Appendix A: Transmit FIFO Port Rates
CN8223
A.2 Port Priority
ATM Transmitter/Receiver with UTOPIA Interface
A.2 Port Priority
The user can define a priority for each port. This priority can range from 0 to 3,
with 0 being the highest priority. A higher priority port will always preempt one
with a lower priority. This priority is programmable for each register in bits 3–2 of
the CELL_GEN_X register.
The user can configure multiple ports to be the same priority. Within the
lowest priority, arbitration is on a “round-robin” basis. For example, if Port 0 is
carrying CBR signals it should be assigned the highest priority. Ports 1 and 2,
carrying less jitter-sensitive data, can be programmed to a lower priority. Port 0
will preempt Ports 1 and 2 when data is available. As long as both ports have data
available, Ports 1 and 2 will alternate transmission in slots where Port 0 is not
active.
Only the lowest priority ports should share a priority. Round-robin scheduling
may break down for multiple ports sharing a priority above a lower priority port.
If the CN8223 has been transmitting at priority 1, and a priority 0 port preempts
this transmission; that priority 0 port will not share the channel with other ports. It
will continue winning arbitration until it no longer has data available. At that
time, another priority 0 or lower priority channels will be allowed to transmit.
A.3 Summary
The CN8223’s transmit rate shaping is controlled via a user-assigned priority and
rate for each FIFO port. The CN8223 scheduling attempts to provide a fair
arbitration algorithm and to limit burst transmission by a single channel on the
line. A system designer should consider each data stream’s characteristics when
choosing the port, priority, and rate to assign.
A-2
Conexant
10046C
CONEXANT [ CONEXANT SYSTEMS, INC ]
