Eng. Saleh Al-Sarraf
April, 2012
Comparing the 802.11 Technologies
IEEE 802.11 standard is known commonly as Wi-Fi or WLAN.
This standard is consists of family of different IEEE wireless standards:
802.11, 802.11a, 802.11b, and 802.11g. Actually, there are many other
types is this wireless family also which are used for special purposes. We
will try to identify the basic characteristics, the modulation techniques
used, and the advantages and disadvantages of 802.11a, 802.11b, and
802.11g.
There are many ways to compare the various 802.11 standards. A few of
them are listed here. Let’s explore each of these in some detail. IEEE802.11 Wireless LANS use two different frequency ranges as shown here.
IEEE-802.11B and G use the 2.4-GHz range. Thus, B and G fall in the
upper part of the Ultra-High Frequency (UHF) portion of the
electromagnetic spectrum. IEEE-802.11A uses the 5-GHz range. This
falls near the low end of the Super High Frequency (SHF) band of the
spectrum.
5GHz IEEE-802.11A
2.4GHz IEEE-802.11B & G
UHF
VLF
LF
MF
HF
VHF
UHF
3-30 30-300 300-3000 3-30 30-300 300-3000 300-3000
MHz
kHz
kHz
kHz MHz MHz
MHz
More specifically each service occupies a relatively small and
unlicensed band of frequencies. IEEE-802.11B and G occupy a band
called the Industrial, Scientific, Medical (ISM) band of the UHF
spectrum. This is a catch-all band for various unlicensed UHF services
including cordless phones and microwave ovens. The 802.11A channels
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Eng. Saleh Al-Sarraf
April, 2012
occupy a band called the Unlicensed National Information Infrastructure
band of the SHF spectrum. U-NII is a catch all band for unlicensed SHF
services. Fortunately it is not nearly as widely used as the ISM band. For
some
long
forgotten
reason,
Unlicensed
National
Information
Infrastructure is usually abbreviated with a hyphen between the U and
the N, as in U-NII.
ISM IEEE-802.11B & G
UHF
300-3000
MHz
SHF
3-30
GHz
U-NII IEEE-802.11A
The wireless networking services are most often compared by their
speed. The speeds of various 802.11 standards are confusing for several
reasons. First, they are generally expressed in “raw-data” speeds. This is
the top speed that the data can reach at the physical layer of the OSI
model. Unfortunately, the actual data rate that can be extracted at the
network layer of the OSI model is about half the maximum. Even so, the
absolute best case speed is usually given. Second, some speeds are
mandatory, others are optional. Third, the top speed is under ideal
conditions. In practice, when errors occur, the wireless devices revert to a
slower, more reliable speed.
Here is a brief rundown. The original 802.11 Standard specified
speeds of 1 and 2 Mbps. It is highly unlikely that you will ever see a
piece of equipment that was built to this original standard. However, B
and G equipment still have these speeds both for backward compatibility
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Eng. Saleh Al-Sarraf
April, 2012
purposes and because they work well at the extreme limits of the range.
The 802.11B standard added the 5.5 and 11 Mbps rates.
The 802.11G standard duplicates all the “B” rates in order to
maintain backward compatibility. G’s top speed is normally listed as 54
Mbps. However, this is an optional speed not required by the IEEE
standard. However, it is required by the Wi-Fi Standard and virtually all
G equipment has the 54 Mbps rate.
The 802.11A standard requires speeds of 6, 12, and 24 Mbps, with
optional speeds up to 54 Mbps. Here again, almost all “A” equipment has
the 54 Mbps speed. The speeds to remember are these: Raw data speeds
of 11 Mbps for 802.11B and 54 Mbps for “G” and “A”. Actually speeds
are about half the raw data speed. Experts will argue the exact value of
the actual speed, but these are good round numbers that are easy to
remember and are not too controversial. In practice, many experts claim
that on average, the actual speed of G is slightly less than that of A. Part
of the reason for this is that G pays a price to maintain backward
compatibility with B. The A standard does not have this burden.
Raw
Actual
802.11B
11 Mbps
About 6 Mbps
802.11G
54 Mbps
About 24 Mbps
802.11A
54 Mbps
About 28 Mbps
All of the 802.11 standards use some method of “spreading” the data
across the relatively wide channel. The 802.11 standards specify three
different methods for “spreading” the data. FHSS is no longer used by
802.11 and never was to any great extent. However, it is used with other
wireless technologies. IEEE-802.11B uses DSSS only, while 802.11A
uses OFDM only. IEEE-802.11G uses both DSSS and OFDM. Let’s
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Eng. Saleh Al-Sarraf
April, 2012
quickly review how these three techniques differ. Recall that FHSS
spreads the data across the frequency spectrum of the channel by
transmitting briefly on one narrow frequency and then hopping to
another. To illustrate the point, here you see four different jumps up and
down the frequency range. At time T0, the equipment transmits on
frequency 1. An instant later at time T1, it hops up the frequency range to
frequency 5. At time T3, it jumps back down range to frequency 2, then
back up range again to frequency 4, and finally back to frequency 3. For
simplicity, only 5 frequencies are shown. In reality many additional
frequencies are used.
3
4
2
5
1
T
4 T
3
T
2 T T
1
0
Time
Frequency
Let’s compare this to the DSSS technique. Here you see a three
dimensional representation of a single 802.11B channel using DSSS.
Recall that with DSSS, there is a single carrier but it is so wide that it
covers the entire channel plus adjacent channels. Power is shown as the
vertical axis. Much of the power is concentrated in the center. Power
drops off as we approach the edges of the band. But there are unwanted
sidebands on each side of the main carrier. A rough analogy is an
Interstate highway but with a single very wide lane.
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Eng. Saleh Al-Sarraf
April, 2012
By contrast, OFDM transmits multiple carriers in parallel. Here we see
the same bandwidth shown earlier with DSSS and FHSS. But the
available bandwidth is used differently. Instead of a single very wide
carrier with its power concentrated in the middle of the band, OFDM uses
multiple narrow carriers each with its own peak of power. Going back to
our highway analogy, whereas DSSS uses the entire width of the highway
as a single lane, OFDM divides the same width into multiple lanes. For
clarity, only four lanes are shown here. In reality, 802.11A and 802.11G
divide each channel into 52 different lanes. However, only 48 of these
lanes carry data.
Power
Time
Frequency
Here is a brief review of standard versus spreading technique. It is
interesting to note that the original (and long obsolete) 802.11 standard
allowed either Direct Sequence Spread Spectrum (DSSS) or Frequency
Hopping Spread Spectrum (FHSS). Today, FHSS is all but forgotten by
the newer 802.11 standards, although it is used by Bluetooth and some
other wireless technologies. The 802.11B equipment available today uses
only DSSS. By contrast, 802.11A uses only Orthogonal Frequency
Division Multiplexing (OFDM). The 802.11G standard dictates DSSS for
those speeds that are “B” compatible. Obviously, it has to, otherwise it
wouldn’t be compatible. But the “G” standard also specifies OFDM for
its additional and generally higher speeds.
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