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Configuring QoS
Introduction to QoS
Quality of Service (QoS) reflects the ability of a network to meet customer needs. In an internet, QoS
evaluates the ability of the network to forward packets of different services.
The evaluation can be based on different criteria because the network provides various services.
Generally, QoS performance is measured with respect to bandwidth, delay, jitter, and packet loss ratio
during packet forwarding process.
Networks without QoS guarantee
On traditional IP networks without QoS guarantee, devices treat all packets equally and handle them
using the first in first out (FIFO) policy. All packets share the resources of the network and devices. How
many resources the packets can obtain completely depends on the time they arrive. This service is called
"best-effort". It delivers packets to their destinations as possibly as it can, without any guarantee for delay,
jitter, packet loss ratio, and so on.
This service policy is only suitable for applications insensitive to bandwidth and delay, such as Word
Wide Web (WWW) and email.
QoS requirements of new applications
The Internet has been growing along with the fast development of networking technologies.
Besides traditional applications such as WWW, email and FTP, network users are experiencing new
services, such as tele-education, telemedicine, video telephone, videoconference and Video-on-Demand
(VoD). Enterprise users expect to connect their regional branches together with VPN technologies to
carry out operational applications, for instance, to access the database of the company or to monitor
remote devices through Telnet.
These new applications all have special requirements for bandwidth, delay, and jitter. For example,
videoconference and VoD require high bandwidth, low delay and jitter. As for mission-critical
applications, such as transactions and Telnet, they do not require high bandwidth but do require low
delay and preferential service during congestion.
The emerging applications demand higher service performance of IP networks. Better network services
during packets forwarding are required, such as providing dedicated bandwidth, reducing packet loss
ratio, managing and avoiding congestion, and regulating network traffic. To meet these requirements,
networks must provide more improved services.
Congestion: causes, impacts, and countermeasures
Network congestion is a major factor contributed to service quality degrading on a traditional network.
Congestion is a situation where the forwarding rate decreases due to insufficient resources, resulting in
extra delay.