Wireless LANs a) [4 marks] What is the difference between an infrastructure basi

Question

Wireless LANs a) [4 marks] What is the difference between an infrastructure basic service set and an independent basic service set? Which of these uses an access point? What would you use an independent service set for? b) [2 marks] What is a distribution system with regards to an extended service set? c) [1 marks] What is the purpose of various spread-spectrum techniques? d) [4 marks] Describe frequency hoping spread spectrum. e) [4 marks] How is coordination achieved using the distributed coordination function? How is coordination achieved using the point coordination function?

Explanation / Answer

a)Answer:

Diffrences:

Independent Basic Service Set:

• Independent BSS are an ad-hoc network that contain no Access Points. Since they do not use Access Points they can not connect to any other basic service set

Infrastructure Basic Service Set:

• An Infrastructure BSS can communicate with other stations not in the same basic service set by communicating to each other through Access Points

c)Answer:

Spread spectrum is a form of wireless communications in which the frequency of the transmitted signal is deliberately varied. This results in a much greater bandwidth than the signal would have if its frequency were not varied.

A conventional wireless signal has a frequency, usually specified in megahertz (MHz) or gigahertz gigahertz), that does not change with time (except for small, rapid fluctuations that occur as a result of modulation). When you listen to a signal at 103.1 MHz on an FM stereo receiver, for example, the signal stays at 103.1 MHz. It does not go up to 105.1 MHz or down to 99.1 MHz. The digits on the radio's frequency dial stay the same at all times. The frequency of a conventional wireless signal is kept as constant as the state of the art will permit, so the bandwidth can be kept within certain limits, and so the signal can be easily located by someone who wants to retrieve the information.

There are at least two problems with conventional wireless communications that can occur under certain circumstances. First, a signal whose frequency is constant is subject to catastrophic interference. This occurs when another signal is transmitted on, or very near, the frequency of the desired signal. Catastrophic interference can be accidental (as in amateur-radio communications) or it can be deliberate (as in wartime). Second, a constant-frequency signal is easy to intercept, and is therefore not well suited to applications in which information must be kept confidential between the source (transmitting party) and destination (receiving party).

To minimize troubles that can arise from the above mentioned vulnerabilities of conventional communications circuits, the frequency of the transmitted signal can be deliberately varied over a comparatively large segment of the electromagnetic radiation spectrum. This variation is done according to a specific, but complicated mathematical function. In order to intercept the signal, a receiver must be tuned to frequencies that vary precisely according to this function. The receiver must "know" the frequency-versus-time function employed by the transmitter, and must also "know" the starting-time point at which the function begins. If someone wants to jam a spread-spectrum signal, that person must have a transmitter that "knows" the function and its starting-time point. The spread-spectrum function must be kept out of the hands of unauthorized people or entities

d)Answer:

FREQUENCY-HOPPING SPREAD SPECTRUM

In a Frequency Hopping spread spectrum system, a transmitter "hops" between the available frequencies according to a predefined algorithm or program, which can be either randomly created or planned before being executed. The transmitter and the receiver, both are synchronized to the samecentre frequency. A small portion of data is transmitted on a narrowband at a frequency. Then, the transmitter tunes to another frequency and transmits again. Hence the transmitter is capable of hopping its frequency over a given bandwidth many times a second, transmitting on one particular frequency for a certain period of time, then hopping to another frequency and transmitting again. .

The Frequency Hopping Spread Spectrum transmitter is a pseudo-noise PN code controlled frequency synthesizer. The transmitted data from the transmitter hops from one frequency value to another depending on the pseudo-random input from the code generator. Varying this instantaneous frequency gives an output spectrum that is spread over the generated range of frequencies. An important factor in Frequency Hopping Spread Spectrum is the rate at which these hops occur. The minimum time required to change from one frequency to another depends on the information bit rate

A Frequency Hopping Spread Spectrum communication is as follows:
1. The transmitting party sends a request for the algorithm through a predefined frequency channel.

2. The receiving party sends a number, known as a seed.

3. The transmitting party uses seed as a variable in a predefined algorithm. It calculates the sequence of frequencies that will be used.

4. The transmitting party sends a synchronization signal through the first frequency in the calculated sequence and then on the basis of the predefined algorithm the frequency hops to another frequency .

5. The communication begins between both the receiving and the transmitting party and change their frequencies in accordance to the calculated code , starting at the same point in time

A commonly used data modulation is the M-ary (MFSK) in which case the the message data modulates a fixed frequency carrier.The carrier frequency is made to hop from one frequency value to another.Hence this method involves a 2 step modulation process Data modulation and frequency hop modulation.A large number of carrier frequencies are generated in a frequency synthesizer which is determined with the help of a PN generator.

Here the data is a MPSK modulated with some carrier frequency .The PN generator has an output of PN codes of word size k-bits.Here the modulated data is mixed with the outputs of the frequency synthesizer for hopping and transmission through the channel.

The receiver signal is first developed by using a mixer with the input coming from the same frequency synthesizer.The developed signal is then applied to the demodulator for recovery of data.

There are two types of Frequency hopping

Slow frequency hopping

Fast frequency hopping

In slow-frequency hopping there is one or more symbols per hop while in fast hop there are several frequency hops per message symbol.Slow hop is more susceptible to jamming than fast hopping.The reason is that in slow hopping the jammer has more time to decide which frequency to jam where in fast frequencing hopping by the time the jammer knows about the frequency and is about to jam it,the frequency quickly hops to another frequency and hence jammig becomes tough.

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