Though 5GHz has many advantages, it also has problems. The most important of these is compatibility:

The different frequencies mean that 802.11a products aren't interoperable with the 802.11b base. To get around this, the IEEE developed 802.11g, which should extend the speed and range of 802.11b so that it's fully compatible with the older systems.

The standard operates entirely in the 2.4GHz frequency, but uses a minimum of two modes (both mandatory) with two optional modes. The mandatory modulation/access modes are the same CCK (Complementary Code Keying) mode used by 802.11b (hence the compatibility) and the OFDM (Orthogonal Frequency Division Multiplexing) mode used by 802.11a (but in this case in the 2.4GHz frequency band). The mandatory CCK mode supports 11Mbps and the OFDM mode has a maximum of 54Mbps. There are also two modes that use different methods to attain a 22Mbps data rate--PBCC-22 (Packet Binary Convolutional Coding, rated for 6 to 54Mbps) and CCK-OFDM mode (with a rated max of 33Mbps).

The obvious advantage of 802.11g is that it maintains compatibility with 802.11b (and 802.11b's worldwide acceptance) and also offers faster data rates comparable with 802.11a. The number of channels available, however, is not increased, since channels are a function of bandwidth, not radio signal modulation - and on that score, 802.11a wins with its eight channels, compared to the three channels available with either 802.11b or 802.11g. Another disadvantage of 802.11g is that it also works in the 2.4 GHz band and so due to interference it will never be as fast as 802.11a.