Cell Relay Archive

[Walt Weber <waltweber@sprintmail.com>]

Paul Deneault wrote:

> Thanks for the response.  So let's see.  If I set a PCR=55000c/s and
> SCR=45000c/s with MBS at 1200
> cells over a cell rate of 46200 will be tagged with CLP=1 ?

Close.  Assume no cells have been sent.  The first 1200 cells above SCR (and less
than PCR) will be switched unchanged.  After that, cells that arrive at a rate >
SCR < PCR will be tagged....but it's a little more complex than that.  Since the
ratio of your PCR to SCR is about 5/4, about 1 cell out of 5 will be tagged,
assuming you are sending at PCR.  At least, this is what I've seen from ATM
switches.



> Hmmm.  Ok here's
> another question.
> What should the CDVT be in this instance ? even if you'd have to guess ?  I
> know it depends on the
> interface.  Let's say a T3.  Would CDVT=20000 usec make sense ?

20000 is a large CDVT for a DS3, especially with a cell rate of about half a
DS3.  It all depends on how 'clumped' your cells are.  If the traffic is shaped
to PCR, you could probably get by with a CDVT of 250 or less.  Shaping makes you
cells evenly spaced so CDVT isn't a big issue.  On the other hand if you don't
shape, you need to find out how clumped you cells are to determine CDVT.
Normally, it is calculated for the number of back-to-back cells in the cell
stream.


>
>
> Cheers,
> Paul
>
> Walt Weber wrote:
>
> > Paul Deneault wrote:
> >
> > > Hello,
> > >
> > > Could somebody please explain the relationship between PCR, SCR and MBS
> > > has it pertains
> > > to a VBR channel ? Ok so I establish a VBR channel with the settings
> > > PCR=55000 cells/sec
> > > SCR=32000 cells/sec
> > > MBS=1200 cells/sec
> > >
> > > Do these numbers make sense ?  what should they be ? is there a rule of
> > > thumb ? Outside of the "suck it and see" approach, is there a forumale
> > > to use ?
> > >
> > > Cheers,
> > > Paul
> >
> > Don't know about rules of thumb, but....
> >
> > ATM switches in the network I work with police vbr as follows:  Cells at or
> > below SCR are passed with no change.  Cells above PCR that exceed the CDVT
> > are discarded.  Cells between PCR and SCR are passed with no change until
> > the MBS value is exceeded, then these cells are tagged.
> >
> > BTW, the switches I am familiar with measure MBS in cells, not cells per
> > second.
> >
> > I"m not familiar with routers, but it seems to me that if you are running
> > IP over ATM AAL5 and want the all your traffic to end up in high priority
> > ATM cells, you should have a MBS that matches with the size of the largest
> > IP packet.  Then you can send IP packets at SCR that burst up to PCR.  In
> > other words, each packet is sent at PCR, but the packets are spaced to keep
> > the average rate at SCR.  On the other hand, if you want faster
> > transmission and can live with tagging of ATM cells, you can set up the
> > CDVT to allow a whole IP packet at the line rate.  You can then send at PCR
> > with bursts at line rate.