which are reserved for overhead functions (Section, Line, and Path). That leaves 86 for actual user payload. To create virtual tributaries, the payload capac- ity of the SPE is divided into seven 12-column pieces, called virtual tribu- tary (VT) groups. Math majors will be quick to point out that 7x12=84, leaving two unassigned columns. These columns, shown in Figure 3-57, are indeed unassigned, and are given the rather silly name of fixed stuff. Now comes the fun part. Each of the VT groups can be further subdi- vided into one of four different VTs to carry a variety of payload types, as shown in Figure 3-58. A VT1.5, for example, can easily transport a 1.544 Mbps signal within its 1.728 Mbps capacity, with a little room left over. A VT2, meanwhile, has enough capacity in its 2.304 Mbps structure to carry a 2.048 Mbps European E1 signal, with a little room left over. A VT3 can transport a DS1C signal, while a VT6 can easily accommodate a DS2, again, each with a little room left over. 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 3-57 SONET fixed stuff Columns 1-4, Section, Line, & Columns 30 and 59, "Fixed Stuff" Path Overhead VT Type Columns/VT Bytes/VT VTs/Group VTs/SPE Figure 3-58 Bandwidth SONET virtual tributary payload transport VT1.5 1.728 2.304 3.456 6.912 One aspect of virtual tributaries that must be mentioned is the mix- and-match nature of the payload. Within a single SPE, the seven VT groups can carry a variety of different VTs. However, each VT group can carry only one VT type. That "little room left over" comment (earlier) is, by the way, one of the key points that SONET and SDH detractors point to when criticizing them as legacy technologies, claiming that in these times of growing com- petition and the universal drive for efficiency, they are inordinately wasteful of bandwidth, given that they w