PCI Express is a layered protocol, consisting of a transaction layer, a data link layer, and a physical layer. The Data Link Layer is subdivided to include a media access control (MAC) sublayer. The Physical Layer is subdivided into logical and electrical sublayers. The Physical logical-sublayer contains a physical coding sublayer (PCS). (Terms borrowed from the IEEE 802 model of networking protocol.)
Configuration/Operating System Layer —Leverages the standard mechanisms defined in the PCI Plug-and-Play specification for device initialization, enumeration, and configuration. This layer communicates with the software layer by initiating a data transfer between peripherals or receiving data from an attached peripheral. PCI Express is designed to be compatible with existing operating systems, but future operating system support is required for many of the technologys advanced features.
Software Layer —Generates read and write requests to peripheral devices. PCI Express maintains initialization and runtime software compatibility with PCI. Like PCI, the PCI Express initialization model allows the operating system to discover add-in hardware devices and allocate system resources. PCI Express retains the PCI configuration space and the programmability of I/O devices. In fact, all operating systems will boot without modification on a PCI Express system. The PCI runtime software model is also preserved, enabling existing software to execute unchanged.
Transaction Layer —Transports read and write requests from the software layer to the link layer using a packet-based protocol, and matches response packets to the original software requests. The transaction layer supports 32-bit and extended 64-bit memory addressing. It also supports PCI memory, I/O, and configuration address spaces, as well as a new message space for in-band messages such as interrupts and resets. This message space eliminates the need for numerous PCI and PCI-X sideband signals.
Link Layer —Adds sequencing and error detection cyclic redundancy codes (CRCs) to the data packets to create a reliable data transfer mechanism between the system chip set and the I/O controller.
Physical Layer —Implements the dual simplex PCI Express channels. Implementations are flexible and various technologies and frequencies may be used. In this way, initial silicon technology can be replaced easily with future implementations that are backward compatible. For example, fiber-optic technology might be used to increase the data transfer rate.
Mechanical Layer —Defines various form factors for peripheral devices.