Fibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. It started use primarily in the supercomputer field, but has become the standard connection type for storage area networks (SAN) in enterprise storage. Despite its name, Fibre Channel signaling can run on both twisted pair copper wire and fiber-optic cables.

Fibre Channel Protocol (FCP) is a transport protocol (similar to TCP used in IP networks) which predominantly transports SCSI commands over Fibre Channel networks.

Fibre Channel topologies

There are three major Fibre Channel topologies, describing how a number of ports are connected together. A port in Fibre Channel terminology is any entity that actively communicates over the network, not necessarily a hardware port. This port is usually implemented in a device such as disk storage, an host bus adapter (HBA) on a server or a Fibre Channel switch. (In the computer storage field, a Fibre Channel switch is a network switch compatible with the Fibre Channel (FC) protocol. It allows the creation of a Fibre Channel fabric, that is currently the core component of most SAN. The fabric is a network of Fibre Channel devices which allows many-to-many communication, device name lookup, security, and redundancy. FC switches implement zoning, a mechanism that disables unwanted traffic between certain fabric nodes.)

-switch Qlogic with optical Fibre Channel connectors installed

Fibre Channel Host Bus Adapter (64-bit PCI-X card)

• Point-to-Point (FC-P2P). Two devices are connected directly to each other. This is the simplest topology, with limited connectivity.
• Arbitrated loop (FC-AL). In this design, all devices are in a loop or ring, similar to token ring networking. Adding or removing a device from the loop causes all activity on the loop to be interrupted. The failure of one device causes a break in the ring. Fibre Channel hubs exist to connect multiple devices together and may bypass failed ports. A loop may also be made by cabling each port to the next in a ring.

o A minimal loop containing only two ports, while appearing to be similar to FC-P2P, differs considerably in terms of the protocol.

o Only one pair of ports can communicate concurrently on a loop.

o Maximum speed of 8GFC.

• Switched fabric (FC-SW). All devices or loops of devices are connected to Fibre Channel switches, similar conceptually to modern Ethernet implementations. Advantages of this topology over FC-P2P or FC-AL include:

o The switches manage the state of the fabric, providing optimized interconnections.

o The traffic between two ports flows through the switches only; it is not transmitted to any other port.

o Failure of a port is isolated and should not affect operation of other ports.

o Multiple pairs of ports may communicate simultaneously in a fabric.

Layer

Fibre Channel does not follow the OSI model layering, but is split similarly into 5 layers, namely:

• FC4 — Protocol Mapping layer, in which application protocols, such as SCSI or IP, are encapsulated into a PDU for delivery to FC2.
• FC3 — Common Services layer, a thin layer that could eventually implement functions like encryption or RAID redundancy algorithms;
• FC2 — Network layer, defined by the FC-PI-2 standard, consists of the core of Fibre Channel, and defines the main protocols;
• FC1 — Data Link layer, which implements line coding of signals;
• FC0 — PHY, includes cabling, connectors etc.;

Layers FC0 through FC2 are also known as FC-PH, the physical layers of Fibre Channel.

Fibre Channel routers operate up to FC4 level (i.e. they may operate as SCSI routers), switches up to FC2, and hubs on FC0 only.

Fibre Channel products are available at 1, 2, 4, 8, 10 and 20 Gbit/s; these protocol flavors are called accordingly 1GFC, 2GFC, 4GFC, 8GFC, 10GFC, or 20GFC. The 16GFC standard was approved by the T11 committee in 2010, and products are expected to be available in 2011. Products based on the 1, 2, 4, 8 and 16GFC standards should be interoperable and backward compatible. The 10 Gbit/s standard and its 20 Gbit/s derivative, however, are not backward compatible with any of the slower speed devices, as they differ considerably on FC1 level in using 64b/66b encoding instead of 8b/10b encoding, and are primarily used as inter-switch links. While the 16GFC standard also uses the 64B/66B encoding, it provides backward compatibility with 4 and 8GFC.

Ports

The following types of ports are defined by Fibre Channel:

• node ports

o N_port is a port on the node (e.g. host or storage device) used with both FC-P2P or FC-SW topologies. Also known as Node port.

o NL_port is a port on the node used with an FC-AL topology. Also known as Node Loop port.

o F_port is a port on the switch that connects to a node point-to-point (i.e. connects to an N_port). Also known as Fabric port. An F_port is not loop capable.

o FL_port is a port on the switch that connects to a FC-AL loop (i.e. to NL_ports). Also known as Fabric Loop port.

o E_port is the connection between two fibre channel switches. Also known as an Expansion port. When E_ports between two switches form a link, that link is referred to as an inter-switch link (ISL).

o EX_port is the connection between a fibre channel router and a fibre channel switch. On the side of the switch it looks like a normal E_port, but on the side of the router it is a EX_port.

o TE_port * a Cisco addition to Fibre Channel, now adopted as a standard. It is an extended ISL or EISL. The TE_port provides not only standard E_port functions but allows for routing of multiple VSANs (Virtual SANs). This is accomplished by modifying the standard Fibre Channel frame (vsan tagging) upon ingress/egress of the VSAN environment. Also known as Trunking E_port.

o VE_Port an INCITS T11 addition, FCIP interconnected E-Port/ISL, i.e. fabrics will merge.

o VEX_Port a INCITS T11 addition, is a FCIP interconnected EX-Port, routing needed via lsan zoning to connect initiator to a target.

• general (catch-all) types

o Auto or auto-sensing port found in Cisco switches, can automatically become an E_, TE_, F_, or FL_port as needed.

o Fx_port a generic port that can become a F_port (when connected to a N_port) or a FL_port (when connected to a NL_port). Found only on Cisco devices where oversubscription is a factor.

o G_port or generic port on a switch can operate as an E_port or F_port. Found on Brocade and McData switches.

o L_port is the loose term used for any arbitrated loop port, NL_port or FL_port. Also known as Loop port.

o U_port is the loose term used for any arbitrated port. Also known as Universal port. Found only on Brocade switches.

(*Note: The term "trunking" is not a standard Fibre Channel term and is used by vendors interchangeably. For example: A trunk (an aggregation of ISLs) in a Brocade device is referred to as a Port Channel by Cisco. Whereas Cisco refers to trunking as an EISL.)