Communications providers saw a need for a permanent shared network technology that offered very low latency and jitter at much higher bandwidths. Their solution was Asynchronous Transfer Mode (ATM). ATM has data rates beyond 155 Mbps. As with the other shared technologies, such as X.25 and Frame Relay, diagrams for ATM WANs look the same. 

ATM is a technology that is capable of transferring voice, video, and data through private and public networks. It is built on a cell-based architecture rather than on a frame-based architecture. ATM cells are always a fixed length of 53 bytes. The 53 byte ATM cell contains a 5 byte ATM header followed by 48 bytes of ATM payload. Small, fixed-length cells are well suited for carrying voice and video traffic because this traffic is intolerant of delay. Video and voice traffic do not have to wait for a larger data packet to be transmitted.

The 53 byte ATM cell is less efficient than the bigger frames and packets of Frame Relay and X.25. Furthermore, the ATM cell has at least 5 bytes of overhead for each 48-byte payload. When the cell is carrying segmented network layer packets, the overhead will be higher because the ATM switch must be able to reassemble the packets at the destination. A typical ATM line needs almost 20% greater bandwidth than Frame Relay to carry the same volume of network layer data.

ATM offers both PVCs and SVCs, although PVCs are more common with WANs.

As with other shared technologies, ATM allows multiple virtual circuits on a single leased line connection to the network edge.

Interactive Media Activity Checkbox: WAN Technologies Asynchronous Transfer Mode (ATM) Upon completing this activity, the student will be able to identify the characteristics associated with ATM connections.

Web Links Asynchronous Transfer Mode (ATM) Switching http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/atm.htm