Overview of the 802.11g Specification
Published: October 13, 2003
Immediately after the Institute of Electrical and Electronics Engineers (IEEE) approved the 802.11b standard in July 1999, they started work on the faster 802.11g standard. The final specification for 802.11g includes both mandatory and optional features. Although the Wi-Fi Alliance requires 54 Mbps for certification, the official 802.11g standard includes a requirement for a data rate of only up to 24 Mbps. The additional data rates of 36, 48, and 54 Mbps are an optional component of the IEEE-approved standard. For competitive reasons, it's unlikely that any vendor would produce gear capable of only the minimum 24 Mbps.
A key difference between 802.11b and 802.11g wireless technologies is the modulation type. Complementary Code Keying (CCK) is used for 802.11b. Orthogonal Frequency Division Multiplexing (OFDM) is used for the higher data rates of 802.11g and CCK is used for the lower 802.11g data rates. Optional support for another modulation called Packet Binary Convolutional Code (PBCC) is also included in the 802.11g standard (22 Mbps to 33 Mbps). Texas Instruments included this modulation in 802.11b+ products that are available from some vendors (D-Link, USR for example). PBCC was unsuccessfully proposed as the official standard for 802.11g.
CCK modulation was included along with ODFM as a requirement in 802.11g to insure backward compatibility and co-existence with 802.11b. Although ODFM modulation is also used in 802.11a, 802.11g is not compatible with it because it operates in a different frequency band.
The table below illustrates the feature sets of each of the three versions of the 802.11 standard. 802.11g has to balance two separate modulation types to provide backwards compatibility with 802.11b.
802.11 Technologies Comparison
Max Mbps data rate | 11 | 54 | 54 |
Modulation Type | CCK | ODFM | CCK and ODFM |
Supported Data Rates | 1, 2, 5.5, 11 Mbps | 6, 9, 12, 18, 24, 36, 48, 54 Mbps | OFDM: 6, 9, 12, 18, 24, 36, 48, 54 Mbps CCK: 1, 2, 5.5, 11 |
Frequencies | 2.4–2.497 GHz | 5.15–5.35 GHz 5.425–5.675 GHz 5.725–5.875 GHz | 2.4–2.497 GHz |
Backward Compatibility for 802.11b Devices
The balancing of CCK and ODFM modulation requires a safety mechanism to control the traffic. The earlier 802.11b standard uses a Request to Send/Clear to Send (RTS/CTS) mechanism to determine if clear transmission is possible. The earlier 802.11b standard isn't ODFM-aware and therefore can only see other 802.11b transmissions. So the official 802.11g standard requires a protection mechanism for mixed b/g operation.
The Wi-Fi Alliance tests for one of two signaling methods as part of certification, RTC/CTS and CTS-to-Self. CTS-to-Self protection sends a CTS message using an 802.11b rate to clear the air, followed by sending data using an 802.11g data rate. These protection mechanisms exact a performance penalty. This means that an 802.11g-capable device must hold back 802.11g ODFM traffic to allow 802.11b clients to transmit.
How Fast Is 802.11g?
The actual maximum throughput in a mixed b/g environment of an 802.11g client is limited to between 14 and 15 Mbps. This is still approximately three times faster than 802.11b. The specification also allows for a G Performance mode (G only) where throughout can reach 20-24 Mbps. An additional B only mode has been added for use in all 802.11b environments.
Several vendors are extending the throughput even further with packet bursting, a technology that enhances speed, but is not part of the official approved 802.11g standard. In mixed 802.11b/g networks, vendors are claiming that this technology improves the throughout of 802.11g products by about 30% and in 802.11g-only networks up to 50%. It is unlikely that vendors will implement this speed boosting technology in the same way. As a result, don't count on full interoperability when packet bursting is used.
