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Radio
Capacity
Each GSM radio channel (ARFCN) is a 200kHz TDMA carrier divided
into 8 time slots, or physical channels, capable of transmitting
one voice call at 16kbps. The first channel, time slot 0, is
reserved for signaling.
GSM MultiFrame from 3GPP TS
45.001 (Click
to enlarge)
Range
Standard GSM is limited to 35km (21 miles) from the base station
due to it's timing advance requirements. The Timing Advance
(TA) value ranges from 0-63 per timeslot, with each TA value
representing a change of 550 meters away from the base station. 3GPP TS 45.010 states: "The maximum timing advance value TAmax shall be 63".
Based on this, 64 TA multiplied by 550m per TA, equals: 64 x
550m = 35,200 meters
Download 3GPP TS 45.010 (R7):
http://www.3gpp.org/ftp/Specs/latest/Rel-7/45_series/45010-740.zip
Some equipment vendors have "extended range" features that allow
the timing advance from another timeslot to be borrowed,
therefore doubling the standard range of 35km to 70km. This is
done at the sacrifice of capacity however, as the timeslot that
the TA is borrowed from can no longer be used for voice or data
traffic.
For example, Motorola offers extended range of up to 120km based
on 219 TA's (4 timeslots):
Link
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