Page 46 - From GMS to LTE

32 From GSM to LTE-Advanced Pro and 5G

Burst 1 Burst 2 Burst 3

t

Without control, a burst arrives too late from subscribers at a far
distance and overlaps with a burst of the next timeslot.
Figure 1.26 Time shift of bursts of distant subscribers without timing advance control.

about the network. The channel is monitored by all mobile devices which are switched
on but currently not engaged in a call or signaling connection (idle mode), and broadcasts,
among many other things, the following information:
– the MCC and MNC of the cell;
– the identification of the cell, which consists of the location area code (LAC) and the
cell ID;
– to simplify the search for neighboring cells for a mobile device, the BCCH also con-
tains information about the frequencies used by neighboring cells. Thus, the mobile
device does not have to search the complete frequency band for neighboring cells.
The Paging Channel (PCH) is used to inform idle subscribers of incoming calls or
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SMS messages. As the network alone is aware of the location area the subscriber is
roaming in, the Paging message is broadcast in all cells belonging to the location area.
The most important information element of the message is the IMSI of the subscriber
or a temporary identification called the Temporary Mobile Subscriber Identity
(TMSI). A TMSI is assigned to a mobile device during the network attach procedure
and can be changed by the network every time the mobile device contacts the net-
work once encryption has been activated. Thus, the subscriber has to be identified
with the IMSI only once and is then addressed with a constantly changing temporary
number when encryption is not yet activated for the communication. This increases
anonymity in the network and prevents eavesdroppers from creating movement pro-
files of subscribers.
The Random Access Channel (RACH) is the only common channel in the uplink
●
direction. If the mobile device receives a message via the PCH that the network is
requesting a connection establishment or if the user wants to establish a call or send
an SMS, the RACH is used for the initial communication with the network. This is
done by sending a Channel Request message. Requesting a channel has to be done via
a ‘random’ channel because subscribers in a cell are not synchronized with each other.
Thus, it cannot be ensured that two devices do not try to establish a connection at the
same time. Only when a dedicated channel (SDCCH) has been assigned to the mobile
device by the network can there no longer be any collision between different sub-
scribers of a cell. If a collision occurs during the first network access, the colliding
messages are lost and the mobile devices do not receive an answer from the network.
Thus, they have to repeat their Channel Request messages after expiry of a timer that
is set to an initial random value. This way, it is not very likely that the mobile devices
will interfere with each other again during their next connection establishment
attempts because they are performed at different times.