ATM Pace Control
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
40-5
40.1.3
Programming APC Scheduling Table Size and NCITS
The size of the APC scheduling table is defined by the minimum bit rate desired for a single connection
and the number of cells transmitted in a time slot.
Defining:
P
Cell scheduling bit rate (usually equal to the PHY bit rate)
min_rate
Minimum data rate at which a channel can be scheduled
max_rate
Maximum data rate at which a channel can be scheduled
M
Minimum APC scheduling table size allowable for a particular configuration
The maximum transmit data rate for a single channel supported by the APC is:
The minimum transmit data rate for a single channel supported by the APC is:
For example, assuming a PHY transceiver with a transmit data rate of 51.84 Mbps, and assuming that the
APC is to be configured to use all of this transmit bandwidth, then P = 51.84Mbps. Also assume that no
single ATM channel (virtual connection) will ever require more than 25% of this bandwidth. By this
assumption, (P / max_rate) is 4, and therefore NCITS is chosen to be 4, using the max_rate formula
described above. Also, because the application has no single connection requiring a bandwidth less than
32kbps (i.e. min_rate = 32kbps), the minimum APC scheduling table size is therefore equal to:
This example deals entirely in integers; however, note that while M must be an integer, NCITS may
include an integer and a fraction. In any other application of this analysis, NCITS may be tuned in order
to achieve an exact integer value M.
Note that a fractional value of NCITS does not mean that a fractional number of channel numbers will be
written to the transmit queue. It means that the number of cells scheduled per APC timeout will vary
around the average defined by NCITS. For example, if NCITS were programmed to 2.5, then 2 cells would
be scheduled during one iteration of the APC algorithm, followed by 3 cells the next time, followed by 2,
and so on.
max_rate
P
NCITS
------------------
=
min_rate
P
M
1
–
(
)
NCITS
--------------------------------------
=
M
51.84Mbps
32kbps
----------------------------
4
----------------------------
1
+
406
=
=
Содержание PowerQUICC MPC870
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