Netis ST3326(ST-3302) [85/118] Chapter9 qos

Chapter9: QoS

Introduction to QoS

Quality of Service (QoS) is a concept concerning service demand and supply. It reflects the ability

to meet customer needs. Generally, QoS does not focus on grading services precisely, but on

improving services under certain conditions.

In an internet, QoS refers to the ability of the network to forward packets. The evaluation on QoS

of a network can be based on different aspects because the network may provide various

services.

Generally, QoS refers to the ability to provide improved service by addressing the essential issues

such as delay, jitter, and packet loss ratio in the packet forwarding process.

Traditional Packet Forwarding Service

In traditional IP networks, packets are treated equally. That is, the FIFO (first in first out) policy is

adopted for packet processing. Network resources required for packet forwarding is determined

by the order in which packets arrive. All the packets share the resources of the network. Network

resources available to the packets completely depend on the time they arrive. This service policy

is known as Best-effort, which delivers the packets to their destination with the best effort, with

no assurance and guarantee for delivery delay, jitter, packet loss ratio, reliability, and so on.

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 Applications and New Requirements

With the expansion of computer network, more and more networks become part of the Internet.

The Internet gains rapid development in terms of scale, coverage and user quantities. More and

more users use the Internet as a platform for their services and for data transmission.

Besides the traditional applications such as WWW, E-mail, and FTP, new services are developed

on the Internet, such as tele-education, telemedicine, video telephone, videoconference and

Video-on-Demand (VoD). Enterprise users expect to connect their regional branches together

using VPN techniques for coping with daily business, for instance, accessing databases or manage

remote equipments through Telnet.

All these new applications have one thing in common, that is, they have special requirements for

bandwidth, delay, and jitter. For instance, bandwidth, delay, and jitter are critical for

videoconference and VoD. As for other applications, such as transaction processing and Telnet,

although bandwidth is not as critical, a too long delay may cause unexpected results. That is,

they need to get serviced in time even if congestion occurs.