[Asterisk-Users] RTP vs cRTP vs IAX

[Brian Capouch]

Jean-Michel Hiver wrote:
> Hi List,
> 
> I have seen this:
> 
> http://www.convergence.com.pk/iax2/trunked.html
> 
> According to this table, using trunking, you can have 16 channels with 
> 171.7 kbps bandwith using  g.729 + IAX2 trunking? Sounds too good to be 
> true...
> 
> Any comments on this?
> 

If I'm reading the little blue/green chart at the top correctly, the 
calculations on this page use some sort of special IP that only has 12 
octets of header, instead of the 20 octets the IP standard specifies.

So the sum of 20 octets of IP header plus 8 octets of UDP header would 
be 28 octets/224 bits, instead of the 160 bits shown there.

They also show an RTP header size of 8 octets; I think it should be 12.

Those would have an effect, I think, on all the derived calculations.

I also see a descrepancy in the bitrate they use for iLBC.  Depending on 
the sample size, its bitrate, according to the blurb on the front page 
of its website, is either 13.3kbs (30ms/sample) or 15.2 (20ms/sample). 
The convergence site states 9kbs.

So either I've got some wrong info, or they do. . . .

B.