Storage Basics: Fibre Channel, Part Two Page 2


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Loops, Fabric Switches, and Directors
Fibre channel can be implemented in a variety of ways. The simplest method is to connect a Fibre channel Host Bus Adapter (HBA) to a Fibre channel device such as a storage array. This creates a very high-speed interface between two devices. Where it is necessary to connect more than two devices (which is very common), Fibre channel switches are used as connectivity points on the storage network.

Fibre channel switches are somewhat like Ethernet switches in that they provide the connectivity point that allows multiple devices to communicate. Interestingly, Fibre channel switches, like Ethernet switches, also create a single point of failure. As with regular Ethernet switches, the number of ports on Fibre channel switches varies from vendor to vendor.

When it comes to Fibre channel switches, there are three main categories of switches: loops, Fabric switches, and directors. Loop switches are perhaps the most basic of the three and are used to connect an FC-AL to the Fabric. A typical loop switch provides eight ports. Fabric switches are high-speed switches interconnected to form the Fibre channel fabric. A typical Fabric switch offers 16 to 32 ports. Finally, directorsare the "Cadillacs" of Fibre channel switches, offering high bandwidth, high availability (99.999%) and high performance. Not surpisingly, directors are the most expensive of Fibre channel switches, but in environments where downtime is critical, they are invaluable. Typical directors offer 32 or more ports.

Fibre Channel Ports
Fibre Channel devices can have a variety of ports on them which perform different functions. The most common type of port is an N_Port (node port), which you will find on devices such as Fibre channel HBAs or on a storage device such as a disk array. An N_Port can only be connected to another N_Port, thereby creating a point to point link between devices, or to an F_Port (discussed next) on a fibre channel switch.

F_Ports (fabric ports) are found on the Fibre channel switch. F_Ports can only be used for connectivity to the N_Port of a Fibre channel device. In addition to the F-Port, the Fibre channel switch has an E_Port (expansion port), which performs the same basic function as the uplink port on a Ethernet switch in that it allows multiple Fibre Channel switches to be connected to each other. E_Ports can only be attached to other E_Ports.

Once you add arbitrated loop capabilities to your fibre channel network, you then get L_Ports (Loop Ports). L_Ports are used in the FC-AL topology and are part of FC-AL nodes. There are two different types of L_Ports, NL_ and FL_Ports. NL_ Ports are used to connect a node to the FC-AL topology and can only be connected to other NL_Ports or to FL_Ports. FL_Ports are used to connect the FC-AL loop to the Fibre channel switch.


Although some other technologies like iSCSI threaten to loosen its stranglehold on the storage environment, Fibre Channel is so widely implemented that it is likely to endure all newcomers to remain the dominant connectivity technology for the foreseeable future.

» See All Articles by Columnist Mike Harwood

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