MSC (Mobile Switching Control):
A Mobile Switching Center (MSC) is a telecommunications
switch deployed in mobile communications networks, to provide call control,
processing and access to the Public Switched Telephone (fixed) Network. 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.
In the GSM mobile phone system, in
contrast with earlier analogue services, fax and data information is sent
directly digitally encoded to the MSC.
There
are various different names for MSCs in different contexts which reflect their
complex role in the network.
The Gateway MSC (G-MSC) is
the MSC that determines which visited MSC the subscriber who is being called is
currently located at. It also interfaces with the PSTN. All mobile to
mobile calls and PSTN to mobile calls are routed through a G-MSC.
The visited MSC (V-MSC) is
the MSC where a customer is currently located. The VLR
associated with this MSC will have the subscriber's data in it.
The anchor MSC is the MSC from
which a handover has been initiated. The target MSC
is the MSC toward which a Handover should take place.
BTS (Base Transceiver Station):
The base
transceiver station or BTS, contains the equipment for transmitting
and receiving radio signals (transceivers), antennas, and equipment for encrypting and decrypting communications
with the base station
controller (BSC). Typically a BTS for anything other
than a picocell will have several transceivers
(TRXs) which allow it to serve several different frequencies and different sectors of the
cell (in the case of sectorised base stations).
A BTS is controlled by a parent BSC
via the "base station control function" (BCF). The functions
of a BTS vary depending on the cellular technology used and the cellular
telephone provider. Frequency hopping is often used to increase
overall BTS performance. Several
hopping sequences are available, and the sequence in use for a particular cell
is continually broadcast by that cell so that it is known to the handsets.
BSC (Base Station Controller):
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. A key function of the BSC is to act as a concentrator where many different low
capacity connections to BTSs (with relatively low utilization) become reduced
to a smaller number of connections towards the mobile
switching center (MSC) (with a high level of utilization). Overall,
this means that networks are often structured to have many BSCs distributed
into regions near their BTSs which are then connected to large centralized MSC
sites.
It also provides all the required data
to the operation support subsystem (OSS) as well as to the performance
measuring centers.
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 modeling of the signal
propagation as well as traffic projections.
Home location
register (HLR):
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. There can be several logical, and
physical, HLRs per public land
mobile network (PLMN), though one international
mobile subscriber identity (IMSI)/MSISDN pair can be associated with
only one logical HLR (which can span several physical nodes) at a time.
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 the primary key to each HLR record.
The next important items of data
associated with the SIM are the MSISDNs, which are the telephone numbers used by mobile phones to
make and receive calls. The primary MSISDN (Mobile Subscriber Integrated
Services Digital Network Number) is the number used for making and receiving
voice calls and SMS, but it is possible for a SIM to have other secondary
MSISDNs associated with it for fax and data calls. Each
MSISDN is also a primary key to
the HLR record. The HLR data is stored for as long as a subscriber remains with
the mobile phone operator.
Visitor location
register (VLR):
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.
The data stored in the VLR has either
been received from the HLR, or collected from the MS (Mobile station). In practice, for
performance reasons, most vendors integrate the VLR directly to the V-MSC and,
where this is not done, the VLR is very tightly linked with the MSC via a
proprietary interface. Whenever an MSC detects a new MS in its network, in
addition to creating a new record in the VLR, it also updates the HLR of the
mobile subscriber, apprising it of the new location of that MS. If VLR data is
corrupted it can lead to serious issues with text messaging and call services.
Data
stored include:
- IMSI (the subscriber's identity
number).
- Authentication
data.
- MSISDN
(the subscriber's phone number).
- GSM
services that the subscriber is allowed to access.
- access
point (GPRS) subscribed.
- The
HLR address of the subscriber.
Other
GSM core network elements connected to the VLR :
The VLR connects to
the following elements:- The
V-MSC to pass required data for its procedures; e.g., authentication or
call setup.
- The
HLR to request data for mobile phones attached to its serving area.
- Other
VLRs to transfer temporary data concerning the mobile when they roam into
new VLR areas. For example, the temporal mobile subscriber
identity (TMSI).
Equipment
identity register (EIR):
The equipment
identity register is often integrated to the HLR. The EIR keeps a
list of mobile phones (identified by their IMEI) which are to be banned from
the network or monitored. This is designed to allow tracking of stolen mobile
phones. In theory all data about all stolen mobile phones should be distributed
to all EIRs in the world through a Central EIR. It is clear, however, that
there are some countries where this is not in operation. The EIR data does not
have to change in real time, which means that this function can be less distributed
than the function of the HLR. The EIR is a database that contains information
about the identity of the mobile equipment that prevents calls from stolen,
unauthorized or defective mobile stations. Some EIR also have the capability to
log Handset attempts and store it in a log file.
Authentication
centre (AUC):
The authentication centre (AUC) is a
function to authenticate
each SIM card that attempts to connect to the
GSM core network (typically when the phone is powered on). Once the
authentication is successful, the HLR is allowed to manage the SIM and services
described above. An encryption key
is also generated that is subsequently used to encrypt all wireless
communications (voice, SMS, etc.) between the mobile phone and the GSM core
network.
If the authentication fails, then no
services are possible from that particular combination of SIM card and mobile
phone operator attempted. There is an additional form of identification check
performed on the serial number of the mobile phone described in the EIR section
below, but this is not relevant to the AUC processing.
Proper implementation of security in
and around the AUC is a key part of an operator's strategy to avoid SIM cloning.
The AUC does not engage directly in
the authentication process, but instead generates data known as triplets for
the MSC to use during the procedure. The security of the process depends upon a
shared secret between the AUC and the SIM
called the Ki. The Ki is securely burned into the SIM
during manufacture and is also securely replicated onto the AUC. This Ki
is never transmitted between the AUC and SIM, but is combined with the IMSI to
produce a challenge/response for identification purposes and an encryption key
called Kc for use in over the air communications.
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