Traffic Policing Application Note 
Size: 616 KB
Date: April 2000

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ATM networks have been touted as "one of the few networks that can provide the real-time and quality of service guarantees required for new multimedia data types". However, as the networks become larger, busier, and more complex, fulfilling this quality of service guarantee will not be a simple task. What are the end effects or the service layer effects if the quality of service (QoS) is less than expected? If cells are lost in a data communications application, retransmission may be required. If cells are delayed, timers may expire causing even more delays. In a video-on-demand service, if the cell delay variation in the ATM network increases due to congestion, the buffers in a set-top box may begin to underflow causing blanking or jagged movements.

Fortunately there has been a great deal of forethought by members of the international standards bodies and industry forums to mitigate potential problems and to ensure that users of ATM networks get the QoS they expect.

ATM layer traffic control is designed to avoid network congestion: preventative traffic management. Network congestion is a state when the network cannot meet the negotiated network performance objectives for established connections or for new connection requests. The goal of traffic control is to protect the network, and other users of the network, so that everyone receives the performance that they pay for and depend on.

There are six traffic control functions that are important to maintaining network performance:

• network resource management
• connection administration control
• selective cell discarding
• traffic shaping
• explicit forward congestion indication
• usage parameter control

This application note focuses on one of the key ATM traffic control functions, Usage Parameter Control (UPC), commonly referred to as traffic policing.

To place UPC in the correct context, this note discusses traffic contracts which determine, for each connection, what requirements the UPC will uphold. The Generic Cell Rate Algorithm is discussed to understand what it means when a traffic contract series, for example, a peak cell rate. A significant portion of this note considers the importance and the methods of testing UPC mechanisms. This note finishes with a brief comment on some of the current standards activities in policing Available Bit Rate (ABR) services.