802.11

802.11

A family of IEEE standards that extend the common wired Ethernet local network standard into the wireless domain. The 802.11 standards are widely known as "Wi-Fi" because the Wi-Fi Alliance provides certification for 802.11 products. There have been four major 802.11 standards relating primarily to speed and designated by single letter suffixes, such as the latest and fastest 802.11n. Two medium speed standards are 802.11a and 802.11g, and the slow speed is 802.11b. For more about Wi-Fi networks, see wireless LAN and Wi-Fi. Following are 802.11 specifications.

Very Slow Speeds (1997)
The first 802.11 specifications included two spread spectrum methods in the unlicensed 2.4 GHz band: 1 Mbps frequency hopping (FHSS) and 1 and 2 Mbps direct sequence (DSSS). It also included an infrared method. Both FHSS and infrared were dropped by the Wi-Fi Alliance, but 1 Mbps DSSS method is still used by access points to advertise themselves (see beaconing).

11b (1999) - Slow Speed
Using DSSS and the 2.4 GHz band, 802.11b boosted speed to 11 Mbps while retaining the slower DSSS modes to accommodate weak signals. It was the first major wireless local network standard, and many laptops were retrofitted with 11b network adapters. Later, 11b was built into the laptop motherboard.

11a (1999) - Medium Speed
Using orthogonal FDM (OFDM), 802.11a transmits up to 54 Mbps. It uses the 5 GHz band and is not backward compatible with the slower 11b.

11g (2003) - Medium Speed
Using orthogonal FDM (OFDM) transmission, 11g increased speed from 11 to 54 Mbps. Both 11b and 11g use the 2.4 GHz band and are compatible, which is why equipment is often designated as 802.11b/g. If 11b and 11g devices communicate, it is done at the slower 11b speed.

11n (2009) - Fastest Speed
The 802.11n standard uses multiple antennas for speeds of 150 Mbps and more. Since 11n can operate in both spectrum bands, it is compatible with previous 11b/g and 11a standards (see 802.11n).

Multiple Channels
To allow nearby access points to operate without interference, 802.11 divides the spectrum into 19 channels for 11n, 12 for 11a and only three for 11b and g. The 11b/g standards use overlapping channels, and only channels 1, 6 and 11 can be used in the U.S.

Infrastructure and Ad Hoc Modes
An 802.11 system works in two modes. In "infrastructure" mode, wireless devices communicate to a wired LAN via base stations known as "access points." Each access point and its wireless devices are known as a Basic Service Set (BSS). An Extended Service Set (ESS) is two or more BSSs in the same subnet.

In "ad hoc" mode, also known as "peer-to-peer" mode, wireless devices communicate with each other directly without an access point. This is an Independent BSS (IBSS).

An additional mode was added in 2009 that enables two devices to communicate with each other directly (see Wi-Fi Direct).

Throughput Varies
The speed of 802.11 systems is distance dependent. The farther away the remote device from the base station, the lower the speed (see chart below). Also, the actual data throughput is generally no more than half of the rated speed because 802.11 uses a collision "avoidance" technology (CSMA/CA) rather than the collision "detection" method (CSMA/CD) in wired Ethernet. Wired systems can detect a collision, but wireless cannot, thus, the CSMA/CA method waits for an acknowledgment from the other end to determine if the packet was transmitted properly. A 54 Mbps rated speed yields only about 27 Mbps in real throughput.

In addition, access points that support a mixed 11b and 11g network drop the throughput to 18 Mbps to start with and wind up with approximately 6 to 9 Mbps total when clients are transmitting. See 802.11 timeline, wireless LAN, ISM band, CCK/OFDM, 802.16 and 802.15.

 TYPE (Band)
                     Max**
             Max     Indoor
 Modulation  Speed   Range

 11b (2.4 GHz)
    DSSS    11 Mbps  150 ft

 11g (2.4 GHz)
    OFDM    54 Mbps  170 ft

 11a (5 GHz)
    OFDM    54 Mbps   95 ft

 11n (2.4 & 5 GHz)
    OFDM   600 Mbps  230 ft

 ** outdoor 2x to 4x farther

Speed Is Distance Dependent

The farther the distance between transmitter and receiver, the lower the bit rate in an 802.11 system. (Chart and data courtesy of Intersil Corporation.)

Wireless LAN

In "infrastructure" mode, the laptops transmit to access points. In "ad hoc" mode, they communicate with each other in peer-to-peer fashion.

Wireless Components

The access point connects to the network, and the adapters are inserted into the client machines. Although one mostly thinks "laptops" when talking about wireless, desktop computers can also be made wireless to avoid running cables. (Images courtesy of Cisco Systems, Inc.)