This slide shows the trace from a Finisar Analyser. It shows a frame being communicated between two ports in an Arbitrated Loop. The frame is a port login, or PLOGI—the frames that two ports use to exchange information about their Fibre Channel operating characteristics.
The highlighted areas in this slide show that this port is using the Alternate BB credit method and has opted to grant a Login Credit of zero. Therefore, before the transmitting port can send a frame it must wait for the first R_RDY to be received so that it can increment its Credit_CNT and send the first frame. It must wait for a second R_RDY before it can send a second frame.