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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 telephone 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
(556), which was introduced in the 1970s. SS6 was followed by signaling system no. 7
(557) 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
Figure 12.4 The SS7 stack.
first three levels of the SS7 hierarchy. The telephone user part (TUP) is a protocol for telephony call control and for trunk maintenance. The integrated service user part (ISUP) is a protocol for call control and trunk maintenance for both the telephone network and N-ISDN (see following Section 12.1.4). It supports signaling for calls in which either one or both parties are N-ISDN users. The signaling connection control part (SCCP) is a protocol that provides functions for the transfer of messages that are not trunk related. The combination of MTP and SCCP corresponds to the OSI Layers 1, 2, and 3. The transaction capabilities application part (TCAP) provides for operations that are not related to individual trunks and involve two signaling points. TCAP provides an interface to TC-users.
12.1.4 Narrowband ISDN (N-ISDN)
The integrated service data network (ISDN) was a concept of a future network put forward in the 1980s. This future network will be capable of providing a wide range of services for voice and non-voice applications, and all of these services will be provided by one network, rather than different networks. ISDN was seen as evolving progressively from the digital telephone network to include additional functions and network features, such as circuit-switching and packet-switching for data. The fundamental building block of ISDN was the 64-Kbps connection.
VOICE OVER ATM AND MPLS
Figure 12.5 An N-ISDN user.
The first generation of ISDN is referred to as the narrowband ISDN (N-ISDN). In this network, ISDN users can communicate with each other in circuit-switching mode and packet-switching mode. As shown in Figure 12.5, an ISDN user can operate multiple 64Kbps digital terminal equipment (TE) of differing types, such as digital telephone (PCM), high-speed facsimile terminal, and high-speed computer modem. TEs are connected to the local exchange via a digital subscriber line (DSL). The DSL is a two-wire or four-wire line that allows simultaneous transmissions in both directions. The transmission rate in one direction is 144 Kbps.
The 144-Kbps bit stream is divided into two 64-Kbps B-channels and one 16-Kbps D-channel. The B-channels are used for circuit-mode communications. They can also be used to carry digital information, such as digitized voice, computer data, and video. Both B-channels can be used at the same time, permitting the user to operate two phones or a phone and a computer modem at the same time. The two B-channels can also be linked together to provide an aggregate 128-Kbps data channel. The D-channel is used for signaling between the user and the local exchange. The digital subscriber signaling system no. 1 (DSS1) is used (see Section 12.1.5). In addition, the D-channel can be used for low speed packet-switching. The B-channels and the D-channel are full-duplex.
This basic access scheme is known as the basic rate and it was standardized by ANSI in 1988. Because it makes use of two B-channels and a D-channel, it is also known as 2B + D. The basic rate was intended for residential users and very small offices.
As in the TDM schemes (see Section 2.1), the basic rate access transmission is organized into fixed-length frames that repeat continuously. Each frame is 48 bits long and it repeats every 250 ^sec. The frame structure from the N-ISDN user to the local exchange is shown in Figure 12.6. Fields B1 and B2 contain data from the two B-channels. They are 8-bit long, and they repeat twice in the frame. The remaining fields (D, F, and L) are 1-bit long. D repeats four times within the frame, and is used to carry data from the D-channel. F and L carry a framing bit and a dc balancing bit, respectively. The F-L combination is used to synchronize the receiver at the beginning of the frame. Each frame contains 16 bits from each B-channel, and four bits from the D-channel. The bit rate is 144 Kbps, but due to the additional overheads bits, the total bit rate is 192 Kbps.