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GSM Security Overview (Part 1)

Explore the evolution of mobile telephony, from the first handheld cellular phone in 1973 to the modern GSM network architecture. Learn about GSM generations, frequency reuse, time division multiple access (TDMA), Code Division Multiple Access (CDMA), and the transition towards 3G networks. Discover the architecture components such as Base Transceiver Station (BTS), Mobile Switching Center (MSC), Home Location Register (HLR), Visitor Location Register (VLR), Equipment Identity Register (EIR), and Authentication Center (AuC). Delve into GSM channels, Full-Rate coding, and the use of TDMA and FDMA for communication. Uncover the significance of SS7 signaling protocol and Mobile Application Part (MAP) in GSM networks.

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GSM Security Overview (Part 1)

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  1. GSM Security Overview (Part 1) Wireless telephone history Yuri Sherman

  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? MSC BS PSTN HLR, VLR, AC, EIR

  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 CDMAfor 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

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