PCI Express (PCIe) is the next generation system bus technology. Built on the serial interface architecture, PCIe provides faster, cooler and low power consuming interfaces. PCIe allows for higher throughput than the traditional PCI and PCI-X interfaces. Next generation I/O technologies like Infiniband, 4Gbps Fibre Channel, 10 Gb Ethernet and high end video cards can work at their full potential only on PCIe expansion busses. PCIe busses are made of individual data paths known as lanes. The speed of a PCIe bus is indicated by its width or the number of lanes, generally indicated as, x1, x2, x4, x8 & x16 and read as “by 1, by 2, by 4, by 8 and by 16”. A card of lesser lanes can be used in any expansion slot with higher number of lanes. For example, a x4 card can be used in a x16 slot but the vice versa is not possible. Sometimes PCIe busses are configured for lesser number of lanes than the expansion slot it self. For example, a slot can be x8 wide but configured for x4 width. So even if a x8 card is placed in the slot, it will still function at x4 speed. However, this allows of full flexibility in utilizing all available expansion slots on your system. PCI and PCI-X are bus based technologies and cause contention among PCI devices attached to the same bus. Since the PCIe technology is point to point and full duplex, even after fully populating all the available slots, all the slots are guaranteed to perform at their peak performance unlike the PCI and PCI-X busses.

At a software level, PCI, PCI-X and PCI Express are compatible so existing drivers and interfaces do not require extensive rewrites for new PCI Express cards using similar hardware. More information available at PCI SIG website.