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News and comments on community broadband networks, the communities deploying them and the technologies that support them. Published by Denise Frey and Al Bonnyman.

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Tuesday, October 28, 2003


Using multimode fiber for FTTH -- not a promising idea

Every so often I hear someone proposing using multimode fiber for FTTH (fiber to the home) networks. I'm not sure where this idea comes from -- perhaps its' use in 10 Gigabit Ethernet local area network (LAN) applications or the availability of cheap optoelectronic components for use with multimode fiber.

Trying to build a multimode FTTH system and probably always will be a poor idea. There are several reasons to stick with single-mode fiber:

1. It's less expensive than multimode (and probably always will be). Multimode fiber needs careful (and possibly slower) preform manufacturing to get bandwidths as high as 2000 MHz-km. It also requires greater use of rare-earth dopants.

2. Those same dopants also give multimode its' higher attenuations (and thus reduced distances). If you read Corning's brochure for its' high bandwidth Infinicor multimode fiber, you'll see Corning is advocating it for premises links <300 m when used in 10 Gigabit Ethernet links. (This is not likely to change in the future unless some new rare earths are discovered)

3. Single-mode fiber has extraordinarily high bandwidth. Most of the fiber being sold now has reduced or zero water peaks, allowing the use of twenty wavelengths each carrying 10 or 40 MBps data streams using CWDM -- coarse wavelength division multiplexing.

4. Existing cable TV, telco and power utility fiber already installed in the U.S. is 99.999% single-mode. The most likely locations to place a box with PON splitters probably already are served by singlemode fiber to an existing telco box or cable TV node. Using multimode downstream of the splitter would require running new fiber upstream and abandoning the existing fiber.

5. Less tangible, but probably a factor is that all the likely players -- telcos, cable TV operators, power utilities -- now use only single-mode and will likely resist any switch. This would be especially true of telcos.

High bandwidth multimode fiber is good stuff, just not for a FTTH application.

A major reason to use multimode in a premises fiber build is that many poorly capitalized "Mom and Pop" building wiring contractors don't have more expensive singlemode splicing equipment and they employ very low cost, less-skilled labor. That's not true in the outside plant world where telcos, cable TV operators, power utilities and their contractors own 1000s of single-mode fusion splicers.

Also, multimode fiber is easier to connectorize with lower cost connectors. Premises fiber builds are connector-rich with many patch panels, etc. Companies are frequently reorganized, offices reconfigured, LAN equipment moved, etc. Subdivisions -- except trailer parks -- are not often reorganized.

Connector-rich network designs are not used in the outside plant environment, which is likely to remain a splice-only environment for a long time. Some carriers are uncomfortable deploying connectors in areas with wide temperature swings because of past issues with change in connector loss. Additionally, many lasers don't like the back reflections that come from some connectors.


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