A Review of Multimedia Networking

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8.4. ATM

The ATM technology referred to in section 7.6 is equally effective in Local and Wide Area Networks. However in the Wide Area context is one of many possible services offered by telecommunications operators which have been mentioned in this section. The costs of public ATM provision are not yet known. The costs of the first phases of SuperJANET which employs ATM between some twelve institutions over 34 or 155 synchronous digital links from BT have been funded by Research Councils. It is reasonable to assume that costs of subscribing to ATM services will be related to the required bandwidth and other user requirements such as quality of service. Competitors to ATM will include fixed links, Frame Relay and SMDS.

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 :-

A cell loss ratio of less than 1.7 x 10 -9
A cell transfer delay across the network of 150 micro second per switch. plus transmission and propagation time.
A cell delay variation for one cell in 108 of no more than 250 msec

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:-

Constant bit rate, connection-orientated with timing
Variable bit rate, connection-orientated with timing
Variable bit rate, connection-orientated without timing
Variable bit rate, connectionless without timing
Un-restricted variable bit rate, connection-orientated or connectionless

Each of these categories are further specified through network provider objectives for various ATM performance parameters. These parameters may include cell transfer delay, cell delay variation, and cell loss ratio. The connection traffic descriptor specifies characteristics of the data generated by the user of the connection. This information allows the ATM network to commit the resources necessary to support the traffic flow with the quality of service the user expects. Characteristics defined in the ATM Forum UNI specification include peak cell rate, sustainable cell rate, and maximum and minimum burst sizes.

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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