In previous posts in this series, we’ve performed the analysis necessary to classify traffic, determine how it will route, and compute the volume of traffic the WAN link will be carrying. Thanks to Erlang traffic models, we have also determined the number of simultaneous calls the WAN needed to be sized. While all of this information is useful, there’s one last leap to get to the end goal of this exercise.
For calls going across a WAN, the lower bandwidth G.729 codec is usually used. There are exceptions, of course: faxes, TTY/TTD, and modem calls would require G.711. That being said, we’ll just assume G.729 for now.
This codec has a payload bit rate of 8kbps, but there are also RTP, UDP, and IP headers (overhead) to be considered. Basically, we’re looking at 40 bytes per packet. If we put 20ms of audio samples in each packet, then the packet rate is 50 packets per second. The math looks like this:
Packets per second * Header size in bytes * 8 bits per byte / 1000 bits per kilobit
50 * 40 * 8 / 1000 = 16kbps
Add the 8kbps payload bit rate and you’re almost done. There are additional headers whose size depends upon the WAN technology in use. For example, a T1 MPLS circuit would add 6 bytes of MPLS headers, plus 6 bytes of layer 2 PPP headers, for a total of 12 bytes per packet, which works out to 4.8kbps. Add that to the 16kbps of upper layer headers and the 8kbps of payload, and we have a total of 28.8kbps.
All that’s left now is to multiply the bandwidth per call by the number of simultaneous calls the link needs to support. Last time we determined that was 31 calls, for a bandwidth total of 892.8kbps.
As Columbo (remember him?) might have said, “just one more thing.” What happens if most the calls are G.729 voice calls but some of the calls are going to be faxes? After all, the phone bills we used in the first posting in this series didn’t necessarily differentiate between voice calls and faxes. We could re-analyze the call data using logs from the fax machines. That would be the most precise of the options. On the other end of the spectrum, we could use “management’s informed estimation.” Which you use is going to be your call, but it’s something you should consider.