EXFO FTB-800 Series for FTB-1 - Manual (Page 150)

Test Setup - Test Configurator, Timer, and System
140 FTB-700G/800 Series
Interface - Port (Fibre Channel)
Buffer to Buffer Flow Control
Buffer to Buffer (BB) Flow Control, also called Flow Control Management,
manages the control of data traffic between the local and remote fibre
channel ports based on the network, distance, and traffic congestion.
Each remote port advertises the buffer size (BB_Credit) during the login.

Enable check box when selected allows to set or use buffers to receive
frames. The Enable check box is force selected when the Login -
Enable check box is selected.

Available BB_Credit is the number of frame buffers that the remote
port has available to receive frames from the local port: 1 to 65535
(default is 10). The Available BB_Credit is not editable when the
Login - Enable check box is selected but its value received from the
remote interface through the login process is displayed. If the login
fails, the Available BB_Credit displays “–”.