Specification Sheet

Interfaces
18 Datasheet, Volume 1 of 2
PCI Express* uses packets to communicate information between components. Packets
are formed in the Transaction and Data Link Layers to carry the information from the
transmitting component to the receiving component. As the transmitted packets flow
through the other layers, the packets are extended with additional information
necessary to handle packets at those layers. At the receiving side, the reverse process
occurs and packets get transformed from their Physical Layer representation to the
Data Link Layer representation and finally (for Transaction Layer Packets) to the form
that can be processed by the Transaction Layer of the receiving device.
2.2.1.1 Transaction Layer
The upper layer of the PCI Express* architecture is the Transaction Layer. The
Transaction Layer's primary responsibility is the assembly and disassembly of
Transaction Layer Packets (TLPs). TLPs are used to communicate transactions, such as
read and write, as well as certain types of events. The Transaction Layer also manages
flow control of TLPs.
2.2.1.2 Data Link Layer
The middle layer in the PCI Express* stack, the Data Link Layer, serves as an
intermediate stage between the Transaction Layer and the Physical Layer.
Responsibilities of Data Link Layer include link management, error detection, and error
correction.
Figure 2-1. PCI Express* Layering Diagram
Figure 2-2. Packet Flow through the Layers