PLANET XGS3-24042 - Manual (Page 179)

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Chapter 20 Egress QoS Configuration
20.1 Introduction to Egress QoS
In traditional IP networks, all packets are treated in the same way. All network equipments treat them by the
first-in-first-out policy and try best effort to send them to the destination. However, it does not guarantee the
performance like reliability and transmission delay. Network develops so fast that new demand has been
raised for the quality of service on IP network with the continual emergence of new applications. For example,
delay-sensitive services like VoIP and video put higher demands on packet transmission delay and users
cannot accept too long transmission delay (by contrast, E-mail and FTP services are not sensitive to the time
delay). In order to support services with different service requirement like voice, video and data service, the
network is required to be able to distinguish between different communications and provide appropriate
service. The traditional best-effort IP network cannot identify and distinguish various kinds of communications
while this ability is the very premise of providing differentiated services for different communications.
Therefore, the best-effort service mode of traditional network cannot meet the demand of applications. The
emergence of QoS techniques is committed to solve this problem.
Egress PolicyMap is the QoS policy in egress which performs QoS control of packets in the egress direction
and provides better service for specified network communication with kinds of techniques. Egress PolicyMap
includes class-map and policy-map, of which class-map is used for selecting packets to operate and
policy-map is used for specifying the operation to use. Not all equipments support Egress QoS currently.
20.1.1 Egress QOS Terms
Egress QoS: Achieving QoS on egress of port.
Inner_vid: VLAN ID brought by the TAG near the header of network layer when double TAGs exist.
Outer_vid: VLAN ID brought by the TAG near the header of network link layer when double TAGs exist. The
TAG is considered to be outer tag by default when only one TAG exists.
Outer_tpid: Protocol type of the network link layer header indicating the type of outer tag.