MT90500 Datasheet, PDF (45/159 Pages) Mitel Networks Corporation – Multi-Channel ATM AAL1 SAR

MT90500
4.3.1.3 Transmit Event Scheduler Overview
4.3.1.3.1 Introduction
The distinctive characteristic of AAL1, and the other Constant Bit Rate techniques supported by the MT90500,
is that they carry isochronous data, i.e. data that arrives at the SAR at a constant rate. For AAL1 Nx64 (an
AAL1 VC carrying ‘N’ TDM channels), the SAR has to transmit exactly N bytes in ATM cells for every N bytes
that arrive in TDM frames. The TDM port delivers one byte for each of N TDM channels every frame, 8000
times a second. If “on average” the SAR transmits less than, or more than, N * 8000 bytes/s the Transmit
Circular Buffer will eventually overrun or underrun (where “on average” is determined by the size of the
Transmit Circular Buffer). The MT90500 TX_SAR meets this requirement by using “schedulers” that are tied to
the TDM frame rate (125 µs). The MT90500 scheduler ensures that N bytes are sent out in ATM cells for every
frame of the TDM port.
4.3.1.3.2 Fixed TDM Payload Schedulers
The simplest case of Constant Bit Rate traffic is a VC using cells of fixed TDM payload size. This includes
• CBR-AAL0 cells, which always carry 48 TDM payload bytes or a fixed partial-fill TDM payload,
• AAL1 N=1 cells, which always carry 47 TDM payload bytes or a fixed partial-fill TDM payload,
• partially-filled cells, which always carry the same number of TDM payload bytes (4 to 47),
• CBR-AAL5 cells, which always carry the same number of TDM payload bytes (8, 16, 24, 32 or 40).
For a cell of constant TDM payload size ‘M’ carrying ‘N’ TDM channels, it can be seen that one cell of ‘M’ bytes
must be sent every M/N frames:
TDM in = M/N frames * N bytes/frame = M bytes
ATM out = one M-byte cell = M bytes
TDM in = ATM out
For this case of fixed TDM payload size, we could create a scheduler that was M/N frames long, and program it
to send one cell. Let us consider a simple example (simpler than typically required of a MT90500 scheduler). If,
for example, the SAR was to carry 6 TDM channels (N = 6) in a CBR-AAL0 48-byte cell (M = 48), we could use
a 8 frame scheduler (48/6 = 8) with one cell event programmed:
8 frames * 6 bytes/frame = 48 bytes = one 48-byte cell
If the Transmit Circular Buffers are considered, it can be seen that after one frame the six Transmit Circular
Buffer will each contain one byte of TDM data. After two frames, each of the six Transmit Circular Buffers will
contain two bytes of TDM data. After 8 frames each of the six Transmit Circular Buffers will contain eight bytes,
enough TDM bytes to fill a 48-byte cell, therefore the scheduler is programmed send a 48-byte cell every 8
frames.
For many values of N, however, M/N will not be an integer number of frames. So we could use a scheduler M
frames long, and program it to send N cells (spread out over M frames). Over M frames, with N bytes arriving
each frame, the SAR receives M * N bytes:
TDM in = M frames * N bytes/frame = M * N bytes
The SAR will send N cells of M bytes:
ATM out = N cells * M bytes/cell = M * N bytes
TDM in = ATM out
In our example of 6 TDM channels over CBR-AAL0, we could use a 48 frame scheduler, programmed with 6
cell events:
48 frames * 6 bytes/frame = 288 bytes = 6 cells * 48 bytes/cell
So over the course of 48 TDM frames the SAR receives 288 TDM bytes, and the SAR sends exactly 6 cells
containing a total of exactly 288 bytes. The Transmit Circular Buffers return to a constant level, and the
scheduler meets the requirements for CBR.
To support CBR-AAL0, one of the MT90500 schedulers can be set to a length of 48 (long end = 47, long/short
= 0), and N cell events programmed in. A typical application of the MT90500 would use a longer scheduler, set
to a multiple of 48 (see below). To support AAL1 N=1, (TDM payload ‘M’ = 47) one of the MT90500 schedulers
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