User's Manual

18-2
Drop Precedence: When processing the packets, firstly drop the packets with the bigger drop precedence,
the ranging is 0-1. It’s shortening is Drop-Prec or DP.
Classification: The entry action of QoS, classifying packet traffic according to the classification information
carried in the packet and ACLs.
Policing: Ingress action of QoS that lays down the policing policy and manages the classified packets.
Remark: Ingress action of QoS, perform allowing, degrading or discarding operations to packets according to
the policing policies.
Scheduling: QoS egress action. Add the packets to the corresponding egress queue according to the
internal priority. And then decide sending and dropping according to Drop Precedence, sending algorithm and
queue weight of egress queue.
18.1.2 QoS Implementation
To implement the switch software QoS, a general, mature reference model should be given. QoS can not
create new bandwidth, but can maximize the adjustment and configuration for the current bandwidth resource.
Fully implemented QoS can achieve complete management over the network traffic. The following is as
accurate as possible a description of QoS.
The data transfer specifications of IP cover only addresses and services of source and destination, and
ensure correct packet transmission using OSI layer 4 or above protocols such as TCP. However, rather than
provide a mechanism for providing and protecting packet transmission bandwidth, IP provide bandwidth
service by the best effort. This is acceptable for services like Mail and FTP, but for increasing multimedia
business data and e-business data transmission, this best effort method cannot satisfy the bandwidth and
low-lag requirement.
Based on differentiated service, QoS specifies a priority for each packet at the ingress. The classification
information is carried in Layer 3 IP packet header or Layer 2 802.1Q frame header. QoS provides same
service to packets of the same priority, while offers different operations for packets of different priority.
QoS-enabled switch or router can provide different bandwidth according to the packet classification
information, and can remark on the classification information according to the policing policies configured, and
may discard some low priority packets in case of bandwidth shortage.
If devices of each hop in a network support differentiated service, an end-to-end QoS solution can be created.
QoS configuration is flexible, the complexity or simplicity depends on the network topology and devices and
analysis to incoming/outgoing traffic.