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. 

Spread Spectrum

No License Necessary "BUT"

    The 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.

    SPREAD 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 interfering signal.

    Spread-spectrum 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.

    Directional 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.

    Unfortunately, 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 cheaper.

    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.  

    TOWER 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 so.

    Compounding 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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