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Assembling Your Wifi Network: Wireless Standards

An understanding of wireless standards will help you set up your own home network or setting up your office's.

As with most computer technology, manufacturers have come out with about a dozen mutually incompatible ways for you to assemble your wireless network. Below are some of the more common wireless standards fighting for your dollar? They were created by a number of industry and standards organizations, such as the Institute of Electrical and Electronics Engineers, Inc., which is famous for its painfully user-unfriendly standards names, such as "802.11b."

802.11b: Currently the most popular standard, 802.11b is used by many wireless access providers. If you want to connect to such a wireless network while at the beach for instance, this is the standard for you.

802.11b operates in the 2.4GHz radio frequency range. Its maximum bandwidth (the amount of data that can pass through a channel per second) is 11 Mb/s (Megabits per second), and the maximum effective range is 50 meters, though the actual effective range can vary considerably. Note: the bandwidth decreases with range-you will only get that maximum bandwidth of 11Mb/s right at the wireless access point.

The available frequency range at 2.4GHz is quite narrow and offers a maximum of three separate data channels. Data channels are just like radio channels-you can only have so many in a frequency range; however, unlike radio channels, more than one user can use a single channel.

802.11g: The 802.11g standard is an upgrade to the 802.11b. It also operates in the 2.4GHz radio frequency range. The maximum bandwidth is 34 Mb/s and the maximum range is 50 meters, though, again, the actual effective range can vary quite a bit. And once again, the bandwidth decreases with range.

The 802.11g standard is backwards compatible with its predecessor, which means any 802.11b equipment you buy now should still be usable when 802.11g becomes more affordable and you decide to upgrade your network.

802.11a: Unlike 802.11b and 802.11g, the 802.11a standard uses the 5GHz radio frequency range and isn't compatible with any of the other available standards. The maximum bandwidth is 54Mb/s. I've seen different maximum effective ranges listed for the 802.11a standard-from 10m to 50m. Note: The bandwidth you get with the 802.11a standard falls off much faster as you get further from the wireless access point than with the two previous standards. And as you get towards a range of 50m, you're probably dropping down to a bandwidth similar to what you would get with the 802.11b standard at the same range.

The frequency range available at 5GHz is wider than at 2.4GHz, and will offer you from 8 to 12 separate data channels (depending on where you live in the world).

Wi-Fi: The Wi-Fi alliance is a non-profit organization created in 1999 to certify the inter-operability of wireless equipment based on the 802.11 series of wireless standards. This means a Wi-Fi certified 802.11b adaptor is certified to work with other Wi-Fi certified 802.11b equipment. The Wi-Fi website currently lists equipment certified to work under the 802.11a and 802.11b standards, and Wi-Fi certified products for the 802.11g standard should be available in the future. Make sure the equipment you get is Wi-Fi certified-it should say so on the box.

Bluetooth: Bluetooth is a wireless standard designed more as cable replacement technology than a wireless network. You can use it to communicate between devices such as cell phones, PDAs (personal digital assistants), and computers. Bluetooth uses radio waves in the 2.4GHz frequency. The maximum effective range at 10m is much shorter than that of the 802.11b standard, and the bandwidth is only 4 Mb/s. As with the other standards noted, the actual performance can vary, and the bandwidth decreases with range. Note: Fewer devices can be hooked up to a Bluetooth network than to an 802.11b network.

IrDA: IrDA uses infrared waves to communicate. Many laptops, printers, PDAs, digital cameras, pagers, cell phones, and more, come with IrDA ports. The maximum bandwidth is 4Mb/s, though you will usually get a much lower performance. The maximum effective range is about 10m, and the actual performance will vary. Again, the bandwidth decreases with range.

WAP: Wireless Application Protocol. "WAP" is a buzzword you've probably heard a lot if you've been following the news on wireless communications. WAP is compatible with most wireless networks and is used to specify how to communicate with the various handheld devices on the wireless network, such as cell phones, PDAs, and pagers. It allows users of handheld devices to access the Web, email, etc. via the Internet. WAP is not an actual type of wireless network, just a way of communicating via a wireless network. (The acronym "WAP" is sometimes used to refer to a Wireless Access Point.)

These requirements lead to a set of technologies called "spread spectrum" communications which operate at 2.4Ghz and 5Ghz. Instead of picking particular frequency range and using high power levels to send as much data as possible using that frequency, the spread spectrum approach takes a much wider frequency band and sends the data using many different frequencies at relatively low power. Further, based on agreement between the sender and receiver, the frequencies can be changed several times per second in what would appear to be a random pattern. The two most common approaches were called Frequency Hopping and Direct Sequence.

 In Frequency Hopping Spread Spectrum (FHSS), the bandwidth is divided into channels (the 2.4Ghz range is divided into 79 channels in the US). Once a wireless connection is established, the receiver and transmitter agree on one of several frequency hopping patterns. Based on the current channel and a simple mathematical calculation, both the receiver and transmitter jump to the next channel in the sequence at the same time. Unless you know the proper calculation to make, an eavesdropper would have a difficult time following the communication.

While the original goal of changing frequencies was to evade detection (the military obviously used far more than 79 channels), having multiple sequences means that interference among different groups of receivers and transmitters is reduced. As a mater of fact, it is possible to deploy a number of base stations with overlapping coverage areas, and have a wireless network card associate itself with the base station with the strongest signal. In this way, a network of base stations operate much like a cellular telephone network, handing mobile base stations off from one to the other.

In Direct Sequence Spread Spectrum (DSSS) the frequency range is divided into fewer channels. Instead of transmission at high power and moving quickly from one channel to another, the data is "spread out" on a predefined pattern with a power level so low that the signal appears to be background noise. The only way to reconstruct the data was to know the subtle patterns in the background noise. To make a simple analogy, it is like hearing an AM radio station which is too far away. It all sounds like static, but there is enough of a pattern to the static that you know that people are talking but you cannot discern what they are saying. In DSS, the circuitry in the network cards knows the exact pattern in the "static" and reconstructs the data. Again, a feature that was originally designed to evade detection results in a wireless network technology that is tolerant of interference from other sources such as a wireless phone, microwave oven, or unlicensed equipment operating in the same frequency range.

In the late 1990's, there was a great deal of debate as to which of the technologies was superior with excellent arguments for all of the technologies. But the debate was resolved (for now) as low-cost equipment from Lucent and Apple came out using IEEE 802.11 DSSS technology. In order to be compatible with the existing (and growing) installed user base, nearly all new wireless networking products came out supporting the DSSS technology.

Another factor which caused DSSS to be more broadly accepted was the throughput. Originally, both DSSS and FHSS operated at 1Mbps. Many people felt that the minimum acceptable bandwidth was 10Mbps (as fast as Ethernet). It turned out that it was easier to push DSSS technology to 11Mbps than the FHSS technology because of the way the FCC set the rules for the use of the 2.4Ghz unlicensed frequency range. The FHSS community pushed to have the FCC rules relaxed to boost the performance of FHSS, but by the time it was resolved DSSS had a strong foothold in the consumer market.

The FHSS approach still has a distinct advantage when there is a high density of access points and mobile workstations. Even though the speed of an individual FHSS connection is slower, if there are many simultaneous connections, FHSS will make better overall use of the frequency.

Wireless networking is the ultimate expression of home networking. It frees you from your "computer dungeon" and allows you to be productive in more pleasant surroundings. The market for wireless equipment has settles down and a wide range of interoperable products exist based on the IEEE 802.11 Direct Sequence Spread Spectrum (DSS) technology. You can easily purchase a kit with several network cards and an access point and build a nice wireless network as an add-on to your existing network.

Frequency assignments for commercial applications are in the 900 MHz, 2.4 GHz, and 5 GHz bands. A hub antenna is housed in a wireless access point that ideally is centrally located (see Figure) where line-of-sight connectivity can be established with the various terminal antennas.

While line-of-sight is not strictly required, it is desirable in consideration of signal quality. This is particularly so at the higher frequencies, which suffer greater attenuation (i.e., loss of signal strength).

The wireless access point then connects to the servers, peripherals and other hosts via cabled connections, although wireless connectivity is possible. Multiple hub antennas can be interconnected by wires for communications between rooms, floors, buildings, and so on.

This scenario involving access points is known as infrastructure mode, and is typical. In ad hoc mode, or peer-to-peer mode, the wireless devices (e.g., laptop clients) communicate directly with each other.

In order to serve multiple workstations simultaneously, spread spectrum radio technology commonly is employed to maximize the effective use of the limited bandwidth supported by the narrow frequency ranges available to WLANs.

Frequency Hopping Spread Spectrum (FHSS) involves scattering packets of a data stream across a range of frequencies in a carefully choreographed hop sequence, rather than using a single transmission frequency. A side benefit of spread spectrum is that of increased security, since the signal is more difficult to intercept.

While the raw aggregate bandwidth of an RF LAN generally is described as falling into a range (e.g., 1-11 Mbps) sensitive to link quality at any given time, the effective throughput generally is considerably less due to issues including overhead and error control mechanisms.

Some wireless LANs also use Direct Sequence Spread Spectrum (DSSS) transmission, which calls for the signal to be transmitted simultaneously over several frequencies, thereby increasing the likelihood that the signal will get through to the receiving antenna.

Regardless of the frequency range employed, buildings are full of metal and other sources of interference that combine to reduce the effectiveness of RF-based wireless LANs. Lead paint, metal studs, nails, foil-backed insulation and even glass windows with metal content all can cause interference.

For that matter, any dense physical matter (e.g., walls, floors, ceilings, your four-year-old son, and the neighbor's cat) will cause some amount of attenuation, as it absorbs, reflects and scatters some signal energy to various degrees. The denser the physical matter, the worse the effect.

Some WLANs use frequencies (e.g., 902 MHz, 2.4 GHz, and 5.7 GHz) in the unlicensed ISM (Industrial/Scientific/Medical) bands. This approach avoids expensive and lengthy licensing processes through regulatory authorities such as the FCC (U.S.), but involves significant potential for interference from other such systems in proximity.

A wide variety of other devices (garage door openers, bar code scanners and industrial microwave ovens) also use the same frequencies. As these LANs (and other devices in the ISM band) operate at fairly low power levels, the actual risk of interference is relatively slight, but it does exist. As the popularity of such LANs has increased, situations have developed in which such interference has, indeed, become an issue.

Although it is somewhat unusual, infrared light (Ir) also can serve as the transmission medium. An Ir-based WLAN system generally requires line-of-sight between the light source and receiver, although you often can bounce a signal off of a wall or two without affecting connectivity too significantly.

Whether based on RF or Ir at the physical layer, wireless LANs are a relatively immature technology that is a long way from being ubiquitous, but that is gaining acceptance rapidly. While acquisition costs aren't necessarily all that low compared to wired LANs, configuration and reconfiguration costs are virtually zero since there are few, if any, cables and wires to consider.

Definitely on the positive side of the equation, wireless offers the advantage of portability, particularly in the case of the unlicensed frequencies.

On the negative side, bandwidth is limited since the frequency range is limited and throughput is limited and also because link quality can be poor and retransmissions can be necessary. Further, security is a real concern affecting any RF-based transmission system.

802.11b (Wi-Fi)

The most common WLANs currently are those conforming to the IEEE 802.11b specification. Not only are they increasingly deployed in private enterprise applications, but also in public applications such as airports and coffee shops.

Dubbed Wi-Fi (Wireless Fidelity) by the Wireless Ethernet Compatibility Alliance (WECA), 802.11b includes three transmission options, one of which is Ir-based and two of which are RF-based.

802.11b employs DSSS modulation using the Barker code chipping sequence. Each bit is encoded into an 11-bit Barker code (e.g., 10110111000), with each resulting data object forming a chip. The chip is put on a carrier frequency (i.e., a small frequency range that carries the signal) in the 2.4 GHz range, and the waveform is modulated using one of several techniques.

Knowing that the 802.11a specification operates at radio frequencies between 5.15 and 5.875 GHz, and that the 802.11b and 802.11g specification operates at radio frequencies in the 2.4 to 2.497 GHz range, we can see that the 802.11a has a wider frequency band, allowing more channels and more overall throughput. The wider frequency band allows 802.11a to support up to eight non-overlapping channels. However, 802.11b/g standards support only up to three non-overlapping channels.

Each channel is capable of carrying the maximum throughput for its standard. Therefore, the 802.11b and 802.11g standards have a maximum of three non-overlapping channels, each carrying 11 Mbps throughput (33 Mbps total) and 54 Mbps (162 Mbps total) throughput, respectively. The 802.11a standard has a maximum of eight non-overlapping channels carrying a maximum 54 Mbps throughput each (432 Mbps total). (For more on Wi-Fi standards, check out this Quick Reference guide.)

The frequency ranges and channels may vary by country. In the U.S., the 802.11b (2.4 GHz) standard operates on 11 channels. All but three of those channels are overlapping channels. Channels one, six, and eleven are the only non-overlapping channels.

Most manufactures set their default channel to one of the non-overlapping channels. D-Link products, for example, default to channel six. You have the option of selecting which channel your WLAN operates on in order to avoid interference from other wireless devices that operate in the 2.4 GHz frequency range. Examples of this would be 2.4 GHz cordless phones and X-10 wireless products.

The biggest advantage of 802.11b is that it's the most widely deployed wireless LAN technology and provides good wall penetration and indoor range. The advantage of 802.11a is that it provides increased network capacity and interferes with other wireless devices far less than 802.11b products do.

So to answer your question, unless you have a specific reason for changing it, I would suggest you keep your wireless channel configured for the manufacturer's default settings. If, however, you must change it, for the best performance I would suggest trying to use one of the other non-overlapping channels first: Channels one, six, and eleven. Other then interference issues, there really is no other reason or advantage to selecting another channel.

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