ATM Quality of Service

Traffic flows through an ATM network are characterised by a number of traffic descriptors. The peak cell rate (PCR) is the maximum rate at which a source may submit cells to the network. The PCR is the reciprocal of the minimum cell spacing T; thus PCR = 1/T. Cells that do not exceed the PCR are deemed to be conforming. While a source may submit conforming cells to the network, the effects of multiplexing could cause jitter in the cell spacing, resulting in nonconformance of cells as they propagate through the network. The network defines a cell delay variation tolerance (CDVT) that allows small bursts over the PCR.

Over a period of time greater than T, the sustained cell rate (SCR) defines the limit at which conforming cells may enter the network. A flow may be allowed to burst over the SCR (up-to the PCR). The burst “period” is defined by the maximum burst size parameter (MBS). The ATM Forum defines the following service categories:

• CBR: Constant bit rate

• VBR: Variable bit rate

• ABR: Available bit rate

• UBR: Unspecified bit rate

The CBR service category is for time-sensitive applications that require a fixed capacity, as specified by the PCR. Conforming traffic may burst over the PCR up to the CDVT. This service category is typically for circuit emulation, voice traffic or constant bit rate video.

For VBR traffic, conforming traffic may burst over the SCR up to the MBS. Like the CBR service category, it may burst over the PCR up to the CDVT.

The ABR service category is for traffic sources that can adapt their transmission rates to the conditions of the network through flow-control methods. As well as a PCR, the ABR service category also has a Minimum Cell Rate (MCR).

The UBR service category is defined by the PCR but there is no guaranteed throughput. Applications using this service category must have a high tolerance to delay. QoS is best-effort. In this chapter we focus on CBR and VBR service categories.