Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Figure 3.1 Circuit-switched network model

The main features of circuit switched-architectures such as Time Division

Multiplex (TDM), Frame Relay, and ATM are:

■■ Central control, such as the Intelligent Network (IN) for TDM in each

network for setting up the paths across each of the respective networks.

■■ Telecom standards are focused on interfaces. The most common inter-

faces are standard user-to-network interfaces (UNI) and network-to-

network interfaces (NNI).

■■ Services must be supported by the features in all NNIs, UNIs, and in

all the central control units.

■■ New services require new standards support in all UNIs, NNIs, and

central controllers.

■■ No service intelligence is presumed in the user devices. Users have no

control over the applications or the choice of services, except for the ser-

vices made available by subscription by the service provider, such as

the local telephone company or the mobile phone company.

These features are an impediment for the richness and time-to-market for

new services, as well as for user choice of services. Experience has shown that

this type of network is not favorable for innovation, since independent devel-

opers cannot get easy access to the central control platforms of the service

providers or influence the interface standards in a timely manner.

Following are some features that are more of an engineering concern and are

also reflected in the high cost of service:

■■ The circuit-switched nature requires the keeping of state for a connec-

tion or call in every switch, and in several switch subsystems in every

switch, as well as in every central control unit. State is also kept in all

networks in the path between the parties. State requires expensive pro-

cessing and memory in all network elements and components where

state is kept.

Central

Control

SW SW

NNI

Central

Control

SW SW

UNI NNI UNI

Central

Control

USER SW SW USER

SW: CIRCUIT SWITCH

User has little

control

Services supported by

interfaces and central

controllers

ITU-T Central Network Control:

POTS, X.25, ISDN, BISDN, FR, ATM, GSM, H.323, H.248...

40 Chapter 3