ATM Layer Service Categories

The ATM Forum and ITU-T have defined several distinct service categories at the ATM layer (1,4). The categories defined by the ATM Forum include constant bit rate (CBR), real-time variable bit rate (VBR-rt), non-real-time variable bit rate (VBR-nrt), available bit rate (ABR), and unspecified bit rate (UBR). ITU-T defines four service categories, namely, deterministic bit rate (DBR), statistical bit rate (SBR), available bit rate (ABR), and ATM block transfer (ABT).

The first of the three ITU-T service categories correspond roughly to the ATM Forum’s CBR, VBR, and ABR classifications, respectively. The fourth service category, ABT, is solely defined by ITU-T and is intended for bursty data applications. The UBR category defined by the ATM Forum is for calls that request no quality of service guarantees at all. Figure 4 lists the ATM service categories, their quality of service (QoS) parameters, and the traffic descriptors required by the service category during call establishment (1,4).

Figure 4. ATM layer service categories

The constant bit rate (or deterministic bit rate) service category provides a very strict QoS guarantee. It is targeted at real-time applications, such as voice and raw video, which mandate severe restrictions on delay, delay variance (jitter), and cell loss rate. The only traffic descriptors required by the CBR service are the peak cell rate and the cell delay variation tolerance. A fixed amount of bandwidth, determined primarily by the call’s peak cell rate, is reserved for each CBR connection.

The real-time variable bit rate (or statistical bit rate) service category is intended for real-time bursty applications (e.g., compressed video), which also require strict QoS guarantees. The primary difference between CBR and VBR-rt is in the traffic descriptors they use. The VBR-rt service requires the specification of the sustained (or average) cell rate and burst tolerance (i.e., burst length) in addition to the peak cell rate and the cell delay variation tolerance. The ATM Forum also defines a VBR-nrt service category, in which cell delay variance is not guaranteed.

The available bit rate service category is defined to exploit the network’s unused bandwidth. It is intended for non-real-time data applications in which the source is amenable to enforced adjustment of its transmission rate. A minimum cell rate is reserved for the ABR connection and therefore guaranteed by the network. When the network has unused bandwidth, ABR sources are allowed to increase their cell rates up to an allowed cell rate (ACR), a value that is periodically updated by the ABR flow control mechanism (to be described in the section entitled ‘‘ATM Traffic Control’’). The value of ACR always falls between the minimum and the peak cell rate for the connection and is determined by the network.

The ATM Forum defines another service category for non-real-time applications called the unspecified bit rate (UBR) service category. The UBR service is entirely best effort; the call is provided with no QoS guarantees. The ITU-T also defines an additional service category for nonreal-time data applications.

The ATM block transfer service category is intended for the transmission of short bursts, or blocks, of data. Before transmitting a block, the source requests a reservation of bandwidth from the network. If the ABT service is being used with the immediate transmission option (ABT/IT), the block of data is sent at the same time as the reservation request.

If bandwidth is not available for transporting the block, then it is simply discarded, and the source must retransmit it. In the ABT service with delayed transmission (ABT/DT), the source waits for a confirmation from the network that enough bandwidth is available before transmitting the block of data. In both cases, the network temporarily reserves bandwidth according to the peak cell rate for each block. Immediately after transporting the block, the network releases the reserved bandwidth.