1
Cellular Concept
Traditional mobile service was structured similar to
television broadcasting: One very powerful transmitter located at the highest
spot in an area would broadcast in a radius of up to fifty kilometers. The
Cellular concept structured the mobile telephone network in a different way.
Instead of using one powerful transmitter many low-powered transmitter were
placed through out a coverage area. For example, by dividing metropolitan
region into one hundred different areas (cells) with low power transmitters
using twelve conversation (channels) each, the system capacity could
theoretically be increased from twelve conversations using one hundred low
power transmitters.
The cellular concept employs variable low power levels,
which allows cells to be sized according to subscriber density and demand of a
given area. As the populations grows, cells can be added to accommodate that
growth. Frequencies used in one cell cluster can be reused in other cells.
Conversations can be handed over from cell to cell to maintain constant phone
service as the user moves between cells.
The cellular system design was pioneered by during’70s by
Bell Laboratories in the United
States , and the initial realization was
known as AMPS (Advanced Mobile Phone Service). The AMPS cellular service was
available in United States
in 1983. AMPS is essentially generation 1 analog cellular system in contrast to
generation 2 digital cellular systems of GSM and CDMA (1S-95).
Cells :
A cell is the basic geographic unit of cellular system. The
term cellular comes from the honeycomb areas into which a coverage region is
divided. Cells are base stations transmitting over small geographic areas that
are represented as hexagons. Each cell size varies depending upon landscape.
Because of constraint imposed by natural terrain and man-made structures, the
true shape of cell is not a perfect hexagon.
A group of cells is called a cluster. No frequencies
are reused in a cluster.
Features of Digital Cellular Systems:
·
Small
cells
·
Frequency
reuse
·
Small,
battery-powered handsets
·
Performance
of handovers
Cellular System Characteristics
General
|
Cellular radio systems allow the subscriber to place
and receive telephone calls over
the wire-line telephone network where ever cellular coverage is provided.
Roaming capabilities extend service to users traveling outside their
“outside” home service areas.
|
characteristics of digital
cellular systems
|
The
distinguishing features of digital cellular systems compared to other mobile
radio systems are:
§
Small cells
A cellular system uses many base stations with
relatively small coverage radii (on
the order of a 100 m to 30 km).
§
Frequency reuse
The spectrum allocated
for a cellular network is limited. As a result there is a limit to the number
of channels or frequencies that can be used. For this reason each frequency
is used simultaneously by multiple base-mobile pairs. This frequency reuse
allows a much higher subscriber density per MHz of spectrum than other
systems. System capacity can be further increased by reducing the cell size
(the coverage area of a single base station), down to radii as small as 200
m.
§
Small, battery-powered handsets In addition to
supporting much higher densities than previous systems, this approach enables
the use of small, battery-powered handsets with a radio frequency that is
lower than the large mobile units used in earlier systems.
§
Performance of handovers
In cellular systems, continuous
coverage is achieved by executing a “handover” (the seamless transfer of the
call from one base station to another) as the mobile unit crosses cell
boundaries. This requires the mobile to change frequencies under control of
the cellular network.
|
Frequency Reuse :
Why frequency reuse
|
The spectrum
allocated for a cellular network is limited. As a result there is a limit to
the number of frequencies or channels that can be used. A cellular network
can only provide service to a large number of subscribers, if the channels
allocated to it can be reused. Channel reuse is implemented by using the same
channels within cells located at different positions in the cellular network
service area.
Radio
channels can be reused provided the separation between cells containing the
same channel set is far enough apart so that co-channel interference can be kept
below acceptable levels most of the time. Cells using the same channel set
are called co-channel cells.
|
Cell clustering
|
The figure on
the opposite page shows an example. Within the service area (PLMN), specific
channel sets are reused at a different location (another cell). In the
example, there are 7 channel sets: A through G. Neighboring cells are not
allowed to use the same frequencies. For this reason all channel sets are
used in a cluster of neighboring cells. As there are 7 channel sets, the PLMN
can be divided into clusters of 7 cells each. The figure shows three
clusters.
The number of
channel sets is called K. K is also called the reuse factor. In the figure,
K=7. Valid values of K can be found using equation (where i and j are
integers):
K=i²+j²+I*j
Explaining
this equation is beyond the scope of this course. Some constraints to K are
provided later in this chapter. Note that in the example: Cells are shaped
ideally (hexagons). The distance between cells using the same channel set is
always the same.
|
Other cell clusters
|
The figure on the opposite page shows some examples of
possible clusters. The more cells in a cluster, the greater the separation
between co-channel cells when Other
clusters are deployed. The idea is to keep co-channel cell separation the
same throughout the system area for cells of the same size. Some valid
cluster sizes that allow this are: 1, 3, 4, 7, 9 and 12.
|
Procedure for locating co-channel
cells
|
It is always possible to find cells using the same channel
set, if only the value of K is known. The following procedure is used.
In the figure on the opposite page an example is shown
with K = 19.
|
Signal attenuation With distance
|
Frequencies can be reused throughout a service area
because radio signals typically attenuate with distance to the base station
(or mobile station). When the distance between cells using the same
frequencies becomes too small, co-channel
Interference might occur and lead to service interruption
or unacceptable quality of service.
|
Step
|
Action
|
1
|
Use the integer values i and j from the equation, and
start
With
the upper left cell. Through this cell, draw the j-axis.
|
2
|
Draw
the i-axis. To find the starting point for the i-axis, count j cells down the
j-axis. In the example, one has to count 2 cells down (j=2). The positive
direction of the i-axis is always two cell faces (120 degrees) relative to
the positive direction of the j-axis.
|
3
|
Find
the first co-channel cell. It is found by counting i cells in the positive i-axis
direction. In the example, i = 3.
|
4
|
Find
the other co-locating cells by repeating the previous steps. The
Starting
point is again at the upper left cell, but now choose another
Direction
for the j-axis (e.g. rotate the j-axis with 60 degrees, which is one cell
face). As each cell has 6 faces, one will find 6 co-channel cells around the
starting cells. These are the nearest located co-channel cells.
|
Capacity/Performance Trade-offs :
n If K increases, then performance
increases
n If K increases, then call capacity
decreases per cell
The number of sites to cover a given area with a given high
traffic density, and hence the cost of the infrastructure, is determined
directly by the reuse factor and the number of traffic channels that can be
extracted from the available spectrum. These two factors are compounded in what
is called spectral efficiency of the system. Not all systems allow the same
performance in this domain: they depend in particular on the robustness of the
radio transmission scheme against interference, but also on the use of a number
of technical tricks, such as reducing transmission during the silences of a
speech communication. The spectral efficiency, together with the constraints on
the cell size, determines also the possible compromises between the capacity
and the cost of the infrastructure. All this explains the importance given to
spectral efficiency.
Many technical tricks to improve spectral efficiency were
conceived during the system design and have been introduced in GSM. They
increase the complexity, but this is balanced by the economical advantages of a
better efficiency. The major points are the following:
The control of the transmitted power on the radio path aims
at minimizing the average power broadcast by mobile stations as well as by base
stations, whilst keeping transmission quality above a given threshold. This
reduces the level of interference caused to the other communications;
Frequency hopping improves transmission quality at slow
speeds through frequency diversity, and improves spectral efficiency through
interferer diversity;
Discontinuous transmission, where by transmission is
suppressed when possible, allows a reduction in the interference level of other
communications. Depending on the type of user information transmitted, it is
possible to derive the need for effective transmission. In the case of speech,
the mechanism called VAD (Voice Activity Detection) allows transmission
requirements to be reduced by an important factor (typically, reduced by half);
The mobile assisted handover, whereby the mobile station
provides measurements concerning neighboring cells, enables efficient handover
decision algorithms aimed at minimizing the interference generated by the cell
(whilst keeping the transmission quality above some threshold).
References:1. The GSM system for mobile
communication-Michel Mouly & Marie- Bernadette Pautet.
2. GSM system Engineering-Asha Mehrotra (Artech House Publisher).
2 GSM
ARCHITECTURE
INTRODUCTION
A GSM system is basically designed as a combination of three
major subsystems: the network subsystem, the radio subsystem, and the operation
support subsystem. In order to ensure that network operators will have several
sources of cellular infrastructure equipment, GSM decided to specify not only
the air interface, but also the main interfaces that identify different parts.
There are three dominant interfaces, namely, an interface between MSC and the
base Transceiver Station (BTS), and an Um interface between the BTS and MS.
GSM NETWORK STRUCTURE
Every telephone network needs a well-designed structure in
order to route incoming called to the correct exchange and finally to the
called subscriber. In a mobile network, this structure is of great importance
because of the mobility of all its subscribers [1-4]. In the GSM system, the
network is divided into the following partitioned areas.
·
GSM service area;
·
PLMN service area;
·
MSC service area;
·
Location area;
·
Cells.
The GSM service is the total area served by the combination
of all member countries where a mobile can be serviced. The next level is the
PLMN service area. There can be several within a country, based on its size.
The links between a GSM/PLMN network and other PSTN, ISDN, or PLMN network will
be on the level of international or national transit exchange. All incoming
calls for a GSM/PLMN network will be routed to a gateway MSC. A gateway MSC
works as an incoming transit exchange for the GSM/PLMN. In a GSM/PLMN network,
all mobile-terminated calls will be routed to a gateway MSC. Call connections
between PLMNs, or to fixed networks, must be routed through certain designated
MSCs called a gateway MSC. The gateway MSC contains the interworking functions
to make these connections. They also route incoming calls to the proper MSC
within the network. The next level of division is the MSC/VLR service area. In
one PLMN there can be several MSC/VLR service area. MSC/VLR is a role
controller of calls within its jurisdiction. In order to route a call to a
mobile subscriber, the path through links to the MSC in the MSC area where the
subscriber is currently located. The mobile location can be uniquely identified
since the MS is registered in a VLR, which is generally associated with an MSC.
The next division level is that of
the LA’s within a MSC/VLR combination. There are several LA’s within one MSC/VLR
combination. A LA is a part of the MSC/VLR service area in which a MS may move
freely without updating location information to the MSC/VLR exchange that
control the LA. Within a LA a paging message is broadcast in order to find the
called mobile subscriber. The LA can be identified by the system using the
Location Area Identity (LAI). The LA is used by the GSM system to search for a
subscriber in a active state. Lastly, a LA is divided into many cells. A cell
is an identity served by one BTS. The MS distinguishes between cells using the
Base Station Identification code (BSIC) that the cell site broadcast over the
air.
MOBILE STATION
The MS includes radio equipment and
the man machine interface (MMI) that a subscribe needs in order to access the
services provided by the GSM PLMN. MS can be installed in Vehicles or can be
portable or handheld stations. The MS may include provisions for data communication
as well as voice. A mobile transmits and receives message to and from the GSM
system over the air interface to establish and continue connections through the
system .
Different type of MSs can provide
different type of data interfaces. To provide a common model for describing
these different MS configuration, ”reference configuration” for MS, similar to
those defined for ISDN land stations, has been defined.
Each MS is identified by an IMEI that
is permanently stored in the mobile unit. Upon request, the MS sends this
number over the signaling channel to the MSC. The IMEI can be used to identify
mobile units that are reported stolen or operating incorrectly.
Just as the IMEI identities the
mobile equipment, other numbers are used to identity the mobile subscriber.
Different subscriber identities are used in different phases of call setup. The
Mobile Subscriber ISDN Number (MSISDN) is the number that the calling party
dials in order to reach the subscriber. It is used by the land network to route
calls toward an appropriate MSC. The international mobile subscribe identity
(IMSI) is the primary function of the subscriber within the mobile network and
is permanently assigned to him. The GSM system can also assign a Temporary
Mobile Subscriber Identity (TMSI) to identity a mobile. This number can be
periodically changed by the system and protect the subscriber from being
identified by those attempting to monitor the radio channel.
Functions of MS
The primary
functions of MS are to transmit and receive voice and data over the air
interface of the GSM system. MS performs the signal processing function of
digitizing, encoding, error protecting, encrypting, and modulating the
transmitted signals. It also performs the inverse functions on the received signals
from the BS.
In order to transmit voice and data
signals, the mobile must be in synchronization with the system so that the
messages are the transmitted and received by the mobile at the correct instant.
To achieve this, the MS automatically tunes and synchronizes to the frequency
and TDMA timeslot specified by the BSC. This message is received over a
dedicated timeslot several times within a multiframe period of 51 frames. We
shall discuss the details of this in the next chapter. The exact synchronization
will also include adjusting the timing advance to compensate for varying
distance of the mobile from the BTS.
The MS monitors the power level and
signal quality, determined by the BER for known receiver bit sequences
(synchronization sequence), from both its current BTS and up to six surrounding
BTSs. This data is received on the downlink broadcast control channel. The MS
determines and send to the current BTS a list of the six best-received BTS
signals. The measurement results from MS on downlink quality and surrounding
BTS signal levels are sent to BSC and processed within the BSC. The system then
uses this list for best cell handover decisions.
MS keeps the GSM network informed of
its location during both national and international roaming, even when it is
inactive. This enables the System to page in its present LA.
The MS includes an equalizer that
compensates for multi-path distortion on the received signal. This reduces
inter-symbol interface that would otherwise degrade the BER.
Finally, the MS can store and display
short received alphanumeric messages on the liquid crystal display (LCD) that
is used to show call dialing and status information. These messages are limited
to 160 characters in length.
Power Levels
These are five different categories of
mobile telephone units specified by the European GSM system: 20W, 8W, 5W, 2W,
and 0.8W. These correspond to 43-dBm, 39-dBm, 37-dBm, 33-dBm, and 29-dBm power
levels. The 20-W and 8-W units (peak power) are either for vehicle-mounted or
portable station use.
The MS power is adjustable in 2-dB
steps from its nominal value down to 20mW (13 dBm). This is done automatically
under remote control from the BTS, which monitors the received power and
adjusts the MS transmitter to the minimum power setting necessary for reliable
transmission.
SIM Card
As described in the first chapter, GSM subscribers are
provided with a SIM card with its unique identification at the very beginning
of the service. By divorcing the subscriber ID from the equipment ID, the subscriber may never own the GSM
mobile equipment set. The subscriber is identified in the system when he
inserts the SIM card in the mobile equipment. This provides an enormous amount
of flexibility to the subscribers since they can now use any GSM-specified mobile
equipment. Thus with a SIM card the idea of “Personalize” the equipment
currently in use and the respective information used by the network (location
information) needs to be updated. The smart card SIM is portable between Mobile
Equipment (ME) units. The user only needs to take his smart card on a trip. He
can then rent a ME unit at the destination, even in another country, and insert
his own SIM. Any calls he makes will be charged to his home GSM account. Also,
the GSM system will be able to reach him at the ME unit he is currently using.
The SIM is a removable SC, the size
of a credit card, and contains an integrated circuit chip with a
microprocessor, random access memory (RAM), and read only memory (ROM). It is
inserted in the MS unit by the subscriber when he or she wants to use the MS to
make or receive a call. As stated, a SIM also comes in a modular from that can
be mounted in the subscriber’s equipment.
When a mobile subscriber wants to use
the system, he or she mounts their SIM card and provide their Personal
Identification Number(PIN), which is compared with a PIN stored within the SIM.
If the user enters three incorrect PIN codes, the SIM is disabled. The PIN can
also be permanently bypassed by the service provider if requested by the subscriber.
Disabling the PIN code simplifies the call setup but reduces the protection of
the user’s account in the event of a stolen SIM.
International
Mobile Subscriber Identity.
An IMSI is assigned to each authorized GSM user. It consists
of a mobile country code (MSC), mobile network code (MNC), and a PLMN unique
mobile subscriber identification number (MSIN). The IMSI is not
hardware-specific. Instead, it is maintained on a SC by an authorized
subscriber and is the only absolute identity that a subscriber has within the
GSM system. The IMSI consists of the MCC followed by the NMSI and shall not
exceed 15 digits.
Temporary Mobile Subscriber Identity
A TMSI is a MSC-VLR specific alias that is designed to
maintain user confidentiality. It is assigned only after successful subscriber
authentication. The correlation of a TMSI to an IMSI only occurs during a
mobile subscriber’s initial transaction with an MSC (for example, location
updating). Under certain condition (such as traffic system disruption and
malfunctioning of the system), the MSC can direct individual TMSIs to provide
the MSC with their IMSI.
Mobile Station ISDN Number
The MS international number must be dialed after the
international prefix in order to obtain a mobile subscriber in another country.
The MSISDN numbers is composed of the country code (CC) followed by the
National Significant Number (N(S)N), which shall not exceed 15 digits.
The Mobile Station Roaming Number (MSRN)
The MSRN is allocated on temporary basis when the MS roams
into another numbering area. The MSRN number is used by the HLR for rerouting
calls to the MS. It is assigned upon demand by the HLR on a per-call basis. The
MSRN for PSTN/ISDN routing shall have the same structure as international ISDN
numbers in the area in which the MSRN is allocated. The HLR knows in what
MSC/VLR service area the subscriber is located. At the reception of the MSRN,
HLR sends it to the GMSC, which can now route the call to the MSC/VLR exchange
where the called subscriber is currently registered.
International Mobile Equipment Identity
The IMEI is the unique identity of the equipment used by a
subscriber by each PLMN and is used to determine authorized (white),
unauthorized (black), and malfunctioning (gray) GSM hardware. In conjunction
with the IMSI, it is used to ensure that only authorized usera are granted
access to the system. An IMEI is never sent in cipher mode by MS.
BASE STATION SYSTEM
The BSS is a set of BS equipment (such as transceivers and
controllers) that is in view by the MSC through a single A interface as being
the entity responsible for communicating with MSs in a certain area. The radio
equipment of a BSS may be composed of one or more cells. A BSS may consist of
one or more BS. The interface between BSC and BTS is designed as an A-bis
interface. The BSS includes two types of machines: the BTS in contact with the
MSs through the radio interface and the BSC, the latter being in contact with
the MSC. The function split is basically between transmission equipment, the
BTS, and managing equipment at the BSC. A BTS compares radio transmission and
reception devices, up to and including the antennas, and also all the signal
processing specific to the radio interface. A single transceiver within BTS
supports eight basic radio channels of the same TDM frame. A BSC is a network
component in the PLMN that function for control of one or more BTS. It is a
functional entity that handles common control functions within a BTS.
A BTS is a network component that serves one cell and is
controlled by a BSC. BTS is typically able to handle three to five radio
carries, carrying between 24 and 40 simultaneous communication. Reducing the
BTS volume is important to keeping down the cost of the cell sites.
An important component of the BSS that is considered in the
GSM architecture as a part of the BTS is the Transcoder/Rate Adapter Unit
(TRAU). The TRAU is the equipment in which coding and decoding is carried out
as well as rate adoption in case of data. Although the specifications consider
the TRAU as a subpart of the BTS, it can be sited away from the BTS (at MSC),
and even between the BSC and the MSC.
The interface between the MSC and the BSS is a standardized
SS7 interface (A-interface) that, as stated before, is fully defined in the GSM
recommendations. This allows the system operator to purchase switching
equipment from one supplier and radio equipment and the controller from
another. The interface between the BSC and a remote BTS likewise is a standard
the A-bis. In splitting the BSS functions between BTS and BSC, the main
principle was that only such functions that had to reside close to the radio
transmitters/receivers should be placed in BTS. This will also help reduce the
complexity of the BTS.
Functions
of BTS
As stated, the primary responsibility of the BTS is to
transmit and receive radio signals from a mobile unit over an air interface. To
perform this function completely, the signals are encoded, encrypted,
multiplexed, modulated, and then fed to the antenna system at the cell site.
Trans-coding to bring 13-kbps speech to a standard data rate of 16 kbps and
then combining four of these signals to 64 kbps is essentially a part of BTS,
though, it can be done at BSC or at MSC. The voice communication can be either
at a full or half rate over logical speech channel. In order to keep the mobile
synchronized, BTS transmits frequency and time synchronization signals over
frequency correction channel (FCCH and BCCH logical channels. The received
signal from the mobile is decoded, decrypted, and equalized for channel
impairments.
Random access detection is made by
BTS, which then sends the message to BSC. The channel subsequent assignment is
made by BSC. Timing advance is determined by BTS. BTS signals the mobile for
proper timing adjustment. Uplink radio channel measurement corresponding to the
downlink measurements made by MS has to be made by BTS.
BTS-BSC Configurations
There are several BTS-BSC configurations: single site;
single cell; single site; multicell; and multisite, multicell. These
configurations are chosen based on the rular or urban application. These
configurations make the GSM system economical since the operation has options
to adapt the best layout based on the traffic requirement. Thus, in some sense,
system optimization is possible by the proper choice of the configuration.
These include omni directional rural configuration where the BSC and BTS are on
the same site; chain and multidrop loop configuration in which several BTSs are
controlled by a single remote BSC with a chain or ring connection topology;
rural star configuration in which several BTSs are connected by individual
lines to the same BSC; and sectorized urban configuration in which three BTSs
share the same site amd are controlled by either a collocated or remote BSC.
In rural areas, most BSs are installed to provide maximum
coverage rather then maximum capacity.
Transcoder
Depending on the relative costs of a transmission plant for
a particular cellular operator, there may be some benefit, for larger cells and
certain network topologies, in having the transcoder either at the BTS, BSC or
MSC location. If the trascoder is located at MSC, they are still considered
functionally a part of the BSS. This approach allows for the maximum of
flexibility and innovation in optimizing the transmission between MSC and BTS.
The transcoder is the device that takes 13-Kbps speech or
3.6/6/12-Kbps data multiplexes and four of them to convert into standard
64-Kbps data. First, the 13 Kbps or the data at 3.6/6/12 Kbps are brought up to
the level of 16 Kpbs by inserting additional synchronizing data to make up the
difference between a 13-Kbps speech or lower rate data, and then four of them
are combined in the transcoder to provide 64 Kpbs channel within the BSS. Four
traffic channel can then be multiplexed on one 64-Kpbs circuit. Thus, the TRAU
output data rate is 64 Kpbs. Then, up to 30 such 64-Kpbs channels are
multiplexed onto a 2.048 Mpbs if a CEPT1 channel is provided on the A-bis
interface. This channel can carry up to 120-(16x 120) traffic and control
signals. Since the data rate to the PSTN is normally at 2 Mbps, which is the
result of combining 30-Kbps by 64-Kbph channels, or 120- Kbps by 16-Kpbs
channels.
BSC
The BSC, as discussed, is connected to the MSC on one side
and to the BTS on the other. The BSC performs the Radio Resource (RR)
management for the cells under its control. It assigns and release frequencies
and timeslots for all MSs in its own area. The BSC performs the intercell
handover for MSs moving between BTS in its control. It also reallocates
frequencies to the BTSs in its area to meet locally heavy demands during peak
hours or on special events. The BSC controls the power transmission of both
BSSs and MSs in its area. The minimum power level for a mobile unit is
broadcast over the BCCH. The BSC provides the time and frequency
synchronization reference signals broadcast by its BTSs. The BSC also measures
the time delay of received MS signals relative to the BTS clock. If the
received MS signal is not centered in its assigned timeslot at the BTS, The BSC
can direct the BTS to notify the MS to advance the timing such that proper
synchronization takes place. The functions of BSC are as follows.
The BSC may
also perform traffic concentration to reduce the number of transmission lines
from the BSC to its BTSs, as discussed in the last section.
SWITCHING SUBSYSTEMS: MOBILE
SWITCHING CENTER AND GATEWAY SWITCHING CENTER
The network and the switching subsystem together include the
main switching functions of GSM as well as the databases needed for subscriber
data and mobility management (VLR). The main role of the MSC is to manage the
communications between the GSM users and other telecommunication network users.
The basic switching function of performed by the MSC, whose main function is to
coordinate setting up calls to and from GSM users. The MSC has interface with
the BSS on one side (through which MSC VLR is in contact with GSM users) and
the external networks on the other (ISDN/PSTN/PSPDN). The main difference
between a MSC and an exchange in a fixed network is that the MSC has to take
into account the impact of the allocation of RRs and the mobile nature of the
subscribers and has to perform, in addition, at least, activities required for
the location registration and handover.
The MSC is a telephony switch that performs all the
switching functions for MSs located in a geographical area as the MSC area. The
MSC must also handle different types of numbers and identities related to the
same MS and contained in different registers: IMSI, TMSI,ISDN number, and MSRN.
In general identities are used in the interface between the MSC and the MS,
while numbers are used in the fixed part of the network, such as, for routing.
Functions of MSC
As stated, the main function of
the MSC is to coordinate the set up of calls between GSM mobile and PSTN users.
Specifically, it performs functions such as paging, resource allocation,
location registration, and encryption.
Specifically, the call-handling function of paging is
controlled by MSC. MSC coordinates the set up of call to and from all GSM
subscribers operating in its areas. The dynamics allocation of access resources
is done in coordination with the BSS. More specifically, the MSC decides when
and which types of channels should be assigned to which MS. The channel
identity and related radio parameters are the responsibility of the BSS, The
MSC provides the control of interworking with different networks. It is
transparent for the subscriber authentication procedure. The MSC supervises the
connection transfer between different BSSs for MSs, with an active call, moving
from one call to another. This is ensured if the two BSSs are connected to the
same MSC but also when they are not . In this latter case the procedure is more
complex, since more then one MSC in involved. The MSC performs billing on calls
for all subscribers based in its areas. When the subscriber is roaming
elsewhere, the MSC obtains data for the call billing from the visited MSC.
Encryption parameters transfers from VLR to BSS to facilitate ciphering on the
radio interface are done by MSC. The exchange of signaling information on the various interface toward the other network elements
and the management of the interface themselves are all controlled by the MSC.
Finally, the MSC serves as a SMS gateway to forward SMS messages from Short
Message Service Centers (SMSC) to the subscribers and from the subscribers to
the SMSCs. It thus acts as a message mailbox and delivery system.
VLR
The VLR is collocated with an MSC. A MS roaming in an MSC
area is controlled by the VLR responsible for that area. When a MS appears in a
LA, it starts a registration procedure. The MSC for that area notices this
registration and transfers to the VLR the identify of the LA where the MS is
situated. A VLR may be in charge of one or several MSC LA’s. The VLR
constitutes the databases that support the MSC in the storage and retrieval of
the data of subscribers present in its area. When an MS enters the MSC area
borders, it signals its arrival to the MSC that stores its identify in the VLR.
The information necessary to manage the MS is contained in the HLR and is
transferred to the VLR so that they can be easily retrieved if so required.
Data Stored in VLR
The data contained in the VLR and in the HLR are more or
less the same. Nevertheless the data are present in the VLR only as long as the
MS is registered in the area related to that VLR. Data associated with the
movement of mobile are IMSI, MSISDN, MSRN, and TMSI. The terms permanent and
temporary, in this case, are meaningful only during that time interval. Some
data are mandatory, others are optional.
HOME
LOCATION REGISTER
The HLR is a database that permanently stores data related
to a given set of subscribers. The HLR is the reference database for subscriber
parameters. Various identification numbers and addresses as well as
authentication parameters, services subscribed, and special routing information
are stored. Current subscriber status including a subscriber’s temporary
roaming number and associated VLR if the mobile is roaming, are maintained.
The HLR provides data needed to route calls to all MS-SIMs
home based in its MSC area, even when they are roaming out of area or in other
GSM networks. The HLR provides the current location data needed to support
searching for and paging the MS-SIM for incoming calls, wherever the MS-SIM may
be. The HLR is responsible for storage and provision of SIM authentication and
encryption parameters needed by the MSC where the MS-SIM is operating. It
obtains these parameters from the AUC.
The HLR maintains record of which supplementary service each
user has subscribed to and provides permission control in granting services.
The HLR stores the identification of SMS gateways that have messages for the
subscriber under the SMS until they can be transmitted to the subscriber and
receipt is knowledge.
Some data are mandatory, other data are optional. Both the
HLR and the VLR can be implemented in the same equipment in an MSC
(collocated). A PLMN may contain one or several HLRs.
AUTHENTICATION CENTER
The AUC stores information that is necessary to protect
communication through the air interface against intrusions, to which the mobile
is vulnerable. The legitimacy of the subscriber is established through
authentication and ciphering, which protects the user information against
unwanted disclosure. Authentication information and ciphering keys are stored
in a database within the AUC, which protects the user information against
unwanted disclosure and access.
In the authentication procedure, the key Ki is never
transmitted to the mobile over the air path, only a random number is sent. In
order to gain access to the system, the mobile must provide the correct Signed
Response (SRES) in answer to a random number (RAND )
generated by AUC.
Also, Ki and the cipher key Kc are never transmitted across
the air interface between the BTS and the MS. Only the random challenge and the
calculated response are transmitted. Thus, the value of Ki and Kc are kept
secure. The cipher key, on the other hand, is transmitted on the SS7 link
between the home HLR/AUC and the visited MSC, which is a point of potential
vulnerability. On the other hand, the random number and cipher key is supposed
to change with each phone call, so finding them on one call will not benefit
using them on the next call.
The HLR is also responsible for the “authentication” of the
subscriber each time he makes or receives a call. The AUC, which actually
performs this function, is a separate GSM entity that will often be physically
included with the HLR. Being separate, it will use separate processing
equipment for the AUC database functions.
EQUIPMENT IDENTIFY REGISTER
EIR is a database that stores the IMEI numbers for all
registered ME units. The IMEI uniquely identifies all registered ME. There is generally
one EIR per PLMN. It interfaces to the various HLR in the PLMN. The EIR keeps
track of all ME units in the PLMN. It maintains various lists of message. The
database stores the ME identification and has nothing do with subscriber who is
receiving or originating call. There are three classes of ME that are stored in
the database, and each group has different characteristics.
·
White List: contains those IMEIs that are known
to have been assigned to valid MS’s. This is the category of genuine equipment.
·
Black List: contains IMEIs of mobiles that have
been reported stolen.
·
Gray List: contains IMEIs of mobiles that have
problems (for example, faulty software, wrong make of the equipment). This list
contains all MEs with faults not important enough for barring.
INTERWORKING
FUNCTION
·
GSM provided a wide range of data services to
its subscribers. The GSM system interface with the various forms of public and
private data networks currently available. It is the job of the IWF to provide
this interfacing capability.
The IWF, which in essence is a part of MSC, provides the
subscriber with access to data rate and protocol conversion facilities so that
data can be transmitted between GSM Data Terminal Equipment (DTE) and a
land-line DTE.
ECHO CANCELER
EC is used on the PSTN side of the MSC for all voice
circuits. The EC is required at the MSC PSTN interface to reduce the effect of
GSM delay when the mobile is connected to the PSTN circuit. The total
round-trip delay introduced by the GSM system, which is the result of speech
encoding, decoding and signal processing, is of the order of 180 ms. Normally
this delay would not be an annoying factor to the mobile, except when
communicating to PSTN as it requires a two-wire to four-wire hybrid transformer
in the circuit. This hybrid is required at the local switching office because
the standard local loop is a two-wire circuit. Due to the presence of this
hybrid, some of the energy at its
four-wire receive side from the mobile is coupled to the four-wire transmit
side and thus retransmitted to the mobile. This causes the echo, which does not
effect the land subscriber but is an annoying factor to the mobile. The
standard EC cancels about 70 ms of delay.
During a normal PSTN (land-to-land call), no echo is
apparent because the delay is too short and the land user is unable to
distinguish between the echo and the normal telephone “side tones” However,
with the GSM round-trip delay added and without the EC, the effect would be
irritating to the MS subscriber.
OPERATION
AND MAINTENANCE CENTER
The
OMC provides alarm-handling functions to report and log alarms generated by the
other network entities. The maintenance personnel at the OMC can define that
criticality of the alarm. Maintenance cover both technical and administrative
actions to maintain and correct the system operation, or to restore normal
operations after a breakdown, in the shortest possible time.
The fault management functions of the OMC allow network
devices to be manually or automatically removed from or restored to service.
The status of network devices can be checked, and tests and diagnostics on
various devices can be invoked. For example, diagnostics may be initiated
remotely by the OMC. A mobile call trace facility can also be invoked. The
performance management functions included collecting traffic statistics from
the GSM network entities and archiving them in disk files or displaying them
for analysis. Because a potential to collect large amounts of data exists,
maintenance personal can select which of the detailed statistics to be
collected based on personal interests and past experience. As a result of
performance analysis, if necessary, an alarm can be set remotely.
The OMC provides system change control for the software
revisions and configuration data bases in the network entities or uploaded to
the OMC. The OMC also keeps track of the different software versions running on
different subsystem of the GSM.
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