What is GSM

What is GSM (Global System for Mobile communication)?

GSM (Global System for Mobile communication) is a digital mobile telephone system that is widely used in Europe and other parts of the world. GSM uses a variation of time division multiple access (TDMA) and is the most widely used of the three digital wireless telephone technologies (TDMA, GSM, and CDMA). GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either the 900 Mhz [the current Australian allocation] or 1800 MHz frequency band.

GSM is the de facto wireless telephone standard in Europe and, according to IEEE Spectrum, is being used in more than 150 networks in 86 countries. Since many GSM network operators have roaming agreements with foreign operators, users can often continue to use their mobile phones when they travel to other countries.

Why can't I use my phone more the 35 km's from a base station?

Basically once the Base Station is more then 35 km's away the TDMA signalling fails because of the time required to reply.

To allow maximum number of users access, each band is subdivided into 124 carrier frequencies spaced 200KHz apart, using FDMA techniques. By applying TDMA techniques, each of these carrier frequencies is further subdivided into time slots which provide each user with the carrier frequency for approximately 0.577ms. This equates to approx 217 jumps per second, but amongst a very small frequency range so encryption is a must for proper security of calls. In fact it is not exactly that, it is hopping 13 times every 60 ms, which gives 13/0.06 per second. 0.577ms = 13 frames/60 ms /8 time slots

As soon as you get more then 35km from the base station, the phone cannot respond in time (eg in that time slot) so the base station starts handling another call.

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