GSM - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 273

Although this may appear to be simple, it is extremely complicated to achieve.
Data is sent through multiple channels, and simply walking around outside can
be complicated for the mobile telephone network, as users regularly disconnect
from one tower while connecting to a new tower. This is all handled transpar-
ently by the telephone and the telephone network, resulting in what appears to
be a seamless network. The truth is, at any one moment, a telephone, or device
in a mobile network, may not send and receive data.
GSM
The i rst generation of mobile communications, known as 1 G, was a simple
technology that allowed full-duplex voice communication (full-duplex meaning
that you could talk and listen at the same time). A simple system, it worked
extremely well for people that needed to be on the move and connected continu-
ously. Most 1 G telephones were car phones; relatively large devices that ran
on a car's battery, but allowed users to do what the telephones were designed
for—talking.
The 1 G network was entirely analog, but was only called 1 G when a new
technology was needed; it was then known as the second generation, or 2 G and
replaced 1 G.
In 1981, the European Conference of Postal and Telecommunications
Administration (known as CEPT) created a new committee, the Groupe Spéciale
Mobile, based in Paris. The GSM name would later be known as Global System
for Mobile Communication, and its logo would become the de facto standard
in almost all countries.
GSM changed quite a few technical aspects; all communications were now
digital instead of analog. By using digital technology, communications could be
compressed, using less bandwidth, allowing more users access to the network.
Because mobile devices were becoming truly mobile and smaller, phone's radio
emission strength was reduced, requiring more and more cells to allow com-
munications. Cell towers were now cheap to produce, so this wasn't a problem,
as was the cost to pay for safe devices that could be placed in a pocket and
used all day.
One of the changes that the GSM specii cation proposed was something that
is still in use: a SIM card. A SIM card contains a unique serial number, operator
network information, subscriber information, temporary network information,
and two passcodes for the user: the PIN and PUK. By using a SIM card, users
can choose their mobile operator, and mobile operators can sometimes “lock”
mobile phones to their network.
The original GSM specii cation did not include data transfer but was rapidly
modii ed to allow SMS messages, just one such method that uses digital data.
SMS, short for Short Message System, is a technique to send 160 characters to