
MT90500
47
Using for an example the case of N=6, we would program the AAL1 scheduler with 6 cell events over 46/47
frames:
(7 turns * 47 frames/turn * 6 bytes/frame) + (1 turn * 46 frames/turn * 6 bytes/frame)
= 375 frames * 6 bytes/frame
= 2250 bytes
and
6 cells/turn * (7 turns * 47 bytes/cell + 1 turn * 46 bytes/cell)
= 6 cells/turn * (375 bytes/turn)
= 2250 bytes
To support AAL1 Nx64, one of the MT90500 schedulers can be set up with one turn of 46 and 7 turns of 47
(long end = 46, short end = 45, long/short = 7). We program N cells into the first 46 frames of the scheduler,
and we know that the 8 turns of the scheduler will take 375 frames, during which we will send N * 8 cells,
containing N * 375 bytes of TDM data. A typical application of the MT90500 might have the last of the three
schedulers set this way to support AAL1 Nx64.
4.3.1.3.4
Other Considerations
The Transmit Circular Buffer operation of the MT90500 is slightly more complex than the theoretical examples
above. The MT90500 uses a four-frame buffer to optimize TDM data transfers, and this means that the Transmit
Circular Buffers are actually written four bytes at a time, every four frames. For this reason the schedulers in
the MT90500 operate on the quad-frame by default. In addition, the long/short operation of the AAL1 scheduler
requires that an extra byte be held in the Transmit Circular Buffer. Finally, the number of frames between cells
must be an integer. These extra considerations mean that in general a cell will not be transmitted when there
are exactly M/N bytes in the Transmit Circular Buffer, but at some point afterwards. This adds slightly to the
transmission delay, but does not require attention on the part of the user, except in the programming of the
“Circ. Buff. Pnt.” field of the Transmit Control Structure (see below).