Corporate Networks Private Telephone Networks

www.pragsoft.com Chapter 10: Telephone Networks 139 customer is an organization with numerous outlets throughout the country e.g., a fastfood chain. The service is characterized by a single number with the 800 prefix which, when dialed from anywhere in the country, is redirected to the SCP for handling. A typical service profile would use the origin of the call to direct it to the nearest outlet.

10.5. Further Reading

Martin 1988, Tanenbaum 1989, Black 1989, and Stallings 1991 provide highly readable introductory descriptions of telephone networks. Detailed descriptions of telecommunication concepts protocols can be found in Schwartz 1987, Freeman 1989, Martin 1990, Bellamy 1991, Spragins et al 1991, Van Duuren et al 1992, Heldman 1993, and Heldman 1994. Modarressi and Skoog 1990 and Jabbari 1991 describes SS7 and its use for ISDN and intelligent networks. Bush and Parsosns 1992 provide a readable account of PBX systems. 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.