 analog telephone call. It transports a human voice from 300 to 3300 Hz, 3 kHz of analog bandwidth. This bandwidth is enforced by the telephone receiver and by the local exchange office and is not a characteristic of the analog line itself. That human voice can be replaced by any other analog signal that fits between 300 and 3300 Hz, such as a modem or fax signal. Analog trunks in the United States are bundled together in L-carrier. Each call has 3 kHz of bandwidth, and analog circuits modify the sound by shifting its frequencies into the ultrasonic range for transport. Table 13.1 shows the L-carrier range as of 1972.2 As recently as 1990, the telephone network in the United States was mostly analog L-carrier. Competition forced the rapid conversion of the PSTN to all-digital trunks during the 1990s, and L-carrier is, for all intents and purposes, an obsolete technology. 13.1.2 Protocols for digital transport The basic building block for digital transport protocols, both in North America and in Europe, is 64 kbps. This is referred to as the DS-0 in North America and the E-0 in Eu- rope. This 64-kbps building block is transmitted in 8-bit blocks at a transmission rate of 8000 blocks per second. When used for voice transmission, each 8-bit block contains an 8-bit sample of the analog voice signal. The method used for taking the sample is different in North America from that used in Europe. The procedure used in North 1The reader will observe that framing, addressing, error detection, and the application (trans- ported information) are attributes of most transport protocols, and the natural tendency is to as- sign these attributes to upper layers of the OSI model. 2There was an L-2 system used briefly before World War II. 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 Telephone Transport TABLE 13.1 The North American Analog