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 > > > > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=56957&t=56942 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
