Introduction To Telecommunications

There are two types of communication networks: circuit-switched networks and packed-switched networks. In circuit-switched networks, a dedicated physical circuit between the calling and called party is set up at the start of a call, and released when the call has ended. Telephone networks are circuit-switched networks. Today, these networks are used for speech and other purposes, such as facsimile, and are usually referred to as telecommunication networks.

Initially, all communication networks were circuit-switched networks. Data communication networks made their appearance around 1970. In these networks, a call consists of short data bursts (packets) followed by relatively long silent intervals, and does not require a dedicated physical circuit. Internet is an example of a data communication network.

Today, the terms "telecommunication network" and "data communication network" usually imply circuit-mode and packet-mode networks, respectively.

This book is about signaling in telecommunication networks. To understand signaling, it is necessary to be familiar with some basic telecommunication concepts and terms. This chapter presents an overview of telecommunication networks. It is intended as an introduction, and sets the stage for the later chapters.


Figure 1.1-1 shows a small part of a telecommunication network. It consists of exchanges, trunks, and subscriber lines. Trunks are circuits between exchanges,

Local Exchange

Figure 1.1-1 Partial view of a telecommunication network.

Local Exchange

Intermediate Exchange

Figure 1.1-1 Partial view of a telecommunication network.

and the group of trunks between a pair of exchanges is known as a trunk group (TG). Subscriber lines (SL) are circuits between a subscriber S and his local exchange (A, B, C). Exchanges D and E do not have subscriber lines, and are known as intermediate, tandem, toll, or transit exchanges.

Calls. A call requires a communication circuit (connection) between two subscribers. Figure 1.1-2 shows a number of connections in the network of Fig. 1.1-1 that involve subscriber Sp. In Fig. l.l-2(a), Sp is on a call with Sq who is attached to the same exchange. Calls of this type are known as intraexchange calls. The circuit for the call consists of the subscriber lines SLP and SLq, and a temporary path in exchange A. Cases (b) and (c) are calls between Sp and

Sp A sq

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