Beta Draft Confidential
About ATM Logical Ports
About the Oversubscription Factor
ATM Services Configuration Guide for CBX 3500, CBX 500, GX 550, and B-STDX 9000 1/19/052-19
About the Oversubscription Factor
The oversubscription factor percentage enables you to optimize the number of PVCs
and SVCs you can configure on the network by allowing you to oversubscribe the
logical ports. If you configure oversubscription for the VBR classes of service (CoS),
QoS is no longer guaranteed.
The CAC algorithm determines effective bandwidth of a virtual circuit (PVC and
SVC). For a VBR circuit, the CAC uses the circuit’s PCR, SCR, and MBS values. For
CBR circuits, the CAC uses the PCR of the circuit. UBR circuits are assigned 100 cps
of bandwidth for load and reroute purposes, since it is a “best effort” service.
PVC routing is determined by either an OSPF algorithm or the network administrator
(if you manually define the circuit path). Each time a PVC attempts to come up after
configuration, OSPF reserves the required bandwidth on the port. OSPF deducts the
amount of reserved bandwidth from the available virtual bandwidth pool for the
applicable CoS.
The available virtual bandwidth can become negative in extreme situations. For the
variable bit rate-non-real time (VBR-NRT) queue, if a number of trunks fail, PVC
rerouting may cause the available virtual bandwidth value to become negative.
Existing PVCs can be rerouted over a negative virtual bandwidth trunk. However, new
PVCs cannot traverse trunks that have a negative virtual bandwidth. Any PVC that
fails during the time of the reroute is considered to be a new PVC when it attempts to
come up after the trunk is rerouted.
Since inter-LAN traffic is bursty in nature, not all network traffic uses the network
resources at precisely the same time. Basically, the higher you set the oversubscription
factor, the less guarantee there is that user data will get through on the port; the
trade-off is that you can provision more circuits on that port. If, however, all network
traffic attempts to use the network resources at precisely the same time (for example,
during multiple file transfer sessions over the same trunk), some traffic may be
delayed or even dropped.
Note – To ensure QoS, monitor the network closely before you modify
oversubscription values to exceed the minimum value of 100%. If you adjust the
oversubscription percentage, monitor the cell-loss ratio to be sure the new setting does
not affect QoS.
Note – Appendix A describes how to tune the CAC to optimize your network. If you
tune the CAC properly, you can optimize network resources without adversely
affecting QoS.
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