Chapter One
INTRODUCTION TO TELECOMMUNICATIONS
There are two types of communication networks: circuit-switched networks and packet-switched networks. In circuit-switched networks a dedicated physical (digital or analog) circuit between the calling and called party is set up at the start of a call and released when the call has ended. Traditional telephone networks are circuit-switched networks and collectively form the switched-circuit network (SCN). 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 networks, consisting of a number of nodes connected by digital links, made their appearance around 1970. In these networks, a call (or session) consists of a series of short data bursts (packets) followed by relatively long silent intervals. A physical circuit therefore does not have to be dedicated to a single data call but can be shared by several simultaneous calls. The Internet is an example of a data network.
The terms "telecommunication network" and "data network" usually imply circuit-mode and packet-mode, respectively. However, advances in packet technology are making possible voice communication in data networks, in what is called convergence of voice and data. The long-term trend is toward packet communication for voice, video, and data, so the word "telecommunication" is also used sometimes to denote converged networks.
This book is about signaling in communication networks. The first nineteen chapters are dedicated to signaling in telecommunication networks, with "telecommunication" used in the traditional sense. The last three chapters are dedicated to signaling in packet networks, with focus on the convergence of voice and data.
To understand signaling it is necessary to be familiar with some basic telecommunication concepts and terms. This chapter presents an overview of telecommunication networks (in the SCN sense). It is intended as an introduction and sets the stage for later chapters.
1.1 TELECOMMUNICATION NETWORKS
1.1.1 Introduction
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, and the group of trunks between a pair of exchanges is known as a trunk group (TG). Subscriber lines (SLs) are circuits between a subscriber S and the 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 [S.sub.p]. In Fig. 1.1-2(a), [S.sub.p] is on a call with [S.sub.q] 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 S[L.sub.p] and S[L.sub.q] and a temporary path in exchange A. Cases (b) and (c) are calls between [S.sub.p] and subscribers attached to other local exchanges (interexchange calls). The circuit in case (b) consists of S[L.sub.p], a temporary path across exchange A, trunk [T.sub.1], a temporary path across exchange B, and S[L.sub.r]. The connections of Fig. 1.1-2 are set up (switched "on") at the start of a call and released (switched "off") when the call ends.
Setup and Release. The setup and release of connections in telecommunication networks are triggered by signals. Starting and ending a call involve signaling between the subscribers and their local exchanges and, for interexchange calls, signaling between the exchanges along the connection.
Figure 1.1-3 shows the signaling for the setup of the connection of Fig 1.1-2(b). Subscriber [S.sub.p] sends a request-for-service signal to exchange A (by lifting the handset of a telephone) and then signals the digits of the telephone number of [S.sub.r] (with the dial or keyset of the telephone).
From the received number, exchange A determines that [S.sub.r] is served by exchange B, and that the call is to be routed out on a trunk in group T[G.sub.1] (Fig. 1.1-1). It then searches for an idle trunk in this group and finds trunk [T.sub.1]. Exchange A now seizes the trunk and sends a seizure signal, followed by signals that represent digits of the called number, to exchange B. It then sets up a path between S[L.sub.p] and [T.sub.1].
When exchange B receives the seizure signal and the called number, it checks whether [S.sub.r] is idle. If this is the case, it sends a ringing signal on S[L.sub.r], and a ringing-tone signal on [T.sub.1], to inform [S.sub.p]. When [S.sub.r] lifts the handset of the telephone, an answer signal is sent to exchange B, which then stops the ringing signal and ringing tone, sets up a path between [T.sub.1] and S[L.sub.r], and signals to exchange A that the call has been answered.
The connection is now complete and allows speech or other communications between the subscribers. At the end of the call, another signaling sequence takes place to release the connection.
One-Way and Bothway Trunk Groups. In Fig. 1.1-1, there is at most one trunk group between two exchanges. Let us consider the group T[G.sub.1]. The network should allow calls originating at A with destination B and calls originating at B with destination A. Therefore, both exchanges are allowed to seize trunks in T[G.sub.1]. A trunk group whose trunks can be seized by the exchanges at both ends is known as a bothway trunk group.
A pair of exchanges can also be interconnected by two one-way trunk groups. The trunks in one-way groups can be seized by one exchange only. For example, exchanges A and B could be interconnected by two one-way trunk groups T[G.sub.1A] and T[G.sub.1B], whose trunks can be seized by A and B, respectively.
Both arrangements are used in actual networks. Two-way groups have an economic advantage because, for a given traffic intensity, the number of trunks of a bothway trunk group can be smaller than the total number of trunks in the one-way groups.
In bothway groups, it can happen that the exchanges at both ends of a trunk group seize the same trunk at the same time (double seizure). There are several alternatives to deal with a double seizure. For example, it can be arranged that one exchange continues the setup, and the other exchange backs off (tries to. seize another trunk for its call). The signaling on bothway trunks includes provisions to alert the exchanges when a double seizure occurs.
1.1.2 Networks
In everyday life, we think of "the" telecommunication network that allows us to speak, or send faxes and other data, to just about anybody in the world. In fact, the telecommunication network is an aggregation of interconnected networks of several types.
Networks can be classified as shown in Fig. 1.1-4. In the first place, the (global) telecommunication network consists of national networks and the international network. In turn, a national network is a combination of public and private networks. Public networks are for general use; private networks can be used only by employees of the organization (an airline company, the U.S. government, etc.) that owns the network. A public network consists of a "fixed" network and a number of "cellular mobile" networks. In the United States, the fixed public network-known as the public switched telecommunication network (PSTN)-consists of about 150 LATA (local access and transport area) networks (the network of Fig. 1.1-1 is a LATA network), interconnected by networks that are known as IC (interexchange carrier), or long-distance, networks.
We now examine the interconnections of these networks. LATA and IC networks are interconnected by internetwork trunk groups-see Fig. 1.1-5. Some local exchanges (A) have a direct trunk group to an exchange of an IC, other exchanges (B, C, D, E) have access to the IC network via an intermediate (tandem) exchange in their respective LATAs.
A cellular network has one or more mobile switching centers (MSCs)-see Fig. 1.1-6. Each MSC is connected by an internetwork trunk group to a nearby tandem exchange T of a fixed (LATA) network. When a mobile station is making a call, it uses a radio channel of a nearby MSC.
Private Branch Exchange (PBXs). These are exchanges owned by government agencies, businesses, and so on and located in buildings that belong to these organizations. A PBX enables the employees in a building to call each other and to make and receive calls from subscribers served by the public network. A PBX is connected by an access line group (ALG) to a nearby local exchange (Fig. 1.1-7).
An organization with PBXs in several cities can establish a private network that consists of the PBXs and a number of tie trunk groups (TTG) between the public local exchanges to which their PBXs are attached. A TTG is a "private" group that is leased by the LATA operator and is dedicated to private-network calls. In Fig. 1.1-7, the connection for a call between public branch exchanges X and Y uses a trunk of TT[G.sub.1] and is switched in the public local exchanges A and B.
Today there are also virtual private networks (VPNs). They appear to a business as a private network but use the trunks of the public networks.
International Calls. Figure 1.1-8 shows the interconnection of long-distance (IC) networks in different countries. For a call from country A to country C, an IC network in country A routes the connection to an international switching center (ISC). An ISC has national trunk groups to exchanges of its IC and international trunk groups to ISCs in foreign countries.
The term "international network" refers to the combination of the ISCs and their interconnecting trunk groups.
1.1.3 Telecoms
We shall use the term telecom to denote a company that owns and operates a public telecommunication network. Until recently, the telecoms in most countries were government-owned monopolies that operated an entire national network. In recent years, a number of countries have started to privatize their telecoms and to allow competition by newly formed telecoms.
The networks in the United States are operated by investor-owned telecoms. Until 1984, the Bell System was the largest telecom, operating practically the entire long-distance network and many-but by no means all-local networks. Independent (non-Bell) telecoms, such as GTE, United Telecoms, and a host of smaller companies, operated the other local networks.
The division of the U.S. public network into local access and transport areas (LATAs) and interexchange carrier (IXC or IC) networks took place in 1984, due to government actions that broke up the Bell System. As a result, competing IXCs started offering long-distance and international service, while local exchange carriers (LECs) provided service on a regional basis, in LATAs. A typical LEC owned a number of adjacent predivestiture local networks. Competition for local service was allowed but was slow in developing because of high entry barriers. The latest development is that IXCs and LECs are allowed to provide both local and long-distance service. In addition, new wireline, cable, and wireless companies can provide competing telephone service. All that has resulted in mergers between LECs, IXCs, cable companies, and wireless companies.
1.1.4 Synonyms
Since telecommunication terms originated rather independently in different countries, technical literature in English still uses different terms for the same concepts, depending on whether the authors are from the United States, from the United Kingdom, or from other English-speaking countries, or documents are translations from other languages. Some frequently used synonyms are listed below:
Subscriber, customer, user
Subscriber line, line, loop
Local exchange, local office, central office, end office
Intermediate exchange, tandem exchange, toll exchange, transit exchange
International switching center, gateway, international exchange
Trunk, junction, circuit
Telecom, administration, carrier, operating company, telephone company, telco, service provider
Exchange, switch
Switchblock, switch fabric
1.2 NUMBERING PLANS
This section explores the formats of the numbers (sometimes called addresses) that identify the subscribers of telecommunication networks.
Subscriber Numbers (Directory Numbers). The geographical area of a nation is divided into several numbering areas, and subscriber numbers (SNs) identify subscriber lines within a particular numbering area. A SN consists of an exchange code (EC) that identifies an exchange within a numbering area, followed by a line number (LN):
SN = EC-LN
National Numbers. Within a country, a subscriber is identified by a national number (NN), consisting of an area code (AC), which identifies the numbering area, followed by a subscriber number:
NN = AC-SN = AC-EC-LN
International Numbers. Worldwide, a subscriber is known by an international number (IN) that consists of a country code (CC), followed by a national number:
IN = CC-NN
The generic format for the international numbering plan is specified by ITU-T in Rec. E.164. Three types of numbering schemes are supported by E.164, all with the CC component having up to three digits and the total IN number having a maximum of 15 digits:
1. Numbering plan for geographic areas (subscriber numbers)
2. Numbering plan for global services (e.g., Freephone numbers)
3. Numbering plan for networks (other than the telephone network)
For item 1 the CC may have one, two, or three digits and the NN breakdown is left open for national definitions. For global services and for networks, the CC must be three digits long. For networks, the NN is divided into an identification code (IC) of one to four digits and a subscriber number.
When subscriber [S.sub.1] calls a subscriber located in the same numbering area, the number dialed is a SN. If the called subscriber lives in the same country but in a different area, [S.sub.1] has to dial a NN. If the called party lives in another country, [S.sub.1] needs to dial an IN. To allow the local exchange to interpret what is being dialed, prefixes may need to be prepended to the numbers of a numbering plan. Prefixes are part of the dialing plan, described in Section 3.7.
National numbering plans define the formats of subscriber and national numbers. Most countries have their own numbering plans. However, the United States, Canada, and a number of Caribbean countries are covered by a common plan that was introduced in the mid-1940s.
1.2.1 North American Numbering Plan (NANP)
The North American territory is divided into numbering plan areas (NPAs), which are identified by three-digit numbering plan area codes, most often called simply area codes, AC(3).
Each area covers a state, or part of a state, but never crosses a state boundary. Lightly populated states (Alaska, New Mexico, etc.) have one NPA, while more heavily populated states (Illinois, New York, etc.) are divided into several NPAs. The territory of a NPA is not identical to the service area of a LATA (LATA boundaries were established much later, after the breakup of the Bell System). Some (but not all) of the AC(3) numbering space in the 8XX and 9XX ranges is used for services where destinations are not directly linked to a geographical area ("800 service" and "900 service"). The 700 NPA is also available for individual carriers who want to introduce new services.
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Excerpted from Signaling in Telecommunication Networksby John G. van Bosse Fabrizio U. Devetak Copyright © 2005 by John Wiley & Sons, Ltd. Excerpted by permission.
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