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GSM John Alamia Overview GSM (Global System for Mobile Communications) is a common second generation cell phone standard. GSM is a digital standard that was adopted over the original analog system. The GSM network operates at 850 MHz or 1900 MHz bands in the US. Modulation

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GSM

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GSM

John Alamia


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Overview

  • GSM (Global System for Mobile Communications) is a common second generation cell phone standard.

  • GSM is a digital standard that was adopted over the original analog system.

  • The GSM network operates at 850 MHz or 1900 MHz bands in the US.


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Modulation

  • GSM uses GMSK or Gaussianminimum shift keying for modulation

  • GMSK is a kind of continuous frequency shift keying


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GMSK

  • The on-off binary signal is first transformed to a polar binary signal.

  • The polar binary signal is filtered such that a Gaussian-shaped signal is produced.

  • Frequency modulation is then applied to the signal


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Band Usage (900MHz Band)

  • The uplink frequency band is 890-915 MHz

  • The downlink frequency band is 935-960 MHz.

  • The 25 MHz bandwidth of each is divided into 124 carrier frequency channels each spaced 200 kHz apart.

  • Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel.

  • Full rate is 13kbps; Half rate is 6kbps


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Advantages

  • Digital data allows for better more effective use of bandwidth

  • Radio signals use less power, this allows batteries to last longer

  • Digital voice encoding allowed for digital error checking to be used.

  • Digital services such as email and SMS or Short Message Service (text messages) were added.


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Security

  • GSM uses some simple encoding algorithms but has been found to be less effective then originally believed.

  • Previous analog systems provided virtually no security. Phones were prone to being “cloned” with analog systems.

  • GSM prevents cloning problem but also stops legitimate uses of this property.


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Disadvantages

  • The weaker digital signal may not be able to reach cell tower

  • Digital signal will drop out faster then an analog signal.


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