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SmartCell ZX-250 User Guide 4-17
Switch Administration Traffic Management
4.6 TRAFFIC MANAGEMENT
This section describes how the SmartCell ZX-250 manages bandwidth and congestion. It briefly describes console
commands that affect how the SmartCell ZX-250 manages traffic. This section also provides guidelines for setting
some traffic control parameters.
2p›F For information on troubleshooting traffic congestion problems, see Chapter 5,
“Troubleshooting.”
The SmartCell ZX-250 has extensive abilities for managing the flow of traffic. Traffic management includes all
operations performed by the SmartCell ZX-250 that ensure optimum switch throughput, where throughput is based on
rate of packet loss, available bandwidth, and traffic processing overhead. Under most conditions, the SmartCell
ZX-250 can efficiently and automatically manage switch traffic. However, if necessary, you can adjust the switch
traffic management parameters. For example, it might be necessary to adjust parameters for a port that carries a large
amount of CBR traffic or a very large number of simultaneous connections.
The SmartCell ZX-250 provides console commands that affect traffic flow on a global, port, or category of service
level. These console commands affect switch traffic flow by controlling
U Bandwidth allocation
U Call Admission Control (CAC) policies
U The service category for a connection
U Buffer memory allocation
U Threshold settings for anti-congestion routines
%"¤›apm Do not change traffic control settings unless you have expert-level experience
with ATM switching. Back up the switch configuration before making changes.
Also, make notes of the changes you make to the traffic control parameters.
4.6.1 Traffic Descriptors
Traffic characteristics of an ATM source are signaled through a set of traffic descriptors during connection
establishment. The SmartCell ZX-250 uses traffic descriptors for resource allocation during call set up and guarantees
the Quality of Service (QoS) across the connection. The source traffic descriptor is a set of parameters that describes
the expected bandwidth utilization of a connection. You can set these parameters,
U Peak Cell Rate (PCR)
U Sustainable Cell Rate (SCR) and Maximum Burst Size (MBS)
U Minimum Cell Rate (MCR) and Initial Cell Rate (ICR) — signaled through UNI4.0 signaling only
Traffic descriptors vary for each QoS. If a connection is bi-directional, a traffic descriptor has to be assigned to each
direction and need not be the same in both directions.
SmartCell ZX-250 user data cells are classified according to the state of a cell loss priority (CLP) bit in the header of
each cell. A CLP 1 cell has a lower priority than a CLP 0 cell and is discarded first. Source traffic descriptors can
specify CLP 0 cell traffic, CLP 1 cell traffic, or the aggregate CLP 0+1 traffic.