ere designed when the compa- nies that delivered them were monopolies and less concerned about such things than they are now. We will discuss this issue in a later section of the book. For now, though, suffice it to say that one of the elegant aspects 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 of SONET is its ability to accept essentially any form of data signal, map it into standardized positions within the SPE frame, and transport it effi- ciently and at very high speed to a receiving device on the other side of town or the other side of the world. Creating the Virtual Tributary Superframe You will recall that when DS1 frames are transmitted through modern networks today, they are typically formatted into extended superframes in order to eke additional capability out of the comparatively large per- centage of overhead space that is available. When DS1 or other signals are transported, via an STS-1 formatted into VT groups, four consecutive STS-1s are "ganged together" to create a single VT superframe, as shown in Figure 3-59. To identify the fact that the frames are behaving as a VT superframe, certain of the overhead bytes are modified to indicate the change. SONET Synchronization SONET relies on a timing scheme called "plesiochronous timing." As I implied earlier, the word sounds like one of the geological periods that we Figure 3-59 VT superframe STS-1 #1 STS-1 #2 STS-1 #3 STS-1 #4 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 all learned in geology classes (Jurassic, Triassic, Plesiochronous, Plas- ticene). "Plesiochronous" derives from Greek, and means "almost timed." Other words that are commonly tossed about in thi