Intelligent Networks Private Telephone Networks

140 Communication Networks Copyright © 2005 PragSoft

11. Integrated Services Digital Network

Current trends in telecommunication are toward integration of voice and data services. So far these services have been available separately, requiring separate subscription, communication links, and equipment. It has long been acknowledged that the integration of these services will result in significant flexibility and cost benefits to both service users and service providers. The Integrated Service Digital Network ISDN is a major attempt to realize these objectives. The three most important ingredients of ISDN have already been discussed in earlier chapters: circuit switching, packet switching, and common channel signaling SS7. This chapter looks at the rest of the ISDN technology. We will start with some basic ISDN concepts, including its channels, reference points, functional groupings, and services. We will then describe the ISDN protocol architecture in relation to the OSI model, and discuss various ISDN standards. Finally, we will examine the potential future of ISDN within the context of global communication networks.

11.1. Basic Concepts

ISDN provides a fully integrated digital network for voice and data communication. It supports both circuit and packet switching. Figure 11.123 illustrates the overall arrangement of a typical ISDN network, which brings together a mix of old and new technologies and devices. Each ISDN switch consists of an exchange termination part, which performs the necessary circuit switching functions, and a packet handler, which performs the necessary packet switching functions. The packet handlers implement X.25 and are connected to a public packet switched network via X.75. The exchange terminations are interconnected via tandem exchanges. STPs and SCPs provide network intelligence, and were described in the previous chapter. Subscriber access is provided via a network termination andor terminal adapter NTTA. This provides the connectivity for a variety of user devices, including ISDN phones, Plain Old Telephone Sets POTS, LANs, PBXs, and X.25 terminals. www.pragsoft.com Chapter 11: Integrated Services Digital Network 141 Figure 11.123 ISDN network overview. Packet Handler Exchange Termination ISDN Switch Packet Handler Exchange Termination ISDN Switch Public Packet Switched Network Tandem Exchange SCP STP To other ISDN exchanges To other ISDN exchanges ISDN Phone POTS PBX NTTA LAN X.25 Access U X.75 X.75

11.1.1. ISDN Channels

Subscriber access to ISDN is via digital channels, of which there are three types: • B channels are used for carrying user data digitized voice or computer- generated data at 64 kbps. This data rate is more than necessary in many situations e.g., compressed digitized voice can be transmitted using less bandwidth. Consequently, a B channel is sometimes subdivided into smaller subchannel. Whether there is a subdivision or not, the network treats the whole thing as one channel. All subchannels therefore are between the same two end- points and follow the same route. • D channels are primarily used for common channel signaling purposes. They are typically associated with B channels and carry the control signals for B channel calls. D channels are also used for packet-switched data communication. A D channel may operate at 16 or 64 kbps. • H channels are used in a high-speed trunk capacity. They are suitable for applications that require higher than 64 kbps data rates. Multi-media applications e.g., audio, video, and graphics multiplexed over the same channel are examples. H channels are divided into three categories depending on their speed: • H0 operates at 384 kbps = 6 B channels