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Technology Stocks : LAST MILE TECHNOLOGIES - Let's Discuss Them Here -- Ignore unavailable to you. Want to Upgrade?

To: ftth who wrote (4720)	7/18/1999 2:33:00 PM
From: wonk	Read Replies (1) \| Respond to of 12823

Thanks Dave. I am familiar with Freeman's book but do not have a copy. I guess it time for a all-day research foray at the local University library. I think this article adds something to the discussion here and over on the GBLX thread: ...System design Unlike other optical LAN systems, the Lucent product operates at 1.55 µm, leveraging existing telecom technology such as modulators and distributed feedback (DFB) lasers. To reduce the power density of the beam sufficiently to satisfy safety regulations, the signal must go through a beam expander, so the design incorporates high power erbium/ytterbium fiber amplifiers to increase signal strength to required levels for transmission through the atmosphere. The modulated beam at the exit port of the transmitter is about 3 cm in diameter, with a divergence of about 0.5 mrad; after a transmission distance of 4 km, the beam diameter increases to about 2 m. A 20-cm-aperture telescope at the receiver end collects the signal and focuses it into a multimode fiber, which passes it through to the demultiplexer. After separation into individual channels, the signals pass through to lensed avalanche photodiode receivers. The system operates at 2.5 Gbit/s (OC-48) over four channels, for an aggregate data rate of 10 Gbit/s. Atmospheric degradation and security An obvious issue with an optical wireless LAN system is the need for line-of-sight positioning, as well as signal degradation due to atmospheric absorption and scattering. "We are susceptible to atmospheric disturbances," acknowledges Jim Auborn, director of the communications technologies department, but he notes that the system is surprisingly robust. "Rain isn't as bad as heavy fog." Snow and hail also present problems, but the group maintains that they can engineer the system to operate effectively over shorter distances even in adverse conditions. "We're transmitting on the order of 0.5 to 1 W per channel," says Auborn. "In typical weather, we lose about 20 dB to geometric spreading over 4 km. We can probably accommodate another 40 dB of loss due to atmospheric effects." Initially, atmospheric degradation would increase the signal bit error rate (BER), but the system could compensate by increasing transmission power or decreasing data rate. Operating at 1.55 µm, rather than the 700 to 800 nm wavelength range more typical of other optical wireless systems, actually renders the system less susceptible to atmospheric degradation. Another advantage is in the regulatory arena—at 1.55 µm, safety-imposed power limits are 50 to 100 times higher than at the shorter wavelengths. With more power, the system can transmit over greater distances, and more reliably. In the wireless arena, security is always an issue. The types of encryption techniques used in RF systems can be incorporated in the optical system as well, but the primary weapon of optical wireless is the wide exit port beamwidth, and the geometric spreading of the beam during transit. The beam expansion reduces the optical power to a level that is difficult to detect. "In order to receive the signal," says Auborn, "somebody is going to have to get right in the line of sight." Safety interlocks designed to power down the system in the event of a beam interrupt by a window washer, for example, would also act to protect the system in the event of interception. "It's probably not as secure as fiber, but it's much more secure than RF."... www2.photonicsonline.com{3C6F48EE Certainly, I take Frank's cautionary comments to heart. However, if one could get availability up to 4-5 nines via a mesh configuration, I think they've got something here. I'm intrigued. ww