Even more important, its data transport format is based on the slot, which is called a cell in ATM. This slot has format very similar to the ATM cell, which we will discuss at length in this chapter. It even has the same number of octets, 53; 48 of these carry the payload. This is identical to ATM. This "cell" idea even may be found in Bellcore's SMDS (switched multimegabit data service). In each case the slot or cell was 53 octets long. DQDB introduced a comparatively new concept of the HCS or header check sequence for detecting errors in the header. In neither case was there a capability of detecting errors in the payload. SMDS/DQDB left it to layer 3. Slots or cells carry pieces of messages. The first "piece" 1 Compromise might be a better term. 20 North America, PSTN BER can be expected to be better than 5 X 10-10 Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman ASYNCHRONOUS TRANSFER MODE of a message is identified by a BOM (beginning of message); all subsequent pieces, but not the last, are identified with the COM (continuation of message), and the last piece or slot is identified with an EOM (end of message). There is also an MID (message identifier) appended to all pieces of a common message. This is done SO that there is no confusion as to which transaction a cell belongs. 20.2 INTRODUCTION TO ATM ATM is an outgrowth of the several data transmission format systems discussed above, although some may argue this point. Whereas the formats described above ostensibly were to satisfy the needs of the data world, 3 ATM (according to some) provides an optimum format or protocol family for data, voice, and image communications, where cells of medium can be intermixed through the network as illustrated in Figure 20.1. It would really seem to be more of a compromise from our perspective. Typically, these ATM cells can be transported on SONET, SDH, E1/DS1, and other digital formats. Cells can also be transported contiguously without an und