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VoIP on ATM/IMA

How can I determine vbr-rt parameters for implementing VoIP. There are some formulas that include the Peak Cell Rate, Average Cell Rate and Cell Burst values, but I don't know if this formulas are intended only for VoATM.

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New Member

Re: VoIP on ATM/IMA

When you are defining an ATM vpi/vci pair on a virtual path, regardless of whether you are talking about IMA or DS3, OCx, etc, class of service can usually be defined UBR, VBR-RT, VBR-NRT, or CBR. If we are talking about VoIP over ATM, and it sounds like we are, you can implement QoS at layer 3 with atm and/or frame-relay traffic shaping and policy based routing. Peak cell rate, sustained cell rate, and burst size in cells are determined by the provisioning of the virtual path in the service provider network. You will get these from the carrier/telco and shape your paths accordingly in the router. You will want to make sure you can get large packet pings through the network without packet loss. Ping with a 1500 byte packet, and if you are 'out of shape' you will see intermittent packet loss. If SCR, PCR, and burst are not set properly to match the service provider network, you will not be able to reliably pass pings (you will get long ACKs in TCP) like the one described above through the network and voice (as well as data) will subsequently suffer.

Pat

New Member

Re: VoIP on ATM/IMA

Indeed, I'm applying LLQ to this PVC (reserving 1336 kbps) by means of a policy-map and a service-policy output to the PVC. However, I need to know if the parameters for defining vbr-rt (PCR, SCR, MBS) must be configured accordingly to the reserved bandwidth (a priority command applied on the policy-map.) The Web information indicates a serie of formulas to obtain the PCR, SCR and MBS using a 16kbps factor to reflect (I think) the VoATM bandwidth (BW) requirement on each parameter but MBS. If my implementation use VoIP on ATM/IMA links, do I need to reflect a VoIP BW requirement on that formulas?

Regards!

Gerardo.

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