Fibre Channel overview


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In the past 10 years, Ethernet-based local area networks (LANs) have progressed a great deal, particularly in their data throughput capabilities. Most data communications functions take place on either a network or channel connection.

Network connections, such as Ethernet, provide a means to transport data from point A to point B using a path that is shared with other traffic. In fact, most network-based data transport doesn't care what type of connection is required to make the data readable at its destination; that is left up to another process defined within the data-link stack of the Open Standards Interconnection (OSI) specification. In contrast, channel-based connections provide direct or switched point-to-point connections between the source and destination.

From an operational perspective, network-based communication is slower than that over a channel because it is typically software intensive.

Current versions of Ethernet support devices, called network switches, replace the network hub but allow dedicated paths or channels to be established between network devices, thus reducing the potential for collisions to occur. However, Ethernet is still a network-based delivery method, and while it's efficient at carrying large amounts of data, it is not efficient for use in transporting more real-time data such as establishing direct I/O connections with disk drives.

In 1994, the American National Standards Institute (ANSI) established Fibre Channel as a standard. It combines the best of network and channel-based data transport.

How does it work?

Fibre Channel is based on an interconnection scheme called the fabric. The fabric essentially acts as an active switching system that can manage and route connections. The fabric consists of one or more Fibre Channel switches interconnected through one or more ports. Ports on the fabric are called F_ports.



The capabilities of different communications channels.




Fibre Channel requires at least one link between two nodes. Fibre Channel devices are connected by hardware devices called N_ports, which are usually the physical termination between the device and the media used to attach to the Fibre Channel fabric. This is similar to how network interface cards (NIC) connect to the unshielded twisted pair cabling or fiber connections on an Ethernet network. The N_Port also contains all of the software and hardware to manage the Fibre Channel protocol. Each node connected to the fabric typically contains two ports, but must contain at least one port. The ports can act as transmitters and receivers. Fibre Channel nodes only need to manage their connection with the corresponding F_Port on the fabric.

Fibre Channels can support not only traditional network protocols such as TCP/IP, but also protocols such as the Small Computer System Interface (SCSI). For example, Fibre Channel permits network servers, located in one part of a building, direct access to arrays of disk drives and other high-performance storage media, separated by a room or a continent. This is the fundamental basis of Storage Area Networks.

Fibre Channel topologies

Fibre Channel can be deployed in one of three different topologies: point-to-point, arbitrated-loop and cross-point (also called fabric-switched).

The point-to-point topology is simply a connection between two N_ports, where one of those N_ports is a server (also called the initiator). This method is limited to only two nodes and therefore not able to grow with the network. The point-to-point topology may be used where it is desirable to connect a single server to a single remote disk array.

The Arbitrated Loop (AL) topology has gained wider acceptance due to its lower per-port cost and ability to share media. AL breaks the Fibre Channel into loops, each loop supporting as many as 126 nodes and one fabric port. Note that devices supporting AL_ports on a node are called NL_ports and ports on the fabric are called FL_ports. The AL topology works like that of Ethernet: only one device may send data at a given time. Where Ethernet uses a concept called contention to transmit data, AL uses arbitration. A node must win an arbitration to send data. The winner of that arbitration is called the loop-master and has the ability to send data for that instant. Adding too many devices to a loop may significantly degrade its performance.

The AL topology can be interconnected by using a hub or by daisy-chaining the devices. As with any network topology, daisy-chaining devices on a network could be dangerous because if one of the connections break, the entire segment is compromised.

Cross-point topology provides the highest performance for Fibre Channel. Cross-point topology is simply a fabric based on a number of interconnected switches. Each switch typically contains from eight to 64 ports, with the largest called director switches. The real benefit with this configuration is increased performance and its ability to scale as large as needed. While adding more devices to an AL topology may decrease performance, adding more switches in a cross-point topology actually increases its performance.

While Fibre Channel doesn't subscribe to the OSI standard used with traditional networking, it does subscribe to a similar five-layered architecture. Those five layers are:

  1. FC-4 defines the interface between Fibre Channel and upper level protocols.

  2. FC-3 defines functions such as file encryption and compression.

  3. FC-2 defines Fibre Channel flow control, data encapsulation, classes of service information and the physical models for components.

  4. FC-1 describes the ordering of data into sets and defines the encoding and decoding schemes of data into bytes.

  5. FC-0 describes the physical media including the ports and cabling.

The next generation of Fibre Channel promises to be even faster and more efficient. Its natural ability to interface external disk arrays with network servers make it useful for users that require access to large amounts of storage, quickly and reliably.


McNamara, BE Radio's consultant on computer technology, is president of Applied Wireless, New Market, MD.


All of the Networks articles have been approved by the SBE Certification Committee as suitable study material that may assist your preparation for the SBE Certified Broadcast Networking Technologist exam. Contact the SBE at (317) 846-9000 or go to www.sbe.org for more information on SBE Certification.




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