North American Network Operators Group

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Re: UBR at MAE-East ATM, anyone?

  • From: Kevin Oberman
  • Date: Tue Apr 18 12:58:33 2000


your description of how the MAE ATM NAPs work is not quite in line
with either my understanding or the ATMF traffic management specs.

As you state, the connections to the NAPS is ABR, but you then say 
"with PCR being twice SCR". If the connection is ABR (and it is), there
is not PCR or SCR. There is only MCR (minimum cell rate). This really
means that you are guaranteed that the bandwidth reserved by the
MCR will be available to you. Any traffic beyond this is treated on a
best-effort basis, exactly as if it was UBR. The only limit on what
you may inject (above MCR) is the line rate and there is no guarantee
that any of this traffic will make it through the fabric.

The best way to think of MCR is that it is much like CIR on a frame
relay circuit and that an MCR of zero, like a CIR of zero, can be a
practical and functional connection. At least a functional as a UBR

Since the MCR is committed bandwidth, it really can't properly support
over-booking. The sum of all of the MCRs for all of the PVCs connected
to the line should not exceed the capacity of the line, 96000 CPS for
DS-3 (PLCP), 353207 CPS for OC-3, or 1412830 CPS for OC-12. But, beyond
these limits, any further traffic injected on any PVC will be
forwarded on a best-effort basis exactly as it would if the switch was
running UBR.

A big problem is that many of the folks at the MAEs don't seem to
realize this and set the MCRs to excessively large values. How high
they should be set depends on how critical it is that a given
connection run without drops under peak loads. 

For most of our peerings (not those with the major NSPs where we have
multi-megabits flowing most of the time) we set the MCR on our PVCs to
a rather small value. I typically use 2600 CPS or about 1 Mbps and
these circuits typically run very clean.

Don't wish for the Ameritech UBR model (though I am very happy with
the AADS NAP). Educate people that they should not over-provision
their PVCs. If the port is running idle, they are are simply wasting
valuable (and expensive) capacity. And, if your router does not support
the specification of ABR PVCs with an MCR, keep this in mind when
specifying PCR and SCR, since these values are not really meaningful
in an ABR world. I typically set PCR to line rate and SCR to MCR with
a 100 cell MBS, but YMMV and I have not done enough testing to have
confidence that they are near optimal, only that they work pretty

R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: [email protected]			Phone: +1 510 486-8634