Skip Each frame in Asterisk starts out as 20mS of audio or 160 words of signed linear samples at a frame rate of 50 per second. Each 20mS 160 word sample is encoded in GSM which results in a compressed payload of 33 bytes. Check. Now, on to that 33 byte packet you have to add 4 bytes for the IAX protocol header, 20 bytes for the IP header, 8 bytes for the UDP header, and 18 bytes for the ethernet header. So now we have 33+4+20+8+18 or 83 bytes. At a frame rate of 50 per second, this means that we will have 83*50 or 4150 bytes per second which when multiplied by 8, is 33,200 bits per second. Throwing in 5% for IAX control frame overhead, and occasional IAX control frame retries, we get 34860 bytes or about 35 KB/sec. I have verified this with ntop running on my firewall machine and the numbers are very close to the calculations. Steve WA6ZFT http://site.asteriskguide.com/bandcalc/bandcalc.php Skip WB6YMH wrote:
all,
There is a 44 page document on dl.allstarlink.org which may answer a lot of questions about how to configure app_rpt and Asterisk on an ACID system. Hopefully, this will answer a lot of the same questions we keep seeing on the list.
Excellent!
But ... this brings up a new question. Where do your bandwidth numbers come from for the chart on page 6? Is that the total bandwidth in *both* directions? The numbers are about a double of the 17kbps I calculate for EchoLink.
{ (8000 samples/second) / (160 samples/gsm frame) / (4 gsm frames/packet) = 12.5 packets/second. or 1 packet every .08 seconds. Each packet consists of an IP header and an UDP header so (12.5 packets/second) * (20+8+12+(33 * 4)) bytes/packet) * 8 bits/byte = 17200 bits/second.}
73's Skip WB6YMH
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