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  • 1. Basic Concepts of Cellular Networks and Mobile IP Aug 31, 2005
  • 2. • Evolution of Cellular Networks • Architectures – AMPS – GSM • Security Mechanisms in GSM Cellular Networks: Agenda
  • 3. Origin of Wireless Communications • Wireless communications gained popularity in 1930’s – Mainly used for public safety by police and other government organizations – Not connected to the PSTN (Public Switching Telephone Networks) • First public mobile telephone service started in 1946 in United States – Using a single high power transmitter and large tower to cover an area of 50 km
  • 4. Concept of Cellular Networks • A single high power transmitter services one larger area  multiple low power transmitters service multiple smaller areas (Cells) • Frequency can be reused by cells far away from each other  improve usage • A set of cells that do not share frequency form a cluster • The cluster is then replicated throughout the desired communication area
  • 5. Evolution of Cellular Networks 1G 2G 3G 4G2.5G Analog Digital Circuit-switching Packet-switching
  • 6. 1G Systems • Goal: To develop a working system that could provide basic voice service • Time frame: 1970-1990 • Technology: FDMA/FDD • Example Systems: – Advanced Mobile Phone System (AMPS-USA) – Total Access Communication System (TACS-UK) – Nordic Mobile Telephone (NMT-Europe) • Incompatible analog systems
  • 7. 2G Systems • Goal: Digital voice service with improved quality and also provide better data services • Time Frame: 1990- 2000 • Technology: TDMA/TDD, CDMA • Example Systems: – Global System for Mobile (GSM-Europe) – IS-136(TDMA) – IS-95 (CDMA)
  • 8. • Goal: To provide better data rates and wider range of data services and also act as a transition to 3G • Time frame: 2000-2002 • Systems: – IS-95B – High Speed Circuit Switched Data (HSCSD) – General Packet Radio Service (GPRS) – Enhanced Data rates for GSM Evolution (EDGE) 2.5G Systems
  • 9. • Goal: High speed wireless data access and unified universal standard • Time frame: 2002- • Two competing standards – One based on GSM, IS-136 and PDC known as 3GPP – Other based on IS-95 named 3GPP2 • Completely move from circuit switching to packet switching • Enhanced data rates of 2-20Mbps 3G Systems
  • 10. • Future systems • Goal: – High mobility, High data rate, IP based network – Hybrid network that can interoperate with other networks 4G Systems
  • 11. AMPS • 1G system developed by Bell Labs • Analog system used FDMA/FDD • 40Mhz of spectrum • 842 channels • rate: 10kbps
  • 12. Public Switched Telephone Network MTSO (MSC) BTS BTS BTS BTS MTSO: Mobile Telecommunication Switching Office Also known as MSC (Mobile Switching Center) BTS: Base Transceiver Station AMPS: Architecture
  • 13. Public Switched Telephone Network MTSO (MSC) BTS BTS BTS BTS Paging message Paging message Paging message Paging message AMPS: Conventional Telephone  Cell Phone
  • 14. • Call arrives at MSC via the PSTN • MSC then sends out a paging message via all BTS on the FCC (Forward Control Channel). • The paging message contains subscriber’s Mobile Identification Number (MIN) • The mobile unit responds with an acknowledgement on the RCC (Reverse Control Channel) • MSC directs BS to assign FVC (Forward Voice Channel) and RVC (Reverse Voice Channel) AMPS: Conventional Telephone  Cell Phone
  • 15. • Subscriber unit transmits an origination message on the RCC • Origination message contains – MIN – Electronic Serial Number – Station Class Mark – Destination phone number • If BTS receives it correctly then it is passed on to MSC • MSC validates the information and connects the call AMPS: Cell phone initializes a call
  • 16. • GSM system consists of three interconnected sub- systems – Base station Subsystem • Mobile station (MS) • Base Transceiver Station (BTS) • Base Station Controllers (BSC) – Network Switching Subsystem (NSS) • Mobile Switching Center (MSC) • Home Location Register (HLR) • Visitor Location Register (VLR) • Authentication center (AUC) – Operation Support Subsystem • Operation Maintenance Centers GSM: Architecture
  • 17. Base Station Subsystem BSC BSC BTS BTS BTS BTS BTS BTS BTS BTS •BSCs connect the MS to the NSS •The BTS provides last mile connection to the MS and communication is between the BTS and MS •Handover between BTS within same BSC is handled by the BSC GSM
  • 18. BSC BSC BTS BTS BTS BTS BTS BTS BTS BTS Base Station Subsystem MSC HLR VLR AUC Public Networks Network Switching Subsystem OSS Operation Support Subsystem GSM
  • 19. • Principles – Only authenticated users are allowed to access the network – No user data or voice communication is transmitted in “clear text” • The subscriber identity module (SIM) card is a vital part of GSM security. It stores – International Mobile Subscriber Identity (IMSI) – Ciphering Key Generating Algorithm (A8) – Authentication Algorithm (A3) – Personal Identification Number – Individual Subscriber Authentication Key (Ki) Security in GSM
  • 20. • Mobile station contains – A5 algorithm and IMEI • The network stores – A3, A5, A8 algorithms • The Authentication Center stores – IMSI – Temporary Mobile Subscriber Identity (TMSI) – Individual Subscriber Authentication Key (Ki) Security in GSM
  • 21. Channel Establishment Identity (TMSI or IMSI) Authentication Request (RAND) Run Authentication Algorithm (RAND) Response (SRES,Kc) Authentication Response (SRES) •RAND is 128 bit random sequence •SRES is signed response generated for authentication Security in GSM: Authentication Network Mobile Station SIM
  • 22. At the Network end At the Mobile user end in the SIM A3 Algorithm RAND (challenge) Ki (128 bit) Transmitted to mobile A3 Algorithm RAND (challenge) Ki (128 bit) A8 Algorithm Kc used for encryption of user data and signaling data Proper authentication completed if result is zero Transmitted back to base station Authentication based on RAND
  • 23. • Ki is known only to the operator who programs the SIM card and is tied to IMSI • IMSI should be transmitted as less as possible. • Only TMSI is used for authentication • TMSI is periodically updated Security in GSM: Authentication
  • 24. • GSM uses symmetric cryptography – Data is encrypted using an algorithm which is seeded by the ciphering key Kc • Kc is known only to base station and mobile phone and is frequently changed • The A5 algorithm is used for ciphering the data • Along with Kc the algorithm is ‘seeded’ by the value based on the TDMA frame • Internal state of the algorithm is flushed after a burst Security in GSM: Data Encryption
  • 25. A5 algorithm Kc (from A8 algorithm) Count (from TDMA frame) User Data Xor Encoded message Security in GSM: Authentication
  • 26. • Why Mobile IP? • Basic Principle of Mobile IP • Route Optimization Mobile IP: Agenda
  • 27. • Internet hosts/interfaces are identified by IP address – Domain name service (DNS) translates host name to IP address – IP address identifies host/interface and locates its network IP Addressing Gateway Host 1 MH ISU: 129.168.*.* Internet Host 2 Gateway PSU: 130.203.*.*
  • 28. • A host move to another network requires different network address – But this would change the host’s identity – How can others still reach the moving host? How can on- going connections to the moving host be not interrupted? • Applications – GPRS (2.5G), 3G cellular networks – Mission-critical applications • IP devices held by police, ambulance, coast guards are always connected when moving – Moving offices, … Problems
  • 29. CH MH Home network MH CHMH = mobile host CH = correspondent host Home network Foreign network Foreign network How to direct packets to moving hosts transparently? Routing for Mobile Host
  • 30. • An analogy: what do you do when moving from one apartment to another? – Leave a forwarding address with your old post-office! – The old post-office forwards mails to your new post- office, which then forwards them to you • Mobile IP: – Two other entities – home agent (old post-office), foreign agent (new post-office) – Mobile host registers with home agent the new location – Home agent captures packets meant for mobile host, and forwards it to the foreign agent, which then delivers it to the mobile host Mobile IP: Basic Idea