09 March 2007

Wi-Fi

Wi-Fi is the marketing name for the IEEE 802.11 standard for wireless networks. As such it is not the trademark of any business; it belongs to the Wi-Fi Alliance.
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T-Mobile Wireless PC Card

Linksys Wireless Router

Netgear Ethernet hub
Wi-Fi is the technology employed so that many computers in a relatively large open area can be connected to the Internet. It consists of a two-way radio adapter for the individual computer and a wireless router. The router, in turn, has a conventional broadband (Ethernet) connection to the Internet.

To access the Internet remotely, one needs to have a wireless network interface controller (WNIC, or wireless card), which is closely analogous to a network interface card (NIC). The WNIC uses an antanae for two-way communication with the base station. In some cases, a password is required to access the network; in a growing number of cases, it is not. When one turns on a laptop/PDA in a Wi-Fi "hotspot," the WNIC detects the network and tried to connect.

In order to create a Wi-Fi network, the establishment needs a wireless router or wireless access points (WAP); in most cases, routers include a WAP, and they are used when the WLAN is very small. If it's larger, such as a business office with dozens of users, then one will need several WAP's. The WAP's are connected via Ethernet to a hub, and interface with the devices—desktops, PDA's, and printers—via radio signals.

Frequency Issues

Wireless routers naturally use digital radio signals, with frequencies of 2.4 or 5.8 GHz (i.e., 5 or 12.5 cm waves). By comparison, GSM cell phones in the USA use a somewhat lower frequency of 1.9 GHz (16 cm waves). These are the highest frequencies currently in common practical use. Generally speaking, higher frequencies permit higher data throughput and smaller antanae, although they also require higher levels of energy and suffer more physical interference. There are over a dozen Wi-Fi standards now, but they are divided between the 802.11a (and derivatives) and the 802.11b (and derivatives); Wi-Fi receivers on computers work with either one or the other. For example, an end user equipped with an 802.11a WNIC will not be able to connect with an 802.11b access point. This is supposed to be resolved with the future 802.11n standard, which will transmit in both frequencies.

(As a result, WNIC's often have multiple radio cards; so do institutional-grade wireless routers.)


The 802.11a uses the 5 GHz frequency; it handles a very high throughput of data (up to 54 Mbps), but has a range of >20 meters.1 The competing 802.11b standard uses the lower 2.4 GHz frequency, has a throughput of 11 Mbps, and a range of >90 meters. In a large structure, therefore, many more access points are required for the 802.11a than others. Both the 'a and 'b standards are pretty much obsolete; equipment for the 'a standard was late shipping, costlier, and incompatible with the 'b-standard routers that establishments preferred to install. The 'b standard was superseded by 'g, which had the 54 Mbps throughput , and the 'n standard is replacing both.


An early headache for Wi-Fi developers was competition for the 2.3-2.4 wavelength with satellite radio.2 At the same time, the US military complained about the use of the 5 GHz band because it could interview with radar.3


Multiplexing and Modulation

Frequency aside, the big initial difference was that 802.11a used orthogonal frequency division multiplexing (OFDM), while 'b used carrier sense multiple access with collision avoidance (CSMA/CA). "Multiplexing" refers to the type of technology used to allow multiple users access to a single scarce data channel—in this case, a microwave-band radio wavelength with just enough variance that it can be reliably distinguished by receivers. This is a fundamental cleavage in different cell phone formats: how to subdivide a crowded radio channel into many different "conversation." In CSMA/CA, data is transmitted in packets; the WNIC and wireless router wait their turn, while in OFDM different "conversations" are combined into a single stream, and modulated out of the stream by a mathematical equation known as a Fast Fourier Transform. (This is known as "frequency multiplexing," or "frequency division multiple access").

Modulation of radio waves refers to the way that the signal is embedded in the wave emissions. In AM radio (500-1200 KHz), the signals are modulated by amplitude: a louder sound requires a more powerful wave than those before or after.4 In FM radio, the signals are modulated by frequency, and soundwaves are modeled by variances of up to 75 KHz (in other words, 98.1 FM really means 98.025-98.175 MHz, with Ravel's "Bolero" starting closer to 98.025 and ending up at 98.175). This works fine with analog radio because the highest pitched sounds audible to the human ear are about 15 KHz, or 1/33 the frequency of the lowest AM radio signals.



Phase Shift
With digital radio, however, modulation is a more sensitive matter. There is more data per wavelength, and the frequency is higher. Accuracy is much more urgent. As it happens, there are several different types of modulation in digital radio.
  1. Frequency Shift Keying (FSK): a binary form of FM in which there are two distinct frequencies.
  2. Phase Shift Keying (PSK): a system in which individual waves are interrupted at their peak and resume at the trough (or any other phase of the wave). Used on the original 802.11 standard.
  3. Complemetary Code Keying (CCK): used in 802.11b standard, this is a complex scheme that combines multiplexing with modulation.5
  4. Orthogonal frequency-division multiplexing: like CCK, a combination of modulation and multiplexing. Used in the 802.11a, 'g, and 'n standards, with extraordinarily high performance.6
It would appear that OFDM has become the universal standard in both Wi-Fi transmissions and digital radio.

Notes

1 Jim Geier, "The BIG Question: 802.11a or 802.11b?," Wi-Fi Planet (24 Jan 2002)

2 Ed Sutherland, "Sirius Spectral Problems for 802.11b," Wi-Fi Planet (19 March 2002)

3 Eric Griffith, "The DOD vs. Wi-Fi?" Wi-Fi Planet (17 Dec 2002)

4 For an excellent article on both analog and digital modulation, see Ian Poole, "Modulation basics, part 1: Amplitude and frequency modulation," "part 2: Phase modulation," and "part 3: Spread spectrum and OFDM," DSP Design (June 2008). Any mistakes in the part about modulation are ones I made, not Mr. Poole.

5 For a useful explanation of CCK, see Bob Pearson, "Complementary Code Keying Made Simple" , Intersil Americas Inc. (2001). CCK is technically related to Code Division Multiple Access (CDMA) multiplexing used on many cell phones. While an interesting subject, both CCK and OFDM are outside the scope of this post.

6 For a useful explanation of OFDM, see Rethnakaran.P & Herbert Dawid, "An Orthogonal Frequency Division Multiplexing" , Digital Communication Solutions, Synopsys Inc (2003).



Additional Reading and Sources

Jim Geier, "802.11 Alphabet Soup," Wi-Fi Planet (5 Aug 2002); old article, but clear intro to Wi-Fi protocols. See also "Wireless LAN glossary" Whatis.com (19 Aug 2005)

Jack Hanlin & Derek Walker, "Wi-Fi," SearchMobileComputing.com (20 Oct 2008)

"Set up a wireless LAN" Hewlett-Packard How to guides

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