Alternative Radio
You can deploy several PTP wireless connections with so-called highly directional antennas (which extend the range of your wireless base station), alternate radio channels and antenna polarization, where you orient your antennas to work together. It's not uncommon to see half a dozen or more remote sites connected to a single hub site using PTP products such as Proxim's Tsunami. But PMP (point-to-multipoint) setups, where a central point serves multiple remote sites, or spokes, make more sense when the density of links is half a dozen or higher. It all depends on the technology and your geography. Multiple PTP links in a single direction (sector) using 2.4-GHz 802.11 technology, for instance, are limited by the bandwidth required per link (22 MHz) and the total available unlicensed bandwidth (83.5 MHz). A PMP system, in contrast, typically uses a polling protocol to support higher-density applications.
Either way, you must decide whether to use licensed or unlicensed spectrum. Licensed gives you exclusive use of the spectrum in a certain geographical area; with unlicensed, you may share the airwaves. Some WMANs, including those based on the developing 802.16 standard, can operate in licensed or unlicensed spectrum.
Many organizations won't use unlicensed WMANs for fear of interference or security problems. But unlicensed systems, usually based on spread-spectrum and OFDM (orthogonal frequency division multiplexing) protocols, are designed to tolerate interference--directional antennas dramatically minimize this problem. Wireless is still vulnerable to jamming, however.
Meanwhile, today's PMP landscape is dotted by proprietary offerings like Proxim's Tsunami MP.11 and Alvarion's BreezeNet DS.11 Outdoor, which operate in the 2.4-GHz band and are loosely based on the 11-Mbps 802.11b, or Wi-Fi, standard. RoamAD uses a modified 802.11 specification to provide the necessary MAC (Media Access Control) layer needed for outdoor and long-range use.Unlike the IEEE's wireless LAN (802.11) and Wireless Personal Area Network (802.15) specifications, 802.16d was designed for geographically dispersed locations in licensed and unlicensed frequencies between 2 and 11 GHz. The higher frequencies offer more bandwidth, but their shorter wavelengths require line-of-sight, which is realistic for back haul of cellular or Wi-Fi links over open terrain, but not for PMP in a dense urban area or where an outdoor antenna isn't desirable.
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