Topology ANSI FDDI Standard

120 Communication Networks Copyright © 2005 PragSoft The FDDI protocol requires that each station maintains three timers for regulating the operation of the ring: • The token holding timer determines how long a transmitting station can keep the token. When this timer expires, the station must cease transmission and release the token. • The token rotation timer facilitates the normal scheduling of token rotation between stations. • The valid transmission timer facilitates recovery from transmission errors.

9.4. Further Reading

Introductory texts on LANs and MANs include: Kessler and Train 1992, Stallings 1993a, Stamper 1993, and Martin et al 1994. Other useful texts include: Chorafas 1989, Fortier 1992, and Hunter 1993. Zitsen 1990 and Layland 1994 discuss the internetworking aspects of LANs. Hegering and Lapple 1993 describe the Ethernet in detail. Gohring and Kauffels 1992 discuss token rings. Jain 1993 and Nemzow 1994 provide detailed descriptions of the FDDI technology. www.pragsoft.com Chapter 10: Telephone Networks 121

10. Telephone Networks

By far the largest communication network on this planet is the global Public Switched Telephone Network PSTN. This network brings together about a billion telephone sets in a highly connected fashion which allows almost every set to be reached by every other set. The global telephone network is the center-piece of the field of telecommunication. Although the telephone system was originally designed to carry analog voice signals, the advent of computers and data communication has lead to it being increasingly used to also carry computer data traffic. At the same time, telephone networks have evolved to take advantage of the digital technology made available by computers. Modern telecommunication networks are designed to carry information in a variety of forms e.g., voice, data, video, fax. Telecommunication and data communication are fast converging. This chapter serves as an introduction to telecommunication. It looks at the various components of telephone networks and the signaling technology which helps operate them. Unlike data networks, telephone networks have been traditionally circuit-switched and largely remain so. More recently, the technology for packet switching has made it possible to also carry real-time conversations in packets. Our discussions in this chapter assume circuit switching. We will first present some fundamental telecommunication concepts, and then discuss signaling. Signaling remains the most complex aspect of telecommunication networks. We will look at common channel signaling and its major standards: SS7. Finally, we will examine private telephone networks and their relevance to public networks.

10.1. Basic Concepts

The most familiar component of a telephone network is the telephone set provided for each user. It is a relatively simple device which can exchange control signals with the network to help establish and release calls, and to send and receive a user’s speech signal. Users are referred to as subscribers , because they subscribe to a service provided by the telephone company. Typically, many subscribers are attached to the same network, and each subscriber can contact every other subscriber. The system employs switches to facilitate connections between subscribers. The arrangement of