Cell Relay Archive

[Walt Weber <waltweber@sprintmail.com>]

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.