Churning THE ATM PASSIVE OPTICAL NETWORK

296 VOICE OVER ATM AND MPLS Table 12.1 ABCD signaling scheme. Frame number Transmitted bit 6 A 12 B 18 C 24 D 30 A 36 B 42 C 48 D exchange C see Figure 12.3. The release guard signal is used to indicate to the receiving exchange that it can use the same trunk for another call. In addition to the signals mentioned above e.g. seizure, wink, answer, clear-forward, clear-back, and release guard, and the dialed digits which are transported using the DTMF scheme, a number of supervisory bits are also used to indicate supervisory line states e.g. on-hook, off-hook, idle, and ringing. Supervisory bits are known as the ABCD signaling bits . The ABCD bits are transferred by robbing the 8th bit of the time slot associated with the voice call every six frames. The robbed bit is used to transmit the A, B, C, and D bits, respectively see Table 12.1. Let us consider a time slot associated with a particular voice call. Then, the A bit will be transmitted by robbing the 8th bit of the slot in the 6th frame, the B bit will be transmitted by robbing the 8th bit of the time slot in the 12th frame, the C bit will be transmitted by robbing the 8th bit of the time slot in the 18th frame, the D bit will be transmitted by robbing the 8th bit of the time slot in the 24th frame, and so on. The four bits provide for a 16-state signaling scheme for each voice channel.

12.1.3 Signaling System No. 7 SS7

As mentioned in the previous section, CAS is an in-band signaling protocol. In the common-channel signaling CCS , all signaling information for establishing and releasing a phone call is carried in messages over a separate packet-switching network. This packet- switching network is known as the signaling network. It consists of signaling points SP and signaling transfer points STP, which are interconnected by signaling links SL. An SP originates, receives, and processes signaling messages. It can be part of either a tele- phone exchange or a database, which is accessed via CCS messages. An STP is an SP that simply switches messages from an incoming SL to an outgoing SL. That is, it does not originate or process signaling messages. The first generation common-channel signaling protocol was signaling system no. 6 SS6 , which was introduced in the 1970s. SS6 was followed by signaling system no. 7 SS7 about ten years later. SS7 consists of several layers see Figure 12.4. The message transfer part MTP provides a reliable transfer service to the protocols running above it. It is divided into three parts: MTP1, MTP2, and MTP3. These three parts occupy the