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.
The Traffic descriptor
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.
Type of Service
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.
{| class="wikitable"
|+ Table 1: ATM Forum Traffic Services
|-
! ATM Forum Traffic Management 4.0 ATM Service Category !! ITU-T I.371 ATM Transfer Capability !! Typical Use
|-
|| Constant Bit Rate (CBR)|| Deterministic Bit Rate (DBR)|| Real-time, QoS guarantees
|-
|| Real-Time Variable Bit Rate (rt-VBR)|| (for further study)|| Statistical mux, real time
|-
|| Non-Real-Time Variable Bit Rate (nrt-VBR)|| Statistical Bit Rate (SBR)|| Statistical mux
|-
|| Available Bit Rate (ABR)|| Available Bit Rate (ABR)|| Resource exploitations, feedback control
|-
|| Unspecified Bit Rate (UBR)|| (No equivalent)|| Best effort, no guarantees
|-
|| (No equivalent)|| ATM Block Transfer (ABT)|| Burst level feedback control
|}
Constant Bit Rate (CBR)
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.
Real-Time Variable Bit Rate (rt-VBR)
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. An example of traffic that requires this type of service category are variable rate, compressed video streams. 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). Real-time VBR connections can be characterized by a Peak Cell Rate (PCR), Sustained Cell Rate (SCR), and Maximum Burst Size (MBS). Cells delayed beyond the value specified by the maximum CTD (Cell Transfer Delay) are assumed to be of significantly reduced value to the application.