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Cells delayed beyond the value specified by the maximum CTD (Cell Transfer Delay) are assumed to be of significantly reduced value to the application. Real-time VBR connections can be characterized by a Peak Cell Rate (PCR), Sustained Cell Rate (SCR), and Maximum Burst Size (MBS). Sources that use rt-VBR connections are expected to transmit at a rate that varies with time (for example, traffic that can be considered bursty). An example of traffic that requires this type of service category are variable rate, compressed video streams. The rt-VBR service category is used for connections that transport traffic at variable rates - traffic that relies on accurate timing between the traffic source and destination.

For telephony and native voice applications CBR provides low-latency traffic with predictable delivery characteristics, and is therefore typically used for circuit emulation. These applications include services such as video conferencing, telephony (voice services) or any type of on-demand service, such as interactive voice and audio. The amount of bandwidth is characterized by a Peak Cell Rate (PCR). CBR is tailored for any type of data for which the end-systems require predictable response time and a static amount of bandwidth continuously available for the life-time of the connection. The CBR service category is used for connections that transport traffic at a constant bit rate, where there is an inherent reliance on time synchronisation between the traffic source and destination. Non-Real-Time Variable Bit Rate (nrt-VBR) Table 1: ATM Forum Traffic Services ATM Forum Traffic Management 4.0 ATM Service Category The remaining three service categories (nrt-VBR, UBR and ABR) are considered non-real-time service categories. CBR and rt-VBR are the real-time service categories. The service categories are characterised as being real-time or non-real-time. These service categories provide a method to relate traffic characteristics and QoS requirements to network behaviour.

The basic differences among these service categories are described in the following sub-sections. These parameters form the traffic descriptor for the connection.Ĭurrently, five ATM Forum-defined service categories exist (see Table 1).