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TRENDnet User's Guide
Advanced Settings
Bandwidth Control
QoS
Each egress port can support up to 8 transmit queues. Each egress transmit queue
contains a list specifying the packet transmission order. Every incoming frame is
forwarded to one of the 8 egress transmit queues of the assigned egress port, based on
its priority. The egress port transmits packets from each of the 8 transmit queues
according to a configurable scheduling algorithm, which can be a combination of Strict
Priority (SP) and/or Weighted Round Robin (WRR).
Typically, networks operate on a best-effort delivery basis, which means that all traffic
has equal priority and an equal chance of being delivered in a timely manner. When
congestion occurs, all traffic has an equal chance of being dropped.
When you configure the QoS feature, you can select specific network traffic, prioritize it
according to its relative importance, and use congestion-management and congestion-
avoidance techniques to give preferential treatment. Implementing QoS in your network
makes network performance more predictable and bandwidth utilization more effective.
The Switch supports 802.1p priority queuing. The Switch has 8 priority queues. These
priority queues are numbered from 7 (Class 7) — the highest priority queue — to 0 (Class
0) — the lowest priority queue.
The eight priority tags specified in IEEE 802.1p (p0 to p7) are mapped to the Switch's
priority queues as follows:
Priority : 0 1 2 3 4 5 6 7
Queue : 2 0 1 3 4 5 6 7
Priority scheduling is implemented by the priority queues stated above. The Switch will
empty the four hardware priority queues in order, beginning with the highest priority
queue, 7, to the lowest priority queue, 0. Each hardware queue will transmit all of the
packets in its buffer before permitting the next lower priority to transmit its packets.
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When the lowest hardware priority queue has finished transmitting all of its packets, the
highest hardware priority queue will begin transmitting any packets it may have received.
QoS Enhancement
You can configure the Switch to prioritize traffic even if the incoming packets are not
marked with IEEE 802.1p priority tags or change the existing priority tags based on the
criteria you select. The Switch allows you to choose one of the following methods for
assigning priority to incoming packets on the Switch:
⚫
802.1p Tag Priority - Assign priority to packets based on the packet's 802.1p
tagged priority.
⚫
Port Based QoS - Assign priority to packets based on the incoming port on the
Switch.
⚫
DSCP Based QoS - Assign priority to packets based on their Differentiated Services
Code Points (DSCPs).
Note: Advanced QoS methods only affect the internal priority queue mapping for the
Switch. The Switch does not modify the IEEE 802.1p value for the egress frames. You can
choose one of these ways to alter the way incoming packets are prioritized or you can
choose not to use any QoS enhancement setting on the Switch.
802.1p Priority
When using 802.1p priority mechanism, the packet is examined for the presence of a valid
802.1p priority tag. If the tag is present, the packet is assigned to a programmable egress
queue based on the value of the tagged priority. The tagged priority can be designated to
any of the available queues.
Ethernet Packet:
6
6
2
DA
SA
Type / Length
6
6
4
DA
SA
802.1Q Tag
Managed Industrial L2 Switch
42-1496
4
Data
FCS
2
42-1496
Type / Length
Data
4
FCS
43
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