2. Introduction to GSM: Mobile Phone Subscribers Worldwide
200
400
600
800
1000
1200
1996
1997
1998
1999
2000
2001
2002
year
Subscribers [million]
GSM total
TDMA total
CDMA total
PDC total
Analogue total
Total wireless
Prediction (1998)

3. Introduction to GSM: Development of Mobile Telecommunication Systems
IS-95
cdmaOne
IS-136
TDMA
D-AMPS
GSM
PDC
GPRS
IMT-DS
UTRA FDD / W-CDMA
EDGE
IMT-TC
UTRA TDD / TD-CDMA
cdma2000 1X
1X EV-DV
(3X)
AMPS
NMT
IMT-SC
IS-136HS
UWC-136
TD-SCDMA
CT0/1
CT2
IMT-FT
DECT
CDMA
FDMA
IMT-MC
cdma2000 1X EV-DO

4. GSM Overview
Several first generation analog cellular systems in Europe but incompatible - limited roaming
ETSI standards for pan-European Global System for Mobile Communications (GSM, originally Group Spe’ciale Mobile 1982) at 900 MHz
1992 GSM is launched
Standards for Digital Cellular System at 1800 MHz (DCS 1800, recently renamed GSM 1800; US version is PCS 1900)

5. GSM: Overview
Simultaneous introduction of essential services in three phases (1991, 1994, 1996) by the European telecommunication administrations  seamless roaming within Europe possible
Today many providers all over the world use GSM (more than 184 countries in Asia, Africa, Europe, Australia, America)
More than 747 million subscribers
More than 70% of all digital mobile phones use GSM

6. GSM, cont.. Objectives: Broad offering of speech and data services
Compatible with wireline networks
Automatic roaming and handoff
Highly efficient use of frequency spectrum
Support for different types of mobile terminal equipment (e.g., cars, portable handsets)
Digital signaling and transmission
Low cost infrastructure and terminal equipment

7. GSM, cont.. 13 recommendations R.00: Preamble
R.01: General structure of recommendations, GSM overview
R.02: Service aspects: types of services
R.03: Network aspects: architecture, call routing, performance objectives
R.04: Mobile/base station interface and protocols

8. GSM, cont..
R.05: Physical layer on radio path: multiple access, channel coding, modulation, transmission
R.06: Speech coding aspects
R.07: Terminal adaptors for mobile stations
R.08: Base station and mobile switching center (MSC) interface
R.09: Interworking with PSTN and packet data networks
R.10: Service interworking, short message service
R.11: Equipment specification
R.12: Operation and maintenance, tariffs, traffic administration

9. GSM, cont.. Summary of features Channel bandwidth 200 kHz
Multiple access
TDMA
Users/carrier 8
Speech coding rate
13 kb/s
FEC coded speech rate
22.8 kb/s

10. GSM, cont.. Summary of service quality requirements
Speech intelligibility
Summary of service quality requirements
90%
Max one-way delay
90 ms
Max handoff gap
150 ms if intercell
Time to alert mobile of
inbound cell
4 sec first attempt, 15 sec final attempt
Release time to called
network
2 sec
Connect time to called
network
4 sec

11. GSM: Performance Characteristics
Communication
Mobile, wireless communication; support for voice and data services
Total mobility
International access, chip-card enables use of access points for different providers
Worldwide connectivity
One number, the network handles localization
High capacity
Better frequency efficiency, 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
Access control, authentication via chip-card and PIN

12. GSM: Mobile Services GSM offers Several types of connections
Voice connections, data connections, short message service
Multi-service options (combination of basic services)
Three service domains
Bearer Services
Telematic Services
Supplementary Services

13. Bearer Services
Telecommunication services to transfer data between access points
Different data rates for voice and data (original standard)
Data service (circuit switched)
synchronous: 2.4, 4.8 or 9.6 kbit/s
asynchronous: bit/s
Data service (packet switched)
asynchronous: bit/s
Today: data rates of approx. 50 kbit/s possible

14. Tele Services I Offered Services
Mobile Telephony primary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz
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)

15. Tele Services II Additional services Non-Voice-Teleservices Fax
Voice mailbox (implemented in the fixed network supporting the mobile terminals)
Electronic mail (MHS, Message Handling System, implemented in the fixed network)
Short Message Service (SMS) alphanumeric data transmission to/from the mobile terminal using the signaling channel, thus allowing simultaneous use of basic services and SMS

16. Supplementary Services
May differ between different service providers, countries and protocol versions
Important services
Identification: forwarding of caller number
Suppression of number forwarding
Automatic call-back
Conferencing with up to 7 participants
Locking of the mobile terminal (incoming or outgoing calls)
...

17. Architecture of the GSM System
GSM is a PLMN (Public Land Mobile Network)
Several providers setup mobile networks following the GSM standard within each country
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

18. GSM: Architecture Overview
OMC, EIR, AUC
Fixed Network
(PSTN)
NSS
with OSS
HLR
GMSC
VLR
MSC
VLR
MSC
BSC
BSC
RSS

19. GSM: Reference Architecture
Radio Subsystem (RSS)

20. GSM General Architecture
PSTN
GSM Public
Land Mobile
Network
(PLMN)
OMC
VLR
MSC
HLR
AUC
NMC A
EIR
BSS
BSS
BSC
Abis
ADC
BTS
BTS
OSS Um MS MS MT
OSS: Operation Subsystem
BSS: Base Station Subsystem
MS: Mobile Station (Mobile User) TE

21. System Architecture: Radio Subsystem
Network and Switching 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 MS MS Um
Abis
BTS
BSC
MSC
BTS A
BTS
MSC
BSC
BTS
BSS

22. GSM General Architecture, cont..
Mobile station (MS) communicates to base stations through radio interface Um
Mobile termination (MT) supports physical channel between MS and base station (radio transmission, channel coding, speech coding)

23. GSM General Architecture, cont..
Terminal equipment (TE), e.g., telephone set.
Contains terminal/user-specific data in form of smart card (subscriber identify module or SIM card), plugs into any GSM terminal like credit card and identifies user to network for personal mobility (in addition to terminal mobility) and security

24. GSM General Architecture, cont..
Base station Subsystem (BSS) communicates with mobile switching center through network interface A
Base Transceiver Station (BTS) handles channel allocation, signaling, frequency hopping, handover initiation, etc.
BTS communicates with BSC using Abis interface

25. GSM General Architecture, cont..
Base station controller (BSC) manages radio channels, paging, handoff for several BTSs
BSC communicates with MSC using A interface
Mobile switching center (MSC) is gateway to PSTN and packet data networks
Performs switching, paging functions, MS location updating, handoff control, etc.

26. GSM General Architecture, cont..
Home location register (HLR) stores subscriber info and part of MS’s location info to route incoming calls to visitor location register (VLR) where mobile is roaming
VLR registers users roaming in its area and assigns roaming numbers

27. GSM General Architecture, cont..
Authentication center (AUC) is accessed by HLR to authenticate a user for service.
It contains authentication and encryption keys for subscribers
Equipment identity register (EIR) allows stolen or fraudulent mobile stations to be identified

28. GSM General Architecture, cont..
Operation subsystem (OSS) contains: Operations and Maintenance Center (OMC), Network Management Center (NMC), and Administration Center (ADC).
These network elements work together to monitor, control, maintain, and manage the network

29. System Architecture: Network and Switching Subsystem
network subsystem
fixed partner networks
Components
MSC (Mobile Services Switching Center):
ISDN (Integrated Services Digital Network)
PSTN (Public Switched Telephone Network)
Databases
HLR (Home Location Register)
VLR (Visitor Location Register)
EIR (Equipment Identity Register)
ISDN PSTN
MSC
EIR
SS7
HLR
VLR
ISDN PSTN
MSC

30. Radio Subsystem
The Radio Subsystem (RSS) comprises the cellular mobile network up to the switching centers
Components
Mobile Stations (MS)
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, managing of network resources, mapping of radio channels (Um) onto terrestrial channels (A interface)
BSS = BSC + sum(BTS) + interconnection

31. Mobile Station Terminal for the use of GSM services
A mobile station (MS) comprises two elements:
ME (Mobile Equipment):
Peripheral device of the MS, offers services to a user
Speaker, microphone, keypad, and radio modem
Usually subsidized by the service provider to encourage new subscribers
SIM (Subscriber Identity Module):
Smart card issued at subscription time
Personalization of the mobile terminal, stores user parameters such as address and type of service
Calls are directed to the SIM rather than the terminal
Stores short messages
Carries a PIN number that needs to be verified to make the information on the card available to the user.

32. Base Station Subsystem
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

33. GSM: Cellular Network segmentation of the area into cells
possible radio coverage of the cell
idealized shape of the cell
use of several carrier frequencies
not the same frequency in adjoining cells
cell sizes vary from some 100 m up to 35 km depending on user density, geography, transceiver power etc.
hexagonal shape of cells is idealized (cells overlap, shapes depend on geography)
if a mobile user changes cells  handover of the connection to the neighbor cell

34. Network and Switching Subsystem
NSS is the main component of the public mobile network GSM
Switching, mobility management, interconnection to other networks, system control
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

35. Network and Switching Subsystem
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 user currently in the domain of the VLR
Equipment Identity Register (EIR): registers GSM mobile stations and user rights. Stolen or malfunctioning mobile stations can be locked and sometimes even localized.

36. Network and Switching Subsystem
Authentication Center (AUC)
Generates user specific authentication parameters on request of a VLR
Authentication parameters used for authentication of mobile terminals and encryption of user data on the air interface within the GSM system

37. Mobile Service Switching Center
The MSC (mobile switching center) plays a central role in GSM
switching functions
additional functions for mobility support
management of network resources
interworking functions via Gateway MSC (GMSC)
integration of several databases

38. Mobile Service Switching Center
Functions of a MSC
specific functions for paging and call forwarding
termination of SS7 (signaling system no. 7)
mobility specific signaling
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

39. Mechanisms to Support Mobility
Registration
Performed as soon as the mobile unit is powered on
Call establishment
Performed when the user initiates or receives a call
Handover
Performed when the MS needs to change its connection point to the network
Security
Protects from fraud and eavesdropping

40. Registration Procedure
As we turn on an MS, it passively synchronizes to the frequency, and frame timings of the closest BS to get ready for information exchange.
The MS then listens to advertisements from the BS regarding system and cell identity to determine its location in the network.
If the current location is not the same, the MS initiates a registration procedure.

41. Registration Procedure: MS turned on in a New MSC Area
1-4: A radio channel is established between the MS and BSS to process the registration
5-8: The NSS authenticates the MS
9-11: A TMSI (Temporary Mobile Subscriber Identity) is assigned and updates are made in the VLR and HLR
12: The temporary radio channel is released.

42. Call Establishment: Mobile Originated Call
1-5: Similar to registration procedure
6-7: Ciphering to protect against eavesdropping
8-15: Similar procedure as in POTS (Plain Old Telephone Service), except for the additional traffic channel assignment procedure (10-11)

43. Mobile Originated Call
1, 2: connection request
3, 4: security check
5-8: check resources (free circuit)
9-10: set up call
VLR 3 4
PSTN 6 5
GMSC
MSC 7 8 2 9 1 MS
BSS 10

44. Call Establishment: Mobile Terminated Call
After dialing, the PSTN directs the call to the MSC of the destination address
The MSC requests routing information from the HLR
Since the MS is in another area, the address of the new MSC is given to the original MSC. It contacts the new MSC.
The VLR of the new MSC initiates a paging procedure in all BSS under the control of the new MSC
After a reply from the MS, a link is established for communication.

45. Mobile Terminated Call
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 4
HLR
VLR 5 8 9 3 6 14 15
PSTN 7
calling
station
GMSC
MSC 1 2 10 13 10 10 16
BSS
BSS
BSS 11 11 11 11 12 17 MS

46. Handoff (Handover in Europe)
Two types
Internal: between BTSs that belong to the same BSSs
External: between two different BSSs controlled by the same MSC
Sometimes there are handoffs between BSSs that are controlled by two different MSCs (the old MSC continues to handle call management)
Reasons to handoff:
Signal strength deterioration at the edge of a cell
Traffic balancing (to easy traffic congestion)

47. Handoff Decision receive level BTSold receive level BTSold
Hysteresis MARGIN MS MS
BTSold
BTSnew

48. Handoff Procedure Mobile-assisted Handoff

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

50. Security in GSM Security Services (Features)
Access Control/Authentication (Identity Authentication)
User is assigned SIM (Subscriber Identity Module) at the subscription time which contains
* IMSI: International Mobile Subscriber Identity
* Individual Subscriber Authentication Key (Secret PIN) (Personal Identification Number)
* and the Authentication Algorithm (Challenge response method)
Identity Confidentiality
voice and signaling encrypted on the wireless link (after successful authentication)
User Data Confidentiality (Privacy/Anonymity)
temporary identity TMSI (Temporary Mobile Subscriber Identity)
newly assigned at each new location update (LUP)
encrypted transmission

51. Security in GSM 3 algorithms specified in GSM
A3 for authentication (“secret”)
A5 for encryption (standardized)
A8 for key generation (“secret”)
A5 uses a key of 128 bits, and the “response to the challenge” is 32 bits long – not very secure
Key information is not shared between systems:
A triple consisting of the random number used in challenge, challenge response, and the data encryption key are exchanged between the VLR and the HLR.
The VLR verifies if the response generated by the MS is the same.

52. Security in GSM Service Request authentication procedure includes:
* Mobile transmitting its Temporary IMSI (TIMSI)
* Network replying with a randomly generated number (RAND)
* Mobile computes the Signed Response (SRES) using the
authentiction algorithm, the key (which is a function of the
frame number) and RAND, and sends the SRES back to the
network
* Network compares the SRES from the mobile with its
computation of the SRES and authenticates a user or not.
Privacy is provided by a Temporary Mobile Subscription Identity (TMSI) valid during its binding to a VLR and computed after the authentication procedure.
IMSI (on the SIM CARD)  15 digits
* MCC (Mobile Country Code) (3 digits)
* MNC (Mobile Network Code) (2 digits)
* MSIN (Mobile Subscriber Identifier) (2 digits)
* NMSI (Network Mobil Subscriber Identifier) (2 digits)

53. The GSM Protocol Architecture

54. GSM Protocol Stack Layer 1: Physical layer
A and A-bis interfaces follow the ISDN standard with 64 kbps digital data per voice user
The new physical later in GSM is for the Um air interface
Specifies how the voice and data are formatted into packets and sent through the radio channel
Specifies radio modem details, structure of traffic and control packets in the air
Modulation and coding techniques, power control methodology, and time synchronization approaches which enable establishment and maintenance of channels

55. Physical Packets Bursts
Normal Burst (NB)
3 tail bits (TBs) at the beginning and end (gap time)
8.25 bits of gap period
Two sets of 58 bits (116) encrypted bits: 114 bits of data and two flags to indicate user traffic or signaling and control
26-bit training sequence for the equalizers
Frequency-correction burst (FB)
Broadcasted by the BSs. The MS uses it to synchronize with the master clock in the system
Synchronization burst (SB)
Forwarded by the BTS. The MS uses it for training of the equalizer, learning of network identity and to synchronize the time slots
Random access burst (RAB)
Use by the MS to access the BS as it registers to the network

56. GSM – TDMA/FDMA (124 Frequencies; in each Freq there are 8 channels (time slots))
MHz
124 channels (200 kHz)
Downlink
frequency
MHz
124 channels (200 kHz)
Uplink
time
Each channel is separated in time via a frame
GSM TDMA Frame 1 2 3 4 5 6 7 8
4.615 ms
Each frame is subdivided into 8 GSM time-slots
GSM Time-Slot (normal burst)
Each slot represents a physical TDM channel and lasts for 577 µs
guard
space
guard
space
tail
user data S
Training S
user data
tail
3 bits
57 bits 1
26 bits 1
57 bits 3
577 µs
Each TDM channel occupies the 200 kHz carrier for 577 ms every ms

57. GSM Physical Channels TDMA frame = 4.615 ms Timeslot 1 2 3 4 5 6 7 8
Frequency 1
Ch 1
Ch 2
Ch 3
Ch 4
Ch 5
Ch 6
Ch 7
Ch 8
Frequency 2
Ch 1
Ch 2
Ch 3
Ch 4
Ch 5
Ch 6
Ch 7
Ch 8 : :
Frequency 124
Ch 1
Ch 2
Ch 3
Ch 4
Ch 5
Ch 6
Ch 7
Ch 8

58. GSM-Channels
The total RF Spectrum (~50 MHz) is located in MHz (upstream) and 935 – 960MHz (downstream)
This spectrum is divided into 124 RF carriers of 200KHz.
The offset of the upstream/downstream pairing is fixed 45 MHz.
Each RF carrier is further divided into 8 time slots (TDMA)-physical channels
There is no pre-assignment of any channels to a specific and exclusive use.

59. GSM-Channels
For a given channel the uplink F_u and downlink F_d frequency can be obtained from
F_u = (N-1) MHz
F_d = (N-1) MHz
with N = 1,2,…, 124.

60. GSM-Channel & Frames
When an MS is assigned to an information channel, a radio channel and a timeslot are also assigned.
Radio channels are assigned in frequency pairs (one for the uplink and other for the downlink).
Each pair of radio channels supports up to 8 simultaneous calls.
Thus, the GSM can support up to 992 simultaneous users with full rate speech coder.
This number can be doubled to 1984 with half rate speech coder.

61. GSM Hierarchy of Frame Structure
Hyperframe (=2048 superframes=2,715,648 frames=21,725,184slots) 1 2
...
2045
2046
2047
3 h 28 min s
Superframe (=51 multiframes)) 1 2
... 48 49 50
6.12 s 1
... 24 25
Multiframe (traffic or control multiframes) 1
... 24 25
120 ms 1 2
... 48 49 50
235.4 ms
Frame: 8 burst of time slots 1
... 6 7
4.615 ms
slot
burst
577 µs

62. GSM Logical Channel Structure
TCH
CCH
CBCH
TCH/F
TCH/H
BCH
CCCH
DCCH
FCCH
SCH
BCCH
PCH
AGCH
RACH
ACCH
SDCCH
SACCH
FACCH

63. GSM Channel Types
3 groups of logical (i.e., Virtual Circuits) channels, TCH, CCH and CBCH, which are realized on top of physical channels.
TCH (Traffic Channel)
To carry voice or data traffic of the users
CCH (Control Channel)
For control and signaling functions
CBCH (Cell Broadcast Channel)
For broadcast functions from a service center to a MS in a cell area.

64. GSM Logical Channels Traffic (TCH) Channels:
Two-way, carrying voice and data
Full-rate traffic channels (TCH/F)
Half-rate traffic channels (TCH/H)
Full rate channel may carry 13 kb/s speech or data at 12, 6, or 3.6 kb/s
Half rate channel may carry 6.5 kb/s speech or data at 6 or 3.6 kb/s

65. GSM Logical Channels, cont..
CCH consists of 3 groups of logical control channels, BCH, CCCH and DCCH
BCH (Broadcast Channel):
Point-to-multipoint downlink only.
Contains three sub-channels, BCCH, FCCH and SCH
BCCH (Broadcast Control Channel):
Used by the BTS to broadcast synchronization parameters, available services, and cell ID.
I.o.w. sending cell identities, organization info about common control channels, cell service available, etc.

66. GSM Logical Channels FCCH (Frequency Correction Channel):
An MS uses it to synchronize its carrier frequency and bit timing.
SCH (Synchronization Channel):
Used by the BTS to broadcast frame synchronization signals to all MSs;
I.o,w., send TDMA frame number and base station identity code to synchronize MSs.

67. GSM Logical Channels, cont…
CCCH (Common Control Channel) One way:
Consists of three sub-channels, PCH, AGCH and RACH. These channels are used for paging and access
- PCH (Paging Channel): Used by BTS to page the MS
AGCH (Access Grant Channel):
to assign MSs to stand-alone dedicated control channels for initial assignment; Used by the MS to access the BTS for call establishment
RACH (Random Access Channel): for MS to send requests for dedicated connections; for the acknowledgement from the BTS to the MS after a successful attempt by MS using RCH.

68. GSM Logical Channels, cont…
DCCH (Dedicated Control Channel): bi-directional point-to-point -- main signaling channels. Consist of two sub-channels, SDCCH and ACCH
SDCCH (Stand-alone dedicated control channel):
for service request, subscriber authentication, equipment validation, assignment to a traffic channel; Call establishment and mobility management

69. GSM Logical Channels, cont…
ACCH consists of two sub-channels, SACCH and FACCH
SACCH (slow associated control channel): for out-of-band signaling associated with a traffic channel, e.g., signal strength measurements; Assigned to each TCH and SDCCH. Used to exchange parameters between the BTS and the MS to maintain the link.
FACCH (fast associated control channel): for preemptive signaling on a traffic channel, e.g., for handoff messages; Used to support fast transitions in the channel when SACCH is not adequate.