DSSS and FHSS - Spread Spectrum modem
FHSS and DSSS are resistant to interference from conventional radio transmitters. The FHSS signal doesn't stay in one place on the band, FHSS can elude a jammer – (a transmitter designed to block radio transmissions on a given frequency). DSSS avoids interference by configuring the spreading function in the receiver to concentrate the desired signal but spread out and dilutes any interfering signal.
DSSS and FHSS
No License Necessary "BUT"
usable portion of the radio spectrum is huge ranging from approximately 6 KHz to
300 GHz. Because radio is so well suited to information transmission, however,
almost all of the spectrum is already reserved for specific uses. Alas, because
bandwidth is so scarce and valuable, the frequencies allocated for unlicensed
networking are what one might call junk frequencies ones commercial users are
unlikely to want. The 2.4- GHz band, for example, is subject to interference
from microwave ovens, and the signals have difficulty penetrating trees, heavy
snow, or anything at all that contains water. (the water absorbs a portion of
the signal and is heated, just as in a microwave oven.) The 900-mHz band is
often plagued by interference from medical and scientific equipment, cordless
phones, wireless stereo speakers, and similar devices.
Unlicensed users of the band are considered to be secondary users. They
take a back seat to licensed, or primary, users, who can transmit stronger
signals and are subject to fewer restrictions. A high-power primary user such as
a TV station or a vehicle location system can render an unlicensed frequency
band useless to anyone else in the vicinity, including wireless LANS. Unlicensed
users have no recourse - even if they've already spent tens of thousands of
dollars on wireless networking equipment. The
requirements imposed by the regulations on unlicensed wireless networking
equipment are relatively simple. First, the strength of the signal is
limited-usually to less than I watt. Second, the signal must be transmitted
using one of two spread-spectrum methods. The signal must either be spread out
over a certain range of frequencies or hop among a certain minimum number of
narrow slots each second.
SPECTRUM The idea of spread-spectrum radio transmission was proposed by the
military who was seeking ways to prevent radio signals from being monitored or
blocked by hostile parties. The two inventors came up with the notion of
changing the frequency of a transmission at regular intervals faster than the
enemy could retune. A special
receiver that knew the frequency-hopping pattern could follow it and pick up the
entire transmission. The hopping patterns were controlled by the punched holes
in piano rolls became known as frequency-hopping spread spectrum (FHSS).
Later, as digital logic became popular, an- other kind of spread spectrum was
developed direct-sequence spread spectrum (DSSS). In this method of
transmission, the signal does not hop from one frequency to another but is
passed through a spreading function and distributed over the entire band at
once. DSSS usually provides slightly higher data rates and shorter delays than
FHSS, because the transmitter and receiver don't have to spend time retuning.
Both FHSS and DSSS are resistant to interference from conventional radio
transmitters. Because the signal doesn't stay in one place on the band, FHSS can
elude a jammer – (a transmitter designed to block radio transmissions on a
given frequency). DSSS avoids interference by configuring the spreading function
in the receiver to concentrate the desired signal but spread out and dilutes any
radio is good at dodging interference from conventional sources – (signals
that stay in one narrow area of the frequency band and don’t move. it
doesn’t always do as well when
there are other spread ,spectrum systems operating nearby, though. The more
frequency-hopping transmitters operating on a band, the more likely it is that
one or more of them will hop to the same frequency at the same time, garbling
data that must be retransmitted. DSSS is better at resisting noise up to a
certain point. However, if the combined interference throughout the band rises
above a certain level, throughput dramatically drops-nearly to zero.
Unfortunately, it only takes a few nearby FHSS systems to cripple a DSSS system.
On the other hand, because a DSSS system is always transmitting on every
frequency in the band, a nearby FHSS system may be unable to find any clear
channel to hop to. In the presence of interference, FHSS usually degrades more
gracefully than DSSS, but neither works well when competing at close range.
antennas can sometimes help a node focus on the system with which it must
communicate and ignore interference from others. However, as a general rule,
when two transmitters compete for the same bandwidth, the one that expends more
energy per bit of data wins the battle.
because the total energy each device can emit is limited by the FCC, the
transmitter trying to send data at the highest speed. say - 10 Mbps instead of 1
Mbps - loses. The newest equipment can be hobbled by gear that's older and
Another problem that can plague wireless networks is called the hidden
transmitter problem. Suppose a wireless network is implemented as a sort
of Ethernet-in-the-air-any node can speak as long as it doesn’t hear anyone
else transmitting. This works well if all the nodes can hear one another.
However, in real life, physical obstacles sometimes prevent some nodes in a
network from being able to tell when another is transmitting.
OF BABEL If all the transmitters on a given band would follow the same protocol,
the number of collisions would be small. Unfortunately, there are many wireless
networking - each of these schemes uses a unique protocol that the others do not
under- stand. So although devices of the same type will cooperate, devices of
different types won’t know how to keep out of one another's way. (Many older
wireless devices, including proprietary wireless LANs and cordless phones,
observe no etiquette at all and so will never avoid collisions.) Because the FCC
regulations do not require users of a given band to observe a common etiquette,
many devices do not cooperate even when it might be in their best interest to do
this problem is another loop-hole in the FCC regulations. The regulations limit
the strength of the signal that each transmitter can emit but don’t limit the
total number of transmitters any one user can operate, nor how close together
those transmitters can be. For example, the Metricom Ricochet network, which
operates on the 900-MHz unlicensed band, uses hundreds of transmitters attached
to utility poles to pro- vide wireless Internet service to an entire
neighborhood or city. To penetrate building walls and overcome interference,
each transmitter can increase its power, if necessary, to the FCC-allowed
maximum. The result: If you try to operate wire- less networking equipment in
the same unlicensed public band, you may find your equipment swarmed by dozens
of transmitters, each cranking up the power to drown you out.
Many wireless LANs allow nodes to roam as much as 1,000 feet from the base station. Unfortunately, the farther you stray from the hub, the more likely you are to find yourself closer to a source of interference than to your own base station. This phenomenon sometimes causes perplexing interference problems in city office buildings. Two tenants who each install wireless LANs on their floors can make life insurable for one another. Likewise, a 900-mHz or 2.4-GHz cordless phone in a nearby office can sometimes render a wireless LAN inoperable.
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