2010/03/06

Calculating the number of VoIP channels you can fit on an ADSL line

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 ยต-law) VoIP:

BytesReason
8AAL5 trailer
2PPP framing for PPPoA
20IPv4 header
8UDP header
12RTP header
160G.711 voice data
This adds up to 210 bytes; cells carry 48 bytes each, and can only be used for one packet, so we need 5 cells, with 30 bytes padding. There are 50 of these packets in a second of VoIP, so each call needs 250 cells/second, or 106 kbit/s of sync rate.

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.