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Dedicating bandwidth for voice

My question is how to dedicate certain bandwidth for voice over a 64kbps serial leased line which presently carrying data as well as voice. Whenever there is a data transfer, we are getting lot of disturbance to voice. On both the ends of 64kbps leased line we are using cisco 3640 series router. I'm thinking of dedicating some bandwidth for voice.

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There are three immediate problems plus a number of smaller issues.

First, realize that each voice circuit will use somewhere between 14-24 kilobits per second (depending on the IP tricks you use to minimize the voice bandwidth). If you have two voice circuits active, you are using about 28 kilobits per second. That leaves about 30 to 36 kilobits per second for data, which isn't much.

The second problem is that a 64 kbps line will be S-L-O-W. A 64-byte VoX packet is going to take 80 or 90 milliseconds (from memory) to serialize onto the circuit. This will give you a LOT of delay. (At least you aren't on frame relay and have to worry about the jitter buffers etc.) A lot of delay will cause echo on the voice and will annoy users. If you are running an international link, add more delay for the propagation of the signal (speed of light is only so fast, but electrons go a lot slower...)

The third problem, and the easiest to solve, is implementing quality of service for the link. Have a look at this web page and modify your router configs according to this document on Cisco's Web site. It has all the information you need. I suspect you will be able to cookie cutter the configs to your situation:

http://www.cisco.co m/warp/public/788/voice-qos/voip-mlppp.html

This page also discusses the issues I raised above in more detail.

If you have any more questions, drop on over to the Network Administration Discussion Forum and I'll drop by every now and then and try to answer your questions. Look forward to seeing you there!!!


This was first published in July 2001

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