Signaling

Signaling

The use of signals for controlling communications E.g., call setup, connection, connection teardown, and

billing Early signaling

Signaling in the period from 1890 to 1976 was limited to setup and release of connections between two subscribers

Signals were carried by the same circuit that carried the speech – also known as channel associated signaling

Early Signaling

Initially automatic signaling was possible only for calls between subscribers served by the same exchange – subscriber signaling

Later on it became possible to dial calls between subscribers served by nearby exchanges

National long distance calls needed operator assistance until 1950s

International direct distance dialing, which requires signaling on international trunks became possible in 1960s

Subscriber Signaling

Subscriber signaling

Signaling between subscriber and local exchange is known as subscriber signaling

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.

Calling Party Control

Release of connection is usually under control of calling party If the calling party clears first, the exchange

immediately releases the connection If the called party clears first, the exchange

starts a timer of say 30-60 sec, and releases the call when it receives a clear forward, or an expiration of timer, whatever occurs first.

Forward and Backward Signals

Forward signals are the signals which are sent in the direction of call set up. Request for service, digits of called number are all

examples of forward signals

Backward signals are the signals in the other direction Dial tone, ringing tone, and answer are backward

signals

Groups of Subscriber signals

Supervision signals Signals sent by subscribers to local exchanges

Request for service, disconnect by calling party Answer, disconnect by called party

Address signals Signals 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 call

Tones and Announcements Audible backward signals (dial tone, ringing tone, busy tone

etc.) sent by the exchange to calling subscriber.

Signaling equipment at the local exchange

Signaling equipment at the local exchange

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.

Tone and announcement circuits TAC It have memory that store all tones and

announcements that can be sent to a subscriber.

Line Circuit (LC)

Reception of supervision signals

Line circuit LC can be in two states Normal state

When the telephone is involved in a call 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). LC states are changed on command from the

processor

Call states

The exchange processor keeps track of the call state of the telephones, and store these values in its temporary memory

Major call states are: Idle Dialing Calling Ringing Called

Determination of signal type

Present call state

Change in hook status

Supervision signal

Idle To off-hook Request for service

Dialing

Calling To on hook Clear forward

Ringing To off hook Answer

Called To on hook Clear back

Reception of address signals

Recognition time Electrical disturbances on a subscriber line can result in

off-hook/on-hook pulses on lines that are on-hook/off-hook Processor takes no action until the new hook state has persisted

for predetermined recognition time (20-40 ms) Hookswitch flash

This is a fifth supervision signal, sent by a subscriber who in is the calling or called state, to request an action from the local exchange

Exchanges usually interpret on-hooks of 0.1-1.0 seconds as flashes

Longer on hooks are considered as clear-forward or clear-back signals

Reception of address signals

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

On receipt of first digit, dial tone is turned off Path between DRC and subscriber line is

released when the complete called number has been received

Reception of address signals

Digit imitationWhen a key on keypad is depressed, the

transmitter is disabled. However, there are intervals (between digits) when no key is pressed. During these intervals, transmitter is enabled, and may pick up speech or noise in the vicinity of calling subscriber.

That’s why the DTMF frequencies have been chosen to minimize noise imitation.

Ringing signals

When the subscriber line has to be rung, the processor sets the LC to the ringing state.

Switch S connects a common ringing voltage source (RS) to the A-wire of the subscriber line.

Tones, Announcements and Ringing

Tone Formats

Dial tone Usually it is a continuous tone

Ringing tone Ringing signal have the same cadences as the ringing tone

Busy tone Sent by local exchange to called subscriber

Tones and announcement in other failed setups other than busy Congestion Invalid or non-working called number

Invalid area or exchange code Number unallocated

Subscriber signaling for supplementary services

Subscription to supplementary services has a monthly fee

Every local exchange has a database with entries for each subscriber, listing the supplementary services for that subscriber

Services are invoked by dialing special digit sequences called feature access code

These codes starts with an asterisk (*) or a hash (#)

Supplementary services

Call waiting service Call forwarding service Three way calling Distinctive ringing Selective call rejection Caller ID etc.

Resource

Chp 2 and 3 of “Signaling in Telecommunication Networks”John G. van Bosse