local Digital trunk exchange High capacity exchange optical transmission systems Figure 2.16 Two-layer network and links between trunk and local exchanges. Introduction to Telecommunications Network Engineering The transmission systems that interconnect trunk exchanges make up a transmission or transport network. Its basic purpose is simply to provide a required number of channels (or data transmission capacity) from one exchange site to another. Exchanges use these channels of the transport net- work for calls that they route from one exchange to another on subscriber demand. The trunk exchanges are usually located in major cities. They are digital and use the international common channel signaling standard SS7 to exchange routing and other signaling information between exchanges. The transmission lines between exchanges have conventionally carried TDM tele- phone channels, as explained in Chapter 4. Currently the use of IP networks for connections among exchanges is increasing and it requires media gateways (MGWs) between the exchange and IP network to take care of signaling and real-time transmission through the IP network. International Network Each country has at least one international switching center to which trunk exchanges are connected, as shown in Figure 2.17. Via this highest switching hierarchy level, international calls are connected from one country to another and any subscriber is able to access any of the other more than 2 billion sub- scribers around the world. High-capacity optical systems interconnect international exchanges or switching centers of national networks. Submarine cables (coaxial cable or optical cable systems), microwave radio systems, and satellites connect conti- nental networks to make up the worldwide telecommunications network. The first submarine cable telephone system across the north Atlantic Ocean was installed in 1956, and it had the capacity of 36 speech channels. Modern optical submarine systems have a capacity of several hundred thou- sand speech channels and new hi