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Decisions, decisions

As the performance of optical fiber cabling evolves, end users are faced with an increased number of choices.


March 1, 2001  


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Telecommunications cabling standards and performance continue to evolve, and there are now as many choices available for optical fiber as there are for copper. So how is an end user to make an informed decision?

To start, the user must choose between multimode fiber or singlemode fiber. To make this choice, one needs to know which applications will run over the fiber, the relative economics of the optical equipment and the maximum distances between equipment. Most LAN applications up to 622 Mbps use light emitting diode (LED) light sources that are designed to work over multimode fibers. They provide operating distances that range from 300 to 2000 metres and are more economical than applications using laser light sources that are designed to work over singlemode fibers.

Laser light sources are used for Gigabit applications, because LEDs are not fast enough to transmit the narrow pulses of light that are required for gigabit speeds. There are two versions of Gigabit Ethernet over fiber. One version is called 1000BASE-SX. It uses a Vertical Cavity Surface Emitting Laser (VCSEL) that operates at 850 nm wavelength of light and is designed to run over multimode fiber. This version is a more economical alternative than the other version, 1000BASE-LX, which uses a conventional laser light source operating at a 1300 nm wavelength of light and works over both multimode and singlemode fiber.

Generally speaking, multimode fiber has an advantage in that that it is backward compatible with most legacy LAN applications that use LEDs, and is also more economical than corresponding applications that run over singlemode fiber. Singlemode fiber has an advantage in that it can support high-speed applications at much longer distances. Multimode fiber tends to be a good choice for intra-building backbone applications and singlemode fiber is a good choice for inter-building applications.

WHICH FIBER?

Once a decision has been made to use multimode fiber, the user must then determine which mutimode fiber (type and grade) to install.

The forthcoming revision of the TIA 568-B.1 and B.3 standard recognizes both 50/125 micron and 62.5/125 micron multimode fiber for horizontal and backbone cabling. This is a change from the current TIA 568-A standard, which only recognizes 62.5/125 micron multimode fiber. The reason for the change is that 50/125 micron fiber provides a higher bandwidth than 62.5/125 fiber at the 850 nm wavelength window. Consequently, it can support distances that are twice as long for the 1000BASE-SX Gigabit Ethernet application (see Table 1).

The bandwidth of multimode fibers has traditionally been characterized using an Over Fill Launch (OFL) test method, which is representative of LED light source transmission. A new method for measuring bandwidth of multimode fibers is currently under development at TIA. This method uses a Restricted Mode Launch (RML) technique, which is more representative for laser light transmission.

THINKING ABOUT TOMORROW

The final criterion that the user must consider is support for future applications. What type of fiber will be needed for the next generation 10 Gbps Ethernet standard? The IEEE 802.3ae working group is currently developing four different Physical Media Dependent (PMD) specifications for 10 Gbps Ethernet over fiber. One of theses specifications (10GBASE-LX4) will be designed to operate over standard 62.5/125 and 50/125 micron multimode fibers, using a Wide Wavelength Division Multiplexing (WWDM) technology for distances up to 300 metres. This option will be relatively expensive, but will accommodate the installed base of multimode fibers.

The most economical option under development is 10GBASE-SR/WR, which will be designed to operate at 10 Gbps serial data rates, using an 850 nm VCSELs over multimode fibers. This option can support distances of less than 100 metres over conventional multimode fibers, and up to 300 metres over the next generation 50/125 micron multimode fibers. The specification for the next generation multimode fiber is currently under development in TIA TR 42.8 subcommittee. The remaining two options for 10 Gbps Ethernet (10GBASE-LR/LW and 10GBASE-ER/EW) will be designed to operate over singlemode fiber for distances extending from 2 km to 40 km.CS

TABLE 1. Operating distances for gigabit ethernet over fiber
Worst Case 1000BASE-SX
@ 850 nm 62.5 mm MMF 50 mm MMF Unit
OFL bandwidth 160 200 400 500 MHz.km
Operating distance 220 275 500 550 m
Worst Case 1000BASE-LX
@ 1300 nm 62.5 mm 50 mm SMF Unit MMF MMF
OFL bandwidth 500 400 500 N/A MHz.km
Operating distance 550 550 550 5000 m

Paul Kish is Director of IBDN Systems & Standards at NORDX/CDT in Pointe Claire, PQ. He is also Chair of the TR-42 engineering committee.

Disclaimer: The information presented is the author’s view and is not official TIA correspondence.


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