PCI Express System Architecture

Traffic Classes and Virtual Channels

During initialization a PCI Express device-driver communicates the levels of QoS that it desires for its transactions, and the operating system returns TC values that correspond to the QoS requested. The TC value ultimately determines the relative priority of a given transaction as it traverses the PCI Express fabric. Two hardware mechanisms provide guaranteed isochronous bandwidth and differentiated services:

• Virtual Channel Arbitration

• Port Arbitration

These arbitration mechanisms use VC numbers to manage transaction priority. System configuration software must assign VC IDs and set up the association between the traffic class assigned to a transaction and the virtual channel to be used when traversing each ...