802.11

802.11

A family of IEEE standards for wireless LANs that extend wired Ethernet (802.3) into the wireless domain. The 802.11 standard is more widely known as "Wi-Fi" because the Wi-Fi Alliance, an organization independent of IEEE, provides certification for products that conform to 802.11 (see Wi-Fi Alliance).

The first 802.11 specifications were introduced in 1997 and included two spread spectrum methods for transmission in the unlicensed 2.4GHz 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 the 1 Mbps DSSS method is still used by access points to advertise themselves (see beaconing).

802.11b (11b)
In 1999, 802.11b boosted speed to 11 Mbps using DSSS. The 1 and 2 Mbps DSSS modes were retained so that devices could throttle down to lower speeds when signals become weak.

802.11a and 802.11g - (11a and 11g)
Using the orthogonal FDM (OFDM) transmission method, two higher-speed standards followed 802.11b that provide up to 54 Mbps: 802.11a transmits in the 5 GHz frequency range and is not backward compatible with 11b, but 11g transmits in the same 2.4 GHz range and is compatible with 11b. If 11b and 11g devices are communicating, it is done at the slower 11b speed.

Multiple Channels
802.11 systems divide the spectrum into channels so that nearby access points can operate on different channels without interference, but 11b and 11g use overlapping channels. Out of the 11 channels (in the U.S.), only channels 1, 6 and 11 can be used, effectively allowing only three access points to operate near each other. However, 11a uses 12 non-overlapping channels, allowing 12 access points to operate in the same vicinity.

802.11n (11n)
The 802.11n standard, expected in 2009, uses multiple antennas for speeds up to 100 Mbps and more. An interim specification was approved in 2007 so that equipment could work together (see 802.11n).

Two Modes of Operation
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).

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 wireless LAN, ISM band, CCK/OFDM, 802.16 and 802.15.

                               Spread
           Cell                Spectrum
   Speed   Radius  Modulation  Coding
   (Mbps)  (ft.)   Method      Method

   802.11b (DSSS Transmission, 2.4 GHz)
    1     **       BPSK      Barker (11 chip)
    2     **       QPSK      Barker (11 chip)
    5.5   150      QPSK      CCK (8 chip)
   11     115      QPSK      CCK (8 chip)

   802.11g (OFDM Transmission, 2.4 GHz)
   See chart below for cell radius differences.

   802.11a (OFDM Transmission, 5 GHz)
    6      95      BPSK      Rate 1/2 Convolutional
    9      75      BPSK      Rate 3/4 Convolutional
   12      75      QPSK      Rate 1/2 Convolutional
   18      58      QPSK      Rate 3/4 Convolutional
   24      48      16 QAM    Rate 1/2 Convolutional
   36      35      16 QAM    Rate 3/4 Convolutional
   48      25      64 QAM    Rate 2/3 Convolutional
   54      22      64 QAM    Rate 3/4 Convolutional

   802.11n (OFDM Transmission, 2.4 & 5 GHz)
   Expected in 2009 (see 802.11n).

   ** used as a fallback rate
   Data courtesy of Intersil Corporation.

Speed Is Distance Dependent

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

Wireless LAN

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

Wireless Components

These are 802.11g products from Linksys. The access point is connected to the network, and the wireless adapters are inserted into the client machines. Although most people think laptops when they talk about wireless, adding wireless to a remote desktop computer eliminates the need to run cables. (Images courtesy of Cisco Systems, Inc.)