e two communicating devices. The mobile station and the BSS/PCU combination use the air interface to communicate in a secure manner, with reliable data transfer across a hostile medium (that being the air). These combined interfaces and protocols provide more efficient radio resource utilization. From this interface, the next step is for the mobile sta- tion to communicate across these interfaces to the SGSN, covered in Chap- ter 7, "X.25, Internets, Intranets, and Extranets." Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. CHAPTER X.25, Internets, Intranets, and Extranets X.25, Internets, Intranets, and Extranets Chapter 7 Objectives Upon completion of this chapter, you should be able to Understand the benefits of circuit-switched VS. packet-switched networks. Discuss the original concept of X.25 packet switching. Understand the services of the Internet and intranet. Describe how the TCP/IP works. Describe how data is transferred across the IP networks. Modes of Switching In any given network, switching points are interconnected to a form of mesh through which voice and data calls are routed from one device to another, based on the terminal's address. The addressing methods are somewhat unique to the network being used. The network provides con- nectivity for every device in a way that is consistent with the infrastructure. As a user dials a call, for example, resources (circuit-switched) are allocated for the duration of the connection. Each network, however, uses a different form of switching, but could be an overlay to a different network. This is the case with many of the data networks today. These data networks are over- lays to the circuit-switched telephone network regardless of the traffic they carry. GPRS is similar in that the IP-based GPRS data network is an over- lay to the GSM circuit-switched network. Circuit Switching Circuit