Number of Channels required for G711 call

[ciscovoipsupport]

Hi All,

Was going through bandwidth calculator here.. I am trying to find out how many channels are required to make one G711 call which is compressed. Here is what i know.

Packet Calculation:

Total packet size (bytes) = (MP header of 6 bytes) + ( compressed IP/UDP/RTP header of 2 bytes) + (voice payload of 20 bytes) = 28 bytes

Total packet size (bits) = (28 bytes) * 8 bits per byte = 224 bits

PPS = (65536 codec bit rate) / (160 bits) = 436 pps

Bandwidth per call = voice packet size (224 bits) * 436 pps = 95kbps

Infrastructure:

Lets a i just have a single T1 channel available out of 23 of a PRI.

Channel capacity is 64kbps. Bandwidth per call requirement is app 95kbps.

What will happen? Will the call go through? or Do i require 2 channels to make a successful call.

[Ayodeji Okanlawon · Accepted solution]

You are mixing things up. Those headers are purely ip headers, they dont affect TDM transmissions. Hence those calculations have no impact on how many channels are supported on a T1 or e1 channel. What affects how many calls you can make over a T1/E1 channel is simply the following

1. PVDM available

2. Codec complexity used on the dsp..

eg if you have a single e1 card which supports 30 channels and you have provisioned a PVDM3-16 on your gateway, assuming you are using the default codec complexity of flex then you will only have 16 channels available out of 30.

I wrote a document detailing something similar to this here...Have a read..

https://supportforums.cisco.com/document/123126/cisco-pvdm2-and-pvdm3-dsp-creditmips-allocation

[Carlo Poggiarelli]

Hi. +5 to my buddy Deji. T1 you should have up to 23 b-channels which means up to 23 g711 voice calls HTH regards Carlo

[ciscovoipsupport]

Thank you for the link Deji.. 
Couple of other questions..

Q1. So in what scenarios do you calculate the Bandwidth per call ..
Why do we need to know the bandwidth required, if all i need is PVDMs and number of channels.

From what i read bandwidth calculation is required only in IP network and not on TDM network?

Q2. Is PVDMs still required if i do not use any compression or complexity on VGW?


 

[Ayodeji Okanlawon]

Q1..Bandwidth calculation is required in an IP network before you call gets to the PSTN gateway. Eg, branch sites connecting to your central cucm over the WAN. You need to know how many calls can transverse the WAN based on the bandwidth available and the bandwidth each call will use..

Q2. You need PVDM for IP to TDM encoding. You need dsp to modulate ip traffic to TDM traffic. Your cisco IP solution is IP based. However once you need to connect to the PSTN either via digital E1/T1 or analogue FXO/BR1, you  need dsp for encode all your ip traffic to that medium. This doesn't depend on compression or complexity ( I am not sure I know what you mean by this)