One Stop Systems EB3200, User Manual

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ExpressBox 3200 | 9 Appendix B PCI Express General Information 82
9.7.1 PCI Express Performance Table
8b/10b encoding
8b/10b encoding is a telecommunications line code in which each eight-bit data byte is converted to a 10-bit transmission character. PCI
Express 1.0 and 2.0 use the 8b/10b encoding system (which is the same encoding used by Fast Ethernet, i.e., 100 Mbps, networks)
8b/10b encoding was invented by IBM and is used in transmitting data on enterprise system connections, gigabit Ethernet and over fiber
channel. This encoding supports continuous transmission with a balanced number of zeros and ones in the code stream. 8b/10b can also detect
single-bit transmission errors.
To convert a figure given in bits per second (bps) to bytes per second (B/s) you need to divide it by eight, since a byte is a group of eight bits.
However, because of the 8b/10b encoding, we need to make this division by 10 rather than eight. This is the reason why, with a clock of 2.5
GHz and 5 GHz, the x1 bandwidth of these connections are 250 MB/s and 500 MB/s, respectively, and not 312.5 MB/s and 625 MB/s. The
two extra bits added are called “overhead,” and they “eat” 20% of the channel bandwidth.
128b/130b encoding
PCI Express 3.0 uses a different encoding system, called 128b/130b. As you can deduce, this encoding system transmits each 128 bits of data as
a 130-bit number, which offers a far lower overhead. To transmit 128 bits of data, PCI Express 3.0 needs only two extra bits, while with the
previous revisions, 32 extra bits are needed (two for every eight bits). Because of this lower overhead requirement, PCI Express 3.0 can achieve
double the PCI Express 2.0 bandwidth with a clock rate of 8 GHz instead of 10 GHz.