3Com Switch 8800 Advanced Software V5 Configuration Guide

60
QOS OVERVIEW
When configuring QoS, go to these sections for information you are interested in:
■ “Introduction” on page 827
■ “Traditional Packets Forwarding Application” on page 827
■ “New Requirements Caused by New Applications” on page 827
■ “Congestion: Causes, Impact, and Countermeasures” on page 828
■ “Traffic Management Technologies” on page 829
Introduction Quality of Service (QoS) measures the service performance of service providers in
terms of client satisfaction. Instead of giving accurate marks, QoS emphasizes
analyzing what good or imperfect services are, and they come in what kind of
circumstances, so as to provide a cutting edge improvement.
In the Internet, QoS evaluates service performance for network packet forwarding.
Due to various services offered by the network, the evaluation for QoS will be
based on different aspects accordingly. Generally, QoS evaluates the service
performance for those network core requirements during packet forwarding
process, such as delay, jitter and packet loss ratio.
Traditional Packets
Forwarding
Application
On traditional IP networks, the devices treat all packets identically and handle
them with the first in, first out (FIFO) policy, assigning forwarding resources by
arrival sequence of packets. All the packets share the resources of the network
devices. How many resources the packets can obtain will completely depend on
the time they arrive.
This service policy is called Best-effort, which delivers the packets to their
destination as it can, without any assurance and guarantee for delivery delay, jitter,
packet loss ratio, reliability and so on for packet forwarding.
The traditional Best-Effort service policy is only suitable for applications insensitive
to bandwidth and delay, such as WWW, file transfer and e-mail.
New Requirements
Caused by New
Applications
With the fast development of the network, more and more networks access the
Internet. The Internet has been expanded in terms of its scale, coverage and users
quantities. More and more users use Internet as their data transmission platform
to implement various applications.
Apart from traditional applications of WWW, e-mail and FTP, network users try to
expand some new applications, such as tele-education, telemedicine, video
telephone, videoconference and Video-on-Demand (VoD), on the Internet. And