Congestion: Causes, Impact, And Countermeasures; Causes - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide
Those new applications have one thing in common, i.e. high requirements for
bandwidth, delay, and jitter. For instance, videoconference and VOD need the
assurance of wide bandwidth, low delay and jitter. As for mission-critical applications,
such as transaction and Telnet, they may not require wide bandwidth but do require
lower delay and be handled by priority during congestion.
The new emerging applications demand higher requirements for service performance
of IP network. Better network services during packets transmission are required other
than simply delivering the packets to their destination, such as providing user-specific
bandwidth, reducing packet loss ratio, avoiding congestion, regulating network traffic,
setting priority of the packets. To meet those requirements, the network should be
provided with better service capability.

1.4 Congestion: Causes, Impact, and Countermeasures

Network congestion is a key factor to degrade the service quality of the traditional
network. Congestion refers to such a fact that the service rates are decreased due to
relative deficiency of the resources supply (leading to extra delay).

1.4.1 Causes

Congestion will easily occur in complex packet switching circumstances in the Internet,
with two cases illustrated in the following Figure:
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Traffic congestion at interfaces
with different speed
Figure 1-1 Diagram for traffic congestion
1) The packet streams enter a router from a high speed link and are forwarded via a
low speed link;
2) Packet streams enter to a router from several interfaces with a same speed and
are forwarded through an interface with the same speed as well;
When traffic arrives at wire speed, congestion may occur for network resource
bottleneck.
Besides the bottleneck of link bandwidth, congestion will also be caused by resources
deficiency in normal packet forwarding, such as the deficiency of assignable processor
time, buffer and memory. In addition, congestion may occur if the arrival traffic is not
managed efficiently and the assignable network resources are inadequate.
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10M 100M
Traffic congestion at interfaces
with the same speed
3Com Corporation
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Chapter 1 QoS Overview
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