Occurrence And Influence Of Congestion And The Countermeasures; Occurrence Of Congestion - H3C S7500E Series Operation Manual

Operation Manual – QoS
H3C S7500E Series Ethernet Switches
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 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.
1.4 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.

1.4.1 Occurrence of Congestion

Congestion is very common in a complicated environment of packet switching on
Internet. The diagram below gives two examples:
Figure 1-1 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
1-2
Chapter 1 QoS Overview