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Telecom Tutorials
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STANDARDIZATION & ARCHITECTURE
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 The GSM standard was developed by the Groupe
SpecialMobile, which was an initiative of the Conference of
European Post and Telecommunications (CEPT)
administrations.
 The responsibility for GSM standardization now resides with
the
Special Mobile Group (SMG) under the European
Telecommunication Standard Institute (ETSI).

 Fully digital system utilizing the 900MHz frequency band.
 TDMA over radiocarriers(200 kHz carrier spacing)
 8 full rate or 16 half rate TDMA channels per carrier
 User/terminal authentication for fraud control
 Encryption of speech and data transmissions over the radio
path
 Full international roaming capability
 Low speed data services (upto 9.6kb/s)
 Compatibility with ISDN for supplementary services
 Support of short message services(SMS)

 GSM supports a range of basic and supplementary services,
and these services are defined analogous to those for
ISDN(i.e.,bearer services, teleservices, and supplementary
services).
 The most important service supported by GSM is Telephony.
 Other services derived from telephony included in the GSM
specification are emergency calling and voice messaging.
 Bearer services supported in GSM include various
asynchronous and synchronous data services for information
transfer.
 Teleservices based on these bearer services include group 3
fax and short message service(SMS)
 The data capabilities of GSM have now been enhanced to
include high speed circiut-switched data(HSCSD) and general
packet radio service (GPRS).

 Call offering services  call forwarding
 Call resrtiction services  call barring
 Call waiting service
 Call hold service
 Multi party service  tele conferencing
 Calling line presentation restriction services
 Advice of charge service
 Closed user group service

 The GSM System comprises of  Base Transceiver Station
(BTS), Base Station Controllers (BSC), Mobile Switching
Centers (MSC), and set of registers (databases) to assist in
mobility management and security functions.
 All signaling between the MSC and the various registers
(databases) as well as between the MSCs takes place using
the Signaling System 7(SS7) network, with the application
level messages using the Mobile Application Protocol (MAP)
designed specifically for GSM.
 The MAP protocol utilizes the lower layer functions from the
SS7 protocol stack.

HLR/ACVLR
VLR
EIR
PSTN
MSC MSC
BSC
D
F
CB
G
E
Abis
BTS
BTS
BTS
A

 The GSM mobile stations are portable radiotelephony units
that can be used on any GSM sysytem as vehicular and
handheld terminals.
 Power levels supported by the GSM mobile station currently
range from 0.8 to 8.0 W ,and power saving techniques are
used on the air interface to extend battery life.
 At the time of manufacture, an international mobile
equipment identity (IMEI),is programmed into the terminal.
 A subscriber identity module (SIM) is required to activate and
operate a GSM terminal. The SIM may be contained within the
MS , or it may be a removable unit.
 The international mobile subscriber identity (IMSI) is
programmed into the SIM at the time of service provisioning,
along with the appropriate security parameters and
algorithms.

 The base station system comprises a base station controller
(BSC) and one or more subtending base transceiver stations
(BTS).
 The BSS is responsible for all functions related to the radio
resource (channel) management.
 Range of functions performed by the BSS 
 Radio resource control
 Configuration of radio channels
 Selection, allocation and deallocation of radio channels
 Monitoring of radio channel busy/idle status
 Encryption of radio interface
 Frequency hopping and power control
 Assignment of frequency-hop sequence and start time
 Assignment of effective radiated power (ERP) values to mobile
stations

 Handoff management
 Collect signal quality data from adjacent BSSs
 Analyze signal quality data and determine handoff need
 Keep MSC informed regarding handoff activity
 Digital signal processing
 Transcoding and rate adaption
 Channel coding and decoding

 The Mobile Switching Centre or MSC is a sophisticated
telephone exchange which provides circuit-switched calling,
mobility management, and GSM services to the mobile
phones roaming within the area that it serves. This means
voice, data and fax services, as well as SMS and call divert.
 There are various different names for MSCs in different
contexts which reflects their complex role in the network, all
of these terms though could refer to the same MSC, but doing
different things at different times.
 A Gateway MSC is the MSC that determines which visited MSC
the subscriber who is being called is currently located. It also
interfaces with the Public Switched Telephone Network. All
mobile to mobile calls and PSTN to mobile calls are routed
through a GMSC.

 The Visited 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

 The Home Location Register or HLR is a central database that
contains details of each mobile phone subscriber that is
authorized to use the GSM core network.
 There is one HLR in one Public Land Mobile Network. HLR is a
single database but can be maintained as separate databases
when the data to be stored is more than the capacity.
 More precisely, the HLR stores details of every SIM card
issued by the mobile phone operator. Each SIM has a unique
identifier called an IMSI which is one of the primary keys to
each HLR record.
 The next important items of data associated with the SIM are
the telephone numbers used to make and receive calls to the
mobile phone, known as MSISDNs. The main MSISDN 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.

 Examples of other data stored in the HLR in a SIM record is:
 GSM services that the subscriber has requested or been given
 GPRS settings to allow the subscriber to access packet
services
 Current Location of subscriber (VLR and SGSN)
 Call divert settings applicable for each associated MSISDN.
 The HLR data is stored for as long as a subscriber remains
with the mobile phone operator.
 The HLR is a system which directly receives and processes
MAP transactions and messages. If the HLR fails, then the
mobile network is effectively disabled as it is the HLR which
manages the Location Updates as mobile phones roam
around.

 Visitor Location Register (VLR) is a database - part of the GSM
mobile phone system - which stores information about all the
mobiles that are currently under the jurisdiction of the MSC
(Mobile Switching Center) which it serves.
 Of all the information it stores about each MS (Mobile
Station), the most important is the current LAI (Location Area
Identity) which identifies under which BSC (Base Station
Controller) the MS is currently present. This information is
vital in the call setup process.
 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.

 When mobile equipment is stolen or lost, the operator or
owner will typically contact the Central Equipment Identity
Register (CEIR) which blacklists the device in all operator
switches so that it will in effect become unusable, making
theft of mobile equipment a useless business.
 The IMEI number is not supposed to be easy to change,
making the CEIR blacklisting effective. However this is not
always the case: IMEI may be easy to change with special
tools and operators may even flatly ignore the CEIR blacklist.

 The Authentication Centre or 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).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.
 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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