Fiber Distributed Data Interface (Networking)

The Fiber Distributed Data Interface (FDDI) is a high-speed network that employs a counter-rotating token-ring technology for fault tolerance. It provides interconnectivity between computers and peripherals, including the interconnection of LANs, within a building or campus environment. Originally conceived to operate over multimode fiber-optic cable, FDDI can run over single-mode fiber-optic cable, shielded twisted pair copper, and even unshielded twisted pair copper wiring.

Figure 45

Frame format of FDDI.

FDDI uses two counter-rotating rings: a primary ring and a secondary ring. Data traffic usually travels on the primary ring. The secondary ring operates in the opposite direction and is available for fault tolerance. If appropriately configured, stations may transmit simultaneously on both rings, thereby doubling the bandwidth of the network to 200 Mbps.

Frame Format

The ANSI X3T12 standard defines a multifield frame format for FDDI (Figure 45), and is composed of the following:

PREAMBLE Stations need to synchronize with the signal’s frequency, which is done by means of 64 preamble bits.

STARTING DELIMITER This one-byte field indicates the start of a frame. FRAME CONTROL This one-byte field is used for access and frame control.

DESTINATION ADDRESS This six-byte field contains the destination MAC address.

SOURCE ADDRESS This six-byte field contains the source MAC address.

INFORMATION FIELD This variable-length field (up to 4478 bytes) contains the user data.

FRAME CHECK SEQUENCE This four-byte field contains the checksum for error control.

ENDING DELIMITER This four-bit field indicates the end of a frame.

FRAME STATUS This 12-bit field indicates the status of a frame, such as error detected, addressed recognized, and frame copied.

Token Passing

FDDI uses a timed token protocol, while 802.5 token ring uses a priority/reservation token access method. This means there are some differences in frame formats and in how a station’s traffic is handled. Management of the rings is also different.

With FDDI, a timed token-passing access protocol passes frames of up to 4,500 bytes in size, supporting up to 1,000 connections over a maximum multimode fiber path of 200 km (124 miles) in length. When FDDI is run over copper wire, such as Category 5 cabling, the distance between connections must be less than 100 meters.

Each station along the path serves as the means for attaching and identifying devices on the network, regenerating and repeating frames sent to it. Unlike other types of LANs, FDDI allows both asynchronous and synchronous devices to share the network. FDDI stresses reliability and its architecture includes integral management capabilities, including automatic failure detection and network reconfiguration.

Any change in the network status—such as power-up or the addition of a new station—leads to a “claim” process during which all stations on the network bid for the right to initialize the network. Every station indicates how often it must see the token to support its synchronous service. The lowest bid represents the station that must see the token most frequently. That request is stored as the Target Token Rotation Time (TTRT). Every station is guaranteed to see the token within two times the number of TTRT seconds of its last appearance.

This process is completed when a station receives its own claim token. The winning station issues the first unrestricted token, initializing the network on the first rotation. On the second rotation, synchronous devices may start transmitting. On the third and subsequent rotations, asynchronous devices may transmit, if there is available bandwidth. Errors are corrected automatically via a beacon-and-recovery process during which the individual stations seek to correct the situation.

Standards

These processes are defined in a set of standards (X3T12) sanctioned by the American National Standards Institute (ANSI). The standards address four functional areas of the FDDI architecture:

PHYSICAL MEDIA DEPENDENT Data is transmitted between stations after converting the data bits into a series of optical pulses. The pulses are then transmitted over the cable linking the various stations.

The PMD sublayer describes the optical transceivers, specifically the minimum optical power and sensitivity levels over the optical data link. This layer also defines the connectors and media characteristics for point-to-point communications between stations on the FDDI network. The PMD sublayer is a subset of the physical layer of the OSI reference model, defining all of the services needed to transport a bit stream from station to station. It also specifies the cabling requirements for FDDI-compliant cable plant, including worst-case jitter and variations in cable plant attenuation.

PHYSICAL LAYER The Physical Layer (PHY) protocol defines those portions of the physical layer that are media independent, describing data encoding/decoding, establishing clock synchronization, and defining the handshaking sequence used between adjacent stations to test link integrity. It also provides the synchronization of incoming and outgoing code-bit clocks and delineates octet boundaries as required for the transmission of information to or from higher layers. These processes allow the receiving station to synchronize its clock to the transmitting station.

MEDIA ACCESS CONTROL FDDI’s data link layer divides is divided into two sublayers. The Media Access Control (MAC) sublayer governs access to the medium. It describes the frame format, interprets the frame content, generates and repeats frames, issues and captures tokens, controls timers, monitors the ring, and interfaces with station management.

The Logical Link Control (LLC) sublayer, while not part of the FDDI standard, is required for proper ring operation and is part of the IEEE 802.2 standard. In keeping with the IEEE model, the FDDI MAC is fully compatible with the IEEE 802.2 Logical Link Control (LLC) standard. Applications that can currently interface to the LLC and operate over existing LANs, such as IEEE 802.3 CSMA/CD or 802.5 token ring, should be able to operate over an FDDI network.

The FDDI MAC, like the 802.5 token-ring MAC, has two types of protocol data units, a frame and a token. Frames carry data (such as LLC frames), while tokens control a station’s access to the network. At the MAC layer, data is transmitted in four-bit blocks called 4B/5B symbols. The symbol coding is such that four bits of data are converted to a five-bit pattern, therefore the 100-Mbps FDDI rate is provided at 125 million signals per second on the medium. This signaling type is employed to maintain signal synchronization on the fiber.

STATION MANAGEMENT The Station Management (SMT) facility provides the system management services, detailing control requirements for the proper operation and interoperability of stations on an FDDI ring.

It acts in concert with the PMD, PHY, and MAC layers. The SMT facility manages connections, configurations, and interfaces. It defines services such as ring and station initialization, fault isolation and recovery, and error control. SMT is also used for statistics gathering, address administration, and ring partitioning.

Equipment Classes

Three classes of equipment are used in the FDDI environment: single attached stations (SASs), dual attached stations (DASs), and concentrators (CONs).

A DAS physically connects to both rings, while a SAS connects only to the primary ring via a wiring concentrator. In the case of a link failure, the internal circuitry of a DAS can heal the network using a combination of the primary and secondary rings. If a link failure occurs between a concentrator and a SAS, the SAS becomes isolated from the network.

These equipment types may be arranged in any of three topologies: dual ring, tree, and dual ring of trees. In the dual ring topology, DASs form a physical loop, in which case all the stations are dual-attached. In a tree topology, remote SASs are linked to a concentrator, which is connected to another concentrator on the main ring. Any DAS connected to a concentrator performs as a SAS. Concentrators may be used to create the network hierarchy known as a dual ring of trees. This topology offers a flexible hierarchical system design that is efficient and economical. Devices requiring highly reliable communications attach to the main ring, while those that are less crucial attach to branches off the main ring. Thus, SAS devices can communicate with the main ring, but without the added cost of equipping them with a dual-ring interface or a loop-around capability that would otherwise be required to ensure the reliability of the ring in the event of a station failure.

Failure Protection

FDDI provides an optional bypass switch at each node to overcome a failure anywhere on the node. In the event of a node failure, it is bypassed optically, removing it from the network. Up to three nodes in sequence may be bypassed; enough optical power will remain to support the operable portions of the network.

In the event of a cable break, the dual counter-rotating ring topology of FDDI allows the redundant cable to handle normal 100-Mbps traffic. If both the primary and secondary cables fail, the stations adjacent to the failures automatically loop the data around and between rings, thus forming a new C-shaped ring from the operational portions of the original two rings. When the fault is healed, the network reconfigures itself.

FDDI concentrators normally offer two buses corresponding to the two FDDI backbone rings. Fault tolerance is also provided for stations that are connected to the ring via a concentrator because the concentrator provides the loop-around function for attached stations.

Last Word

An extension of FDDI, called FDDI-2, offers Hybrid Mode, which uses a 125 ^sec cycle structure to transport isochronous (i.e., real-time) traffic, in addition to synchronous and asynchronous frames. However, FDDI is limited by distance, while ATM is a highly scalable broadband networking technology that spans both LAN and WAN environments. This means that ATM will eventually dominate corporate networks, especially for running multimedia applications.