Sprechen Sie CCK?
Although it would be great to say that draft-802.11g products really do provide the 5X throughput that vendors love to feature so prominently in their advertising and product packaging, it turns out that reality isn't that simple. The reasons involve things such as the incomplete nature of draft specifications (and the loopholes they provide), competitive pressures that cause products to ship before they should, and just plain errors and misunderstandings in design and implementation.
Whatever the reason, the fact is that the throughput you see from draft-11g products may vary significantly from product to product from now and probably right up through the release of the spec. The heart of the throughput issue lies in the "protection" mechanism defined in the draft-802.11g spec. To understand what's going on, I'll need to peel the onion a layer or two, so bear with me.
Although 802.11b and g equipment operate in the same 2.4GHz frequency band, they primarily use different modulation schemes. 802.11b uses a method called Complementary Code Keying (CCK), and 802.11g uses Orthogonal Frequency Division Multiplexing (OFDM). In simple terms, its like two people being able to hear each other, but unable to communicate because they speak two different languages. In their great wisdom, the IEEE Task Group G quickly realized the basic problem, and mandated two fixes to bridge the gap.
First, they said that 802.11g products must support both the OFDM and CCK modulation methods, giving the newer standard the ability to speak the old one's language. (A third method, PBCC, was also included, but as an optional method.) But just as a bi-lingual person must receive a clue to switch to the language of the person who is trying to speak to them, a similar mechanism was needed so that 802.11g stations would be able to understand that an 802.11b station was trying to communicate, and clue the 11g station to switch to CCK to have the conversation. This second compatibility feature is known as "protection".