Qos Overview - 3Com 8807 Configuration Manual

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QoS Overview

Conventional packet network treats all packets equally. Each switch/router
processes all packets in First-in-First-out (FIFO) mode and then transfers them to
the destination in the best effort, but it provides no commitment and guarantee to
such transmission performance as delay and jitter.
With fast growth of computer networks, more and more data like voice and video
that are sensitive to bandwidth, delay and jitter are transmitted over the network.
This makes growing demands on quality of service (QoS) of networks.
Ethernet technology is a widely-used network technology dominant for
independent LANs and many LANs based on Ethernet are organic parts of the
Internet. In addition, Ethernet access is becoming one of the major access modes
for Internet users. Therefore it is inevitable to consider Ethernet QoS if we want to
achieve point-to-point global QoS solution. Ethernet switching devices then
naturally need to provide different QoS guarantee for different types of services,
especially for those which are sensitive to delay and jitter.
The following terms are involved in QoS.
Flow
It refers to all packets passing thought the switch.
Traffic classification
Traffic classification is the technology that identifies the packets with a specified
attribute according to a specific rule. Classification rule refers to a packet filtering
rule configured by an administrator. A classification rule can be very simple. For
example, the switch can identify the packets of different priority levels according
to the ToS (type of service) field in the packet headers. It can also be very complex.
For example, it may contain information of the link layer (layer 2), network layer
(layer 3) and transport layer (layer 4) and the switch classifies packets according to
such information as MAC address, IP protocol, source address, destination address
and port ID. Classification rule often is limited to the information encapsulated at
the packet header, rarely using packet contents.
Packet filtering
Packet filtering refers to filtering operation applied to traffic flow. For example, the
deny operation drops the traffic flow which matches the classification rule and
allows other traffic to pass. Ethernet switches use complex classification rules, so
that traffic flow can be filtered purposefully to enhance network security.
There are two key steps in packet filtering:
Step 1: Classify the traffic at the port according to a specific rule.