The Upper Layer of the architecture is the Transaction Layer. The main responsible of this layer is to begin the process of turning request or completion from device core into PCI Express transactions.
On the Transmit side, the transaction layer receives request or completion data from the core, and turns that information into out going PCI Express transaction.
On the Receive side the transaction layer accepts the incoming PCI Express transactions from its Data Link Layer.
Transaction Layer uses TLP-Transaction Layer Packet to communicate request and complete data into other PCI Express devices. Each TLP has a Header to identify the type of transaction. The Transaction Layer request device generates the TLP and completion device consumes the TLP. Transaction Layer has several other operations which include “Flow Control” and “Power Management”.
Header identifies the information for the transaction (types). PCI Express architecture defines four transaction types.
· Memory Transaction
· I/O Transaction
· Configuration Transaction
· Message Transaction
Where as DLP and ECRC (End to End CRC) are optional.
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