Fiber Channel Arbitrated Loop (FC-AL)

A fiber channel arbitrated loop (FC-AL) is a high-speed fiber channel topology that connects devices using a loop topology. It is similar to a token ring network, where a token or passing system is used to prevent data from colliding when two or more data streams are sent at the same time. An FC-AL passes data by using a one-way loop technique.

FC-AL technology eliminated the need for expensive fiber channel switches and allowed several servers and PC storage devices to be connected. Prior to 2007, switches were seldom used because of their expense. Since that time, the price of switches has decreased dramatically, so that the FC-AL system is now rarely used for server-to-storage I/O. It is still frequently used on the back end of certain disk array controllers.

A fiber channel arbitrated loop is also known as an arbitrated loop.

The FC topology has three major fiber channels that connect ports:

• Switched Fabric: A network topology that connects devices using crossbar switches.
• Point-to-Point: A network system that connects one device to another, allowing two-way data communication
• Arbitrated Loop: Connects devices in a loop, and can have only two devices communicating at the same time. The is the most common fiber channel used in FC-AL.

A fiber channel arbitrated loop can connect up to 127 devices with one port attached to the fabric or structure. The FC-AL is constructed so that only one port can transmit data at a time. Prior to transmitting data, a port has to arbitrate with all other ports in the loop before communication begins.

The FC-AL system uses the arbitration signal to choose the port. Once the arbitration signal selects a port, it can use the fiber channel, a gigabit-speed network topology that is used mostly for network storage. The FC-AL topology has several attributes including the following:

• All devices on the loop share the same bandwidth
• It can be cabled using a hub or a loop technique
• Supports a private loop with no fabric port for NL_Ports
• Supports a public loop connected to a fabric for FL_Ports
• When a port malfunctions in the loop, all ports stop working
• A hub maintains a logical ring but allows cabled star topology
• Two ports function like an arbitrated loop and not a point-to-point
• Supports up to 127 ports (devices) with one port attached to the fabric
• Has a serial architecture that is compatible with small computer system interface (SCSI)