Generic cell rate algorithm

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.

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.

However, while there are possible advantages in understanding this leaky bucket description, it does not necessarily result in the best (fastest) code if implemented directly.

This is evidenced by the relative number of actions to be performed in the flow diagrams for the two descriptions (figure 1).

The virtual scheduling algorithm, while not so obviously related to such an easily accessible analogy as the leaky bucket, gives a clearer understanding of what the GCRA does and how it may be best implemented.

This version of the algorithm works because τ defines how much earlier a cell can arrive than it would if there were no jitter: see leaky bucket: delay variation tolerance.

This replacement of the process with an RTC is possible because ATM cells have a fixed length (53 bytes), thus T is always a constant, and the calculation of the new bucket level (or of TAT) does not involve any multiplication or division.

As a result, the calculation can be done quickly in software, and while more actions are taken when a cell arrives than are taken by the token bucket, in terms of the load on a processor performing the task, the lack of a separate update process more than compensates for this.

Hence, applying the GCRA to limit the bandwidth of variable length packets without access to a fast, hardware multiplier (as in an FPGA) may not be practical.

This may be best understood where the transmission on an VBR VC is in the form of fixed length messages (CPCS-PDUs), which are transmitted with some fixed interval or the Inter Message Time (IMT) and take a number of cells, MBS, to carry them; however, the description of VBR traffic and the use of the dual leaky bucket are not restricted to such situations.

Similar reference algorithms where the high and low priority cells are treated differently are also given in Annex A to I.371 .

Figure 1: Equivalent versions of the generic cell rate algorithm
Figure 2: Example cell timings on a VBR connection
Figure 3: Reference algorithm for Sustainable Cell Rate (SCR) and Peak Cell Rate (PCR) for CLP = 0 + 1 cell flow