ATM Option System Reference - Comtech EF Data

ATM Option
Configuring the Cell Rate Parameters for Variable Bit Rate
Service Categories
To define the cell rate on the PVC connection, Variable Bit Rate service categories (RT-
VBR and NRT-VBR) require the Peak Cell Rate (PCR), Sustainable Cell Rate (SCR) and
Maximum Burst Size (MBS). The other service categories require a PCR definition only.
The NetPerformer performs no validation of bandwidth overload, e.g. going over the
permitted PCR. Correct configuration is therefore very important.
In general, to ensure an adequate cell rate:
• Set the Sustainable cell rate (SCR) parameter to the typical bandwidth requirement
of the VC on the PVC
• Set the Maximum burst size (MBS) in cells to the same value as the Peak cell rate
parameter. This assumes that a traffic burst will last one second.
Example 1:
• The ATM switch is VBR with a PCR of 2000 and SCR of 1000
• The NetPerformer PVC is UBR with a PCR of 2000
• In high traffic situations the NetPerformer must transmit at 2000 cells per second
• However, since the ATM switch allows an SCR of only 1000, it will demand
more cells.
• The NetPerformer will lose data, and the line could go down.
Example 2:
• The ATM switch is VBR with a PCR of 2000 and SCR of 1000
• The NetPerformer is UBR with a PCR of 1000
• Therefore, the rate of the NetPerformer will never surpass the SCR of the switch,
and no data will be lost.
NOTE: A PVC that is configured as either CBR or RT-VBR has higher priority than a
PVC in NRT-VBR or UBR. This is not configurable.
2-20 Memotec Inc.