Arbitrated loop, also known as FC-AL, is a Fibre Channel topology in which devices are connected in a one-way loop fashion in a ring topology. Historically it was a lower-cost alternative to a fabric topology. It allowed connection of many servers and computer storage devices without using then very costly Fibre Channel switches. As of 2007 the cost of the switches dropped considerably, so FC-AL is rarely used for a server-to-storage communication. It is however still commonly utilized on backend of some disk array controllers.
- It is a serial architecture that is compatible with SCSI, handling up to 127 ports (devices). One port may optionally connect a loop to fabric switch port.
- The bandwidth on the loop is shared among all ports.
- Only two ports may communicate at a time on the loop. One port wins arbitration and may open one other port in either half or full duplex mode.
- Arbitrated loop with only 2 ports is a valid one, and while having the same physical topology as point-to-point still acts as a loop protocol-wise.
- Fibre Channel ports capable of arbitrated loop communication are NL_port (node loop port) and FL_port (fabric loop port), collectively referred to as the L_ports. Physical connectors on the hub are not considered ports in terms of the protocol.
- An arbitrated loop with no fabric port (with only NL_ports) is a private loop.
- An arbitrated loop connected to a fabric through FL_port, is a public loop.
- NL_Port must provide fabric logon (FLOGI) and name registration facilities to initiate communication with other node through the fabric (to be an initiator).
Arbitrated loop can be physically cabled in a ring fashion or using a hub. The physical ring ceases to work if one of the devices in the chain fails. The hub on the other hand, while maintaining a logical ring, allows a star topology on the cable level. Each receive port on the hub is simply passed to next active transmit port, bypassing any inactive or failed ports.
Fibre Channel hubs therefore have one other very useful function: They provide bypass circuits that prevent the loop from breaking if one device fails or is removed. If a device is removed from a loop (for example, by pulling its interconnect plug), the hub’s bypass circuit detects the absence of signal and immediately begins to route incoming data directly to the loop’s next port, bypassing the missing device entirely. This gives loops at least a measure of resiliency—failure of one device in a loop doesn’t cause the entire loop to become inoperable.