Applications and the Future - Codes: The Guide to Secrecy from Ancient to Modern Times - page 340

example, 802.11a is a standard for WLANs operatingin the 5 GHz 9.9 radio fre-
quency range and having a data rate of 9-54 Mbps (see Footnote 3.11 on page
160). This is a successor to the widely accepted standard, 802.11b 9.10 , which
operates in the 2.4-GHz range. The modulation 9.11 scheme chosen for 802.11b is
called Complementary Code Keying (CCK) allowingfor higher data speeds and
less transmission errors. CCK is comprised of 64 eight-bit code words employed
to encode data for 5.5 and 11 Mbps in the 2.4-GHz band. Unique properties of
the code words permit them to be differentiated from one another by a receiver,
even if there is a lot of channel noise. CCK only functions with Direct-Sequence
Spread Spectrum (DSSS), which is a transmission technology wherein a signal
to be sent is combined with a higher data rate bit sequence to make it more ro-
bust. This sequence is a redundant bit pattern for each bit sent, thus alteration
duringtransmission is minimized; and if there is such an alteration, the original
data may be recovered via the redundancy.
Sometimes the task group and standard are referenced interchangeably in
the literature. We will occasionally separate the two. For instance, 802.11i is a
proposed standard (see page 345), which we discuss at length, so for convenience
and to avoid confusion, we use the well-known notation TGi to denote the task
group for the standard implicit in 802.11i; and similarly TGn for 802.11n.
WLAN Standards : In June of 2003, 802.11gwas iven oLcial status
as the third standard for WLAN encoding. This standard saw wider adoption
than that of 802.11a due to the limited range of 802.11a, and the full backward
compatibility of 802.11gwith 802.11b, the latter of which had already seen
a very wide adoption rate, whereas 802.11a is not backward compatible with
802.11b. Furthermore, 802.11gemploys data rates of 54 Mbps. 802.11a/b/gare
more than adequate for wireless Internet access and for the sharingof small files.
However, when bigfiles come into play, they are slower than most methods. For
these larger files and other applications requiring higher bandwidth, we need
a new standard. The followingnot-yet-developed standard is the follow-up to
802.11g.
TGn was created in September of 2003 with a goal of substantially increasing
9.9 A GHz is a unit of frequency equal to 10 9 Hertz. A Hertz is the international unit for
measuring frequency, equivalent to the older unit of cycles per second .
9.10 The first widespread commercial use of 802.11b was made by Apple Computer under the
name Airport . TM
9.11 Modulation is the varying of some characteristics (for instance, amplitude, frequency,
or phase) of an electrical carrier wave in order to embed information in it. For instance,
amplitude modulation (AM) is that in which the amplitude (magnitude) of a carrier signal
is varied to encode it with information; whereas frequency modulation (FM) is a method of
embedding data onto an alternating-current (AC) wave by varying the instantaneous frequency
of the wave, but the amplitude remains the same. An illustration from the “Old West” is a
stream of smoke (as carrier) from a fire being modulated by waving a blanket to send a “smoke
signal”. A modulator is a device that superimposes data on a wave. It uses an oscillator to
create a wave, which it then combines with the data to create the carrier wave.
After the advent of the first wireless transmitters were put into use in the early twenti-
eth century using radiotelegraphy (Morse code), it was modulation that made it possible to
broadcast music and voices, which came to be known as radio. With the modern use of data
communications, cell phones and the like, the term “wireless” has come back into vogue.