Transcript Signaling - BS(SE) GCUF

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



Address signals


Signals sent by the calling subscriber when dialing the called
party number
Ringing


Request for service, disconnect by calling party
Answer, disconnect by called party
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
Calling
To on hook
Clear forward
Ringing
To off hook
Answer
Called
To on hook
Clear back
Dialing
Reception of address signals

Recognition time



Electrical disturbances on a subscriber line can result in offhook/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 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


Ringing tone


Usually it is a continuous 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