IEEE 802.11 Standards

IEEE 802.11 Standards

IEEE 802.11 is an industry standard for a shared, wireless LAN that defines the physical (PHY) layer and Media Access Control (MAC) sublayer for wireless communications. Figure 1-1 shows the relation of the IEEE 802.11 standard relative to the IEEE 802 specification and the Open Systems Interconnection (OSI) reference model.

Figure 1-1. The IEEE 802.11 standard.

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. Figure 1-2 shows the 802.11, 802.11b, 802.11a, and 802.11g standards that exist at the PHY layer. These standards are described in the sections that follow.

Figure 1-2. The standards for 802.11 at the PHY layer.

More Info
This book does not describe the details of IEEE PHY encoding and transmission techniques or the details of 802.11 frame formats and MAC management. For more information, see 802.11 Wireless Networks, The Definitive Guide by Matthew S. Gast, Sebastopol, CA: O Reilly & Associates, 2002.

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.

NOTE
802.11b uses the same frequency band as that used by microwave ovens, cordless phones, baby monitors, wireless video cameras, and Bluetooth devices.

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.

802.11g

IEEE 802.11g, a relatively new standard at the time of the publication of this book, operates at a bit rate as high as 54 Mbps, but uses the S-Band ISM and OFDM. 802.11g is also backward-compatible with 802.11b and can operate at the 802.11b bit rates and use DSSS. 802.11g wireless network adapters can connect to an 802.11b wireless AP, and 802.11b wireless network adapters can connect to an 802.11g wireless AP. Thus, 802.11g provides a migration path for 802.11b networks to a frequency-compatible standard technology with a higher bit rate. Existing 802.11b wireless network adapters cannot be upgraded to 802.11g by updating the firmware of the adapter they must be replaced. Unlike migrating from 802.11b to 802.11a (in which all the network adapters in both the wireless clients and the wireless APs must be replaced at the same time), migrating from 802.11b to 802.11g can be done incrementally.

Like 802.11a, 802.11g uses 54 Mbps in ideal conditions and the slower speeds of 48 Mbps, 36 Mbps, 24 Mbps, 18 Mbps, 12 Mbps, and 6 Mbps in less-than-ideal conditions.