Description: The scheduling discipline applied to the queue.

qosSPQ - Strict Priority Queueing

Queues are serviced in strict priority order. Packets on the queue are servicedd and transmitted until either the queue is empty or a packet is found in a higher queue, in the latter case the higher priority queue take precedence.

Individual PCP (CoS) bits or DSCP markings in each packet are not observed within the queue and are not therefore transmitted first according to their relative priority. Instead it is possible to map certain VLANs, PCP bits and DSCP marking to certain queues to achieve priority transmission.

The mapping of VLAN, PCP bits or DSCP markings to a specific queue are managed using tables :

sub10EthMgmtQoSVlanTable sub10EthMgmtQoSPCPTable sub10EthMgmtQoSDSCPTable

qosDWRR - Deficit Weighted Round Robin

A weighting is allocated to each queue which defines the number of bytes that can be transmitted from that queue in a single visit of the scheduler. This is used to calculate a quantum which is the number of bytes that can always be transmitted in each visit of the scheduler.

A deficit counter is also maintained across visits from the scheduler, at each visit the quantum is added to the deficit. This ensures that on average a queue that is not loaded to its intended capacity will utilise the calculated deficit at times when the queue is busy. At startup Deficit counter is initialised to zero.

On each visit of the scheduler packets are transmitted according to the following rules...

Deficit = Quantum + Deficit.

while (Queue not empty and PktLength <= Deficit) Transmit Pkt Deficit = Deficit - PktLength

Remaining Deficit is carried forward to the next visit of the scheduler.

Effective behaviour of DWRR is as follows:

1. Available bandwidth is shared fairly when queues are not saturated 2. Lower priority saturated queues will transmit at their allocated service rate. 3. Any remaining bandwidth is shared amongst unsaturated queues.

NB. In a mixed configuration where some queues are configured as Strict Priority (SP) and some configured as Deficit Weighted Round Robin (DWRR), then queues configured as SP should all be higher in scheduling priority than those configured as DWRR. This is to avoid the lower priority queues having transmission precedence over higher priority queues.