New transport methods

When considering methods to move information from one point to another, it is interesting to reflect on how our needs have changed over just the past 20 years. It doesn't seem long ago that we had two options to get program information to the transmitter — equalized telephone lines or analog STL systems. Remote broadcasts were limited to equalized telephone or dial-up POTS lines. Life was simpler then, but now we need to move huge amounts of digital encoded program audio streams along with the usual amount of typical data generated by Internet, e-mail and video conferencing and voice over IP applications.

Wireless networks provide untethered convenience. Photo courtesy of Linksys.

It is interesting that there hasn't been a marked increase in the amount of new commercially available data transport methods during the past five years, particularly with regard to choices of wired connections. The major advancements, and where most of the research and development money has been spent, is in the non-licensed microwave and optical link markets.

Wireless microwave

In 1997, the IEEE adopted the 802.11 standard that provides for a wireless interface between a base station and wireless devices at data rates of 1Mb/s or 2Mb/s either frequency-hopping spread spectrum (FHSS) or direct-sequence spread spectrum (DSSS) on one of 11 channels designated within the 2.4MHz frequency spectrum. The 802.xx standard essentially defines an Ethernet protocol to be used in conjunction with a wireless medium.

In 1999, an enhancement to the 802.11 standard designated 802.11b was approved. This standard increased throughput to 11Mb/s with the capability to reduce its speed as needed to account for poor signal conditions in intervals of 5.5Mb/s, 2Mb/s or 1Mb/s. The only modulation scheme 802.11b uses is DSSS. This standard is also called Wi-Fi, which is becoming widely deployed within schools, businesses and a multitude of local hotspots such as those found in coffee shops and some hotels.

The 802.11a standard permits the use of the 5.8MHz frequency spectrum. 802.11a uses orthogonal frequency division multiplexing (OFDM) modulation to provide as much as 54Mb/s of data throughput in one of eight possible channels. While 802.11a may also be called Wi-Fi, it is in reality not compatible with 802.11b systems operating in the 2.4MHz band.

The 802.11g standard permits data rates up to 54Mb/s in the 2.4MHz band using a combination of ODFM for rates above 20Mb/s and DSSS for rates below 20Mb/s. Certified as a Wi-Fi device, 802.11 will ultimately replace 802.11b.

Blue Tooth is the code name for yet another wireless standard that provides wireless communications on the 2.45MHz band using FHSS modulation. Probably the biggest negative for Blue Tooth is that it does not support IP and TCP/IP protocols, which may limit its use to certain proprietary applications or for localized communications such as that between a cell phone and laptop PC, for example.

FSO links are proving to be practical and do not require frequency coordination. Photo courtesy of Lightpointe.

Free-space optics

Free-space optics (FSO) is actually a technology first developed by Alexander Graham Bell in the late 19th century before he demonstrated the telephone. At that time he had created a wireless optical system that traveled about 600 feet and was aptly named the Photophone. The technology emerged again in the 1960s when significant improvements permitted it to be used for military applications.

FSO is an optical line-of-sight wireless method to send data between different locations. It uses a laser to send signals through space, similar to a traditional microwave radio link, but operating in the infrared spectrum. An FSO transmitter system consists of a digitally modulated low-power (about 50mw) infrared laser diode focused through a high-performance lens. The laser is focused on a highly sensitive photon detector-based receiver based on PIN-diodes or a newer technology (derived from an older device) called avalanche photodiode that allow the receiver to work under lower signal conditions. Commercially available FSO systems operate full-duplex, similar to radio-based DS-1/DS-3 links.

Are FSOs as reliable as RF radio systems operating in the microwave spectrum? Of course, similar to microwave systems, FSOs need to have a clear line-of-sight. However, unlike microwave systems, light can be blocked and reflected by a number of environmental conditions such as rain, snow and fog. Current FSO systems uses a number of advanced techniques to mitigate some of the problems caused by weather using a combination of multiple-beam transmitters and automatic tracking receivers. If the beam is blocked, such as that caused by birds or another object in the path, the IP stream would be retransmitted until the connection is re-established.

Similar to fiber optic technology, FSO is capable of supporting bandwidths far in excess of what can be provided by microwave-based systems — up to 2.5Gb/s and 4,000 meters.

The use of RF-based and optical wireless systems is growing at a steady rate and is expected to surpass that of wired connections within the next 10 years. Additional spectrum, strengthened security, equipment prices and overall ease and cost to deploy wireless systems will no doubt drive this trend.

While most discussions of wireless systems deal with point-to-point connections, many manufacturers are producing base stations that not only communicate with remote clients but also with other base stations. This ability allows the creation of local meshed networks that can be propagated through a small campus or an entire region, similar to that of cellular telephone networks. Meshed networks are inherently much more reliable and robust compared to that of a simple point-to-point connection due to the ability to re-route traffic based on usage and most available connections to the switched network.

McNamara is president of Applied Wireless, Elkins Park, PA.

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