CCRM Closed-loop Flow Control
ATM Services Configuration Guide for CBX 3500, CBX 500, GX 550, and B-STDX 9000 1/19/055-9
Beta Draft Confidential
CCRM Closed-loop Flow Control on a UNI (Traffic Shaping)
For information on CCRM closed-loop flow control on a UNI, see “Per-VC Traffic
Shaping” on page 5-16.
CCRM Cell Generation
Any port on an IOM can generate CCRM cells. CCRM cells are generated at the
RM cell generation rate.
When a CCRM cell is generated:
• The Direction bits (DIR bits) and backward indicator (BI) bits are set, indicating
that this is a switch-generated backward RM cell.
• The CI and NI bits are set according to the current congestion status of the VC.
The destination ATM switch periodically sends backward binary notification through
CCRM cells to the source ATM switch, indicating the state of the destination ATM
switch’s queue for a VC. The binary notification is reflected in the CI and NI bits of
the CCRM cell. The CCRM cell indicates either a cell rate increase or decrease. The
source ATM switch then responds by adjusting the cell rate accordingly for that VC
and terminates the CCRM cell.
CCRM Cell Termination
When you configure the FCP to terminate CCRM cells, the FCP decides whether or
not to increase or decrease the ACR. This decision is based upon one or more of the
following:
• The local port congestion state
• The current ACR being above the fair bandwidth for the VC
• The CI and NI state in the CCRM cell
The fair bandwidth for a VC is the proportional allocation of the total bandwidth for
managed (non-real time) circuits. This allocation is based on the MCR of the VC
relative to all of the managed VCs. The total, non-real time bandwidth is the total port
bandwidth, less the bandwidth allocated to unmanaged (real-time) circuits and
point-to-multipoint (PMP) non-real time circuits.
Note that the FCP can increase the ACR well beyond its fair bandwidth. Once other
circuits attempt to use that bandwidth (which causes a congestion condition), the FCP
will throttle back the ACR towards the fair bandwidth for the circuit until the
congestion condition is removed.
For general RM cell termination considerations, see “RM Cell Termination” on
page 5-6.
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