The transmitter is required to keep a running tally of available buffers at the receiver port: • The transmitter takes the login credit and maintains a CREDIT_CNT, which it decrements for each frame sent and increments for every credit received.
• Receiver Ready (R_RDY) is the receiver’s notification (called the BB_Credit) that a frame has been processed and the buffer is available. It does this by sending an R_RDY control signal to the transmitting port.
• When the transmitting port receives R_RDY it knows that it can send another frame.
• When a transmitting port runs out of credits it must stop sending frames, otherwise there will not be a buffer available at the receiving port and the frames will be lost.
This slide shows that: • Port A has a received a login credit of 16 by Port B • Port B has received a login credit of 4 from Port A.
See what happens to the Credit_CNT as a frame is sent by Port A and an R_RDY is received.