Fiber Distributed Data Interface
Fiber Distributed Data Interface (FDDI) provides a 100 Mbit/s optical standard for data transmission in a local area network that can extend in range up to 200 kilometers (120 mi). Although FDDI logical topology is a ring-based token network, it does not use the IEEE 802.5 token ring protocol as its basis; instead, its protocol is derived from the IEEE 802.4 token bus 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, although it can use copper 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 which offered the same 100 Mbit/s speeds, but at a much lower cost and, since 1998, by Gigabit Ethernet due to its speed, and even lower cost, and ubiquity.
FDDI, as a product of American National Standards Institute X3T9.5 (now X3T12), conforms to the Open Systems Interconnection (OSI) model of functional layering of LANs using other protocols. FDDI-II, a version of FDDI, adds the capability to add circuit-switched service to the network so that it can also handle voice and video signals. Work has started to connect FDDI networks to the developing Synchronous Optical Network (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 mi). FDDI has a larger maximum-frame size (4,352 bytes) than standard 100 Mbit/s Ethernet 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). 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.
[edit] 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 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.
FDDI Transmission Media FDDI uses optical ï¬ber as the primary transmission medium, but it also can run over copper cabling. Asmentioned earlier, FDDI over copper is referred to as Copper-DistributedData Interface (CDDI). Optical ï¬ber has several advantages over copper media. In particular, security, reliability, and performance all are enhanced with optical ï¬ber media because ï¬ber does not emit electrical signals. A physical medium that does emit electrical signals (copper) can be tapped and therefore would permit unauthorized access to the data that is transiting the medium. In addition, ï¬ber is immune to electrical interference from radio frequency interference (RFI) and electromagnetic interference (EMI). Fiber historically has supported much higher bandwidth (throughput potential) than copper, although recent technological advances have made copper capable of transmitting at 100 Mbps. Finally, FDDI allows two kilometers between stations using multi-mode ï¬ber, and even longer distances using a single mode. FDDI deï¬nes two types of optical ï¬ber: single-mode and multi-mode. A mode is a ray of light that enters the ï¬ber at a particular angle. Multi-mode ï¬ber uses LED as the light-generating devices, while single-mode ï¬ber generally uses lasers. Multi-mode ï¬ber allowsmultiplemodes of light to propagate through the ï¬ber. Because thesemodes of light enter the ï¬ber at different angles, they will arrive at the end of the ï¬ber at different times. This characteristic is known as modal dispersion. Modal dispersion limits the bandwidth and distances that can be accomplished using multi-mode ï¬bers. For this reason, multi-mode ï¬ber is generally used for connectivity within a building or within a relatively geographically contained environment. Single-mode fiber allows only onemode of light to propagate through the fiber. Because only a single mode of light is used,modal dispersion is not present with single-mode fiber. Therefore, single-mode is capable of delivering considerably higher performance connectivity and over much larger distances, which is why it generally is used for connectivity between buildings and within environments that are more geographically dispersed.
FDDI Speciï¬cations FDDI speciï¬es the physical and media-access portions of the OSI reference model. FDDI is not actually a single speciï¬cation, but it is a collection of four separate speciï¬cations each with a speciï¬c function. Combined, these speciï¬cations have the capability to provide high-speed connectivity between upper-layer protocols such as TCP/IP and IPX, and media such as ï¬ber-optic cabling. FDDI’s four speciï¬cations are the Media Access Control (MAC), Physical Layer Protocol (PHY), Physical-Medium Dependent (PMD), and Station Management (SMT). The MAC speciï¬cation deï¬nes how themediumis accessed, including frame format, token handling, addressing, algorithms for calculating cyclic redundancy check (CRC) value, and error-recovery mechanisms. The PHY speciï¬cation deï¬nes data encoding/decoding procedures, clocking requirements, and framing, among other functions. The PMD speciï¬cation deï¬nes the characteristics of the transmission medium, including ï¬ber-optic links, power levels, bit-error rates, optical components, and connectors. The SMT speciï¬cation deï¬nes FDDI station conï¬guration, ring conï¬guration, and ring control features, including station insertion and removal, initialization, fault isolation and recovery, scheduling, and statistics collection. FDDI is similar to IEEE 802.3 Ethernet and IEEE 802.5 Token Ring in its relationship with the OSI model. Its primary purpose is to provide connectivity between upper OSI layers of common protocols and the media used to connect network devices.
[edit] Standards
FDDI standards include:
[edit] References
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C". This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.gdfgdf
