Page 72 - From GMS to LTE

58 From GSM to LTE-Advanced Pro and 5G

A‐MSC in command in all the different scenarios, it is ensured that during the lifetime
of a call only the G‐MSC, the A‐MSC and at most one R‐MSC are part of a call. In addition,
tandem switches might be necessary to route the call through the network or to a roam-
ing network. However, these switches purely forward the call and are thus transparent
in this procedure.
Finally, there is also a handover case in which the subscriber who is served by an R‐MSC
returns to a cell which is connected to the A‐MSC. Once this handover is performed, no
R‐MSC is part of the call. Therefore, this scenario is called a subsequent handback.
From the mobile device point of view, all handover variants are performed in the
same way, as the handover messages are identical for all scenarios. To perform a hando-
ver as quickly as possible, however, GSM can send synchronization information for the
new cell in the handover message. This allows the mobile device to immediately switch
to the allocated timeslot instead of having to synchronize first. This can only be done,
however, if the current and the new cells are synchronized with each other, which is not
possible, for example, if they are controlled by different BSCs. As two cells that are
controlled by the same BSC may not necessarily be synchronized, synchronization
information is by no means an indication of what kind of handover is being performed
in the radio and core network.

1.9 The Mobile Device

Owing to the progress of miniaturization of electronic components during the
mid‐1980s, it was possible to integrate all components of a mobile device into a single
portable device. A few years later, mobile devices had shrunk to such a small size that
the limiting factor in future miniaturization was no longer the size of the electronic
components. Instead, the space required for user interface components like display and
keypad limited a further reduction. Because of continuous improvement and miniaturi-
zation of electronic components, it is possible to integrate more and more functionalities
into a mobile device and to improve the ease of use. While mobile devices were at first
used only for voice calls, the trend today is toward feature‐rich devices that also include
telephony functions. This section, therefore, first describes the architecture of a, from
today’s perspective, simple voice‐centric phone. Afterward, the architecture of a modern
smartphone will be discussed.

1.9.1 Architecture of a Voice‐Centric Mobile Device

Simple GSM devices for voice and SMS communication can be built with only a few
parts today and can therefore be produced very cheaply. The simplest GSM mobile
phones can thus be bought today for less than 20 euros.
Figure 1.46 shows the principle architecture of such a device. Its core is based on a
baseband processor, which contains a reduced instruction set (RISC), a CPU and a digital
signal processor (DSP).
The RISC processor is responsible for the following tasks:

handling of information that is received via the different signaling channels (BCCH,
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PCH, AGCH, PCH and so on);
call establishment (DTAP);
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