ATM stands for Asynchronous Transfer Mode, a cell-switched network technology used for building high-speed backbones. ATM breaks data into fixed-size cells of 53 octets. Five octets are used for the cell header; the remaining 48 are available for data. The fixed size allows an ATM switch to handle the cells quickly and efficiently. An ATM switch is allowed to drop cells as necessary if the switch’s capacity is exceeded. There is a mechanism for distinguishing between cells that can be discarded and high-priority cells that should not be discarded (although even high-priority cells can be discarded if there is no alternative).
Sending IP packets over an ATM network presents some interesting problems. Each packet must be broken into multiple cells, since most IP packets won’t fit into 48 octets. If any of the cells are dropped, the packet won’t make it through the network undamaged. Therefore, the packet will probably be resent (it will certainly be re-sent if it’s a TCP segment). In turn, this means dumping many additional cells into a switch that is already suffering from congestion. This problem is less serious if your ATM network is designed for minimal cell loss.
ATM requires special (and sometimes expensive) hardware to run. Some of the common ATM hardware products provided by Cisco include:
ATM Network Processor Module (4500/4700-series routers)
ATM Interface Processor (7500-series routers)
ATM Port Adapter (7500-series routers)
ATM-DXI allows ATM over a non-ATM interface, ...