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PCI Express accelerates toward 8GT/s

Aug 13, 2007 — by LinuxDevices Staff — from the LinuxDevices Archive — views

Now that electromechanical specs and compliance tests are available for PCI Express (PCIe) 2.0, the PCI-SIG (special interest group) has outlined a forthcoming version 3.0 specification designed to support up to 8GT/s (gigatransfers per second). PCIe 2.0 compliant products are expected this year, followed by PCIe 3.0 products in 2010.

The PCIe 2.0 specification already doubles the throughput of the original PCIe spec to 5GT/s. The forthcoming PCIe 3.0 spec will up the ante even more, to 8GT/s. Further, since the 8b/10b encoding required in earlier versions will be removed, PCIe 3.0 will actually be able to transfer twice as much data, given that 8b/10b overhead accounted for 20 percent of throughput, the group suggested.

The PCI-SIG apparently considered defining PCIe 3.0 as a 10GT/s specification, but opted for the slower speed after taking into consideration implementation cost and complexity, among other factors. The group believes that 8GT/s can be achieved using mainstream silicon process technology and low-cost materials, while maintaining “full mechanical compatibility,” it said.

Al Yanes, PCI-SIG chairman, stated, “Experts in the PCIe Electrical Workgroup analyzed both 10GT/s and 8GT/s and after careful consideration of power, implementation complexity, and silicon area, recommended 8GT/s. This allows us to satisfy the next-generation performance requirements for all existing PCIe applications while maintaining backward compatibility, and at the same time broadening the adoption of this pervasive technology into new and emerging applications and usage models.”

The PCI-SIG is an industry group formed in 1992 around PCI (peripheral component interconnect), the expansion bus technology that all but replaced the original ISA bus during the 90s. More recently, the group in 2004 defined the PCI Express bus used in most commodity PCs today.

The current version of PCI Express supports throughputs of up to 2.5GT/s, and bandwidths of up to 16 lanes. Thus, a PC expansion slot with a single-lane PCIe interface has a throughput of 2.5Gbps — about three times that of PCI, and plenty to support gigabit Ethernet. Meanwhile, a slot with a 16-lane PCIe interface supports throughputs of 40Gbps — about double that of an 8x AGP (advanced graphics port) interface.


The final PCIe 3.0 specifications are expected in late 2009, and “could be seen in products starting in 2010,” the PCI-SIG said. The PCI-SIG expects products based on PCIe 2.0 and 3.0 to be backward-compatible with those based on the current version of the spec, it said.

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