Page 44 - From GMS to LTE

30 From GSM to LTE-Advanced Pro and 5G

TDMA frame, 4.615 ms

0 123456701 2 3456 7 0 12

TS-0 TS-2 TS-3 TS-4 TS-5 TS-6 TS-7

t
Figure 1.24 Arrangement of bursts of a frame for the visualization of logical channels in Figure 1.25.

these messages do not have to be sent very often, no dedicated physical bursts are
allocated to the FACCH. Instead, user data is removed from a TCH burst. To inform
the mobile device of this, the stealing bits to the left and right of the training sequence,
as shown in Figure 1.23, are used. This is the reason why the FACCH is not shown in
Figure 1.25.
The Slow Associated Control Channel (SACCH) is also assigned to a dedicated
●
connection. It is used in the uplink direction to report signal quality measure-
ments of the serving cell and neighboring cells to the network. The network then
uses these values for handover decisions and power control. In the downlink direc-
tion, the SACCH is used to send power control commands to the mobile device.
Furthermore, the SACCH is used for timing advance control, which is described in
Section 1.7.4 and Figure 1.26. As these messages are only of low priority and the
necessary bandwidth is very small, only a few bursts are used on a 26‐multiframe
pattern at fixed intervals.
The Standalone Dedicated Control Channel (SDCCH) is a pure signaling channel
●
that is used during call establishment when a subscriber has not yet been assigned a
TCH. Furthermore, the channel is used for signaling that is not related to call estab-
lishment, such as for the location update procedure or for sending or receiving a text
message (SMS).
Besides the dedicated channels, which are always assigned to a single user, there are a
number of common channels that are monitored by all subscribers in a cell:
The Synchronization Channel (SCH) is used by mobile devices during network and
●
cell searches.
The Frequency Correction Channel (FCCH) is used by the mobile devices to cali-
●
brate their transceiver units, and is also used to detect the beginning of a
multiframe.
The Broadcast Common Control Channel (BCCH) is the main information channel
●
of a cell and broadcasts SYS_INFO messages that contain a variety of information