Cell Relay Archive

[Jon Gauthier <jgauthier@mitre.org>]

What class of service (CoS) do you need on the connection? CBR, VBR-rt,
VBR-nrt, ABR, UBR, UBR+? And what is the application? And is this
connection thru a public network or private?

You need specifics before you try to figure PCR. A CBR connection is
rate limited to PCR, as opposed to SCR (sustained cell rate) or MCR
(minimum cell rate). An ATM switch's call admission control (CAC)
algorithm allocates bandwidth differently depending on the traffic type.
A 15 Mbps CBR VC has a PCR of 15 Mbps with a low cell delay variance
tolerance (CDVT). A 15 Mbps VBR-rt connection may have an SCR of 15 Mbps
and a PCR of 20 Mbps, with a somewhat higher CDVT. A 15 Mbps UBR+
connection may have an SCR of 15 Mbps, an implied PCR of 148 Mbps, and a
maximum burst size (MBS) of 700 cells. You see - many very different
VCC's can be called a 15 Mbps connection...

On the other hand, you may be asking "My ATM switch needs its PCR
parameter in cells per second, not Mbps. How do I convert 15 Mbps to
cells per second?"

In that case, first find out from your ATM service carrier if 15 Mbps
includes cell headers or payload only. Here in the USA one carrier may
give you 15 Mbps "clear" by actually giving you a 16.56 or a 16.91 Mbps
VC. If it includes cell headers, then divide 15,000,000 by (53
bytes/cell x 8 bits/byte). Then the answer is 35,378 cps. If it is
payload only, then if the traffic is AAL-1, use a 47 byte payload (or
39,894 cps); if AAL-5, use a 48 byte payload (equates to 39,063 cps). 

A full SONET OC-3c line rate is 155.52 Mbps. SONET Payload available to
ATM is only 149.76 Mbps, or 352,207 cps. ATM payload available to the
various AALs is only 135.632 Mbps. Check out
http://www.msci.magic.net/papers/overhead.pdf for good information of
actual data rates available using different protocols... 

If I'm still way off base to what you're asking, then - nevermind! ;^)

Anupam wrote:
> 
> In article <3b2fec5f.28559520@news.atl.mediaone.net>, no@spam.invalid says...
> >
> >On Tue, 19 Jun 2001 20:01:51 GMT, anupam@yahoo.nospam.pls.com (Anupam)
> >wrote:
> >
> >>Hi all!
> >>
> >>Please help a newbie here. How does one
> >>figure out the appropriate PCR (peak cell rate)
> >>calculation for a 15 Mbps VC?
> >>
> >What is the lowest port speed of either end?
> 
> It is 155 Mbps (OC3 interface) SC-to-SC fiber cabling on both end.
> I carved out 15Mbps PVC but dunno how calculation work for PCR.