 of frame three, and so on, until it has built an interleaved frame that is now three times the size of an STS-1: 9X270 Figure 3-55 Byte inter- leaving in SONET 270 Bytes Bytes One STS-3 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 bytes instead of 9x90. Interestingly (and impressively!), the STS-3 is still generated 8,000 times per second. The technique I have just described is called a single-stage multiplex- ing process, because the incoming payload components are combined in a single step. There is also a two-stage technique that is commonly used. For example, an STS-12 can be created in either of two ways. Twelve STS-1s can be combined in a single-stage process to create the byte inter- leaved STS-12; alternatively, four groups of three STS-1s can be com- bined to form four STS-3s, which can then be further combined in a second stage to create a single STS-12. Obviously, two-stage multiplexing is more complex than its single-stage cousin, but both are used. Special note: The overall bit rate of the STS-N system is NxSTS-1. However, the maximum bandwidth that can be transported is STS-1 - but N of them can be transported! This is analogous to a channelized T1. The STS-Nc Frame Let's go back to our Fast Ethernet example mentioned earlier. In this case, 51.84 Mbps is inadequate for our purposes, because we have to transport the 100 Mbps Ethernet signal. For this we need what is known as a concatenated signal. One thing you can say about SONET: It doesn't hurt for polysyllabic vocabulary. On the long, lonesome stretches of outback highway in Australia, unsuspecting car drivers often encounter a devilish vehicle known as a "road train." Imagine an 18-wheel tractor-trailer (see top drawing, Figure 3-56, for a remarkable illustration) barreling down the highway at 80 miles per hour, but now imagine that it has six traile