When a device is started, the transmitter section will start to emit a continuous stream of IDLE characters. This generates a square wave signal with 50% duty-cycle at 1/12 of the Baud rate on the outgoing link. The receiver on the other side of the link will use this character frequency to calibrate the receiver delay-locked-loop. Following the link calibration there is an exchange of control characters to insure that both ends of the link have been started successfully.
After correct operation of the link has been established, each end sends a number of flow control characters corresponding to its receive buffer size. The current implementations have buffering for at least two flits, i.e. 64 characters.