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

[John Todd]

>  >    - 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.

I'll try this shortly after I return from dinner.  I can get another 
phone into the equation, I suppose, but I'll test to see if results 
with all of the codecs are >5% out of expected trends from my tests 
with two phones.  If not, I'll give a quick summary and not do the 
whole experiment cycle over again.

>  > 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.

Yes, that's a real head-scratcher.  As I said, I did it three times 
to verify, and the results were very close.

>  > 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.

There do seem to be some clusters:
   G.711, GSM, G.729, and Speex are all within 5% of 100pps with 
either one or two channels

   ILBC is at 67pps with one channel, but jumps to 102pps with two channels
   LPC10 is at 89pps with one channel, and jumps to 101pps with two channels

This may result in some skewed test results.  I will re-test the 
outliers again (ILBC, LCP10) to see if there was some confounding 
factor in my original results.

>  > -------------
>>  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...

I'll try with three channels.

>  > 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.

[snip]

I could dig through the manuals, but can you give me a quick pointer 
on how to do this?  What are the knobs I can turn with speex?


JT