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2.11 QoS function
The QoS function is a function to secure the quality of the communication by priority control and rewriting of
priority control.
In the priority control function of this device, there is a function where ACL is not used and the function where
ACL is used.
Given below is the explanation from basic priority control function where ACL is not used.
2.11.1 Priority control function
Priority control function is a function which does queuing for the packet and outputs according to the priority of
the mapped queue. The priority control function is configured with each function such as determination of user
priority to input packet, mapping to queue in this device for the user priority and priority control of queue.
The user priority to input packet is determined by the default priority for IEEE802.1p compliant CoS and the
reception packet without tag. Moreover, when the qos classification command is used, the user priority can be
determined by using upper 3 bits of TOS field of IPv4 (IP Precedence) and upper 3 bits of TC of IPv6.When 'qos
classification' is validated, user priority by the upper 3 bit of TOS and TC is preferred than the user priority as per
default priority for reception packet without CoS or Tag.
For example, in case of frame with VLAN tag which carries the below mentioned IP packet, user priority is
determined by Precedence (= upper 3 bits of DSCP) of TOS field when qos classification is validated, and user
priority is determined by CoS (Priority of Tag control information) when qos classification is invalidated.
DA
The packet having user priority is queued in the multiple queues of the output port (including the device
address port) that is mapped to that priority. Mapping of the user priority value and the queue in this device
can be changed when the number of queues is 4. The queue has the priority of 0～3 respectively and the
priority increases with the increase in number.
The queued packet is output according to priority control method of the queue. Priority control method is
selected from Strict Priority Queuing (Strict) or Weihted Round Robin (WRR) or Weighted Deficit Round Robin
(WDRR).
SA
VLAN
Tag
protocol
control information
Priority
CFI
3 bit
1 bit
TYPE
IP header
TOS field
VID
Precedence
12 bit
3 bit
IP data
CRC
D
T
R
Unused
1 bit 1 bit 1 bit
2 bit
DS field
DSCP
CU
6 bit
2 bit
Page 25 of 71
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