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On Wireless Computer Networks / IEEE 802.11a Protocol

The work life, whether related with academic interests or business concerns, is getting more dependent on computer and information technologies day by day and this increased the demand for mobility and freedom. People would like to access information at any time and anywhere they would like due to the requirements and essentials of academic and business life. The demand driven by people is met by wireless network applications. In this article, information on 802.11a standard of wireless computer networks and how it compares to 802.11b shall be given. Let's look over 802.11 and 802.11b standards.

802.11 is a standard that was developed by Institute of Electrical and Electronics Engineers (IEEE) in 1997. The products based on 802.11 standard can operate on a band of 2.4 Ghz, which is named as ISM (Industry, Science, Medicine) in U.S. and which can be used without license in Turkey as well. Moreover, these products can reach up to 1 Mbsp to 2 Mbps bandwidth.

There are two types of technologies whose standards are developed by IEEE: 802.11 and 80211.b. 802.11, uses FHSS (Frequency Hopping Spread Spectrum) modulation technique at 2.4 Ghz band and it has a bandwidth of maximum 2 Mbps. Despite its limited bandwidth, 802.11 provide a solid and secure system infrastructure. Since all the bandwidth and frequency hopping technique is used, it is barely probable that it will be disturbed by noise and interferences. It is possible to use 15 access points at the same coverage area. However, 802.11b uses DSSS (Direct Squence Spread Spectrum) modulation technique at 2.4 Ghz band and it has a shared bandwidth of maximum 11 Mbps. Although, 802.11b provides more bandwidth, the drawback is that it can easily be disturbed by the interferences and noises of the systems operating at the same frequency spectrum. It is possible to use on the systems where this modulation technique is implemented, 3 different simultaneous access points at 3 different channels within the same coverage area.

Wireless networking wasn't accepted initially for some reasons: throughput (1Mbps/2Mbps) was much too slow compared to the most prevalent (10Mbps) wired Ethernet standard; the first wireless products didn't work well together with wireless products from other vendors, and lastly, the hardware was significantly more expensive than wired ones. However, during the last years, these concerns were addressed. These factors boosted the acceptance of wireless networking and it enjoyed a rapid increase of its demand. Firstly, market forces including competition and vastly increased volumes have driven down the costs of 802.11b wireless network equipment; secondly, products based on the 802.11b standard with 11 Mpbs bandwidth are now a major rival to Ethernet and lastly, interoperability concerns have been addressed by WECA's (Wireless Ethernet Compatibility Allience) Wi-Fi (Wireless Fidelity) certification program.

But 802.11b isn't perfect. The major problems of 802.11b can be explained as follows: limited bandwidth which is incapable of supporting a data environment that includes nowadays more video, graphics, and sound; lack of support for qos - quality of service - that would enable real-time data transfer and lack of support for prioritization; radio interference from other devices and networks, (included among these interference issues is hardware that use Bluetooth which operates at 2.4 Ghz radio spectrum with 1 Mpbs maximum bandwidth when utilized in "Personal Area Networks - PANs". It seems that Bluetooth will finally become commonly available for connecting peripheral devices to each other without wires and 2.4GHz wireless networks may suffer from that) and lastly, security concerns

Middle East Technical University Computer Center is committed to implement the latest developments of technology. Therefore, Computer Center launched the 802.11b standard in the campus at the end of 1999. As a result of these efforts, the units and divisions (Sports Hall, Microelectronic Technologies, Office of Equipment and Medical Center) within campus that could not be connected to the local area network via traditional wired methods were connected to the LAN via point-to-point wireless network connection. CC Network Group has initiated a test on implementation of wireless network within campus in 2002 and created a network that operates in "workgroup" mode. The system is yet in the testing period and operates in workgroup mode via an access point and an antenna that are placed on the top of MM Building. Mobile or stationary computers with network cards that are compatible with 802.11b standard can establish wireless connection within a radius of 3,2 km far away from MM Building. The tests showed that the bandwidth reached 4 Mbps in places where the location is close to MM Building. Shall an emergency case occur and one or more of the units/divisions experience access problems, METU CC Network Support Group owns an access kit in reserve to meet the urgent needs.

802.11 and 802.11b standards are briefly explained in this article to provide a broader perspective for the main issue of this article; the 802.11a standard, which METU CC Network Support Group plans to implement in the campus this year. (You can have more information about 802.11b wireless network standard a http://www.bidb.odtu.edu.tr/index.php?go=ng&sub=wireless).

Despite the fact that IEEE's main aim in developing 802.11a standard was not solve the problems of 802.11b, because of the reasons mentioned above, it would not be incorrect to say that 802.11a standard will be quite popular in the future. In fact both of the standards (802.11b and 802.11a) were released in 1999 by IEEE; however, 802.11b became more popular on the market. The two advantages of 802.11a are that it supports more throughputs and it has more canal capacity with more bandwidth in both features as compared to 802.11b.

As compared to 802.11b, 802.11a operates not on 2.4 Ghz ISM band, but on 5 Ghz band which is named as U-NII (Unlicensed National Information Infrastructure) in U.S. and 802.11a supports 54 Gbps bandwidth. Another main difference of 802.11a is that as a modulation technique 802.11a supports OFDM (Orthogonal Frequency Division Multiplexing). Let's examine the factors that make the difference between these two standards in detail:

Increased Bandwidth:

The key advantage of 802.11a is that nominal speed for 802.11a is 54Mbps. However, overhead factors use up a significant part of the bandwidth. According to the chipset vendors, actual maximum 802.11a bandwidth should range from 22 to 26Mbps. Within the IEEE specification for 802.11a there is a higher speed mode that can be implemented for greater bandwidth, variously called "turbo" mode or "2X" mode by different vendors, which is expected to add about a 25% to 50% improvement in available bandwidth. The higher speed mode isn't standardized, so interoperability between products isn't guaranteed in turbo or 2X mode.

802.11a, like 802.11b can operate at different speed levels. 802.11b's speeds are 11, 5.5, 2, and 1Mbps. 802.11a has a maximum rated speed of 54Mbps but drops back to 48, 36, 24, 18, 12, 9, and 6Mbps.

802.11a 802.11b 802.11
Standard Approved September 1999 September 1999 July 1997
Available Bandwidth 300 Mhz 83.5 Mhz 83.5 Mhz
Unlicensed Frequencies of Operation 5.15-5.35 Ghz, 5.725-5.825 Ghz 2.4 - 2.4835 Ghz 2.4 - 2.4835 Ghz
Overlapping Channels 4 (indoor), 4 (indoor /outdoor) 3 (indoor / outdoor) 3 (indoor / outdoor)
Rate per Channel 6,9,12,18,24,36,48,54 Mbps 1,2,5,5,11 Mbps 1,2 Mbps
Modulation Type OFDM (Orthogonal Frequency Division Multiplexing) DSSS (Direct Sequence Spread Spectrum) FHSS, DSSS (Frequency Hopping Spread Spectrum)

Table: 802.11, 802.11b [http://www.atheros.com]

Additional Transmission Channels:

A second reason for additional overall bandwidth with 802.11a is channel support. With 802.11b, three channels are available for simultaneous operation in the 2.4 to 2.4835GHz frequency band (there are eleven center frequencies specified 2.412, 2.417, 2.422, 2.427, 2.432, 2.437, 2.442, 2.447, 2.452, 2.457, and 2.462 GHz, but because there is a required 25MHz spacing between active channels, only three are typically used at one time). In 802.11a, however, eight channels can operate simultaneously in the two lower bands of the 5GHz spectrum used in the U.S., 5.15 to 5.25GHz and 5.25 to 5.35GHz. The center points for the eight channels, each of which is 20MHz wide and can support 52 carrier signals, are 5.18, 5.2, 5.22, 5.24, 5.26, 5.28, 5.30, and 5.32 GHz. Regardless of the number of available channels, an 802.11a access point only uses one channel at a time, which if managed correctly can help you get the most from available bandwidth. If you have multiple access points, you can support more users.

One of the difference of 802.11a from 802.11b is that it uses Orthogonal Frequency Division Multiplexing - OFDM as a modulation technique. OFDM is a fdm - frequency division multiplexing) technique for transmitting large amounts of digital data over a radio wave. OFDM works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. OFDM reduces the amount of crosstalk in signal transmissions and it is tolerant to channel noise. Therefore, it is very suitable for most of the wireless applications.

While explaining 802.11b, we have mentioned above about Wi-Fi certification program developed by Wireless Ethernet Compatibility Alliance (WECA) with mission of guaranteeing interoperability between different products of various companies. A similar certification program is applicable to 802.11a products. It is called Wi-Fi5


The project of Hiperlan/2, which is a standard that is implemented in Europe, is designed by Broadband Radio Access Networks (BRAN) division of European Telecommunications Standard Institute (ETSI) and accepted by ETSI. 8Like 802.11a, it operates at 5 Ghz frequency spectrum (5.15-5.35 and 5.470-5.725 Ghz) and supports maximum 54 Mbps bandwidth. This frequency band is used without proprietary in most of Europe. A forum named Hiperlan/2 Global Forum (H2GF) is created to support Hiperlan/2 standard. This forum aims to make Hiperlan/2 more widespread and popular. The organization also aims to achieve the interoperability of different hardware of different vendors.

The main differences of Hiperlan/2 as compared to 802.11a are that it supports safety/security concerns (such as authentication and encryption) and traffic prioritization. This is a significant concern for real-time data transfer like video conferencing and sound transmission. As apart from that Hiperlan/2 support various different backbone networks such as Ethernet, ATM, and UMTS.


M. Suna Ylmaz

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