d with DS-1's 24, for a data rate of 2.048 Mbps. The trunk side of local exchange offices is connected to other switches almost exclu- sively by using digital facilities. The basic building block for these digital facilities is the DS-1 (or E-1, where deployed). Since the actual number of trunks required in LEC and IXC offices is large, the carrier facilities used typically operate at higher rates than DS-1 or E-1. This is accomplished by multiplexing multiple DS-1s or E-1s onto a larger facility. A digital signal hierarchy has been established for these higher rates. For instance, four DS-1s are multiplexed to form a DS-2, and seven DS-2s are multiplexed to form a DS-3. The DS-2 is, in practice, an intermediate step in the hierarchy, and transmission facility actually used will carry a DS-3, consisting of 28 DS-1s. Optical 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. The PSTN and Telephone Switching The PSTN and Telephone Switching facilities may be used to carry even higher rates. Internationally, E-1 facilities are also multiplexed up to higher rates. 11.3.2 Switching Modern digital switches are designed to switch individual calls in 8-bit samples at a rate of 8000 sample per second. Analog signals, such as those common on two-wire lines, are converted to four-wire signals using hybrids and digitized to the 64-kbps rate. Digital channels derived from the time slots present on digital facilities are presented directly to the switch. Two channels, one for each direction of the call, must be switched to have a complete call. Since the incoming and outgoing legs of the call may not be in synchronization with the internals of the switch, buffers are used by the switch to store the 8-bit sam- ples until the switch is ready to accept them for switching and also prior to placing the sample in the outgoing time slot. The buffering capability of