Re: QOS Bc Be and Tc

Hi Carlos,

I know Bc/Be are both amounts of data..But as per my knowledge Bc/Be does
need to happen in a single Tc and they relate directly to CIR/PIR; they are
in fact the amount of data that can be sent during one Tc interval.

http://qospf.wordpress.com/2008/06/01/ccie-cert-guide-notes-qos-part-iv/
*Be = Tc * (PIR  CIR);

*The maximum Be is in fact found by replacing the PIR by the AR (Access Rate
of the interface).

Best Regards,

On Wed, Aug 18, 2010 at 10:41 PM, Carlos G Mendioroz <tron_at_huapi.ba.ar>wrote:

> Karim,
> Be does not need to happen in one Tc, as your message implies.
> Also, Be is not a rate, is just an ammount of burst.
> That's the difference between one rate and two rate policers,
> and so PIR is not directly related to Be.
>
> -Carlos
>
> karim jamali @ 18/8/2010 16:20 -0300 dixit:
>
> Hi Masroor,
>>
>> Well the simplest and easiest way to think about it (whether with
>> policing/shaping) is that you are connected via a Fastethernet interface
>> to
>> a provider (however most probably you won't have 100 Mbps bought from the
>> provider), and one thing about interfaces they are either ON or OFF. So
>> you
>> can tell it please only send 2 Mbps or 3Mbps..Thus the only way to control
>> is to turn it ON/OFF in such a manner that you will be sending the rate
>> you
>> already bought.
>>
>> Thus the CIR is what you already agreed upon with the Provider. Bc is the
>> same as the CIR however within a smaller time frame Tc. Suppose for
>> instance
>> you are given 2Mbps (CIR) from a provider, and your Tc is 125 ms..Your
>> router will divide this one sec into 8 intervals (each a Tc long) and in
>> that Tc you will be only sending Bc (what is allowed to send in one Tc).
>> However note that to send the Bc you will require only a part of the Tc
>> for
>> instance half. Thus what is actually happening is you are sending for half
>> a
>> Tc, and putting your interface off for another half of the Tc with the aim
>> of getting an average of 2Mbps.
>>
>> Calculation Wise: 2Mbps/1 sec = Bc/Tc=CIR/1sec thus you can calculate the
>> Bc.
>> Then check how much time it takes you to send the Bc (based ont he
>> interface
>> speed).
>> time required= Bc/(Access Rate of interface) = (this time will be
>> definitely
>> less than Tc)
>>
>> Thus in every Tc interval you will be ON for this time required/off for
>> the
>> remaining time. Note that for VOIP applications it is advised that the Tc
>> be
>> small so that your interface doesn't turn off for a long time and break
>> the
>> 150ms rule.
>>
>> Regarding Be, is in fact what happens if you exceed the rate that your ISP
>> has given you..It happens in a way that suppose for one of the Tc
>> intervals,
>> you undersubscribed your utilization, i.e. you didn't reach your Bc limit,
>> the provider might be kind enough to keep you some balance, and in another
>> time you may be allowed to burst above Bc. Thus in one Tc it might be
>> possible that you send (Bc + Be) the Be being the access in your account.
>> Same formulas/same concept with CIR but this time with PIR. Supposing that
>> the ISP tells you I am comitted to serve you 2Mbps but at times you may be
>> allowed to get a peak PIR 3Mbps.
>>
>>
>> Hope this helps..I guess for more information you can refer to INE's blog
>> as
>> Petr's posts are great.
>> http://blog.ine.com/page/3/?s=QOS+Shaping
>>
>> Best Regards,
>>
>> On Wed, Aug 18, 2010 at 9:52 PM, masroor ali <masror.ali_at_gmail.com>
>> wrote:
>>
>> hi,
>>>
>>> Can anyone explain in easy wordings that what is Bc Be and Tc
>>>
>>> --
>>> Regards,
>>> Masroor Ali
>>>
>>>
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>>
>>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina
>

--
KJ
Blogs and organic groups at http://www.ccie.net