Different wireless technologies that WLANs use

Laser/Infrared

    Both laser and infrared transmission fall under that same category in that the method of transmission and  reception are similar.  Networks are created when optical transmitter and receiver are in the line-of-sight and are able to make the hand shake. In much the same way that your CD player reads the 1's and 0's on the disk data is sent and received by way of packets (packs) of 1's and 0's.  Below is a diagram that will explain how information is exchanged between local networks.

                                                Optical Management Interface (OMI)
                                                            Figure 1. Optical Management Interface (OMI)
                                                                        Ref:  http://www.eagleopt.com/omi.htm
 
        Implementation of such a system can be very cheap or very costly.  Just transceivers themselves can range from thousands of dollars for infrared systems to much higher for laser.  Of course price is strongly dependent on the utility requirements.  Therefore it's crucial for management to weigh the many different factors:
        Pros:

• Very high data transfer rate, up to Gbps for laser and 155Mbps for infrared(1.25 miles)
• Long range in data transfer:  hundreds of miles for laser and upto 2.5 miles for infrared (10Mbps)
• Multiple channel access
• Digital transmission
• Low cost device (cheap IR-LEDs)

        Cons:

• Data transfer rate is inversly proportional to distance
• Large and bulky units prevents mobile networking
• Must have direct-line-of-sight
• Greatly affected by the change in the transmitting medium (air, stray sunlight, clouds)
 Article links: Laser/infrared networking Research and Technology,  FAQs

Micro-Wave (mW)

    Microwave radios have traditionally had a perception of being reliable and fast, but also complicated to install and maintain. Some of today's newer microwave systems have made significant improvements. They are light weight, compact and easy to install and maintain. In fact, the some of the smaller microwave systems require only a 2.5 inch post mounted on the roof or inside an office window to operate effectively. These factors reduce the time it takes to install as well as the related cost. In most cases, all that is required to physically install a microwave system are details such as placement of the antennas, type of mount to be used, and placement of the cable runs from the outdoor radio unit to the indoor data interface unit.    Microwave systems require a clear line-of-site between the two locations. This implies that there can be no obstacles or obstructions in the path of the microwave signal. In order to accomplish this,
microwave antennas are located as high as possible. Generally, microwave antennas are mounted on roofs of buildings, radio towers, or other tall structures. In the event that a clear line-of-site cannot be established, systems are sometimes repeated over objects (i.e., other buildings) or reflected around an object using specific microwave passive repeaters. In many cases it is necessary to perform a site survey in order to assure a clear line-of-site.
    Microwave system performance can be affected by environmental and atmospheric factors. The most irevalent factor that affects microwave signals is rain, especially in the higher frequency bands (above 18 GHz). Rain has the adverse property of absorbing microwave energy and attenuating the signal path. Other factors such as snow, fog, smog, temperature inversions, etc., have minimal effects on microwave performance.
    Even with these environmental factors, microwave communications can offer greater than 99.995 %availability. In order to assure reliable communications, microwave systems are designed to
accommodate the rain factor in several ways. First, the worst case rain rate is identified for a specific region. Most often, microwave path designers plan for rainfalls that exceed the 0.01 % rain rate. This amount can range anywhere from one inch per hour in Arizona to four inches per hour in Florida. Second, the maximum path distance is calculated with the worst case rainfall. This means that systems that can operate up to three miles in Arizona may only be able to operate up to one mile in Florida. Third, antenna size is determined to achieve the desired reliability factor. The larger the antenna size, the higher the focusing factor, or gain. A 99.995 % availability translates into about 26 total minutes of statistical outage in one year.
    Most microwave LAN systems operate in the 23 GHz band and require licensing by the Federal
Communications Commission (FCC). This licensing procedure is a simple process and assures the
user of interference free operation. The licensing process consists of two steps: First, a frequency
coordination is performed to determine an available frequency. Many search firms are available in the U.S. that can perform this task. They basically have access to a regularly updated database that
identifies other existing microwave users in various geographic areas. The geographic coordinates of
your intended site is submitted to one of these search firms to allow them to accurately recommend a unique frequency that has minimal interference potential. Site coordinates are generally obtained from a site survey. Second, a License Request form 402 is filed with the FCC along with the frequency coordination report. After reviewing your submittal, the FCC grants your station the license to operate that specific microwave system at that specific site using the requested frequencies. The process takes about two weeks for frequency coordination and 60 days for the FCC license approval. This entire process can be handled by most search firms.

        Below is a summary of  the pros and cons for microwave LAN units:
                Pros:

• Operating at 24GHz frequency domain, not FCC regulated.
• Can relocate system without relicencing through FCC
• Moderate data transfer rate of up to 20Mbps
• Up to 10 miles transmission range with 2feet high antennae
     Cons:

•  Microwaves are like sound waves which can have interference problem under
•  high traffic condition.
•  Large antennae prevents hand held mobile units
•  Must operate within same room, cannot penetrate walls
 Microwave Networking,
Wireless Networking over Microwave,
10 Questions on Microwave Networking

Radio Frequency (RF)

 Narrowband Technology    A narrowband radio system transmits and receives user information on a specific radio frequency. Narrowband radio keeps the radio signal frequency as narrow as possible just to pass the information.Undesirable crosstalk between communications channels is avoided by carefully coordinating different users on different channel
frequencies.
Spread Spectrum
    Most wireless LAN systems use spread-spectrum technology, a wideband radio frequency technique developed by the military for use in reliable, secure, mission-critical communications systems. Spread-spectrum is designed to trade off bandwidth efficiency for reliability, integrity, and security. In other words, more bandwidth is consumed than in the case of narrowband transmission, but the tradeoff produces a signal that is, in effect, louder and thus easier to detect, provided that the receiver knows the parameters of the spread-spectrum signal being broadcast. If a receiver is not tuned to the right frequency, a spread-spectrum signal looks like background noise. There are two types of spread spectrum radio: frequency hopping and direct sequence.
    A private telephone line is much like a radio frequency. When each home in a neighborhood has its own private telephone line, people in one home cannot listen to calls made to other homes. In a radio system, privacy and noninterference are accomplished by the use of separate radio frequencies. The radio receiver filters out all radio
signals except the ones on its designated frequency.
    An interesting note to the development history of Spread Spectrum is that the idea of didn't really come from the think tank of the armed forces or its affiliates.  Instead, the idea of spreading the signal into this wideband to guide
long range missiles without being intercepted by enemies came as a result of a dinner discussion between an actress and a composer.  Check this story out at the Secret Communications System link.

back to main menu

Frequency Hopping

Frequency-Hopping Spread Spectrum TechnologyFrequency-hopping spread-spectrum (FHSS) uses a narrowband carrier that changes frequency in a pattern known to both transmitter and receiver. Properly synchronized, the net effect is to maintain a single logical channel. To an unintended receiver, FHSS appears to be short-duration impulse noise.
 


 
 Figure 2a. Frequency Hopping Spread Spectrum(Ref:  http://www.wlana.com/intro/introduction/wirels.html)

Direct Sequencing

Direct-Sequence Spread Spectrum TechnologyDirect-sequence spread-spectrum (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 (and, of course, the more bandwidth required). 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 rejected (ignored) by most narrowband receivers.


 

                                                Fig. 2b.  Direct Sequencing Spread Spectrum.

                                     (Ref:   http://www.wlana.com/intro/introduction/wirels.html)