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.
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:
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
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