Electronic Design

Optical Wireless Technology Bumps LANs To 100 Mbits/s

There's a new way to get 100-Mbit/s speeds in a LAN: optical wireless. Wireless LANs certainly aren't new, but they've never even come close to replacing the old-style wired LAN. The new 802.11b Ethernet wireless LAN standard shows great promise for widespread use because of its interoperability between different vendors and its declining cost. But at a maximum of 11 Mbits/s, it can't compete with CAT5 wired Ethernet, which routinely operates at 100 Mbits/s.

Maybe now it can. Texas Instruments recently announced a reference design for a 100-Mbit/s Ethernet-compliant, full-duplex wireless LAN using optical techniques. The design combines TI's DSP and microelectromechanical-systems (MEMS) capability.

Here's how an optical wireless link works. Each PC has a network interface card that connects to an external optical transceiver module containing a laser and photodetector. This unit is positioned so it can see a wall- or ceiling-mounted optical transceiver and switch unit. This "basestation" switch contains a MEMS optical mirror that's electromechanically controlled by a DSP chip (see the figure). The algorithms let the mirror direct and track the invisible 850-nm infrared beam. Even if users move their transceivers, the system will track and find them to maintain the connection. A basestation can handle eight to 24 users.

The big benefit of wireless optical is the dramatic cost reduction afforded by eliminating the need for wiring. Furthermore, people and their PCs may be moved or repositioned quickly and easily without the cost of moving or adding cables. Optical wireless also is ideal for leased facilities or temporary situations like trade shows. The primary downside is that optical networking requires a clear line-of-sight path for communications, though this isn't a problem in many cases. Distances up to 50 m can be achieved.

While this optical system will most likely be found in LANs, it additionally can be employed in other wireless applications. These include short-range free-space optical links between buildings or in the "last-mile" telephone connections to homes and businesses. The 100-Mbit/s rate makes this system useable in nondata applications like video transmission, where speed is essential.

This optical method blows the doors off other wireless technologies. IrDA, another optical method, is good for 16 Mbits/s at up to 2 m. Radio systems work well, but they still come up short. The IEEE-802.11b-standard system provides up to 11 Mbits/s up to 90 m and through walls. Even the newer 802.11a wireless Ethernet standard in the 5-GHz band maxes out at 54 Mbits/s. The HomeRF standard is good for 1.6 to 10 Mbits/s up to 50 m. Bluetooth offers up to 1 Mbit/s over 10 to 30 m.

The optical wireless solution, though, offers a dedicated and switched point-to-point connection. All the other systems are shared multipoint connections at much lower speeds. What's not to like? TI's business manager for optical wireless solutions, Matt Harrison, says that the reference design and related components will be available this quarter. For more information, go to www.ti.com/sc/alp or call (800) 477-8924, ext. 4500.

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