Arbitrated loop

Arbitrated loop, also known as FC-AL, is a Fibre Channel topology that requires no fibre channel switches. Devices are connected in a one-way loop fashion in a ring topology.

It is a serial data architecture that is compatible with SCSI, handling up to 126 devices per port (NL port). It handles 127 devices if counting the 1 port connected to an optional fabric switch port. Only one connection may be a switch port.
The bandwidth on the loop is shared among devices.
Only 2 devices on the loop are communicating at any one time.
Naturally if only 2 devices are left in the loop, the topology becomes Point-to-Point.
Arbitrated Loop can be connected in a daisy chained fashion or a hub fashion. The daisy chain fashion will raise problems if one of the devices in the chain failed. The hub fashion will allow the loop to continue in case any device on the loop failed. An enhanced low-cost solution is also using a loop switch (as opposed to a fabric switch). Loop switches provide considerable performance improvements over hubs without much additional cost.
Arbitrated Loop uses NL_port, FL port.
An Arbitrated Loop with a fabric port, FL_Port, is considered a public loop.
An Arbitrated Loop with no fabric port, FL_Port, is considered a private loop.
NL_Port must provide Logon and Name registration facilities to login into the Fabric.
F/NL Port - For use with a non-Fabric switched Loop topology, Provides loop logon services, Provides name registration services.

See also