NEC mPD98410 User Manual page 199

Q.13
How much is the output (re-queuing) throughput of the multi-cast queue?
A.13
The HTT & control memory is accessed in the following three ways during re-queuing of the multi-cast cell (= cell
storage address copy):
• The cell storage address is read.
• HTT Area-B is read.
• The cell storage address for each output queue is written.
In the case of multi-cast of 1:7, for example, two read operations and seven write operations are necessary for
processing one multi-cast cell. Because the µ PD98410 requires an idle period once between a read operation and a
write operation, ten accesses must be made to process one multi-cast cell.
Incidentally, the µ PD98410 operates using 44SWCLK as a basic cycle, and allocates at least 8 memory accesses
per basic cycle to multi-cast processing.
To process eight multi-cast cells, therefore, 56 cells (= 8 ports × 7 directions) are re-queued by using 80 access
timings (= 10 times × 8 cells). In this case, the re-queuing throughput is about 1.7 [Gbps] where SWCLK = 33 [MHz].
This exceeds the switching capability of 1.256 [Gbps]. As a result, cells are congested in the output queue.
56 (cells) × 53 (octets/cells) × 8 (bits/octets)
80 (times) 1
×
33 × 10
6
8 (times)
Next, cell congestion in the multi-cast queue is explained.
As described above, multi-cast of 1:7 requires 10 access timings for processing one multi-cast cell. This means
that a cell is read once from the multi-cast queue each time processing has been performed 10 times. Therefore, the
read speed of the multi-cast queue is equivalent to 254.4 [Mbps].
1 (cell) × 53 (octets/cells) × 8 (bits/octets)
10 (times) 1
×
33 × 10
6
8 (times)
If the total speed of multi-cast connection (total throughput of input cells) exceeds 254.4 [Mbps], cells are
increasingly congested in the multi-cast queue. If the speed does not exceed 254.4 [Mbps], up to eight cells are
congested in the multi-cast queue.
The value of 254.4 [Mbps] sounds as if the capability were low. If there are eight multi-cast connections of 1:7,
however, cells are congested in the output queue rather than in the multi-cast queue. Therefore, this value is
considered sufficient.
CHAPTER 7 FAQ (Frequently Asked Questions)
(sec/SWCLK) × 44 (SWCLK)
(sec/SWCLK) × 44 (SWCLK)
= 1.7808 × 10
9
(bps)
= 254.4 × 10
6
(bps)
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