[asterisk-users] Bandwidth requirements

[Brian Candler]

On Thu, Oct 05, 2006 at 10:53:14AM +0200, raphael Jacquot wrote:
> Brian Candler wrote:
> 
> >However on ADSL, you have to add the 15% ATM cell tax. And you would be 
> >wise
> >to add 20% headroom (i.e. so your line is not more than 80% full)
> 
> ATM cell tax is actually 10% as there's 5 header bytes for each 53 bytes 
> cell,

Only if all cells are filled. On average there will be half a cell empty at
the end of each packet.

A common case is 1500-byte packets; these will take 32 cells, or 1696 bytes
total, giving a tax of 13%. Mix some smaller packets in with that and you
get a higher tax. So I tend to work on 15% as a rule of thumb.

However it's worse for VoIP as has been pointed out. e.g. if you are sending
60 byte packets (20 IP, 8 UDP, 12 RTP, 20 G729) then they will take two
53-byte cells, so you pay 46 extra bytes to carry 60 bytes of IP; the tax is
then 77%

(You will also have encapsulation, e.g. PPPoA, but that probably fits in the
wasted space without needing another cell)

Regards,

Brian.