By Jim Geier
As 802.11a products began shipping months ago, more and more companies have been taking advantage of 802.11a’s superior performance. 802.11a radios transmit at 5GHz and send data up to 54Mbps using OFDM (orthogonal frequency division multiplexing). The results have been very good. 802.11a products deliver excellent performance.
Inside 802.11a
Before discussing benefits and implications of 802.11a, let’s take a look at how 802.11a devices operate.
802.11a defines one of several different 802.11 Physical Layers (PHYs). The actual name of 802.11a is the “High Speed Physical Layer in the 5GHz band,” commonly referred to as the “OFDM PHY.” Another popular PHY of course is 802.11b, which most companies have been installing for the past couple years. Others include 802.11 FHSS (frequency hopping spread spectrum) and 802.11 IR (infrared).
No matter which 802.11 PHY you deploy, the MAC (medium access control) Layer is the same. The MAC Layer manages and maintains communications between 802.11 radio NICs and access points by coordinating access to a shared radio channel. The MAC Layer is actually a program that runs on a processor; whereas, the PHY involves digital communications circuitry and an RF (radio frequency) modulator to prepare data for transmission over the air medium.
The 802.11a PHY is quite different than 802.11b, which uses direct sequence spread spectrum (DSSS). 802.11a specifies the use of OFDM to support higher data rates.
OFDM divides the data signal across 48 separate sub-carriers to provide transmissions of 6, 9, 12, 18, 24, 36, 48, or 54Mbps of which 6, 12, and 24Mbps are mandatory for all products. For each of the sub-carriers, OFDM uses PSK (phase shift keying) or QAM (quadrature amplitude modulation) to modulate the digital signal depending on the selected data rate of transmission. In addition, four pilot sub-carriers provide a reference to minimize frequency and phase shifts of the signal during transmission. This form of transmission enables OFDM to operate extremely efficiently, which leads to the higher data rates, and minimize the affects of multi-path propagation.
The operating frequencies of 802.11a in the U.S. fall into the national information structure (U-NII) bands: 5.15-5.25GHz, 5.25-5.35GHz, and 5.725-5.825GHz. Within this spectrum, there are twelve, 20MHz channels, and each band has different output power limits. The Code of Federal Regulations, Title 47, Section 15.407, regulates these frequencies in the U.S.
OFDM is becoming very popular for high speed transmission. In addition to being selected as the basis for the 802.11g PHY, OFDM is the basis for the European-based HiperLAN/2 wireless LAN standards. In fact the 802.11a PHY is very similar to the HiperLAN/2 PHY. In addition, OFDM has also been around for a while supporting the global standard for asymmetric digital subscriber line (ADSL).
The following are benefits of 802.11a:
Higher performance. By far the top reason for choosing 802.11a is the need to support higher end applications involving video, voice, and the transmission of large images and files. In addition, 802.11a does a superior job of supporting densely populated areas of users having lower bandwidth needs, such as surfing the Internet. 802.11a can deliver data rates up to 54Mbps and there’s enough room in the 5GHz spectrum to support up to 12 access points operating in the same area without causing interference between access points. This equates to 432Mbps (12 X 54Mbps) total data rate performance. Even the upcoming 802.11g standard, which will deliver 54Mbps data rates in the 2.4GHz band doesn’t come close to the performance of 802.11a. With 802.11g, the same problem exists as with 802.11b: You have only three non-overlapping channels for setting access point frequencies, which severely limits capacity.
Less RF interference.The growing use of 2.4GHz cordless phones and Bluetooth devices is crowding the radio spectrum within many facilities. This significantly decreases the performance of 802.11b wireless LANs. Cordless phones wreak enough havoc to cause companies to either ban the use of the phones or not install wireless LANs. The use of 802.11a operating in the relatively un-crowded 5GHz band avoids this interference. Of course non-802.11 devices will eventually occupy the 5GHz band as well; however, there’s much more room with 12 non-overlapping channels to limit interference with the other devices.
The following are drawbacks of 802.11a:
Less range. The superior performance of 802.11a offers excellent support for bandwidth hungry applications, but the higher operating frequency equates to relatively shorter range. Even with this limitation, however, 802.11a can sometimes deliver better performance than 802.11b at similar ranges from the access point. For example at ranges of 100 feet, 802.11a may deliver 24Mbps, but 802.11b devices at the same range are operating at 5.5Mbps.
If you’re planning to deploy 802.11b networks for 11Mbps throughout the facility, it’s very likely that you can install 802.11 access points at the same locations and still achieve 6 to 12Mbps data rates. As a result, you can install approximately the same number of 802.11a access points as 802.11b and likely have similar performance. When needs for higher performance occur in the future, you can add more access points to increase the coverage to 54Mbps throughout the facility. This approach enables you to grow into a longer term, higher performing solution while spreading the costs over time.
Limited interoperability. 802.11a doesn’t talk to 802.11b. For example, an end user equipped with an 802.11a NIC will not be able to connect with an 802.11b access point. The 802.11 standard offers no provisions for interoperability between the different physical layers. The solution to this problem is multimode radio cards that support multiple 802.11 PHYs, such as 802.11a/b, 802.11a/g, etc. These cards should be available on the market by the end of 2002. As a result, an 802.11a/b radio within an end user device will automatically sense whether the access point is 802.11a or 802.11b and then communicate accordingly.
Likewise, an access point can also deploy a dual 802.11a/b solution, enabling interoperability with end user devices equipped with either an 802.11a or 802.11b radio. In the meantime, however, you can still install 802.11a wireless LANs. This assumes, however, that you’re able to implement 802.11a radios in the user devices. Some devices today, such as bar code scanners, come equipped with 802.11b cards that you can’t easily change.
Higher prices.The current list prices of 802.11a products are approximately 30 percent higher than 802.11b, but the price gap should close over the next couple years. The higher price today, nevertheless, causes some companies to install 802.11b in order to lower initial costs. The problem is that the primary migration path for these companies to deliver higher data rates in the future will be to upgrade their 802.11b access points to 802.11g. It’s not clear when 802.11g products will be available, though, because the standard still requires major work before IEEE ratification and FCC approval takes place. 802.11a is available today and operates in a much less crowded part of the spectrum that includes higher capacity. 802.11a is clearly a better long term solution, especially when future performance needs are not very well known. It’s better to pay a little more now for a better solution rather than a lot more later to replace hardware.
Of course the your decision on which 802.11 PHY to support depends on requirements of your specific wireless LAN application. Based on the benefits, I highly recommend using 802.11a unless requirements dictate otherwise. It’s always better to have too much performance rather than not enough, especially for large numbers of users and higher end applications.
Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs (SAMs, 2001), and regularly instructs workshops on wireless LANs.
Reprinted from 80211-planet.com.
