ATM - by the best providers on the web - Asynchronous transfer mode

Asynchronous transfer mode (ATM) consists of a set of hosts connected by ATM links to an ATM switch. An ATM switch gets its data from the hosts connected to it and passes this data to the destination. The destination host can either be an end-point, or it can be an intermediate ATM switch on the way from the data's source to it's destination.

Data is transmitted over an ATM network in ATM cells. A cell is a fixed size at 53 byte data structure that contains 48 bytes of data and 5 bytes of control information. Each cell's control information includes a virtual circuit number. This number is used by ATM switches to determine where the cell should be sent, and is used by receiving hosts to determine which buffers should receive the data (thus allowing inbound network data to be demultiplexed in hardware).

The virtual circuit number is composed of two numbers: the virtual channel identifier (VCI) and the virtual path identifier (VPI). All data sent over an ATM network is associated with a virtual circuit. There are two types of virtual circuits: permanent virtual circuits (PVC) and switched virtual circuits (SVC). Permanent virtual circuits are usually set up in an ATM switch by a network administrator. Switched virtual circuits are connections that are established as needed through the use of signalling protocols

The 48 byte data area of an ATM cell is quite small when compared to the data area of an Ethernet or FDDI packet. ATM includes a number of ATM adaptation layers (or AAL). AAL5 allows a host to send and receive frames of up to 64K in size. When a host sends a large AAL5 frame, the ATM host-network interface breaks it up into ATM cells. When the cells arrive at the receiving host, these cells are reassembled by the receiving machine's ATM host-network interface. AAL5 allows hosts to send and receive frames and not worry about the small ATM cells.

One advantage of ATM is its use of virtual circuits makes it easier to provide network performance guarantees to applications. Each active virtual circuit on an ATM network can be allocated a fixed portion of the network's bandwidth.

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