IEEE 802.11 Standards

IEEE 802.11 is an industry standard for a shared, wireless LAN that defines the PHY and MAC sublayer for wireless communications.

802.11 MAC Sublayer

At the MAC sublayer, all the IEEE 802.11 standards use the carrier sense multiple access with collision avoidance (CSMA/CA) MAC protocol. A wireless station with a frame to transmit first listens on the wireless frequency to determine whether another station is currently transmitting (carrier sense). If the medium is used, the wireless station calculates a random backoff delay. After the random backoff delay, the wireless station again listens for a transmitting station. By instituting a random backoff delay, multiple stations that are waiting to transmit do not end up trying to transmit at the same time (collision avoidance).

The CSMA/CA scheme does not prevent all collisions, and it is difficult for a transmitting node to detect that a collision has occurred. Depending on the placement of the wireless access point (AP) and the wireless clients, distance or radio frequency (RF) barriers can also prevent a wireless client from sensing that another wireless node is transmitting (known as the hidden station problem).

To better detect collisions and solve the hidden station problem, IEEE 802.11 uses acknowledgment (ACK) frames and Request to Send (RTS) and Clear to Send (CTS) messages. ACK frames indicate when a wireless frame is successfully received. When a station wants to transmit a frame, it sends an RTS message that indicates the amount of time it needs to send the frame. The wireless AP sends a CTS message to all stations, granting permission to the requesting station and informing all other stations that they are not allowed to transmit for the time reserved by the RTS message. The exchange of RTS and CTS messages eliminates collisions due to hidden stations.

802.11 PHY Sublayer

At the physical (PHY) layer, IEEE 802.11 defines a series of encoding and transmission schemes for wireless communications, the most prevalent of which are the Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), and Orthogonal Frequency-Division Multiplexing (OFDM) transmission schemes.

IEEE 802.11

The bit rates for the original IEEE 802.11 standard are 2 and 1 Mbps using the FHSS transmission scheme and the S-Band Industrial, Scientific, and Medical (ISM) frequency band, which operates in the frequency range of 2.4 to 2.5 GHz. Under good transmission conditions, 2 Mbps is used; under less-than-ideal conditions, the lower speed of 1 Mbps is used.

802.11b :

The major enhancement to IEEE 802.11 by IEEE 802.11b is the standardization of the physical layer to support higher bit rates. IEEE 802.11b supports two additional speeds, 5.5 Mbps and 11 Mbps, using the S-Band ISM. The DSSS transmission scheme is used in order to provide the higher bit rates. The bit rate of 11 Mbps is achievable in ideal conditions. In less-than-ideal conditions, the slower speeds of 5.5 Mbps, 2 Mbps, and 1 Mbps are used.

802.11a :

IEEE 802.11a (the first standard to be ratified, but just now being widely sold and deployed) operates at a bit rate as high as 54 Mbps and uses the C-Band ISM, which operates in the frequency range of 5.725 to 5.875 GHz. Instead of DSSS, 802.11a uses OFDM, which allows data to be transmitted by subfrequencies in parallel and provides greater resistance to interference and greater throughput. This higher-speed technology enables wireless LAN networking to perform better for video and conferencing applications.

Because they are not on the same frequencies as other S-Band devices (such as cordless phones), OFDM and IEEE 802.11a provide both a higher data rate and a cleaner signal. The bit rate of 54 Mbps is achievable in ideal conditions. In less- than-ideal conditions, the slower speeds of 48 Mbps, 36 Mbps, 24 Mbps, 18 Mbps, 12 Mbps, and 6 Mbps are used.

IEEE 802.11 Operating Modes

IEEE 802.11 defines two operating modes :

• Infrastructure Mode
• Ad hoc mode

Regardless of the operating mode, a Service Set Identifier (SSID), also known as the wireless network name, identifies the wireless network. The SSID is a name configured on the wireless AP (for infrastructure mode) or an initial wireless client (for ad hoc mode) that identifies the wireless network. The SSID is periodically advertised by the wireless AP or the initial wireless client using a special 802.11 MAC management frame known as a beacon frame.

Infrastructure Mode

In infrastructure mode, there is at least one wireless AP(Access Point) and one wireless client. The wireless client uses the wireless AP to access the resources of a traditional wired network. The wired network can be an organization intranet or the Internet, depending on the placement of the wireless AP.

Ad hoc Mode

In ad hoc mode, wireless clients communicate directly with each other without the use of a wireless AP.

Ad hoc mode is also called peer-to-peer mode. Wireless clients in ad hoc mode form an Independent Basic Service Set (IBSS). One of the wireless clients, the first wireless client in the IBSS, takes over some of the responsibilities of the wireless AP. These responsibilities include the periodic beaconing process and the authentication of new members. This wireless client does not act as a bridge to relay information between wireless clients.

Ad hoc mode is used to connect wireless clients together when there is no wireless AP present. The wireless clients must be explicitly configured to use ad hoc mode. There can be a maximum of nine members in an ad hoc 802.11 wireless network.