SIP Demystified.pdf

Source: SIP Demystified

CHAPTER

Signalling in

the Circuit-

Switched

Network

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Signalling in the Circuit- Switched Network

Chapter 1

The telephone network, also known as the Public Switched Telephone Net-

work (PSTN), reaches almost every country in the world. We can find tele-

phone sets almost in every house, including simple analog phones, more

advanced Integrated Services Digital Network (ISDN) phones, cordless

phones, cellular phones, and even satellite phones. Among technologies,

telephone systems are astonishingly widespread.

All these phones have something in common. They use a circuit-switched

network to communicate between them. The PSTN has been around for a

long time, and its users are happy with it. People can talk to each other

across oceans and still feel in proximity. Even in long-distance calls between

two mobile phones, it is possible to understand what the other person is

saying relatively well.

Besides, the PSTN is a highly-reliable network. When somebody picks up

a phone in order to make a call, most of the time the call goes through. Tele-

phone switches rarely crash, and when they do, backup systems take over

to continue providing service to the users.

In short, people trust the PSTN. They have grown to expect trouble-free

everyday use, and they are confident that in the case of an emergency, the

PSTN will connect them to an ambulance, the police, or the fire department.

Taking into account all these features of the PSTN, one might think that

the procedures and mechanisms used by the telephone network would be

emulated in any network providing a similar service. Since the PSTN works

so well, let us model any voice network after it. Specifically, let us use

PSTN-like signalling protocols—they work well.

This assumption is wrong, however, and we will see why for a different

environment, such as the Internet, PSTN-like protocols are not suitable.

Material differences in the PSTN architecture and the Internet architec-

ture require completely new signalling protocols rather than the evolution

of trusty, old ones.

This chapter contains a brief PSTN-signalling history that explains how

signalling protocols have evolved from analog to digital. We will see that

beyond the differences in transport technology used by the PSTN and the

Internet—the former is circuit-switched while the latter is packet-switched

—differences in their respective paradigms for governance and operation

make different signalling design mandatory.

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Signalling in the Circuit- Switched Network

Signalling in the Circuit-Switched Network

The Origins of Circuit-Switching

A telephone network aims to provide its users with certain services. Many

services fall into the category of telephone services, but the first and most

important one is the transmission of voice between users. Any user of the

network has to be able to call any other user attached to it. All telephone

networks must fulfill this requirement.

The first telephone network that enabled two users to communicate con-

sisted of two telephone sets joined by a cable. That system provided both

users of the network with adequate voice service, but as soon as more users

wanted to be able to use the network, the number of telephone sets

increased. The best way to add a new user to the network was to install new

cables from the telephone set of the newcomer to all the already-installed

telephone sets, creating what is known as a fully meshed topology, where

every telephone set is directly connected to every other telephone (see

Figure 1-1).

Figure 1-1

A fully meshed

topology.

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Signalling in the Circuit- Switched Network

Chapter 1

Unfortunately, a fully meshed topology presents a number of disadvan-

tages in the telephone context. Failure to scale is a very important draw-

back. An individual connection from each user to any other user who

potentially can be called has to be established. A physical connection to

every single user is affordable as long as the system is small, but as the

number of users grows, the number of connections grows in an exponential

manner. Cables are expensive and difficult to install. Therefore, adding new

users to the network becomes very expensive very quickly.

Cost aside, if a single telephone set has as many cables connected to it as

there are users in the network, routing becomes complicated to manage.

Each telephone set has to maintain a huge routing table that indicates

which cable corresponds to which destination in order to route the calls.

Due to their limitations of scalability and management, early systems

evolved from a fully meshed topology to a star topology. In a star topology,

all telephone sets are connected to a single central unit called a switch (see

Figure 1-2) and all calls are routed through it from source to destination.

The switch connected cable from the call’s originator to cable at the call’s

receiver. At first it was operated manually, by a now-quaint figure known as

a “switchboard operator” who plugged wires into sockets.

Figure 1-2

Star topology.

SWITCH

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Signalling in the Circuit- Switched Network

Signalling in the Circuit-Switched Network

Old though it may be, this technology is still in use and still referred to

as circuit switching. Obviously, circuit switching facilitates the addition of

new users to the system, because it reduces the “cost” to a single cable

between the switch and the new telephone set. Management of the system

is streamlined. The switch still has to handle a number of circuits, but the

user equipment—the telephone set—remains simple.

The next step in circuit-switching is to connect together several switches

building a circuit-switched network (see Figure 1-3). In such a network,

each user’s telephone set is connected to the switch nearest to it. That

switch is connected to another, and so on across the system.

In telephony, switches are also referred to as exchanges. Therefore, the

closest switch to a telephone set is called its local exchange. Calls between

two telephones are routed first to the local exchange of the caller and then

traverse other exchanges until reaching the local exchange of the callee,

which can announce the call to him by ringing his telephone. Circuit-

switching overcame the limitations of the first telephone system and

remains the dominant technology for voice transmissions even now. But

circuit-switching technology evolved repeatedly from that very first net-

work, and at present, several hierarchical levels of switches are imple-

mented all together to make up the PSTN, a worldwide communication

network.

Figure 1-3

Hierarchical topology.

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