Quality of Service (QoS)
33
Horizon Compact Plus Release 1.0.1
Wireless Ethernet Product User Manual
– Volume 2
8.5.1
Cut Through
Any traffic assigned to a Cut Through queue is serviced at the highest priorty (regardless of the order of
that particular queue among other priority queues) by stopping the in-transition frames from other queues.
If a frame is being forwarded over the air from another queue and traffic arrives on the Cut-Through
queue, the transmission of the traffic from the other queue is stopped, and the frames from the Cut
through Queue are inserted for transmission. After the transmission of the Cut Through frames is
complete, the frames from the other queue which was stopped (i.e. being cut-through) is resumed. This
results in extremly low latency and jittter in the Cut Through frames.
In
Priority Queuing
mode any one of the priority queues (Q8 to Q1) can be selected as a cut-through
queue.
In
Weighted Fair Queuing
, only when Expedite Mode is on, any one of the configured expedite queues
can be selected as a Cut Through queue.
8.5.2
Strict Priority Queuing
If the Strict Priority Queuing policy is enabled in the Horizon Compact Plus system, the scheduling
mechanism can be described as follows:
1.
Select the highest priority queue (Q8 to Q1
) which has a frame in it, and hasn’t used up its CIR
budget
2.
Send that frame
The priority queues are serviced by the scheduler always in the order Q8, Q7, Q6, Q5, Q4, Q3, Q2, Q1
with Q8 always being polled first out of the eight queues.
The operation of the Scheduler is affected by both the user-configurable CIR and CBS settings as
described above. In the priority queuing mode the CIR of each of the eight queues may be assigned up to
100% of the system speed (system current speed).
Figure 8-1 CoS Queues can be allocated a CIR and a Committed Burst Size (CBS)
.
