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Visitor
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Registered: ‎12-19-2018

Calculating PCIe Bandwith - WP350

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Currently I am trying to take a deeper look into the inner workings of PCIe.

I've found your whitepaper on "Understanding Performance of PCI Express Systems (WP350 (v1.2))".

In the Chapter "Symbol Encoding" on page 2 it is explained what the 8b10b encoding is, and that this symbol encoding is the cause of 20% loss of throughput. So far, so good...

In Chapter "Write Transaction Throughput" on page 12 an example calculation is done for the throughput of 200 write transactions (Example 1). But what I am missing in this calculation is the consideration of the 8b10b encoding. Shouldn't there be a 20% addition somewhere in Step 4, caused by the used encoding, or am I missing something?

 

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Explorer
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Registered: ‎08-14-2013

It's considered in the byte transfer time of 4 ns.  The raw bit rate is 2.5 GT/s after 8b/10b encoding.  Before 8b/10b encoding, it is 2 Gb/s (see table 2).  It takes 8 bits / 2 Gb/s = 4 ns to transfer each byte. 

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Explorer
Explorer
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Registered: ‎08-14-2013

It's considered in the byte transfer time of 4 ns.  The raw bit rate is 2.5 GT/s after 8b/10b encoding.  Before 8b/10b encoding, it is 2 Gb/s (see table 2).  It takes 8 bits / 2 Gb/s = 4 ns to transfer each byte. 

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Visitor
Visitor
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Registered: ‎12-19-2018
I really don't know how i missed that (multiple times). Thanks!
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