual telephone units called stations, all of which are connected to a central answering station. The central answering station is connected to one or more local loop lines, or trunks, back to the local exchange. 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.

18-2: Telephone System Private Telephone System: 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).

18-2: Telephone System Private Telephone System: Private Branch Exchange PBX provides baseband interconnections to all the telephones in an organization. The PBX offers the advantages 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 Centrex. This service performs the function of a PBX but uses special equipment and special trunk lines.

18-2: Telephone System Figure 18-19: A PBX.

18-3: 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.

18-3: Facsimile 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 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.

18-3: Facsimile Figure 18-20: Components of a facsimile system.

18-3: Facsimile How Facsimile Works Today’s 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.

18-3: Facsimile How Facsimile Works The transmission 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.

18-3: Facsimile How Facsimile Works The 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 machine’s modem demodulates the signal that is then processed to recover the original data. The data is decompressed and printed out.

18-3: Facsimile Figure 18-22: Block diagram of modem fax machine.

18-3: Facsimile Most fax machines use charge-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.

18-4: Paging Systems Paging is a radio communication system designed to signal individuals wherever they may be. Paging systems operate in the simplex mode. They broadcast signals or messages to individuals who carry small battery-operated receivers. To contact an individual with a pager, make a telephone call. A paging company will send a radio signal that will be received by the pager.

18-4: Paging Systems The paging receiver has a built-in audible signaling device or silent vibrator that inform the person that he or she is being paged. Some paging receivers have a small LCD screen on which a telephone number is displayed. This tells the paged individual which number to call. The newest pagers are two-way devices that receive data or send data in the form of numerically coded messages or short alphanumeric text.

18-4: Paging Systems Paging System Operation The paging business is closely allied with the telephone business, because the telephone system provides the initial and final communication process. 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.

18-4: Paging Systems Paging System Operation 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.

18-4: Paging Systems Figure 18-27: The paging process.

18-4: Paging Systems Paging Protocols 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. Thus the most widely used digital paging format is the FLEX system, developed by Motorola. REFLEX and INFLEXION are newer, two-way forms of FLEX.

18-4: Paging Systems 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.

18-4: Paging Systems Figure 18-30: A FLEX paging receiver.

18-5: Internet Telephony Internet telephony, also called Internet Protocol (IP) telephony or Voice over Internet Protocol (VoIP), uses the Internet to carry digital voice telephone calls. VoIP almost completely bypasses the existing telephone system. VoIP is a highly complex digital voice system that relies on high-speed Internet connections from cable TV companies, phone companies supplying DSL, and other broadband systems including wireless.

18-5: Internet Telephony VoIP uses the Internet’s vast fiber-optic cabling network to carry phone calls without phone company charges. In large companies, VoIP is replacing traditional telephone service because: It offers the benefits of lower long-distance calling charges It reduces the amount of new equipment needed, since phone service is provided over the same LAN that interconnects the PCs.

18-5: Internet Telephony VoIP Fundamentals There are two basic parts to an IP phone call: The “dialing” process which establishes an initial connection The voice signal flow.

18-5: Internet Telephony VoIP Fundamentals: Voice Signal Flow The voice signal is first amplified and digitized by an analog-to-digital converter (ADC), which is part of a coder-decoder (codec) circuit that also includes a digital-to-analog converter (DAC). The bit stream is processed by a voice encoder that compresses the voice signal. The resulting serial digital signal is put into a special packet by a microcomputer processor running a VoIP protocol and then transmitted by Ethernet over a LAN or via a high-speed Internet connection such as is available from a cable TV company or on DSL.

18-5: Internet Telephony VoIP Fundamentals: Voice Signal Flow From there the signal travels over standard available Internet connections using TCP/IP through multiple servers and routers until it comes to the desired location. At the receiving phone, the process is reversed. The Internet signal gets converted back to Ethernet, and the VoIP processor recovers the original packet. The compressed data is extracted, decompressed by a DSP, and sent to the DAC in the codec where the original voice is heard.

18-5: Internet Telephony VoIP Fundamentals: Voice Signal Flow One of the main problems with VoIP is that it takes a relatively long time to transmit the voice data over the Internet. Also, even though the signals travel at gigabit speeds, the packets pass through numerous routers and servers, each adding transit time or latency. Latency is the delay between the time the signal is transmitted and the time it is received. One party may have to wait a short time before responding to avoid talking while the signal is still be received.

18-5: Internet Telephony VoIP Fundamentals: Link Establishment A special protocol called the session initiation protocol (SIP) is used to get the “packetized” voice data to the desired phone. The protocol sets up the call and then makes sure that the voice packets produced by the calling phone get sent to the receiving phone in a timely manner.

18-5: Internet Telephony Internet Phone Systems Home VoIP: To establish IP phone service in the home, the subscriber must have some form of high-speed Internet service (Cable TV or DSL). In addition, the subscriber must have a VoIP interface. A common example is the Analog Terminal Adapter (ATA). This device connects the standard home telephone to the existing broadband Internet modem.

18-5: Internet Telephony Internet Phone Systems Enterprise IP Phones: IP phones in companies or large organizations are especially designed for VoIP service. The telephone set contains all the ATA circuitry except for the SLIC and connects directly to the available Ethernet connection usually supplied to each desk. No broadband modem is needed. Since most employees will also have a PC connected to the LAN, a two-port Ethernet switch in the phone or PC provides a single Ethernet connection to the LAN that the phone and PC share.

18-5: Internet Telephony Figure 18-32: Analog terminal adapter (ATA) or VoIP gateway.