IEEE 802.11g Standard
In early 2000, the IEEE 802.11 Task Group G was given the job of developing a higher-speed, backward-compatible physical-layer extension to the highly successful IEEE 802.11b standard. The new supplement, designated IEEE 802.11g, was to be compatible with the 802.11 media-access control (MAC), implement all mandatory portions of the existing IEEE 802.11b standard to ensure compatibility and interoperability, and include a maximum data rate of at least 20 Mbits/second. The driving forces behind the task group's work were manifold and significant. Besides the much-needed backward compatibility to 802.11b, they ranged from harmonization of the 2.4- and 5-GHz bands, to applications that stand to benefit from the higher rates at 2.4 GHz. These applications include streaming video in the home and high-speed Internet access and file transfers. After almost a year and a half, the 802.11g group reached a compromise among several proposals, which enabled the adoption of the first 802.11g draft standard last November. The task group made additional technical improvements at its January session and completion of the IEEE 802.11g standard is expected late this year or early in 2003. The IEEE 802.11g draft standard takes the IEEE 802.11b standard requirements and adds extra rate capabilities to extend the data rate in the 2.4-GHz band up to 54 Mbits/s. The mandatory modes of the 802.11b standard-namely, 1- and 2-Mbit/s Barker (code), 5.5- and 11-Mbit/s complementary-code keying (CCK), and a long preamble (192 microseconds)-are required within IEEE 802.11g. In addition, IEEE 802.11g mandates the short preamble (96 microseconds) that is an option in IEEE 802.11b, enabling increased throughput, especially for short packets. The optional 5.5- and 11-Mbit/s packet binary convolutional coding (PBCC) of IEEE 802.11b has been extended in IEEE 802.11g to 22 and 33 Mbits/s. The packet format for the short- and long-preamble modes as well as Barker, CCK and PBCC. To achieve data rates up to 54 Mbits/s, the IEEE 802.11g draft standard has borrowed from IEEE 802.11a. That standard uses orthogonal frequency-division multiplexing (OFDM) in the 5-GHz band to achieve data rates of 6, 9, 12, 18, 24, 36, 48 and 54 Mbits/s. IEEE 802.11g uses the same encoding formatting in the 2.4-GHz band to achieve the same rates, mandating the OFDM rates of 6, 12 and 24 Mbits/s. To compensate for the difference between the 16-microseconds interframe spacing of IEEE 802.11 and the 10-microseconds interframe spacing of IEEE 802.11g, a 6-microseconds virtual-signal extension has been added to the OFDM packets. During this 6 microseconds, no signal is transmitted, but the MAC layer behaves as if there were. This, in effect, enables a longer interframe spacing of 16 microseconds, which is required to decode the OFDM packets.