Fiber distributed data interface
Encyclopedia
Data transmission
Data transmission, digital transmission, or digital communications is the physical transfer of data over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless communication channels, and storage media...
in a local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
that can extend in range up to 200 kilometres (124.3 mi). Although FDDI logical topology is a ring-based token network, it does not use the IEEE 802.5 token ring protocol
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
as its basis; instead, its protocol is derived from the IEEE 802.4 token bus
Token bus
Token bus is a network implementing the token ring protocol over a "virtual ring" on a coaxial cable. A token is passed around the network nodes and only the node possessing the token may transmit. If a node doesn't have anything to send, the token is passed on to the next node on the virtual ring...
timed token protocol. In addition to covering large geographical areas, FDDI local area networks can support thousands of users. As a standard underlying medium it uses optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
, although it can use copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
cable, in which case it may be referred to as CDDI (Copper Distributed Data Interface). FDDI offers both a Dual-Attached Station (DAS), counter-rotating token ring topology and a Single-Attached Station (SAS), token bus passing ring topology.
FDDI was considered an attractive campus backbone technology in the early to mid 1990s since existing Ethernet networks only offered 10 Mbit/s transfer speeds and Token Ring networks only offered 4 Mbit/s or 16 Mbit/s speeds. Thus it was the preferred choice of that era for a high-speed backbone, but FDDI has since been effectively obsolesced by fast Ethernet
Fast Ethernet
In computer networking, Fast Ethernet is a collective term for a number of Ethernet standards that carry traffic at the nominal rate of 100 Mbit/s, against the original Ethernet speed of 10 Mbit/s. Of the fast Ethernet standards 100BASE-TX is by far the most common and is supported by the...
which offered the same 100 Mbit/s speeds, but at a much lower cost and, since 1998, by Gigabit Ethernet
Gigabit Ethernet
Gigabit Ethernet is a term describing various technologies for transmitting Ethernet frames at a rate of a gigabit per second , as defined by the IEEE 802.3-2008 standard. It came into use beginning in 1999, gradually supplanting Fast Ethernet in wired local networks where it performed...
due to its speed, and even lower cost, and ubiquity.
FDDI, as a product of American National Standards Institute
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...
X3T9.5 (now X3T12), conforms to the Open Systems Interconnection
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
(OSI) model of functional layering of LANs using other protocols. FDDI-II, a version of FDDI, adds the capability to add circuit-switched service
Circuit switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...
to the network so that it can also handle voice and video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
signals. Work has started to connect FDDI networks to the developing Synchronous Optical Network
Synchronous optical networking
Synchronous Optical Networking and Synchronous Digital Hierarchy are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or highly coherent light from light-emitting diodes . At low transmission rates data can also be transferred via an...
(SONET).
A FDDI network contains two rings, one as a secondary backup in case the primary ring fails. The primary ring offers up to 100 Mbit/s capacity. When a network has no requirement for the secondary ring to do backup, it can also carry data, extending capacity to 200 Mbit/s. The single ring can extend the maximum distance; a dual ring can extend 100 km (62.1 mi). FDDI has a larger maximum-frame size (4,352 bytes) than standard 100 Mbit/s Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
which only supports a maximum-frame size of 1,500 bytes, allowing better throughput.
Designers normally construct FDDI rings in the form of a "dual ring of trees" (see network topology
Network topology
Network topology is the layout pattern of interconnections of the various elements of a computer or biological network....
). A small number of devices (typically infrastructure devices such as routers and concentrators rather than host computers) connect to both rings - hence the term "dual-attached". Host computers then connect as single-attached devices to the routers or concentrators. The dual ring in its most degenerate form simply collapses into a single device. Typically, a computer-room contains the whole dual ring, although some implementations have deployed FDDI as a Metropolitan area network
Metropolitan area network
A metropolitan area network is a computer network that usually spans a city or a large campus. A MAN usually interconnects a number of local area networks using a high-capacity backbone technology, such as fiber-optical links, and provides up-link services to wide area networks and the...
.
Mitigating failure
FDDI requires this network topology because the dual ring actually passes through each connected device and requires each such device to remain continuously operational.The standard actually allows for optical bypasses, but network engineers consider these unreliable and error-prone. Devices such as workstations and minicomputers that might not come under the control of the network managers are not suitable for connection to the dual ring.
As an alternative to using a dual-attached connection, a workstation can obtain the same degree of resilience
Resilience (network)
In computer networking: “Resilience is the ability to provide and maintain an acceptable level of service in the face of faults and challenges to normal operation.”These services include:* supporting distributed processing* supporting networked storage...
through a dual-homed connection made simultaneously to two separate devices in the same FDDI ring. One of the connections becomes active while the other one is automatically blocked. If the first connection fails, the backup link takes over with no perceptible delay.
Standards
FDDI standards include:- ANSI X3.139-1987, Media Access Control (MAC) — also ISO 9314-2
- ANSI X3.148-1988, Physical Layer Protocol (PHY) — also ISO 9314-1
- ANSI X3.166-1989, Physical Medium Dependent (PMD) — also ISO 9314-3
- ANSI X3.184-1993, Single Mode Fiber Physical Medium Dependent (SMF-PMD) — also ISO 9314-4
- ANSI X3.229-1994, Station Management (SMT) — also ISO 9314-6