The magazine of the Melbourne PC User Group WiFi Update Peter Lange

A lot has happened in wireless networking since we last covered it in a special issue of PC Update in 2001. Back then, the 11 Mbit/s speed of the IEEE's (Institute of Electrical and Electronics Engineers, http://www.ieee.org) technology standard with the cryptic name "802.11b" had replaced the 2 Mbit/s systems we were thrilled about a few years earlier.

In the meantime, new variations of this technology called 802.11a and 802.11g have yet again almost quintupled the speed at which wireless-enabled devices can communicate with each other - from 11 Mbit/s to 54 Mbit/s, and beyond with proprietary solutions some equipment manufacturers have devised. Security features have also been improved, protecting user data from eavesdropping and their wireless networks from unauthorised access. This followed increased awareness and demand from a growing user community, slowly extending from typical home use into the enterprise world as well.

Alphabet Soup

As if 802.11b, a, and g (and their illogical naming sequence) weren't causing enough confusion already, other standards called 802.11h, i, e, and n have evolved, as well as others called 802.1x, 802.15 and 802.16 -some of which are not even comparable to a, b or g at all - accompanied by such terms as WPA, WiMAX, Bluetooth, ZigBee. With this article we want to shed some light on and take a deeper look into that soup bowl. What are the characteristics, advantages or disadvantages of each technology, which one should you get?
802.11b, a, and g are the three main technologies in wireless networking today, so let's take a closer look at them.

802.11 b

802.11b came first, offering a data transfer rate of 11 Mbit/s - the net data rate, or throughput, is about half of that due to a normal overhead for transmission security protocols, optional data encryption etc. The system will automatically fall back to lower data rates when the radio link conditions deteriorate: 5.5, 2 and 1 Mbit/s.

802.11b uses DSSS modulation (Direct Sequence Spread Spectrum) - a method of transmitting a signal by spreading it over a relatively wide part of the available frequency spectrum. An 802.11b "channel" (the spectrum occupied by the transmission of a signal) is about 22 MHz wide - that is around 1000 times more than a conventional radio channel used to transmit speech for example (typically 10 to 25 kHz, AM/FM), and still much wider than a speech or data channel used by your GSM mobile phone or one used by your local FM radio station (200 kHz). The advantage of spread spectrum technologies over conventional transmission is that the signal becomes much less susceptible to interference when it is spread over a wide range of the spectrum - in fact many users can actually use the same channel simultaneously, and the signal strength of a spread spectrum signal can be so low, it can even be below the ambient noise level, which makes it very hard to detect and eavesdrop on. Not surprising then that the technology was developed and first used by the military.

There are 13 such channels available within the 83.5 MHz of spectrum in the 2.4 GHz WiFi band in Australia. Each of them is about 22 MHz wide, but their centre frequencies are only 5 MHz apart, as shown in the table in Figure 1 below. This means that operation of overlapping channels in locations in close proximity to each other is potentially affected by interference, and there are only 3 sets of 3 channels each that do not overlap at all: Channels no. 1, 6, 11, or 2, 7, 12, or 3, 8, 13.
 

As the take up of WiFi rose, interference became more and more of an issue, especially in densely populated areas, CBDs etc. And mind you, other WiFi systems nearby are not the only source of interference. Microwave ovens operate right in the 2.4 GHz WiFi band, with power levels of several hundred watts (as opposed to your WiFi gear with up to 4 watts), and also some cordless telephones. This is the "ISM band" (Industrial, Scientific, Medical), so many other devices operate in it, too, and it is unregulated spectrum, which means it is free to use, but users have to accept interference or sort it out themselves.

802.11a: More, and Less

The interference issue is exactly where 802.11a came in handy when it emerged in late 2001. It also operates in unregulated spectrum, but in the higher 5.2 and 5.8 GHz range. There is a lot more "space" up there: 350 MHz of spectrum in Australia as opposed to the 83.5 in the 2.4 GHz band. The 802.11a channel bandwidth is 20 MHz and the channels are all non-overlapping, with a centre frequency spacing of also 20 MHz. So in theory, there are more channels available than in 802.11b - in practice, however, present-day 802.11a equipment supports up to 12 channels because most of it is manufactured in or for countries with less spectrum available. The risk of interference is still much smaller than in 802.11b though, because of the non-overlapping channel structure and the fact that no microwave ovens or other ISM devices operate in the 802.11a frequency band - this band is however shared with satellites, radar and some other systems.

802.11a also uses a different modulation method than 802.11b, called OFDM (Orthogonal Frequency Division Multiplexing), delivering an almost fivefold increase in speed: 54 Mbit/s maximum data transfer rate, also with automatic fallback in several steps to speeds as low as 6 Mbit/s when radio link conditions deteriorate.

The maximum coverage range of 802.11a is smaller than that of 802.11b systems, firstly because of the higher frequency band used (radio waves propagate "easier" and further at lower frequencies), and secondly because of the more stringent transmitter power limits: While up to 4 watts EIRP (Effective Isotropic Radiated Power) are allowed for 802.11b systems in Australia, the limit for 802.11a systems is 200 milliwatts (mW) EIRP in the lower part of the spectrum (5.15 - 5.35 GHz) and 1 watt EIRP in the upper part of the spectrum (5.725 - 5.875 GHz). Tests have shown, however, that when 802.11a systems do provide coverage, they deliver better data rates than 802.11b systems over the same distance.

An initial disadvantage of 802.11a was its higher power consumption: 20 to 40% more than equivalent 802.11b devices. This is an issue for the battery life of mobile/portable devices like laptops or PDAs powering wireless interface cards. 802.11a equip- ment was also quite a bit more expensive than 802.11b when it first came out.

The primary reason though why 802.11a has not been a big success in the marketplace (yet) is probably the lack of backward compatibility with existing 802.11b systems, due to the different frequency band, channel spacing etc. Vendors were quick to provide dual-mode 802.11a/b devices, but still the take up was very slow -because a new standard had already been announced promising the best of both worlds: 802.11g.

802.11g: The Best Of Both Worlds?

802.11g uses OFDM modulation like 802.11a, thus providing the same increased data rates up to 54 Mbit/s, but it operates in the 2.4 GHz band like 802.11b, resulting in greater coverage ranges due to the more favourable radio propagation in that band and the higher EIRP allowance. Maximum ranges are shorter than those of 802.11b due to the different modulation method, but again, where an 802.11g system does provide coverage at a given distance, it will deliver higher data rates than 802.11b at the same distance.

802.11g provides a "mixed mode" in which legacy 802.11b devices can coexist with new 802.11g devices, allowing for a smooth transition from one technology to the next.

Power management improvements have been developed in the meantime actually leading to lower overall power consumption of 802.11a/g devices compared to 802.11b, so that disadvantage has disappeared.
However, 802.11g is subject to the same inherent interference risks of the 2.4 GHz band as 802.11b - higher user density, other ISM equipment, microwave ovens etc.

802.11g equipment was grabbed off the shelves by consumers much quicker than 802.11a equipment ever was, even before the new standard was fully ratified by the IEEE, when some vendors had come out with equipment based on a draft version of the standard early in 2003 that required firmware updates later on. This development gave the industry a significant boost, at least the consumer segment of it, just in time when much of the initial WiFi hype was beginning to crumble.

Improved Security With WPA

Besides the emergence of 802.11a and g, the other major development in wireless networking in recent years was the improvement of security mechanisms. The data encryption method called WEP (Wired Equivalent Privacy) used initially in the early systems was known to be notoriously weak soon after its introduction. Tools are freely available that claim to be able to crack secret WEP keys within hours or even minutes of "listening" to encrypted data. There are several weaknesses in the implementation of WEP, one of them is the fact that it uses static keys, or at best dynamic keys that change per user and per session.

This and other weaknesses were addressed in the development of WPA (WiFi Protected Access). It uses the TKIP protocol (Temporal Key Integrity Protocol) which can change the encryption key for each packet of data, thus significantly increasing security. In addition, while WEP had virtually no user authentication mechanism, WPA supports 802.1x, the standard for access control used also in wired LANs, and the Extensible Authentication Protocol (EAP).

WPA is regarded as quite safe at the moment, at least as long as relatively short and text-based keys are avoided. A further improved new release called WPA2 is planned for the middle of this year.

WPA is actually a subset of 802.11i, a comprehensive security standard for wireless LANs currently under development. Responding to demand, equipment manufacturers went ahead and adopted this subset before the full 802.11i standard was ratified.

Other 802.11s

No, there is no such thing as a standard called 802.11s - this time it's really just a plural. After clarifying the most important letters in the 802 alphabet soup, a, b, g, i and x, we want to at least mention some of the others here briefly:

802.11e provides Quality of Service (QoS) support for LAN applications, which will be critical for delay-sensitive applications such as Voice over Wireless IP (VoWIP). The standard will provide classes of service with managed levels of QoS for data, voice and video applications.

802.11h provides some additional features to comply with European regulations for 5 GHz wireless LANs (WLANs), such as transmission power control (TPC) and dynamic frequency selection (DFS). Both of these features aim at reducing interference - useful outside of Europe and outside the 5 GHz band as well.

802.11n is a proposal for the next technology in line after 802.11a and g, providing greater data throughput of at least 108 Mbit/s, possibly reaching 320 Mbit/s. The IEEE is specifically talking of throughput here, not just data transfer rate - throughput being what the user actually experiences when for example downloading a file, without counting any of the transmission overheads.

802.15 is a standard for WPANs (Wireless Personal Area Networks) - networks that occupy a much smaller area than a WLAN, a few meters in range, where personal devices communicate with each other. One substandard, 802.15.1, uses the Bluetooth technology that is already quite commonly being used for communication between wireless computer peripherals (printers, keyboards, mice etc). Another sub- standard, 802.15.4 ("ZigBee"), is particularly interesting, defining a low data rate solution (20 to 250 kbit/s) with multi-month to multi-year battery life and very low complexity. Potential applications are sensors, interactive toys, smart badges, remote controls, and home automation.

802.16, or "WiMAX", on the other hand defines WMANs (Wireless Metropolitan Area Networks) and promises data rates of up to 70 Mbit/s over distances of up to 50 km even under non-line-of-sight conditions. This technology could be interesting in particular for WISPs (Wireless Internet Service Providers) as wireless "backhaul" to their WiFi hotspots, an alternative to the costly wired lines that are typically used for this purpose today.

Watch this space for a closer look at some of these exciting new technologies in future issues of PC Update.

About the Author
Peter Lange has worked as a consultant in the wireless telecommunications industry for over 15 years, helping clients worldwide to plan, build and optimise wireless and mobile telecommunications networks.
E-mail: peter.lange@netcontel.com.

Reprinted from the April 2004 issue of PC Update, the magazine of Melbourne PC User Group, Australia

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