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What is Fibre Channel?

Aug 3rd 2008
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A Brief History of Fibre Channel

Fibre Channel is a network technology primarily used for storage networking. Work on defining Fibre Channel started in 1988 as a way to simplify the HIPPI system then in use for similar roles. In 1994, Fibre Channel was received ANSI standard approval.

HIPPI used a massive 50-pair cable with bulky connectors, and had limited cable lengths. Fibre Channel was primarily concerned with simplifying the connections and increasing distances, rather than to increasing transmission speeds. Later, designers added the goals of connecting parallel SCSI disk storage over serial Fibre Channel, providing higher speeds and a greater number of connected devices. Fibre Channel designers have since added support for any number of “upper layer” protocols, including SCSI, ATM, and IP, with SCSI being the predominant usage.

Today, Fibre Channel has since become the standard connection type for storage area networks (SANs) in enterprise storage. Despite common connotations of its name, Fibre Channel signaling can run on both twisted-pair copper wire and optical fibre cables.

Fibre Channel Protocol (FCP) is the interface protocol of SCSI on the Fibre Channel.

Fibre Channel Topologies

There are three major Fibre Channel topologies:

  • Point-to-Point (FC-P2P). Two devices are connected back to back. 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. Often an arbitrated loop between two ports will negotiate to become a P2P connection, but this is not required by the standard.
  • Switched fabric (FC-SW). All devices or loops of devices are connected to Fibre Channel switches, similar conceptually to modern Ethernet implementations. The switches manage the state of the fabric, providing optimized interconnections. Very limited security is available in today’s fibre channel switches.

Fibre Channel Layers

Fibre Channel is a layered protocol, consisting of 5 layers, namely:

  • FC0: the physical layer. Includes cables, fiber optics, connectors, pinouts etc.
  • FC1: the data link layer. Implements the 8b/10b encoding and decoding of signals.
  • FC2: the network layer. Defined by the FC-PI-2 standard, consists of the core of Fibre Channel, and defines the main protocols.
  • FC3: the common services layer. A thin layer that could eventually implement functions like encryption or RAID.
  • FC4: the Protocol Mapping layer. A layer whereby other protocols, such as SCSI, are encapsulated into an information unit for delivery to FC2.

FC0, FC1, and FC2 are also known as FC-PH, the physical layers of fibre channel.

Fibre Channel Transmission Speeds

Fibre Channel products are currently available at 1 Gbit/s, 2 Gbit/s and 4 Gbit/s. At the time of writing, an 8 Gbit/s standard is under development and a 10 Gbit/s standard has been ratified but is currently only for use with switch interconnects. No 10 Gbit/s initiator or target products are expected based on the 10 Gbit/s standard.

Products based on the 1, 2, 4 and 8 Gbit/s standards should be interoperable, and backward compatible. The 10 Gbit/s standard, however, is not expected to be backward compatible with any of the slower speed devices.

Fibre Channel Ports

The following types of ports are defined by Fibre Channel:

  • 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 InterSwitch Link or ISL.
  • 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.
  • F_port is a fabric connection in a switched fabric topology. Also known as Fabric port. An F_port is not loop capable.
  • FL_port is the fabric connection in a public loop for an arbitrated loop topology. Also known as Fabric Loop port. Note that a switch port may automatically become either an F_port or an FL_port depending on what is connected.
  • G_port or generic port on a switch can operate as an E_port or F_port.
  • L_port is the loose term used for any arbitrated loop port, NL_port or FL_port. Also known as Loop port.
  • N_port is the node connection pertaining to hosts or storage devices in a Point-to-Point or switched fabric topology. Also known as Node port.
  • NL_port is the node connection pertaining to hosts or storage devices in an arbitrated loop topology. Also known as Node Loop port.
  • TE_port is a term used for multiple E_ports trunked together to create high bandwidth between switches. Also known as Trunking Expansion port.

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