1.6 Equal-Sized Packets Model
A networking model in which packets are of equal size can also be constructed. Equal-sized packets, or cells, bring a tremendous amount of simplicity to the networking hardware, since buffering, multiplexing, and switching of cells become extremely simple. However, a disadvantage of this kind of networking is the typically high overall ratio of header to data. This issue normally arises when the message size is large and the standard size of packets is small. As discussed in Section 1.3, the dominance of headers in a network can cause delay and congestion.
One of the networking technologies established using the equal-sized packets model is asynchronous transfer mode (ATM). The objective of ATM technology is to provide a homogeneous backbone network in which all types of traffic are transported with the same small fixed-sized cells. One of the key advantages of ATM systems is flexible processing of packets (cells) at each node. Regardless of traffic types and the speed of sources, the traffic is converted into 53-byte ATM cells. Each cell has a 48-byte data payload and a 5-byte header. The header identifies the virtual channel to which the cell belongs. However, because the high overall ratio of header to data in packets results in huge delays in wide area networks, ATM is rarely deployed in networking infrastructure and therefore we do not expand our discussion on ATM beyond this section.