From: Navin MS (navin_ms07@yahoo.com)
Date: Thu Jun 08 2006 - 01:16:12 ART
Yes, When it comes to plain English.. a burst is thought of something in excess that a router is
sending into the FR cloud. BUT from a frame-relay perspective, it simply says how many bits in
burst can be sent in one Time interval. Bc just gives us bursting capability which is very much
required given the sporadic nature of real-time traffic. I have an easy alternate way of
understanding this. One can look at it this way.
For a customer there are 4 options...
1) Send traffic at a constant rate of CIR (in which case Bc of 4000 is unused). Though you were
allowed to burst, you didn't :)
2) Send a burst of Bc bits per Time interval (in which case you are using Bc of 4000 bits). When
you do this, you are effectively sending CIR bps (Bc/Tc) as agreed.
3) Send a burst of (Bc + Be) bits per Time interval (if Be is configured, in which case you are
using Bc of 4000 and Be of 2000 say). When you do this, you are effectively sending more than CIR.
That is, your Tx rate = (Bc+Be)/Tc which is greater than (Bc/Tc).
4) Send traffic which is constant for some part of the Time interval and random burst for some
other part of Time interval. Eg: For a period of say, Tc/2 you sent 3000 bits at a constant
bit-rate and another Tc/2 you send a random burst of 1000 bits. Together, you sent 4000 bits in
that interval. THAT CONFORMS TO CIR. Now, if you send anything more than 4000 bits, you will be
using the extended Burst capability (Be).
Similar reasoning goes for policing too.
Just my 2 cents.
Naveen.
--- Godswill Oletu <oletu@inbox.lv> wrote:
> Roberto,
>
> Though, I have not checked the question to see if it is shaping or policing,
> but if we assume that it is policing as you have just said, I am lost at
> how:
>
> Police cir 128000 bc 4000
>
> Will result in a burst, as the original poster indicated. The command above
> is just telling the router that, in order not to create a scheduling
> concern, please kindly send 4000 bytes per burst or timing interval (Tc).
>
> Bc is committed burst, you have paid for 128Kbps and you are telling the
> router to allow you to burst your traffic from 0 to 4000 per timing interval
> (Tc), and this commitment is based on your CIR. The concept of burst as we
> understand it in everyday English only start coming into play with Be or
> excess burst.
>
> Also, if you look at the computation you just did; you are equating CIR to
> be the same the same value as Bc, but using different metrics, which might
> not be the correct thing to do.
>
> The CIR given was 128kbps, convert this to bytes will be be 128000 / 8 =
> 16000 bytes and you divided that by 4 to get 1/4Bc (4000 bytes) that was
> asked for.
>
> Let assume that a full Bc was asked for instead of 1/4Bc, you see that your
> computation will make Bc = 16000 bytes and this is the same amount of
> traffic our CIR is allowing us to transmit per second. In a one second
> traffic snapshot, the difference between 128kbps and 16000 bytes is the same
> as the difference between $1 and 100 cents.
>
> As you can see, that might not be the correct formula to computer Bc either
> in policing or in shaping.
>
> HTH
> Godswill Oletu
>
>
> ----- Original Message -----
> From: "Roberto Fernandez" <rofernandez@us.telefonica.com>
> To: "Brian McGahan" <bmcgahan@internetworkexpert.com>; "Godswill Oletu"
> <oletu@inbox.lv>; "Leigh Harrison" <ccileigh@gmail.com>; "Andi Bennett"
> <bigandibennett@yahoo.com>
> Cc: <ccielab@groupstudy.com>
> Sent: Wednesday, June 07, 2006 2:21 PM
> Subject: RE: FRTS Lab 1 - 7.2 question.
>
>
> Friends,
>
> Well... checking into 7.2 question of Version 3 WB, it is not a question
> about FRTS it is about policing.
>
> With FRTS Bc is bits, but with policing is Bytes, thus 128 kbits/4 = 32
> kbits => 4K Bytes
>
> The answer says
>
> Police cir 128000 bc 4000
>
>
>
> Best Regards,
> Roberto
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Brian McGahan
> Sent: Tuesday, June 06, 2006 9:45 AM
> To: Godswill Oletu; Leigh Harrison; Andi Bennett
> Cc: ccielab@groupstudy.com
> Subject: RE: FRTS Lab 1 - 7.2 question.
>
> > I think it has to do the value of the default 'Tc' (the Tc value used
> when
> > none is supplied) which is 125ms.
>
> In this case yes, but not always. The default Tc is a function
> of the CIR. The higher the CIR the lower the maximum (and default) Tc.
> Change the "frame-relay cir" value in a map-class with no other values
> configured and look at the "show traffic-shape" output. As the CIR goes
> up you will see the Tc go down.
>
>
> HTH,
>
> Brian McGahan, CCIE #8593
> bmcgahan@internetworkexpert.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987 x 705
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>
>
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of
> > Godswill Oletu
> > Sent: Tuesday, June 06, 2006 7:23 AM
> > To: Leigh Harrison; Andi Bennett
> > Cc: ccielab@groupstudy.com
> > Subject: Re: FRTS Lab 1 - 7.2 question.
> >
> > LH,
> >
> > Maybe, you are talking about 'fragments' which are in bytes,
> otherwise,
> > 'CIR' are in bits/second; 'Bc' in Bits and 'Be' in Bits.
> >
> > I think it has to do the value of the default 'Tc' (the Tc value used
> when
> > none is supplied) which is 125ms.
> >
> > Be=(AR- CIR) * Tc/1000
> > =(128000 * 125/1000
> > =16000
> >
> > 1/4 Be = Be / 4 = 16000/ 4
> > = 4000
> >
> > My only problem is that, the orignal posted mentioned 'Bc' instead of
> > 'Be'.
> > I do not see how 'Bc' will result in a Burst, except I am missing
> > something
> > here or in the question. 'Bc' simply tell us how much need to be
> > transmitted
> > per timing interval to conform to our 'CIR'; which is in our case
> every
> > 125ms and to conform to our 'CIR' of 128K, that value is 16000. 'Be'
> tell
> > us
> > that, after fulfilling our allocated quota and we have room, how much
> > above
> > CIR should we Burst?
> >
> > Since, the Access Rate (AR) was not given in the task, it is safe to
> > assume
> > that 'AR' is equal to 'CIR', making both 'Bc' & 'Be' to be equal and
> 1/4
> > of
> > Be will be 4000.
> >
> > Please, set me straight if I am missing something here.
> >
> > Thanks.
> > Godswill Oletu
> >
> >
> > ----- Original Message -----
> > From: "Leigh Harrison" <ccileigh@gmail.com>
> > To: "Andi Bennett" <bigandibennett@yahoo.com>
> > Cc: <ccielab@groupstudy.com>
> > Sent: Tuesday, June 06, 2006 6:39 AM
> > Subject: Re: FRTS Lab 1 - 7.2 question.
> >
> >
> > > Hey there Andi,
> > >
> > > CIR is in bits, Bc is in bytes
> > >
> > > 128,000 / 4 = 32,000 _bits_ per interval
> > >
> > > 32,000 / 8 = 4,000 _bytes_ per interval
> > >
> > > Hope that helps a bit,
> > >
> > > LH
> > >
> > > Andi Bennett wrote:
> > > > Dear group.
> > > > Being a bit crap at FRTS can somebody explain the answer to the
> Lab
> > 1
> > 7.2 question.
> > > >
> > > > 128K CIR and allow for 1/4th for burst.
> > > >
> > > > How does this come up with bc = 4000 ?
> > > >
> > > > Am I missing something here?
> > > > Cheers
> > > > A.
> > > >
> > > >
> > > >
> > > > Send instant messages to your online friends
> > http://uk.messenger.yahoo.com
> > > >
> > > >
> >
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