ATM Traffic contract/Service contract

Via: Wikipedia

Another key ATM concept is that of the traffic contract. When an ATM circuit is set up each switch is informed of the traffic class of the connection.

ATM traffic contracts are part of the mechanism by which "Quality of Service" (QoS) is ensured. There are four basic types (and several variants) which each have a set of parameters describing the connection.

• CBR - Constant bit rate: you specify a Peak Cell Rate (PCR), which is constant.
• VBR - Variable bit rate: you specify an average cell rate, which can peak at a certain level for a maximum interval before being problematic.
• ABR - Available bit rate: you specify a minimum guaranteed rate.
• UBR - Unspecified bit rate: your traffic is allocated all remaining transmission capacity.

VBR has real-time and non-real-time variants, and is used for "bursty" traffic.

Most traffic classes also introduce the concept of Cell Delay Variation Tolerance (CDVT) which defines the "clumping" of cells in time.

Traffic contracts are usually maintained by the use of "Shaping", a combination of queuing and marking of cells, and enforced by "Policing".

Traffic shaping

Traffic shaping is usually done at the entry point to an ATM network and attempts to ensure that the cell flow will meet its traffic contract.

Traffic policing

To maintain network performance it is possible to police virtual circuits against their traffic contracts. If a circuit is exceeding its traffic contract, the network can either drop the cells or mark the Cell Loss Priority (CLP) bit (to identify a cell as discardable further down the line). Basic policing works on a cell by cell basis, but this is sub-optimal for encapsulated packet traffic (as discarding a single cell will invalidate the whole packet). As a result, schemes such as Partial Packet Discard (PPD) and Early Packet Discard (EPD) have been created that will discard a whole series of cells until the next frame starts. This reduces the number of redundant cells in the network, saving bandwidth for full frames. EPD and PPD work with AAL5 connections as they use the frame end bit to detect the end of packets.