On Wed, Oct 04, 2006 at 07:51:14PM -0400, hugolivude wrote: > Age old question it seems but I haven't been able to get a handle on > it yet. Let's assume I'm using a g729 codec. If I wanted to handle > 20 simultaneous calls, how much bandwidth would I need? Is there a > general formula for this? > I tried this caluclator: > [1]http://www.voip-calculator.com/calculator/eipb/ > I wasn't sure what "Packet Duration to select so I took the default > 20ms (2 samples) - whatever that means.
A 20ms packet duration means that 20ms of audio is stuffed into one IP packet. Since each packet carries 1/50th of a second of audio, that means you're generating 50 packets per second for each channel. With g729 your audio is 8000 bits per second. The overhead on each packet is 20 bytes (IP) + 8 bytes (UDP) + 12 bytes (RTP) = 40 bytes or 320 bits. So your bandwidth requirement per channel is: - 8000 bits per second for payload - 320x50 = 16000 bits per second for overhead making a total of 24000 bits per second. 20 simultaneous calls is therefore 480,000 bits per second. That is a bit of an underestimate though, because it doesn't include any layer 2 framing overhead (i.e. for encapsulating the IP frames in the underlying medium). For example, if it were HDLC serial on a leased line, that would be just 2 bytes per frame for flags, maybe a couple of bytes for CRC, plus occasional bit-stuffing. However on ADSL, you have to add the 15% ATM cell tax. And you would be wise to add 20% headroom (i.e. so your line is not more than 80% full) As you can see, the packetisation overhead is twice as large as the useful data you're transporting. You can reduce this by increasing the packet duration, but that increases the latency of your audio (and ADSL links already add 20-30ms of latency themselves). Too much latency is objectionable to users. I have read that if you use IAX2 trunking it's able to combine audio from multiple streams into a single packet, thus sharing the overhead between them, but I have no experience of this myself. HTH, Brian. _______________________________________________ --Bandwidth and Colocation provided by Easynews.com -- asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users