Frame Relay Terms

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Mar 29th, 2009
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Hi there

This is Samiullah Channa

I have a problem with understanding the Frame Relay Terms e.g:- (CIR, AR, Burst Rate, MBR, Burst Mode).

COuld you please guide me that how do these terms work?

I am simply confused with these terms>



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Giuseppe Larosa Sun, 03/29/2009 - 04:56
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Hello Samiullah,

FR provides a way to build logical circuits PVCs (one or more).

First of all there is a point-to-point serial link that connects the router (FR DTE) to the FR switch (FR DCE).

the speed of the access link to the FR cloud is the AR= access rate.

For each PVC, logical circuit some parameters are defined:

CIR = Committed Information Rate, this is the "guaranteed" rate that the FR provider agrees to provide to the VC in any conditions.

Data traffic has a bursty nature.

In addition to CIR the provider if his network has free resources can agree to allow for some bursts over the CIR.

To define how bursts are treated other parameters are defined:

the burst rate defines the peak rate for the VC.

Burst rate <= Access rate

MBR should be the maximum length of a burst expressed in packets or bytes see it as a train of packets sent at burst rate.

I'm not sure about what burst mode means.

Notice that the sum of all VCs rates (CIRs) can be greater then the access rate in this case (usually on central site) the link is oversuscribed.

Hope to help


sam_channa Sun, 03/29/2009 - 11:23
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Hello Sir Giuseppe, it's so nice of you and I am satisfied now, just one more little confusion is there, that which of the two things do we purchase:





lamav Sun, 03/29/2009 - 11:41
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You purchase both from the service provider.

The AR is the physical link speed, such as T1, T3, etc.

CIR is the rate at which the service provider guarantees to send your traffic.



sam_channa Sun, 03/29/2009 - 22:09
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sir thanks a lot, kindly tell me which other types of Access Services I can buy?

And I f I buy the Ar but I will just Access the CIR shall I still pay for Both?

If Pay for both then what is the advantage of buying T1/E1?



thotsaphon Sun, 03/29/2009 - 11:46
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AR stands for Access Rate. It's the rate that you've purchased from ISP. Let's say 1024 bps.

CIR stands for Committed Information Rate. ISP will guarantee you at this rate. let's say they will guarantee you at 984 bps. The excess traffic might be dropped at ISP.

All depend on a service contract you signed when you bought.



CriscoSystems Sun, 03/29/2009 - 21:43
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Hi Samiullah

As an additional explanation, each frame relay frame has a "discard-eligible (DE)" bit in its frame header. If you transmit data that exceeds your CIR, each excess frame has the DE bit set to 1. This way, when the service provider's network is congested, their equipment examines each frame and drops the ones whose DE bit is set to 1.


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