C
HAPTER
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| Introduction
Package Contents
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IEEE 802.1
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Q
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The SMCGS501 and SMCGS801 switches support 802.1p priority queuing
Quality of Service, which is an implementation of the IEEE 802.1p standard.
With the 802.1p QoS function, you can reserve bandwidth for important
functions that require a lot of bandwidth or have a high priority, such as VoIP
(Voice-over Internet Protocol), web browsing applications, or video
conferencing. The Switches have separate hardware queues on on each physical
port to which packets from various applications are mapped and priorities
assigned.
The illustration below shows how 802.1p priority queuing is implemented on the
switches.
Figure 1: Mapping QoS on the Switches
There are four priority queues labeled TC0, TC1, TC2 and TC3. The untagged
packets and the eight IEEE 802.1p priority values (defined by the standard) are
mapped to the four priority queues on the switch. TC3 has the highest priority of
the four priority queues, while TC0 has the lowest priority. The untagged packets
and eight priority values, specified in IEEE 802.1p, are mapped to the switch’s
priority queues as follows:
◆
Untagged packets, and packets with priority 1 and 2 are assigned to the
switch’s TC0 queue.
◆
Packets with priority 0 and 3 are assigned to the switch’s TC1 queue.
◆
Packets with priority 4 and 5 are assigned to the switch’s TC2 queue.
◆
Packets with priority 6 and 7 are assigned to the switch’s TC3 queue.
The switches use Weighted Robin Round (WRR) for scheduling. The WRR queue-
scheduling algorithm schedules all the queues in turn with every queue assured
a certain service time. For WRR mode, the default weight values of TC0, TC1,
TC2 and TC3 are 1:2:4:8.
Tag
untag
0
1
2
3
4
5
6
7
Priority Queue
Weight Value
1
2
4
8
TC0
TC1
TC2
TC3
