Data transmission in computer networks

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Why can waves with a very low frequency follow the earth's surface? Why are they not used for data transmission in computer networks?

Answers:

Radio waves which are below 2 MHz follow the ground. Following are the reason why its happened,

  1. Diffraction
  2. Current induced in the Earth's surface, which slows waves near the earth, and because of this wave front tilt downward.

These can be some reason that why Low frequency are not used for data transmission in computer networks.

Low frequencies - à lower data rates ( Nyquist/Shannon)

As more power is needed to transmit and receive low frequencies, for this large antenna are needed.

Penetration of Lower frequencies to material is high.

Why does the ITU-R only regulate 'lower' frequencies(up to some hundred GHz) and not higher frequencies (in the THz range)?

Answers:

THz range frequencies, such as infrared, visible light, don't interfere with other transmission because ther are are blocked easily when obstacles are presents.

So only the standard safety regulations applied to the systems.

Its not easy to generate the higer freuencies when the radio systems stay beyond the 100 GHz.

What are the different approaches in reulation regarding mobile phone systems in Europe and the US? What are the consequences?

Answers:

European approach

  • Classical approach which was that, standardization and regulate before any products were available.
  • ETSI which is founded by EU governments to harmonize national regulations.
  • All countries had to obey or follow the standards set by ETSI.

American Approach

  • Companies develop the systems, then market success try to force the standard, or companies itself try to standardize.
  • FCC is the example.

Affects of different Approaches

  • Some standards failed and some succeeded only in Europe, HIPERLAN 1(completely failed), and ISDN (Succeeded). Some become the worldwide (GSM).
  • US approach is better for the most of the systems, i.e. initial products, and after it, standardized. WLAN 802.11 is a good example for the USA approach. But standards about mobile market which are well established in Europe but its not even known in US. i.e. free roaming, MMS, GPRS roaming.

Why is the international availability of the same ISM bands important?

Answers:

International availability of same ISM band is important because of

  1. Interoperability.
  2. large enough market to make the consumer devices inexpensive.

What are the main problems of signal propagation? Why do radio waves not always follow a straight line? Why is reflection both useful and harmful?

Answers:

Following are the reason which cause the signal propagation,

  1. Attenuation
  2. Scattering
  3. Diffraction
  4. Reflection
  5. Refraction

In the presence of large buildings, without multipath propagation or reflection of signal we can't receive radio signal. Inter symbol interference (ISI) is due to multipath propagation.

Why, typically, is digital modulation not enough for radio transmission? What are general goals for digital modulation? What are typical schemes?

Answers:

FDM is used worldwide for separating different systems. So all the radio system must use analogue modulation for modulating the digital signal onto a carrier frequency. Most of the radio stations want to transmit at the same time, because of this reason all the signal should be modulation onto different carrier frequencies.

Other reason behind digital modulation,

  1. Antenna and medium characteristics.

Characteristics for digital modulation are

  1. Spectral efficiency
  2. Power efficiency
  3. Robustness.

Typical schemes are

  1. ASK
  2. PSK
  3. FSK

What are the main benefits of a spread spectrum system? How can spreading be achieved? How can DSSS systems benefit from multi-path propagation?

Answers:

Benefits of a Spread Spectrum System:

  1. Robust against interference
  2. Inherent security
  3. Basis for CDMA technologies
  4. If the signal level is low enough then it can be used as background of existing systems.

By Chipping sequence or frequency hopping (XORing) a bit, we can achieve Spreading. Guard spaces are now the orthogonality of the chipping sequences or hopping patterns. Higher Orthogonality-à Lower the correlation of spread signals, or lower the collision probability of frequency hopping systems.

To recombining signals travelling along different paths, DSSS system typically use rake receivers.

What limits the number of simultaneous users in a TDM/FDM system compared to a CDM system? What happens to the transmission quality of connections if the load gets higher in a cell, i.e., how does an additional user influence the other users in the cell?

Answers:

References:

  1. Jochen Schiller, " Mobile Communication", 2nd ed., Addison-Wesley