858 CHAPTER 65: QOS OVERVIEW
Telnet do not necessarily require high bandwidth but they are highly dependent on
low delay and need to be processed preferentially in case of congestion.
The emergence of new services brings forward higher requirements for the service
capability of the IP network. In the delivery process, users hope to get better
services, such as dedicated bandwidth for users, reduced packet loss rate,
management and avoidance of network congestion, control of network traffic,
provision of packet priority, and so on, instead of just having packets delivered to
the destination. To meet these requirements, the network service capability need
to be further improved.
Occurrence and
Influence of
Congestion and the
Countermeasures
QoS issues that traditional networks face are mainly caused by congestion.
Congestion means reduced service rate and extra delay introduced because of
relatively insufficient resource provisioned.
Occurrence of
Congestion
Congestion is very common in a complicated environment of packet switching on
Internet. The diagram below gives two examples:
Figure 255 Traffic congestion
1 Packets enter a switch over a high-speed link and are forwarded out over a
low-speed link.
2 Packets enter a switch through multiple interfaces of the same rate at the same
time and are forwarded out on an interface of the same rate.
If the outbound traffic exceeds the line rate, the traffic encounters the bottleneck
of resources and congestion occurs.
Besides bandwidth bottleneck, any insufficiency of resources for packet
forwarding, such as insufficiency of assignable processor time, buffer size, and
memory resources can cause congestion. In addition, congestion will also occur if
the traffic that arrives within a certain period of time is improperly controlled and
the traffic goes beyond the assignable network resources.
Influence of Congestion Congestion may cause a series of negative influences:
■ Congestion increases delay and delay jitter in packet delivery.
■ Excessively high delay will cause retransmission of packets.
■ Congestion decreases the effective throughput of the network and the
utilization of the network resources.
Traffic congestion on
interfaces of different rates
Traffic congestion on
interfaces of the same rates
100M
100M
100M
100M
100M
10M
To Next Page
Loading...
