ATM Access Egress Queuing and Scheduling

After the ATM pseudowire is terminated at the access egress, all the ATM cells are mapped to the default queue, which is queue 1, and queuing is performed per SAP. ATM access egress queuing and scheduling applies to the 16-port T1/E1 ASAP Adapter card, 32-port T1/E1 ASAP Adapter card, and 2-port OC3/STM1 Channelized Adapter card with atm/ima encapsulation. ATM access egress queuing and scheduling applies to the 4-port OC3/STM1 Clear Channel Adapter card and 4-port DS3/E3 Adapter card with atm encapsulation.

Once the per-SAP queuing takes place, the ATM Scheduler services these queues in the fashion and order defined below, based on the service categories assigned to each of these SAPs.

At access egress, CBR and rt-VBR VCs are always shaped, since there is no option to the user to turn shaping off. Shaping for nrt-VBR is optional.

Strict priority scheduling in an exhaustive fashion takes place for the shaped VCs in the order listed below:

1. CBR (always shaped)

2. rt-VBR (always shaped)

3. nrt-VBR (when shaped, user-configurable for shaped or unshaped)

UBR traffic is not shaped. To offer maximum flexibility to the user, nrt-VBR unshaped (also known as scheduled) is implemented.

ATM traffic is serviced in priority order. CBR traffic has the highest priority and is serviced ahead of all other traffic. After all of the CBR traffic has been serviced, rt-VBR traffic is serviced. Then, nrt-VBR traffic is serviced.

After scheduling all the other traffic from the CBR and VBR service categories, UBR is serviced. If there is no other traffic, UBR can burst up to the line rate. Scheduled nrt-VBR is treated the same way as UBR. Both UBR and unshaped nrt-VBR are scheduled using the weighted round-robin scheduler.

The scheduler weight assigned to queues hosting scheduled nrt-VBR and UBR traffic is determined by the configured traffic rate. The weight used by the scheduler for UBR+ VCs is dependent on the Minimum Information Rate (MIR) defined by the user. UBR with no MIR traffic has an MIR of 0.

Similarly, the scheduler weight is dependent on the Sustained Information Rate (SIR) for scheduled nrt-VBR. Weight used by the scheduler is programmed automatically based on the user-configured MIR/SIR value and is not user-configurable.

For UBR+, Table: Scheduler Weight Values (WRR) based on MIR for T1/E1 ASAP Adapter Cards and 2-port OC3/STM1 Channelized Adapter Card and Table: Scheduler Weight Values (WRR) based on MIR for the 4-port OC3/STM1 Clear Channel Adapter Card are used to determine the weight of a UBR+ VC. These tables are also applicable to scheduled nrt-VBR weight determination. Instead of the MIR, the SIR is used to determine the scheduler weight.

The access egress ATM scheduling behavior is shown in Table: ATM Scheduling and Relative Priorities . For UBR traffic, the scheduler weight of the lowest possible value is always used, which is the value of 1. Only cell-based operations are carried out.