ATM

ATM (Asynchronous Transfer Mode) is a packet technology, which means that all the information sent over an ATM network is broken down into discrete packets.

Unlike other packet technologies, ATM employs uniformly sized packets, called cells, each of which is 53 bytes long. Because the cells are all the same size, cell delay at ATM switches is more predictable and manageable.

ATM was designed specifically to handle broadband applications efficiently and at the same time let users give certain types of traffic priority treatment on the network. For example, voice traffic, which cannot tolerate much delay, can be marked "high priority" with a guaranteed bandwidth and minimal delay. Less sensitive traffic, such as electronic mail, can be marked for lower priority.

The first five bytes of an ATM cell, known as the header, contain an address that tells where the cell wants to go in the network. The header also specifies the priority of the "payload" information carried in the remaining 48 bytes. ATM networks are linked together by a series of ATM switches that take in cells from various sources and switch them out again.

There are two ways to do establish connections between the switches. In a Permanent Virtual Circuit, network managers pre-set paths from switch to switch ahead of time. PVCs were the initial way to establish virtual circuits between sites before the technology supporting switched virtual circuits was developed. In a Switched Virtual Circuit, the cells travel along paths that are set up on the fly and torn down again after a designated period. The advantage of this approach is that switched virtual circuits occupy switching capacity only while they are set up, increasing switch efficiency and so payback on investment.

In either case, when a network device, such as a router, wants to send cells into an ATM network, it requests a virtual circuit. That request is passed from the initial ATM switch to other ATM switches in the network, with each switch determining whether it can handle the request. If all switches along the path can accommodate the request, the virtual circuit is established. If not, the request must be repeated. That virtual circuit will support a certain quality of service that has been preset in the ATM network switches based on user requirements.

The most common ATM qualities of service are known as Constant Bit Rate, Variable Bit Rate, Available Bit Rate and Unspecified Bit Rate.

Additional resources

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