Wide area network interfaces will operate at 155.52 Mbps and 622.08 Mbps, both requiring optical fibre interfaces. The standards for ATM were first developed by the International Telecommunications Union (ITU) in the 1980's. Co-odination of the implementation of ATM is in the hands of the ATM Forum which consists of a wide cross section of companies. The ATM Forum has developed implementors specifications to try to insure that equipment manufactured by several companies can inter-operate. The latest is the UNI Specification (Version 3.1).
ATM uses small constant size packets to reduce and control delay. Control of the priorities of packets in ATM switches enables guaranteed delivery of information. ATM can emulate ISDN channels and Ethernet characteristics. ATM is seen as a universal technology which can be used over physical LANs and WANs and may be able to carry both asynchronous and isochronous data. ATM may also be delivered over ISO-Ethernet.
ATM network technology has strong industrial support and is already carrying traffic over the academic SuperJANET network. ATM can support different speeds, traffic types and quality of service matched to applications. ATM cells coming from a user are guaranteed delivery at the other end with a high probability and low delay. A cell is a short block of data 53 octets in length including 5 octets overhead. The performance aims are :-
ATM users have a dedicated connection to a high speed ATM switch. Switched virtual circuits are set up by the switch to a destination. Additionally ATM users can select a preferred network provider to service the connection.
ATM signalling establishes a "hard state" in the network for a call. "Hard state" implies that the state of a connection in intermediate switching equipment can be set and once established it will be maintained until a message is received by one of the ends of the call requesting a change in state for the connection. As a result, an ATM end system (this could be a workstation with an ATM adapter or a router with an ATM interface) receives guaranteed service from the ATM network. The ATM network is responsible for maintaining the connection state. ATM termination points must be responsible for changing the state of the connection, and specifically informing the ATM network to establish, alter, or close the connection.
Each ATM end point in a network has an ATM address associated with it to support dynamic connection establishment via signalling. These addresses are hierarchical in structure and globally unique. As a result, these addresses are routed. This allows ATM networks to eventually support a large number of ATM endpoints once a routing architecture and protocols to support it become available.
Several classes of ATM service have been defined:-
The variable and constant bit rate, connection-orientated with timing services are most appropriate to the transport of real time multimedia. However other services could be useful for transfer of multimedia material in less than 'real' time.
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