duces tromboning. There is another function of inter-MSC pipes. Some wireless carriers may wish to support roamer terminations and mobile-to-mobile calls without having to use expen- sive capacity leased from the PSTN. If the wireless carrier establishes additional voice capacity among MSCs, then that carrier can serve roamer terminations and mobile-to- mobile calls end-to-end on its own transport facilities, possibly at reduced cost. 8 If a vendor is doing this, then the total capacity for the pipe between two MSCs should be the sum of what is needed for handoffs and roaming plus what is needed for mobile-to- mobile communications. Although the two functions may be supported on the same physical channel, their logical functions are separate. In addition to the separation of the two data links between two MSCs, the ANSI-41 channel capacity must be dedicated to ANSI-41 and not used for other purposes. 13.2.4 MSC to PSTN If you are an old-fashioned telephony engineer, then you can relax for a while as we dis- cuss MSC-to-PSTN transport. The connection between MSC and PSTN, at least at the time we are writing this book, is ordinary circuit-switched pulse code modulation (PCM) on DS-0 trunks bundled 24 to a DS-1 or E-0 trunks bundled 30 to an E-1. To the PSTN, the MSC looks like a local exchange office or a PBX. The MSC-to-PSTN trunks do not carry handoff traffic because that is handled by dedicated MSC-to-MSC links. Incoming calls to roamers come into the home MSC and are forwarded to the serving MSC just like any other forwarded call. The signaling sys- tem may be smart enough to redirect the call to the serving MSC without it having to make a detour into the home MSC and back out again. As we will discuss in the next chapter, signaling eliminates much inefficiency in telephone transport. The MSC-to-PSTN trunk group is ordinary not only in its format but also in its size. An MSC with 100 base stations with a capacity of 500 calls each, for example, will re- quire tens of thousands of trunks, several DS-4 or E-4 lin