[Asterisk-Users] Calculating required bandwidth

[Andrew Kohlsmith]

On December 16, 2004 01:52 pm, Race Vanderdecken wrote:
> The quick tyrannical answer,

And wrong -- I am taking the time to correct it not so much to slam you but 
more for list posterity -- just because the codec rate is 64kbps doesn't mean 
that's what's actually on the wire, even if you ignore signalling.

> Each T1 channels is 64Kbps, 1,544,000 / 24 = 64,333

each T1 has 24 channels of 8 bit data plus one frame bit.
24*8+1 = 193 bits per T1 frame.  Frames are sent 8000 per second.  8000*193 = 
1544000 bits per second.  There's your T1 raw rate.

You can't use that frame bit for yourself so 24*8*8000 = 1536000 bits per 
second.  That's your T1 data rate; that's what you can actually use.

Now.  Running IP on a T1 you have certain overheads.  UDP frame overhead is 4 
bytes, plus your TCP overhead of 12 bytes, for a total of 16 bytes (128 
bits).  G.711 is 64kbps data rate, but Asterisk sends only 20ms per packet in 
an attempt to balance data throughput and effect of lost packets.

so 64kbps / 50 is 1280 bits of audio per packet, plus 128 bits of overhead for 
1408 bits per packet.  50 of these per second of audio gives you 70400bps for 
one second of G.711 VOIP audio.

so now take your T1 data rate of 1536000bps and divide your audio rate into it 
for an answer of 21 channels of G.711 VOIP audio.

Now that was straight UDP audio -- there was no signalling overhead and it 
wasn't SIP RTP.

RTP has 12 octets all its own, and still need 12 bytes of IP overhead, so it 
is actually costlier: I'll spare you all the calculations but it's 20 
channels of SIP G.711 audio per T1, likely with enough room for 
signalling.  :-)

Regards,
Andrew "the tyrant's tyrant" Kohlsmith