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 = 6.5 Kbps x (20+44)/20 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. _______________________________ Priscilla Oppenheimer www.troubleshootingnetworks.com www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=56952&t=56942 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
