What Is Fibre Channel Arbitrated Loop and How’s It Working [MiniTool Wiki]

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About Fibre Channel Arbitrated Loop

Fibre Channel Arbitrated Loop (FC-AL) is a high-speed Fibre Channel topology using a loop topology for connected devices. It is similar to Token Ring Network, which uses a token or delivery system to prevent data from colliding when sending two or more streams simultaneously.

In this post of MiniTool, you can more information about Fibre Channel Arbitrated Loop.

In fact, FC-AL uses one-way loop technology to pass data. Historically, it is a low-cost alternative to structural topologies. It allows many servers and computer storage devices to be connected without the need for very expensive Fibre Channel switches. That makes the cost of switch has dropped dramatically.

Hence, FC-AL became rare in server-to-storage communications, but it is still very common in the storage system.

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Another thing you need to know is that Fibre Channel technology is used in early SAN. The Fibre Channel was developed in 1988, used for improving the transmission bandwidth of the hard disk protocol, focusing on fast, efficient and reliable transmission of data. By the end of 1990s, FC SANs began to be widely used on a large scale.

Features of Fibre Channel Arbitrated Loop

FC topology has three main ports for the fiber channel:

  • Switch fabric: It is a network topology which uses a crossbar to connect devices.
  • Peer-to-peer: The network system can connect one device to another, allowing mutual data communication
  • Arbitrated loop: Connect devices in a loop and only two devices can communicate at the same time. This is the most commonly used Fibre Channel in FC-AL.

FC-AL can connect with 127 devices at most with one port connected to the fabric or structure. The structure of FC-AL only allows a port to pass data at a time. Before transmitting data, the port must arbitrate with all other ports in the loop prior to the beginning of communication.

The FC-AL system chooses ports according to arbitration signals. Once the arbitration signal chooses a port, it can use Fibre Channel, a gigabit speed network topology primarily used for network storage. The attributes of FC-AL mainly contain:

  • It is a serial architecture that can be used as a transport layer in a SCSI network and supports up to 127 devices. The loop can be connected to the Fibre Channel fabric through one of its ports.
  • It can be cabled with a hub or a loop technique.
  • All the bandwidth in the loop is sharable among all the ports.
  • There can only be two ports communicating at a time in a loop. Once a port wins arbitration, it can open another port in half-duplex or full-duplex mode.
  • A loop with two ports is valid and has the same physical topology as point-to point, but still act as a protocol loop.
  • It supports a private loop with no fabric port for NL-Ports.
  • It supports a public loop connected to a fabric for FL-Ports
  • The fiber loop port, or the FL port on the switch, is connected to the NL port (node ​​ring) on ​​the HBA to support loop operation. With the FC-AL architecture, when a device joins the FC-AL, or when any errors occur or need to be reset, the loop must be reinitialized. In this process, all communications must be temporarily suspended. Due to its addressing mechanism, FC-AL is theoretically limited to 127 nodes.

How FC-AL Works

The arbitrated loop can be physically connected in a circular fashion or using a hub. If the device in the chain fails, the physical ring will stop working. On the other hand, the hub allows a star topology at the cable level while maintaining the logical ring.

Each receive port on the hub is simply passed to the next active transport port, bypassing any inactive or failed ports.

Therefore, fibre channel hubs have another feature providing a bypass circuit to prevent loop disconnection when a device fails or is removed. If you don’t remove the device from the loop, the hub’s bypass circuit detects no signal and immediately begins routing incoming data directly to the next port of the loop. It bypasses the lost device completely. 

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