MT90883 Datasheet, PDF (53/97 Pages) Zarlink Semiconductor Inc – TDM to Packet Processors

MT90880/1/2/3
Data Sheet
Options
Class of Service
Class 3
Class 2
Class 1
Class 0
Option 1 (default)
SP
SP
SP
WFQ
Option 2
SP
SP
WFQ
WFQ
Option 3
SP
WFQ
WFQ
WFQ
Option 4
WFQ
WFQ
WFQ
WFQ
Table 19 - Configuration of Packet Queues
Queues defined as strict priority take precedence over queues designated as using weighted fair queuing.
Hence, in option 3, Class 3 gets all the bandwidth it needs, and the other classes are allocated a proportion of
the remaining bandwidth. In effect, option 1 defaults to strict priority across all four queues, since Class 0 only
gets serviced when there are no packets in the remaining SP queues.
Strict Priority
The Strict Priority queue system ensures that the queue with the highest priority is empty before transmitting
any data from the next highest priority. For example in option 1, if the three queues for Classes 3, 2 and 1 each
contained packets to transmit, all the packets in the Class 3 queue would be transmitted before packets from
any other classes. The packets in the Class 2 queue will then be transmitted before any packets from Class 1. If
more packets arrive in Class 3 while Class 2 or Class 1 packets are being transmitted, the new packet in Class
3 is automatically sent once the current packet has completed transmission. Any packets still in the Class 2 or
Class 1 queues must wait until the Class 3 queue is empty again before they can be transmitted.
Weighted Fair Queuing
Weighted Fair Queuing allocates a portion of the available link bandwidth to each class. This prevents higher
priority queues from "hogging" the link, since once they have used their allocated bandwidth, the other queues
are given access to the link.
In the MT9088x family, each class assigned to WFQ by the option settings is given a weight between 1 and 64.
The bandwidth of each LAN port is 100 Mbs, and the weights assigned to each class represent a proportion of
the bandwidth assigned to the queue. The sum of the weights must equal 64 for correct functionality.
For example, if option 4 is selected the following weights might be programmed to set the appropriate
bandwidth as shown in Table 20:
Class
Weight
% Bandwidth allocated
Class 3
32
Class 2
24
Class 1
06
Class 0
02
TOTAL
64
50.0 Mbs
37.5 Mbs
9.4 Mbs
3.1 Mbs
100 Mbs
Table 20 - Example of Weighted Fair Queuing
Weighted Fair Queuing doesn't set a maximum figure on the amount of bandwidth a queue gets allocated. If
there are no other queues requiring service, a queue will continue to be serviced even if its bandwidth allocation
is exceeded. The bandwidth allocation is only used to guarantee a minimum allocation to a particular traffic
class, should it be required.
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