ry @ 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 3 The most common form of multiplexed access and transport is T-car- rier, or E-carrier outside the United States. Let's take a few minutes to describe them. Framing and Formatting in T1 The standard T-Carrier multiplexer, shown in Figure 3-38, accepts inputs from 24 sources, converts the inputs to PCM bytes, then time-division multiplexes the samples over a shared four-wire facility, as shown in Figure 3-39. Each of the 24 input channels yields an eight-bit sample, in Figure 3-38 T-Carrier multiplexer channel banks, showing DSO cards Figure 3-39 Regenerators A time-division multiplexer in action 8-bits per sample, 24 samples per frame + frame bit = 193 bits. 8,000 frames are generated per second, yielding 1.544 Mbps. 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. Telephony Telephony round-robin fashion, once every 125 microseconds (8,000 times per sec- ond). This yields an overall bit rate of 64 Kbps for each channel (eight bits per sample X 8,000 samples per second). The multiplexer gathers one eight-bit sample from each of the 24 channels, and aggregates them into a 192-bit frame. To the frame it adds a frame bit, which expands the frame to a 193-bit entity. The frame bit is used for a variety of purposes that will be discussed in a moment. The 193-bit frames of data are transmitted across the four-wire facil- ity at the standard rate of 8,000 frames per second, for an overall T1 bit rate of 1.544 Mbps. Keep in mind that 8 Kbps of the bandwidth consists of frame bits (one frame bit per frame, 8,000 frames per second); only 1.536 Mbps belongs to the user. Beginnings: D1 Framing The earliest T-Carrier equipment was referred to as D1 and was consid- erab