How to calculate QPI bandwidth?

I want to calcuate the QPI bandwidth of the Intel E5-2600 processor.

Wikipediea shows that the formula should be:

CPU frequency × 2 bits/Hz (double data rate) × 16(20) (data bits/QPI link width) × 2 (unidirectional send and receive operating simultaneously) ÷ 8 (bits/byte)

Therefore, the QPI bandwidth of E5-2600 is 20.8GB/s. But according to intel's specification, giga transfers per second is 8GT/s, which rendering a 32GB/s QPI bandwidth:

8 GT/s x 16 x 2 / 8

Can you tell me what's wrong with the original formula, and how to calculate the QPI bandwidth exactly?

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