Need help understanding bandwidth and VoIP

Answered Question
Jul 16th, 2007
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Hello,

I need some help understanding bandwidth, I've always had difficulty keeping the units of bandwidth measurement straight.


Voice quality is directly impacted by the codec used. Default in CallManager is G7.11, which takes up 80Kb/s for each voice call. Does this mean that a 1.5/384 circuit (which translates to 1,572 Kbps/384 Kbps) will be able to support about 4 calls using G7.11? (384/80 = 4.7 calls, rounded down to 4 calls)


Does this also mean that a PRI, which will run at 1.5Mb/s up and down, which translates to 1572 Kb/s up and down will suport just under 20 calls without degradation?


-Shikamaru

Correct Answer by paolo bevilacqua about 10 years 1 week ago

Hi,


Your calculation for ADSL is correct.


But, PRI (primary rate interface) is not a data circuit, is instead an access to telco network that uses T1 as physical layer, just like a data circuit does. So you would say correctly, a data T1 would support up to 20 calls of VoIP w/ G.711 codec. Then, you can put in play VAD that drastically reduces bandwidth allocation using the same codec. Remember however, that generally users don't like VAD.


See:

http://cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml


And if you are a registered customer, the interactive calculator reference in the above document.


Hope this helps, please rate post if it does

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Correct Answer
paolo bevilacqua Mon, 07/16/2007 - 08:49
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Hi,


Your calculation for ADSL is correct.


But, PRI (primary rate interface) is not a data circuit, is instead an access to telco network that uses T1 as physical layer, just like a data circuit does. So you would say correctly, a data T1 would support up to 20 calls of VoIP w/ G.711 codec. Then, you can put in play VAD that drastically reduces bandwidth allocation using the same codec. Remember however, that generally users don't like VAD.


See:

http://cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml


And if you are a registered customer, the interactive calculator reference in the above document.


Hope this helps, please rate post if it does

shikamarunara Mon, 07/16/2007 - 08:55
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Okay, good to see that my thinking is on the right track. I get confused when people starting using MegaBits and MegaBytes or KiloBits and KiloBytes to describe circuits. My next question is; while my DSL circuit is more than enought to support the single VoIP call that my VoIP phone makes (provided it's using G7.11, I am going to call my provider and check), the other traffic going through the cicuit, like my VNC sessions and the SMTP connections going to my email server, throws a monkey wrench into the mix. Is there a program that you can recommend that will help me determine bandwidth utilization on my DSL circuit? I would like to use one that will help me maintain graphs.


Thanks

-Shikamaru

paolo bevilacqua Mon, 07/16/2007 - 09:03
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Hi,

Usually consumer-grade ADSL is good enough to do simultaneosly all the things you mentioned.


To monitor your circuit, I would recommend PRTG that is free for personal use on a single device.


Thanks for the nice rating and good luck!

shikamarunara Mon, 07/16/2007 - 09:05
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In the normal scheme of things, an ADSL circuit is more than enough, I know. But, with the host I'm running off the circuit this is not the normal scheme of things. You should see my NAT translation table :(


Thanks for the help, p.


-Shikamaru

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