Introduction to GSM technology and GSM trainer ST-2133
GSM:
GSM stands for Global system for Mobile communication. It is the way of
communication in which many mobile devices can exchange data specially voice signal
with out bewing connected with each other through a wired medium. GSM can be
elaborated by the following layout.
The main components of a GSM network are explained below.
1) Mobile Station (MS):
Mobile station is any mobile equipment that is carried by the subscriber. It may be a
laptop, a mobile phone or any other device that is connected to the cellular network. The
mobile equipment contains a SIM, which stands for “Subscriber’s identification module”
which contains the entire customer related information (identification, secret key for
authentication, etc.). Each mobile station also have IMEI #, it is the “International Mobile
Equipment identity”. The IMEI of every mobile equipment is unique and it is used by
the GSM network to identify valid devices and therefore can be used for stopping a stolen
phone from accessing the network in that country. For example, if a mobile phone is
stolen, the owner can call his or her network provider and instruct them to "blacklist" the
phone using its IMEI number.
2) Base Station Subsystem (BSS):
The BSS consist of two parts the BTS and the BSC both of them are explained
briefly.
a) Base Transceiver Station (BTS):
The cell size can be found by the BTS. It connects the MS with the BSC. Data is
encrypted before sending so that it can’t be leeched, and decrypted when it is received
both the encryption and decryption is done by the BTS. It also handles the transcoding of
the received data as well as the uplink radio channel power management. One BTS has
up to 16 transceivers.
b) Base Station Controller (BSC):
The base station controller (BSC) provides, classically, the intelligence behind the
BTSs. Typically a BSC has tens or even hundreds of BTSs under its control. The BSC
handles allocation of radio channels, receives measurements from the mobile phones, and
controls handovers from BTS to BTS (except in the case of an inter-BSC handover in
which case control is in part the responsibility of the anchor MSC). A key function of the
BSC is to act as a concentrator where many different low capacity connections to BTSs
(with relatively low utilisation) become reduced to a smaller number of connections
towards the mobile switching center (MSC) (with a high level of utilisation). Overall, this
means that networks are often structured to have many BSCs distributed into regions near
their BTSs which are then connected to large centralised MSC sites.
The databases for all the sites, including information such as carrier frequencies,
frequency hopping lists, power reduction levels, receiving levels for cell border
calculation, are stored in the BSC. This data is obtained directly from radio planning
engineering which involves modelling of the signal propagation as well as traffic
projections.
3) Mobile Switching Center (MSC):
The MSC sets up and releases the end-to-end connection, handles mobility and
hand-over requirements during the call and takes care of charging and real time pre-paid
account monitoring. It also processes requests for service connections from mobile
devices and land line callers, and routes calls between the base stations and the public
switched telephone network (PSTN). The MSC receives the dialed digits, creates and
interprets call processing tones, and routes the call paths.
4) Registers:
There are two types of registers used in the GSM network to keep the record of
the uses.
a) Home Location Register (HLR)
b) Visitor Location Register (VLR)
• Home Location Register:
The home location register (HLR) is a central database that contains details of
each mobile phone subscriber that is authorized to use the GSM core network. The
HLRs store details of every SIM card issued by the mobile phone operator. Each SIM
has a unique identifier called an IMSI which is theprimary key to each HLR record.
Other data stored in the HLR is
o GSM services that the subscriber has requested or been given.
o GPRS settings to allow the subscriber to access packet services.
o Current location of subscriber (VLR and serving GPRS support
node/SGSN).
o Call divert settings applicable for each associated MSISDN.
• Visitor Location Register:
The visitor location register is a database of the subscribers who have roamed into the
jurisdiction of the MSC (Mobile Switching Center) which it serves. Each base
station in the network is served by exactly one VLR, hence a subscriber cannot be
present in more than one VLR at a time.
Data stored in VLR include:
o IMSI (the subscriber's identity number).
o Authentication data.
o MSISDN (the subscriber's phone number).
o GSM services that the subscriber is allowed to access.
o Access point (GPRS) subscribed.
o The HLR address of the subscriber.
6) SIM:
A subscriber identity module or subscriber identification module (SIM) is
an integrated circuit which securely stores the service-subscriber key (IMSI) used to
identify a subscriber on mobile telephony devices (such as mobile
phones and computers).
• IMSI:
SIM cards are identified on their individual operator networks by a
unique IMSI. Mobile operators connect mobile phone calls and communicate with their
market SIM cards using their IMSIs. The format is:
o The first 3 digits represent the Mobile Country Code (MCC).
o The next 2 or 3 digits represent the Mobile Network Code (MNC). 3 digit
MNC codes are allowed by E.212 but are not widely implemented.
o The next digits represent the mobile station identification number. Normally
there will be 10 digits but would be fewer in the case of a 3 digit MNC or if
national regulations indicate that the total length of the IMSI should be less
than 15 digits.
