Kelvin Hilton

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FMC2nd Generation Mobile TelecommunicationsPart
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• Kelvin Hilton
• k.c.hilton_at_staffs.ac.uk

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Objectives

• Understanding
• Origins
• Multiplexing (SDM/FDM/TDM/CDM)
• TDMA / CDMA
• Introducing GSM as a 2G technology
• GSM Architecture

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Origins of 2nd Generation Systems

• Lessons that were nearly learned from 1G
• Need for standardisation
• Two contrasting approaches
• Europe
• Because of European Union, decided to develop a
  pan-European standard for the next generation of
  mobile networks
• Rest of the World
• Particularly the US and countries in US sphere of
  influence allowed market forces to remain
  dominant
• Japan, far-east went in a different direction

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Origins of 2nd Generation Systems

• Europe
• In 1982 groupe spéciale mobile (GSM) founded to
  develop next generation standard
• Soon renamed global system for mobile
  communications
• Tasked the European Telecommunications Standards
  Institute (ETSI) with responsibility of
  specifying and maintaining the standard
• 5,000 pages long the 1st version was released in
  1991

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Origins of 2nd Generation Systems

• USA
• Original AMPS analogue system upgraded to digital
  in 1991 (D-AMPS)
• US military had been using a secure system for
  some time, this was made commercially available
  in 1991 called Code Division Multiple Access
  (CDMA)
• Japan
• Originally developed the Japanese Digital
  Cellular (JDC) system in 1993
• Later renamed Personal Digital Cellular (PDC)

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Origins of 2nd Generation Systems

• In Europe the frequency allocated for the new 2nd
  Generation systems was originally 900 MHz
• In US no new spectrum was initially allocated for
  D-AMPS CDMA both competing at 850 MHz, GSM was
  allocated at 1900 MHz (900 was owned by RAM
  Mobile for Mobitex)
• Soon realised that 900 MHz could not offer
  sufficient bandwidth in metropolitan areas

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Origins of 2nd Generation Systems

• Again solution adopted was different
• In 1994, Europe allocated a new frequency, 1800
  MHz (known as Digital Cellular System (DCS) 1800)
• Used smaller cells and more efficient codecs
• US companies had to concentrate on squeezing more
  out of the available bandwidth by improving the
  technology
• Because of the standards based approach GSM
  became the dominant worldwide technology

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Origins of 2nd Generation Systems

• GSM now used in over 190 countries
• Over 400 different providers worldwide
• Has over 70 of world market
• Estimated 800 million GSM users worldwide
• In US
• 135 million CDMA
• 107 million TDMA
• 16 million GSM!!
• Japan
• PDC has 60 million users

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GSM as a 2nd Generation System

• GSM principally designed to service voice
• Data not seen as a major revenue generator
• However, does provide limited data services
• 3 categories of services defined
• Bearer services
• Tele services
• Supplementary services

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GSM Bearer Services

• Original GSM allowed for 9600 bit/s non-voice
  services
• Permits transparent / non-transparent,
  synchronous or asynchronous data transmission
• Transparent bearer services
• Used only at the physical layer
• May use FEC
• Non-transparent bearer services
• Use services of the Transparent bearer
• Utilise link control for retransmission, etc
• Uses the bearer services to interwork with PSTN,
  ISDN, X.25, etc

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GSM Tele Services

• Voice encryption
• Messaging (SMS, EMS, MMS)
• Basic data communication (eg Fax)
• High-quality voice delivery using 3.1 kHz
  bandwidth
• Codecs for voice and modem
• Standard free of charge emergency number, has
  highest priority, automatically connects to
  nearest emergency center

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GSM Supplementary Services

• Similar to ISDN networks
• May include
• User identification
• Call redirection
• Call forwarding
• Closed User Groups
• Mulitparty calls

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GSM Architecture

• Consists of three subsystems

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Radio Subsystem (RSS)

• Comprises all radio specific entities

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Radio Subsystem (RSS) (1)

• Mobile Station (MS, the phone)
• Comprises of all equipment needed for
  communication with GSM network

MS
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Mobile Station (MS)

• Consists of
• Subscriber Identity Module (SIM)
• Stores user-specific data
• Card type
• Subscription type therefore which services user
  can/cannot access
• Personal Identification Number (PIN)
• A PIN unblocking key (PUK) used if the SIM is
  locked accidentally
• Authentication key (Ki)
• International mobile subscriber identity (IMSI)
  permanent
• The cipher key (Kc) set once phone is logged on
  to network
• Temporary mobile subscriber identity (TMSI) when
  user is not on home network used with the
  Location area identification (LAI) to locate the
  user on any GSM network in the world

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Mobile Station (MS)

• cont
• Without the SIM only emergency calls are allowed
• International Mobile Equipment Identity (IMEI)
• Unique ID for the device used for theft
  protection
• For GSM 900 phone has transmit power of 2W
• For GSM 1800, 1W due to smaller cells
• Apart from the phone interface MS may also
  consist of
• Display, speaker, microphone, programmable keys,
  computer modem, IrDA, Bluetooth, etc

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Radio Subsystem (RSS) (2)

• Base Transceiver Station (BTS)
• Comprises of all the radio equipment
• Antennas
• Signal processors
• Amplifiers

BTS
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Base Station Transceiver (BTS)

• BTS manages a radio cell

• Using sectorized antenna may manage several cells
• GSM cell can be anything from 100m to 35km

Radio Cell
BTS
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Radio Subsystem (RSS) (3)

• MS and BTS are connected by the Um interface
  (ISDN U interface for mobile)

Radio Cell
MS
Um
BTS
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Radio Subsystem (RSS) (3)

• Base Station Controller (BSC) manages a
  collection of BTSs
• Reserves radio frequencies
• Handles handover between BTSs
• Performs paging of MSs
• Multiplexes the radio channels onto the fixed
  network
• Communicates with the BTSs using the Abis
  interface

Abis
BSC
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BCS / BTS Tasks
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Radio Subsystem (RSS) (4)

• The BSC and its managed BTSs and connected MSs
  is called a Base Station Subsystem (BSS)

BSS
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Base Station Subsystem (BSS)

• GSM networks consist of n BSSs
• BSS performs all the necessary functions for
  maintaining a radio connection to a MS
• Coding/decoding of voice traffic
• Rate adaptation between the wireless and fixed
  network

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Network Switching Subsystem (NSS)

• heart of the GSM system Schiller, 2002

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NSS

• Connects the wireless and fixed networks together
• Performs handovers between BSSs
• Supports
• All functions necessary for worldwide
  localisation of users
• Charging accounting
• Roaming
• Consists of switches and databases

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NSS

• Mobile services switching centre (MSC)
• High-performance digital ISDN switches
• Setup connections with other MSCs
• Connect to the BSCs over the A interface
• Form the fixed backbone of the GSM network
• MSC usually manages a group of BSCs in a
  geographical area
• Handles
• All signalling necessary for connection setup
  release
• Handover between MSCs
• All supplementary services (eg call forwarding)
• Uses SS7

MSC
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NSS (2)

• Gateway MSC (GMSC)
• Special node that handles connections to other
  fixed networks
• PSTN
• ISDN
• Using special additional interworking functions
  (IWF) can connect to public data networks such as
  X.25

MSC
GMSC
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NSS (3)

• Gateway MSC (GMSC) connects to fixed networks

MSC
GMSC
IWF
PSTN
ISDN
PDN
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NSS

• Home location register (HLR) is the most
  important database in a GSM system
• Stores all relevant user data including
• Mobile Subscriber ISDN number (MSISDN)
• Details of subscription permissions
• Call forwarding
• Roaming
• GPRS
• Subscribers ISMI
• Users location area (LA)
• Mobile subscriber roaming number (MSRN)
• Users current VLR (see following) and MSC
• Only 1 customer HLR record worldwide
• Real-time database has to provide data within
  certain time bounds

HLR
MSC
GMSC
IWF
PSTN
ISDN
PDN
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NSS

• Visitor location register (VLR) associated with
  particular MSC
• Dynamic real-time database that stores data on
  users in a particular LA associated with the MSC
• IMSI
• MSISDN
• HLR address
• When new MS enters an LA controlled by the MSC
  the VLR copies data from user HLR
• Not uncommon for a VLR to hold data on 1million
  subscribers!

HLR
MSC
VLR
GMSC
IWF
PSTN
ISDN
PDN
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Operation Subsystem (OSS)

• Functions for network operation maintenance

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OSS (1)

• Authentication center (AuC)

AuC
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Authentication Center (AuC)

• Due to the vulnerability of mobile networks to
  attack, GSM specification separates out the
  algorithms for key generation into a OSS network
  entity
• Used by the HLR to authenticate a user
• May be a securely partitioned part of the HLR

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OSS (2)

• Operation and maintenance center (OMC)

AuC
OMC
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Operation and Maintenance Centre (OMC)

• Monitors and controls all other network entities
• Via the O interface using SS7 with X.25
• Typically manages
• Traffic monitoring
• Status reports
• Subscriber security management
• Accounting and billing
• Uses the concept of telecommunications management
  network (TMN) specified by ITU-T

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OSS

• Equipment identity register (EIR)
• Database of all IMEIs for the network
• Contains a blacklist of any MS that has been
  reported stolen or is currently locked
• White list contains all valid MSs
• Gray list contains all MSs that may not be
  functioning correctly

AuC
OMC
EIR
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GSM Network
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Questions ?