Egress Shaping/Scheduling

Each SAP of an ATM VLL has an associated egress ATM traffic descriptor. The default traffic descriptor has service category UBR with zero MIR, resulting in endpoints associated with this descriptor being scheduled at the lowest priority on the ATM MDA. Egress traffic may be shaped or scheduled, depending on the configuration of the egress ATM traffic descriptor associated with the SAP. Table 1 describes how the different service categories, and shaping settings, and priorities affect egress transmission rates.

Shaping applies to CBR, rtVBR, and nrtVBR service categories and results in cells being transmitted in such a way as to satisfy a downstream ATM UPC function. For example, the transmission rate is limited (in the case of CBR, there is a hard limit of PIR, while rtVBR/nrtVBR transmits at SIR with short (constrained by MBS) bursts of up to PIR), and the inter-cell gap is also controlled.

Service categories UBR and rtVBR are scheduled on the WRR scheduler with the configured rates (MIR for UBR+) determining the weight applied to the flow. Weights are between 1 and 255 and are determined by a formula applied to the configured rate. UBR flows (for example, those with no MIR) receive a weight of 1 and the maximum weight of 255 is reached by flows with configured rates of around 8 Mb/s. Scheduling does not apply a limit to the transmission rate; the available port bandwidth is shared out by the scheduler according to the weight, so if the other flows are quiescent, one flow may burst up to port bandwidth.

Shaping and scheduling of egress ATM VLL traffic is performed entirely at the ATM layer and is, therefore, not forwarding-class-aware. If the offered rate is greater than can be transmitted toward the customer (either because the shaping rate limits transmission or because the SAP does not receive sufficient servicing in the weighed round-robin used for scheduled SAPs), the per-VC queue begins to discard traffic. These discards trigger the congestion control mechanisms in the MDA queues or in the IOM service egress queues associated with the SAP. For AAL5 SDU VLLs, these discards occur at the AAL5 SDU level. For N-to-1 VLLs, these discards occur at the level of the cell or a block of cells when cell concatenation is enabled.