Generic Cell Rate Algorithm - Dual Leaky Bucket Controller

Dual Leaky Bucket Controller

Multiple implementations of the GCRA can be applied concurrently to a VC or a VP, in a dual leaky bucket traffic policing or traffic shaping function, e.g. applied to a Variable Bit Rate (VBR) VC. This can limit ATM cells on this VBR VC to a Sustained Cell Rate (SCR) and a Maximum Burst Size (MBS). At the same time, the dual leaky bucket traffic policing function can limit the rate of cells in the bursts to a Peak Cell Rate (PCR) and a maximum Cell Delay Variation tolerance (CDVt): see Traffic Contract#Traffic Parameters.

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. In this case, the average cell rate over the interval of IMT is the SCR (=MBS/IMT). The individual messages can be transmitted at a PCR, which can be any value between the bandwidth for the physical link (1/δ) and the SCR. This allows the message to be transmitted in a period that is smaller than the message interval IMT, with gaps between instances of the message.

In the dual leaky bucket, one bucket is applied to the traffic with an emission interval of 1/SCR and a limit value τ_SCR that gives an MBS that is the number of cells in the message: see leaky bucket#Maximum Burst Size. The second bucket has an emission interval of 1/PCR and a limit value τ_PCR that allows for the CDV up to that point in the path of the connection: see leaky bucket#Delay Variation Tolerance. Cells are then allowed through at the PCR, with jitter of τ_PCR, up to a maximum number of MBS cells. The next burst of MBS cells will then be allowed through starting MBS x 1/SCR after the first.

If the cells arrive in a burst at a rate higher than 1/PCR (MBS cells arrive in less than (MBS - 1)/PCR - τ_PCR), or more than MBS cells arrive at the PCR, or bursts of MBS cells arrive closer than IMT apart, the dual leaky bucket will detect this and delay (shaping) or drop or de-prioritize (policing) enough cells to make the connection conform.

Figure 3 shows the reference algorithm for SCR and PCR control for both Cell Loss Priority (CLP) values 1 (low) and 0 (high) cell flows, i.e. where the cells with both priority values are treated the same. Similar reference algorithms where the high and low priority cells are treated differently are also given in Annex A to I.371 .