PCI Express, or PCIe, (formerly known as Arapaho or 3GIO for 3rd Generation I/O, not to be mistaken for PCI-X or PXI) is an implementation of the PCI computer bus that uses existing PCI programming concepts, but bases it on a completely different and much faster serial physical-layer communications protocol. The physical-layer consists not of a bus, but of a network of serial interconnects (because synchronization of parallel connections is hindered by timing skew) much like twisted pair ethernet. A single hub with many pins on the mainboard is used, allowing all kinds of switching and parallelism.
PCI Express slots (from top to bottom: x4, x16, x1 and x16), compared to a traditional 32-bit PCI slot (bottom), as seen on DFI's LanParty nF4 SLI-DRIt is supported primarily by Intel, who started working on the standard as the Arapahoe project after pulling out of the InfiniBand system.
PCI Express is intended to be used as a local interconnect only. As it is based on the existing PCI system, cards and systems can be converted to PCI Express by changing the physical layer only – existing systems could be adapted to PCI Express without any change in software. The higher speeds on PCI Express allow it to replace almost all existing internal buses, including AGP and PCI, and Intel envisions a single PCI Express controller talking to all external devices, as opposed to the northbridge/southbridge solution in current machines.
The PCIe link is built around a bidirectional, serial (1-bit), point-to-point connection known as a "lane". This is in sharp contrast to the PCI connection, which is a bus-based system where all the devices share the same unidirectional, 32-bit, parallel bus.
PCI Express is a layered protocol, consisting of a Transaction Layer, a Data Link Layer, and a Physical Layer. The Physical Layer is further divided into a logical sublayer and an electrical sublayer. The logical sublayer is frequently further divided into a Physical Coding Sublayer (PCS) and a Media Access Control (MAC) sublayer (terms borrowed from the OSI model of networking protocol).
As of 2005, PCI Express appears to be well on its way to becoming the new backplane standard in personal computers. There are several explanations for this, but the principal reason is that it was designed to be completely transparent to software developers - an operating system designed for PCI can boot in a PCI Express system without any code modification. Other secondary reasons include its enhanced performance and strong brand recognition.
Most of the new graphics cards from both ATI Technologies and NVIDIA use PCI Express. NVIDIA uses the high-speed data transfer of PCIe for its newly developed Scalable Link Interface (SLI) technology, which allows two graphics cards of the exact same chipset and model number to be run at the same time, allowing increased performance. ATI Technologies has also developed a dual-GPU system based on PCIe called Crossfire.
Most new Gigabit Ethernet chips and some 802.11 wireless chips also use PCI Express. Other hardware such as RAID controllers and network cards are also starting to make the switch. In 2005, Apple updated both the consumer iMac and workstation PowerMac to use PCI Express exclusively, hence supplanting the AGP and PCI-X connectivity that they had formerly utilized.
ExpressCard is just starting to emerge on laptops. The problem is many laptops only have one slot and it is impossible to give up the existing legacy Cardbus for the new ExpressCard slot. Desktops do not have this problem as they have multiple slots and can support PCI Express and the legacy PCI slots concurrently
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