Wireless Transmission Media

Wireless Transmission Media
Wireless LANs employ radio frequency (RF) and infrared (IR) electromagnetic
airwaves4 to transfer data from point to point. The Federal Communications
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AM Broadcast
900 Mz
Band
2.4 Mz
Band
5 Mz
Simple Wireless LAN Band
configuration.
Industrial, Scientific and
Medical (ISM) Bands
Cellular
(840 MHz)
Viable
Light
X-Rays
7-1 WLAN overview. (courtesy of Wireless LAN Alliance)
Commission (FCC) and a general world agreement set aside the radio frequencies
that are available for unlicensed commercial use. These Industrial Scientific and
Medical (ISM) bands include the 900-MHz, 2.4-GHz, and 5-GHz bands that are used
by many commercial wireless communication devices. The majority of emerging
WLAN devices are designed to operate in the 2.4-GHz band due to global availability
and reduced interference.5
Several transmission mediums are capable of transferring data across airwaves.
Like most technologies, they each have their own benefits and limitations. Infrared
systems and narrowband radio systems are the leading technologies being used by
the wireless industry.
Infrared Systems
While capable, infrared (IR) systems do not make for a practical enterprise WLAN
solution and therefore are not widely employed, IR is able to transfer data by taking
advantage of those frequencies located in close proximity to, but beneath visible
light on the electromagnetic spectrum. These high bands face the same limitations
as visible light in that they cannot penetrate nontransparent objects such as walls,
floors, and ceilings. As a result, WLANs transmitting via IR are restricted to operating,
at best, within the same room, and could be further limited to a short-range lineof-
sight restriction.6
Narrowband Radio Systems
Narrowband radio systems transmit and receive data on a specific radio frequency.
Different users communicate on alternative frequencies or channels to ensure some
level of privacy and avoid interference. Radio receivers are constructed to listen only
for their designated frequency and to filter out all others. The natural limitation to
this system should be clear: If another transceiver is operating at the same frequency
and within range, interference will occur and data will no doubt be lost or corrupted.
Another downside of implementing narrowband technology is that, at least in the
United States, a license must be obtained from the FCC for each site where it is to be
implemented.
Wideband Radio Systems: Spread Spectrum
Instead of using a single frequency, the Spread-Spectrum technology, as its name
suggests, traverses the frequency band to reliably transmit data. Originally employed
by the military, Spread Spectrum distributes the signal over a wide range of frequencies
uniformly, thus consuming more bandwidth in exchange for reliability, integrity,
and security of communications. This so-called wideband usage lets devices avoid
interference and other signal noise in a way not possible with narrowband transmissions.
The benefits come with a price. By their nature, wideband communications are
noisier and therefore easier to detect; luckily, to an improperly tuned receiver a
Spread-Spectrum signal appears as nothing more than background noise.7
Spread Spectrum comes in two forms: Frequency-Hopping Spread Spectrum
(FHSS) and Direct-Sequence Spread Spectrum (DSSS). Of the two, frequencyhopping
is less costly to deploy; however, direct-sequence has the potential for more
The Wireless Local Area Network (WLAN) 331
widespread use. This can be attributed to the higher data rates, greater range, and
built-in error correction capabilities of DSSS.
Frequency-Hopping Spread Spectrum (FHSS)
FHSS successfully mitigates the effects of interference by attaching the data signal
to a shifting carrier signal. This modulated carrier signal literally hops, as a function
of time, from one frequency to the next across the band. Each transceiver is programmed
with a hopping code that defines the order and range of frequencies used.
To properly communicate, each device must be configured with the same hopping
code to ensure that signals are sent and received at the correct time and on the
proper frequency.8 As a result, synchronized transceivers effectively create a logical
communications channel with data rates reaching 2 to 3 Mbps and a range of 1,000
feet without installing repeaters.9
For interference to occur, the conflicting narrowband signal would need to be
broadcast at the same frequency and at the same time as the hopping signal. Should
errors in transmission occur on one frequency, the signal will be re-broadcast on a
different frequency at the next hop, as shown in Figure 7-2. To receivers that are not
programmed with the appropriate hopping code, FHSS transmissions appear to be
short duration impulse noise.10 Distinct hopping codes can be implemented on the
same WLAN to prevent sub-WLANs from interfering with one another. FHSS-based
WLANs are best for supporting a high number of clients when ease-of-installation is
key and either outdoors or in relatively open indoor facilities.11
Direct-Sequence Spread Spectrum (DSSS)
DSSS infuses a redundant bit pattern into each bit being transferred. The inserted
bits are referred to as a chip or a chipping code.12 By including the chip, a receiver is
able to perform data recovery routines on signals based on statistical analysis. A
greater number of bits in the chipping code will result in a signal that is less likely to
be negatively affected by interference. As it is increasing the signal size, DSSS
requires more bandwidth to operate, generally using three non-overlapping frequen-
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2.44
2.40 GHz 2.42 GHz
200 ms 200 ms
The cow jumped over the moon the moon
200 ms
2.44 GHz 2.46 GHz
7-2 Frequency Hopping Spread Spectrum (FHSS). (Courtesy of Anyware Network
Solutions)
cies to communicate. The error-correcting capability prevents DSSS from needing to
retransmit data that may have been corrupted while en route, as shown in Figure 7-3.
Recall that FHSS systems countered interference by trying to avoid signal collisions
through constant motion, essentially attempting to out-pace conflicts. While
this is a successful method, it limits data throughput to relatively small packets
because the modulation technique has adverse affects on larger data rates. To compensate,
DSSS systems include error-correcting bits, thus removing the need to hop
frequencies and to retransmit in the event of an error. As a result data rates up to 11
Mbps and ranges up to several miles can be achieved with DSSS.13