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20-20
ATM Switch Router Software Configuration Guide
OL-7396-01
Chapter20 Configuring Frame Relay to ATM Interworking Port Adapter Interfaces
Configuring Frame Relay to ATM Resource Management
MBS = [Bc/8 ( 1/(1 –(CIR/PIR))) + 1 ] [OHB (n)]
= [4000/8 (1/(1 -(32000/64000))+1] [2/64]
= [500 ( 1 / 0.5 ) +1] [2/64]
= [500 (2 +1)] [2/64]
= [1500][2/64]
= 46.875
Rounded of to next integer
MBS = 47
The Bc and Be values must be at least equal to the frame-size (calculated in bits). The Bc value indicates
how long the VC can accommodate a burst above CIR. It depends entirely on the source of the traffic,
how bursty it is, and how much the administrator will allow the VC to burst. There is no problem if the
Bc, Be values are configured higher than the input burst coming from the VC.
Note If you configure a high value for Bc and if you have enabled Overflow-Queuing then switching to
Overflow-Queuing will be delayed by the factor (Bc – Frame-size [of the incoming traffic]).
Roughly, the value is related to the number of frames the VC can accommodate with a continuous burst
without tagging DE based on (CIR, Bc) [dropping based on ((PIR-CIR), Be)]. So, the Bc and Be values
should always be more than the frame-size of the largest frame that is expected on the VC. If the interface
bandwidth is high compared to the CIR then it is better have a larger Bc value. Similarly, Be (PIR-CIR)
should be considered.
The following scenario describes when you might need to have higher Bc and Be values:
Usually the CIR is much less than the interface-rate. On a serial interface you get a complete frame at
the interface-rate than at the configured CIR since you need to send a complete frame and start sending
the next frame. In the event the other VCs have nothing to send, that bandwidth is used to send the traffic
on the serial interface (provided the incoming traffic is not shaped). In that event, you should expect
more frames to be dumped on to the Frame Relay ATM module and expect them to be shaped and sent.
If the module is expected to accommodate more frames without dropping them due to UPC the best
solution is to increase Bc and Be values.
PVC Connection Traffic Rows
Permanent virtual channel (PVC) connection traffic rows, or stable rows, are used to specify traffic
parameters for PVCs.
Note PVC connection traffic rows cannot be deleted while in use by a connection.