Fibre Channel

Posted on Mar 18, 2009 in Knowledge Base

Fibre Channel, or FC, is a high-speed network technology (commonly running at 2-, 4-, 8- and 16-gigabit per second rates) primarily used to connect computer data storage. Fibre Channel is standardized in the T11 Technical Committee of the International Committee for Information Technology Standards (INCITS), an American National Standards Institute(ANSI)-accredited standards committee. Fibre Channel was primarily used in supercomputers, but has become a common connection type for storage area networks (SAN) in enterprise storage. Despite its name, Fibre Channel signaling can run on twisted pair copper wire in addition to Fiber-Optic Cables.

Fibre Channel Protocol (FCP) is a transport protocol (similar to TCP used in IP networks) that 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 portin Fibre Channel terminology is any entity that actively communicates over the network, not necessarily ahardware port. This port is usually implemented in a device such as disk storage, an HBA on a server or a Fibre Channel switch.

  • 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 ringnetworking. 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.
    • A minimal loop containing only two ports, while appearing to be similar to FC-P2P, differs considerably in terms of the protocol.
    • Only one pair of ports can communicate concurrently on a loop.
    • 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:
    • The switches manage the state of the fabric, providing optimized interconnections.
    • The traffic between two ports flows through the switches only, it is not transmitted to any other port.
    • Failure of a port is isolated and should not affect operation of other ports.
    • Multiple pairs of ports may communicate simultaneously in a fabric.
Attribute Point-to-Point Arbitrated loop Switched fabric
Max ports 2 127 ~16777216 (224)
Address size N/A 8-bit ALPA 24-bit port ID
Side effect of port failure Link fails Loop fails (until port bypassed) N/A
Mixing different link rates No No Yes
Frame delivery In order In order Not guaranteed
Access to medium Dedicated Arbitrated Dedicated

Layers

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

  • FC4 – Protocol-mapping layer, in which application protocols, such as SCSI or IP, are encapsulated into aPDU for delivery to FC2.
  • FC3 – Common services layer, a thin layer that could eventually implement functions like encryption or RAIDredundancy 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, 16 and 20 Gbit/s; these protocol flavors are called accordingly 1GFC, 2GFC, 4GFC, 8GFC, 10GFC, 16GFC or 20GFC. The 16GFC standard was approved by the INCITS T11 committee in 2010, and those products became available in 2011. Products based on the 1GFC, 2GFC, 4GFC, 8GFC and 16GFC standards should be interoperable and backward compatible. The 1GFC, 2GFC, 4GFC, 8GFC designs all use8b/10b encoding, while the 16GFC standard uses 64b/66b encoding. Unlike the 10GFC and 20GFC standards, 16GFC provides backward compatibility with 4GFC and 8GFC.

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.