what the business proposition can afford to pay. Also, the underlying transmission facility can exhibit drastically different performance. For example, satellite or terrestrial radio channels are far more vul- nerable to errors than fiber. Bit error rate in transmission channels is dependent on receiver sensitivity or the ability of the receiver to detect low-level signals, or distinguish between noise and signal. When the RF carrier signal at the receiver antenna terminal is low, the receiver can easily mistake a noise spike or pulse for a digital signal transition. Fiber behaves the same way, except that it has several orders of magnitude greater sensitivity than radio receivers do, plus many times the bandwidth possible on any given transmission path. In practice, order an E3 or DS3 from your friendly service provider, connect the codecs, and operate the network. What about ATM and IP framing and protocols? There are as many possibilities as there are protocols. It is instructive and enlightening to compare two more-ATM over PDH/SONET/SDH and IP over PDH/SONET/SDH. APPENDIX ATM Transmission The case for ATM transport is more complex than straight TDM but less complicated than IP. To further simplify, assume constant bit rate transport and use of ATM adaptation layer 1 (AAL1) for any number of reasons that area beyond the scope of this Appendix. Here's the story and an example: Recall the ATM transport mechanism is a 53-byte cell. 47 or 48 bytes of the 53 are used for payload, and the rest is overhead. The most important thing to recognize and not ignore is the fact that ATM network access on a (PDH) DS3 must fit within the clear channel rate of a DS3 as outlined above (i.e., [44,736,000]- [526,306]=44,209,694bps, which is a brick wall boundary that the network will not allow to be exceeded under any circumstances). This is the maximum potential size ATM payload that can be carried by the TDM DS3, layer 1 network facility. The ATM payload crossing the network channel interface cannot exceed the precise number o