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MT90500
148
16 possible VCs
128 possible VCs
1024 possible VCs
0.0625 Kbyte
0.5 Kbytes
4 Kbytes
B. RX_SAR Control Structures
Each control structure begins with 12 bytes of control data, and is then followed by 2 bytes of information for
each TDM channel. For the purposes of these calculations, all RX_SAR control structures are designed to
occupy an integer number of 16-byte blocks.
16 VCs (16 TDM channels per VC)
128 VCs (4 TDM channels per VC)
1024 VCs (1 TDM channel per VC)
0.75 Kbyte (16 * ROUNDUP(12 + 16 * 2) = 768 bytes)
4 Kbytes (128 * ROUNDUP(12 + 4 * 2) = 4096 bytes)
16 Kbytes (1024 * ROUNDUP(12 + 1 * 2) = 16384 bytes)
C. Receive Circular Buffers
A 64-byte buffer provides 8 ms of buffering capability (1 byte = 125
μ
s) while a 1024-byte buffer provides up to
128 ms of buffering. In this example, we give sizes for a 64-byte and a 1024-byte (64 / 1024) buffer for each
channel.
256 TDM channels
16 / 256 Kbytes
512 TDM channels
32 / 512 Kbytes
1024 TDM channels
64 / 1024 Kbytes
D. External Memory to Internal TDM Memory Structure
Four bytes of RAM are required for every TDM channel transmitted.
256 TDM channels
512 TDM channels
1024 TDM channels
1 Kbyte
2 Kbytes
4 Kbytes
E. Receive Data Cell FIFO holding 64 cells
(Refer to Section 4.5.4.)
64 at 64 bytes each
4 Kbytes
Sub-total external memory space requirements to support the ATM to TDM receive process:
Minimum requirements (256 channels, 64-byte buffers):
~22 Kbytes
Maximum requirements (1024 channels, 1024-byte buffers):
1052 Kbytes
Total Memory Requirements
Total external memory size requirements to support both the TDM to ATM transmit process and the ATM
to TDM receive process:
Minimum requirements:
44.7 Kbytes (256 X 64 kbps bidirectional channels with 16 VCs)
Maximum requirements:
1142.4 Kbytes (1024 X 64 kbps bidirectional channels with 1024 VCs)
Where the memory requirements are close to the physical size of the provisioned memory, the designer should
pay special attention to what memory address boundaries some structures may not cross. (See Section 7.4.2.)