Comparing 802.11a vs. 802.11b/g Performance

Some articles have been written recently in various online publications comparing the performance of 802.11a and 802.11b/g. Everyone agrees that 802.11a and 802.11b/g have different performance characteristics, but not everyone agrees on exactly what those differences are. In this month’s article, we at Connect802 run through the common arguments for and against 802.11a and 802.11b/g and we weigh in with our experience on the topic.

First, a run-down of the basic facts.

802.11b and 802.11g both operate in the 2.4 GHz ISM frequency band. They use the same channel layout and are subject to the same power output restrictions. 802.11g offers higher data rates (maxing out at 54 Mbps) compared to 802.11b (maxing out at 11 Mbps).

802.11a operates in the 5 GHz UNII frequency band. It has a different channel layout and different power output restrictions than 802.11b and 802.11g. Like 802.11g, 802.11a uses OFDM and offers data rates up to 54 Mbps.

Now, on to the comparison.

Environmental Noise

Given that 802.11a and 802.11g both use OFDM and both offer the same data rates, performance between them should be pretty much the same, right? Not necessarily. Some people argue that 802.11a should have less range than 802.11g, all other things being equal, because 802.11a uses higher-frequency signals, and (in fact) higher-frequency signals lose signal power over shorter distances than lower-frequency signals. Since 802.11a’s frequency is roughly double that of 802.11b/g’s, the argument goes that 802.11a’s range ought to be significantly less than that of 802.11b/g’s. Basing conclusions on these arguments would lead one to believe that an 802.11a network always requires more access points in a given area than a corresponding 802.11g network, As it turns out, these arguments just don’t hold up in the real-world.

Connect802’s experience is that 802.11a performance is usually as good as or better than 802.11b/g’s in the type of office, commercial, and residential areas that we usually encounter. The limiting factor is not the issue of power loss at higher frequency. The reason for 802.11a being a workable alternative to 802.11g is that the 2.4 GHz frequency band is usually very crowded with interfering devices such as other 802.11 networks, 2.4 GHz cordless phones, microwave ovens, and Bluetooth devices. These devices create a background “hum” of noise that reduces the performance of 802.11b/g networks in the area.

Spectrum analyzer screen shows background interference from frequency-hopping devices.

In comparison, the 5 GHz band is totally clean.

By way of example, imagine trying to hold a conversation in a quiet living room versus a crowded auditorium. Your voice would carry less and you would have to talk louder in the auditorium. In comparison, the 5 GHz frequencies used by 802.11a are almost always totally free of interfering devices. The 802.11a devices are talking in the quiet living room; the 802.11b/g devices are talking in the crowded auditorium. Even though the 802.11a signals experience more attenuation, our experience is that 802.11a can usually achieve as good or better range and throughput compared to 802.11b/g in the most common environments.

An exception to this finding would be an environment where the 2.4 GHz frequencies were relatively uncluttered. Examples of this might be a rural environment without lots of other wireless transmitters or a warehouse whose metal and cement walls block out most external signals. In such an environment, the 2.4 GHz spectrum might be just as clean as the 5 GHz spectrum, allowing it to perform up to its full potential.

Power Output Restrictions

The 2.4 GHz ISM band used by 802.11b/g has different FCC power output restrictions than the 5 GHz UNII band used by 802.11a. 2.4 GHz ISM transmitters are allowed significantly more output power than most 5 GHz UNII transmitters. Whereas 802.11b/g transmitters could have up to 1 Watt (1000 mW) of output power, 802.11a transmitters are usually limited to 40 mW or 250 mW depending on which channel they’re using. On paper, then, it looks like 2.4 GHz has another advantage: more power equals more range and higher data rates.

But once again, real-world considerations mean that things aren’t as simple as they might at first seem. Very few 802.11b/g devices actually take advantage of the increased output power that is available to them. Most 802.11b/g transmitters have a maximum output power of no more than 100 mW, and output powers of 15 to 30 mW are typical in our experience. Why don’t they take advantage of the greater output power that’s available to them? Engineering and business concerns. A higher-powered device is more expensive to engineer and has decreased battery life. It also gets hotter! An output power of about 30 mW seems to be the “sweet spot” that the industry has settled on for 802.11b/g transmitters. Of course, higher-powered devices do exist, but they’re relatively rare.

What power output do 802.11a transmitters use? In our experience, 40 mW is typical. So, real-world 802.11a power output is comparable to real-world 802.11b/g power output, even though 802.11b/g is allowed to use more power in most circumstances. (We say, “in most circumstances,” because some of the channels in the 5 GHz UNII band do allow for higher power output than the 2.4 GHz ISM band, but these channels are not allowed to be used by typical 802.11 client devices).

Channel Allocation

The number of available channels is one place where 802.11a clearly wins over 802.11b/g. 802.11b/g allocates 14 channels, of which 11 are available for use in the United States. Since these channels overlap with each other, a maximum of three of them can be used simultaneously: 1, 6, and 11. 802.11a, on the other hand, currently has no fewer than 23 available channels, and they don’t overlap, which means that all of them can be in use at the same time without performance degradation! Clearly, 802.11a offers more overall capacity and can support more user and access point density than 802.11b/g.

This difference will become even more relevant as 802.11n phases into the marketplace. 802.11n uses double-width channels to achieve its highest data rates. In the 2.4 GHz ISM band, only one of these double-width channels can be supported, severely limiting deployment options. The 5 GHz band, with its much wider bandwidth, can support many more of these channels.

Summary and Conclusion

On paper, 802.11b/g has certain advantages over 802.11a, but Connect802’s experience is that, in the real world, 802.11a provides as good or better performance than 802.11b/g. The primary reason for this is the congested nature of the 2.4 GHz ISM frequency band that is used by 802.11b/g. Why, then, aren’t 802.11a networks more popular? We conclude that it’s because of a lack of market adoption. As long as the majority of client devices only support 802.11g, there isn’t a compelling reason to install an 802.11a wireless network, no matter how good its performance might be. Nevertheless, if you’re in a situation where you know that the clients of a wireless network can be 802.11a devices, don’t let the theoretical disadvantages of 802.11a prevent you from deploying it.

Connect802 can provide wireless designs for 802.11b/g, 802.11a, or any other frequency band that you desire.

What’s The Story With WiMAX?

WiMAX is defined as Worldwide Interoperability for Microwave Access by the WiMAX forum, an organization that was formed to promote conformance and interoperability of the IEEE 802.16 standard. The WiMAX forum describes 802.16 as, “a standards-based technology enabling delivery of last mile wireless broadband access as an alternative to cable and DSL.”

WiMAX antennas on a tower

Although WiMAX is often compared to and confused with Wi-Fi (802.11), the two technologies address fundamentally different sets of needs. WiMAX is capable of providing symmetrical bandwidth over many kilometers, whereas Wi-Fi’s bandwidth is often asymmetrical and typically ranges up to only a few hundreds of feet. WiMAX has capabilities for high-speed backhaul links, whereas Wi-Fi hotspots are typically backhauled with relatively low-speed DSL or cable modem links, meaning that these hotspots can’t really offer as much speed as their 54 Mbps data rates suggest. Finally, Wi-Fi performance suffers when line of sight to the antenna is compromised. By comparison, WiMAX has provisions for using lower-frequency signals to decrease the need for line-of-sight, similar to how cell phones operate.

In the same way that 802.11 has various amendments (802.11a, 802.11b, 802.11g, and so forth), there is more than one version of 802.16. But the different 802.16 variants differ more than do the 802.11 variants. The major ways in which 802.16 variants differ are: fixed vs. mobile receivers, line of sight vs. non line of sight, and licensed versus unlicensed frequency bands. Some variants of 802.16 require that the radios be in a fixed location, while others allow for mobile receivers. Some variants require line of sight between the two antennas, while others don’t. Some variants use licensed frequency bands, while others use the same unlicensed frequency bands used by 802.11a.

This creates quite a variety of potential services that 802.16 can provide, and contributes to some of the confusion with regard to what exactly WiMAX is. It might make more sense to think of WiMAX as a suite of services, as opposed to a single wireless technology. One service might offer data rates around 70 Mbps over distances of a few miles using high-powered, fixed point-to-point links in licensed frequency bands. Another service might offer data rates around 5 Mbps over distances of hundreds of feet using lower-powered links to mobile users in unlicensed frequency bands.

When you hear speculation about WiMAX’s capabilities, be aware that the speaker might be combining features in a way that would be difficult or impossible to implement in the real world. For example, it’s unlikely that you’ll find a configuration that offers point-to-multipoint data rates of 70 Mbps over distances of up to 70 miles in unlicensed frequency bands. In general, higher data rates and longer distances require licensed frequency bands, point-to-point operation, and/or line-of-sight. Configurations that work in unlicensed frequencies or allow for mobile operation typically only offer lower data rates Nevertheless, WiMAX does offer services that are difficult to achieve otherwise. Although real-world deployments are limited, most of its promise today seems to be in providing a replacement for last-mile wired services such as telephone, DSL/cable/ISDN broadband, and even as a replacement for cable television.

Finally, be aware that truly WiMAX-certified equipment is relatively rare, especially in the United States. The first equipment to officially receive the WiMAX certification operated in frequency bands that were not usable in the U.S. because they had already been allocated. Interestingly, the 2.5 GHz frequency band that WiMAX uses has been allocated to Sprint/Nextel, which has announced that it plans a WiMAX rollout over the next few years.