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What is 100BASE FX?


New technologies generate great excitement, but sometimes older technologies continue to serve important needs. An example of such a technology is 100BASE-FX. Although 100BASE-FX was developed in the mid-1990s, it is still in use today, 10 years later, when we also have Gigabit and 10-Gigabit available. Why? One reason is that 100BASE-FX has the longest range over multi-mode fiber optic cable of any Ethernet technology. While 100BASE-FX can reach two kilometers using any quality of multi-mode fiber, the Gigabit maximum range is 550 meters, and 10-Gigabit maximum range is 300 meters on only the highest-quality multi-mode fiber. 


100BASE-FX networks are wired together in a star topology using fiber-optic cabling and 100-Mbps fiber-optic hubs or Ethernet switches. The maximum length of any segment of fiber-optic cabling connecting a station (computer) to a hub is 412 meters. The grade of fiber-optic cabling used is usually two-strand multi-mode fiber-optic cabling, with one strand carrying transmitted data and the other strand receiving data. However, you can also use two-strand single-mode fiber-optic cabling. If multi-mode fiber-optic cabling is used, the variety used is typically a grade with a 62.5-micron core diameter. Repeaters can be used to extend the length of cabling and for interfacing between 100BASE-FX /TX and 100BaseT4 segments. The maximum allowable distances with repeaters are 2 kilometers using multimode fiber-optic cabling and 10 kilometers using single-mode fiber-optic cabling. Only one or two repeaters can be used per collision domain, depending on whether Class I or Class II repeaters are used.

Benefits of 100BASE-FX:

After a decade of existence, there are still good reasons to use 100BASE-FX. There are many applications where higher speed is not needed and where there is existing multi-mode fiber in place. When 100 Megabits/sec is adequate, there are several benefits of 100BASE-FX: 
  • can extend the network to greater distances than copper cabling can support 
  • noise immunity (fiber is immune to external interference) 
  • security (fiber is difficult, though not impossible, to tap) 
  • electrical immunity (there are no grounding issues with fiber) 
Factories can be difficult environments for computer networks due to many sources of external interference that can affect the signal on copper cabling. For factories, fiber-based networks are often the only option. In large campus environments, fiberoptic cabling is required to reach the distances between buildings. Because of the FDDI legacy, many of those fiberoptic links are constructed of multimode fiber with distances no greater than two kilometers. When 100-Megabit communication is adequate, this campus example is a perfect fit for 100BASE-FX. Department stores and grocery chains are other examples where 100-Megabit communication is adequate, but the challenges are long distances from the computer room to the point-of-sale devices and cash registers. Again, 100BASE-FX is ideal for such a network.Military and other high-security networks often standardize on fiberoptic networking because of its inherent security: signals do not "leak" from the cables, and the cables are very difficult to tap without being detected.

Finally, cost is a big benefit of 100BASE-FX, because 100BASE-FX typically uses LEDs instead of higher-cost lasers. With less expensive transceivers at each end of the link, the overall system cost is lower. Existing multimode fiber segments further reduce the cost by allowing customers to re-use that fiber instead of installing new fiber. 

What is the range of 100BASE FX running in half-duplex?

100Base-FX is simply Fast Ethernet over fiber. Originally, the specification was known as 100Base-X over CDDI (Copper Data Digital Interface) or FDDI (Fiber Data Digital Interface). Because the signaling is so vastly different, these two technologies were split into 100Base-FX and 100Base-TX. 100Base-FX runs over multi-mode fiber. Now 100Base-T4 was the specification created to upgrade 10Base-T networks over Cat-3 wiring to 100 Mbps without having to replace the wiring. Using four pairs of twisted pair wiring, two of the four pairs are configured for half-duplex transmission (data can move in only one direction at a time).

Gigabit Ethernet:

The fastest form of Ethernet is currently Gigabit Ethernet, also known as 1000BaseT over Cat5 or higher grade cable, using all four pairs of the cable. It uses a physical star topology with logical bus. There is also 1000Base F, which runs over multi-mode fiber optic cabling. Data transmission is full-duplex, but half-duplex is also supported.


When using 100Base-FX with repeaters for backbone cabling runs, Ethernet switches cannot be more than 412 meters apart when running in half-duplex mode and 2 kilometers apart when running in full-duplex mode.


Even 10 years after its creation, 100BASE-FX is a viable technology with compelling reasons for its use. There are many situations where existing multi-mode fiber is available, and that fiber can be used to extend the network with 100BASE-FX links. Also, the long range of 100BASE-FX allows connectivity over multi-mode fiber where Gigabit does not reach. Some proprietary equipment is designed for 100BASE-FX, and those legacy solutions are still in use today. And military installations that prefer fiber optic connectivity for security reasons still benefit from the long distances 100BASE-FX supports over multi-mode fiber optic cabling.  For reasons of low cost components, ability to re-use existing multi-mode fiber and support for long distances, 100BASE-FX will be in use for many years to come — and ProCurve will continue to support this proven technology even as we bring new switches to market. 

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Fast Ethernet 
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