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WiFi Principles

Electromagnetic Spectrum

The Electromagetic Spectrum

The electromagnetic spectrum encompasses all of the frequencies from the lowest sound waves up to gamma rays. Frequencies are measured in hertz (Hz). A one Hz signal oscillates at the rate of 1 cycle per second. A one kilohertz (KHz) signal oscillates 1000 times per second. Next is megahertz (MHz) which represents 1 million cycles per second. A GHz is 1 billion cycles per second or 1000 MHz. The normal range of human hearing is roughly 20 Hz to 20,000 Hz. The radio frequencies are a portion of the spectrum above sound waves, starting at 30,000 Hz up to 300 GHz.


How is it done?

WiFi starts at an access point, which is usually connected to the network by wire. Digital data from the network is encoded onto a radio signal that is transmitted over the air. Using sophisticated wireless network protocols (specifically IEEE 802.11a/g/n/ac) mobile devices can log on and establish a link therefore becoming a client on the network the same as those plugged in by wires. With wired connections, the number of users is limited by the number of physical ports on the router. With WiFi the number is theoretically unlimited. In practice though too many WiFi users will drain the access point’s resources, slowing down the rate of data transfer.

For home and small office use, the access point is usually incorporated into a router although you can purchase them as stand alone units. Larger office environments may use multiple access points to get the coverage needed. Dedicated access points called hot spots are common at coffee shops, airports, shopping malls and restaurants that provide Internet service for their customers on the premises.

Generally speaking any client that is connected to the LAN with wires will have the same level of service no matter where they are located. With WiFi, the further away one gets from the access point the less reliable coverage becomes. As the radio signal gets weaker the 802.11 protocol lowers the data rate to compensate. Loading of web pages gets slower due to the reduced data rate and missed packets of data that have to be resent. For situations where longer than normal range is required we need to employ different strategies.


Spread Spectrum

Due to the limited number of channels available and the extreme number of users, WiFi obviously cannot use the traditional method of one user per channel the way AM and FM broadcast stations work. To get around this, a technique known as spread spectrum is employed. The idea is to spread the signal out over a relatively wide range of frequencies.

There are a variety of ways to accomplish this including frequency hopping and pseudo-random noise sequencing. To simplify, the receiver knows where the transmitter is at any given time so it is able to sync up while rejecting undesired users. The user on the next channel will sometimes receive your signal but the information is rejected by their receiver since it’s not expecting anything at that point in time. The power output is also spread out over a greater bandwidth so a normal radio trying to listen in will only hear noise.

WiFi communications do not have a continuous duty cycle. Multiple users can be on the same channel since their devices will not sync up with each other. With more users the noise floor will appear to rise from its natural level. A number of other services share the 2.4 GHz band such as cordless telephones and microwave ovens that also impede wireless network performance. There will be a point where spread spectrum receivers reach their limit of being able to pick their desired signals out of the noise. In this case you need to switch to another, less crowded channel. Luckily, with the low-power nature of WiFi equipment most signals are severely limited in range so unless you live in a very densely populated area, the chances of finding a usable channel for your access point is very good.

Another advantage of spread spectrum is its ability to reject interference from non-communication devices like microwave ovens. On a normal radio if a persistent interfering signal is present you have no choice but to change channels. This would be a major inconvenience to WiFi. The sum up, network performance is limited by distance from the access point, number of users, interference from other devices and environmental factors.

Data rates on an 802.11g or n can be as high as 54Mb/sec but remember this is the speed between your device and the router. Most people’s Internet connection is much slower. On our Local Area Network however, we can transfer data directly between devices with what’s known as an Ad-hoc or computer to computer network connection and take advantage of the higher data rates. Gamers on the same network will find this useful.

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