Congestion: Causes, Impact, And Countermeasures; Causes - 3Com MSR 50 Series Configuration Manual

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HAPTER O
Congestion: Causes,
Impact, and
Countermeasures

Causes

VERVIEW
Apart from traditional applications of WWW, e-mail and FTP, network users try to
expand some new applications, such as tele-education, telemedicine, video
telephone, videoconference and Video-on-Demand (VoD), on the Internet. And
the enterprise users expect to connect their regional branches together to develop
some operational applications through VPN technology, for instance, to access the
database of the company or monitor their remote equipment via Telnet.
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 service performance of IP network.
Better network services during packets forwarding 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.
Network congestion is a key factor to degrade the service quality of the traditional
network. Congestion refers to such a fact that the forwarding rates are decreased
due to relative deficiency of the resources supply (leading to extra delay).
Congestion will easily occur in complex packet switching circumstances in the
Internet, with two cases illustrated in the following figure:
Figure 456 Traffic congestion on interfaces with different speeds
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Figure 457 Traffic congestion on interfaces with the same speed
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The packet streams enter a device from a high speed link and are forwarded via
■
a low speed link;
The packet streams enter a device from several interfaces with a same speed
■
and are forwarded through an interface with the same speed as well.
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