Cell Relay Archive

[Paul Deneault <pdeneault@sympatico.ca>]

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 ? 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 ?

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.