FDDI Network : Fiber Distributed Data Interface

FDDI Network : In this article, I am going to tell you about a very interesting LAN network, which is named FDDI Network. FDDI means Fiber Distributed Data Interface. FDDI refers to a group of networking specifications that were standardized by ANSI throughout the 1980’s, is a Local Area Network standard designed for transmission over both single-mode and multi-mode fiber-optic cable.

FDDI networks is a specification that describes a 100 Mbps token ring using fiber optics media. FDDI is considered similar to the IEEE 802.5 standard and FDDI was created by ANSI in the mid-1980s. Now I am going to tell what is a FDDI network? What are the advantages and disadvantages of FDDI network? To get complete information about FDDI Network, you have to read the complete article.

What is FDDI Network?

FDDI network is a high performance optical fiber LAN network, which covers an area of 200 km. The data speed in FDDI network is around 100 Mbps and about 1000 stations are connected to it. Due to the high band-width of the FDDI network, it is used as the backbone of copper LANs connections, which means that it plays an important role in the connection. 

Another special feature of FDDI networks is that multimode fiber can be used in this network, because using single-mode fiber for a network running at 100 Mbps speed increases the extra cost which is not needed. FDDI uses LEDs instead of lasers in networks, but this is not only because LEDs are cost efficient but also because FDDI is sometimes used to connect directly to user workstations. Now let's try to understand the topology of FDDI network.

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FDDI Network Topology

Backbone - this applications use the LAN standard because it provides 100 Mb/s bandwidth (Ethernet runs at 10 Mb/s and Token Ring runs at 16 Mb/s). The design of FDDI is such that it does not call more than one error in 2.5x10pow10 bits.

As shown in below figure, FDDI is a redundant-ring, token passing network that provides high-speed connections between groups of slower network devices. The FDDI back-bone refers to the central dual ring structure or two types of ring. The outer ring is usually called A and the inner ring is called B. Only the A ring is used for primary network transmission, it is also called the token ring and the information transmitted on this ring in only one direction. The second ring called as B, in this ring information transmitted in the opposite direction and is used for fault tolerance.

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If an imbalance develops in the primary ring due to a break in the fiber cable or the device is turned off, the secondary ring is activated. The network then ceases transmission on the B ring. As shown in the example below, this scheme provides fault tolerance while maintaining an intact ring structure.

As with the Token Ring LAN, the FDDI allocates access to the LAN using the token passing method. Like Ethernet and Token Ring, FDDI is a shared LAN. The total bandwidth of the LAN is divided equally among the devices connected to it. If more devices are added, the less bandwidth each device gets.

Most networks today use concentrators, which connect multiple devices to a central FDDI backbone. It is possible to "stack" these concentrators to connect multiple nodes (or desktops) to the network. As the following example shows, the number of connections to the desktop is much higher than the number of connections to the FDDI backbone.

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FDDI Standards

The MIC (Media Interface Connector) standard was defined specifically for use in FDDI networks. To reduce the possibility of simple wiring errors (for example, connecting an A-ring cable to a box's B-ring transceiver) the Standards Committee designed special keys for each connector/port mating.

Keying options are based on the specific part of the FDDI LAN that fits the data communication link. I will tell about the main option further.

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FDDI Encoding Scheme

4B/5B encoding scheme uses by FDDI LAN that is 80 percent efficient. This means that for every 4 bits of useful data sent, 5 signal elements are actually transmitted. This 4B/5B encoding scheme needed a serial signal rate of 125 MBd to transmit 100 MB/s. As a result the serial signal for the FDDI link is 125 MBd.

Other connector types approved by the Standards Committee are the ST connector and SC connector for FDDI, shown below. CDDI (Copper Distributed Data Interface, which is FDDI over twisted-wire pair) has emerged as a low-cost technology link with a maximum length of 100 meters. The backbone of the FDDI network, which has a maximum link length of 2 km between devices, will continue to use fiber optics.

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FDDI Structure

The standard FDDI link is made up of four different layers: the MAC (Media Access Control) layer, the PHY (Physical) layer, the PMD (Physical Media Dependent) layer, and the SMT (Station Management). These all layers are described in the below table.

FDDI products are available in the market some products had a MIC connector as their optical interface and 2 rows of 11 pins for their electrical, board interface. Some other products had (1 row of 13 pins) these offered at a lower price. Even smaller FDDI products (1 row of 9 pins, with a dual SC connector) also available in the market . Customers of FDDI MIC transceivers are increasingly using 1 X 9 foot print products due to limited board space and low cost.

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FDDI Network Summary

There are two types of fiber rings in FDDI cabling, one transmits in clockwise direction and the other transmits in anti-clockwise direction. If one cable breaks in some condition, then we can use the other cable as a backup. And if under some circumstances both brakes happen at the same time (like in case of fire and accident in cable duct) then we can join two rings in a single ring, we can do this about twice.

Two classes of stations have been defined by FDDI. We use Class A station to connect both the rings. And we use class B stations (which are slightly cheaper) to connect only one ring. The FDDI network uses 4 out of 5 encoding schemes. As details mentioned above each group of 4 MAC symbols are encoded as a group of 5 bits on the medium. In 32 combinations distribution are like- 16 for data, 3 for delimiters, 2 for control, 3 for hardware signaling and 8 unused.

The basic FDDI protocol is very similar or very matching to the 802.5 protocol. To transmit the data, each station must first capture the token. When the station gets the token, it transmits its data frame and removes the token as soon as the data frame starts moving around. The difference between FDDI and 802.5 is that the stations in the token ring cannot produce a new token until their frame is rotated and returned.

In FDDI, the station is allowed to generate a new token as soon as it finishes its transmission. One reason for this is that FDDI has 1000 stations connected and is spread over an area of   200 km, so to save time and prevent the ring from solidifying, the station has given the permission to produce a new token immediately after transmission.

FDDI networks have high bandwidth, so FDDI quickly became a popular choice for use in universities and businesses. While FDDI was once the world's fastest LAN technology for many years, it was later superseded by Fast Ethernet. Which used to offer 100 Mbps speeds that too at a much lower cost. FDDI can be run at 100 Mbps speed at 100 stations within 200 km. FDDI supports 10 Mbps in Ethernet via network and 4 Mbps in token ring. The distance between two nodes in an FDDI network should be less than one kilometer.

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Use of FDDI Network

FDDI is mainly used to create high speed backbone in the network. Network backbone refers to the network architecture for the data transmission in the network. Which is made from advanced technology, which has the highest capacity in the network, the performance of the network depends on the backbone of the network. If the backbone of the network fails, the whole network fails.

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Advantage Of FDDI Network

➤High Bandwidth: The capacity of data transmission in FDDI network is very high. The bandwidth of the FDDI network can be up to 250 Gbps in a network where FDDI is used.

➤High Security: Fiber optics cable is used in making FDDI network, the network security of the finished fiber optics cable is very high, due to which the security of FDDI network is very strong.

➤Cable Distance: FDDI network uses fiber optics cables. A long fiber optics cable can transmit data over a much longer distance than other media, due to which the signal in FDDI network can be transmitted up to 200 km.

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Disadvantage Of FDDI Network

➤FDDI is called the complex technology of the network because the installation and maintenance of this type of network requires a lot of technical skill.

➤FDDI is a very expensive technology. Because in this connector and adapter are used with fiber optics cable. Which is very expensive.

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Last Word

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