The ATM Paradigm

ATM is using short fixed-sized packets, or cells, which are transported at regular time intervals, so as to minimise the processing time associated with their processing, buffering and routing. It is a connection-oriented network, usually operating at very high speeds.

ATM provides a mechanism for logical connections, called Virtual Circuit Connections (VCC). A VCC is setup between two end users through the network, and a variable rate, full-duplex flow of cells is exchanged over the connection [Sta98]. A Virtual Path Connection (VPC) is a bundle of VCCs that have the same end points, and it helps to restrain the signalling and control cost of multiple connections by grouping them into a single unit, as shown in Figure 3.2.

  

Figure: ATM connection relationships, source: [Sta98].

The real-time services of ATM are classified to constant and variable bitrate services, or CBR and VBR respectively. ATM defines a range of QoS guarantees for its connections. These can be generally described by the term traffic control, and refer to the actions taken to avoid congestion conditions or to minimise congestion effects, and include [Sta98]:

• Resource management using virtual paths, which is allocating the available network resources in such a way as to separate traffic flows according to their required service characteristics.
• Connection admission control, which ensures that a new connection can get the QoS agreed in the traffic contract, otherwise it rejects the request.
• Usage parameter control, which monitors established connections to determine whether the traffic conforms to the traffic contract.
• Selective cell discard, which discards low-priority cells tagged by the data source or the usage parameter control.
• Traffic shaping, which regulates and smooths out the traffic flow so it does not exceed the agreed QoS parameters.

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