Peripheral Component Interconnect Express - PCI Express (PCI-E)

Techopedia explains Peripheral Component Interconnect Express - PCI Express (PCI-E)

The new PCI-E 3.0 technology is different from the former PCI, PCI-X and AGP boards in many ways:

• Communication consists of data and status-message traffic being packetized and depacketized.
• Data is sent via paired point-to-point serial links, called lanes, allowing data movement in both directions simultaneously and allowing more than one pair of devices to communicate simultaneously.
• PCI-E slots contain from one to 32 lanes in powers of 2 (1, 2,4, 8 etc.). Each “lane” is a pair of data transfer lines, one for transmitting and one for receiving, and is composed of 4 wires. The number of lanes in a slot is denoted by an x before it, e.g. x16 designates a 16-lane PCI-E card.
• Higher bandwidth is provided by channel grouping – using multiple lanes for a single device.
• Serial buses transmit data faster than parallel buses due to the latter’s limitation requiring data to arrive simultaneously at their destination (This has to do with the frequency and wavelength of a single bit). With serial buses there is no requirement for signals to arrive simultaneously.
• PCI-E follows a layered protocol composed of 3 layers: a transaction layer, a data link layer, and a physical layer.

The following are the rates of transmission and bandwidth for the various PCI-E busses. These rates are for total transmission in both directions, 50% being in either direction:

• PCI Express 1x 500 MB/s
• PCI Express 2x 1000 MB/s
• PCI Express 4x 2000 MB/s
• PCI Express 8x 4000 MB/s
• PCI Express 16x 8000 MB/s (x16 cards are the largest size in common use.)
• PCI Express 32x 16000 MB/s

By comparison a PCI card has bandwidth of 132 MB/s; AGP 8x: 2,100 MB/s; USB 2.0: 60 MB/s; IDE: 100 to 133 MB/s; SATA: 150 MB/s; SATA II: 300 MB/s; Gigabit Ethernet: 125 MB/s; and Firewire 800: approx. 100 MB/s.