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RE: VoIP data rates [7:56942] posted 11/06/2002
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Priscilla Oppenheimer wrote:
> 
> Matthew Webster wrote:
> > 
> > Hi all,
> > 
> > if I am transporting RTP packets over a frame relay network
> > what are my packet sizes? I ask because if you are using a
> > G723.1 codec with an encoding of 6.3kbits/s, then presumably
> > that excludes the UDP, IP and FR headers, but includes the RTP
> > packets. Or does it? Is the converse true in that the
> > 6.3kbits/s includes the UDP/IP/FR headers?
> > 
> > cheers,
> > Matthew.
> 
> You are comparing apples and oranges when you compare a bit
> rate (kbits/s) to a frame size (bits, or usually in bytes).
> 
> But that's OK. We can work around this, and, as you have
> discovered, it's necessary to work around it because CODECs
> like G723.1 specify a bit rate for the encoded voice, whereas
> what we really need to know is how much bandwidth is required
> for one voice stream, encapsulated in FR/IP/UDP/RTP.
> 
> The 6.3 kbps for G.731.1 is just for the coded, compressed
> voice, per the CODEC standard. It doesn't include bandwidth
> used by encapsulating protocols.
> 
> You would have to know how many voice samples get stuffed into
> one frame. I believe Cisco VoIP puts 20 bytes of encoded voice
> in one packet. To be exact you would also have to know if RTP
> is being compressed or not. If it's not, the IP/UDP/RTP header
> adds 40 bytes. So it approximately doubles the frame size.
> 
> If you double 6.3 kbps, you get about 13 Kbps.
> 
> Here's the math:
> 
> actual_BW = CODEC_BW x
> (payload_length+encap_length)/(payload_length)
> 
> actual_BW = 6.3 Kbps x (20+44)/20 = 20.16 kbps

The extra 4 bytes were for the Frame Relay header, BTW.
> 
> Or, you could find a chart at Cisco that simply tells you how
> much bandwidth is required for each of the CODECs for the
> variety of technologies, VoIP, VoFR, VoATM. Let me see if I can
> find one.... Shoot, I can't. I'll get back to you on that. I
> know it's on CCO somewhere.

Here's a URL. It shows bandwidth consumption calculations with PPS, but I
have seen Cisco docs that do it the way I did too by calculating overhead...

http://www.cisco.com/warp/public/788/pkt-voice-general/bwidth_consume.htm

Priscilla


> 
> _______________________________
> 
> Priscilla Oppenheimer
> www.troubleshootingnetworks.com
> www.priscilla.com
> 
> 
> 
> 
> 




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