rs-in effect, a 98- wheeler. These things give passing a whole new meaning. If a road train is rolling down the highway pulling three 50-foot trailers (middle draw- ing), then it has the ability to transport 150 feet of cargo-but only if the cargo is segmented into 50-foot chunks. But what if the trucker wants to transport a 150-foot-long item? In that case, a special trailer must be installed that provides room for the 150-foot payload (bottom drawing). If you understand the difference between the second and third draw- ings, then you understand the difference between an STS-N and an 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 Chapter 3 Figure 3-56 STS-1 Trucking Australian road trains and concatenated STS-1s STS-3 Trucking STS-3 Trucking STS-3 Trucking STS-3c Trucking STS-Nc. The word "concatenate" means "to string together," which is exactly what we do when we need to create what is known as a super-rate frame-in other words, a frame capable of transporting a payload that requires more bandwidth than an STS-1 can provide, such as our 100 Mbps Fast Ethernet frame. In the same way that an STS-N is analogous to a channelized T1, an STS-Nc is analogous to an unchannelized T1. In both cases, the customer is given the full bandwidth that the pipe pro- vides; the difference lies in how the bandwidth is parceled out to the user. Overhead Modifications in STS-Nc Frames When we transport multiple STS-1s in an STS-N frame, we assume that they may arrive from different sources. As a result, each frame is inserted into the STS-N frame with its own unique set of overhead. When we create a concatenated frame, though, the data that will occupy the combined bandwidth of the frame derives from the same source-if we pack a 100 Mbps Fast Ethernet signal into a 155.53 Mbps STS-3c frame, there's only one signal to pack. It's pretty obvious, then,