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To access the contents, click the chapter and section titles.
Wireless Networking Handbook
Laser-based systems offer more privacy than radio links. Someone wanting to receive the laser data signal would have to physically place himself directly in the beams path (see fig. 3.4). Also, the eavesdropper would have to capture the light to obtain the data, significantly attenuating or completely disrupting the signal at the true destination. This means he would have to put himself next to the laser modem at either end of the link by standing on top of the building or climbing to the top of a tower. Physical security, such as fences and guards, can effectively eliminate this type of sabotage. A small number of companies sells laser-based wire less MAN components. Heres an overview of the leading products:
LCI's LACELCI (Laser Communications Incorporated) has a product line called Laser Atmospheric Communication Equipment (LACE). LACE is a laser-based system that operates in the near infrared region of the spectrum at a wavelength of 820 nanometers. LCI recommends a maximum range of 1 Km (3340 feet) for LACE, but the actual transmission distance depends on atmospheric conditions, such as rain and smog, and desired data rates. Faster data rates limit the range in order to keep errors at an acceptable level (better than 10-9 error rate). LCIs wireless MAN products interface with a variety of protocols, such as 10 Mbps Ethernet, 4 or 16 Mbps, 2.0 Mbps V.35, T-1/E-1, RS 232/422/423, 34 Mbps E-3, 45 Mbps T-3, 51 Mbps SONET or ATM, 100 Mbps Fast Ethernet 802.3U, 100 Mbps FDDI, and 155 Mbps SONET or ATM. The units that house the laser modems are weather-tight and mount on the top of a building or tripod. LACE will work through glass; however, the glass surface will reduce the light intensity by approximately four percent. In order for the light to penetrate the glass, the beam must be nearly perpendicular to the pane of glass, and the glass cannot have any infrared reflecting coating. SILCOM's FreespaceSILCOM has a laser link product called Freespace that operates using infrared light. Freespace interfaces with and supports 10 Mbps Ethernet, full-duplex Ethernet, or 4/16 Mbps Token Ring at up to 300 meters (1000 feet). Freespace has a built-in, uninterruptable power supply to keep the link running up to three hours if power fails. The unit also protects itself against accumulating ice and snow by automatically warming at temperatures below 34°F (1°C). Freespace has a unit that you attach to the outside of the building with a clear line of sight to the unit at the opposite end of the link. A built-in telescopic sight and signal strength meter eases installation. You can place a repeater, router, or bridge inside the building and attach it to the head unit via an optical fiber cable. Wireless MAN Case StudiesMany companies are beginning to reap the benefits of wireless networks by seeing better returns on initial investments of wireless MAN hardware in comparison to other forms of connectivity, such as optical fiber installations and 56 Kbps and T1 leased services. The following case studies show how some companies are using wireless MAN products. Wireless ATM/POS Data Communications Network1
First Security Bank of New Mexico (formerly First National Bank in Albuquerque) has one of the largest wireless ATM (Automated Teller Machine) networks in the western US with over eighty digital wireless modems in operation. The bank has four point-to-multipoint wireless systems currently installed throughout the metropolitan Albuquerque area. Two of the systems each serve approximately 20 ATMs at branch offices. The third system serves about 20 stand-alone ATMs at convenience stores, supermarkets, hotels, sport facilities, travel centers, and so on. The fourth system connects an electronic benefit transfer (EBT) system between the banks data center and eight point-of-sale (POS) terminals in supermarkets throughout the city. One of the systems accommodates a recreational vehicle (RV) equipped with two ATMs. Believed to be the first of its kind in the world, the RV is available for special events such as conventions, the annual Hot Air Balloon Fiesta, and state and county fairs where no other banking facility is available. Figure 3.5 illustrates the system configuration. This system uses the RAN products available from Multipoint Networks.
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