Generic cell rate algorithm

The generic cell rate algorithm (GCRA) is a leaky bucket-type scheduling algorithm for the network scheduler that is used in Asynchronous Transfer Mode (ATM) networks. It is used to measure the timing of cells on virtual channels (VCs) and or Virtual Paths (VPs) against bandwidth and jitter limits contained in a traffic contract for the VC or VP to which the cells belong. Cells that do not conform to the limits given by the traffic contract may then be re-timed (delayed) in traffic shaping, or may be dropped (discarded) or reduced in priority (demoted) in traffic policing. Nonconforming cells that are reduced in priority may then be dropped, in preference to higher priority cells, by downstream components in the network that are experiencing congestion. Alternatively they may reach their destination (VC or VP termination) if there is enough capacity for them, despite them being excess cells as far as the contract is concerned: see priority control. The generic cell rate algorithm (GCRA) is a leaky bucket-type scheduling algorithm for the network scheduler that is used in Asynchronous Transfer Mode (ATM) networks. It is used to measure the timing of cells on virtual channels (VCs) and or Virtual Paths (VPs) against bandwidth and jitter limits contained in a traffic contract for the VC or VP to which the cells belong. Cells that do not conform to the limits given by the traffic contract may then be re-timed (delayed) in traffic shaping, or may be dropped (discarded) or reduced in priority (demoted) in traffic policing. Nonconforming cells that are reduced in priority may then be dropped, in preference to higher priority cells, by downstream components in the network that are experiencing congestion. Alternatively they may reach their destination (VC or VP termination) if there is enough capacity for them, despite them being excess cells as far as the contract is concerned: see priority control. The GCRA is given as the reference for checking the traffic on connections in the network, i.e. usage/network parameter control (UPC/NPC) at user–network interfaces (UNI) or inter-network interfaces or network-network interfaces (INI/NNI) . It is also given as the reference for the timing of cells transmitted (ATM PDU Data_Requests) onto an ATM network by a network interface card (NIC) in a host, i.e. on the user side of the UNI . This ensures that cells are not then discarded by UPC/NCP in the network, i.e. on the network side of the UNI. However, as the GCRA is only given as a reference, the network providers and users may use any other algorithm that gives the same result. The GCRA is described by the ATM Forum in its User-Network Interface (UNI) and by the ITU-T in recommendation I.371 Traffic control and congestion control in B-ISDN . Both sources describe the GCRA in two equivalent ways: as a virtual scheduling algorithm and as a continuous state leaky bucket algorithm (figure 1). The description in terms of the leaky bucket algorithm may be the easier of the two to understand from a conceptual perspective, as it is based on a simple analogy of a bucket with a leak: see figure 1 on the leaky bucket page. However, there has been confusion in the literature over the application of the leaky bucket analogy to produce an algorithm, which has crossed over to the GCRA. The GCRA should be considered as a version of the leaky bucket as a meter rather than the leaky bucket as a queue.