MNS-BB
Software User Guide
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allowing it to be used in the LAN, MAN, and WAN. DiffServ works by tagging each packet (at the
originating device or an intermediate switch) for the requested level of service it requires across the
network.
DiffServ inserts a 6-bit DiffServ code point (DSCP) in the TOS (type of service) field of the IP header, as
shown in the picture above. Information in the DSCP allows nodes to determine the per hop behavior
(PHB), which is an observable forwarding behavior for each packet. Per hop behaviors are defined in
according to:
•
Resources required (e.g., bandwidth, buffer size)
•
Priority (based on application or business requirements)
•
Traffic characteristics (e.g., delay, jitter, packet loss)
Nodes implement PHBs through buffer management and packet scheduling mechanisms. This hop-by-hop
allocation of resources is the basis by which DiffServ provides quality of service for different types of
communications traffic.
11.5
PQ: Priority Queuing
PQ
ensures that important traffic gets the fastest handling at each point where it is used.
It was designed to give strict priority to important traffic. Priority queuing can flexibly prioritize
according to network protocol (for example IP, IPX, or AppleTalk), incoming interface, packet size,
source/destination address, and so on. In PQ, each packet is placed in one of two queues—high or
low—based on an assigned priority. Packets that are not classified by this priority list mechanism fall
into the normal queue.
Note
: LE2425A and LEV2525A Switches support two priority queues, 1 (low) and 0 (high)
During transmission, the algorithm gives higher-priority queues absolute preferential treatment over
low-priority queues.
11.6 QoS
Management
The introduction discussed a common method (but by no means the only method) for QoS
management.
For baselining a network, you can use RMON probes and an application (such as Traffic Director) to
develop a good understanding of traffic characteristics. RMON probes provide more complete
information. In addition, targeted applications should be baselined (this is commonly measured by
response time). This information helps to validate any QoS deployment. From this data, QoS policy is
set and deployed.
Once deployed, it is important to evaluate the QoS policies and deployment and to decide whether
additional services are needed. In addition, RMON probes should still continue to monitor the
network because the traffic characteristics likely will change. A constant look at network traffic will
help with changing trends and allow a network administrator to address new network requirements
