Networks are more frequently being prepared to allow specification of the quality of service needed by users.

For example: - a typical voice telephone call may give a QoS parameter for throughput of 64kbps and time delay less than 500msec. A user transmit video might need throughput of 2Mbps.

ATM QoS SPECIFICATIONS:

There are three kinds of QoS specifications, which are shown as follows:

CONSTANT BIT RATE (CBR):

It is defined for video and audio, since these have predefined maximum speed.

VARIABLE BIT RATE (VBR):

It is defined for compressed video and audio where the data rate relays on the level of compression that may be achieved.

AVAILABLE BIT RATE (ABR):

It is used for typical data functions and allows need of whatever bandwidth is available at a required time.