ATM BASICS - Chapter 4 docx

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Chapter 4

ATM Services

The chapter explains and summaries a range of concepts and mechanisms,

which allow ATM for supporting of large variety of services. It presents the

concept of Quality of Service in ATM from the services perspective. It

describes QoS categories and traffic classes, QoS parameters as well as traf￾fic parameters that are used in ATM devices. The chapter also explains

basic traffic management mechanisms, which ensure that requested QoS is

delivered to the customers. The reading of the chapter is advised prior to

discussing ATM applications.

Before presenting different ATM services and applications, one has to real￾ize the great capabilities of ATM technology in terms of provisioning guar￾anteed bandwidth and strict QoS conditioning. In fact whenever ATM ser￾vices are discussed, the QoS objectives and mechanisms are concerned. ATM

was designed to deliver the QoS in a way incomparable to any former tech￾nology. The state-of-the-art transmission and switching constraints impli￾cated a very detailed and coarse definition of the ATM QoS. The unique com￾bination of capabilities and mechanisms allow for ensuring that all types of

applications can be given a suitable set of parameters. Therefore, different

services can be supported simultaneously with adequate QoS treatment.

4.1 Quality of Service Categories

Since ATM supports transport of several different connection and traffic

types, as well as service types, a service classification was developed.

Initially, ITU-T took into account three factors that are used to differentiate

between services. Service classes introduced by ITU-T are dependent on the

timing relationship between source and destination, bit rate, and the con￾nection mode. Next ATM Forum presented its own classification of so called

QoS service categories. The relations between two classification models are

given in the Table 4-1.

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Table 4-1, QoS Categories and Service Classes

ATM Forum defined also an updated version of UBR QoS

category called Differentiated UBR.