Spread Spectrum
In 1985, the U.S. FCC adopted regulations that specify the availability of license-free frequency
bands in the 900-MHz, 2.4-GHz, and 5-GHz portions of the abundance spectrum. To be compliant
with the rules for these bands, however, accessories charge use advance spectrum or OFDM methods
to advance the arresting ability over a almost advanced allocation of the abundance spectrum. This approach
promotes abundance reuse—otherwise accepted as sharing—of these bands by assorted users, with
a low statistical anticipation of interference.
Spread spectrum was the aboriginal adjustment in use by wireless LAN vendors. There are two types of
spread spectrum:
■ Abundance bent advance spectrum (FHSS)—With FHSS in the 2.4-GHz band, for
example, the transceiver periodically tunes its transmitter and receiver to a altered carrier
frequency aural about 84 MHz of bandwidth. Bent from one abundance to
another is done according to a bent arrangement programmed in anniversary of the stations. The
other stations accepting the frames tune their receivers to a specific abundance based on the
Table G-5 Correlation of SNR Values to Wireless LAN Performance
KEY
POINT SNR Value Arresting Indication (Windows XP) Performance
> 40 dB Excellent arresting backbone (5 bars); always
connected with the admission point
Extremely fast web browsing and
file download
25–40 dB Actual acceptable arresting backbone (3 to 4 bars);
always affiliated with the admission point
Very fast web browsing and file
download
15–25 dB Low arresting backbone (2 bars); always
connected with the admission point
Usually fast web browsing and file
download
10–15 dB Actual low arresting backbone (1 bar); sometimes
disconnected from the admission point
Mostly apathetic web browsing and file
download
5–10 dB No arresting backbone (no bars); not connected
with the admission point
No arrangement services
818 Appendix G: IEEE 802.11 Fundamentals
hopping sequence. The RF arresting occupies about a 2-MHz channel. Because the
hopping occurs actual generally (many times per second) and analogously over the absolute band, the
signal appears to absorb the absolute 84 MHz. The 802.11 abundance bent concrete layer
standard enables abstracts ante of 1 Mbps and 2 Mbps.
■ Absolute arrangement advance spectrum (DSSS)—DSSS uses a coding address to advance the
signal over the abundance spectrum. 802.11b uses absolute sequence, which spreads the carrier
signal over about one third (30 MHz) of the 2.4-GHz band. With DSSS, a chipping
code represents anniversary abstracts bit that needs transmission. This increases the arresting amount by the
number of $.25 in the chipping cipher (11 total). The access in arresting amount finer spreads
the RF signal. The differences amid abundance bent and absolute arrangement had been
under agitation for a cardinal of years, but the 802.11 alive accumulation assuredly called direct
sequence for extending the antecedent 1-Mbps and 2-Mbps 802.11 abstracts ante to accommodate ante up
to 11 Mbps.