From: elping (elpingu@xxxxxxxxxx)
Date: Thu Jun 20 2002 - 00:34:58 GMT-3
Larry:
This is why I love this list ....you took the time to write all this down
clearly...
I thank you...
I undertand now .....
El ping
Larry Roberts wrote:
> Shaping mechanisms, such as Frame Relay Traffic Shaping (FRTS) and Committed
> Access Rate (CAR) use a token bucket mechanism. The token bucket basically
> regulates the amount of data that can be sent during a time period. Data can
> only be sent if there are enough free tokens in the token bucket equivalent
> to that amount of data. If there are enough tokens, the data is sent and the
> appropriate amount of tokens are removed from the Token Bucket. If there is
> not enough tokens, the router will buffer the data until the Token Bucket
> has enough tokens to send the data and then remove the appropriate amount of
> tokens. There are many terms and formulas you must understand to grasp this
> concept:
>
> CIR (Committed Information Rate) - This is the average amount of data that
> either you (or your service provider) wants you to be able to send
> Bc - (Committed Burst) - This is the amount of data that can be sent during
> a time interval. There is really no bursting at this point. This is just the
> amount of data that you are allowed to send each time interval to meet your
> CIR.
> Be - (Excess Burst) - Here is where bursting happens. This is normally the
> difference between your CIR and actual line access rate (or another
> pre-defined parameter by your service provider).
>
> Be + CIR = the depth of the token bucket
> Bc = the rate at which tokens are added to the bucket
>
> Now, in order to put all of this together we need one more important piece
> of data. That piece of data is the time interval or Tc.
> Tc = Bc/CIR
> This usually comes out to 0.125 or 125ms. This is the default setting on
> Cisco routers.
>
> Example - I have a 128k Frame Relay circuit. I have configured the CIR as
> 64k. By default, the Tc is 125ms. So, 0.125 times 64,000 = 8,000. Therefore,
> my default Bc is 8,000. Therefore, my token bucket fills at this rate and
> allows me to send 8k every 125ms. 125ms is one eighth of a second. So, 8,000
> times 8 equals 64,000 or 64k per second. Now, my actual line speed is 128k.
> I can configure a Be of 64k. This will allow my Token Bucket to fill to a
> capacity of 128k. This means that if I don't send any data for a period of
> 128,000 divided by 8,000 = 16 time intervals, which equals 2 seconds, my
> token bucket will fill to capacity and during the next time interval I can
> send 128k worth of data or burst to 128k. However, I would then go into a
> period of quite time where I couldn't send any data at all until the token
> bucket filled back up to CIR, which would be CIR (64,000) divided by Bc
> (8,000) = 8 time intervals or one second.
>
> Now, this is probably the perfect example of how to do this right. However,
> when you get into higher data rates, the quite time after the burst can get
> quite lengthy. Also, a Tc of 125ms works great for data, but this should be
> adjusted to 10ms if you are running Voice across the Frame Relay circuit.
>
> HTH,
> Larry Roberts
> CCIE #7886 (R&S / Security)
>
> ----- Original Message -----
> From: "elping" <elpingu@acedsl.com>
> To: <ccielab@groupstudy.com>
> Sent: Tuesday, June 18, 2002 11:43 PM
> Subject: CIR, Be, Bc
>
> > Can some one post an explanation of these frame relay
> > CIR, Be, Bc
> >
> > I am a bit confused on the whole thing ...i did not find the doc on
> > cisco helpfull
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