ATM Subsystem Traffic Management
High-speed ATM networks support a variety of applications with different traffic and
quality of service (QOS) requirements. For example, multimedia and time critical data
applications require guaranteed levels of delay and throughput, while other applications
can tolerate variations in delay and throughput (LAN traffic). This diversity requires
different congestion management methods.
The 3Com ONcore ATM subsystem supports the ATM Reserved Bandwidth (RB)
service type of traffic.
In the Reserved Bandwidth (RB) service, an application needs to establish a traffic
contract with the network before transmitting data. The traffic contract consists of a
specified QOS class and a set of traffic descriptors. Through resource allocation, the
network either provides the desired QOS for the ATM connection or refuses the call.
For this method, the source must be accurately modelled and able to precisely describe
its traffic pattern. The allocated bandwidth is usually less than the peak rate in order to
benefit from statistical multiplexing gains which may cause congestion. A source
policing scheme ensures that the source conforms itself to the contract by means of a
"leaky bucket" rate control.
The use of selective and global backpressure for traffic management in the ATM
subsystem gives the 3Com ONcore ATM network an added value. Selective
backpressure temporarily stops one virtual connection. Global backpressure
temporarily stops an ATM link.
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