Understanding Cos - D-Link DES-1228/ME User Manual

The picture above shows the default priority setting for the Switch. Class-3 has the highest priority of the four priority classes of
service on the Switch. In order to implement CoS, the user is required to instruct the Switch to examine the header of a packet to
see if it has the proper identifying tag. Then the user may forward these tagged packets to designated classes of service on the
Switch where they will be emptied, based on priority.
For example, lets say a user wishes to have a video conference between two remotely set computers. On the receiving end, the
administrator instructs the Switch to examine packets for this tag, acquires the tagged packets and maps them to a class queue on
the Switch. Then in turn, the administrator will set a priority for this queue so that will be emptied before any other packet is
forwarded. This results in the end user receiving all packets sent as quickly as possible, thus prioritizing the queue and allowing
for an uninterrupted stream of packets, which optimizes the use of bandwidth available for the video conference.

Understanding CoS

The Switch has four priority classes of service. These priority classes of service are labeled as 3, the high class to 0, the lowest
class. The eight priority tags, specified in IEEE 802.1p are mapped to the Switch's priority classes of service as follows:
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Priority 0 is assigned to the Switch's Q1 class.
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Priority 1 is assigned to the Switch's Q0 class.
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Priority 2 is assigned to the Switch's Q0 class.
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Priority 3 is assigned to the Switch's Q1 class.
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Priority 4 is assigned to the Switch's Q2 class.
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Priority 5 is assigned to the Switch's Q2 class.
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Priority 6 is assigned to the Switch's Q3 class.
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Priority 7 is assigned to the Switch's Q3 class.
For strict priority-based scheduling, any packets residing in the higher priority classes of service are transmitted first. Multiple
strict priority classes of service are emptied based on their priority tags. Only when these classes are empty, are packets of lower
priority transmitted.
For weighted round-robin queuing, the number of packets sent from each priority queue depends upon the assigned weight. For a
configuration of 4 CoS queues, A~D, with their respective weight value: 4~1. When each queue has 20 outbound packets, they are
DES-1228/ME Layer 2 Fast Ethernet Managed Switch
Figure 8- 1. An Example of the Default CoS Mapping on the Switch
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