Page 65 - From GMS to LTE

Global System for Mobile Communications (GSM) 51

frame. Finally, it is also not possible to use a lossy data compression scheme for data
calls. Therefore, the TRAU operates in a transparent mode for data calls. If the
data stream can be compressed, this has to be performed by higher layers or by the data
application itself.
With a radio receiver or an amplifier of a stereo set, the different states of a GSM
connection can be made audible. This is possible as the activation and deactivation of
the transmitter of the mobile device induce an audible sound in the amplifier part of
audio devices. If the GSM mobile device is held close enough to an activated radio or an
amplifier during the establishment of a call, the typical noise pattern can be heard,
which is generated by the exchange of messages on the signaling channel (SDCCH). At
some time during the signaling phase, a TCH is assigned to the mobile device at the
point at which the noise pattern changes. As a TCH burst is transmitted every 4.615
milliseconds, the transmitter of the mobile device is switched on and off with a frequency
of 217 Hz. If the background noise is low enough or the mute button of the telephone is
pressed, the mobile device changes into DTX mode for the uplink part of the channel.
This can be heard as well, as the constant 217 Hz hum is replaced by single short bursts
every 0.5 seconds.
For incoming calls, this method can also be used to check that a mobile device has
started communication with the network on the SDCCH one to two seconds before
ringing. This delay is due to the fact that the mobile device first needs to go through the
authentication phase and the activation of the ciphering for the channel. Only afterward
can the network forward further information to the mobile device as to why the channel
was established. This is also the reason why it takes a much longer time for the alerting
tone to be heard when calling a mobile device compared to calling a fixed‐line phone.
Some mobile devices possess a number of interesting network‐monitoring functionali-
ties, which are hidden in the mobile device software and are usually not directly accessible
via the phone’s menu. These network monitors allow visualization of many procedures and
parameters that have been discussed in this chapter, such as the timing advance, channel
allocation, power control, cell ID, neighboring cell information, handover and cell reselec-
tion. Various web pages can be found on the Internet that explain how these monitors can
be activated, depending on the type and model of the phone. As the activation procedures
are different for every phone, it is not possible to give a general recommendation. However,
by using the manufacturer and model of the phone in combination with terms like ‘GSM
network monitor’, ‘GSM netmonitor’ or ‘GSM monitoring mode’, it is relatively easy to dis-
cover if and how the monitoring mode can be activated for a specific phone.

1.8 Mobility Management and Call Control

As all components of a GSM mobile network have now been introduced, the following
section gives an overview of the three processes that allow a subscriber to roam throughout
the network.

1.8.1 Cell Reselection and Location Area Update
As the network needs to be able to forward an incoming call, the subscriber’s location
must be known. After the mobile device is switched on, its first action is to register with
the network. Therefore, the network becomes aware of the current location of the user,