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QOS OVERVIEW
Introduction Quality of Service (QoS) is a concept generally existing in occasions where service
supply-demand relations exist. QoS measures the ability to meet the service needs
of customers. Generally, the evaluation is not to give precise grading. The purpose
of the evaluation is to analyze the conditions where the services are good and the
conditions where the services still need to be improved, so that specific
improvements can be implemented.
In Internet, QoS measures the ability of the network to deliver packets. The
evaluation on QoS can be based on different aspects because the network
provides diversified services. Generally speaking, QoS is the evaluation on the
service ability to support the critical indexes such as delay, delay jitter and packet
loss rate in packet delivery.
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, FTP and E-mail.
New Requirements
Brought forth by New
Services
With the fast development of computer networks, more and more networks are
connected into Internet. Internet extends very quickly in scale, coverage and the
number of users. More and more users use the Internet as a platform for data
transmission and develop various applications on it.
Besides traditional applications such as WWW, FTP, and E-mail, Internet users also
try to develop new services on Internet, such as tele-education, tele-medicine,
video phones, video conferencing, and video on demand (VOD). Enterprise users
also hope to connect their branch offices in different locations through the VPN
technology to develop some transaction applications, such as to access to the
database of the company or to manage remote switches through Telnet.
The new services have one thing in common: they all have special requirements for
delivery performances such as bandwidth, delay, and delay jitter. For example,
video conferencing and VOD require the guarantee of high bandwidth, low delay
and low delay jitter. Some key services such as the transaction handling and the
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