Basic Concepts of Cellular
Networks and Mobile IP
Aug 31, 2005
• Evolution of Cellular Networks
• Architectures
– AMPS
– GSM
• Security Mechanisms in GSM
Cellular Networks: Agenda
Origin of Wireless Communications
• Wireless communications gained popularity in
1930’s
– Mainly used for public safety by...
Concept of Cellular Networks
• A single high power
transmitter services one
larger area  multiple low
power transmitters ...
Evolution of Cellular Networks
1G 2G 3G 4G2.5G
Analog Digital
Circuit-switching Packet-switching
1G Systems
• Goal: To develop a working system that could
provide basic voice service
• Time frame: 1970-1990
• Technology...
2G Systems
• Goal: Digital voice service with improved quality
and also provide better data services
• Time Frame: 1990- 2...
• Goal: To provide better data rates and wider
range of data services and also act as a
transition to 3G
• Time frame: 200...
• Goal: High speed wireless data access and
unified universal standard
• Time frame: 2002-
• Two competing standards
– One...
• Future systems
• Goal:
– High mobility, High data rate, IP based
network
– Hybrid network that can interoperate with
oth...
AMPS
• 1G system developed by Bell Labs
• Analog system used FDMA/FDD
• 40Mhz of spectrum
• 842 channels
• rate: 10kbps
Public
Switched
Telephone
Network
MTSO
(MSC)
BTS
BTS
BTS
BTS
MTSO: Mobile Telecommunication Switching Office
Also known as...
Public
Switched
Telephone
Network
MTSO
(MSC)
BTS
BTS
BTS
BTS
Paging
message
Paging
message
Paging
message
Paging
message
A...
• Call arrives at MSC via the PSTN
• MSC then sends out a paging message via all
BTS on the FCC (Forward Control Channel)....
• Subscriber unit transmits an origination
message on the RCC
• Origination message contains
– MIN
– Electronic Serial Num...
• GSM system consists of three interconnected sub-
systems
– Base station Subsystem
• Mobile station (MS)
• Base Transceiv...
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...
BSC
BSC
BTS
BTS
BTS
BTS
BTS
BTS
BTS
BTS
Base Station Subsystem
MSC
HLR VLR AUC
Public Networks
Network Switching Subsystem...
• Principles
– Only authenticated users are allowed to access the
network
– No user data or voice communication is transmi...
• Mobile station contains
– A5 algorithm and IMEI
• The network stores
– A3, A5, A8 algorithms
• The Authentication Center...
Channel Establishment
Identity (TMSI or IMSI)
Authentication Request (RAND) Run Authentication
Algorithm (RAND)
Response
(...
At the Network end
At the Mobile user end in the SIM
A3 Algorithm
RAND (challenge)
Ki (128 bit)
Transmitted to mobile
A3 A...
• Ki is known only to the operator who programs
the SIM card and is tied to IMSI
• IMSI should be transmitted as less as p...
• GSM uses symmetric cryptography
– Data is encrypted using an algorithm which is seeded
by the ciphering key Kc
• Kc is k...
A5 algorithm
Kc (from A8 algorithm)
Count
(from TDMA frame)
User Data
Xor
Encoded
message
Security in GSM: Authentication
• Why Mobile IP?
• Basic Principle of Mobile IP
• Route Optimization
Mobile IP: Agenda
• Internet hosts/interfaces are identified by IP address
– Domain name service (DNS) translates host name to IP
address
– ...
• A host move to another network requires different
network address
– But this would change the host’s identity
– How can ...
CH
MH
Home network
MH
CHMH = mobile host CH = correspondent host
Home network Foreign network
Foreign network
How to direc...
• An analogy: what do you do when moving from one
apartment to another?
– Leave a forwarding address with your old post-of...
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Class 1

  1. 1. Basic Concepts of Cellular Networks and Mobile IP Aug 31, 2005
  2. 2. • Evolution of Cellular Networks • Architectures – AMPS – GSM • Security Mechanisms in GSM Cellular Networks: Agenda
  3. 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. 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. 5. Evolution of Cellular Networks 1G 2G 3G 4G2.5G Analog Digital Circuit-switching Packet-switching
  6. 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. 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. 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. 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. 10. • Future systems • Goal: – High mobility, High data rate, IP based network – Hybrid network that can interoperate with other networks 4G Systems
  11. 11. AMPS • 1G system developed by Bell Labs • Analog system used FDMA/FDD • 40Mhz of spectrum • 842 channels • rate: 10kbps
  12. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 25. A5 algorithm Kc (from A8 algorithm) Count (from TDMA frame) User Data Xor Encoded message Security in GSM: Authentication
  26. 26. • Why Mobile IP? • Basic Principle of Mobile IP • Route Optimization Mobile IP: Agenda
  27. 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 129.168.105.126 129.168.105.124 ISU: 129.168.*.* Internet Host 2 Gateway 130.203.4.112 PSU: 130.203.*.*
  28. 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. 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. 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
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