Perspective on QOS/TC/VC and Arbitration

PCI does not include any QoS-related features similar to those defined by PCI Express. Many questions arise regarding the need for such an elaborate scheme for managing traffic flow based on QoS and differentiated services. Without implementing these new features, the bandwidth available with a PCI Express system is far greater and latencies much shorter than PCI-based implementations, due primarily to the topology and higher delivery rates. Consequently, aside from the possible advantage of isochronous transactions, there appears to be little advantage to implementing systems that support multiple Traffic Classes and Virtual Channels.

While this may be true for most desktop PCs, other high-end applications may benefit significantly from these new features. The PCI Express specification also opens the door to applications that demand the ability to differentiate and manage system traffic based on Traffic Class prioritization.