List and briefly define the PCIe protocol layers.

Physical: Consists of the actual wires carrying the signals, as well as circuitry and logic to support ancillary features required in the transmission and receipt of the 1s and 0s.

Data link: responsible for reliable transmission and flow control.

Transmission:  Generates and consumes data packets sued to implement load/store data transfer mechanisms and also manages the flow control of those packets between the two components on a link.