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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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slide1

GSM

John Alamia

overview
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
Modulation
  • GSM uses GMSK or Gaussian minimum shift keying for modulation
  • GMSK is a kind of continuous frequency shift keying
slide4
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
band usage 900mhz band
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
advantages
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.
security
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.
disadvantages
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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