From: Dennis J. Hartmann (dennisjhartmann@hotmail.com)
Date: Sat Mar 26 2005 - 17:48:13 GMT-3
For those of you who were contemplating why ABR was the right answer
(original E-Mail below) and not UBR, I believe I have found the answer with
a little guidance from Tim (ccie2be@nyc.rr.com).
The answer is definetely ABR based on ABR's ability to throttle down
to MCR in response to congestion experienced (CI) messages. ABR also
provides a bandwidth guarantee of MCR, while UBR does not. The best
explanation I found is that UBR is like UDP traffic at layer 4 (spray and
pray).
The question from Netmasterclass.net is worded in such a way that
you have to provide a minimum bandwidth guarantee, but no guarantee of cell
delivery.
Sincerely,
Dennis J. Hartmann
White Pine Communications
dh8@pobox.com / 914-980-0316
CCSI/CCIP/CCNP/CCDP/CCNA/CCDA
Cisco IP Voice Support & Design Specialist
Cisco Optical, VPN & IDS Specialist
MCSE
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Dennis J. Hartmann
Sent: Friday, March 25, 2005 8:56 AM
To: ccielab@groupstudy.com
Subject: ATM ABR/UBR
The following is part of a scenario from www.netmasterclass.net:
If I was asked a question to provide s minimum cell rate (MCR) and peak
cell rate (PCR) on a PVC without making an explicit bandwidth guarantee
ensuring cell loss is kept to a minimum using a feedback mechanism
(congestion control/CAC I believe), would I use ABR or UBR???
The answer I have from this practice lab, points to the use of ABR, but
I configured UBR+ with a MCR and PCR. I'm not sure what the difference is
between ABR and UBR, but I believe it has something to do with the feedback
mechanism. Other than the ABR definition I found on /univercd (below), I
was not able to find anything on UBR and congestion control feedback.
Does anyone have additional information and the why behind this being
ABR and not UBR?
In ABR transmission, the peak cell rate (PCR) specifies the maximum value of
the allowed cell rate (ACR), and minimum cell rate (MCR) specifies the
minimum value for the ACR. ACR varies between the MCR and the PCR and is
dynamically controlled using congestion control mechanisms.
Any feedback is appreciated.
Sincerely,
Dennis J. Hartmann
White Pine Communications
CCSI/CCIP/CCNP/CCDP/CCNA/CCDA
Cisco IP Voice Support & Design Specialist
Cisco Optical, VPN & IDS Specialist
MCSE
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