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GSMGlobal System for Mobile Communications

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GSM

• New rapidly expanding & successful technology.• Fully Digitized technology for better speech quality.• Available in almost every part of the world.• Encryption of user information.• Very well defined interfaces makes truly open system.• Single number operation with World wide Roaming.

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Architecture of GSM

• Mobile Station (MS)• Mobile Hand set is one of the most complicated GSM device. It provides user

the access to the Network. Each handset has unique identity number called IMEI.

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SIM – Subscriber Identity Module

SIM is a removable module goes into the mobile handset. Each SIM has unique number called International Mobile Subscriber Identity (IMSI).It has built in Micro-computer & memory into it. It contains the ROM of 6 to 16KB,RAM of 128 to256 bytes and EEPROM of 3 to 8KB

Base Transceiver Station (BTS)• BTS has a set of Transceivers to

talk to MS.

• Capacity of BTS depends on no of Transceivers.

• Interface between MS & BTS is called Air I/f

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• Base Station Controller (BSC)

• BSC controls several BTSs.

• BSC manages channel allocation, & Handover of calls from one BTS to another BTS.

• BSC is connected to MSC via A interface.

• Interface between BSC & BTS is called A’bis I/f.

• BSC has database for all of its BTS’s parameters.

Mobile Switching Centre (MSC)• MSC is heart of the entire network

connecting Fixed line network to Mobile network.

• MSC manages all call related functions and Billing information.

• MSC is connected to HLR & VLR for subscriber identification & routing incoming calls.

• MSC capacity is in terms of no of subscribers.

• MSC is connected to BSC at one end and Fixed Line network on other end.

• Call Detail Record (CDR) is generated for each & every call in the MSC.

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• Visiting Location Register (VLR)

• Active Users are registered in VLR.

• It has temporary data base of all the active subscribers used for their call routing.

• HLR validates subscriber before registration.

• MSC ask VLR before routing incoming call.

• Home Location Register (HLR)

• All Subscribers data is stored in HLR.

• It has permanent data base of all the registered subscribers.

• HLR has series of numbers for all subscribers.

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• Authentication Centre (AUC)

• Authentication is a process to verify the subscriber SIM.

• Secret data & verification algorithm are stored in to the AUC.

• AUC & HLR combined to authenticate the subscribers.

• Subscriber authentication can be done on every call, if required.

• Equipment Identity Register (EIR)

• All subscriber's mobile handset data is stored in EIR.

• MSC asks mobile to send it IMEI & then checks it with data available in EIR.

• EIR has different classification for mobile handsets like, White list, Grey list & Black list.

• According to category the MS can make calls or can be stopped from making calls.

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• Operation and Maintenance centre (OMC)• All Previously described elements are connected to

OMC.• OMC monitors health of all network elements and

carries out maintenance if required.• OMC keeps track of all faults/errors occurred in

system.• OMC can do traffic analysis for surveillance

purpose.

Voice mail service centre – provides voice mail service.Short message service centre – provides SMS service.

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Features provided by gsm

• SMS & SMS delivery report• Call waiting• Caller ID hiding• Long Distance Prefix• Call Forwarding• Forward All• Forward if busy• Forward if not answered• Forward if unreachable

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Call Processing in GSM

• Mobile Originated Call• Mobile originated calls goes to BTS first & then to BSC.

• BSC forwards this call to MSC.

• MSC does authentication & call routing as per dialled digits

Mobile Terminated call• Mobile terminated calls comes to MSC first, where HLR/VLR inquiries carried out and as per information the

MS is paged in suitable BSC.

• BSC forwards this page to all children BTS where actual paging is done.

• After BTS gets response from Mobile it allocates channel for this call.

• After end of the call BTS informs BSC & MSC

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Working of Gsm

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GSM Frequency Bands

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GSM Carrier frequencies

• Frequency is divided into timeslots for individual phones. This allows eight full-rate or sixteen half-rate speech channels per radio frequency. These eight radio timeslots (or burst periods) are grouped into a TDMA frame. Half-rate channels use alternate frames in the same timeslot. The channel data rate for all 8 channels is 270.833 Kbit/s, and the frame duration is4.615 ms.• The transmission power in the handset is limited to a maximum of 2

watts in GSM 850/900 and 1 watt in GSM 1800/1900.

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Voice codecs

• GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 6.5 and 13 Kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate(6.5 Kbit/s) and Full Rate (13 Kbit/s). These used a system based on linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.

• As GSM was further enhanced in 1997 with the Enhanced Full Rate (EFR) codec, a 12.2 Kbit/s codec that uses a full-rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full-rate channels, or less robust but still relatively high quality when used in good radio conditions on half-rate channel.

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Gsm system security

• GSM uses several cryptographic algorithms for security. The A5/1, A5/2, and A5/3 stream ciphers are used for ensuring over-the-air voice privacy. A5/1 was developed first and is a stronger algorithm used within Europe and the United States; A5/2 is weaker and used in other countries. Serious weaknesses have been found in both algorithms: it is possible to break A5/2 in real-time with a cipher text-only attack. The system supports multiple algorithms so operators may replace that cipher with a stronger one.

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Open source implementation of gsm

Several open-source software projects exist that provide certain GSM features:• gsmd daemon by Openmoko• OpenBTS develops a Base transceiver station• The GSM Software Project aims to build a GSM analyser for less than

$1,000• OsmocomBB developers intend to replace the proprietary baseband

GSM stack with a free software implementation

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