In the last few years a new type of local area network has appeared. This new type of LAN, which is the wireless LAN, provides an alternative to the traditional LANs based on twisted pair, coaxial cable, and optical fiber. The wireless LAN serves the same purpose as that of a wired or optical LAN: to convey information among the devices attached to the LAN. But with the lack of physical cabling to tie down the location of a node on a network, the network can be much more flexible -- moving a wireless node is easy. As opposed to the large amount of labor required to add or move the cabling in any other type of network. Also going wireless may be a better choice where the physical makeup of the building makes it difficult or impossible to run wire in the building.
Wireless networks are ideal for portable computers. Using wireless connections allows portable computers to still be portable without sacrificing the advantages of being connected to a network. These machines can be setup virtually anywhere within the building.
Wireless networks can be used in combination with cabled LANs. In that all the machines that will require relative mobility will be connected wirelessly, while the stations that are for the most part permanant can be connected through cable.
Wireless LANs use one of three transmission techniques: spread spectrum, narrowband microwave, and infrared.
Spread spectrum is currently the most widely used transmission technique for wireless LANs. It was initially developed by the military to avoid jamming and eavesdropping of the signals. This is done by spreading the signal over a range of frequencies, that consist of the industrial, scientific, and medical (ISM) bands of the electromagnetic spectrum. The ISM bands include the frequency ranges at 902 MHz to 928 MHZ and at 2.4 GHz to 2.484 GHz, which do not require an FCC license.
The first type of spread spectrum developed is known as
frequency hopping spread spectrum
. This technique broadcasts the signal over a seemingly random series of radio frequencies. A receiver, hopping between frequencies in synchronization with the transmitter, receives the message. The message can only be fully received if the series of frequencies is known. Because only the intended receiver knows the transmitter's hopping sequence, only that receiver can successfully receive all of the data. Most vendores develop their own hopping-sequence algorithms, which all but guarantees that two transmitters will not hop to the same frequency at the same time.
Even though the FCC has made some rules for frequency hopping spread spectrum technologies. The FCC dictates that the transmitters must not spend more than 0.4 seconds on any one channel every 20 seconds in the 902 MHz band and every 30 seconds in the 2.4-GHz band. Also, the transmitters must hop through at least 50 channels in the 902-MHz band and 75 channels in the 2.4-GHz band--a channel consists of a frequency width which is determined by the FCC. The IEEE 802.11 committee has drafted a standard that limits frequency hopping spread spectrum transmitter to the 2.4-GHz band.
The other type of spread spectrum communication is called
direct sequence spread spectrum
, or pseudonoise. This method seems to be the one that most wireless spread-spctrum LANs use. direct sequence transmitter spread their transmissions by adding redundant data bits called "chips" to them. Direct sequence spread spectrum adds at least ten chips to each data bit. Like a frequency hopping receiver, a direct sequence receiver must know a transmitter's spreading code to decipher data. This spreading code is what allows multiple direct sequence transmitters to operate in the same area without interference. Once the receiver has all of the data signal, it uses a correlator to remove the chips and collapse the signal to its original length.
As with frequency hopping spread spectrum, the FCC has also set rules for direct sequence transmitters. Each signal must have ten or more chips. This rule limits the practical raw data throughput of direct sequence transmitters to 2 Mbps in the 902-MHz band and 8Mbps in the 2.4-GHz band. Unfortunately, the number of chips is directly related to a signal's immunity to interference. In an area with lots of radio interference, you'll have to give up throughput to avoid interference. The IEEE 802.11 committee has drafted a standard of 11 chips for direct sequence spread spectrum.
Frequency hopping radios currently use less power than direct sequence radios and generally cost less. While direct sequence radios have a practical raw data rate of 8 Mbps and frequency hopping radios have a practical limit of 2 Mbps. So if high performance is key and interference is not a problem, go with direct sequencing. But if a small, inexpensive portable wireless adapter for a notebook or PDA is needed a the frequency hopping method should be good enough. With either method of spread spectrum the end result is a system that is extremely difficult to detect, does not interfere with other services, and still carries a large bandwidth of data.
Microwave technology is not really a LAN technology. It's main use is to interconnect LANs between buildings. This requires microwave dishes on both ends of the link. The dishes must be in line-of-sight to transmit and collect the microwave signals. Microwave is used to bypass the telephone company when connecting Lans between buildings.
One major drawback to the use of microwave technology is that the frequency band used requires licensing by the FCC. Once a license is granted for a particular location, that frequency band cannot be licensed to anyone else, for any purpose, within a 17.5 mile radius.
Infrared LANs use infrared signals to transmit data. This is the same technology used in products like remote controls for televisions and VCRs. These LANs can be setup using either a point-to-point configuration or a sun-and-moon configuration where the signals are diffused by reflecting them off of some type of surface.
The major advantage of infrared is its ability to carry a high bandwidth, but its major disadvantage is that they can easily be obstructed, since light cannot pass through solid objects.
| Spread Spectrum | Narrowband Microwave | Infrared | |
| Frequency | 902MHz to 928 MHz ; 2.4 GHz to 2.4385 GHz ; 5.725 GHz to 5.825 GHz | 18.825 GHz to 19.205 GHz | 3 x 10^14 Hz |
| Maximum coverage | 105 to 800 feet, or up to 50,000 square feet | 40 to 130 feet, or up to 5000 square feet | 30 to 80 feet |
| Line of sight required | No | No | Yes |
| Transmit power | Less than 1 W | 25 mW | N/A |
| License required | No | Yes | No |
| Interbuilding use | Possible with antenna | No | Possible |
| Rated speed (% of 10 Mbps wire) | 20% to 50% | 33% | 50% to 100% |
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802.11 uses a contention mechanism to allow stations to share a wireless channel, based on carrier-sense multiple access (CSMA), like 802.3. 802.11 cannot use all of 802.3 because it is not possible in the wireless environment for a station to listen and transmit on the same channel as would be required for the collision detection (CD) used in 802.3. Because of this a statin on a wireless LAN will not be able to determine that a collision has occured until the end of the packet transmission, making collisions more expensive in 802.11 than in 802.3. The 802.11 MAC uses a collision avoidance mechanism to reduce the probablility of collisions. The 802.11 MAC is designed to operate over multiple physical layers, and does not specify various media-dependent parameters.
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For a more detailed description of the IEEE 802.11and other standards used with wireless networking see the paper by Antonio DeSimone and Sanjiv Nanda, Wireless Data: Systems, Standards, Services,
Baltzer Journals, 1995.
Other Wireless Related Sites
References on Wireless
ARTICLES
Scott, Benjamin L. ; "Answers from thin air" ; Inform v 8 n 3 Mar 1994. p 26-28
Harasty, Daniel J. ; Chang, Li Fung ; "Architecture alternatives fro wireless data services" ; Annual International Conference on Universal Personal Communications Record 1994. IEEE, Piscataway, NJ, USA, 94TH0622-1. p 310-314
Zubrzycki, J.T. ; "MBS - a wireless network for digital video" ; IEE Conference Publication n 397 1994. IEE, Stevenage, Engl. p 266-271
Pahlavan, Kavek; Levesque, Allen H. ; "Wireless data communications" ; Proceedings of the IEEE v82 n 9 Sept 1994. p 1398-1430
Abramson, Norman ; "Multiple access in wireless digital networks" ; Proceedings of the IEEE v82 n 9 Sept 1994. p1360-1370
Bantz, D.F. ; "Wireless LAN Design Alternatives," IEEE Network, March/April 1994, p. 43-53.
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"Wireless networks," Baltzer Science Publishers, c1995-
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