3Com Switch 8800 Advanced Software V5 Configuration Guide

828 CHAPTER 60: QOS OVERVIEW
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.
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).
Causes Congestion will easily occur in complex packet switching circumstances in the
Internet, with two cases illustrated in the following figure:
Figure 246 Traffic congestion causes
1 The packet streams enter a device from a high speed link and are forwarded via a
low speed link;
2 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.
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.
Impact Congestion may cause the following negative effects: