IEEE 802.11g

- an overview or tutorial about the 802.11g Wi-Fi standard providing 54 Mbps data transfer rates at 2.4 GHz

After the introduction of Wi-Fi with the 802.11a and 802.11b standards, the 802.11b standard became the most popular operating in the 2.4 GHz ISM band. This standard proved to be the most popular despite the faster operating speed of the a variant of the standard because the cost of producing chips to operate at 2.4 GHz were much less than ones to run at 5 GHz.

In order to provide the higher speeds of 802.11a while operating on the 2.4 GHz ISM band, a new standard was introduced. Known as 802.11g, it soon took over from the b standard. Even before the standard was ratified, 802.11g products were available on the market, and before long it became the dominant Wi-Fi technology.

802.11g specifications
The 802.11g standard provided a number of improvements over the 802.11b standard which was its predecessor. The highlights of its performance are given in the table below.

	802.11g
Date of standard approval	June 2003
Maximum data rate (Mbps)	54
Modulation	CCK, DSSS, or OFDM
RF Band (GHz)	2.4
Channel width (MHz)	20

Summary of 802.11g Wi-Fi Specification

802.11g operation
Like 802.11b, its predecessor, 802.11g operates in the 2.4 GHz ISM band. It provides a maximum raw data throughput of 54 Mbps, although this translates to a real maximum throughput of just over 24 Mbps.

Although the system is compatible with 802.11b, the presence of an 802.11b participant in a network significantly reduces the speed of a net. In fact it was compatibility issues that took up much of the working time of the IEEE 802.11g committee.

A variety of modulation schemes can be sued by 802.11g. For speeds of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps Orthogonal frequency Division Multiplexing (OFDM) is used, but for 5.5 and 11 Mbps it uses Complementary Code Keying (CCK), and then for 1 and 2 Mbps it uses DBPSK/DQPSK+DSSS.

The maximum range that can be achieved by 802.11g devices is slightly greater than that of those using 802.11b, but the range at which the full 54 Mbps can be achieved is much shorter than the maximum range of an 802.11 device. Only when signal levels and interference levels are low can the maximum specified performance be achieved.

Note on OFDM: Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each another there is no mutual interference. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period. This means that when the signals are demodulated they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution. The data to be transmitted is split across all the carriers and this means that by using error correction techniques, if some of the carriers are lost due to multi-path effects, then the data can be reconstructed. Additionally having data carried at a low rate across all the carriers means that the effects of reflections and inter-symbol interference can be overcome. It also means that single frequency networks, where all transmitters can transmit on the same channel can be implemented. For more information on OFDM click here