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RELEASED
PM73122 AAL1GATOR-32
DATASHEET
PMC-1981419
ISSUE 7
32 LINK CES/DBCES AAL1 SAR PROCESSOR
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
157
Figure 72 ADDQ_FIFO Word Structure
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Empty
Unused
Offset
Queue Number
It is necessary to understand the basics of the frame-based scheduling
performed by the chip to best utilize the Offset field in the ADDQ_FIFO. Read
Section 11.2.1.2 and pay particular attention to how the Transmit Calendar works
and to Section 11.2.1.2.1 and the information on clumping. The purpose of the
offset field in the ADDQ_FIFO is to reduce the amount of clumping.
In frame mode (SDF-FR), the first cell of a queue can be scheduled relative to a
reference value. When REF_VAL_ENABLE=0 in the LIN_STR_MODE memory
mapped register, the reference value is always frame 0. When
REF_VAL_ENABLE =1, the reference value is based upon the configuration
consisting of all 24 (T1) or 32 (E1) queues configured identically as single DS0
with no pointer, full cells, and no CAS connections which will be scheduled to
build cells during frame 0, 47, 94, 13, etc.. The reference value increments by
47 frames every time the current t_data_buffer frame write pointer is equal to the
current reference value. When the microprocessor wants to add a queue, it
writes the queue number and an offset to the ADDQ_FIFO. This offset is then
added to the current reference value, and the first cell is scheduled in the
resulting frame. For example, if two single DS0 queues are scheduled one right
after the other with offsets of 0 and 1 respectively, there will never be any
clumping because the first queue will be scheduled during frames 0, 47, 94, 13,
etc, and the second queue will be scheduled during frames 1, 48, 95, 14, etc..
Note that the reference value is optimized to the configuration consisting of all 24
(T1) or 32 (E1) queues configured identically as single DS0 with no pointer, full
cells, and no CAS. This is the configuration that is most likely to experience
clumping. By using the offset field, cells can be prevented from being scheduled
within the same frame even if queues were added some time ago. Software can
guarantee this by assigning an offset equal to queue number(4:0), or by keeping
track of which offsets are available. For non-single DS0 queues, the offset is
beneficial for initial offsets if all queues have the same configuration, but won’t
guarantee that a newly added queue won’t overlap with a queue started some
time ago. However, for non-single DS0 queues, there are less queues active per
line, and therefore clumping is less of an issue.
In multiframe mode (SDF-MF), the REF_VAL_ENABLE bit should be set only for
lines which have all queues configured identically as single DS0 queues with full
cells. When enabled, the reference value is based upon a configuration
consisting of all 24 (T1) or 32 (E1) queues configured identically as single DS0