re dedicated to IP: Chapters 8 and 9.) As a result of these var- ious problems, many vendors have dropped out of the market. ATM Interfaces ATM is a very high-bandwidth, high-performance system that uses a uniform 53-byte cell: 5 bytes of addressing information and 48 bytes of payload. The benefit of the small cell size is reduced latency in transmitting through the network nodes. The disadvantage of the small cell size is the increased overhead. But remember that ATM was built in support of the vision of Broadband ISDN, and the second set of standards in support of Broadband ISDN was SDH/SONET. This means that ATM was created with an eye toward the deployment of fiber, which offers tremen- dous capacities and hence makes bandwidth less of an issue. An ATM network is connection oriented, which for purposes of real-time, mul- timedia, and time-sensitive traffic is very important because it allows controlled latencies. It operates over a virtual circuit path, which leads to great efficiency in terms of network management. Payload error control is done at the endpoints, and some limited error control procedures are performed on the headers of the cells within the network itself. ATM supports asynchronous information access: Some applications consume a high percentage of capacity (e.g., video-on-demand) and others consume much less (e.g., e-mail); thus, ATM allows multirate connections. Packet-Switched Networks Finally, ATM has a highly defined and structured set of QoS definitions, as dis- cussed later in this chapter. The ATM Layers As discussed in the following sections, ATM has three main layers (see Figure 7.18): the physical layer, the ATM layer, and the ATM adaptation layer. The Physical Layer The physical layer basically defines what transmission media are supported, what transmission rates are supported, what physical interfaces are supported, and what the electrical and optical coding schemes are for the ones and zeros. Like the OSI model's physical layer, it's a definition of the physical elements of getti