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PCI Express System Architecture by MindShare, Inc, Ravi Budruk, Don Anderson, Tom Shanley

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Switch Cut-Through Mode

PCI Express supports a switch-related feature that allows TLP transfer latency through a switch to be significantly reduced. This feature is referred to as the 'cut-through' mode. Without this feature, the propagation time through a switch could be significant.

Without Cut-Through Mode

Background

Consider an example where a large TLP needs to pass through a switch from one port to another. Until the tail end of the TLP is received by the switch's ingress port, the switch is unable to determine if there is a CRC error. Typically, the switch will not forward the packet through the egress port until it determines that there is no CRC error. This implies that the latency through the switch is at least the time to clock the ...

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