Re: Shaping and calculating Be value for a 30 second burst

Please see inline:

Muzammil Malick @ 25/04/2010 19:53 -0300 dixit:
> Hi
>
> I, ok so I was one of the below until a few days ago:
>
> /if you ask legendary network engineers (60+ y.o!) they'd say "Bc is CIR
> per TC
> and Be is AIR-CIR per Tc". Actually their training was simple and
> effective...hence again you can say both Bc and Be are referring to the
> number
> of bits and not rates. Technically it's true to say my Bc or my Be is 32000
> bits but it's not true when people say hey the Bc is now 32Kbps./
>
> but after reading emails on GS I have come to understand that I do not
> really understand this aswell as I should.
>
> Carlos, I answered a question on traffic shaping the other day in which
> it stated "allow burst up to port speed if credit has been accumulated"
> You replied to the same thread saying that because Be was not Tc related
> the question should state how long Be applies for.
> In the question in this thread the question states that bursting is
> possible for 30 seconds.
>
> My question is how does stating how long we can burst up to line rate
> affect the Be?

The math of how much you are allowed to transmit is done synchronically,
i.e., at regular intervals that we call Tc.
Say Tc is 1 sec. Bc = 8 means you can send 8 bits (1 byte) per second (Tc).
Now, this is fine if you transmit at the same time all the time, but
what if you tx in a bursty way ? Well, you may be allowed to save some
tx allowance by transmitting at a lower pace (say, not transmitting at
all). How much can you save ? Be.

If you have a Be of 1000, can you tx at 1Kps ? May be, may be not.
Because you have another limit which is your access rate.

>
> e.g if AIR=128k CIR=64k Bc=8000
>
> if we can Be to port speed for 1 second the Be will be 8000, if we can
> Be to port speed for 30 seconds the Be will be 8000.

Hmm, first thing I would calculate is Tc. This is CIR/Bc, i.e. 125ms.
Being you AR 128k, that means that every Tc you can transmit, at most,
16000 bits. And in 1 second there are 8 Tcs, in 30 seconds there would
be 240 Tcs.
Given that you have Bc=8000, with just 8000 bits more per Tc you can
reach AR. To keep your Tx at AR for 240 Tcs you need 240 x 8000,
or 1.920.000.

> Doesn't this all just depend on how much credit is accumulated?

Yes, and that is Be exactly. Be is not how much you can overspeed in
one Tc...

If this sounds complicated, just go stare the Stallings book cover
until you get the idea of how this works. :)

http://www.pearsonhighered.com/educator/academic/product/0,,0139737448,00%2ben-USS_01DBC.html

-Carlos

>
> Thanks in advance
>
> On 25 April 2010 22:56, Carlos G Mendioroz <tron_at_huapi.ba.ar
> <mailto:tron_at_huapi.ba.ar>> wrote:
>
> I would second the move to understand it.
> Bc is Tc related. Tc is your clock, Bc is how much credit you get
> at every tick to transmit.
> Be is not Tc related, so can be much larger than Bc, or even 0.
> And it is how much credit you can save as long as you spend (transmit)
> less than your Bc bits. You can not spend all of it at once, since
> no matter how much credit you have, AR is a hard limit on the speed
> of the link.
>
> Now Bc/Tc is a base rate that is accounted (CIR). If you want to rev up,
> your only need credit for the part over CIR up to AR. Hence the AR-CIR
> count.
>
> -Carlos
>
> Ladee Geek @ 25/04/2010 16:10 -0300 dixit:
> > I have a question about determining the Be for generic traffic
> shaping.
> > I've looked through the QoS exam guide, QoS configuration guide,
> the doc cd
> > and haven't seen enough to be able to figure this one out.
> >
> > AR - 1.544 mb
> > Cir - 768k
> > Tc - 125 ms
> > burst for 30 sec up to line rate
> >
> > I understood Be to be AR - Cir.
> >
> > The answer key for this task states that the Be is calculated by
> taking
> > the line rate and multiplying it by 30 sec?
> >
> > 1544000bps*30s = 46320000bits
> >
> > There's just no way to put that many bits on the wire in 1 sec.
> Or is the
> > Be the number of tokens to be replenished in 1 sec ( same as 5790000
> > bits/Tc)
> >
> > And while I am at it, if you've found a good reference please pass
> it on.
> >
>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar <mailto:tron_at_huapi.ba.ar>>
> LW7 EQI Argentina
>
>
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-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net