Intro to GSM

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GSM Security Overview (Part 1):

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

It all started like this:

It all started like this First telephone (photophone) – Alexander Bell, 1880 The first car mounted radio telephone – 1921

Going further:

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

But what’s cellular?:

But what’s cellular? HLR, VLR, AC, EIR MSC PSTN BS

Cellular principles :

Cellular principles Frequency reuse – same frequency in many cell sites Cellular expansion – easy to add new cells Handover – moving between cells Roaming between networks

Generation Gap:

Generation Gap Generation #1 – Analog [routines for sending voice] All systems are incompatible No international roaming Little capacity – cannot accommodate masses of subscribers

Generation Gap(2):

Generation Gap(2) Generation #2 – digital [voice encoding] Increased capacity More security Compatibility Can use TDMA or CDMA for increasing capacity

TDMA:

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

CDMA :

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.

Back to Generations:

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

The future is now:

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.

GSM:

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

GSM Overview:

GSM Overview

Into the architecture:

Into the architecture Mobile phone is identified by SIM card. Key feature of the GSM Has the “secret” for authentication

Into the architecture(2):

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

Into the architecture(3):

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

Into the architecture(4):

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

More GSM:

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

Sharing:

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.

Channels:

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.

Channels(2):

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)

SS7:

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