Page 33 - From GMS to LTE

Global System for Mobile Communications (GSM) 19

Table 1.3 Basic services of a GSM network.

Basic service Description
Telephony If this basic service is activated, a subscriber can use the voice telephony
services of the network. This can be partly restricted by other
supplementary services that are described below
Short messaging service If activated, a subscriber is allowed to use the SMS
(SMS)
Data service Different circuit‐switched data services can be activated for a subscriber
with speeds of 2.4, 4.8, 9.6 and 14.4 kbit/s data calls
FAX Allows or denies a subscriber the use of the FAX service, which can be
used to exchange FAX messages with fixed‐line or mobile devices

their mobile network operator. This is a great advantage for subscribers and for competition
between mobile operators, but it also implies that it is no longer possible to discern the
mobile network to which the call will be routed from the NDC. Furthermore, the
introduction of MNP also increased the complexity of call routing and billing in both
fixed‐line and mobile networks, because it is no longer possible to use the NDC to
decide which tariff to apply to a call. Instead of a simple call‐routing scheme based on
the NDC, the networks now have to query an MNP database for every call to a mobile
subscriber to find out if the call can be routed inside the network or if it has to be
forwarded to a different national mobile network.
Apart from the IMSI and MSISDN, the HLR contains a variety of information about
each subscriber, such as which services they are allowed to use. Table 1.3 shows a number
of ‘basic services’ that can be activated on a per subscriber basis.
In addition to the basic services described above, the GSM network offers a number
of other services that can also be activated on a per subscriber basis. These services are
called supplementary services and are shown in Table 1.4.
Most supplementary services can be activated by the network operator on a per
subscriber basis and allow the operator to charge an additional monthly fee for some
services if desired. Other services, like multiparty, can be charged on a per use basis.
Although some network operators made use of this in the early years of GSM, most
services are now included as part of the basic monthly fee.
Most services can be configured by the subscriber via a menu on the mobile device.
The menu, however, is just a graphical front end for the user and the mobile device
*
translates the user’s commands into numerical strings which start with an ‘ ’ character.
These strings are then sent to the network by use of an Unstructured Supplementary
Service Data (USSD) message. The codes are standardized in 3GPP TS 22.030 [13] and
are thus identical in all networks. As the menu is only a front end for the USSD service,
the user can also input the USSD strings himself/herself via the keypad. After pressing
the ‘send’ button, which is usually the button that is also used to start a phone call after
typing in a phone number, the mobile device sends the string to the HLR via the MSC,
where the string is analyzed and the requested operation is performed. For example, call
forwarding to another phone (e.g. 0782 192 8355) while a user is already engaged in
**
*
another call – call forward busy (CFB) is activated with the following string: 67
07821928355# + call button.