This American National Standard for FC-AL specifies signaling interface enhancements for ANSI X3, FC-PH-x to allow L_Ports to operate with an Arbitrated Loop topology. This standard defines L_Ports that retain the functionality of Ports as specified in ANSI X3, FC-PH-x. The Arbitrated Loop topology attaches multiple communicating points in a Loop without requiring switches.

The Arbitrated Loop topology is a distributed topology where each L_Port includes the minimum necessary function to establish a Loop circuit. A single FL_Port connected to an Arbitrated Loop allows multiple NL_Ports to attach to a Fabric.

When an L_Port is operating on a Loop with at least one other L_Port, the L_Port uses the protocol extensions to ANSI X3, FC-PH-x that are specified in this standard.

When an L_Port is connected with an N_Port or an F_Port, the L_Port communicates using the protocol defined in ANSI X3, FC-PH-x.

Each L_Port may use a self-discovering procedure to find the correct operating mode without the need for external controls.