 that we don't need three sets of overhead to guide a single frame through the maze of the network. For example, each frame has a set of bytes that keep track of the payload within the synchronous payload envelope. Since we only have one payload, therefore, we can eliminate two of them. And the Path Overhead that is unique to the payload can similarly be reduced, since there is a column of it for each of the three formerly indi- vidual frames. In the case of the pointer that tracks the floating payload, 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 the first pointer continues to perform that function; the others are changed to a fixed binary value that is known to receiving devices as a concatenation indication. The details of these bytes will be covered later in the overhead section. Transporting Subrate Payloads: Virtual Tributaries Let's now go back to our Australian road train example. This time he is carrying individual cans of Fosters Beer. From what I remember about the last time I was dragged into an Aussie pub (and it isn't much), the driver could probably transport about six cans of Fosters per 50-foot trailer. So now we have a technique for carrying payloads smaller than the fundamental 50-foot payload size. This analogy works well for under- standing SONET's ability to transport payloads that require less band- width than 51.84 Mbps, such as T1 or traditional 10 Mbps Ethernet. When a SONET frame is modified for the transport of subrate pay- loads, it is said to carry virtual tributaries. Simply put, the payload enve- lope is chopped into smaller pieces that can then be individually used for the transport of multiple lower-bandwidth signals. Creating Virtual Tributaries To create a "virtual tributary-ready" STS, the synchronous payload enve- lope is subdivided. An STS-1 comprises 90 columns of bytes, four of 