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LAN IN THE SKY

The invisible technology options
There is a wide range of diverse, often incompatible, technology options available for WLAN manufacturers to choose from. This creates hindrances in interoperability. Here's a look at the advantages and disadvantages of each.

Narrowband technology: A narrowband radio system transmits and receives user information on a specific radio frequency. The radio signal frequency is kept as narrow as possible just to help pass the information efficiently. Undesirable crosstalk between communication channels is avoided by carefully coordinating different users on different channel frequencies.

In a radio system, privacy and non-interference are accomplished by the use of separate radio frequencies. The radio receiver filters out all radio signals except the ones on its designated frequency.

Spread spectrum technology: Most WLAN vendors build systems that function on spread spectrum technology. It is a wideband radio frequency technique originally developed by the military for use in reliable and secure mission-critical communication systems. Spread spectrum is designed to provide reliability, integrity, and security, but at the cost of bandwidth efficiency. It consumes more bandwidth than any narrowband transmission, but the signal generated is louder and easier to detect. This is only possible if the receiver knows the parameters of the spread spectrum signal being broadcast. If the receiver is not tuned to the right frequency, a spread spectrum signal appears like background noise. There are two types of spread spectrum radio technology. They are FHSS (Frequency Hopping Spread Spectrum) and DSSS (Direct Sequence Spread Spectrum).

FHSS uses a narrowband carrier that changes frequency in a pattern known to both transmitter and receiver. When the changing pattern is properly synchronized across both ends, the net effect is to maintain a single logical channel. To an unintended receiver, FHSS appears to be short-duration impulse noise.

DSSS generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a chip or chipping code. The longer the chip, the greater the probability that the original data can be recovered.

But a long chip also needs more bandwidth.

Even if one or more bits in the chip are damaged during transmission, statistical techniques embedded in the radio can recover the original data without the need for retransmission. To an unintended receiver, DSSS appears as low-power wideband noise and is ignored by most narrowband receivers.

Infrared Technology: Infrared (IR) systems use very high frequency-light waves to carry data. The waves are just below visible light in the electromagnetic spectrum. IR cannot penetrate opaque objects and is either direct (line-of-sight) or diffuse technology. Inexpensive direct systems provide a very limited range up to three feet, and are typically used for PANs (Personal Area Networks). High performance direct technology is impractical for mobile users and is only implemented on fixed sub networks.

Soutiman Das Gupta can be reached at soutimand@networkmagazineindia.com

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