VoIP: Basics and fundamentals: bandwidth for an H.323 call

[greeg]

Hi there,

A very basic question for my first post

What do you think if I said that an H.233, G.711 codec, call requires 84 Kbps (in IP) to establish call. As we are in full duplex mode, I can consider that it is a 168 Kbps VIRTUAL bandwith, but with a real 84 Kbps CAC setup on ipbx, right? so each RTP channel can use 80 Kbps, at the same time, on my 84 Kbps ip link? As in Ethernet where our NIC are 100 Mbs full-dpulex right?

thanks!
GreeG

[greeg]

As i’m lookin also on google, i’ve found 2 different poitn of view:

bandcalc.com/

This is the number of unidirectional channels, so for the total bandwidth used for media during a simple two-way call, you would specify 2 in this field.

erlang.com/bandwidth.html

Therefore, as a general rule of thumb, it can be assumed that header information will add 16kbps to the bandwidth requirement for voice over IP. For example, if an 8kbps algorithm such as G.729 is used, the total bandwidth required to transmit each voice channel would be 24kbps.

So, as you see, the first tell to count two times the codecs, and the second tells that a 24 Kbpw BW is able to handle 2 G.729 RTP channel (so, you not have to provision 48 Kbps…)

I’m a bit lost…

[greeg]

Hi,

I think i’ve found the explantion: ISDN.

When you have a basic 2B + D, you only use ONE channel B (64kb/s, fullduplex) for talking to anybody… Actually, the secret lives in the full-duplex notion, that specifies taht de bw is available for downstream and upstream…

Am I Right?

[mazzic]

When you have a basic 2B + D, you only use ONE channel B (64kb/s, fullduplex) for talking to anybody…

That has to do with analog voice communication. When your using the ISDN with your voice service, one B channel carries the phone call.