If a service (or an
application) wants to use an ATM network to transport a particular kind of
traffic, it must first inform the network about what kind of traffic is to be
transported, and the performance requirements of that traffic. The ATM service
categories provide a method to relate traffic characteristics and QoS
requirements to network behavior. Hierarchical shaping requires that all VCs
belonging to a given VP have the same ATM service category. This hierarchy
ensures that packets are shaped first at the VC level and then at the VP level.
At the interface level, the priority is based on the ATM service category. The
service category is determined by the first PVC that is configured under a VP.
The priority is in the following order:
Constant Bit Rate
Unspecified Bit Rate + (UBR+)
Variable Bit Rate
Non Real-Time (VBR-nRT)
Variable Bit Rate (VBR-RT)
If no service
category is specified at the VC level, the default is unshaped UBR. The ATM
Hierarchical Shaping feature is supported for the all the above ATM traffic
For VBR-nRT traffic
the output sustainable cell rate (SCR) value that is configured will be taken
into account for VC shaping. For UBR traffic, the output peak cell rate (PCR)
value that is configured will be taken into account for VC shaping.
The ATM Hierarchical
Shaping feature supports over-subscription. Over-subscription occurs when the
sum of the configured rate of UBR and VBR PVCs exceeds the line rate.