 The flow control frame has three PCI bytes specifying the interval between subsequent frames and how many consecutive frames may be sent ("Block Size").The initial byte contains the type (type = 3) in the first four bits, and a flag in the next four bits indicating if the transfer is allowed (0 = Clear To Send, 1 = Wait, 2 = Overflow/abort).The next byte is the block size, the count of frames that may be sent before waiting for the next flow control frame.