ATM QoS

The five ATM service classes are:

1.    constant bit rate (CBR)

2.    variable bit rate—real-time (VBR-rt)

3.    variable bit rate—non-real-time (VBR-nrt)

4.    available bit rate (ABR)

5.    unspecified bit rate (UBR)

Constant Bit Rate

Sources of CBR traffic are expected to send at a constant rate.

The source’s peak rate and average rate of transmission are equal.

CBR class is designed for customers who need real-time audio or video services. CBR is a relatively easy service for implementation

Variable Bit Rate

The VBR class is divided into two subclasses: real-time (VBR-rt) and non-real-time (VBR-nrt).

VBR-rt is designed for users who need real-time services (such as voice and video transmission) and use compression techniques to create a variable bit rate.

The traffic generated by the source is characterized by a token bucket, and the maximum total delay required through the network is specified.

VBR-nrt bears some similarity to IP’s controlled load service. The source traffic is specified by a token bucket.

VBR-nrt is designed for users who do not need real-time services but use compression techniques to create a variable bit rate

Unspecified Bit Rate

UBR class is a best-effort delivery service that does not guarantee anything. UBR allows the source to specify a maximum rate at which it will send.

o   Switches may make use of this information to decide whether to admit or reject or negotiate with the source for a less peak rate.

Available Bit Rate

ABR apart from being a service class also defines a set of congestion-control mechanism.

The ABR mechanisms operate over a virtual circuit by exchanging special ATM cells called resource management (RM) cells between the source and destination.

ABR allows a source to increase or decrease its allotted rate as conditions dictate.

ABR class delivers cells at a minimum rate. If more network capacity is available, this minimum rate can be exceeded.

ABR is suitable for applications that are bursty in nature.