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 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

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 IdleTo off-hookRequest for service Dialing CallingTo on hookClear forward RingingTo off hookAnswer CalledTo on hookClear 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 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 imitation  When 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