Cell Relay Archive

[vkaushik@duettech.com (Vikas_Kaushik)]

>> · If there is a CAC for UBR, the CAC function knows the entire network
>> topology (how did it get to know the topology?).
TRUE.  If the backbone is an ATM network the topology is known by the exchange of PNNI Protocol Topology State Packets (PTSPs).  In fact it is true for all the service categories, there is nothing specific about it for UBR connections.



>> · If the CAC function doesn't know the entire network topology, it asks
>> at connection setup (what signaling message is that?).
Sorry, not clue on this.  I would like to hear more on this.



>> · UBR specifies a PCR and CDVT, but in almost all implementations that's
>> just set to the constraints of the physical interface for any VCC.
Very true.  CAC and PCR enforcement arre not mandatory for UBR setup.  Even if PCR is specified, it may be neglected by the network.  However, it is suggested that PCR be negotiated across the netwok.  The negotiated value gives an estimate of the bottleneck along the connection.  The application may pump data into the network accordingly.



>> · Barring any large buffers or packet discard schemes, UBR traffic from
>> a high bit-rate source will suffer a high cell loss, if sent to a low
>> bit-rate destination.  Any controls on the bit-rate are from the upper
>> layer protocols/application at the ends of the connection.
Once again, very true.  UBR connection does not imply any commitments from the network regarding quality of service. Once again, ensuring fairness to connections and handling congestion control are local to a particular switch.