plz look below, should definetely clarify...
802.11b/2.4GHz vs. 802.11a/5GHz
There are currently two major WLAN standards, and both operate using radio frequency (RF) technology. The two standards have heretofore been colloquially referred to as 802.11b and 802.11a -- together they're collectively called Wi-Fi. To reduce confusion, however, the wireless standard group called the Wi-Fi Alliance will refer to the two technologies as 2.4GHz and 5GHz, respectively, as least on product packaging. These monikers refer to the frequency band that each technology utilizes.
In the alphabet, "a" comes before "b." In the world of wireless networking though, "b" definitely came before "a." The 802.11b specification was the first to be finalized and reach the marketplace.
802.11b/2.4GHz devices operate in an unlicensed radio band and transmit data on the same frequency as some household appliances, including some cordless phones and even microwave ovens. The 802.11b specification provides for a bandwidth rating of 11 Megabits per second (Mbps). This is just a theoretical maximum, however. Wireless networks, as well as wired LANs, never let you obtain that level of performance, or even close to it. The actual throughput you can expect to obtain from an 802.11b network will typically be between 4 and 5Mbps.
This level of performance is more than sufficient for most rudimentary computing tasks. When you consider that a typical broadband DSL or cable modem connection might provide you with from 600kbps to 1.6Mbps of downstream bandwidth, you can see that the speed of 802.11b is not be an impediment to activities like Web browsing, e-mail, file transfer, running applications, and even streaming Internet-based audio and video.
On the other hand, it's not hard to envision scenarios where your bandwidth needs might be greater -- when you want to quickly transfer very large files like graphics, audio, or video or stream those same audio and video files, like your collection of MP3s or home movies on your hard disk.
If you often see the need for more speed, consider 802.11a. Products based on this 5GHz specification offer higher performance. 802.11a has a maximum bandwidth of 54Mbps, almost five times that of 802.11b. Like its predecessor though, you won't see anything near that in the real world. Instead, expect a maximum throughput of between 20 and 25Mbps -- still five times what you get from 802.11b.
The performance of both 802.11b and 802.11a decreases as your distance from the antenna increases. This degradation is neither linear nor granular; in other words, you don't lose half the performance when your distance doubles, and the performance doesn't decline in small increments as you move farther away.
Instead, each wireless specification has a handful of pre-defined bandwidth levels at which it can operate (802.11b has four, while 802.11a has seven). The bandwidth levels drop markedly as you move further away, and by the time you are at the extreme ranges, the bandwidth available is only a small fraction of the maximum.
When indoors, 802.11b signals can travel as far as 150 meters (492 feet). Outdoors, 11b range is over three times greater-- 500 meters (1640 feet, or nearly 1/3 of a mile). The outdoor ranges are higher because there are fewer obstacles, like walls, to absorb or block the radio signal. At either of these extreme ranges, the bandwidth available is a mere 1Mbps, which would yield throughput closer to that of your broadband connection. That low level of throughput could hamstring your networking activities.
On the other hand, for 802.11b to operate in its maximum bandwidth mode of 11Mbps, the distance indoors can be no more than 50 meters (164 feet); outdoors it should be 250 meters (820 feet).
When it comes to the relationship between performance and range, 802.11a behaves in much the same way as 802.11b. That is to say, there is an inverse relationship, so performance goes down as distance goes up.
The trade-off the speed of 802.11a offers is lower range. Indoors, 802.11a allows for a maximum range of only about 100 meters (about 300 feet). Outdoors, the range jumps to over 350 meters (1200 feet). Like 802.11b, when you are using 802.11a equipment at extreme range, you can only communicate at the lowest speed supported, which in this case is 6Mbps.If you want the full 54Mbps bandwidth, your range indoors is limited to a mere 18 meters (60 feet), and outside to approximately 30 meters (100 feet).
With either technology you lose 50% or more of your range in order to enjoy wireless data transfer at the fastest rate possible. The bottom line is that figures for maximum range, like those for maximum bandwidth, should be taken with a healthy dose of sodium chloride. When evaluating the performance and range ratings of wireless networking products, treat them as you would the gas mileage rating on a car. Remember that your mileage will vary.
In addition to the obvious differences in range, another differentiating factor between 802.11b and 802.11a is the quality, or let's call it robustness, of their signals.
Because of the higher frequency (and thus shorter wavelength) that they use, 802.11a signals have a much tougher time penetrating solid objects like walls, floors, and ceilings. As a result, the price for 802.11as higher speed is not only shorter range but a weaker and less consistent signal.
In much of our testing of 802.11a products, we have often seen the signal strength fluctuate wildly, and in some cases disappear altogether, even though we were not that far from the access point, and certainly within the published range of the device.
By contrast, although 802.11b signal strength can also vary, it is much less common, and we've never completely lost a signal unless we were at the extreme edge of the device's range.
Each of the wireless LAN standards has an extra or enhanced mode that provides an increase in performance. These modes are not official standards, and they require that the network be operated under certain conditions or with particular equipment.
802.11b+ 22Mbps mode
Some of the latest 802.11b/2.4GHz products utilize a particular Texas Instruments chipset, the ACX1000, that uses an enhanced form of modulation, which doubles the maximum bandwidth from 11 to 22Mbps. Testing has indicated that this doubling of bandwidth yields only a 50% throughput increase, however, from about 4Mbps up to 6Mbps.
802.11a/5GHz "Turbo" mode
Most 802.11a/5GHz devices using chipsets from Atheros support a "Turbo" mode that can raise the data rate from 54 to 72Mbps -- 108Mbps in some newer products. In order to utilize this enhanced mode, you need to be using hardware from the same vendor on both sides of the connection.
Use of the turbo mode renders only a small increase in real-world speed over the 54Mbps mode, and this increase comes at the expense of range, which is further diminished when the wireless network is operating in Turbo mode.
802.11g is a wireless LAN specification that has been the subject of discussion and debate since before the 802.11a spec was released earlier this year -- companies like Texas Instruments wanted their technology to be the cornerstone of "g" for example. 802.11g has been under development for some time, and while not yet finalized, it is nearing completion. The first products based on the draft of the specification are expected to emerge at the end of 2002. Many 802.11b-based products out today will be upgradeable to 11g with firmware changes.
It's impossible to say for sure what kind of performance or range 802.11g products will have. However, the goal of 802.11g is to provide performance comparable to the 54Mbps of 802.11a, while maintaining compatibility with 802.11b (and similar range as well). This compatibility is maintained because 802.11g operates in the same 2.4GHz frequency as 802.11b. So, 802.11b and 802.11g devices will be able to communicate with each other, but when they do the 802.11g will be no faster than the 802.11b product it is working with -- they'll both be at the slowest speed common to each.
Therefore, if you've already got an 802.11b network in place, 802.11g's backward compatibility will preserve your investment in existing hardware. This is in contrast to the situation when 802.11a emerged. Because it uses a completely different frequency and type of modulation than 802.11b, users wanting to upgrade to 802.11a needed to buy entirely new hardware.
Just recently, dual-mode access points and NICs have started to appear that simultaneously support both 802.11a/5GHz and 802.11b/2.4GHz. It's very likely that many of the first 802.11g products will also be multimode products able to operate as either 802.11g (and by extension, 802.11b) or 802.11a devices.
How to choose?
As you can see, despite the superficial similarities between 802.11a and 802.11b WLAN standards, there are still significant differences between the two concerning the issues of speed, range, quality of signal, cost, and upgradeability.
So which of the two should you choose?
In the majority of cases, for a typical home network 2.4GHz will be the way to go, given its combination of good speed, range, reasonable cost, and upgrade potential. If you absolutely need higher speeds than even 22Mbps 802.11b can offer you, a 5GHz WLAN will do the job, but you'll need to factor in not only the significantly reduced range, but the fact that the signal may be excessively absorbed or reflected in the interior of your home.
Edited by armjan, 26 May 2005 - 12:39 AM.