Introduction to GSM: Mobile Phone Subscribers Worldwide

2. 1200 1000 GSM total 800 TDMA total CDMA total 600 PDC total Subscribers [million] Analogue total Total wireless 400 Prediction (1998) 200 0 year 1996 1997 1998 1999 2000 2001 2002 Introduction to GSM:Mobile Phone Subscribers Worldwide

3. CT0/1 FDMA AMPS CT2 NMT IMT-FT DECT IS-136 TDMA D-AMPS EDGE TDMA IMT-SC IS-136HS UWC-136 GSM GPRS PDC IMT-DS UTRA FDD / W-CDMA IMT-TC UTRA TDD / TD-CDMA CDMA IMT-TC TD-SCDMA IS-95 cdmaOne IMT-MC cdma2000 1X EV-DO cdma2000 1X 1X EV-DV (3X) 2G 3G 1G 2.5G Introduction to GSM: Development of Mobile Telecommunication Systems

4. GSM Overview • Several first generation analog cellular systems in Europe but incompatible - limited roaming • 1987-1989 ETSI standards for pan-European Global System for Mobile Communications (GSM, originally Group Spe’ciale Mobile 1982) at 900 MHz • 1992 GSM is launched • 1990-1993 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. Channel bandwidth 200 kHz Multiple access TDMA Users/carrier 8 13 kb/s Speech coding rate 22.8 kb/s FEC coded speech rate GSM, cont.. Summary of features

10. GSM, cont.. Speech intelligibility 90% Summary of service quality requirements 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: 300 - 1200 bit/s • Data service (packet switched) • synchronous: 2.4, 4.8 or 9.6 kbit/s • asynchronous: 300 - 9600 bit/s • Today: data rates of approx. 50 kbit/s possible

14. Tele Services I • Offered Services • Mobile Telephonyprimary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz • Emergency Numbercommon 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) HLR NSS with OSS 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 AUC HLR NMC A EIR BSS BSS BSC ADC Abis BTS BTS OSS Um MS MS OSS: Operation Subsystem BSS: Base Station Subsystem MS: Mobile Station (Mobile User) MT TE

21. 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 System Architecture: Radio Subsystem RadioSubsystem Network and SwitchingSubsystem MS MS Um Abis BTS MSC BSC 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 networksubsystem fixed partnernetworks • 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) ISDNPSTN MSC EIR SS7 HLR VLR ISDNPSTN 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. cell GSM: Cellular Network segmentation of the area into cells • 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 possible radio coverage of the cell idealized shape of the 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. 1, 2: connection request 3, 4: security check 5-8: check resources (free circuit) 9-10: set up call PSTN Mobile Originated Call VLR 3 4 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. PSTN 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 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 ProcedureMobile-assisted Handoff

49. Handoff Procedure MSC MS BTSold BSCold 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