WILLTEK 4400 Mobile Phone Tester Series, Руководство пользователя

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Chapter 2 GSM Call Mode Operation
GSM basics
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4460 GSM System Option and 4468 EDGE System Option Version 12.20
Timing advance
As said above, timing is extremely critical in GSM. All transmissions between
mobile and base station have to be perfectly synchronized. Due to the RF signal
propagation time (some 3.3 μs per km distance), the mobile receives the signals
transmitted by the base station with a short time delay. Of course, the mobile
will synchronize to the base station’s timing. However, it has no means of
detecting the delay due to signal propagation, so it will be back in timing.
Should the mobile transmit now, the signals will arrive at the base station ‘too
late’ and may cause interference in subsequent time slots. To avoid that, the base
station measures the delay of the bursts received from the mobile in relation to
its internal timing and then commands the mobile to send its bursts ‘a little’
earlier. In GSM terms, this is called ‘assigning the mobile a timing advance’.
Example: Let’s assume that there has been no timing advance set. The mobile is
10 km away from the base station. Due to signal propagation time, the bursts
transmitted by the base station will arrive at the mobile with a delay of some
T (tail bits)
The three tail bits bear no information. Consequently, no
data will be lost if the burst does not exactly “catch” the
time slot or if the bursts of the mobiles transmitting in
the previous or subsequent time slots are slightly too
long or not perfectly synchronized.
Notes:
– The propagation time required by a radio wave to
travel 1 km is approx. 3.3 μs (the duration of one bit
in GSM is approx. 3.7 μs).
– Usually, mobiles are on variable distances to the base
station.
– To avoid that a burst of a mobile ‘comes in too late’,
the mobile is commanded by the base station to use a
timing advance. This means that the mobile will send
its burst some microseconds earlier. After the propa-
gation delay, it will then arrive at the base station in
due time.
– The ‘timing advance’ is explained in detail below.
S (stealing
flag)
The stealing flag will be set when a burst does not con-
tain call data but signaling data.
Training Seq.
The training sequence is a bit sequence known by both
transmitter and receiver. The receiver of a burst looks for
this sequence in the data stream. Thus, the receiver can
easily allocate the useful data within the burst.
Encrypted
data
This is the ‘payload’ of the burst, often also called the
useful data. The data transmitted here is
– encrypted to safeguard it against interception,
– partly encoded so that incorrectly transmitted bits
can be detected and even corrected.
Note: Please refer to section “Voice coding” for more
details.