What Is a Fiber Distributed Data Interface?

Optical Fiber Distributed Data Interface (FDDI) is a set of protocols for sending digital signals on optical cables by the American National Standards Institute (ANSI). FDDI uses a dual ring token, and the transmission rate can reach 100Mb / s. With support for high-bandwidth and long-distance communication networks, FDDI is often used as a backbone network. ^[1]

Optical fiber distributed data interface FDDI is a high-speed backbone network using optical fiber as the transmission medium. It can be used to interconnect a single computer and a local area network.

FDDI standard adopted

Essentially FDDI consists of four sub-sections, each with its own specific function. Together, these parts enable FDDI to provide high-speed connections between upper-layer protocols (such as TCP / IP, IPX) and media (such as fiber optic cables).

The four sub-specifications of FDDI are

The working principle of FDDI is mainly reflected in the three main processes of FDDI. These three main processes are the establishment of site physical connection, ring initialization, and data transmission.

(1) Establishment of site physical connection

During normal operation of the FDDI network, the station management SMT has been monitoring the operating status of the ring and managing the activities of all stations. The connection management module in the station management SMT is responsible for establishing the end-to-end physical connection on the bidirectional optical cable between each pair of PHY / PMD of the station.

The station tests the quality of the physical connection by sending and receiving certain line state sequences to exchange information such as port types and connection rules with neighboring stations. During the testing process, once a fault is detected, the cause of the fault is determined using trace diagnostics, the fault is isolated, and the network is reconfigured.

(2) Ring initialization

After the site's physical connection is complete, the ring is then initialized. Before performing the initialization of the ring, first determine the target token cycle time TTRT of the system. Each station can propose its own TTRT value with the help of the declaration frame. The system determines the TTRT value according to the established competition rules, and completes the initialization of the ring through the site selected for the TTRT value. The process of determining the TTRT value is usually called the declaration process. The declaration process is used to determine the TTRT value.

(3) Data transmission

FDDI data transmission includes three processes: data sending, receiving, and deleting.

In the FDD1 ring network, a station that wants to send data must wait for the token to arrive at the site and capture the token before sending one or more data frames until all data is sent or until the THT times out. Finally, the site releases a new token.

Each station on the FDDI ring is listening to the frame passing by this station at any time. The station compares the destination address in the frame with the address of this station to decide whether to receive the frame. If the destination address of the frame matches the site address, the station After receiving the frame, the "A" flag bit of the FS field of the frame is set to "1", indicating that the destination station exists; while receiving the frame, the station also performs error checking on the frame. If no error is found, the station copies the data field in the frame and sets the "C" flag bit in the FS field of the frame to "1", indicating that the frame has been received by the destination station; if a CRC error is found, the frame is The "E" flag of the FS field is set to "1".

In the FDDI ring network, when a station receives a frame, it also forwards the frame to the next station. The sending station will continue to listen to frames passing by the station after sending the data. When the sending site detects that the source address of a data frame is the same as the address of this site, it immediately stops forwarding the frame and sets it as an invalid frame, so that some of the frame information that has been forwarded to the ring is forwarded to the next site. It is discarded as frame fragments to prevent these frame fragments from continuing to be transmitted on the ring. Then, the sending station is responsible for deleting the rest of the frame from the ring and detecting the FS field in the frame at the same time. If both the "A" and "C" flags in the FS field are "1", the frame has been successfully received by the destination site; if only the "A" flag is "1", the destination site is present but not received This frame indicates that an error occurred during the transmission of the frame; if the "A" and "C" flags are both "0", it indicates that the destination address in the frame is wrong.

Normally, only the primary ring works, and the backup ring acts as a backup. Once a network failure occurs, whether it is a line failure or a site failure, the FDDI network will automatically rebuild the double ring into a single ring by winding to ensure that the network will not be interrupted. This is an important feature that distinguishes FDDI from other LANs.

The FDDI network defines two types of sites: one is a single connection site (SAS) or class B site; the other is a dual connection site (DAS) or class A site. A single connection site is connected to the main ring of the FDDI dual ring through a concentrator. The concentrator can connect multiple SAS sites at the same time, and ensure that any failure or power failure of any SAS site will not affect the FDDI network. A dual-connected site can be connected to both loops at the same time. ^[1]

The main performance indicators of FDDI are as follows:

(1) Use 802.2LLC protocol, compatible with IEEE802LAN.

(2) Use the token passing MAC protocol based on the IEEE 802.5 token ring standard.

(3) Use double-ring topology to ensure the network has fault tolerance.

(4) Multi-mode fiber, single-mode fiber or twisted pair can be used as the transmission medium.

(5) The transmission rate is 100Mbps, and the transmission rate of optical signal symbols is 125Mbps.

(6) The number of connections is not more than 1,000. If each node is dual-connected, it is not more than 500.

(7) The maximum station spacing is 2km, and the maximum loop length is 100km.

(8) With the ability to dynamically allocate bandwidth, it can support both synchronous and asynchronous data services.

(9) The maximum packet length is 4500 bytes. ^[2]

FDDI can be applied in the following environments.

(1) Computer room network, also known as back-end network, is used in the connection between the large computer and high-speed peripherals in the computer room, as well as environments that require high reliability, transmission rate, and system fault tolerance.

(2) The backbone network of the office or building group, also known as the front-end network, is used to connect a large number of small computers, workstations, personal computers and various peripherals.

(3) The backbone network of the campus network or enterprise network, which is used to connect small computers, servers, workstations, personal computers and local area networks in various buildings in the campus or enterprise.

(4) Multiple campus networks or enterprise backbone networks are used to connect multiple campus networks and enterprise networks that are geographically separated by several kilometers, and become a regional backbone network that interconnects multiple campus networks and enterprise networks. ^[2]