With its acronym in English (Fiber Distributed Data Interface defined by Abbreviationfinder.org), FDDI is a set of ISO and ANSI standards for the transmission of data in local or wide area computer networks (LAN) using fiber optic cable. They emerged in the mid-1980s to support high-speed workstations, which had pushed the capabilities of existing [Ethernet] and Token Ring technologies to the limits of their capabilities.
They are implemented by means of a star physics (the most normal) and double token ring logic, one transmitting in a clockwise direction (main ring) and the other in the opposite direction (backup ring or back up), which offers a speed of 100 Mbps over distances of up to 200 meters, supporting up to 1000 connected stations. Its most normal use is as a backbone technology to connect copper LAN networks or high-speed computers to each other. Traffic on each ring travels in opposite directions. Physically, rings are made up of two or more point-to-point connections between adjacent stations. The two rings of the FDDI are known as primary and secondary. The primary ring is used for data transmission, while the secondary ring is generally used as a backup. Two types of stations are distinguished in an FDDI network: Class B stations, or stations of a connection (SAS), are connected to a ring, while those of Class A, or double connection stations (DAS), connect to both rings. SASs connect to the primary ring through a hub that provides connections for multiple SASs. The hub ensures that if there is a power failure or interruption in any given SAS, the ring is not interrupted. This is particularly useful when connecting to the PC ring or similar devices that are frequently switched on and off. FDDI networks use a token transmission mechanism similar to that of Token Ring networks, but also accepts the real-time allocation of network bandwidth, by defining two types of traffic:
- Synchronous Traffic: It can consume a portion of the total 100 Mbps bandwidth of an FDDI network, while asynchronous traffic can consume the rest.
- Asynchronous Traffic: It is assigned using an eight-level priority scheme. Each station is assigned an asynchronous priority level.
Synchronous bandwidth is allocated to stations that require continuous transmission capacity. This is useful for transmitting voice and video information. The remaining bandwidth is used for asynchronous transmissions. FDDI also allows extended dialogues, in which stations can temporarily use all of the asynchronous bandwidth. The FDDI priority mechanism can block stations that cannot use synchronous bandwidth and have too low an asynchronous priority. Regarding encoding, FDDI does not use the Manchester system, but instead implements an encoding scheme called 4B / 5B scheme, in which 5 bits are used to encode Therefore, sixteen combinations are data, while the others are for control. Due to the potential length of the ring, a station can generate a new frame immediately after transmitting another, instead of waiting for its return, so it may be the case that there are several frames in the ring at the same time. The signal sources for FDDI transceivers are LEDs (light emitting diodes) or lasers. The former are typically used for runs between machines, while the latter are used for primary backbone runs.
Frames in FDDI technology have a particular structure. Each frame is made up of the following fields:
- Start delimiter, which indicates the beginning of a frame, and is made up of signaling patterns that distinguish it from the rest of the frame.
- Frame control, which contains the size of the address fields, whether the frame contains asynchronous or synchronous data and other control information.
- Destination address, which contains the physical address (6 bytes) of the destination machine, which can be a unicast (singular), multicast (group) or broadcast (each station) address.
- Source address, which contains the physical address (6 bytes) of the machine that sent the frame.
- Frame check sequence (FCS), a field that completes the originating station with a calculated cyclic redundancy check (CRC), the value of which depends on the content of the frame. The destination station recalculates the value to determine if the frame has been corrupted during transit. The frame is discarded if it is corrupted.
- End delimiter, which contains symbols that indicate the end of the frame.
- Frame status, which allows the source station to determine if an error has occurred and if the receiving station recognized and copied the frame.
Media on FDDI networks
FDDI specifies a 100 Mbps two-ring LAN with token transmission, which uses a fiber optic transmission medium. Although it works at higher speeds, FDDI is similar to Token Ring. Both network configurations share certain characteristics, such as their topology (ring) and their method of access to the medium (token transfer). One of the characteristics of FDDI is the use of fiber optics as a transmission medium. Fiber optic offers several advantages over traditional copper cabling, for example:
- Safety: fiber does not emit electrical signals that can be intercepted.
- Reliability: fiber is immune to electrical interference.
- Speed - Optical fiber has a much higher performance potential than copper cable.
There are two kinds of fiber: single mode (also called single mode); and multimode. Single-mode fiber allows only one mode of light to propagate through it, while multimode fiber allows for multiple modes of light to propagate. The modes can be represented as beams of light rays entering the fiber at a certain angle. When multiple modes of light propagate through the fiber, they can travel different distances, depending on their angle of entry. As a result, they do not arrive at their destination simultaneously; This phenomenon is called modal dispersion. Singlemode fiber can accommodate higher bandwidth and allows longer cable lengths than multimode fiber. Due to these characteristics, singlemode fiber is often used for inter-building connectivity while multimode fiber is most often used for intra-building connectivity. Multimode fiber uses the LEDs as light-generating devices, while single-mode fiber generally uses lasers.
FDDI (Fiber Distributed Data Interface) basically consists of a fiber optic ring per token pass. Token-ring passing refers to the method by which a node connected to the FDDI ring accesses it. The ring topology is physically implemented with fiber optics. Nodes cannot transmit data until they take over. This token is actually a special frame that is used to indicate that a node releases the token. When a node detects that frame and has data to transmit, it captures the frame by removing it from the ring, and releases it when it ends or when its time of possession of the token ends. FDDI provides high-speed interconnection between local area networks (LAN), and between these and wide area networks (WAN). The main applications have focused on the interconnection of Ethernet LAN networks and of these with X.25 WAN networks. Both in connection with these network technologies and with others, they all connect directly to the main FDDI network (backbone). Another application is the interconnection of high-speed remote peripherals to mainframe- type computers. To ensure operation, when a computer is disconnected, damaged, or shut down, a mechanically operated optical switch bypasses the node, removing it from the ring. This security, together with the fact that speeds of 100 Mbps are compatible with distances of 100 km, make FDDI an optimal technology for a large number of applications.