802.11a becomes a contender

Network managers have long chosen 802.11b as their preferred wireless LAN standard because there have been no significant contenders. With vendors releasing 802.11a access points and radio network interface cards, there's finally a decision that needs to be made on whether to deploy 802.11a or 802.11b. When you look at the "tale of the tape" between the two technologies, you'll see that 802.11a becomes a serious contender to the champ.


The edge goes to 802.11a for higher performance. The 802.11a products operate at speeds of up to 54M bit/sec in the 5-GHz frequency band using orthogonal frequency division multiplexing (OFDM). This technology combats impairments such as multipath propagation, which can limit data rates.

The 802.11b standard operates at speeds up to 11M bit/sec in the 2.4-GHz frequency band using direct sequence spread (DSS) spectrum, which is significantly slower and more prone to transmission impairments than 802.11a.


Another edge for 802.11a is its whopping 432M bit/sec total rate performance with eight channels (the specification includes 12 nonoverlapping channels, but current products only support the first eight). Still, it's better than 802.11b, which has 11 channels available, but only three nonoverlapping channels for a total rate performance of 33M bit/sec.


802.11a and 802.11b use a carrier sensing protocol that enables the sharing of a common radio channel. An end user's radio network interface card (NIC) senses the air medium and only transmits if no radio frequency waves above a certain threshold are detected. The presence of a radio frequency signal causes the radio NIC to hold off from transmitting, and an interfering signal strong enough will make the channel appear as busy. As a result, the radio NIC will wait until the interference goes away (which could be minutes, hours or days). Radio frequency interference competes with the transmission of 802.11 frames and can significantly reduce the throughput and availability of a wireless LAN.

And 802.11b systems are more susceptible to radio frequency interference than 802.11a because of many sources of interference in the 2.4-GHz band. "Microwave ovens, Bluetooth devices, 2.4-GHz cordless phones, and other nearby 802.11 wireless LANs can cause significant and damaging [radio frequency] interference with 802.11b systems," says Erwin Noble, director of standards and strategy for Koninklijke Philips Electronics NV's Components division. Most brands of cordless phones can bring a wireless LAN to a standstill while the phones are in use within 75 feet of 802.11 devices.

Radio frequency interference with 802.11a in the 5-GHz band is practically nonexistent. As a result, 802.11a networks have a higher degree of availability. This means that 802.11a can stand on its feet much longer, free from unplanned punches.


Give this one to the champ, at least for the moment. Current list prices have 802.11a components at about 25 percent higher than 802.11b equipment. To minimize costs, most users not requiring higher performance are better off with 802.11b. However, with an 802.11a deployment you will have enough capacity for extra users and the inevitable bandwidth-hungry applications in the future.

"Eventually the price gap between 802.11a and 802.11b will likely become smaller with economies of scale," Noble says. Most wireless chipset suppliers have announced multimode 802.11a/b chipsets, and radio NIC vendors will likely release multimode radio NICs by year-end. Similar to the 10/100M bit/sec Ethernet, the single-priced, dual 802.11a/b radio could become commonplace. This might make the price debate a moot point as users buy one radio NIC for 802.11a and 802.11b.

Existing installations:

Another one for the champ. A problem with the 802.11 standard is that it lacks provisions for interoperability between 802.11a and 802.11b. As a result, it's often not cost-effective for companies having existing 802.11b implementations to migrate to 802.11a.

Some feel that 802.11g is a better upgrade path from 802.11b to achieve higher performance. The 802.11g standard is still under development, but the final version is likely to specify operation up to 54M bit/sec in the 2.4-GHz band using OFDM, and will be backward compatible with 802.11b. You'll likely be able to upgrade most of the recent access points through relatively simple firm-ware upgrades. But remember that 802. 11g is still limited to the three nonoverlapping channels, which don't come close to the higher capacity of 802.11a. And 802.11g has the same issues with radio frequency interference as 802.11b.


Score this as a tie. The higher frequency of 802.11a means it has a lower range than 802.11b networks. However, in practical tests, 802.11a is capable of maintaining data rates higher than 802.11b at similar ranges. Near the access point, 802.11a is 54M bit/sec and 802.11b is 11M bit/sec. As the range increases, 802.11a and 802.11b will step down in data rate to accommodate for lower signal levels, but the data rate of 802.11a will still remain higher than 802.11b at comparable ranges.

802.11b might hang on at longer ranges than 802.11a, but 802.11b will go down to 1M bit/sec, which is barely breathing. Most companies deploying enterprise wireless LANs plan 802.11b coverage to support 11M bit/sec throughout the facility.

This is done to achieve comparable performance to Ethernet. You can replace these access points with the same number of 802.11a access points and have equivalent if not better performance.


This one is also a tie. Most primary wireless LAN security mechanisms, such as Wired Equivalent Privacy and 802.1X, are seated in the media access control layer, which is neutral regarding 802.11a and 802.11b. But some could argue that the lower total effective range of 802.11a (compared with 802.11b) would require an eavesdropper to be closer and possibly within the controlled area of an 802.11a system to receive signals. This would lean the decision toward 802.11a if deploying the network takes into account effective antenna selection and placement.

Bottom line:

In a split decision, the issue of using 802.11a or 802.11b comes down to many factors. If you want high performance and minimal radio frequency interference, then 802.11a is the way to go. An 802.11a network can support existing and future needs for higher performance, making it a cost-effective, long-term strategy.

The 802.11a standard is better for multimedia applications that include video and audio, which may spur movement in the home space for entertainment, and companies looking at videoconferencing applications.

The higher capacity of 802.11a will better support densely populated areas, including airports, convention centers, schools and stock exchanges. The large, concentrated groups of users in these situations will often bog down an 802.11b network.

Cost-conscious companies with existing wireless LAN deployments will likely stick it out with 802.11b until the faster 802.11g comes on the scene. Furthermore, developments in providing for dual-mode (both technologies on the same access point and/or radio cards), and even trimode (a, b and g on the same equipment) might likely cause these decisions to be moot.

Geier provides independent consulting services to companies developing and deploying wireless networks. He can be reached at jimgeier@wireless-nets.com.

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