 application, even the Internet. The result is that that data communications often share transmission fa- cilities and therefore are assigned a circuit on that transmission facility. The circuits provided by the various telephone transport methods can be applied to many purposes. In Chap. 11 we saw the use of circuits as lines for connecting telephones to local ex- change offices. We also saw the use of circuits as trunks to connect switching offices. Circuits may be used to connect private branch exchanges (PBXs) to form a private net- work. These circuits are sometimes referred to as leased lines. The use of the term lines 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. Telephone Transport Key Telephony Concepts in this context may appear confusing because the leased lines connect switches rather than terminating at a fixed point. However, in the telephony context, they are not be- ing switched by the PSTN. Finally, we saw the use of circuits, again called leased lines, to connect data communications equipment. Wireless networks depend heavily on a combination of digital trunks and leased lines for connections between base stations and mobile switching centers (MSCs) and con- nections to the PSTN. 13.1 Telephone Transport Protocols Analog and digital facilities alike rely on protocols, whether it be for the separation of channels through the use of frequency division techniques or the use of framing tech- niques such as DS-1 framing and Asynchronous Transfer Mode (ATM). For those fa- miliar with the Open Systems Interconnection (OSI) model, this raises the question of how the apparent framing and addressing techniques used in transport relate to the OSI model. It can be stated generally that all transport technologies, including ATM, are considered to be OSI layer 1 protocols. 1 13.1.1 Analog transport The simplest protocol is that for the