GSM mobile telephones transmit and receive information in bursts (conversation split into parts) of data, and sent over a physical channel (carrier). By spreading the conversations in *bursts and sent in allocated time slots, over the radio carrier, this is called Time Division Multiplexing.
The benefit GSM gains from using TDMA over the cellular radio network air interface means theoretically eight (8) mobile phone conversations maybe carried over a single carrier; theoretically this could also be stated as eight (8) mobile telephones sharing one carrier (a particular radio frequency.)
Simply exemplifying the benefits of TDMA is not enough and the student should also consider where multiple handsets are active in a particular radio area what impact that would have on a particular mast (BTS base transceiver station) and how the network handles that? Remember, handover may be possible but this is an effect, not the cause. A possible cause is the important factor needed to be known, at first instance.
* A brief discussion and visual about a GSM burst can be found here:
GSM Normal Burst Power/Time Template - http://cellsiteanalysis.blogspot.co.uk/2014/06/gsm-normal-burst-powertime-template.html
