Interfaces
26
Datasheet
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.
The transmission side of the Data Link Layer accepts TLPs assembled by the
Transaction Layer, calculates and applies data protection code and TLP sequence
number, and submits them to Physical Layer for transmission across the Link. The
receiving Data Link Layer is responsible for checking the integrity of received TLPs and
for submitting them to the Transaction Layer for further processing. On detection of TLP
error(s), this layer is responsible for requesting retransmission of TLPs until information
is correctly received, or the Link is determined to have failed. The Data Link Layer also
generates and consumes packets which are used for Link management functions.
2.2.1.3
Physical Layer
The Physical Layer includes all circuitry for interface operation, including driver and
input buffers, parallel-to-serial and serial-to-parallel conversion, PLL(s), and impedance
matching circuitry. It also includes logical functions related to interface initialization and
maintenance. The Physical Layer exchanges data with the Data Link Layer in an
implementation-specific format, and is responsible for converting this to an appropriate
serialized format and transmitting it across the PCI Express Link at a frequency and
width compatible with the remote device.
Figure 2-5. Packet Flow through the Layers
