Mobile Communications Chapter 4: Wireless Telecommunication Systems  GSM  Overview  Services  Sub-systems  Components  IS 95 Overview  Services  Sub-systems  Components  Mobile phone subscribers worldwide 700000 subscribers (x 1000) 600000 Analog total 500000 GSM total 400000 CDMA total 300000 TDMA total PDC/PHS total 200000 total 100000 1996 1997 1998 ICS 243E - Ch4 Wire 1999 2000 2001 Winter 2001 4.2 GSM: Overview  GSM  formerly: Groupe Spéciale Mobile (founded 1982)  now: Global System for Mobile Communication  Pan-European standard (ETSI, European Telecommunications Standardisation Institute)  simultaneous introduction of essential digital cellular 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 130 countries in Asia, Africa, Europe, Australia, America)  more than 100 million subscribers ICS 243E - Ch4 Wire Winter 2001 4.3 Performance characteristics of GSM       Communication  mobile, wireless digital communication; support for voice and data services Total mobility  international access, chip-card enables use of access points of 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  uninterrupted phone calls at higher speeds (e.g., from cars, trains) – better handoffs and Security functions  access control, authentication via chip-card and PIN ICS 243E - Ch4 Wire Winter 2001 4.4 Disadvantages of GSM  There is no perfect system!!  no end-to-end encryption of user data  no full ISDN bandwidth of 64 kbit/s to the user, no transparent B-channel  abuse of private data possible roaming profiles accessible high complexity of the system  several incompatibilities within the GSM standards  ICS 243E - Ch4 Wire Winter 2001 4.5 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 – interface to the physical medium (transparent for example in the case of voice or non transparent for data services) Telematic Services – services provided by the system to the end user (e.g., voice, SMS, fax, etc.) Supplementary Services – associated with the tele services: call forwarding, redirection, etc bearer services MS TE MT R, S GSM-PLMN Um transit network (PSTN, ISDN) source/ destination network TE (U, S, R) tele services ICS 243E - Ch4 Wire Winter 2001 4.6 Bearer Services  Telecommunication services to transfer data between access points R and S interfaces – interfaces that provide network independent data transmission from end device to mobile termination point  U interface – provides the interface to the network (TDMS, FDMA, etc.)   Specification of services up to the terminal interface (OSI layers 13) Transparent – no error control of flow control, only FEC  Non transparent – error control, flow control   Different data rates for voice and data (original standard)  voice service (circuit switched) synchronous: 2.4, 4.8 or 9.6 Kbps  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 ICS 243E - Ch4 Wire Winter 2001 4.7 Tele Services I Telecommunication services that enable voice communication via mobile phones  All these basic services have to obey cellular functions, security measures etc  Offered voice related 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)  Multinumbering several ISDN phone numbers per user possible  ICS 243E - Ch4 Wire Winter 2001 4.8 Tele Services II  Additional services: Non-Voice-Teleservices  group 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 (160 characters) ICS 243E - Ch4 Wire Winter 2001 4.9 Supplementary services Services in addition to the basic services, cannot be offered stand-alone  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 participants  locking of the mobile terminal (incoming or outgoing calls)   ICS 243E - Ch4 Wire Winter 2001 4.10 Data services in GSM II GPRS (General Packet Radio Service)  packet switching  using free slots only if data packets ready to send (e.g., 115 kbit/s using slots temporarily)  standardization 1998  advantage: one step towards UMTS, more flexible  disadvantage: more investment needed  GPRS network elements  GSN (GPRS Support Nodes): GGSN and SGSN  GGSN (Gateway GSN)  interworking unit between GPRS and PDN (Packet Data Network)  SGSN (Serving GSN) supports the MS (location, billing, security)  GR (GPRS Register) user addresses ICS 243E - Ch4 Wire Winter 2001 4.38 GPRS quality of service Reliability class Lost SDU probability Duplicate SDU probability 10-9 10-4 10-2 10-9 10-5 10-5 Delay class Out of sequence SDU probability 10-9 10-5 10-5 Corrupt SDU probability 10-9 10-6 10-2 SDU size 128 byte SDU size 1024 byte mean 95 percentile mean 95 percentile < 0.5 s < 1.5 s [...]... 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 ICS 243E - Ch4 Wire Winter 2001 4.11 GSM: overview OMC, EIR, AUC HLR NSS with OSS VLR MSC GMSC VLR fixed network MSC BSC BSC RSS ICS 243E - Ch4 Wire Winter 2001 4.12 GSM: elements... the new BS to synchronize its receiver to the arrival time of the messages ICS 243E - Ch4 Wire Winter 2001 4.30 4 types of handover 1 MS 2 3 4 MS MS MS BTS ICS 243E - Ch4 Wire BTS BTS BTS BSC BSC BSC MSC MSC Winter 2001 4.31 Handover decision receive level BTSold receive level BTSold HO_MARGIN MS MS BTSold ICS 243E - Ch4 Wire BTSnew Winter 2001 4.32 Handover procedure MSC MS BTSold BSCold measurement... calls)  support of short message service (SMS)  generation and forwarding of accounting and billing information  ICS 243E - Ch4 Wire Winter 2001 4.22 Operation subsystem The OSS (Operation Subsystem) enables centralized operation, management, and maintenance of all GSM subsystems  Components  Authentication Center (AUC)  generates user specific authentication parameters on request of a VLR authentication... network subsystem ICS 243E - Ch4 Wire Winter 2001 4.23 GSM Radio Interface - TDMA/FDMA fr e qu en c y 935-960 MHz 124 channels (200 kHz) downlink 890-915 MHz 124 channels (200 kHz) uplink higher GSM frame structures time GSM TDMA frame 1 2 4 3 5 6 7 8 4.615 ms GSM time-slot (normal burst) guard space tail 3 bits user data S Training S user data 57 bits 1 26 bits 1 57 bits ICS 243E - Ch4 Wire Winter 2001 guard... multiframe 0 1 0 1 24 2 120 ms 25 48 49 50 235.4 ms frame 0 1 6 7 4.615 ms slot 577 µs burst ICS 243E - Ch4 Wire Winter 2001 4.25 GSM protocol layers for signaling Um A Abis MS BTS BSC MSC CM CM MM MM BSSAP RR BTSM RR’ BTSM LAPDm RR’ LAPDm LAPD LAPD radio radio PCM PCM 16/64 kbit/s ICS 243E - Ch4 Wire Winter 2001 BSSAP SS7 SS7 PCM PCM 64 kbit/s / 2.048 Mbit/s 4.26 Mobile Terminated Call 1:  calling... security checks 16, 17: set up connection ICS 243E - Ch4 Wire HLR 4 5 3 6 PSTN Winter 2001 2 GMSC 10 7 VLR 8 9 14 15 MSC 10 13 16 10 BSS BSS BSS 11 11 11 11 12 17 MS 4.27 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 7 MSC 8 2 9 MS ICS 243E - Ch4 Wire Winter 2001 1 10 BSS 4.28 MTC/MOC MS MTC BTS... MSC NSS MSC VLR signaling VLR HLR GMSC ISDN, PSTN IWF PDN O OSS EIR AUC ICS 243E - Ch4 Wire OMC Winter 2001 4.13 GSM: system architecture radio subsystem MS network and switching subsystem MS ISDN PSTN MSC Um BTS fixed partner networks Abis EIR SS7 BTS BSC VLR BTS BTS HLR BSC A BSS MSC IWF ISDN PSTN PSPDN CSPDN ICS 243E - Ch4 Wire Winter 2001 4.14 System architecture: radio subsystem radio subsystem MS... command clear command clear complete ICS 243E - Ch4 Wire HO complete HO complete clear complete Winter 2001 4.33 Security in GSM  Security services  access control/authentication user → SIM (Subscriber Identity Module): secret PIN (personal identification number) SIM → network: challenge response method  confidentiality voice and signaling encrypted on the wireless link (after successful authentication)... (“secret”, open interface) ICS 243E - Ch4 Wire Winter 2001 “secret”: • A3 and A8 available via the Internet • network providers can use stronger mechanisms 4.34 GSM - authentication SIM mobile network Ki AC RAND 128 bit RAND 128 bit RAND Ki 128 bit 128 bit A3 A3 SIM SRES* 32 bit MSC SRES* =? SRES SRES SRES 32 bit Ki: individual subscriber authentication key ICS 243E - Ch4 Wire 32 bit SRES SRES: signed... consisting of  Um BTS Abis BTS BSC BTS (Base Transceiver Station): sender and receiver BSC (Base Station Controller): controlling several transceivers MSC  BTS  A BTS BSC Interfaces MSC BSS ICS 243E - Ch4 Wire Um : radio interface Abis : standardized, open interface with 16 kbit/s user channels  A: standardized, open interface with 64 kbit/s user channels  Winter 2001 4.15 System architecture: network ... (ETSI, European Telecommunications Standardisation Institute)  simultaneous introduction of essential digital cellular services in three phases (1991, 1994, 1996) by the European telecommunication. ..  more than 100 million subscribers ICS 243E - Ch4 Wire Winter 2001 4.3 Performance characteristics of GSM       Communication  mobile, wireless digital communication; support for voice... (PSTN, ISDN) source/ destination network TE (U, S, R) tele services ICS 243E - Ch4 Wire Winter 2001 4.6 Bearer Services  Telecommunication services to transfer data between access points R and S