Telephone Instruments and Signals

Telephone Instruments and Signals 1.INTRODUCTION Communications: • Process of conveying information from one place to another • Requires source, transmitter, receiver, destination and some form of transmission medium • Path may be short Telecommunications: • Tele-distance or far • Long distance communications • Using sound waves

Telephone system           

Invented by Alexander Graham bell and Thomas A. Watson Simple network Just pick up phone – dial a few digits – hold conversation with the person at other end Manufactured in 1920s and the same circuitry works till date Developed for human speech Now transmits data also using modems Global communication is possible This network is called PUBLIC TELEPHONE NETWORK (PTN) PTN interconnects subscribers through one or more switches Hence also called as PSTN (PUBLIC SWITCHED TELEPHONE NETW0RK) PTN consists of several large corporations and hundreds of smaller independent companies jointly referred to as TELCO 2. THE SUBSCRIBER LOOP

 Simplest service is called plain old telephone service (POTS)  Subscribers access the telephone network through a pair of wires called the local subscriber loop or simply called local loop  A local loop is an unshielded twisted pair transmission line (cable pair), consisting of two insulated conductors twisted together  The insulating material is generally polyethylene plastic coating  Conductor coating is a copper wire  Subscriber loop consists of several lengths of copper wire interconnected at junction and cross-connect boxes located in manholes, back alleys or telephone equipment rooms within large buildings and complexes

 Subscriber loop provides a means to connect a telephone set at a subscriber’s location to the closest telephone office, which is commonly called an end office, local exchange office or central office  Once in central office, the subscriber loop is connected to an electronic switching system (ESS), which enables the subscriber to access the public telephone network

3. STANDARD TELEPHONE SET TELEPHONE:  An apparatus for reproducing sound,  especially that of a human voice (speech),  at a greater distance, by means of electricity,  consisting of transmitting and receiving instruments connected by a line or wire  which conveys the electric current  Speech – sound in motion  The basic telephone set is a simple transceiver designed with the primary purpose of converting speech or acoustical signals to electrical signals Features of telephone:  Transmitting and receiving voice signals  Multiple line selection  Hold  Caller ID speaker phone  First telephone is called Butter stamp telephone  Later Western Electric company introduced a telephone set that was the industry standard for nearly four decades – rotary telephone  It is called the Bell System 500-type telephone  Modern Touch tone telephone is called a 2500-type telephone set The quality of transmission over a telephone connection depends on the received volume – the relative frequency response of the telephone circuit – the degree of interference In typical connection, the ratio of the acoustic pressure at the transmitter input to the corresponding pressure at the receiver depends on the following: – translation of acoustic pressure into an electrical signal – losses of the two customer local loop's – central telephone office equipment – cables between central telephone offices

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translation of the electrical signal at the receiving telephone set to acoustic pressure at the speaker output FUNCTIONS OF THE TELEPHONE SET 1. Notify the subscriber when there is an incoming call with an audible signal, such as 1. bell 2. a visible signal such as a flashing light 3. Provide a signal to the telephone network verifying when the incoming call has been acknowledged and answered 2. Convert speech energy to electrical energy in the transmitter and vice versa in the receiver 3. Incorporate some method of inputting and sending destination telephone numbers from the telephone set to the central office switch over the local loop 4. Regulate the amplitude of the speech signal the calling person outputs onto the telephone line 6. Incorporate some means of notifying the telephone office when a subscriber wishes to place an outgoing call (i.e., handset lifted off hook) 7. Ensure that a small amount of the transmit signal is fed back to the speaker, enabling talkers to hear themselves speaking. The feedback signal is sometimes called sidetone or talkback. Side tone helps prevent the speaker from talking too loudly 8. Provide an open circuit to the local loop when the telephone is not in use (on hook) and a closed circuit to the local loop when the telephone is in use (off hook) 9. Provide a means of transmitting and receiving call progress signals between the central office switch and the subscriber, such as – on and off hook – busy – ringing – dial pulses – Touch-Tone signals – dial tone

Telephone Set, Local Loop and Central Office Switching Machines • • • • • • •

A basic telephone set is connected to a central office switching machine (local switch) The basic telephone set requires only two wires (one pair of wires) from the telephone company to operate A pair of wires connecting a subscriber to the closest telephone office is called the local loop One wire on the local loop is called the tip and the other is called the ring When a third wire is used it is called a sleeve – 48V dc is connected is to the ring the tip is connected to ground. A dc voltage was used rather than an ac voltage for several reasons:

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to prevent power supply hum to allow service to continue in the event of a power outage because people were afraid of ac -48V dc is used for supervisory signaling and to provide talk battery for the microphone in the telephone set Ex: On hook, off hook and dial pulses -48V dc is the only voltage required for the operation of a standard telephone Modern telephones are equipped with nonstandard features and enhancements and may require an additional source of ac power Since 1960’s, phone plugs and jacks have gradually been replaced in the home with a miniaturized plastic plug known as RJ-11 and a matching plastic receptacle RJ stands for registered jacks and is sometimes described as RJ-XX RJ is a series of telephone connection interfaces that are registered with the U.S Federal Communications Commission (FCC) Jack some times defines the receptacle and the plug Sometimes it specifies only the plug RJ-11: 6 conductors Each wire 1/16inch by 1/16inch body Only two wires are used for standard telephone circuit to operate The other four wires can be used for a second telephone line and/or for some other special function

• BLOCK DIAGRAM OF A TELEPHONE SET A standard telephone set consists of • A transmitter • A receiver • An electrical network for equalization • Associated circuitry to control side tone levels and to regulate signal power • Necessary signaling circuitry The essential components of a telephone set are • Ringer circuit • On/off hook circuit • Equalizer circuit • Hybrid circuit • Speaker • Microphone • Dialing circuit

Ringer circuit:

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It is an electromagnetic bell, is placed directly across the tip and ring of the local loop Ringer circuit alerts the destination party of incoming calls The audible tone from the ringer must be loud enough to be heard from a reasonable distance and offensive enough to make a person want to answer the telephone as soon as possible In modern telephones, the bell has been replaced with an electronic oscillator connected to the speaker Today, ringing signals can be any imaginable sound, including buzzing, beeping, chiming or your favorite melody

On/off hook circuit: • • • •

Also called switch hook It is a simple single-throw, double-pole (STDP) switch placed across the tip and ring The switch is mechanically connected to the telephone handset so that when the telephone is idle (on hook), the switch is open When the telephone is in use (off hook), the switch is closed, completing an electrical path through the microphone between the tip and ring of the local loop

Equalizer circuit: • • •

Equalizers are combinations of passive components (resistors, capacitors and so on) that are used to regulate the amplitude and frequency response of the voice signals The equalizer helps solve an important transmission problem in telephone set design, namely, The interdependence of the transmitting and receiving efficiencies and a wide range of transmitter currents caused by a variety of local loop cables with different dc resistances

Speaker: • • • • • •

It is a receiver for the telephone Converts electrical signals received from the local loop to acoustical signals that can be heard and understood by a human being Speaker is connected to the local loop through the hybrid network Speaker is typically enclosed in the handset of the telephone along with the microphone It is a transducer Speaker converts electrical energy into mechanical energy and then into acoustic signals

Microphone: • • • • • •

It is the transmitter for the telephone Converts sound signals into electrical signals that are transmitted into the telephone network through the local subscriber loop It is connected to the local loop through the hybrid network It is a transducer The sound waves are first converted into mechanical energy and then this energy is converted into electrical energy Hybrid network or hybrid coil or duplex coil:

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This is a balanced transformer which is used to convert a two-wire circuit (local loop) into a four wire circuit (telephone set) and vice-versa thus enabling full-duplex operation over a two-wire circuit Hybrid network separates the transmitted signals from the received signals • Outgoing voice signals are in the 1V to 2V range, while incoming voice signals are typically half that value • Hybrid network allows a small portion of the transmit signal to be returned to the receiver in the form of a side tone • Insufficient side tones causes the speaker to raise his voice, making the telephone conversation seem unnatural • Too much sidetone causes the speaker to talk too softly, thereby reducing the volume that the listener receives

Dialing circuit: • •

Enables the subscriber to output signals representing digits and this enables the caller to enter the destination telephone number Dialing circuit could be a rotary dialer or an electronic dial-pulsing circuit or a TouchTone keypad, which sends various combinations of tones representing the called digits 4. BASIC TELEPHONE CALL PROCEDURES

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Each subscriber is connected to the switch through a local loop Switch is an electronic switching system (ESS machine) When the calling party’s telephone set goes off hook, the switch hook in the telephone set is released, completing a dc path between the tip and the ring of the loop through the microphone • ESS machine senses a dc current in the loop and recognizes this as an off-hook condition. This operation is called the loop start operation • The amount of dc current produced depends on the wire resistance, which varies with loop length, wire gauge, type of wire and the impedance of the subscriber’s telephone • Typical loop resistance ranges from a few ohms to approximately 1300 ohms, and typical telephone set impedances range from 500 ohms to 1000 ohms The following steps are involved in accessing a telephone system and this method is called POTS (plain old telephone service) 1. Calling station goes off hook 2. After detecting a dc current flow on the loop, the switching machine returns an audible dial tone to the calling station, acknowledging that the caller has access to the switching machine 3. The caller dials the destination telephone no. using one of two methods: mechanical dial pulsing electronic dual-tone multifrequency(Touch – Tone) signals

4. When the switching machine detects the first dialed number, it removes the dial tone from the loop 5. The switch interprets the telephone number and then locates the local loop for the destination telephone number 6. Before ringing the destination telephone, the switching machine tests the destination loop for dc current tot see if it is idle (on hook) or in use (off hook). At the same time, the switching machine locates a signal path through the switch between the two local loops 7a. If the destination telephone is off hook, switching machine sends a station busy signal back to the calling station 7b. If the destination telephone is on hook, the switching machine sends a ringing signal to the destination telephone on the local loop and at the same time sends a ring-back signal to the calling station to give the caller some assurance that something is happening 8. When the destination answers the telephone, it completes the loop causing the dc current to flow 9. The switch recognizes the dc current as the station answering the telephone. At this time the switch removes the ringing and ring-back signals and completes the path through the switch, allowing the calling and called parties to begin their conversation 10. When either end goes on hook, the switching machine detects an open circuit on that loop and then drops the connections through the switch

5. CALL PROGRESS TONES AND SIGNALS •

Call progress tones and call progress signals are acknowledgment and status signals that ensure the processes necessary to set up and terminate a telephone call are completed in an orderly and timely manner • Call progress tones and signals can be sent from – machines to machines – Machines to people – People to machines • When a switching machine outputs a call progress tone to a subscriber, it must be audible and clearly identifiable • Signals can be broadly be classified into two categories; – Station signaling – Interoffice signaling • Station signaling – Exchange of signaling messages over local loops between stations and telephone company switching machines • Interoffice signaling: Exchange of signaling messages between switching machines  Signaling messages can be subdivided further into one of four categories: Altering – indicate a request for service such as going off hook or ringing the destination telephone

Supervising – Provide call status information, such as busy or ring-back signals Controlling – provide call status information such as busy or ring-back signals Addressing – provide the routing information, such as calling and called numbers  Examples of call progress signals are – Dial tone – Dual tone multifrequency tones (DTMF) – Multifrequency tones (MF) – Dial pulses – Station busy – Equipment busy – Ringing – Ring-back – Receiver on hook – Receiver off hook – Receiver-left-off-hook alert

Dial Tone: • •

It is an audible signal comprised of two frequencies: 350Hz and 440Hz The two tones are linearly combined and transmitted simultaneously from the central office switching machine to the subscriber in response to the subscriber going off hook • Dial tone informs subscribers that they have acquired access to the electronic switching machine and can now dial or use Touch-tone in a destination telephone number • After a subscriber hears the dial tone and begins dialing, the dial tone is removed form the line (This is called a breaking dial tone) • Sometimes, a subscriber may go off hook and not receive dial tone • This condition is called no dial tone and occurs when there are more subscribers requesting access to the switching machine than the switching machine can handle at one time Dual-Tone multifrequency (DTMF): • Originally called Touch-Tone • Most efficient means than dial pulsing for transferring telephone numbers from a subscribers location to the central office switching machine • DTMF is a simple two-of-eight encoding scheme where each digit is represented by the linear addition of two frequencies • DTMF is strictly for signaling between a subscribers location and the nearest telephone office or message switching center • DTMF is some times confused with another two-tone signaling system called multifrequency signaling (MF), which is a two-of-six code designed to be used only to convey information between two electronic switching machines  Keypad is comprised of 16 keys and eight frequencies

 Last column indicates special purpose keys and hence are not used in household telephones  Household telephones use two-of-seven tone encoding scheme  The four vertical frequencies are called the low group frequencies  The four horizontal frequencies are called the high group frequencies  The frequency tolerance of the oscillators is ±.5%  The digits 2 through 9 can also be used to represent 24 of the 26 letters (Q & Z are omitted)  The letters were used to identify one local telephone exchange from another such as BR for Bronx, MA for Manhattan and so on  Today letters used to personalize telephone numbers; for example, 1-800-UPS-MAIL equates to the telephone number 1-800-877-6245  When a digit (or letter) is selected, two of the eight frequencies (7 for home phones) are transmitted (one from the low group and one from the high group)  Ex: When 5 is depressed, 770Hz and 1336Hz are transmitted simultaneously  The eight frequencies were purposely chosen so that there is absolutely no harmonic relationship between any of them, thus eliminating the possibility of one frequency producing a harmonic that might be misinterpreted as another frequency  Advantage of Touch-Tone signaling over dial pulsing is speed and control  With Touch-tone signaling, all digits take the same length of time to produce and transmit  Touch tone signaling also eliminates the impulse noise produced from the mechanical switches necessary to produce dial pulses  Dial pulses cannot pass through a central office exchange, whereas DTMF tones will pass through an exchange to the switching system attached to the called number Specifications for DTMF:  Transmit specifications are at the subscribers location  Receive specifications are at the local switch  Minimum power levels are given for a single frequency, and maximum power levels are given for two tones  Minimum duration is the minimum time two tones from a given digit must remain on  The interdigit time specifies the minimum and maximum time between the transmissions of any two successive digits  An echo occurs when a pair of tones is not totally absorbed by the local switch and a portion of the power is returned to the subscriber  The maximum power level of an echo is 10dB below the level transmitted by the subscriber and must be delayed less than 20ms

Multifrequency:

 Multifrequency (MF) tones (Codes) are similar to DTMF signals in that they involve the simultaneous transmission of two tones  MF tones are used to transfer digits and control signals between switching machines, whereas DTMF signals are used to transfer digits and control signals between telephone sets and local switching machines  Mf tones are combinations of two frequencies that fall within the normal speech bandwidth so that they can be propagated over the same circuits as voice. This is called in-band signaling  In-band signaling is rapidly being replaced by out-of-band signaling  MF codes are used to send information between the control equipment that sets up connections through a switch when more than one switch involved in completing a call  MF codes are also used to transmit the calling and called numbers from the originating telephone office to the destination telephone office  The calling number is sent first, followed by the called number Two tone MF codes: MF tones involve transmission of

Frequencies (HZ) Digit or control

Two of six possible frequencies Representing the 10 digits plus two Control signals

700+900

1

700+1100

2

700+1300

3

Digits are transmitted at a rate of Seven per second, and each digit is Transmitted as a 68ms burst

700+1500

4

900+1100

5

900+1300

6

The key pulse(KP) signal is a multiFrequency control tone used to indicate the end of a sequence of Dialed digits

900+1500

7

1100+1300

8

1100+1500

9

1100+1700

Key pulse (KP)

1300+1500

0

1500+1700

Start (ST)

2600Hz

IDLE

STcontrol signal indicates the beginning of the processing of the Signal IDLE signal is a 2600-Hz single Frequency tone placed on a circuit to Indicate the circuit is not currently in use

Dial Pulses: • Dial pulsing is also called rotary pulsing • Originally used to transfer digits from a telephone set to the local switch • Pulsing digits from a rotary switch began soon after the invention of the automatic switching machine • Process begins when the telephone set is lifted off hook, completing a path for current through the local loop • When the switching machine detects the off-hook condition, it responds with dial tone • After hearing the dial tone, the subscriber begins dial pulsing digits by rotating a mechanical dialing mechanism and then letting it return to its rest position • As the rotary switch returns to its rest position, it outputs a series of dial pulses corresponding to the digit dialed • When a digit is dialed, the loop circuit alternately opens (breaks) and closes (makes) a prescribed number of times • The number of switch make/break sequences corresponds to the digit dialed • Dial pulses occur at 10 make/break cycles per second • The central office switch incorporates a special time-out circuit to ensure that the break part of the dialing pulse is not misinterpreted as the phone being returned to its on-hook (idle) condition • All digits do not take the same length of time to dial • EX: Digit 1 requires only one make/break cycle, whereas the digit 0 requires 10cycles • Therefore all telephone no.s do not require the same amount of time to dial or to transmit

Station busy signal • • • • •

Station is a telephone set A station-busy signal is sent from the switching machine back to the calling station whenever the called telephone number is off hook Station busy signal is a two-tone signal comprised of 480Hz and 620Hz The two tones are on for 0.5 seconds, then off for 0.5 seconds Thus, a busy signal repeats at a 60-pulse-per-minute (ppm) rate

Equipment Busy Signal Or Congestion Tone Or No-circuits-available Tone • • •

Signal is sent from the switching machine back to the calling station whenever the system cannot complete the call because of equipment unavailability This condition is called blocking and occurs whenever the system is overloaded and more calls are being placed than can be completed Uses the same two frequencies as the station busy signal, except the equipment busy signal is on for 0.2 seconds and off for 0.3 seconds (120ppm)

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Because equipment busy signal repeats at twice the rate as a station-busy signal, an equipment busy is sometimes called a fast busy, and a station busy is sometimes called a slow busy The telephone company refers to an equipment busy condition as a can’t complete

Ringing: • • • • • •

The ringing signal is sent from a central office to a subscriber whenever there is an incoming call Purpose of ringing signal is to ring the bell in the telephone set to alert the subscriber that there is an incoming call If there is no bell in the telephone set, the ringing signal is used to trigger an another audible mechanism, which is usually a tone oscillator circuit The ringing signal is nominally a 20-Hz, 90-Vrms signal that is on for 2 seconds and off for 4 seconds The ringing signal should not be confused with the actual ringing sound the bell makes The audible ring produced by the bell was originally made as annoying as possible so that the called end would answer the telephone as soon as possible, thus tying up commonusage telephone equipment in the central office for the minimum length of time

Ring-back: • • • •

Ring –back signal is sent back to the calling party at the same time the ringing signal is sent to the called party Ring and ring-back signals are two different signals Ring back signal gives some assurance to the calling party that the destination telephone number has been accepted and processed and is being rung Ring-back signal is an audible combination of two tones at 440Hz and 480Hz that are on for 2 seconds and then off for 4 seconds

Receiver on hook: • • •

When a telephone is on hook, it is not being used and circuit is idle On hook means the telephone handset was literally placed on a hook When a telephone set is on hook, the local loop is open and there is no current flowing on the loop • An on-hook signal is also used to terminate a call and initiate a disconnect Receiver off hook: • When a telephone set is taken off hook, a switch closes in the telephone that completes a dc path between the two wires of the local loop • The switch closes in the telephone that completes a dc path between the two wires of the local loop • The switch closure causes a dc current to flow on the loop • The switching machine in the central office detects the dc current and recognizes it as a receiver off-hook condition • The receiver off-hook condition is the first step to completing a telephone call

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The switching machine will respond to the off-hook condition by placing an audible dial tone on the loop The off-hook signal is also used at the destination end as an answer signal to indicate that the called party has answered the telephone This is sometimes referred to as a ring trip because when the switching machine detects the off-hook condition, it removes the ringing signal

6. CORDLESS TELEPHONES • • • • •

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Operate without cords attached to the handset Occupy a narrow band of frequencies near 1.7 MHz, 60Hz house hold power line for an antenna Early units used FM and were poor in quality and susceptible to interference from fluorescent lights and automobile ignition systems Latest units use 2.4 GHz band Adaptive differential pulse code modulation and spread spectrum technology (SST) are used exclusively in the 2.4GHz band, while FM and SST digital modulation are used in the 902-MHz to 928-MHz band Digitally modulated SST telephones offer higher quality and more security than FM telephones Cordless telephone is a full-duplex, battery-operated, portable radio transceiver that communicates directly with a stationary transceiver located somewhere in the subscriber’s home or office The base station is an ac-powered stationary radio transceiver connected to the local loop through a cord and telephone company interface unit The interface unit functions in much the same way as a standard telephone set in that its primary function is to interface the cordless telephone with the local loop while being transparent to the user Therefore, the base station is capable of transmitting and receiving both supervisory and voice signals over the subscriber loop in the same manner as a standard telephone The base station must also be capable of relaying voice and control signals to and from the portable telephone set is a battery-powered, two-way radio capable of operating in the full-duplex mode Cordless telephone is a full-duplex, battery-operated, portable radio transceiver that communicates directly with a stationary transceiver located somewhere in the subscriber’s home or office The base station is an ac-powered stationary radio transceiver connected to the local loop through a cord and telephone company interface unit

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The interface unit functions in much the same way as a standard telephone set in that its primary function is to interface the cordless telephone with the local loop while being transparent to the user Therefore, the base station is capable of transmitting and receiving both supervisory and voice signals over the subscriber loop in the same manner as a standard telephone The base station must also be capable of relaying voice and control signals to and from the portable telephone set is a battery-powered, two-way radio capable of operating in the full-duplex mode

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7. CALLER ID • • • • • • • •

Enables the destination station of a telephone call to display the name and telephone number of the calling party before the telephone is answered This allows subscribers to screen incoming calls and decide whether they want to answer the telephone The caller ID message is a simplex transmission sent from the central office switch over the local loop to a caller ID display unit at the destination station The caller ID information is transmitted and received using Bell System 202-compatible modems This standard specifies a 1200-bps FSK signal with a 1200-Hz mark frequency (fm) and a 2200-Hz, 90-Vrms ringing signals Therefore to ensure detection of the caller ID signal, the telephone must ring at least twice before being answered The caller ID signal does not begin until 500ms after the end of the first ring and must end 500ms before the beginning of the second ring Therefore, the caller ID signal has a 3-second window in which it must be transmitted FRAME FORMAT • The 500-ms delay after the first ringing signal is immediately followed by the Channel seizure field, which is a 200-ms-long sequence of alternating logic 1sand logic 0s • A conditioning signal field immediately follows the channel seizure field The conditioning signal is a continuous 1200-Hz tone lasting for 130ms, which equates to 156 consecutive logic bits • The protocol for the message type field, message length field and caller ID data field – specifies asynchronous transmission of 16-bit characters framed by one start bit (logic 0) and one stop bit (logic 1) for a total of 10 bits per character • The message type field is comprised of a 16-bit hex code, indicating the type of service and capability of the data message • Only one message type field is currently used with caller ID (04 hex).

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Message type field is followed by a 16-bit message length field, which specifies the total number of characters included in the caller ID data field Caller ID data field uses extended ASCII coded characters to represent a month code (01 through 12), a two-character day code (01 through 31), a two-character hour code in local military time (00 through 23), a two-character minute code (00 through 59), and a variable-length code, representing the caller’s name and telephone number ASCII coded digits consists of two independent hex characters (8 bits each) The first hex character is always 3 (0011 binary), and the second hex character represents a digit between 0 and 9 (0000 to 1001 binary) Ex: Interpret the following hex code for a caller ID message (start and stop bits are not included in hex codes) 04 12 31 31 32 37 31 35 35 37 33 31 35 37 33 36 31 30 35 33 xx 04 message type word 12 decimal (18 chars in the caller ID data field) 31 31 ASCII code for 11 (month of November) 32 37 ASCII code for 27(the 27th day of the month) 31 35 ASCII code for 15 (the 15th hour – 3:00PM) 35 37 ASCII code for 57 (57 minutes after the hour – 3:57PM) 33 31 35 37 33 36 31 30 35 33 10digit ASCII coded telephone number (315 736 1053) xx Check sum (00 hex to FF hex)

8. ELECTRONIC TELEPHONES • • •

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ELECTRONIC TELEPHONES use integrated-circuit technology to perform many of the basic telephone functions It consists of one multifunctional integrated-circuit chip, a microprocessor chip, a TouchTone keypad speaker, a microphone and a handful of discrete devices The major components included in the multifunctional integrated circuit chip are DTMF tone generator, MPU (micro processor unit) interface circuitry, random access memory (RAM), tone ringer circuit speech network, and a line voltage regulator The touch-tone keyboard provides a means for the operator of the telephone to access the DTMF tone generator inside the multifunction integrated-circuit chip The external crystal provides a stable and accurate frequency reference for producing the dual-tone multifrequency signaling tones

9. PAGING SYSTEMS •

Paging systems are simplex wireless communications systems designed to alert subscribers of awaiting messages

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Paging transmitters relay radio signals and messages from wire-line and cellular telephones to subscribers carrying portable receivers Paging signals are transmitted from the paging system to the portable user and never in the reverse direction There are narrow (local), mid (regional) and wide (national) area pagers Narrow-area paging systems operate only within a building or building complex, midarea pagers cover an area of several square miles and wide-area pagers operate worldwide Most pagers are mid-area where one centrally located high-power transmitter can cover a relatively large geographic area, typically between 6 and 10 miles in diameter To contact a person carrying a pager, simply dial the telephone number assigned that person’s portable pager The paging company receives the call and responds with a query requesting the telephone number you wish the paged person to call After the number is entered, a terminating signal is appended to the number, which is usually the #sign The caller then hangs up The paging system converts the telephone number to a digital code and transmits it in the form of a digitally encoded signal over a wireless communications system The signal may be simultaneously sent from more than one radio transmitter times called (simulcasting or broadcasting), as is necessary in a wide-area paging system If the paged person is within range of a broadcast transmitter, the targeted pager will receive the message The message includes a notification signal, which either produces an audible beep or cause the pager to vibrate and the number the paged unit should call is shown on an alphanumeric display Some newer paging units are also capable of displaying messages as well as the telephone number of the paging party Early paging systems used FM Modern paging systems use FSK or PSK Pagers typically transmit bit rates between 200bps and 6400bps with the following carrier frequency bands 138MHz to 175MHz, 267MHz to 284MHz, 310MHz to 330MHz, 420MHz to 470MHz and several frequency slots within the 900-MHz band Each portable pager is assigned a special code, called a cap code, which includes a sequence of digits or a combination of digits and letters The cap code is broadcast along with the paging party’s telephone number If the portable paging unit is within range of the broadcasting transmitter, it will receive the signal, demodulate it, and recognize its cap code Once the portable pager recognizes its cap code, the callback number and perhaps a message will be displayed on the unit Alphanumeric messages are generally limited to between 20 and 40 characters in length

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Early paging systems developed by British Post Office is called Post Office Standardization Advisory Group (POCSAG) and transmitted a two-level FSK signal POCSAG is used an asynchronous protocol, required a long preamble for synchronization The preamble begins with a long dotting sequence to establish clock synchronization Data rates for POCSAG are 512 bps, 1200 bps and 2400bps With POCSAG, portable pagers must operate in the always-on mode all the time, which means the pager wastes much of its power resources on nondata preamble bits European Telecommunications Standards Institute (ETSI) developed the ERMES protocol ERMES transmitted data at a 6250bps rate using four-level FSK (3125 baud) ERMES is a synchronous protocol, which requires less time to synchronize ERMES supports 16 25KHz paging channels in each of its frequency bands FLEX protocol was developed in 1990’s FLEX is designed to minimize power consumption in the portable pager by using a synchronous time slotted protocol to transmit messages in precise time slots With FLEX, each frame is comprised of 128 data frames, which are transmitted only once during a 4-minute period Each frame lasts for 1.875 seconds and includes two synchronizing sequences, a header containing frame information andThe rest of the time, the pager is an ultra-low standby condition When a pager is in the wakeup mode, it synchronizes to the frame header and then adjusts itself to the bit rate of the received signal When the pager determines that there is no message waiting, it puts itself back to sleep, leaving only the timer circuit active pager identification addresses and 11 discrete data blocks Each portable pager is assigned a specific frame (home frame) within the frame cycle that it checks for transmitted messages Thus, a pager operates in the high-power standby condition for only a few seconds every 4 minutes (wakeup time)