The GSM System

1
The GSM System Global System for Mobile
Communications

• Magne Pettersen
• map_at_teleplan.no
• (acknowledgements Per Hjalmar Lehne, Rune Harald
  Rækken, Knut Erik Walter, Anders Spilling)

2
Content

• Introduction
• Network architecture
• Fundamental functionality
• Physical layer / radio interface
• Radio planning
• GSM in the future

3
Content

• Introduction
• Network architecture
• Fundamental functionality
• Physical layer / radio interface
• Radio planning
• GSM in the future

4
GSM status (end 2006)

• 2.18 billion connections in 212 countries
• 82 market share globally
• An incredible industry success!

5
But, let us take a few steps back
6
GSM The idea of a common European mobile
communications system

• 1982 A Nordic group is considering the next
  generation of mobile telephone. NMT (Nordisk
  Mobil Telefon), the analogue first generation
  system has only just been started
• These ideas are presented to CEPT (European
  Conference of Postal and Telecommunications
  Administrations) in June 1982
• September 1982 The first meeting in CEPT GSM
  Groupe Spécial Mobile
• In 1988 ETSI (European Telecommunications
  Standards Institute) is established and the work
  is continued under a new name SMG Special
  Mobile Group

7
GSM - Specifications

• Original specifications for the GSM system
• Good subjective voice quality
• Low terminal and service cost
• Support for international roaming
• Support for handheld terminals
• Support for new services
• Spectrum efficient
• Compatible with ISDN

8
(No Transcript)
9
GSM - Growth

• 1991 First operational GSM network in Finland
  Radiolinja
• 1993 Tele-mobil (later Telenor Mobil) and
  NetCom GSM open their networks in Norway
• 1998 GSM 1800 is deployed to increase capacity
  in cities and other densely populated areas

10
GSM improvements 2.5 G

• The need for data services increase
• In 1998-99 the HSCSD High Speed Circuit
  Switched Data - is standardised. Introduced in
  Norway 1. July 2001 (Telenor)
• I 1999 packet switching using GPRS (General
  Packet Radio Service) is standardised. Introduced
  in Norway 1. February 2001 (Telenor)
• Theoretical data rates up to 171 kbit/s
• "2.5 G" EDGE Enhanced Datarates for GSM
  Evolution
• Standardised in 2001-2002
• Introduced in September 2004 deployment ongoing
  
• Theoretical data rates up to 373 kbit/s

11
Some GSM terminals
Development..
Sony Ericsson W950i the Walkman phone
HTC P4350 Pocket computer running Windows
12
Some more GSM terminals
Nokia N95 with everything, e.g. GPS built in
iPhone Apples Mobile phone initiative
Samsung Blackjack
13
Competing standards

• The CDMA family of standards is the second
  largest group of mobile communications systems
• 340 million connections (November 2006)
• Standard developed in USA
• Strongest standing in the Americas
• Also other

14
Content

• Introduction
• Network architecture
• Fundamental functionality
• Physical layer / radio interface
• Radio planning
• GSM in the future

15
High level network architecture (1/2)
Services / Applications
Core Network (CN)
Access Network (AN)
Ext. network
SIM
ME Mobile equipment
UE User equipment
16
High level network architecture (2/2)

• The network contains functionally of User
  Equipment (UE), Access Network (AN), and Core
  Network (CN)
• User equipment Interfaces the user, handles
  radio functionality
• Access network Communication to and from the
  user equipment, handles all radio related
  functionality in the network
• Core network Communication between access
  network and external networks, handles all
  switching and routing
• Services and applications lie above the network

17
GSM user equipment

• User equipment Mobile equipment (ME) SIM card
• Subscriber Identity Module (SIM) contains
  encryption key and personal data
• The user is uniquely identified through
  International Mobile Subscriber Identity (IMSI)
• The mobile equipment is uniquely identified
  through International Mobile Equipment Identity
  (IMEI)
• Both equipment and user uniquely identified

SIM Subscriber Identity Module
18
GSM Radio Access Network (GRAN)
Packet domain

Gb
Abis
BTS
BSC
A
Circuit domain
BTS
BSC
19
Elements in GSM radio access network

• Base Transceiver Station (BTS)
• The base station, radio access point. The
  coverage area of one BTS is a cell
• Base Station Controller (BSC)
• Controls a number of BTSs. Owns and controls the
  radio resources within its domain
• GRAN must handle interfaces towards both a packet
  switched (packet domain) and a circuit switched
  (circuit domain) part of the core network

20
Some base station equipment
21
Some more base station equipment
Typical micro cell
Typical macro cell
22
Open interfaces access network

• The interfaces between network elements must be
  well defined to achieve open interfaces, i.e.
  different network elements can be delivered by
  different vendors
• Interfaces in GRAN
• Um The air interface between the mobile
  equipment and the BTS
• Abis Interface between BTS and BSC
• A Interface between GRAN and circuit switched
  part of core network (CN).
• Gb Interface between GRAN and packet switched
  part of the core network (CN)

23
GSM core network
External networks
Service platforms
HLR
PSTN/ISDN
A
Gb
IP network
24
Elements in GSM core network

• MSC Mobile Switching Centre
• Switch in the circuit domain. Contains copy of
  service profile for all users currently in the
  MSC coverage area (Visiting Location Register
  VLR, not shown explicitly in figure)
• GMSC Gateway MSC
• Handles all traffic to and from GSM and external
  circuit switched networks, such as PSTN, ISDN or
  other mobile networks
• HLR Home Location Register
• Database containing a master copy of all the
  mobile operators subscribers. There is only one
  logical HLR per GSM network. HLR contains
  information about e.g. permitted services and
  permitted roaming networks
• SGSN Serving GPRS Support Node and GGSN
  Gateway GPRS Support Node have similar
  functionality as MSC / GMSC, but for the packet
  switched part of the network. GGSN handles
  connections to external IP networks
• Also open interfaces between network elements.
  Not discussed here.

25
Content

• Introduction
• Network architecture
• Fundamental functionality
• Physical layer / radio interface
• Radio planning
• GSM in the future

26
Fundamental functionality

• The following functions are described
• Circuit switched connectivity
• Packet switched connectivity
• Mobile messaging
• Security
• Roaming
• Choice of network
• Location update
• Handover

27
Identifying users and mobile terminals

• Identification of users
• Mobile number MS-ISDN number follows numbering
  plan for telephony/ISDN (max. 15 digits)
• Calling number associated with a subscription
• Used on interface towards users
• Identification of a mobile subscription
• IMSI International Mobile Subscription Identity
  max 15 siffer
• Identification of terminal
• IMEI International Mobile Equipment Identity.
  Not used in fundamental service handling, but to
  identify stolen or black listed equipment

28
Circuit Switched connectivity
Mobile network

• Fixed connection and reserved resources while the
  communication lasts.
• (Mobile) telephony
• Circuit switched data, e.g. WAP, mobile office
  solutions using data cards etc.
• Transparent channel with defined performance
• Billing typically per time unit and dependant on
  transport data rate
• Standard GSM up to 14.4 kbit/s (more using HSCSD
  - High Speed Circuit Switched Data)

29
Packet Switched connectivity
Internet or different IP network
Mobile network

• Resources allocated only when data is transferred
  
• Same path through network can be maintained
  (but not necessarily)
• Billing typically dependant on amount of data
  transferred (or fixed tarrifs)
• GPRS Theoretically up to 171 kbit/s, typically
  40 50 kbit/s
• 4 different quality classes for packet bearer
  services

Background Typically automatic download of email, MMS
Interactive Typically web/WAP-browsing, MMS, games
Streaming Network radio, video streaming, web TV
Conversational Voice, video conferencing
30
Mobile messaging formats

• SMS Short Message Service
• Text based service to transfer up to 160
  characters per message (solutions exist to
  connect messages into longer messages, and also
  to carry other types of content ring tones,
  logos)
• MMS Multimedia Messaging Service
• A service for multimedia content, such as text,
  picture, sound, video
• Both SMS and MMS are store and forward
  services, i.e. messages are intermediately stored
  in the network

31
Security functions

• The purpose of security functions is to protect
  users and network against improper and illegal
  use
• Verify that the user has a valid subscription
• Protect the users identity against tracking
• Protection against wiretapping on the radio
  connection
• The mechanisms in GSM are based on secure storage
  of information in the users SIM card

32
Roaming (1/2)
ISDN (country A)
ISDN (country B)

• Circuit switched call to a mobile in a visiting
  network

33
Roaming (2/2)
ISDN (country A)
ISDN (country B)

• Mobile to mobile call in a visiting network
• Effect referred to as tromboning

34
Choice of network

• In GSM the following procedure is followed
• The latest used network is stored on the SIM
• As long as a cell that fulfils the criteria is
  available from this network, the mobile will not
  search for alternatives (the exception is
  national roaming, in which case the mobile will
  periodically search for the home network and
  connect when this becomes available)
• If the previously used network is not available,
  the mobile searches for alternative networks
• The mobiles can perform manually or automatic
  choice of network

35
Location Area / Routing Area (1/2)
HLR
..IMSI gtLAI,RAI ..............
RA 1
LA 1
RA 2
LA 2

• In GSM this is defined as follows
• Location area LA is the area in which the
  network is searching for a registered mobile
  (not currently active) for circuit switched
  services
• Routing area RA Similarly for packet switched
  service

36
Location Area / Routing Area (2/2)

• The dependency between LA and RA is dependant on
  the practical realisation of the network.
  Normally they will be identical
• LA and RA contain a number of cells that can be
  reached from the MSC or SGSN
• LA and RA information for each mobile is stored
  in the HLR (in the home network)
• The mobile is responsible for updating the LA/RA
  information

37
Location update

• A location update is performed when
• The mobile is connecting to a cell and discovers
  that the LAI read is different than the one
  stored in the mobile
• The mobile has been turned on, but not used, for
  a pre-defined period of time since the last
  location update (periodic location update)
• IMSI detach/attach
• An additional function where the mobile informs
  that it is turned on or off (in the same LA),
  saves resources on the radio interface and leads
  to fater response on incoming calls
• Periodic detach
• A network functionality where the network assumes
  that the mobile has been turned off if periodic
  location update has not been performed and no
  other activity has been observed for a
  pre-defined amount of time

38
Handover

• To connect a call or communication session from
  one cell to another (or to a different channel in
  the same cell)
• Is normally performed because the signal level
  from the current cell is becoming to low, but can
  also be done for different reasons, such as too
  much traffic in a cell

39
Types of handover

• Intra cell (to another channel in the same cell)
  (1)
• Inter cell, intra BSC (2)
• Inter BSC, intra MSC (3)
• Inter MSC (4)
• In addition inter system handover can sometimes
  be performed, e.g. GSM to UMTS
• Complicated, special rules apply
• Type of handover has network implications, but
  the algorithms to decide handover are the same

40
Content

• Introduction
• Network architecture
• Fundamental functionality
• Physical layer / radio interface
• Radio planning
• GSM in the future

41
GSM radio interface Main characteristics

• Frequency bands
• GSM 900
• 890 915 MHz Uplink (MS transmit)
• 935 - 960 MHz Downlink (MS receive)
• GSM 1800
• 1710 - 1885 MHz Uplink
• 1805 - 1880 MHz Downlink
• Carrier bandwidth 200 kHz
• Channels / carrier 8
• Multiple access TDMA / FDMA
• Duplex FDD
• Gross bit rate pr carrier 270,833 kbit/s
• Modulation GMSK
• Spectrum efficiency 1.35 bps/Hz

42
Radio parameters

• MS
• Sensitivity -104 (-102) dBm
• Typical 106 dBm
• Max. output power 33 (30) dBmNumbers in
  parenthesis for GSM-1800

• BTS
• Sensitivity -104 (-104) dBm
• Typical 107 dBm
• Max. output power 43 dBm

43
Channels in GSM900
45 MHz
8
7
6
5
4
3
2
4
1
1
2
3
4
1
2
3
4
124
123
200 kHz
890 MHz
960 MHz
935 MHz
915 MHz
MS receive
MS transmit
44
TDMA - principle

• GSM uses TDMA within each carrier
• Each user occupies the entire carrier one time
  slot pr. time frame
• 8 slots per frame

45
GSM Channel structure
25 MHz

• Logical channels built up of physical channels
• Control channels
• Traffic channels
• Logical channels divided between
• Dedicated channels
• Common channels

124 carriers
577 ?s
Burst period
Time slot 1
Physical channel
Time slot 2
..
TDMA frame
4.615 ms
Time slot 8
46
GSM traffic channels
26 frame length 120 ms
TDMA frame length 4.6 ms
Data bit
Training sequence
Data bit
Normal burst

• Traffic channels (TCH) are used to carry voice or
  data
• Typically uses one time slot per frame
• Gross data rate per TCH 22 kbps
• Effective data rate lower because of forward
  error correction

47
Some GSM control channels
BCCH Broadcast Control CHannel Continuously transmitted from the BTS. Contains information about cell identity, frequency etc.
FCCH SCH Frequency Correction CHannel / Synchronisation CHannel Used to correct/synchronise the frequency (FCCH) time synchronise to the frame structure. Each cell has a FCCH and a SCH
RACH Random Access CHannel Used by the mobile to send a request to the network for access. This is a slotted Aloha channel, no pre-allocation possible
AGCH Access Grant CHannel Used by the network to inform the mobile that access has been granted and information about which channel to use
PCH Paging CHannel Used by the network to notify users about incoming calls.
48
Error correction coding in GSM

• The different channels in GSM require different
  degree of protection, and therefore have
  different Forward Error Correction (FEC) schemes
• However, three types of techniques are often
  combined
• Block coding, well suited to detect and correct
  bursts of error
• Convolutional coding, high performance but not
  optimal for bursts of errors
• Interleaving, spreading neighbouring bits out, to
  decorrelate the relative position

49
Block coding

• GSM uses two types of block codes
• Fire code 224 / 184 (control channels only)
• k 184
• t 20
• Parity codes (only error detection, e.g. RACH)
• No block codes used on data channels

50
Convolutional coding

• When choosing depth (register length) in a
  convolutional code there is a trade-off between
  complexity and performance
• GSM uses a register length of 5
• Example of GSM ½ rate convolutional code shown in
  figure (used e.g. on a number of traffic channels)

51
Interleaving

• Whitening process", optimising the conditions
  for the convolutional coder
• Fundamentally important that the interleaving
  spreads the bit errors out
• Interleaving depth improves performance, but also
  increases delay
• GSM Interleaving depth 4 19
• Figure shows example with interleaving depth 4
• Write in vertically, read out horizontally
• On reception, do the reverse process

52
Forward error correction - Overview
53
Modulation

• Assuming that everyone is familiar with digital
  modulation -)
• Considerations upon choosing modulations scheme
• Spectrum efficiency
• Out of band emission (rapid drop off desired to
  limit adjacent channel interference)
• Constant envelope desired for low cost
  amplifiers, e.g. in handheld equipment
• Always a trade off
• In GSM GMSK Gaussian Minimum Shift Keying is
  used

54
GMSK (1/2)

• Leftmost figure show spectrum for MSK, QPSK and
  BPSK
• Rightmost figure shows envelope for different
  QPSK type modulation schemes
• MSK has constant envelope, relatively low
  sidelobes

55
GMSK (2/2)

• GMSK further reduces sidelobes by using a
  Gaussian filter
• Cost introduces inter-symbol-interference (ISI)
• Figures show time and frequency response
• GSM uses BT 0.3

56
Channel equaliser

• Because of reflections, diffractions etc. in the
  radio channel, time dispersion is often
  experienced
• Transmitted signal arriving at the receiver from
  various directions over a multiplicity of paths
• Broadening of transmitted pulse, inter symbol
  interference (ISI)
• Frequency selective fading
• Must be counteracted by using some sort of
  equalisation

57
Maximum likelihood sequence estimator

• GSM uses a Maximum Likelihood Sequence estimator
  (MLSE)
• MLSE looks conceptually like shown in the figure
  below
• The impulse response of the radio channel is
  calculated
• A Viterbi algorithm is used to estimate the most
  likely (Maximum Likelihood - ML) symbol sequence
• MLSE is an optimal technique in terms of removing
  ISI, but the complexity increases exponentially
  with the length of the channel response
• GSM uses a MLSE which operates over 5 bit periods
  (approx. 16 ?s)

58
Power control

• GSM uses power control, adjusting transmit power
  level in accordance with path loss
• Advantages
• Reduces interference
• Reduces power consumption
• Can also be used on downlink
• Manner of operation, GSM
• The system (BSC) measures bit error rate (BER)
• Transmit power adjusted up or down according to
  target value
• Step size 2 dB
• Maximum update interval 60 ms

59
Power control - Example
60
Content

• Introduction
• Network architecture
• Fundamental functionality
• Physical layer / radio interface
• Radio planning
• GSM in the future

61
Fundamentals

• Planning and deploying a GSM network is from an
  operators point of view a question of
• Build as few sites as possible, while maintaining
  required coverage and capacity
• Trade off

62
Coverage limited and capacity limited

• A network can be either
• Coverage limited
• The radio coverage decides the BTS density
• Typically rural areas, large cells, high masts
• Macrocells
• Capacity limited
• The traffic decides the BTS density
• Typically urban areas, small cells, low BTS
  position
• Microcells

63
Frequency reuse

• Frequencies can not be reused in every cell due
  to co-channel interference (CCI)
• A cell cluster uses all the operators
  frequencies (A, B, C, E, F, G, H in Figure)
• Co channel interference level decided by
• Cell clustre size, and thereby Frequency reuse
  distance (D in Figure)
• Propagation properties
• Can be reduced by different techniques
• Sectorisation
• Cell splitting
• Typical cell cluster size in GSM 7

64
Coverage map example

• Unfortunately cell coverage is normally neither
  hexagonal or circular
• Figure shows coverage example from a city centre
• Complicates radio planning

65
Hierarchical cell structures

• In a GSM system it is common that cells of
  different sizes co-exist in that same area
• Picocells, microcells, macrocells
• This is called hierarchical cell structure
• Can make handover (cell change) complicated.
  Often different types of users are reserved for
  one cell type, e.g.
• Users walking indoors on picocell, users walking
  outdoor on microcell, users driving use macrocell

66
Radio planning tools

• Radio planning is most often performed assisted
  by an automated process using a computer
• Underlying functionality
• Digital maps
• Propagation modelling
• System parameters and system performance
• Traffic assumptions and theory
• Often theoretical computer based modelling can be
  tuned by real life data
• Propagation measurements
• Live network traffic data

67
Example Astrix
68
Content

• Introduction
• Network architecture
• Fundamental functionality
• Physical layer / radio interface
• Radio planning
• GSM in the future

69
GSM development

• GPRS and EDGE has introduced packet data and
  support for higher data rates into GSM
• UMTS is a 3G technology building on GSM core
  network, which is backwards compatible with GSM
• GSM-UMTS handover supported
• Almost all UMTS terminals are also GSM terminals
• HSDPA / HSUPA (High Speed Downlink/Uplink Packet
  Access) supports real mobile broadband

70
Trends (1) Convergence

• Mobile communications system become more
  broadband
• At the same time computer network solutions start
  to support mobility (e.g. WiFi, WiMAX)
• Mobile goes broadband and broadband goes mobile?
• Everything comes together?

71
Trends (2) Horizontal integration

• The same services become available on different
  platforms and on different devices
• IP is the glue
• Will mobile circuit switch disappear?

72
(No Transcript)
73
(No Transcript)