(1) Discontinuous Transmission:
GSM speech coder is RELP (Residually Excited Predictive Coder), which is enhanced by including a Long Term Predictor (LTP).
The coder provides 260 bits for 20ms blocks of speech, which yields a bit rate of 13kbps.
In a normal conversation each person speaks on average for less than 40% of the time and maximum period of time there is silence. ThusGSM speech coder takes advantage of this fact and operates in Discontinuous Transmission mode (DTX) by incorporating a Voice Activity Detector (VAD) in speech coder.
In Discontinuous Transmission mode VAD detects the voice in signal and sends signal to speech coder. Thus only when speech coder gets a signal from VAD it gets switched on and for silence it remains off.
This mode provides a longer battery life and reduces instantaneous radio interference since GSM transmitter is not active during silent periods.
A Comfort Noise Subsystem (CNS) is used at the receiving end which introduces background acoustic noise to compensate for the annoying switched muting which occurs due to DTX.
(2) Subscriber Identity Module (SIM):
It is a memory device that stores information such as subscriber’s identification number. The networks and countries where the subscriber is entitled to service, privacy keys and other user specific information.
A subscriber uses SIM with four digit personal ID number to activate service from any GSM phone.
SIM is available as smart card or plug in module, which is less convenient than the SIM card but is removable and portable.
Without SIM installed all GSM mobiles are identical and non-operational. SIM gives GSM subscriber units their identity.
Subscriber can plug their SIM into any suitable terminal such as a hotel phone, public phone , or any portable or mobile and are then able to have all incoming GSM calls routed to that terminal and have all outgoing calls billed to their home phone no matter where they are in the world.
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