Digital Subscriber Signaling

The public switched telecommunication networks in some countries have been converted (often only partially) into Integrated Services Digital Networks (ISDNs) [2,9,10]. An ISDN serves conventional (analog) subscribers and ISDN users. Digital ISDN users can communicate with each other in two modes. In circuit mode, the network sets up a dedicated connection for the call, which can be used for voice and data communication. In packet-mode communication (Chapter 20), the users communicate with short bursts of data, called packets.

There are 64-kb/s ISDN terminals (terminal equipment—TE) of several types, for example, digital telephones, high-speed facsimile terminals, and high-speed computer modems (Fig. 2.1-4). A digital subscriber line (DSL) connects the user's TE to the local exchange. ISDN DSLs are two-wire or four-wire circuits that allow simultaneous information transmission at 144 kb/s in both directions, although, due to overhead bits, the total transmission rate is somewhat higher. Signaling on ISDN lines is message oriented and is described in Chapter 10.

A more recent development is asymmetric digital subscriber lines (ADSLs) and less common variations thereof, such as high bit-rate DSL (HDSL) and symmetric DSL (SDSL), collectively referred to as xDSL. xDSL technology uses high-speed digital modems on regular subscriber loops to create data channels that can achieve rates of several megabits per second, depending on the length of the loop. An ADSL modem creates two bidirectional data channels, one a regular (narrowband) voice channel and the other a high-speed packet data channel. The ADSL data channel has a much higher bit rate in the direction from the local exchange

What Subscriber Loop Signaling
Figure 2.1-4. ISDN digital subscriber line.
Adsl Network Architecture
ADSLconnection packet data connection narrowband voiceconneclion AS: AcccssSystem Figure 2.1-5. ADSL configurations.

to the subscriber than from the subscriber to the local exchange, hence the term "asymmetric," reflecting the fact that in normal communication a far larger volume of data is downloaded from the network than uploaded to it.

The two bidirectional channels are split at the local exchange, where the voice channel is connected to the switchblock and handled like any other subscriber line interface, and the data channel is connected to the packet data network (typically ATM—Chapter 22). A common arrangement is to terminate the ADSL at a digital subscriber line access multiplexer (DSLAM) collocated with the local exchange, but solutions where the local exchange has integrated ADSL interfaces in its line units are also available. With access systems (Section 1.8 and Chapter 6), which cannot transport an ADSL, a remote access multiplexer (RAM) may be interposed at the remote location, to split the voice channel (which connects to the AS) from the data channel (which connects to the packet network). ADSL configurations are shown in Fig. 2.1-5.

A similar approach to xDSL is being used by cable companies, who use the existing coaxial cable access for TV service to offer voice and high-speed data services to subscribers (cable modems).

The general trend is for xDSL and cable modems to replace ISDN as a high-speed digital subscriber interface.

2.1.6 Multiple Interexchange Signaling Systems

For economic reasons, telephone exchanges are kept in service for about 20 years. That means that both the national and the international networks contain different generations of exchanges. Some old-technology exchanges can handle only older interexchange signaling systems and cannot be upgraded to accommodate newer forms of signaling. Modern exchanges using SPC technology (Section 1.7) by and large can be retrofitted to handle newer signaling systems but, depending on their vintage, the process can require a nontrivial investment and long lead times. In some countries, such as the United States, telecoms have shifted a significant portion of the capital investment previously earmarked for new exchanges to equipment based on packet technology (Chapter 20), which requires a whole new set of signaling systems.

As a consequence, a connection routed via one or more intermediate exchanges of different vintages may involve trunks with different interexchange signaling systems. Call-control at such exchanges has to include procedures for signaling interworking between the different signaling systems. One aspect of interworking is the conversion of the formats of individual signals and/or messages. More difficult problems arise when a signal, or an information element in a common-channel signaling message, exists in a new system but not in an older system. Signaling interworking functions can be quite complex and have to be designed with care.

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