Channel Associated Interexchange Signaling

1. Channel Associated Interexchange Signaling

2. 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.

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

5. 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.

6. 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.

7. 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.

8. Groups of Interexchange Signals • 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

9. CAS Signaling Equipment at the Exchanges

10. 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.

11. DMP

12. Interexchange Call-control Definitions • Outgoing and Incoming. • Originating and Terminating Exchanges. • Overlap and En-bloc Address Signaling.

13. 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.

14. 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.

15. 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.

16. 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

17. 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.

18. 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

19. Consider a trunk between exchanges A and B.

20. 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.

21. 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.

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

23. 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

24. 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

26. 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

27. 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

28. 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.

29. 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

31. 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.

32. 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.