H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 425)

43-2
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 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 43-1 Traffic congestion
Traffic congestion on
interfaces of different rates
Traffic congestion on
interfaces of the same rates
100M
100M
100M
100M
100M 10M
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:
z
Congestion increases delay and delay jitter in packet delivery.