LAN 101: Networking Basics

Wireless Ethernet: 802.11n And Bluetooth

802.11n

The latest wireless network standard, 802.11n (also known as Wireless-N), was published in October 2009. 802.11n hardware uses a technology called multiple input, multiple output (MIMO) to increase throughput and range. MIMO uses multiple radios and antennas to transmit multiple data streams (also known as spatial streams) between stations. Unlike earlier 802.11 implementations, in which reflected radio signals slowed down throughput, reflected radio signals can improve throughput as well as increase useful range.

802.11n is the first wireless Ethernet standard to support two frequency ranges or bands:

• 2.4 GHz (same as 802.11b/g)
• 5 GHz (same as 802.11a)

Thus, depending on the specific implementation of 802.11n in use, a dual-band 802.11n device may be able to connect with 802.11b, 802.11g, and 802.11a devices, whereas a single-band 802.11n device will be able to connect with 802.11b and 802.11g devices only.

Wireless-N devices can contain radios in a number of different configurations supported by the standard. The radios are defined or categorized by the number of transmit antennas, receive antennas, and data streams (also called spatial streams) they can support. A common notation has been devised to describe these configurations, which is written as a x b:c, where a is the maximum number of transmit antennas, b is the maximum number of receive antennas, and c is the maximum number of simultaneous data streams that can be used.

The maximum performance configuration supported by the standard is 4 x 4:4, (4 transmit/receive antennas and 4 data streams), which would support bandwidths of up to 600 Mb/s, however no devices are currently on the market using that configuration. Common configurations that are used in Wireless-N devices include 1 x 1:1, 1 x 2:1, and 2 x 2:1, which include radios with 1 or 2 antennas supporting only a single data stream for up to 150 Mb/s in bandwidth. Other common configurations include 2 x 2:2, 2 x 3:2, and 3 x 3:2, which include radios with 2 or 3 antennas supporting up to two data streams for up to 300 Mb/s in bandwidth. Those using more antennas than data streams allow for increased signal diversity and range. The highest performance Wireless-N devices generally available on the market today use a 3 x 3:3 radio configuration, which supports three data streams for up to 450 Mb/s in bandwidth.

802.11n is significantly faster than 802.11g, but by how much? That depends mainly on how many data streams are supported, as well as whether a couple of other optional features are enabled or not. The base configuration uses 20 MHz wide channels with an 800 ns guard interval between transmitted signals. By using channel bonding to increase the channel width to 40 MHz, more than double the bandwidth can be achieved in theory. I say “in theory” because using the wider channels works well under very strong signal conditions, but can degrade rapidly under normal circumstances. In addition, the wider channel takes up more of the band, causing more interference with other wireless networks in range. In the real world I’ve seen throughput decrease dramatically with 40 MHz channels, such that the use of 40 MHz channels is disabled by default on most devices.

Another optional feature is using a shorter guard interval (GI), which is the amount of time (in nanoseconds) the system waits between transmitting OFDM (orthagonal frequency division multiplexing) symbols in a data stream. By decreasing the guard interval from the standard 800 ns to an optional 400 ns, the maximum bandwidth increases by about 10%. Just as with channel bonding (40 MHz channel width), this can cause problems if there is excessive interference or low signal strength, resulting in decreased overall throughput due to signal errors and retries. However, in the real world the shorter guard interval doesn’t normally cause problems, so it is enabled by the default configuration in most devices.

Combining the use of three data streams using standard 20 MHz channels and the standard 800 ns guard interval, the maximum throughput of a Wireless-N connection would be 195 Mb/s. Using the shorter 400 ns guard interval would increase this to up to 216.7 Mb/s. As with other members of the 802.11 family of standards, 802.11n supports fallback rates when a connection cannot be made at the maximum data rate.

The following table compares the standard and optional speeds supported by 802.11n to those supported by 802.11b, 802.11a, and 802.11g.

As you can see from the table, Wireless-N devices supporting four data streams will be able to support up to 600 Mb/s throughput, however the reality today is much less than this. Because there are no devices on the market that support four streams, the maximum throughput advertised by Wireless-N devices today is either 300 Mb/s (using two streams) or 450 Mb/s (using three streams), and those figures also assume both the use of 40 MHz wide channels (not normally recommended) and a short guard interval.

The Wi-Fi Alliance first began certifying products that support 802.11n in its Draft 2 form in June 2007. The 802.11n standard was finally published in October 2009, and 802.11n Draft 2 or later products are considered to be compliant with the final 802.11n standard. In some cases, driver or firmware updates might be necessary to insure ensure full compliance. As with previous Wi-Fi certifications, the Wi-Fi 802.11n certification requires that hardware from different makers interoperate properly with each other. 802.11n hardware uses chips from makers including Atheros, Broadcom, Cisco, Intel, Marvell, and Ralink.

Bluetooth

Bluetooth is a low-speed, low-power standard originally designed to interconnect laptop computers, PDAs, cell phones, and pagers for data synchronization and user authentication in public areas such as airports, hotels, rental car pickups, and sporting events. Bluetooth is also used for a variety of wireless devices on PCs, including printer adapters, keyboards, mice, headphones, DV camcorders, data projectors, and many others. A list of Bluetooth products and announcements is available at the official Bluetooth wireless information website.

Bluetooth devices also use the same 2.4 GHz frequency range that most Wi-Fi devices use. However, in an attempt to avoid interference with Wi-Fi, Bluetooth uses a signaling method called frequency hopping spread spectrum (FHSS), which switches the exact frequency used during a Bluetooth session 1600 times per second over the 79 channels Bluetooth uses. Unlike Wi-Fi, which is designed to allow a device to be part of a network at all times, Bluetooth is designed for ad hoc temporary networks (known as piconets) in which two devices connect only long enough to transfer data and then break the connection. The basic data rate supported by Bluetooth is currently 1 Mb/s (up from 700 Kb/s in earlier versions), but devices that support enhanced data rate (EDR) can reach a transfer rate up to 2.1 Mb/s.

The current version of Bluetooth is 4.0, however versions 2.1 and later supports easier connections between devices such as phones and headsets (a process known as pairing), longer battery life, and improved security compared to older versions. Version 3.0 adds a high-speed mode based on Wi-Fi, while 4.0 adds low energy protocols for devices using extremely low power consumption.

Interference Issues Between Bluetooth and 802.11b/g/n Wireless

Despite the frequency-hopping nature of Bluetooth, studies have shown that Bluetooth 802.11b/g/n devices can interfere with each other, particularly at close range (under 2 meters) or when users attempt to use both types of wireless networking at the same time (as with a wireless network connection on a computer also using a Bluetooth wireless keyboard and/or mouse). Interference reduces throughput and in some circumstances can cause data loss.

Bluetooth version 1.2 adds adaptive frequency hopping to solve interference problems when devices are more than 1 meter (3.3 feet) away from each other. However, close-range (less than 1 meter) interference can still take place. IEEE has developed 802.15.2, a specification for enabling coexistence between 802.11b/g/n and Bluetooth. It can use various time-sharing or time-division methods to enable coexistence. Bluetooth version 2.1 is designed to minimize interference by using an improved adaptive hopping method, whereas 3.0 and later adds the ability to use 802.11 radios for high-speed transfers. Companies that build both Bluetooth and 802.11-family chipsets, such as Atheros and Texas Instruments (TI), have developed methods for avoiding interference that work especially well when same-vendor products are teamed together.