Channel Associated Interexchange Signaling

Channel Associated Interexchange Signaling

Introduction

Channel-associated inter-exchange signaling (CAS)-also known as per-trunk signaling-has been in existence from the beginning of automatic telephony, and was the only form of interexchange signaling until 1976.

Example

• Interexchange signaling sequence for a call from subscriber S1 to subscriber S2

• Exchange A sends the digits, and then cuts through (sets up a path in its switchblock between the subscriber line of S1 and T1).

• Assuming that originating exchange A is responsible for charging the call, it establishes a billing record that includes the calling and called numbers, the date, and the time of answer.

Release

• Exchange A releases its path between S1 and trunk T1, and sends a clear-forward signal to exchange B, which releases its path between T1 and T2, and repeats the clear-forward to exchange C. This exchange then clears its path betweenT2 and called subscriber S2.

• When exchanges B and C have completed the release of respectively T1 and T2, they send release-guard signals (to respectively exchanges A and B). When A and B receive the release-guard, they know that they can again seize respectively T1 and T2 for new calls.

Groups of Interexchange Signals

• Of the four groups of subscriber signals, three also exist in channel-associated interexchange signaling.– Supervision Signals– Address Signals– Tones and Announcements.

• Supervision Signals – The signals in this group represent events that occur

on the trunk, such as, seizure, proceed-to-send, answer, clear-forward, etc.

• Address Signals– The address signals are represented using multi-

frequency (MF) signaling, similar to the DTMF (dual-tone multi-frequency) signaling in telephones.

– The MF frequencies are system-specific, and different from those used in DTMF.

• Tones and Announcements. (Ringing-tone, busy-tone, etc.). – The tones and announcements in interexchange

signaling are the same as in subscriber signaling

Groups of Interexchange Signals

CAS Signaling Equipment at the Exchanges

Control channel (CC)

• Control channel (CC) to the switchblock has two functions. – it transfers processor commands to set up and

release switchblock paths. – it is used for communications between the

processor and DMPs of digital trunks. • The processor can order a DMP to send out a

supervision signal on a specified trunk in the multiplex, and a DMP reports the supervision signals received from the trunks in its multiplex to the processor.

DMP

Interexchange Call-control Definitions

• Outgoing and Incoming.

• Originating and Terminating Exchanges.

• Overlap and En-bloc Address Signaling.

Outgoing and Incoming

• These terms can be applied to trunks, and to exchanges. – An outgoing exchange seizes outgoing trunks,

sends forward signals, and receives backward signals, on its outgoing trunks.

– An incoming exchange receives forward signals, and sends backward signals, on its incoming trunks.

Originating and Terminating Exchanges.

• The originating exchange in a call is the local exchange serving the calling subscriber, and the terminating (or destination) exchange is the local exchange of the called subscriber.

Overlap and En-bloc Address Signaling

• En-bloc register signaling – Once the exchanges receive a proceed-to-send, they

send out the complete called number in one uninterrupted stream.

• Overlap register signaling– Exchange A can seize trunk T1 after receipt of

Exchange Code (EC) from the calling subscriber, and can then send EC to exchange B.

– Similarly successive exchanges send out the initial digits of the called number while still receiving the later digits of the number from the preceding exchange.

CAS Signaling Systems

• Bell System multi-frequency (MF) signaling,

• CCITT No.5 signaling, and

• R2 signaling

– But we will only study Bell System, that too briefly

Bell System multi-frequency (MF) signaling

• Introduced by the Bell System after the Second World War

• This system can be used on FDM analog trunks and TDM digital trunks

• Supervision and address signaling are link-by-link.

Supervision Signaling• A trunk can be in one of the two states

– on-hook (idle) – off-hook (in use) – These states can be different at the exchanges connected by the

trunk – Each exchange continuously sends the trunk state at its end to

the other exchange

• Change in a trunk state is a supervision signal

• Consider a trunk between exchanges A and B.

Release-Guard

• This signaling system does not include a release-guard signal. Therefore, when outgoing exchange A releases the trunk, it starts a timer that expires after 0.75-1.25 s. The exchange does not seize the trunk for a new call until the timer has expired. This gives incoming exchange B the time to release the trunk at its end.

Supervision Signaling on FDM Analog Trunks

• FDM analog trunks can transfer frequencies between 300 and 3400 Hz.

• Exchanges indicate the states of the trunk with a 2600 Hz signaling tone.

Signaling circuitry in the four-wire trunk circuits TC4 of a trunk

Sending and Receiving the Signaling Tone

• STS – signaling tone source• CD - current detector• The sending of the tone is controlled by the exchange

processor, which controls switch X in the TC4

• In the figure, the processor at exchange A has sent an off-hook command to TC4-1, and no tone is sent on the send channel (S) of the trunk. At exchange B, the processor has sent an on-hook command to TC4-2, and switch X connects the tone on the S channel of the trunk

Blocking received signaling tone

• During the conversation, both exchanges indicate off-hook, and no signaling tone is present in either direction.

• However, in other call states, the signaling tone and other voiceband signals (e.g. ringing tone) can be present simultaneously on a trunk

• Solution– Band-elimination filter (BEF): blocks 2600 Hz, but

passes other voiceband frequencies

Blocking received signaling tone

• Consider a call from subscriber S1 to S2– TAC at Exchange C is sending ringing-tone. At points (p),

only ringing-tone is present. At points (q) the ringing-tone and signaling tone are both present.

– There are two reasons why TC4-1 and TC4-3 should pass the ringing-tone (or other voiceband signals), but block the signaling tone.

• In the first place, S1 should hear ringing-tone only.• In the second place, supervision signaling is link-by-link. This

means that the signaling tone from B to A on trunk T1 should be controlled by exchange B. Therefore, TC4-3 has to block the received signaling tone, which otherwise would “leak” into T1

Blue Box Fraud

• Electronic tone generator called blue boxes that had push buttons to generate the different frequencies

• The blue box was used in the following way

• Suppose S has dialed 0 for operator assistance, exchange A does not charge the call, expecting the charging to be done at exchange B

• Before the operator has answered, S sends a burst (say l-2 s) of signaling tone. This is interpreted by exchange B as a clear-forward signal, and the exchange releases trunk T at its end.

• The end of the burst is interpreted at exchange B as a new seizure signal.

• Subscriber S waits about one second after the end of the signaling tone, and then sends the called party’s number.

Supervision Signaling on Digital (PCM) Trunks

• The DMPs maintain frame and superframe (12 T1 frames) synchronization with the incoming bit stream by locking onto the F bits of the frames, which exhibit a repeating 12 bit pattern

• When locked on to this pattern, a DMP can determine the start of each frame, and of each superframe, in the bit stream.

• In frames 6 and 12, bit 8 (the least important bit) of the 24 channels are used for supervision signaling. This is known as bit robbing.

• The effect of bit robbing on the quality of PCM-coded speech is negligible

Supervision Signaling on Digital (PCM) Trunks

• The signaling bits in frames 6 and I2 are known as the Sa and Sb bits.

• The DMPs update their outgoing signaling bits once per superframe.

• The combinations of an Sa and Sb bit could indicate four trunk states. However, the Sb bit in each time slot is set equal to the previous Sa bit

• The bit values 0 and 1 represent respectively on-hook and off-hook.

• The signaling bits cannot be heard by the subscribers, and the subscriber’s speech, or a blue box, cannot corrupt the supervision signals. This avoids the problems associated with in-band signaling.

Address Signaling

• The MF address signals are combinations of two voiceband frequencies- chosen from a set of six frequencies

• Address signaling sequences start with a KP (start-of-pulsing) signal, and end with an ST (end-of-pulsing) signal.