The IB contains a 2.5 Mbyte buffer memory. If this begins to fill to capacity, the IB signals to the OLD by asserting the line XOFF . This can happen asynchronously. When the OLD sees this signal, it sends an XOFF character back to the OLS using the other fibre of the duplex pair. This causes the OLD to inhibit sending acknowledge strobes to the ROC thus preventing data from being read in to the OLS . When the IB \ has cleared some space, it de-asserts XOFF and the OLD sends an XON \ character to the OLS which releases the inhibit on the acknowledge strobe and the data transfers resume.
The IB asserts XOFF when "nearly full" thus leaving sufficient space to read in any words still in the optical "pipeline". Similarly, it de-asserts XOFF only when it has cleared space below the level of "nearly full". This hysteresis prevents the XOFF signal from fluttering.