To wrap up this series on wireless technology I'm going to talk about private, point-to-point, wireless communications using microwave, laser and or satellite systems.Before we begin, it is important to remember that point-to-point means direct line-of-sight. Unlike omni-directional radio signals that transmit in a spherical pattern, if the end points of the link can’t “see” each other the link will not function. Although this may seen obvious, more than one troubleshooting call has found that the reason the point-to-point link stopped working was because someone constructed a building (or other obstruction) directly between the two endpoints.Microwave technology has been around for a long time and has a proven history in terms of equipment reliability, bandwidth and transmission distance. A single microwave link can support both analog (e.g. telephony and video) and digital signal transmissions making it ideal for educational applications and videoconferencing. In the past, one big application for microwave links was "toll bypass." At that time telephone company long-distance charges were much higher so organizations would connect geographically dispersed sites via microwave links for the purpose of "bypassing" telephone company long-distance circuits (in fact, one company even took its name from this: Microwave Bypass Corporation. The payback was pretty easy to justify because although the microwave equipment represented a fairly large one-time capital investment, once the microwave system was installed, there would no longer be any telephone company long-distance bills (at least for calls between sites linked by the microwave system). Further savings could be obtained when employees were allowed to access the organization's PBX at a remote location for the purpose of obtaining local dial tone. For example, a person could dial an access code to connect to a PBX at a remote site and then dial another access code to obtain local dial tone. This way, long-distance telephone charges could be avoided for both intra-company calls and external calls.A typical point-to-point microwave link can span 2 miles (at 30 Gigahertz, GHz) to 30 miles (at 2 GHz) depending on atmospheric conditions (e.g. rain, snow and ice). Although these environmental factors do affect received signal strength, the manufacturer will calculate enough “fade margin” into the signal strength to ensure that a reliable signal is obtained under anticipated worst-case weather conditions.Most systems are modular so that features and bandwidth can be added as needed. A typical link might consist of an analog video channel and a few T1 circuits to voice and data connectivity. Depending on features, expect to budget between $20,000 to $30,000 for a typical microwave link if no tower construction is involved (e.g. rooftop-to-rooftop).Microwave systems require a license from the FCC and frequency coordination to minimize the possibility of interference, but these are usually minor issues. A microwave vendor will normally include these services in the price of the installation or perform them for a nominal fee. No routine preventive maintenance is required.For short distances (up about 3,500 feet) laser technology is a direct competitor to microwave. Laser systems offer very high bandwidth (multiple T3 or 155 Mbps circuits) and do not require a license or frequency coordination. Laser systems may be subject to degraded performance from vibration, heat distortion, blinding by sunlight, dust, fog, and other forms of interference. Alignment stability is often enhanced by the use of a pilot signal that allows the units to correct for minor misalignment. Solid-state laser transmitters should not require periodic maintenance, but the vendor should be required to quote a calculated Mean Time Between Failure (MTBF).If the need is simply to extend a LAN connection from one building to another one solution might be to connect a directional antenna to a garden variety wireless Ethernet (802.11b or 802.11a) Access Point (AP). Although best known for providing wireless Local Area Network (LAN) connectivity in buildings or open environments, the same low-cost technology can be used for a point-to-point link. Although serious security issues have been raised concerning wireless Ethernet, a point-to-point link is relatively secure since, in a properly designed system, the signal is confined to a narrow beam which makes interception relatively difficult. 802.11b (11 Mbps raw bit rate) equipment currently dominates the market, but 802.11a (54 Mbps raw bit rate) equipment is also available. In addition, at least three vendors are offering circuit modules that will allow a single access point to operate as an 802.11b, 802.11a and 802.11g (22 Mbps raw bit rate) access point.
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