From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Sat Apr 22 2006 - 21:33:27 GMT-3
The transmission time is a function of the serialization delay
on the transmit ring of the interface, not of the shaping configuration.
Take the size of the packet in bits and divide it by the interface
clocking in bits per second and you get the amount of seconds it takes
to transmit the packet; this is the serialization delay of that packet.
When shaping is configured all packets sent to the interface go
into the shaping queue. Those that are above the configured fragment
size are fragmented before being sent to the interface to be serialized
(sent). The rate a which packets or fragments are dequeued from the
shaping queue to the transmit ring is based on the CIR, Bc, and Tc. In
this manner if the maximum packet fragment size is less than the Bc it
will take no longer than one Tc for any single packet to be dequeued and
serialized.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
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> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> Kenny
> Sent: Friday, April 21, 2006 1:50 PM
> To: Scott Smith
> Cc: Cisco certification
> Subject: Re: IE Lab10 - FRTS
>
> Hi Scott,
>
> I was searching out for this question of Lab10 as well & happen to
bump
> into
> ur mail. I'm actually more concern on specifically when does shaping &
> fragmentation actually take place.
>
> I read through the Ciscopress QoS 2nd edition book and it seems that
> shaping
> actually occurs before fragmentation. Say there is now congestion on
the
> software queues, and a packet arrives (which is larger than 320bytes)
and
> exceeds the configured Bc, so it gets shaped inside the shaping queue.
> According to the book, it seems that only when the shaping scheduler
> decides
> to de-queue the packet only then it is checked if the packet actually
is
> larger than 320bytes, and if so it is fragmented. So it seems to me
that a
> single packet is actually taking more than 1 interval to be
transmitted
> when
> the packet is in the shaping queue since the actual fragmentation of
the
> packet only occurs after it is being shaped.
>
> Appreciate if u could clarify if I'm looking at this in the wrong
> direction.
> Thanks a lot.
>
> Cheers,
> K
>
> ----- Original Message -----
> From: "Scott Smith" <hioctane@gmail.com>
> To: "Moin, Imran" <imoin@virtela.net>
> Cc: "Group Study (E-mail)" <ccielab@groupstudy.com>
> Sent: Sunday, March 12, 2006 1:47 PM
> Subject: Re: IE Lab10 - FRTS
>
>
> > Tc = 10ms (smallest possible)
> > Bc = CIR * Tc
> > Bc = 256000 * .01 (10/1000 =.01)
> > Bc = 2560
> > fragment = 2560 / 8
> > fragment = 320 (fragment is in bytes so 2560 bits in bytes is 320)
> >
> > Changing the fragment to 320 ensures "that a single packet cannot
take
> > more than one interval to be transmited".
> >
> > -Scott
> >
> > On 3/11/06, Moin, Imran <imoin@virtela.net> wrote:
> >> Hello Folks,
> >>
> >> Can someone please help me understand how the values of Bc and
> >> frame-relay
> >> fragment were derived in Question number 8.1 (FRTS).
> >>
> >> Thanks,
> >> Imran.
> >>
> >>
This archive was generated by hypermail 2.1.4 : Mon May 01 2006 - 11:41:59 GMT-3