Page 81 - From GMS to LTE

Global System for Mobile Communications (GSM) 67

such as the user sending an SMS, the account is continually charged. If the account runs
out of credit, the connection is interrupted and the transmission of further SMS mes-
sages is blocked. In the early years of GSM, the development of these services had been
highly proprietary because of the lack of a common standard. The big disadvantage of
such solutions was that they were customized to work only with very specific compo-
nents of a single manufacturer. This meant that these services did not work abroad as
foreign network operators used components of other network vendors. This was espe-
cially a problem for the prepaid service as prepaid subscribers were excluded from
international roaming when the first services were launched.
To ensure the interoperability of intelligent network components between different
vendors and in networks of different mobile operators, industry and operators stand-
ardized an IN protocol in 3GPP TS 22.078 [36], which is called Customized Applications
for Mobile‐Enhanced Logic, or CAMEL for short. While CAMEL also offers functionality
for SMS and GPRS charging, the following discussion describes only the basic functionality
necessary for circuit‐switched connections.
CAMEL is not an application or a service, but forms the basis for creating services
(customized applications) on an SCP which are compatible with network elements of
other vendors and between networks. Thus, CAMEL can be compared with HTTP.
HTTP is used for transferring web pages between a web server and a browser. HTTP
ensures that any web server can communicate with any browser. Whether the content
of the data transfer is a web page or a picture is of no concern to HTTP because this is
managed on a higher layer directly by the web server and the web client. Transporting
the analogy back to the GSM world, the CAMEL specification defines the protocol for
communication between different network elements such as the MSC and the SCP, as
well as a state model for call control.
The state model is called the Basic Call State Model (BCSM) in CAMEL. A circuit‐switched
call, for example, is divided into a number of different states. For the originator (O‐BCSM),
the following states, which are also shown in Figure 1.54, have been defined:

call establishment;
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analysis of the called party’s number;
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routing of the connection;
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notification of the called party (alerting);
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ongoing call (active);
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disconnection of the call;
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no answer from the called party;
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called party busy.
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For a called subscriber, CAMEL also defines a state model which is called the
Terminating Basic Call State Model (T‐BCSM). T‐BCSM can be used for prepaid
subscribers who are currently roaming in a foreign network to control the call in the
foreign network and to apply real‐time charging.
For every state change in the state model, CAMEL defines a detection point (DP). If a
DP is activated for a subscriber, the SCP is informed of the particular state change.
Information contained in this message includes the IMSI of the subscriber, the current
position (MCC, MNC, LAC and cell ID) and the number that was called. Whether a DP
is activated is part of the subscriber’s HLR entry. This allows the creation of specific
services on a per subscriber basis. When the SCP is notified that the state model has