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.

Causes

Congestion easily occurs in complex packet switching circumstances in the Internet. Figure 453 shows two common cases:

Figure 453 Traffic congestion causes

•The traffic enters a device from a high speed link and is forwarded over a low speed link.

•The packet flows enter a device from several incoming interfaces and are forwarded out of an outgoing interface, whose rate is smaller than the total rate of these incoming interfaces.

When traffic arrives at the line speed, a bottleneck is created at the outgoing interface causing congestion.

Besides bandwidth bottlenecks, congestion can be caused by resource shortage in various forms such as insufficient processor time, buffer, and memory, and by network resource exhaustion resulting from excessive arriving traffic in certain periods.

Impacts

Congestion might bring these negative results:

•Increased delay and jitter during packet transmission

•Decreased network throughput and resource use efficiency

•Network resource (memory in particular) exhaustion and even system breakdown

It is obvious that congestion hinders resource assignment for traffic and degrades service performance. Congestion is unavoidable in switched networks and multi-user application environments. To improve the service performance of your network, you must address the congestion issues.

Countermeasures

A simple solution for congestion is to increase network bandwidth, however, it cannot solve all the problems that cause congestion because you cannot increase network bandwidth infinitely.

A more effective solution is to provide differentiated services for different applications through traffic control and resource allocation. In this way, resources can be used more properly. During resources allocation and traffic control, the direct or indirect factors that might cause network congestion should be controlled to reduce the probability of congestion. Once congestion occurs, resource allocation should be performed according to the characteristics and demands of applications to minimize the effects of congestion.

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