which means that if it starts "late" in the payload envelope, it may well run into the payload envelope of the next frame! In fact, this happens more often than not, but it's OK-SONET is equipped to handle this odd behavior. We'll discuss it shortly. SONET Bandwidth The SONET frame consists of 810 eight-bit bytes, and like the T1 frame is transmitted once every 125 usec (8,000 frames per second). Doing the math, this works out to an overall bit rate of 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 3 810 bytes/frame X 8 bits/byte X 8,000 frames/second = 51.84 Mbps the fundamental transmission rate of the SONET STS-1 frame. That's a lot of bandwidth-51.84 Mbps is slightly more than a 44.736 Mbps DS3, a respectable carrier level by anyone's standard. What if more bandwidth is required, however? What if the user wants to transmit mul- tiple DS3s, or perhaps a single signal that requires more than 51.84 Mbps, such as a 100 Mbps Fast Ethernet signal? Or, for that matter, a payload that requires less than 51.84 Mbps! In those cases, we have to invoke more of SONET's magic. The STS-N Frame In situations in which multiple STS-1s are required to transport multi- ple payloads, all of which fit in an STS-1's payload capacity, SONET allows for the creation of what are called STS-N frames, where "N" rep- resents the number of STS-1 frames that are multiplexed together to cre- ate the frame. If three STS-1s are combined, the result is an STS-3. In this case, the three STS-1s are brought into the multiplexer and byte interleaved to create the STS-3, as shown in Figure 3-55. In other words, the multiplexer selects the first byte of frame one, followed by the first byte of frame two, followed by the first byte of frame three. Then it selects the second byte of frame one, followed by the second byte of frame two, followed by the second byte