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Intro to gsm

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Transcript

  • 1. GSM Security Overview (Part 1) Wireless telephone history
  • 2. It all started like this
    • First telephone (photophone) – Alexander Bell, 1880
    • The first car mounted radio
    • telephone – 1921
  • 3. Going further
    • 1946 – First commercial mobile radio-telephone service by Bell and AT&T in Saint Louis, USA. Half duplex(PTT)
    • 1973 – First handheld cellular phone – Motorola.
    • First cellular net
    • Bahrein 1978
  • 4. But what’s cellular? HLR, VLR, AC, EIR MSC PSTN BS
  • 5. Cellular principles
    • Frequency reuse – same frequency in
    • many cell sites
    • Cellular expansion – easy to add new cells
    • Handover – moving between cells
    • Roaming between networks
  • 6. Generation Gap
    • Generation #1 – Analog [routines for sending voice]
    • All systems are incompatible
    • No international roaming
    • Little capacity – cannot accommodate masses of subscribers
  • 7. Generation Gap(2)
    • Generation #2 – digital [voice encoding]
    • Increased capacity
    • More security
    • Compatibility
    • Can use TDMA or CDMA for increasing capacity
  • 8. TDMA
    • Time Division Multiple Access
    • Each channel is divided into timeslots, each conversation uses one timeslot.
    • Many conversations are multiplexed into a single channel.
    • Used in GSM
  • 9. CDMA
    • Code Division Multiple Access
    • All users share the same frequency all the time!
    • To pick out the signal of specific user, this signal is modulated with a unique code sequence.
  • 10. Back to Generations
    • Generation #2.5 – packet-switching
    • Connection to the internet is paid by packets and not by connection time.
    • Connection to internet is cheaper and faster [up to 56KBps]
    • The service name is GPRS – General Packet Radio Services
  • 11. The future is now
    • Generation #3
    • Permanent web connection at 2Mbps
    • Internet, phone and media: 3 in 1
    • The standard based on GSM is called UMTS. Not yet implemented.
    • The EDGE standard is the development of GSM towards 3G.
  • 12. GSM
    • More than 800 million end users in 190 countries and representing over 70% of today's digital wireless market.
      • source: GSM Association
    • Israel
      • Orange uses GSM
      • Pelephone and Cellcom are about to use GSM
  • 13. GSM Overview
  • 14. Into the architecture
    • Mobile phone is identified by SIM card.
    • Key feature of the GSM
    • Has the “secret” for authentication
  • 15. Into the architecture(2)
    • BTS – houses the radiotransceivers of the cell and handles the radio-link protocols with the mobile
    • BSC – manages radio resources (channel setup, handover) for one or more BTSs
  • 16. Into the architecture(3)
    • MSC – Mobile Switching Center
    • The central component of the network
    • Like a telephony switch plus everything for a mobile subscriber: registration, authentication, handovers, call routing, connection to fixed networks.
    • Each switch handles dozens of cells
  • 17. Into the architecture(4)
    • HLR – database of all users + current location. One per network
    • VLR – database of users + roamers in some geographic area. Caches the HLR
    • EIR – database of valid equipment
    • AuC – Database of users’ secret keys
  • 18. More GSM
    • GSM comes in three flavors(frequency bands): 900, 1800, 1900 MHz. 900 is the Orange flavour in Israel.
    • Voice is digitized using Full-Rate coding.
    • 20 ms sample => 260 bits . 13 Kbps bitrate
  • 19. Sharing
    • GSM uses TDMA and FDMA to let everybody talk.
    • FDMA: 25MHz freq. is divided into 124 carrier frequencies. Each base station gets few of those.
    • TDMA: Each carrier frequency is divided into bursts [0.577 ms]. 8 bursts are a frame.
  • 20. Channels
    • The physical channel in GSM is the timeslot.
    • The logical channel is the information which goes through the physical ch.
    • Both user data and signaling are logical channels.
  • 21. Channels(2)
    • User data is carried on the traffic channel (TCH) , which is defined as 26 TDMA frames.
    • There are lots of control channels for signaling, base station to mobile, mobile to base station (“aloha” to request network access)
  • 22. SS7
    • Signaling protocol for networks
    • Packet – switching [like IP]
    • GSM uses SS7 for communication between HLR and VLR (allowing roaming) and other advanced capabilities.
    • GSM’s protocol which sits on top of SS7 is MAP – mobile application part