Transaction Layer (TL) of PCI Express

This layer’s primary responsibility is to create PCI Express request and completion transactions. It has both transmit functions for outgoing transactions, and receive functions for incoming transactions. On the transmit side, the Transaction Layer receives request data (such as “read from BIOS location FFF0h”) or completion data (“here is the result of that read”) from the device core, and then turns that information into an outgoing PCI Express transaction. On the receive side, the Transaction Layer also accepts incoming PCI Express transactions from its Data Link Layer (refer to fig.). This layer assumes all incoming information is correct, because it relies on its Data Link Layer to ensure that all incoming information is error-free and properly ordered.