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GSM

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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  1. GSM John Alamia

  2. 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.

  3. Modulation • GSM uses GMSK or Gaussian minimum shift keying for modulation • GMSK is a kind of continuous frequency shift keying

  4. 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

  5. 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

  6. 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.

  7. 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.

  8. 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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