9.3. Internet Protocols and ATM

ATM is a link level technology which provides the potential capability for applications at the TCP level to map to a single ATM virtual circuit for transport across an ATM network(s) customised to the network performance and traffic requirements for that application. The future Internet will be comprised of both conventional and "sophisticated" link technologies. The "sophisticated" features of link layers like ATM need to be incorporated into an internet where data travels not only across an ATM network but also several other existing LAN and WAN technologies. ATM allows for each logical channel to have a customisable set of performance and quality of service characteristics. Hence a single ATM link level media appears like an array of link level technologies each with customisable characteristics.

There are several parameters required to map ATM services from a higher level service like IPng [Brazdziunas94]. These ATM parameters can be categorised as: addressing parameters, connection QOS - related parameters, connection management information, and ATM virtual circuit identifier. The first three categories provide support for ATM signalling. The last parameter, a connection identifier that maps IPng packets to ATM virtual circuits, provides support for an ATM virtual circuit per application when the end-to-end connection travels across an ATM subnetwork(s) (this does not assume that ATM is the only type of subnetwork that this connection travels across).

An ATM virtual circuit is established based upon a user's traffic characteristics and network performance objectives. These characteristics which include delay and throughput requirements can only be defined by the application level (at the transport level or above) as opposed to the inter - networking (IPng) level. For instance, a file transfer application transferring a 100 Mbyte file has very different link level performance requirements than a video application. The former requires a high throughput and low error rate connection whereas the latter requires a guaranteed bit rate. Applications will be responsible for reserving the required type of connection from the ATM link.

9.4 ITU-T Standards

Capability to control virtual circuit connections (for ATM) Support of point to point or multipoint and broadcast configurations Change of media from within a call by all parties, e.g. ATM to ISDN Negotiation of Quality of ServiceReconfiguration of a multi-party call Allowing different media to be used with different users in a multi-party call, e.g.. an audio call with one user and a data call with another.

This includes aspects of synchronisation. Different information types may experience different delays through the network. Issues to be addressed in the standard include:

Differential time delay between media carried on separate virtual or physical channels.
Inter - channel synchronisation
Inter - working between different coding schemes, e.g. audio coding schemes
Support for signalling through the network between users

This includes the multiplexing of different media into a single stream onto physical or virtual channels and the ability to change the bandwidth allocated to different media such as audio and video from within a call.

Further issues are raised for network operators by multimedia. Two important ones are charging mechanisms for multimedia services and performance limits.

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