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Appendix A: Transmit FIFO Port Rates
This appendix describes the arbitration mechanism used to control and prioritize
the Transmit FIFO port rate. The CN8223 has two options for its ATM layer
interface: UTOPIA or FIFO mode. UTOPIA mode is discussed in the body of the
data sheet. FIFO mode provides four bidirectional ports. Since they operate
independently, multiple ports may attempt to send data during the same cell slot.
When these collisions occur, the CN8223 implements an arbitration mechanism
to select which port will transmit data.
The CN8223 combines rate control and priority when selecting a port. First,
the rate control circuit determines the eligibility of the port. Then the CN8223
arbitrates between eligible ports on a priority basis. This entire process is repeated
for each cell slot. A cell which has lost arbitration due to priority may still be
determined ineligible for transmission during the next interval.
The priority and rate setting of a channel must not be changed once
transmission has begun. This will cause unpredictable behavior. It could cause a
port to stop transmitting. Users can change both priority and rate for ports during
an “idle” interval. During such an interval, all ports, including unused ports,
should assert FIFO empty flags.
A.1 Rate Control
The CN8223 provides a rate control mechanism to establish fairness. As
described in the specification, each port is assigned a maximum rate through a
register. This rate is based on a 256-cell slot interval. Each port can be allowed to
transmit on a percentage of these cell slots. This is accomplished by assigning a
rate between 0 and 255 in the TX_RATE_xx register. A rate of 0 inhibits
transmission on a port, while a rate of 255 allows transmission in any slot. This
rate establishes an upper bound on an individual port’s rate. It does not guarantee
a rate for ports sharing a priority with, or at a lower priority than, other active
ports.
Table A-1 lists thresholds for each port’s TX_RATE value. In a given cell slot,
these thresholds are compared with the TX_RATE_xx register for the port. If the
TX_RATE value is greater than or equal to the threshold, then the port is eligible
to transmit in that slot. Otherwise, it is ineligible. Note that each port is unique in
its transmission characteristics for a given TX_RATE value. For instance, with a
TX_RATE setting of 127 (50 %), Port 0 will be eligible for eight cells, then
ineligible for eight cells. Port 3 will be eligible every other cell. Both ports may
transmit on 50 % of the cell slots, but the burst length varies.
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