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Address Signals also known as digits and selection signals are forward signals that are sent by the calling subscriber when dialing the called party number.. The telephone is connected t

3 SUBSCRIBER SIGNALING

The vast majority of the customers of telecommunication networks are subscribers who are attached to their local exchanges by analog subscriber lines The signaling between subscriber and local exchange is known as subscriber signaling [l-4] The original, and still predominant, application of subscriber signaling is plain old telephony service (POTS) calling However, subscriber signaling today also supports supplementary services such as call waiting, call forwarding, caller identification, etc

3.1.1 Signaling for an lntraexchange Call

Figure 3.1-1 shows the signaling for an intraexchange call between subscribers

S, and S, The directory number of called subscriber S, is 347-9654

Calling subscriber S1 starts by going off-hook (lifting the handset of the telephone from its cradle) The off-hook is interpreted by the exchange as a

subscriber service) In response, the exchange returns dial-tone, indicatmg that it is ready to receive digits Subscriber S1 then sends the digits of the called number, using the dial or the keypad of the telephone After receipt of 3-4-7, the exchange recognizes one of its exchange codes, and thus knows that it is the destination exchange for the call

The exchange can identify the called subscriber S2 after receipt of the complete called number, and checks whether S2 is free Assuming that this is

47

Signaling in Telecommunication Networks John G van Bosse

ISBNs: 0-471-57377-9 (Hardback); 0-471-22415-4 (Electronic)

@ Cut-through of Path in Exchange

0 Release of Path in Exchange Figure 3.1-l Signaling for an intraexchange call

the case, it sends a ringing signal to alert S,, and informs S, about the call progress with a ringing-tone

When S2 goes off-hook, an answer signal is generated The exchange then cuts through (sets up a path in its switchblock between the subscriber lines) The conversation starts, and the exchange begins to charge S1 for the call At the end

of the call, the subscribers put the handsets back in the cradles of the telephones The signals generated by these actions from the calling and called subscribers are known as respectively the clear-forward and the clear-back signal

Calling Party COWOL The release of connections is usually under control of the calling party In Fig 3.14, the calling party clears first, and the exchange immediately releases the connection

If the called party clears first, the exchange starts a timer of say 30-60 seconds, and releases the call when it receives a clear-forward, or on expiration

of the timer, whatever occurs first This is for the convenience of the called party, who may have picked up a phone in one room, but wants to have the conversation

in another room The called party can then hang up the first phone, move to the other room and pick up the other phone, while the connection stays up

Forward and Backward Signals Call-control signals are categorized as of

which the call is set up (from S1 to S,), and backward signals are sent in the opposite direction The request-for-service signal, and the digits of the called

SIGNALING COMPONENTS IN TELEPHONES 49

number, a re examples

backward signals

of forward signals Dial- tone, ri nging- tone, and answer are

3.1.2 Groups of Subscriber Signals

The signals in the example can be divided into the following four groups Supervision Signals (or line signals) are signals sent by subscribers to local exchanges The forward supervision signals (request-for-service, disconnect by calling party) request the start or end of a connection The backward super- vision signals (answer, disconnect by called party) change the state of a call Address Signals (also known as digits and selection signals) are forward signals that are sent by the calling subscriber when dialing the called party number

Ringing A forward signal sent by the exchange to the called subscriber, to indicate the arrival of a call

Tones and Announcements Audible backward signals (dial-tone, ringing- tone, busy-tone, etc.) sent by an exchange to the calling subscriber, and indicating the progress of a call

This section presents an overview of the circuitry in a telephone, focusing on the components for subscriber signaling

3.2.1 Telephone

The major components in a telephone are shown in Fig 3.2-l The telephone is connected to a line circuit LC in the local exchange by a subscriber line that transfers the subscriber’s speech and the subscriber signaling

Transmitter (microphone) (TR) and receiver (RCV) convert acoustic speech signals to electrical analog signals, and vice versa Transformer (T) and resistor (R) are part of the speech circuit

The signaling functions of a telephone are: the generation (controlled by the subscriber) of supervision signals and digits; the conversion of received electrical tone, and announcement signals into acoustic signals, and- the conversion of the electrical ringing signal into a high-level acoustic signal that can be heard at some distance from the telephone

3.2.2 Supervision Signals

A telephone can

its handset rests

be in two supervision state

in a cradle, and depresses

s When the telephone is not in use, cradle switch (CS)-see Fig 3.2-l

In this state, the telephone is on-hook (this term has been carried over from the time when the receivers of telephones were resting on a hook) When the telephone is on-hook, switch (CS) connects ringer (RR), in series with capacitor (C), to the subscriber line

When a subscriber starts to use the telephone, he lifts the handset out of its cradle In this state, which is known as off-hook, switch (CS) connects transmitter (TR) and receiver (RCV) to the subscriber line

When the telephone is off-hook, direct current can flow in the subscriber line When the telephone is on-hook, capacitor (C) blocks direct current At the exchange, line circuit (LC) determines the supervision state of the telephone from the presence or absence of direct current in the line

Address signaling takes place while the telephone is off-hook (CS closed) There are two types of address signals (digits) Today’s local exchanges handle both signal types

Dial-pulse (DP) Address Signals In early telephones, the address signals were generated by dials [1,2] The dial switch (DS) in Fig 3.2-l(a) is linked mechanically to the dial When the dial is at rest, DS is closed, and the telephone

Iulse address signals B: (60 ms) M: make (40 ms) I: interdigital

presents a path for direct current between points A and B When the dial-after having been rotated by the subscriber- spins back to its rest position, DS opens and closes a number of times, producing a string of breaks in the d.c path The number of breaks in a string represents the value of the digit: one break for value

1, two breaks for value 2, , ten breaks for value 0 The nominal length of a break

is 60 ms (Fig 3.2-2) The breaks in a string are separated by make intervals of nominally 40 ms Consecutive digits are separated by an interdigital interval of

at least some 300 ms

Dual-tone Multi-frequency (DTMF) Address Signals Around 1960, it became practical to place transistor oscillators in telephone sets, and this led to the development of DTMF address signaling [3-51 Figure 3.2-l(b) shows a DTMF telephone, which includes a keypad (KP) that controls a dual-tone oscillator (0)

When a subscriber depresses one of the keys on the keypad (KP), oscillator (0) produces two simultaneous tones A digit is represented by a particular combination of two frequencies; one selected from a low group (697,770,852,

941 Hz) and the other selected from a high group (1209,1336,1477,1633 Hz) This allows 16 digit values, but only 12 of these are implemented on the keypads: digit values 1,2, 0, and the special values * and #

The DTMF frequency combinations have been standardized by CCITT [6]:

Digit Value Frequencies (Hz)

52 SUBSCRIBER SIGNALING

3.2.4 Ringing Signal

When the telephone is on-hook (Fig 3.2-l) and the exchange sends an electrical ringing signal (an alternating current), ringer (RR) produces an audible signal that can be heard in the vicinity of the telephone

In early telephones, the ringers were electromechanical devices Modern telephones have electronic ringers

3.2.5 Tones and Recorded Announcements

These signals have the same electrical characteristics as the speech received during a call Like speech, they are converted into acoustic signals by receiver (RCV)

This section gives an example of the equipment for subscriber signaling at local exchanges We consider a local SPC (stored program control) exchange with a digital switchblock (see Figs 1.7-4 and 1.7-S)

Multiplexed Service Circuits TAC

Figure 3.3-l Equipment for subscriber signaling at a local exchange with a digital switch- block LC: line circuits H: hybrid circuits DMP: digital multiplex port TAC: tone and announce- ment circuits DRC: DTMF receiver circuits

circuits (LC), and are converted into four-wire analog circuits (d) by hybrids (H) First-order PCM multiplexes convert wt of these circuits (m = 24 or 30) into a four-wire digital multiplex circuit (b) that carries PCM-coded speech (1.51) These circuits are attached to digital multiplex ports (DMP) of the switchblock The bit format on (b) is as shown in Fig 1.5-4 Also attached to DMP ports are a number of multiplexed digital sewice circuits

The switchblock provides temporary 64 kb/s digital paths between a service circuit and a subscriber line-more exactly, a PCM multiplex channel (b) associated with the subscriber line These paths transfer PCM-coded tones, DTMF frequencies, and announcements

The switchblock, and the line and service circuits, have control channels (CC)

to the exchange processor This enables the processor to send commands to, and receive information from, these entities

The implementation of subscriber-signaling functions is manufacturer- specific In this example, we assume that the line circuits receive the supervision signals and dial-pulse digits from their lines, and send ringing signals to the lines

We also assume that there are two types of service circuits Tone and

tones and announcements that can be sent to a subscriber When for example a busy-tone has to be sent to a subscriber line, a switchblock path is set up between

an available TAC circuit and the PCM channel associated with the line The processor then orders the circuit to send busy-tone The second type of service circuits are DTMF-digit receiver circuits (DRC) These circuits can provide dial-tone and receive DTMF digits

Figure 3.3-2 shows a line circuit (LC) in some detail The circuit can be in two states, which are changed on command from the processor In the figure, LC is

in its “normal” state Switch (S) connects transformer (T) to the subscriber line When the telephone is involved on a call, the transformer transfers the (analog) speech between the subscribers The LC is set to the ringing state when the telephone has to receive a ringing signal

Hook Status In both LC states, the “hook” state of the telephone is monitored

by current detector (CD), and reported to the exchange processor When the telephone is on-hook (idle), there is no path for direct current between points A and B, and no current flows through CD, which then indicates to the processor that the telephone is on-hook When the telephone is off-hook, there is a path for direct current between points A and B, and a current flows from ground- through transformer (T), the external path (A-B), and current detector (CD)-

to the common battery (BAT) The CD then indicates that the telephone is off-hook

Call States The exchange processor keeps track of the “call” state of the

Multiplex and Switchblock

Figure 3.3-2 Components in line circuit, shown in the normal state T: transformer H: hybrid circuit CD: current detector CC: control channel RS: ringing source BAT: exchange battery S: switch

telephones, and stores these states in its temporary memory We distinguish the following major call states:

Idle: not involved in a call

Dialing: before and during the sending of sending address signals

Calling: involved in a call as calling party, after having sent the address signals Ringing: receiving the ringing signal from the exchange (which indicates an incoming call)

Called: involved in a call as a called party, and having answered

Determination of Signal Type A change in the hook status of a telephone is

a supervision signal The processor determines the signal type, based on the present call state, and the type of hook-status change:

Present Call State Change in Hook Status Supervision Signal Idle

Recognition Time Electrical disturbances on a subscriber line can result in brief off-hook (on-hook) pulses on lines that are on-hook (off-hook) The

SIGNALING EQUIPMENT AT THE LOCAL EXCHANGE 55

processor therefore takes no action until the new hook state has persisted for predetermined recognition time (in the order of 20-40 ms)

subscriber who in is the calling or called state, to request an action from the local exchange

The subscriber generates the flash by momentarily depressing the button of cradle switch (CS) (Fig 3.2-l) This results in a temporary on-hook condition of the telephone The length of a flash varies widely Exchanges usually interpret on-hooks of 0.1-1.0 seconds as flashes, and consider longer on-hooks as clear- forward or clear-back signals The uses of flash signals are discussed later in this section

3.3.4 Reception of Address Signals

On receipt of a request-for-service from a subscriber line, the processor marks the line as “dialing,” selects an idle digit receiver (DRC), orders the switchblock to set up a path between DRC and the line, and commands the DRC to send dial- tone

If the calling subscriber is using a telephone with dial-pulse address signaling, she rotates the dial, and this generates the digits as strings of “break” and

“make” pulses that are detected by current detector (CD) in the line circuit (Fig 3.3-2), and reported to the processor On receipt of the first break, the path between the line and DRC is released

If the calling subscriber is using a telephone with DTMF signaling, she depresses the keys on the keypad This generates DTMF digits that are received

by DRC, and reported to the processor On receipt of the first digit, the dial-tone

is turned off The path between the subscriber line and the DRC is released when the complete called number has been received

Digit receivers have frequency-selective circuits that are tuned to the individual DTMF frequencies, and detect the presence of these frequencies on the subscriber line The receivers accept a digit only if one frequency of the low group, and one frequency of the high group, are present simultaneously for at least 70 ms

Digit Imitation When a key on a keypad telephone is depressed, the transmitter is disabled However, there are intervals (between digits) during DTMF signaling when no key is depressed During these intervals, the transmitter is enabled, and may pick up speech, music, or noise in the vicinity of the calling subscriber These sounds should not imitate DTMF digits The DTMF frequencies have been chosen to minimize digit imitation, by making DTMF tone pairs distinguishable from naturally occurring sounds [5]

Naturally occurring sounds contain tone pairs whose frequency ratios are

“simple” fractions (such as 1:2,3:5,2:3,3:4,4:5, etc.)

Suppose now that a sound has an 1336 Hz component This is detected in the

56 SUBSCRIBER SIGNALING

digit

most

receiver by the frequency-selective circuit tuned to this frequ

likely companion frequencies in this sound are listed below:

Frequency (Hz)

Adjacent DTMF Low-Group Frequencies (Hz)

3.3.5 Ringing Signals

The ringing signal is a high-level 20-25 Hz signal analog, of typically 100 V rms, and designed to drive the electromechanical ringers in the early telephones This ringing signal cannot be provided by a service circuit, because there are no PCM codes to represent these high level voltages The ringing signal is therefore injected into the line circuit (Fig 3.3-2)

When a subscriber line has to be rung, the processor sets the line circuit to the ringing state In this state, switch (S) connects a common ringing-voltage source (RS) to the A-wire of the subscriber line The alternating ringing current passes through the external circuit [subscriber line, capacitor (C) and ringer (RR)-see Fig 3.2-l], and detector (CD) However, this current is not large enough to cause CD to indicate “off-hook.”

When the called telephone goes off-hook, the ringer current and the direct current from battery (BAT) flow through CD An off-hook is reported to the processor, which then changes the LC state to normal

A calling subscriber receives tones or announcements that inform him-about the progress of the set-up of his call These tones originate at the local exchange

of the caller, at intermediate exchanges along the connection, or at the local exchange of the called party (Fig 3.4-l)