From: gladston@br.ibm.com
Date: Wed May 25 2005 - 21:38:15 GMT-3
Let me try.
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So my first question is which one is correct?
Second, are they calculated the same for both Frame-Relay and GTS?
Last, What in the flying heck is ar-cir ?
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Suppose you are said to configure Tc equal 50ms. CIR is given to you, with the value 64000.
By default, TC for FRTS (frame relay traffic shaping) is 125ms, so Bc would be 64000 divided by 8 time slots (there is 8 125ms slots on 1 second). So by default Bc would be 8000 bits. This is how many bits is transmitted over 125ms, giving a total of 64000 bits in 1 second.
If the task say we should change Tc, you need to configure Bc to get the right value for Tc.
As you said, Bc=CIR*TC
Voice
It is recommended to configure a Tc of 10ms for voice, and Be of 0.
Try to understand the reason, so it will be easy to remember than the formula.
Why Be equal 0:
Because if you transmit over what you get from the Service Provider, it will discard when there is congestion. Voice quality will suffer.
Why Tc of 10ms:
Because we do not want to wait 125ms to transmit our voice data.
It can be a little confuse here, but let see what I get: even though TC is 125ms, the rate at which the router transmits is the clock rate.
So, consider we have a WAN circuit of 512kbps (clock rate = AR = access rate), a contracted CIR of 128kbps and a TC of 125ms;
The router can transmit 16kbps on one TC, but the router can transmit 16kbps in much less time than 125ms, because it transmits at Access Rate. Than, it will transmit 16kbps and will wait for the next time slot. Our voice packets will be waiting for it.
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