Applications requiring significant bandwidth like streaming media, on-line gaming, and VoIP do not always render well using Wi-Fi connections. Why is that? Internet access should be the bottleneck. It has far less available bandwidth than an 802.11a/g connection. Or does it?
What affects bandwidth
Shoving data through a cable or fiber is simple compared to transmitting it over the ether. Not being confined also makes it difficult to get a handle on a Wi-Fi connection’s available bandwidth. The following are some of the reasons why:
- Wi-Fi transmissions require considerably more management traffic compared to a wired connection, which reduces the bandwidth available for data.
- Unlike wired Ethernet links, duplex operation (being able to send traffic to both devices simultaneously, doubling bandwidth) is not currently possible with Wi-Fi. Think of two people talking at the same time; neither hears the other.
- Receiving signal strength plays a huge part in determining the throughput data transfer rate, with more signal strength allowing a better rate. Wired connections are affected only by cable length restrictions.
802.11n introduced two new ideas that help with the bandwidth problem. One is to use Multiple-Input/Multiple-Output (MIMO) technology. By its nature, radio signals fan out from the transmitting antenna, bumping into objects on the way to the receiver. Up until MIMO was developed, these out-of-sync radio waves would confuse the receiver; however, MIMO can process the different signals. James Wilson, an Intel engineer, explains in his DeviceForge.com article:
“MIMO technology offers the ability to coherently resolve information from multiple signal paths using spatially separated receive antennas. Multipath signals are the reflected signals arriving at the receiver some time after the original or line of sight (LOS) signal has been received.
Multipath is typically perceived as interference degrading a receiver’s ability to recover the intelligent information. MIMO enables the opportunity to spatially resolve multipath signals, providing diversity gain that contributes to a receiver’s ability to recover the intelligent information.”
Increase channel size
802.11n also introduced another important feature, larger channels. Increasing channel bandwidth from 20 MHz to 40 MHz more than doubles the usable bandwidth. The following slide (courtesy of Intel Labs) illustrates that:
Add in MIMO, and 802.11n substantially increases data transfer rates, as can be seen in the following table (courtesy of Intel Labs):
You may be wondering what the two different measurements mean. Here are their definitions:
- Over-the Air Estimate: Under perfect conditions, it is the amount of data that can be sent from one Wi-Fi radio to another. This is the familiar 54 Mb/s advertised by the 802.11a/g standard.
- MAC Service Access Point Estimate: Is dependent on the physical environment. 802.11a/g under best conditions may have a transfer rate of 22-25 Mb/s. The transfer rate could be low as 1-2 Mb/s in less than optimal situations.
Moving to 802.11n equipment will help considerably if your current system is bandwidth-starved. But, is it enough? Wired technology is rapidly moving from 100 Mb/s Ethernet to Gigabit Ethernet. So, 802.11n with its 100Mb/s limit is still lagging behind.
The next big innovation in Wi-Fi is using multiple data streams. You may have seen the designation 3 x3: 3 (a x b: c). This notation helps identify the device’s capabilities. The first number (a) represents the maximum number of transmit antennas. The second number (b) is the maximum number of receive antennas. Finally, the third number (c) is the maximum number of data streams the radio can process.
The maximum number of data streams allowed by the 802.11n standard is 4 x 4: 4. A device capable of this designation will have an Over-the-Air Estimate bandwidth of 600 Mb/s.
Qualcomm introduced a 4 x 4: 4 chip this past June. Right after 802.11n was ratified, Atheros announced it will have a 3 x 3: 3 chip ready in early 2010. Devices using the Atheros chip will have an Over-the-Air Estimate bandwidth of 450 Mb/s.
Apple is not saying so, but it appears their upgrade to the AirPort Extreme Base Station is a 3 x 3: 3 device. Glenn Fleishman describes what this means in his Ars Technica article:
“The company says only that the latest revision gives you up to 50 percent better Wi-Fi performance and up to 25 percent better range than its immediate predecessor. This conforms with 3×3 antenna arrays, which, even with a two-stream radio, carry data further at higher speeds.”
The only problem is that Intel appears to be the only manufacturer currently offering 3 x3: 3 client network adapters.
Wi-Fi devices that utilize the full potential of 802.11n should eliminate many of the bandwidth problems associated with legacy Wi-Fi devices. The new 802.11n devices will also improve range and reliability. It may be time to unplug the Ethernet cable.