PM73121ꢀAAL1gator II
Data Sheet
PMC-Sierra, Inc.
PMC-980620
,VVXHꢀꢁ
AAL1 SAR Processor
3.2.1 Transmit CDV
The following items affect transmit CDV:
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Cell scheduling,
Contention with other cells scheduled at same time,
Actual cell build time, and
UTOPIA contention.
1. The scheduler has a resolution of 125 µs. In other words, it works off a frame-based clock to
determine whether or not a cell should be sent during the current frame. Therefore, if the ideal
rate of cell transmission is not a multiple of 125 µs, there will be 125 µs of CDV. The sched-
uler will never add more than 125 µs of CDV.
For example, a single DS0 queue with no signaling and using full cells, will need to build a
cell every 47 frames. Therefore, a cell will be scheduled every 47 frames, and the scheduler
will add no CDV.
However, if signaling were added to the single DS0 queue, the extra byte that occurs every 24
bytes (assuming T1 mode) requires compensation. In this case, a cell will be sent every 46 or
47 frames. Therefore, there will be 125 µs of CDV due to the scheduler.
2. Only one cell can be built at a time. Thus if multiple queues are scheduled to send cells during
the same frame, additional CDV will be incurred. Since it takes approximately 8 µs to build a
cell, each cell that is waited for adds 8 µs of delay. When multiple queues are scheduled to
send cells at the same time, the cells will be built in sequential order, starting with 0 and going
to 256. Therefore, in a system that will be adding and dropping queues, the higher number
queues will experience more CDV than the lower number queues, depending on how many
queues are active at the time, and are scheduled within the same frame. Theoretically, it is
possible to have all 256 queues scheduled at the same time, however, this will not occur if
adequate steps are taken.
Minimize CDV due to clumping by the following actions:
Add queues one at a time (PMC-Sierra’s driver does this automatically). When queues are
added at the same time with the same configuration, they are clumped together. By adding
queues one at a time, clumping can be avoided and cell scheduling points can be evenly dis-
tributed. Also, queues are only added in frame 0. Since the transmit buffer is 128 frames (for
E1) or 96 frames (for T1), this equates to 16 ms (for E1) or 12 ms (for T1). Queues should be
added more than 16 ms apart (for E1) or 12 ms (for T1) to prevent them from being clumped
together.
Avoid configurations that will require sending a cell every n frames where n is an integer divi-
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PMC [ PMC-SIERRA, INC ]
