A question that occasionally comes up in the #A&A IRC channel is "how many VoIP channels can I fit on one ADSL line?" The answer is quite simple to calculate, and you can do similar arithmetic for other realtime traffic:
We start with some general knowledge; people who read my previous post on ADSL overheads will be aware that ADSL sync rates are the number of ATM cell bits you can fit down the line. ATM cells are 53 bytes long, so you divide your sync rate by 424 (bits per cell) to get the cell rate. So, all we need to know is how many cells per second in a VoIP call. The table below has details of all the bits in a single 20 millisecond packet of G.711 (a-law or µ-law) VoIP:
|2||PPP framing for PPPoA|
|160||G.711 voice data|
Because we have 30 bytes of padding, we can deduce that the extra 20 bytes overhead of PPPoE won't hurt us; nor will the extra 20 bytes header size of IPv6. However, if we use both IPv6 and PPPoE, we will need one more cell per packet, making it 6 cells for one packet, or 300 cells/second, or 127.2 kbit/s of sync rate per call.
We know we need a little spare capacity for signalling and monitoring, but from this, we can deduce that a 448kbit/s ADSL upstream (IP Stream Standard) can support 4 calls, or 3 if you're using both PPPoE and IPv6. An 832kbit/s upstream supports 8 calls, or 6 if you're using both PPPoE and IPv6. The 2MBit/s I get on Annex M supports 15 calls with PPPoE and IPv6, or 18 calls with either IPv4 or PPPoA.