6
1.3 IEEE 802.1p QoS
The
LP-SG2401
Switch supports 802.1p priority queuing Quality of Service
(QoS), which is an implementation of the IEEE 802.1p standard. With
802.1p QoS function, you can reserve bandwidth for important functions
that require a large bandwidth or have a high priority, such as VoIP (Voice-
over Internet Protocol), web browsing applications or video conferencing.
The switch has separate hardware queues on every physical port which
packets from various applications are mapped to and assigned a priority to.
The illustration below shows how 802.1p priority queuing is implemented
on the switch.
The switch has four priority levels labeled TC0, TC1, TC2 and TC3. The
untagged packets and the eight IEEE 802.1p priority tags defined by the
standard are mapped to the four level queues used on the switch. TC3
has the highest priority of the four priority level queues while TC0 has the
lowest priority on the switch. The untagged packets and eight priority tags,
specified in IEEE 802.1p are mapped to the switch’s priority tags as follows:
The switch uses WRR (Weighted Robin Round) for scheduling. WRR
queue-scheduling algorithm schedules all the queues in turn and every
queue can be assured of a certain service time. The default weight value of
TC0, TC1, TC2 and TC3 is 1:2:4:8.
Tag untag 0
1
2
3
4
5
6
7
Priority Level
TC0
TC1
TC2
TC3
Weight Value
1
2
4
8
Figure 1. Mapping QoS on the Switch
►
The untagged packets, packets with priority tag 1 and 2 are
assigned to the switch’s TC0 level queue.
►
Packets with priority tag 0 and 3 are assigned to the switch’s
TC1 level queue.
►
Packets with priority tag 4 and 5 are assigned to the switch’s
TC2 level queue.
►
Packets with priority tag 6 and 7 are assigned to the switch’s
TC3 level queue.
