t of a network payload is very apparent in SONET/ SDH transmission (see page 98 and 99), in which the payload of 51.8Mbs includes extensible overhead that can accommodate be- tween 1 and 192 51.8Mbs channels. These channels, called STS1 for Synchronous Transport Stream 1, can, in turn, accommodate lower rate channelization to carry legacy PDH payloads. Alternatively, it is possible to map other channel structures capable of carrying program or other content encapsulated in non-PDH structures, such as ATM and IP. PROGRAM CONTENT TRANSPORT ON TDM CHANNELS From the earliest days of TDM, practical encoders and decoders have been and continue to be used to transport NTSC and PAL video and analog audio. These devices are designed to produce an output that runs over DS3 or E3. This means they are hard-wired to produce a nominal 30Mbs payload bitrate. In practice, the payload is an octal number. In the instant example, the maximum APPENDIX payload possible is 30,720,000 bits per second. To provide some measure of tolerance, or guard-band as it's called in RF channels, let's assume the next lower octal number will be our payload rate. Determining this number is a bit of trial and error. Logically, it should be an octal number as well. This means we start with some small number such as 1,048, subtract it from the payload and then divide by 8 to see if the result, 30,718,952, is an octal number. And, yes, that's an octal number. What's important is that the payload fit the channel. It must get transported without errors caused by not enough bandwidth. Incidentally, in this case, it will fit both E3 and DS3. So what happens to the rest of the channel? It's simple; just stuff it with null bits. As long as null bits don't come in the wrong place, which, in this case, is outside the payload, the network doesn't care. What the network looks for are the framing bits. As long as the framing bits are there and in the right place, the network is fat, dumb, and happy. FEC may or may not be required, depending on the requirements and 