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VISM in AAL2 trunking mode

I have a client with using VISM'S in AAL2 trunking mode. I am using the G729a codec for voice compression. When you add the bearer PVC using Cisco Wan Manager, it sets the connection up as VBR rt and the peak cell rate gets rounded off to about 3600 cells/second. I have done the maths, taking into consideration VAD and packetisation delay of 10m sec etc and i get to a figure of around 1800 cells required for a PVC carrying 31 timeslots at G729a. However if i change the cell rate on the PVC to this figure i get bad voice quality. My question is that if we use the default for PCR we are not really getting any benefit as we are only really getting around a 2 to 1 compression ratio. Can any one confirm this and give me some tips.

Thanks, Kevin

New Member

Re: VISM in AAL2 trunking mode

Did you ever find out anything on this? I'm curious what you've learned.

New Member

Re: VISM in AAL2 trunking mode

Yes. Bandwidth utilisation when using Vismsis influenced by a number of things including VAD, packetisation delay, codec type and whether or not you are using subcell multilpexing. We had ours configured for 10 msec paketisation and G739a codec with vad and no subcell multiplexing which effectively meant that each voice channel was using about 43k.

We have now changed our packetisation to 30 msec and we are achieving about 14k per voice channel which is what you would expect with no degradation in voice quality. You need to be careful with changing you packetisation delay as depending on the number of hops you are taking it could affect voice quality. I have a calculator taht works this out. If you want it drop me an e-mail at

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