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PCI Express 3.0 - Double The Speed Without Doubling The Clocks PCI Express 3.0 - Double The Speed Without Doubling The Clocks
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PCI Express 3.0 - Double The Speed Without Doubling The Clocks
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Old 28th August 2008, 14:03   #1
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Default PCI Express 3.0 - Double The Speed Without Doubling The Clocks

As we know, the initial PCIe v1 has 2.5Gbps per direction per link speed - so a 16x PCIe v1 slot with sixteen such bidirectional links would have 2 x 16 x 2.5Gbps = 80Gbps raw bandwidth. After the 8B/10B data encoding used, the available theoretical bandwidth becomes exactly 8GB/s, or 250MB/s per link per direction. The PCIe v2, with 5Gbps per direction per link speed, exactly doubles these numbers.

Now, PCIe v3 goes ahead not with another double (10 Gbps), but with just 8 Gbps, 60 per cent more raw bandwidth. Yeah, it does help use similar tooling and save costs, but how to get double the real bandwidth? Simply. Encoding change - once you use the 'high bandwidth' full-speed PCIe v3 mode, the encoding isn't 8B/10B anymore, but 128B/130B encode, where a two-bit sync character is followed by a 128-bit payload. This, of course, brings the available theoretical bandwidth up by another 25 per cent, to 1GB/s per link per direction, exactly double that of PCIe v2.
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