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Wireless Beyond the Ivy-Covered Halls
8/22/2006
Point to Multipoint (PMP)
This wireless topology is in use in scores of locations across the US, and is less expensive than other large-coverage-area options at today’s cost points. PMP may use either licensed or unlicensed frequencies, depending on the manufacturer. And PMP uses a “base station” topology, in which antennas are sited on one or more tall towers from which signals are sent and received—usually in six sectors of 60 degrees each. (Think of a circle with three lines through it to create six pie-shaped coverage sections.)
Outdoor Wireless Technologies: 6 Caveats
1. Technological change is a constant. Plan for major upgrades every three years.
2. All wireless networks are shared media, so the more users in any given coverage area, the less bandwidth each will have.
3. Although wireless throughput rates keep increasing, so do rates for transmission via fiber-optic cabling. If you need a high-speed, reliable backbone link, think fiber first.
4. An outdoor wireless signal can’t penetrate far into buildings, so if networking is desired throughout a building, an internal LAN or WiFi network would still be needed. Reception in high-rise buildings is a special problem.
5. Wireless networks operate on radio frequencies (RF). Designing coverage areas for RF is as much an art as a science. Best case: 95 percent coverage.
6. Most wireless systems require a clear line of sight (LOS) between the transmitter/receiver units, although some manufacturers now can offer “near line of sight.” The difference is significant, so always verify!
In this type of network, each end user must have an antenna/receiver (usually proprietary to the manufacturer of the PMP system) at his site, pointing at the base station; a line of sight between the two is required. PMP can provide approximately 600-800 Kbps of twoway service per end user. Cautionary note: Although PMP systems are still available for sale, there have been no new installations. It appears the market is moving away from this mostly proprietary technology and toward meshed networking.
Meshed Networks
A meshed network topology permits multiple lower-capacity radios to interconnect, thus allowing greater coverage, plus some ability for the network to “heal itself” when a node fails. BelAir Networks, Cisco Systems, Motorola, Proxim Wireless, Strix Systems, and Tropos Networks are all manufacturers of WiFi-based mesh networking equipment. As of June 2006, Tropos has the largest installed base of larger-scale mesh networks. For a good, basic understanding of mesh networking technology, see the white paper here.
Funding?
Stay tuned for general wireless cost considerations in a future column. In the meantime, to help your staff and students living off campus, why not forge a mutually beneficial arrangement with the city in which your campus resides? Municipalities often have good reasons (which usually include improved public safety communications) to implement outdoor wireless networks, and many are exploring their options right now. This option should at least be in your playbook.
Wendy Chretienis a Network Systems Consultants with Elert & Associates , an independent technology consultancy.
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