eep in mind that the train can only fill to its maximum capacity of seats. We can imagine the SONET/SDH frame as being full of cells in the payload, some cells busy and some idle/unassigned. At the peak traffic period, all cells will be busy, and some traffic (passengers) may have to be turned away. We can go even further with this analogy. Both Washington, DC, and New York City attract large groups of tourists, and other groups travel to business meetings or conventions. A tour group has a chief tour guide in the lead seat (cell) and an assistant guide in the last seat (cell). There may be SO many in the group that they extend into a second car or may just intermingle with other passengers on the train. The tour guide and assistant tour guide keep an exact count of people on the tour. The lead guide wears a badge that says BOM, all tour members wear badges that say COM, and the assistant tour guide wears a badge that says EOM. Each group has a unique MID (message ID). We also see that service is connection-oriented (Washington, DC, to New York City). Asynchronous means that we can keep filling seats on the train until we reach its maximum capacity. If we look up the word, it means nonperiodic whereas the familiar E1/DS1 are periodic (i.e., synchronous). One point that seems to get lost in the literature is that the train has a maximum capacity. It is common to read that ATM provides "bandwidth" on demand. The underlying transmission medium has a fixed bandwidth. We believe the statement is supposed to mean "cell rate capacity" on demand. This is true of course until we reach the maximum capacity. For example, if the underlying transmission format is SONET STS-1, it can provide 86 X 9 octets X 8000/(53) per second or about 116,000 cells per second of payload (see Chapter 19). ASYNCHRONOUS TRANSFER MODE 20.3 USER-NETWORK INTERFACE (UNI) AND ARCHITECTURE ATM is the underlying packet technology of broadband ISDN (B-ISDN). At times in this section, we will use the terms ATM and B-ISDN interchangeably. Figures 20.2 an