From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Mon Feb 09 2004 - 05:12:49 GMT-3
Ahmed,
Tc is in ms, Bc is in bits, and CIR is in bits per second.
Therefore your formula should actually read:
Bc_bits = CIR_bits_per_second * Tc_ms * 1_second/1000_ms
Bc = CIR * Tc/1000
Regarding fragmentation, the goal is to avoid a debt situation in
the shaping algorithm. When Cisco first implemented traffic shaping, there
was a bug in the code when you set the Bc less than the MTU of the
interface. When a packet that was greater than Bc came to the output queue,
it could not be sent because there could never be enough credit to send it
(assuming Bc + Be was less than the packet size). In order to deal with
this case when the MTU is greater than the Bc, the shaping algorithm can go
in to debt. The problem with going into debt however, is that it may
defeat the purpose of setting your Tc to be small.
By setting the MTU (fragment size) on the interface to be the
maximum amount of traffic that can be dequeued per interval (Bc), you are
guaranteeing that the worst serialization delay that a priority packet will
receive is the size of the Tc. In the case that your Bc is CIR/1000, this
serialization delay will be 10ms. This is not to say that the fragment size
relates to the access rate or port speed of the interface though. What the
shaping algorithm is trying to prevent is the sending at the access rate of
the interface for extended periods of time. Therefore your fragment size
should relate to what you are shaping to per interval, which is the Bc.
Also I hate to nitpick here, but the Bc is not "the number of bytes
that can be sent per Tc at the link speed." It is the number of bits per
interval you must average in order to achieve the CIR over the second.
For more info:
http://www.internetworkexpert.com/resources/01700368.htm
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987 x 705
Outside US: 775-826-4344 x 705
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Ahmed Mustafa
> Sent: Sunday, February 08, 2004 10:42 PM
> To: Michael Snyder; ccielab@groupstudy.com
> Cc: 'Bob Sinclair'
> Subject: Re: Quick question on Voip over Frame.
>
> Michael/Bob,
>
> Quick question for my knowledge.
>
> How did you come up with Bc. I used the formula, but getting a different
> value.
>
> Tc = Bc/CIR
>
> 125 = Bc/ 32000
>
> BC = 32000 x 125 =
>
> Can you guys helpe me out here.
>
> Thanks
>
> Ahmed
> ----- Original Message -----
> From: "Michael Snyder" <msnyder@revolutioncomputer.com>
> To: <ccielab@groupstudy.com>
> Cc: "'Bob Sinclair'" <bsin@cox.net>
> Sent: Sunday, February 08, 2004 6:24 PM
> Subject: RE: Quick question on Voip over Frame.
>
>
> > Thanks Bob, Good catch.
> >
> > I used the cir speed for my math, and put it into bits.
> >
> > Double error, glad I'm not playing baseball. :)
> >
> > Checked the command reference, it's in bytes like you said, the ios help
> > doesn't list it.
> >
> >
> > So it is total bytes per second (port speed/8) times (millisecond
> > interval/1000) = fragment size in bytes.
> >
> >
> > -----Original Message-----
> > From: Bob Sinclair [mailto:bsin@cox.net]
> > Sent: Sunday, February 08, 2004 3:52 PM
> > To: Michael Snyder; ccielab@groupstudy.com
> > Subject: Re: Quick question on Voip over Frame.
> >
> > Michael,
> >
> > Since the frame-relay committed burst, Bc, is in bits and the fragment
> > size
> > is in bytes, I don't think it would be correct to say that the fragment
> > size
> > would equal the Bc.
> >
> > The Bc should be the number of bytes that can be sent per Tc at the link
> > speed. For a 64000 bps link and a Tc of 10 milliseconds, that would
> > be a
> > fragment size of 80 bytes.
> >
> > 64000/8 X .01 = 80 bytes
> >
> > HTH,
> >
> > Bob Sinclair
> > CCIE #10427, CISSP, MCSE
> > www.netmasterclass.net
> >
> > ----- Original Message -----
> > From: "Michael Snyder" <msnyder@revolutioncomputer.com>
> > To: <ccielab@groupstudy.com>
> > Sent: Sunday, February 08, 2004 4:11 PM
> > Subject: Quick question on Voip over Frame.
> >
> >
> > > Using the requirements for Voip
> > >
> > > Port Speed 64000
> > > CIR 32000
> > > Tc 10 ms
> > >
> > >
> > > map-class frame-relay voip
> > > frame-relay cir 32000
> > > frame-relay bc 320
> > > frame-relay be 0
> > > frame-relay mincir 32000
> > > no frame-relay adaptive-shaping
> > > frame-relay fragment 320
> > >
> > >
> > > fragment type end-to-end fragment size 320
> > > cir 32000 bc 320 be 0 limit 40 interval 10
> > > mincir 32000 byte increment 40 BECN response no
> > > frags 22 bytes 1561 frags delayed 17 bytes
> > delayed
> > > 1201
> > >
> > >
> > > 1) Will the fragment size always equal Bc?
> > >
> > > 2) Should I use the `frame-relay ip rtp priority 16384 16383 32` with
> > my
> > > class or add my own priority queue to the physical interface?
> > >
> > > There's no firm requirements, just trying to figure out a good way of
> > > doing it.
> > >
> > >
> > http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a
> > > 00800d6788.shtml
> > >
> > >
> > > Michael Snyder
> > > Lead Network Engineer
> > > CCNP/DP, CSS1, MCSE NT/2000
> > > Revolution Computer Systems
> > > (270) 443-7400
> > >
> > >
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