4 The IEEE 802.11 standard
4.3 802.11 PHY Sublayer

At the physical (PHY) sublayer, IEEE 802.11 defines a series of encoding and transmission schemes for wireless communications, the most common of which are the Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), and Orthogonal Frequency Division Multiplexing (OFDM) transmission schemes. Figure 1.13 shows the 802.11, 802.11b, 802.11a, 802.11g, 802.11n and 802.11ac standards that exist at the PHY sublayer. These standards are described in the following sections.

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Figure 1.13 IEEE 802.11standards at the PHY layer

IEEE 802.11

The bit rate for the original IEEE 802.11 standard is 2 Mbps using the FHSS transmission scheme and the ISM frequency band, which operates in the frequency range of 2.4 to 2.5 GHz. However, under less than ideal conditions, a lower bit rate 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 2.4 GHz frequency band. 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.

It is important to note that 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

The IEEE 802.11a operates at a bit rate as high as 54 Mbps and uses the 5 GHz frequency band. 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 devices (such as cordless phones that work at the 2.4 GHz frequency band), 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 operates at a bit rate as high as 54 Mbps, but uses the 2.4 GHz frequency band 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.

802.11n

The IEEE 802.11n standard aims to improve distance (up to 250 m) and network throughput over the two previous standards, 802.11a and 802.11g, with significant increase in the maximum raw data rate from 54 Mbps to 600 Mbps under ideal conditions by adding the multiple-input multiple output technology and channels of 40 MHz, of greater bandwidth. This technology, called MIMO, uses multiple wireless signals and antennas, at the transmitter and receiver. It can be used in the 2.4 GHz or 5 GHz frequency bands.

802.11ac

The 802.11ac standard, an upgrade from 802.11n, provides similar range but increases throughput. It runs on the 5 GHz band and incorporates beam-forming, wide bands and multiple antennas to deliver theoretical data speeds up to 1.3 Gbps, more than double peak rates of 600 Mbps with 802.11n.