ATM Service Categories

Providing desired QoS for different applications is very complex. For example, voice is delay-sensitive but not loss-sensitive, data is loss- sensitive but not delay-sensitive, while some other applications may be both delay-sensitive and loss-sensitive.

To make it easier to manage, the traffic in ATM is divided into five service classes accorcing to various combination requested QoS:

· CBR: Constant Bit Rate

CBR is the service category for traffic with rigorous timing requirements like voice, and certain types of video. CBR traffic needs a constant cell transmission rate throughout the duration of the connection.

· rt-VBR: Real-Time Variable Bit Rate

This is intended for variable bit rate traffic for e.g. certain types of video with stringent timing requirements.

· nrt-VBR: Non-Real-Time Variable Bit Rate

This is for bursty sources such as data transfer, which do not have strict time or delay requirements.

· UBR: Unspecified Bit Rate

This is ATM’s best-effort service, which does not provide any QoS guarantees. This is suitable for non-critical applications that can tolerate or quickly adjust to loss of cells.

· ABR: Available Bit Rate

ABR is commonly used for data transmissions that require a guaranteed QoS, such as low probability of loss and error. Small delay is also required for some application, but is not as strict as the requirement of loss and error. Due to the burstiness, unpredictability and huge amount of the data traffic, sources implement a congestion control algorithm to adjust their rate of cell generation. Connections that adjust their rate in response to feedback may expect a lower CLR and a fair share of available bandwidth.

The available bandwidth at an ABR source at any point of time is dependant on how much bandwidth is remaining after the CBR and VBR traffic have been allocated their share of bandwidth. Figure 1 explains this concept.