The document provides an overview of GSM (Global System for Mobile Communication) technology. It describes how GSM was originally developed to provide roaming cellular voice services across Europe compatible with other networks. It now has over 5 billion subscribers worldwide. The document outlines the key components of GSM including the mobile station, base station, switching center, and databases. It also describes the radio subsystem using frequency division duplex and time division multiple access and the network switching subsystem responsible for mobility management and switching.

1. Chapter 3
Telecommunication Systems/
Cellular Networks

2. GSM Overview
 Formerly: Groupe Spéciale Mobile (founded 1982)
 Now: Global System for Mobile Communication
 Goal : was to provide a mobile phone system that allows
users to roam throughout Europe and provides voice
services compatible to ISDN(Integrated Services
Digital Network) and other PSTN( public switch
telephone network) systems.
 Today many providers all over the world use GSM
(219 countries in Asia, Africa, Europe, Australia, America)
 more than 5 billion subscribers in more than 800 networks
 more than 80% of all digital mobile phones use GSM

3. GSM Technologies
 Today more people use mobile phone
system than the fixed telephones .
 A 2G cellular network (a digital network for voice
communication)
 Circuit switching for voice (mainly)/data (limited)
transmission rate
 Connection-oriented service: establish a communication
path (channel) for point-to-point communication
 Multiplexing
 Frequency division multiplexing (FDM) plus Time division
multiplexing (TDM) (adding to Space division multiplexing,
SDM)
 Uses 124 pair of channels per cell, each channel can
support 8 users through TDM (992 users max actually 500
users)
 Some channels are used for control signals, etc

4. GSM TECHNOLOGY.
 GSM implements SDMA using cells with BTS and assigns an MS to a BTS.
 FDD(Frequency Division Duplex) is used to separate downlink and uplink
 Media access combines TDMA and FDMA.
 In GSM 900, 124 channels, each 200 kHz wide, are used for FDMA
 Due to technical reasons, channels 1 and 124 are not used for transmission
in GSM 900.
 Typically, 32 channels are reserved for organizational data; the remaining
90 are used for customers.
 Each of the 248 channels is additionally separated in time via a GSM
TDMA frame, each 200 kHz carrier is subdivided into frames that are
repeated continuously. The duration of a frame is 4.615 ms.
 A frame is again subdivided into 8 GSM time slots, where each slot
represents a physical TDM channel and lasts for 577 μs. Each TDM channel
occupies the 200 kHz carrier for 577 μs every 4.615 ms.

5. GSM - TDMA/FDMA
1 2 3 4 5 6 7 8
higher GSM frame structures
935-960 MHz
124 channels (200 kHz)
downlink
890-915 MHz
124 channels (200 kHz)
uplink
time
GSM TDMA frame
GSM time-slot (normal burst)
4.615 ms
546.5 µs
577 µs
tail user data Training
S
guard
space S user data tail
guard
space
3 bits 57 bits 26 bits 57 bits
1 1 3

6. GSM Technologies..
 Mobility management
 Two-tier architecture: HLR and VLR
 Location area for location update together with paging for
searching
 Services
 Mainly for voice communication
 Data communication is very limited (i.e., 9.6kbit/s) and
supporting SMS

7. Performance Characteristics of GSM
Comparing with 1G cellular network
 Communication
 Mobile communication for voice and data services
 Total mobility
 International access, chip-card enables use of access points of
different providers (roaming services)
 Worldwide connectivity
 One number, the network handles localization and interoperability
 High capacity
 Better frequency efficiency (frequency reuses), smaller cells, more
customers per cell
 High transmission quality
 High audio quality and reliability for wireless, uninterrupted phone
calls at higher speeds (e.g., from cars, trains)
 Security functions
 encryption, authentication via chip-card and PIN

8. GSM: Mobile Services
GSM offers
several types of connections
 voice connections, data connections, short
message service
 voice communication services
 Basic telephony
 Emergency number
 common number throughout Europe (112);
mandatory for all service providers; free of charge;
connection with the highest priority (preemption of
other connections possible)
 Voice mailbox

9. GSM: Mobile Services…
 Non-Voice-services
 electronic mail ( Message Handling
System, implemented in the fixed network)
 Short Message Service (SMS)
alphanumeric data transmission to/from the
mobile terminal (160 characters) using the
signaling channel, thus allowing
simultaneous use of basic services and
SMS(almost ignored in the beginning now
the most successful add-on!)

10. GSM: Mobile Services….
 Supplementary service :May differ between
different service providers, countries and
protocol versions
 Caller identification: CallerID
 forwarding of caller number:callDiverting
 automatic call-back
 conferencing with up to 7 participants
 locking of the mobile terminal (incoming or
outgoing calls)

11. Architecture of the GSM System
 GSM is a PLMN (Public Land Mobile Network)
 Main components
 MS (mobile station)
 BS (base station)
 MSC (mobile switching center)
 LR (location register)
 Subsystems
 RSS (radio subsystem): covers all radio aspects
 NSS (network and switching subsystem): call forwarding, handover,
switching
 OSS (operation subsystem): management of the network

12. Components
Mobile
Station
Base
Transceiver
Station
switching
center
Databas
e
Manageme
nt

13. GSM: overview
fixed network
BSC
BSC
MSC MSC
GMSC
OMC, EIR,
AUC
VLR
HLR
NSS
with OSS
RSS
VLR

14. GSM: elements and interfaces

15. Um
Abis
A
BSS
radio
subsystem
MS MS
BTS
BSC
BTS
BTS
BSC
BTS
network and
switching subsystem
MSC
MSC
fixed
partner networks
IWF
ISDN
PSTN: public switched
telephone network
PSPDN
CSPDN
SS7
EIR
HLR
VLR
ISDN: Integrated Services
Digital Network
PSTN
GSM: system architecture

16. System architecture: radio subsystem
 Components
 MS (Mobile Station)
 BSS (Base Station Subsystem):
consisting of
 BTS (Base Transceiver Station):
sender and receiver
 BSC (Base Station Controller):
controlling several transceivers
 Interfaces
 Um : radio interface
 Abis : standardized, open interface
with 16 kbit/s user channels
 A: standardized, open interface with
64 kbit/s user channels
Um
Abis
A
BSS
radio
subsystem
network and switching
subsystem
MS MS
BTS
BSC MSC
BTS
BTS
BSC
BTS
MSC

17. Radio subsystem
 The Radio Subsystem (RSS) comprises the
cellular mobile network up to the switching
centers
 Components
 Base Station Subsystem (BSS):
 Base Transceiver Station (BTS):
 radio components including sender, receiver, antenna
 if directed antennas are used one BTS can cover several
cells
 Base Station Controller (BSC):
 switching between BTSs, controlling BTSs,
 mapping of radio channels (Um) onto terrestrial channels (A
interface)
 BSS = BSC + sum(BTS)
 Mobile Stations (MS)
 MS=ME(mobile Equipment ) + SIM( subscriber identity module)

18. Base Transceiver Station and Base Station
Controller
 Tasks of a BSS are distributed over BSC and
BTS
 BTS comprises radio specific functions
 BSC is the switching center for radio channels
Functions BTS BSC
Management of radio channels X
Frequency hopping (FH) X X
Management of terrestrial channels X
Mapping of terrestrial onto radio channels X
Channel coding and decoding X
Rate adaptation X
Encryption and decryption X X
Paging X X
Uplink signal measurements X
Traffic measurement X
Authentication X
Location registry, location update X
Handover management X

19. Mobile station
 Terminal for the use of GSM services
 Mobile Equipment (ME)
 represents physical terminals, such as a mobile or
PDA.
 Subscribers identity module (SIM)
 Stores all user specific data(Static)
 i.e Card-type, subscribed service, personal identity
number(PIN) ,PIN unblocking key (PUK), International
mobile subscriber identity(IMSI) ,phone book.
 MS=ME + SIM

20. System architecture: network and
switching subsystem
• Components
• MSC (Mobile Services Switching
Center):
• IWF (Interworking Functions)
• ISDN (Integrated Services Digital
Network)
• PSTN (Public Switched Telephone
Network)
• PSPDN (Packet Switched Public Data
Net.)
• CSPDN (Circuit Switched Public Data
Net.)
•Databases
• HLR (Home Location Register)
• VLR (Visitor Location Register)
• EIR (Equipment Identity Register)
• Protocol
• SS7 (signaling system no. 7)
network
subsystem
MSC
MSC
fixed partner
networks
IWF
ISDN
PSTN
PSPDN
CSPDN
SS7
EIR
HLR
VLR
ISDN
PSTN

21. Network and switching subsystem
 NSS is the main component of the public mobile network
GSM
 Switching, Hand over , mobility management ( localization ),
 Components
 Mobile Services Switching Center (MSC)
 controls all connections via a separated network to/from a mobile
terminal within the domain of the MSC - several BSC can belong to
a MSC
 Databases (important: scalability, high capacity, low delay)
 Home Location Register (HLR)
central master database containing user data, permanent and
semi-permanent data of all subscribers assigned to the HLR (one
provider can have several HLRs)
 Visitor Location Register (VLR)
local database for a subset of user data, including data about all

22. Home Location Register (HLR)
It stores all user-relevant information.
 This comprises static information, such as the mobile
subscriber ISDN number (MSISDN),
 MSISDN = CC + NDC + SN
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number
 MSISDN is the number which is used for dialing
 subscribed services (e.g., call forwarding, roaming
restrictions, GPRS), and the international mobile
subscriber identity (IMSI).
 IMSI is used to identify a subscriber by the operator.

23. Home Location Register (HLR)….
 Dynamic information is also needed, e.g., the current
location area (LA) of the MS, the mobile
subscriber roaming number (MSRN), the current
VLR and MSC.
 As soon as an MS leaves its current LA, the
information in the HLR is updated.
 This information is necessary to localize a user in the
worldwide GSM network.
 All these user-specific information elements only
exist once for each user in a single HLR, which also
supports charging and accounting.

24. Visitor Location Register (VLR)
 If a new MS comes into an LA the VLR is
responsible for, it copies all relevant information for
this user from the HLR.
 This hierarchy of VLR and HLR avoids frequent HLR
updates and long-distance signaling of user
information.

25. Protocol: standard signaling system No. 7 (SS7)
 The standard signaling system No. 7 (SS7) is used
for controlling the signal
 SS7 covers all aspects of control signaling for digital
networks (reliable routing and delivery of control
messages, establishing and monitoring of calls).
 Features of SS7 are number portability, free
phone/toll/collect/credit calls, call forwarding, three-
way calling (caller, receiver & call waiting)etc.

26. Mobile Services Switching Center
 The MSC (mobile services switching center) plays a
central role in GSM
 switching functions ( b/n BSC)
 Hand over functions for mobility support
 management of network resources
 interworking functions via Gateway MSC (GMSC)
 integration of several databases.
 location registration and forwarding of location
information
 provision of new services (fax, data calls)
 support of short message service (SMS)
 generation and forwarding of accounting and billing
information

27. GSM: elements and interfaces
NSS
MS MS
BTS
BSC
GMSC
IWF
OMC
BTS
BSC
MSC MSC
Abis
Um
EIR
HLR
VLR VLR
A
BSS
PDN
ISDN, PSTN
RSS
radio cell
radio cell
MS
AUC
OSS
signaling
O

28. Operation subsystem
 enables centralized management and
maintenance of all GSM subsystems
 Components
 Authentication Center (AUC)
 Used to protect user identity and data transmission.
 authentication parameters and Encryption keys are generated
and stored
 May be situated in special protected part of the HLR
 Equipment Identity Register (EIR)
 registers GSM mobile stations devices
 Contain 3 list
1. Black list(stolen or locked )
2. Gray list( malfunctioning MS)
3. White list (Valid devices): contain all approved type of

29. Black - Mobile Stations (MS) on the Blacklist
are denied access to the network
• Gray - MSs on the Graylist are allowed on
the network, but may be tracked
• White - MSs on the Whitelist are allowed
access to the network
 Operation and Maintenance Center (OMC)
 control and monitor radio subsystem and
the network subsystem entities via the O
interface .
 OMC function are Traffic monitoring ,
status report of the network entities

30. Databases recap
Mobile Switching Center (MSC) is at core; consists
of several databases
 Home location register (HLR) database – stores
information about each subscriber that belongs to
it
 Visitor location register (VLR) database –
maintains information about subscribers currently
physically in the region
 Authentication center database (AuC) – used
for authentication activities, holds encryption keys
 Equipment identity register database (EIR) –
keeps track of the type of equipment that exists at
the mobile station

32. GSM frequency bands
Type Channels Uplink [MHz] Downlink [MHz]
GSM 850 128-251 824-849 869-894
GSM 900
classical
extended
0-124, 955-
1023
124 channels
+49 channels
876-915
890-915
880-915
921-960
935-960
925-960
GSM 1800 512-885 1710-1785 1805-1880
GSM 1900 512-810 1850-1910 1930-1990
GSM-R
exclusive
955-1024, 0-
124
69 channels
876-915
876-880
921-960
921-925
- Please note: frequency ranges may vary depending on the country!
- Channels at the lower/upper edge of a frequency band are typically not used

33. Example coverage of GSM networks (www.gsmworld.com)
T-Mobile (GSM-900/1800) Germany O2 (GSM-1800) Germany
AT&T (GSM-850/1900) USA Vodacom (GSM-900) South Africa

34. ETMTN (GSM-900) Ethiopia

35. Localization and calling
 always to know where a user currently is ,
GSM performs periodic location updates even if
the MS is not in use( as long as it is logged on
to the GSM network ).
 HLR always contains info about the current
location
 VLR of the MSC informs the HLR about the
location change.

36. Localization…
 To locate the MS several numbers are needed.
 1. Mobile subscriber international ISDN number
(MSISDN)
 Its user number which associated with SIM.
 MSISDN= CC+NDC+SN
 Phone number E.g. +251917111213
 country code (CC)……… (+251) Ethiopia
 National Destination code(NDC)…. ( 917 ) Jimma
 Subscriber number (SN)………………..(111213) individual
 2. International mobile subscriber identity (IMSI)
 IMSI for internal unique identification of a subscriber.
 IMSI= MCC+MNC+MSIN
 Used by the network provider operator
 Mobile country code (MCC): to locate country
 Mobile network code (MNC)-code of network provider
 Mobile subscriber identification number (MSIN)

37. Localization…
 3. Temporary mobile subscriber identity (TMSI)
 To hide the IMSI, which would give away the exact identity of the user signaling
over the air interface.
 To hide the IMSI by BSC
 TMSI is selected by the current VLR and is only valid temporarily and
within the location area of the VLR.
 4. Mobile station roaming number (MSRN)
 Temporary address to hide the identity and location of a subscriber
by MSC
 The VLR generates this address on request from the MSC, and the
address is also stored in the HLR. Contain
 Visitor country code (VCC)
 Visitor national destination code(VNDC)
 5. International mobile Equipment identity (IMEI)
 Unique code to each mobile equipment
 device specific theft protection
 Stored in EIR( Equipment identity register )

38. Mobile Terminated Call(MTC)
 1: calling a GSM subscriber
 2: forwarding call to GMSC
 3: signal call setup to HLR
 4, 5: request MSRN from VLR
 6: forward responsible
MSC to GMSC
 7: forward call to
 current MSC
 8, 9: get current status of MS
 10, 11: paging of MS
 12, 13: MS answers
 14, 15: security checks
 16, 17: set up connection
PSTN
calling
station
GMSC
HLR VLR
BSS
BSS
BSS
MSC
MS
1 2
3
4
5
6
7
8 9
10
11 12
13
16
10 10
11 11 11
14 15
17

39. Mobile Originated Call(MOC)
 1, 2: connection request
 3, 4: security check
 5-8: check resources (free circuit)
 9-10: set up call

40. MTC/MOC
BTS
MS
paging request
channel request
immediate assignment
paging response
authentication request
authentication response
ciphering command
ciphering complete
setup
call confirmed
assignment command
assignment complete
alerting
connect
connect acknowledge
data/speech exchange
BTS
MS
channel request
immediate assignment
service request
authentication request
authentication response
ciphering command
ciphering complete
setup
call confirmed
assignment command
assignment complete
alerting
connect
connect acknowledge
data/speech exchange
MTC MOC

41. GSM Roaming
 The ability for a cellular customer to automatically
make and receive voice calls, send and receive data,
or access other services when travelling outside the
geographical coverage area of the home network, by
means of using a visited network.
 Roaming Agreements between network operators
required .
 National Roaming(visited network in the same country as
the home network)
 International Roaming( visited network is outside the home
country)

42. How Roaming Works
 Subscribe for the roaming service
 roaming agreement is needed b/n operators .
 Phone support the radio frequency
 e.g. 850, 900, 1800 and 1900 MHz
 Switch on your phone in the foreign network.
 Note that when roaming you have to pay both for
calls that you make and receive.

43. GSM coverage and network info
ETHIOPIA
 Network Information
 Operator: Ethiopian Telecommunications Corporation
Also known as: ETMTN
 Currently : Ethio telecom
 Technology: GSM
 Frequency: 900
 Launch Date: APR 1999
 Services
 Short Message Service
 Voice mail
 Voice communication

44. Roaming Partners: ETMTN
 A Afghanistan, Albania, Algeria, Angola, Argentina, Armenia, Australia, Austria,
Azerbaijan, B Bahrain, Bangladesh, Belarus, Belgium, Benin, Bosnia Herzegovina,
Botswana, Brazil, Bulgaria, Burkina Faso, Burundi, C Cameroon, Canada, Chad,
China, Comoros, Congo, Cote D Ivoire, Croatia, Cyprus, Czech Republic, D Denmark,
Djibouit E Egypt, Equatorial Guinea, Estonia, F Finland, France, G Gabon, Gambia,
Georgia, Germany, Ghana, Greece, Guinea, Guinea Bissau, H Hong Kong, Hungary, I
Iceland India, Indonesia, Iran, Iraq, Ireland, Israel, Italy, J Jamaica, Japan, Jordan,
K Kazakhstan, Kenya, Korea, Kuwait, L Latvia, Lebanon, Lesotho, Liberia, Libya,
Liechtenstein, Lithuania, Luxembourg, M Madagascar, Malawi, Malaysia, Mali ,Malta
Mauritius, Mexico, Mongolia, Morocco, Mozambique, N Namibia, Netherlands, Niger,
Nigeria, Norway, O Oman, P Pakistan, Panama, Papua New Guinea, Philippines,
Poland, Portugal, Puerto Rico, Q Qatar, R Romania, Russia, Rwanda, S Saudi
Arabia, Senegal, Serbia, Seychelles, Sierra Leone, Singapore, Slovakia, Slovenia, South
Africa, Spain, Sri Lanka, Sudan, Swaziland, Sweden, Switzerland, Syria,T Tanzania,
Thailand, Togo, Trinidad and Tobago, Tunisia, Turkey, Turkmenistan, U Uganda,
Ukraine, United Arab Emirates, United Kingdom, United States, Uruguay, Uzbekistan, Y

45. Hand over
 Hand over is used to provide continuity of services
while a mobile unit moves from one cell to another.
 hand over is necessary due to the limited power of
the MS and BTS
 Reason for hand over
 > Low signal strength ( out of cell range )
 > Load balancing (too high in one cell)

46. Hand over…
 Handoff is the process of changing the channel
(frequency, time slot, spreading code, or combination
of them) associated with the current connection
while a call is in progress
 Hand off parameters (measurements)
 word error indicator( WEI)
 Demodulation error
 Received signal strength indicator (RSSI)
 Quality indicator (QI)
 Quality with respect to interference
 The handoff algorithm will be based on the value of
this parameters .(threshold value )

47. Types of handover
 In general there are 3 types of handovers for cellular
network.
 Mobile controlled hand over (MCHO)
 Mobile unit measures the parameters and make handover
decision by itself .
 Network controlled hand over (NCHO)
 Base station measures the parameters and makes the
handover decision.
 Mobile Assisted hand over (MCHO)
 Mobile unit measures the parameters and sends the values to
the network.
 Then the base station(BS) makes the hand over decision.
 Used by GSM.

48. 4 types of GSM handover
MSC MSC
BSC BSC
BSC
BTS BTS BTS
BTS
MS MS MS MS
1
2 3 4
1. Intra cell HO
2. Inter cell /Intra-BSC HO
3. Inter-BSC/Intra-MSC HO
4. Inter-MSC HO

49. Handover decision
receive level
BTSold
receive level
BTSnew
MS MS
HO_MARGIN
BTSold BTSnew

50. Hand off decisions algorithms
 Using Relative signal strength
 Switch to BTS with better signal
 Using Relative signal strength and threshold
 Switch to BTS with better signal plus threshold

51. Handover procedure
HO access
BTSold BSCnew
measurement
result
BSCold
Link establishment
MSC
MS
measurement
report
HO decision
HO required
BTSnew
HO request
resource allocation
ch. activation
ch. activation ack
HO request ack
HO command
HO command
HO command
HO complete
HO complete
clear command
clear command
clear complete clear complete

52. High-speed circuit-switched data (HSCSD)
 is an enhancement to the original data transmission
mechanism of the GSM system,
 four times faster than GSM, up to 38.4 kbit/s.
 circuit-switched mode (connection oriented).
 Higher speeds are achieved as a result of superior
coding methods(error correction methods), and the ability
to use multiple time slots to increase data throughput.
 Single data is splitting into 8, to send over all slot time.
 One problem of this configuration is that the MS is
required to send and receive at the same time.
 HSCSD exhibits some major disadvantages. It still uses
the connection-oriented mechanisms of GSM.

53. High Speed Circuit Switched Data
(HSCSD)
53

54. General Packet Radio Service
(GPRS)
 2.5G
 is a packet oriented mobile data service.
 usage charging is based on volume of data.
 Use packet switching method.
 provides data rates of 56-114 kbps
 GPRS extends the GSM Packet circuit switched data
capabilities and makes the following services
possible:
 "Always on" internet access
 Multimedia messaging service (MMS)
 Push to talk over cellular (PoC/PTT)
 Instant messaging (IM)

55.  The main benefit for users of GPRS is the ‘always
on’ characteristic – no connection has to be set up
prior to data transfer.

56. Evolution of GSM…
56
 EDGE (Enhanced Data rate for GSM Evolution)
 2.5 G , also known as Enhanced GPRS (EGPRS)
 new modulation scheme, GMSK(Gaussian minimum-shift keying)
& 8PSK ( 8 phase shift keying)
 384 kbps is the maximum data rate
 designed for service providers that may or may not migrate to
UMTS
 UMTS (Universal Mobile Telecommunications
Systems)
 3G
 144kbps for vehicular access
 384 kbps for wide-area coverage
 2 Mbps for local coverage (stationary)
 WCDMA (wideband CDMA)
 Adopted by Europe and Japan

57. Evolution of CDMA…
 CDMA2000 1X
 2.5G
 Use to supports circuit-switched voice
 It also enables the transmission and reception of data at
rates up to 153 kbps in both directions.
 CDMA2000 1xEV-DO (Evolution-Data Optimized)
 3G and broadband Internet access
 Uses CDMA/TDMA and support up to 3Mbps
 The concept of CDMA2000 1xEV-DO is that is a packet data
only carrier
 voice can be carried either by using Voice over IP, VoIP, or
by using a fall-back to a CDMA2000 1X carrier.
 The fall-back mode is the one that tends to be used more
widely as most operators have a CDMA2000 1X system in
place.
