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7.8

Is there a "rule-of-thumb" for setting the ATM VBR-nrt burst-window size

Asked by hassarfj in Asyncronous Transfer Mode (ATM)

Tags: atm

I need help getting my virtual circuit to burst to full port-speed.  We have configured a 20Mb/s sustained information rate (SIR) virtual circuit between two Cisco 7206 routers connected to a single telco ATM switch via two 40Mb/s DS3 local loops.  The telco says we are permitted to burst to port speed for a short time, (the burst-window), after which we should revert back to the SIR.  Once the router(?) determines no other user needs the bandwidth, it should again try to transmit at port speed.  The best I have done so far is to burst to 24 or 25 Mb/s.

The local loops terminate on the same telco ATM switch, so there is no "cloud" to transit, and this is the only PVC on the link, so there should be no self-induced congestion.  We average 3-5Mb/s on the link, but several times an hour, we have to synchronize some databases and pass a huge block of data in a short time.

Following one telco tech's suggestion, I set the VBR-nrt burst window to 33 cells.  A different tech said we should set it to 4400 cells (!)  A third telco tech said we should calculate how many cells we can pass in 125ms. (11700), and set the window-size accordingly.
What IS the right answer to enable me to send data at the full 40Mb/s rate?  Is there a "rule-of-thumb" for setting the ATM VBR-nrt burst-window size
[+][-]12/13/02 11:59 AM, ID: 7580058Accepted Solution

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Zone: Asyncronous Transfer Mode (ATM)
Tags: atm
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Solution Provided By: quietyakr3
Participating Experts: 2
Solution Grade: A
 
[+][-]12/12/02 11:54 AM, ID: 7574173Expert Comment

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[+][-]12/13/02 07:20 AM, ID: 7578510Author Comment

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[+][-]12/13/02 07:33 AM, ID: 7578577Author Comment

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[+][-]09/11/03 10:35 PM, ID: 9342377Expert Comment

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