Traffic management attempts to reduce the frequency, severity and duration of congestion. Moreover, it also aims at identifying irregular and misbehaving traffic and ensuring, as far as possible, that other users do not suffer as a result. Additionally traffic management attempts to optimise the use of network resources.

Traffic management in ATM supports multiple qualities of service (QoS). The QoS parameters are based on the speed, accuracy and dependability required by the cells being delivered. Thus traffic management supports multiple performance objectives. This is necessary as different types of traffic have different requirements. For example, voice may not tolerate jitter but may tolerate a certain amount of loss whereas file transfer can tolerate jitter and delay but may not tolerate loss.

The QoS is negotiated by the network before a connection is established. Once configured, the network will guarantee to give the agreed quality of service to the end users. The different qualities of service offered by ATM are listed below.

• Constant Bit Rate (CBR) : This transfer capability is characterised by low delay variation and low cell loss. Thus it provides a continuous isochronous stream mainly used for permanent VPs/VCs and real time applications needing low delay variation. It should give performance equivalent to comparable private lines. It is an expensive option as bandwidth is guaranteed between two points even if the line is idle. When there is no data to send, the link will send idle or blank cells at the same rate, effectively still using bandwidth when it is not necessary.
• Variable Bit Rate non-real-time (VBR-nrt) : This transfer capability does not give any guarantee regarding delay variation along the connection. However, if the source shapes its bursts of traffic in a static pattern, low cell loss may be accomplished. This transfer capability is considered suitable for transferring data that is delay tolerant.
• Variable Bit Rate real-time (VBR-rt) : This transfer capability offers lower delay variation than VBR-nrt because these streams of cells are treated with a higher priority. However, the source must shape the traffic in a static pattern.
• Available Bit Rate (ABR) : This transfer capability does not give any guarantee regarding delay variation. Very low cell loss may be expected. However, the source needs to shape its traffic dynamically on the basis of feedback messages from the network. This transfer capability allows a high network utilisation.
• Unspecified Bit Rate (UBR) : This transfer capability provides no guarantees regarding delay, delay variation or cell loss. There are no restrictions on the source behaviour other than its peak sending rate.

When ATM connections are established, the subscribers agree to a traffic contract between them and the service providers. This traffic contract also includes specific traffic parameters. End user devices are obliged to meet the contracted traffic conditions to avoid the cells being discarded by the network, thereby maximising end-to-end performance available to the user.

Traffic shaping occurs at the ingress to the ATM network. Routers or LAN switches contain shaper functions. These accept any input data patterns and process these source streams such that the output cell streams conform to the traffic contract. A traffic shaper also ensures that the cell sequence integrity is maintained.

If the source traffic strays outside the traffic parameters agreed in the traffic contract, the unexpected excess cells from the corresponding source can unfairly utilise the bandwidth of other connections. This can reduce the QoS of these other connections to unacceptably low levels. A policing device is used at the edge of the public network to detect the excess cells that do not conform to the traffic parameters. Once, detected, they are either discarded or their CLP bit is set so that they are carried at a lower priority

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© Omar Bashir, December 1998

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