Traffic contract

If a network service (or application) wishes to use a broadband network (an ATM network in particular) to transport a particular kind of traffic, it must first inform the network about what kind of traffic is to be transported, and the performance requirements of that traffic. The application presents this information to the network in the form of a traffic contract. When a connection is requested by an application, the application indicates to the network: The Type of Service required. The Traffic Parameters of each data flow in both directions. The quality of service (QoS) Parameters requested in each direction. These parameters form the traffic descriptor for the connection. Currently, five ATM Forum-defined service categories exist (see Table 1). The basic differences among these service categories are described in the following sub-sections. These service categories provide a method to relate traffic characteristics and QoS requirements to network behaviour. The service categories are characterised as being real-time or non-real-time. CBR and rt-VBR are the real-time service categories. The remaining three service categories (nrt-VBR, UBR and ABR) are considered non-real-time service categories. 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. 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 amount of bandwidth is characterized by a Peak Cell Rate (PCR). These applications include services such as video conferencing, telephony (voice services) or any type of on-demand service, such as interactive voice and audio. For telephony and native voice applications CBR provides low-latency traffic with predictable delivery characteristics, and is therefore typically used for circuit emulation.