[Asterisk-Users] IAX2 trunking: codec bandwidth comparison notes and results

[Mark Spencer]

>    - The number for "Estimated IP Overhead" was obtained by
> subtracting (additional channel usage) from (single channel usage.)
> This is possibly inaccurate.

It should generally be pretty accurate.  You might try running 3 calls
just to confirm.

> ILBC:
>   one call:   56134.91 bps/67.45 pps    (28.0 kbps)
>   two calls:  98679.11 bps/102.41 pps   (49.3 kbps)
>
>   Thus:
>    For every additional call:       42544 bps (21.2 kbps)
>    Est. IP/IAX2 overhead (1 call):  13590 bps ( 6.7 kbps)
>    Raw number of calls per megabit: 47

Remember ILBC uses a different frame length (thus the lower pps count) and
because it's not going to line up exactly with the G.729 or ulaw frames,
there will be even fewer.  However, I still don't see only 6.7kbps.  That
seems just a bit too low.

> GSM
>   one call:   70958.16 bps/102.13 pps   (35.4 kbps)
>   two calls: 100455.23 bps/102.63 pps   (50.2 kbps)
>
>   Thus:
>    For every additional call:       29497 bps (14.7 kbps)
>     Est. IP/IAX2 overhead (1 call): 41461 bps (20.7 kbps)
>    Raw number of calls per megabit: 68

IP overhead is purely a factor of the number of packets (PPS).  GSM, ulaw,
Speex, and G.729 should all have identical overheads, in principle.

> -------------
> LPC10
>   one call:   43855.44 bps/89.94  pps   (21.9 kbps)
>   two calls:  56059.18 bps/100.81 pps   (28.0 kbps)
>
>   Thus:
>    For every additional call:       12203 bps ( 6.1 kbps)
>    Est. IP/IAX2 overhead (1 call):  31561 bps (15.8 kbps)
>    Raw number of calls per megabit: 164

I would predict LPC10 to be around 4.8, not 6.1....  I wonder why the
discrepency.  Perhaps the IAX overhead is coming into play more here...

> SPEEX
>   one call:   74817.18 bps/101.06 pps   (37.4 kbps)
>   two calls: 109692.68 bps/102.18 pps   (54.8 kbps)
>
>   Thus:
>    For every additional call:       34875 bps (17.4 kbps)
>    Est. IP/IAX2 overhead (1 call):  39941 bps (19.9 kbps)
>    Raw number of calls per megabit: 57

Changing Asterisk's selection of options on Speex could improve this.

>    I am a consultant, and I'm happy to do this kind of work to further
> develop your products or deployments of VoIP/Asterisk systems.  Mail
> me (jtodd @loligo.com) for details.

Your work speaks for itself, but for what it's worth, I also give you my
own thumbs up for your dedicated understanding of the mechanics,
debugging, and testing of Asterisk.

Mark