How PCI Express Works

Devices using PCI share a common bus, but each device using PCI Express has its own dedicated connection to the switch.

PCI Express Connection Speeds

The 32-bit PCI bus has a maximum speed of 33 MHz, which allows a maximum of 133 MB of data to pass through the bus per second. The 64-bit PCI-X bus has twice the bus width of PCI. Different PCI-X specifications allow different rates of data transfer, anywhere from 512 MB to 1 GB of data per second.

A single PCI Express lane, however, can handle 200 MB of traffic in each direction per second. A x16 PCIe connector can move an amazing 6.4 GB of data per second in each direction. At these speeds, a x1 connection can easily handle a gigabit Ethernet connection as well as audio and storage applications. A x16 connection can easily handle powerful graphics adapters.

How is this possible? A few simple advances have contributed to this massive jump in serial connection speed:

  • Prioritization of data, which allows the system to move the most important data first and helps prevent bottlenecks
  • Time-dependent (real-time) data transfers
  • Improvements in the physical materials used to make the connections
  • Better handshaking and error detection
  • Better methods for breaking data into packets and putting the packets together again. Also, since each device has its own dedicated, point-to-point connection to the switch, signals from multiple sources no longer have to work their way through the same bus.