RELEASED
PM73123 AAL1GATOR-8
DATASHEET
PMC-2000097
ISSUE 2
8 LINK CES/DBCES AAL1 SAR
into underrun. In order to prevent the underrun, an additional (47 -
frames_per_cell) frames are buffered before data is sent out. This buffering
when combined with the frames_per_cell + 1 that is buffered on the transmit side
results in a total of 48 frames of buffering. The result is every DBCES
connection has the same amount of delay regardless of the number of active
DS0’s. This delay is (48 + CDVT) * 125 us. The calculation to determine how
much extra buffering is needed in DBCES mode is calculated by RALP every
time the queue exits underrun or exits from an all idle state. This buffer
adjustment guarantees that the delay for a given channel does not change
regardless of how many other channels are active or inactive. This is due to the
fact the total buffering between the transmit side and receive side is always
equal to 48 frames. If there is only one channel active, 48 frames will be
buffered on the transmit side and 0 frames (excluding CDVT) on the receive
side. If 31 channels are active, then 3 frames will be buffered on the transmit
side and 45 frames will be buffered on the receive side.
Figure 51 below shows the DBCES receive side buffering as a function of time.
The y-axis represents the buffer depth in frames and the x-axis is time in
microseconds. The CDVT in this example is set for 1 ms (8 frames) and initially
31 channels are active. The first 6600 us are spent on the DBCES buffering and
CDVT buffering (45 frames + 8 frames = 53 frames).
Once the buffering is complete, data playout begins. In Figure 51 below playout
for 31 channels is represented by the first saw tooth wave. On average a cell is
received every 188 us and it takes 188 us to play a cell’s worth of data onto the
line. The second saw tooth wave represents one active channel. The wave
begins at 8700 us and a cell is received on average every 5858 us. The third
saw tooth wave represents 8 active channels. The wave begins at 20413 us and
on average a cell is received every 731 us.
Note that if the buffer adjustment was not made, the queue would have underun,
when all channels but one went inactive.
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