802.11b&g - 2.4 GHz Frequency Band

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802.11b

IEEE 802.11b standard uses the 2.4 GHz frequency band and was released in parallel with 802.11a.

• standardized the use of:
  • Direct Sequence Spread Spectrum (DSSS)
    • a carrier method
    • some ways inferior to OFDM (802.11a)
    • support a nominal data rate of 11 Mbps
  • Complementary Code Keying (CCK) signal encoding
• 2.4 GHz band is subdivided into up to 14 channels
  • spaced at 5 MHz intervals from 2,412 MHz up to 2,484 MHz
  • Because the spacing is only 5 MHz and DSSS needs 20 MHz channel bandwidth, 802.11b channels overlap quite considerably
  • means that co-channel interference is a real possibility
    • unless widely spaced channels are chosen (1, 6, and 11, for instance)
  • in the Americas, regulations permit the use of channels 1–11 only
  • in Europe, channels 1–13 are permitted
  • in Japan, all 14 channels are permitted

IEEE 802.11g

IEEE 802.11g standard offered a relatively straightforward upgrade path from 802.11b.

• uses the same encoding mechanism and 54 Mbps rate as 802.11a (OFDM)
  • but in the 2.4 GHz band used by 802.11b and with the same channel layout
  • made it straightforward for vendors to design 802.11g devices that could offer backwards support for legacy 802.11b clients
• when in 802.11b compatibility mode,
  • drops back to used DSSS

Info

802.11b requires 22 MHz channels, but 802.11g only requires 20 MHz.

• The channel layout accommodates both standards
• If 802.11b compatibility is disabled, channels 1, 5, 9, and 13 are non-overlapping.