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
Ack. So the difference is EchoLink sends 80 millseconds of audio/GSM per packet and Asterisk/app_prt only sends 20 milliseconds so there are 4 times as many packets with 3 times the header overhead. I excluded the Ethernet header since it's stripped before going out the Wan port (at least for dialup users which is where the calculation is most critical). IRLP GSM sends 200 milliseconds of audio in each packet which is WAY WAY too much IMHO. I have no opinion if 20 milliseconds or 80 milliseconds per packet is "better", I'm just trying to understand the numbers. Thanks for the math! 73's Skip WB6YMH
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
_______________________________________________ App_rpt mailing list App_rpt@lists.illiana.net http://lists.illiana.net/mailman/listinfo/app_rpt
_______________________________________________ App_rpt mailing list App_rpt@lists.illiana.net http://lists.illiana.net/mailman/listinfo/app_rpt