Frequently Asked Questions About the FCP
ATM Services Configuration Guide for CBX 3500, CBX 500, GX 550, and B-STDX 9000
1/19/05
6-17
Beta Draft Confidential
How is fair bandwidth determined?
In the event of network congestion, the FCP on the IMA IOM may, under certain
circumstances, adjust the rates for active circuits evenly among the available
bandwidth, as opposed to their fair bandwidth value. The fair bandwidth value
determines the amount of available Non-Real Time (NRT) bandwidth assigned to a
particular circuit based on the minimum cell rate (MCR) and, for VBR-NRT circuits,
the sustainable cell rate (SCR).
When congestion occurs at a DS1/T1 channel on the IMA IOM, the FCP alleviates the
congestion by throttling back (also known as back pressuring) the rates of all active
NRT circuits that it manages by setting one or more throttle bits.
The following issues affect the operation of the FCP on the 3-Port Channelized DS3/1
IMA and 1-Port Channelized STM-1/E1 IMA IOMs:
•
The time required to poll each of the 84 DS1/T1 channels on the 3-Port
Channelized DS3/1 IMA IOM or the 63 E1 channels on the 1-Port Channelized
STM-1/E1 IMA IOM
By contrast, FCP-enabled IOM1 modules have a maximum of only eight channels
or ports to poll.
•
The effects of the increased delay required to poll each of the 84 DS1/T1 channels
on the 3-Port Channelized DS3/1 IMA IOM or the 63 E1 channels on the 1-Port
Channelized STM-1/E1 IMA IOM
If this delay is long enough, the FCP continues to forward traffic to the egress
port. In the absence of any other flow control mechanisms, such as received RM
cells, the FCP cannot determine whether downstream congestion is present. If this
continues, the non-FCP buffers will overflow, resulting in “silent” cell drops.
To minimize the likelihood of “silent” cell drops, the FCP includes a peripheral
component known as the Port Congestion Monitor (PCM). The PCM monitors the
state of the queue on each of the channels and sets the throttle bits as appropriate.
Note – This information applies only when using the ATM FCP on the following
modules:
– CBX 500 3-Port Channelized DS3/1 IMA IOM and 1-Port Channelized STM-1/E1
IMA IOM.
– CBX 3500 3-Port Channelized DS3/1 IMA Enhanced module and 1-Port
Channelized STM-1/E1 IMA module
Summary of Contents for CBX 3500
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