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G.711 (ulaw)
one call: 164333.75 bps/94.26 pps ( 82.1 kbps)
two calls: 296171.60 bps/101.46 pps (148.0 kbps)
Thus:
For every additional call: 131837 bps (65.9 kbps)
Est. IP/IAX2 overhead (1 call): 32495 bps (16.0 kbps)
Raw number of calls per megabit: 15
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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
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G.729
one call: 60124.33 bps/101.26 pps (30.0 kbps)
two calls: 79496.23 bps/102.85 pps (39.7 kbps)
Thus:
For every additional call: 19372 bps ( 9.6 kbps)
Est. IP/IAX2 overhead (1 call): 40752 bps (20.3 kbps)
Raw number of calls per megabit: 103
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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
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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
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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
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Conclusions:
lpc10 is the clear winner for thrifty use of bandwidth. However,
voice quality is on the robotic-sounding side, and may not be
acceptable for users who are expecting something near toll-quality.
Calls are perfectly understandable, but nuances are lost with this
codec. This was the only codec that had significant quality issues
that were worth mentioning in this study; other codecs have different
sound quality ratings, but their differences are minor enough that
they are not relevant to the scope of this examination. That being
said about call quality, there is certainly a tradeoff to getting
~164 calls in a megabit, which is extremely impressive.
G.729 is the next best from a bandwidth standpoint, and expected
additional channel usage is very close to the quoted bandwidth of
G.729 codec usage - ~9.6kbps in each direction. The next most
efficient codec would be GSM at 14.7kbps, and third would be Speex at
17.4kbps.
Somewhat surprising were the IP/IAX2 overhead figures for ILBC -
they are almost one-third that of some other protocols. I tested
that particular number three times to make sure I did not make any
errors, and all tests were within 3% of each other, so that is an
unexpected but consistent result. Of course, my method for obtaining
these estimated IP/IAX2 overhead figures may be inappropriate - I
welcome comments on the method used to estimate those numbers.
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