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
From an operational perspective, network-based communication is
slower than that over a channel because it is typically software
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
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
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
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
Fibre Channel topologies
Fibre Channel can be deployed in one of three different topologies:
point-to-point, arbitrated-loop and cross-point (also called
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
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
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:
FC-4 defines the interface between Fibre Channel and upper level
FC-3 defines functions such as file encryption and compression.
FC-2 defines Fibre Channel flow control, data encapsulation, classes
of service information and the physical models for components.
FC-1 describes the ordering of data into sets and defines the
encoding and decoding schemes of data into bytes.
FC-0 describes the physical media including the ports and
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.