the Path Over- head is only used by the source and destination multiplexers-the inter- mediate multiplexers don't care about the specific nature of the payload, because they don't have to terminate or interpret it. 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 3-53 Layers of SONET overhead surround the payload prior to trans- mission. The SONET Frame Keep in mind once again that we are doing nothing more complicated than building a T1 frame with an attitude. Recall that the T1 frame was made up of 24, eight-bit channels (samples from each of 24 incoming data streams) plus a single bit of overhead. In SONET, we have a similar con- struct-a lot more channel capacity, and a lot more overhead, but the same functional concept. The fundamental SONET frame is shown in Figure 3-54, and is known as a Synchronous Transport Signal, Level One, or STS-1. It is nine bytes tall and 90 bytes wide, for a total of 810 bytes of transported data including both user payload and overhead. The first three columns of the frame are the Section and Line Overhead, known collectively as the Transport Overhead. The bulk of the frame itself, to the left, is the syn- chronous payload envelope or SPE, which is the "container" area for the user data that is being transported. The data, previously identified as the 90 Bytes Figure 3-54 Fundamental Payload Envelope SONET frame 9 Bytes Payload Transport Overhead (SOH + LOH) 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 payload, begins somewhere in the payload envelope. The actual starting point will vary, as we will see later. The Path Overhead begins when the payload begins: Because it is unique to the p