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Basic Concepts of Cellular Networks and Mobile IP

Basic Concepts of Cellular Networks and Mobile IP. Aug 31, 2005. Cellular Networks: Agenda. Evolution of Cellular Networks Architectures AMPS GSM Security Mechanisms in GSM. Origin of Wireless Communications. Wireless communications gained popularity in 1930’s

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Basic Concepts of Cellular Networks and Mobile IP

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  1. Basic Concepts of Cellular Networks and Mobile IP Aug 31, 2005

  2. Cellular Networks: Agenda • Evolution of Cellular Networks • Architectures • AMPS • GSM • Security Mechanisms in GSM

  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. 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 Concept of Cellular Networks

  5. Evolution of Cellular Networks 2G 3G 4G 1G 2.5G 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. 2.5G Systems • 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)

  9. 3G Systems • 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

  10. 4G Systems • Future systems • Goal: • High mobility, High data rate, IP based network • Hybrid network that can interoperate with other networks

  11. AMPS • 1G system developed by Bell Labs • Analog system used FDMA/FDD • 40Mhz of spectrum • 842 channels • rate: 10kbps

  12. AMPS: Architecture BTS MTSO (MSC) Public Switched Telephone Network BTS BTS BTS MTSO: Mobile Telecommunication Switching Office Also known as MSC (Mobile Switching Center) BTS: Base Transceiver Station

  13. AMPS: Conventional Telephone  Cell Phone BTS MTSO (MSC) Public Switched Telephone Network BTS Paging message Paging message Paging message Paging message BTS BTS

  14. AMPS: Conventional Telephone  Cell Phone • 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)

  15. AMPS: Cell phone initializes a call • 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

  16. GSM: Architecture • 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

  17. GSM BTS BTS BSC BTS BTS BTS BTS BSC BTS BTS Base Station Subsystem • The BTS provides last mile connection to the MS and communication is between the BTS and MS • BSCs connect the MS to the NSS • Handover between BTS within same BSC is handled by the BSC

  18. Network Switching Subsystem GSM HLR VLR AUC BTS BTS BSC BTS BTS MSC Public Networks BTS BTS BSC OSS BTS Operation Support Subsystem BTS Base Station Subsystem

  19. Security in GSM • 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)

  20. Security in GSM • 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)

  21. Security in GSM: Authentication Mobile Station Channel Establishment SIM Identity (TMSI or IMSI) Authentication Request (RAND) Run Authentication Algorithm (RAND) Network Response (SRES,Kc) Authentication Response (SRES) • RAND is 128 bit random sequence • SRES is signed response generated for authentication

  22. Authentication based on RAND At the Network end Transmitted to mobile RAND (challenge) A3 Algorithm Ki (128 bit) Proper authentication completed if result is zero At the Mobile user end in the SIM Transmitted back to base station RAND (challenge) A3 Algorithm Ki (128 bit) Kc used for encryption of user data and signaling data A8 Algorithm

  23. Security in GSM: Authentication • 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

  24. Security in GSM: Data Encryption • 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

  25. Security in GSM: Authentication Xor Kc (from A8 algorithm) A5 algorithm Encoded message Count (from TDMA frame) User Data

  26. Mobile IP: Agenda • Why Mobile IP? • Basic Principle of Mobile IP • Route Optimization

  27. IP Addressing Internet ISU: 129.168.*.* • 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 Gateway PSU: 130.203.*.* Gateway Host 1 MH 129.168.105.126 129.168.105.124 Host 2 130.203.4.112

  28. Problems • 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, …

  29. Routing for Mobile Host MH CH MH = mobile host CH = correspondent host Foreign network Home network How to direct packets to moving hosts transparently? CH Home network Foreign network MH

  30. Mobile IP: Basic Idea • 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

  31. A MH Moves to a Foreign Network CH MH HA MH = mobile host CH = correspondent host HA = home agent FA = foreign agent 130.203.*.* 129.186.*.* Home network Foreign network FA 129.186.105.216 130.203.4.112 • MH discovers a FA in the foreign network. • MH seeks a care-off address from the FA • MH registers/authenticates its care-off address to the HA in its home network.

  32. Packets towards MH CH MH Foreign network Home network HA FA MH = mobile host CH = correspondent host HA = home agent FA = foreign agent • HA receives packets for the MH. • HA tunnels packets to FA • FA decapsulates packets and delivers them to MH

  33. Packet Addressing Packet from CH to MH Source address = address of CH Destination address = home IP address of MH Payload Home agent intercepts above packet and tunnels it Source address = address of HA Destination address = care-of address of MH Source address = address of CH Destination address = home IP address of MH Original payload

  34. If MH Moves Again Foreign network #1 Foreign network #2 CH MH MH FA #1 FA #2 HA Home network • MH registers new address (FA #2) with HA & FA #1 • HA tunnels packets to FA #2, which delivers them to MH • Packets in flight can be forwarded from FA #1 to FA #2

  35. Packets from MH CH MH Foreign network Home network HA FA Mobile hosts also send packets • Mobile host uses its home IP address as source address • Lower latency • Still transparent to correspondent host • No obvious need to encapsulate packet to CH • Triangle Routing

  36. Route Optimization CH MH HA Foreign network Home network FA • When HA receives a packet (from CH) to tunnel to FA: • It sends a binding message to CH with the care-of address of the MH. • CH caches the address, and forward later packets directly to the care-of address.

  37. Route Optimization • When a FA receives a tunneled message, but sees no visitor entry for the mobile host, it generates a binding warning message to the appropriate HA • When a HA receives a warning, it issues an update message to the CH, which removes the care-of address from its cache.

  38. Notice • Topic of next class: Wireless LAN and Mobile Ad Hoc Network • Reminder: pick the papers you want to present (with preferred dates if you want) ASAP.

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