45 QOS Configuration
45.1 QOS Overview
The fast development of the Internet results in more and more demands for multimedia
streams. Generally, people have different service quality requirements for different
multimedia, which requires the network to be able to allocate and dispatch resources
according to the user demands. As a result, the traditional "best effort" forwarding
mechanism cannot meet the user demands. So the QOS emerges.
The QOS (Quality of Service) is used to evaluate the ability for the service provider to meet
the customer demands. In the Internet, the QOS mechanism is introduced to improve the
network service quality, where the QOS is used to evaluate the ability of the network to
deliver packets. The commonly-mentioned QOS is an evaluation on the service ability for the
delay, jitter, packet loss and more core demands.
45.1.1 Basic Framework of QoS
The devices that have no QoS function cannot provide the capability of transmission quality
service, and will not ensure special forwarding priority for certain dataflow. When abundant
bandwidth, all the traffic can be well processed. But when congestion occurs, all traffic also
has an equal chance of being dropped.This kind of forwarding policy is otherwise called the
service of best effect, since the device now is exerting its performance of data forwarding
and the use of its switching bandwidth is maximized.
The device of this module features the QoS function to provide transmission quality service.
This makes it possible to select specific network traffic, prioritize it according to its relative
importance, and use congestion-management and congestion-avoidance techniques to
provide preferential treatment. The network environment with QoS configured is added with
predictability of network performance and allocates network bandwidth more effectively to
maximize the use of network resources.
The QoS of this device is based on the DiffServ (Differentiated Serve Mode) of the IETF
Internet Engineering Task Force. According to the definitions in the DiffServ architecture,
every transmission packet is classified into a category in the network, and the classification
information is included in the IP packet header. The first 6 bits in the TOS (Type Of Service)
field for IPv4 packet header or the Traffic Class field for Ipv6 packet header carry the
classification information of the packet. The classification information can also be carried in
the Link layer packet header. Below shows the special bits in the packet:
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