Visit for more Learning Resources Introduction to GSM -GSM was orignally developed to serve as the cellular service for Europiean contries -Currently it is the worlds most popular standard for new cellular radio and personal communication equipment throughout the world Visit for more Learning Resources 1

GSM Features 1] SIM-Subscriber Identitity Module Which stores information in the form of ID number,N/w and countries where subscriber is entitled to service,privacy keyes and other user specific information Each subscriber have 4 digit personal ID no to activate GSM service 2]On The Air Privacy- The privacy is made possible by encrypting the digital bit stream sent by GSM transmitter by using specific secret cryptographic key. That is only known to cellular carrier This key changes with time for each user Every carrier and GSM eqipment manufacturermust sign the memorandum of understanding before developing GSM eqipment 2

Continue...... The Mou is an international agreement which allows the sharing of cryptographic algorithm and other information between countries and carriers 3

GSM Services GSM services follow ISDN Guidlines and are classified into 1]Teleservices 2]Data service 3]User service 1]Teleservice-Standard Mobile telephony 2]Data service-Computer -to-computer communication and packet switched traffic 3]User service-they are divided into three major categories 1]Telephone service 2]Bearer Service 3]Supplimentary ISDN services 4

Continue..... 1]Telephone services- Including emergency calling,videotext and teletex 2]Bearer services- Supports packet switched protocol and data rates from 300 bps to 9.6 kbps The mode in which data can be transported could be transparent or non transparent mode 5

Continue 3]Supplimentary ISDN service - Digital in nature It includes call diversion,closed user groups and caller identification They also include the short messege service which allow GSM subscriber and base stations to transmit alphanumeric pages of limited length While simultaneously carring normal voice traffic SMS also provides cell brodcasting which allows GSM base Station to repetitively transmit ASCII messages with as many as fifteen strings in concatenating fashion 6

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GSM Architecture It consist three major components 1]Base station subsystem(BSS) 2]Operation and Maintenance Center(OMC) 3]Network and Switching Subsystem(NSS) 8

Base Station subsystem It consist Mobile station (MS),Base station Controllers(BSC),Base Tranreceiver Stations(BTS). BSC- Many base station system controlles by BSC Maintain connection to Ms Coding decoding of voice 9

BSC and BTS BSC provides all control function and physical link between MSC and BTS BTS is responsible for handling radio interface to the mobile station it is connected to MS via Um interface Um interface contains all machnism for wireless interface (TDMA.FDMA) BTS is radio equipment(Anteena) 10

OMC It connect to BSC it perfom function of handovers,system load,blocking rates etc.it also monitor the netwok and dignosis and troubleshoot the GSM network 11

Network Switching subsystem -It is responsible for performing call processing and subscriber related function. It includes 1] MSC-Mobile switching Center 2]HLR-Home location register(HLR) 3]Visitor Location register(VLR) 4] Authentication Center(AUC) 5]Equipment Identity Register(EIR)

MSC 1It handle communication between different MS connected to different BSCs. 2] the function of MSC is to locate different MS and associated BTS,call switching and authentication etc.

HLR It is a database for managing the mobile subscriber It stores permanent data of subscriber Eg.service profile,location information A home subscriber charges are less than roaming subscriber.

VLR It is database which consist temporary information about subscribers used by MSC to provide services to visiting subscriber

AUC This authentication center is used to provide authentication and encryption method that is used to verify the user identity, ensure confidentiality and secrecy of each cell

EIR It contains a list of all valid MS eqipment within the network

GSM Physical Channel (in detail) A single GSM Carrier is divided into eight timeslots. Therefore can support up to eight MS subscribers simultaneously. The timeslots are arranged in sequence and are conventionally numbered 0 to 7. Each repetition of this sequence is called a “TDMA frame”. 18 18

GSM Physical Channel Cntd.... BURST: The information carried in one timeslot is called a “burst”. The timing of the burst transmissions to and from the mobiles is critical. Each time slot of a TDMA frame lasts for duration of 156.25 bit periods or 576.9 µsec or 0.576 ms so a frame takes 4.615 ms GSM’s data transmission rate is 270.83 kbps per carrier frequency. Therefore one bit duration is 3.692 µsec 19 19

Burst structure As it has been stated before, the burst is the unit in time of a TDMA system. Four different types of bursts can be distinguished in GSM: The frequency-correction burst is used on the FCCH. It has the same length as the normal burst but a different structure. The synchronization burst is used on the SCH. It has the same length as the normal burst but a different structure. The random access burst is used on the RACH and is shorter than the normal burst. The normal burst is used to carry speech or data information. It lasts approximately 0.577 ms and has a length of 156.25 bits.

TB Data Training seq. Data TB GP Normál burst (NB) 000 58 bit 26 bit 58 bit 000 8.25 bit time TB 000...0 TB GP Frequency Correction Burst (FB) 000 142 bit 000 8.25 bit time TB Data Sync. seq. Data TB GP Synchronization burst (SB) 000 39 bit 64 bit 39 bit 000 8.25 bit time TB Sync. seq. Data TB GP Access burst (AB)or RACH 000 41 bit 36 bit 000 68.25 bit time

GSM Logical Channels There are two main groups of logical channels, traffic channels and control channels. Run over a physical channel, but not necessarily in all its time slots Traffic & Control channels are further classified into groups. 22 22

Traffic Channels Traffic channels can carry either Speech or Data In addition to these full-rate TCHs, there are also half-rate TCHs defined. 23 23

Full Rate Traffic Channels Half Rate Traffic Channels Traffic Channels Cntd.... Full Rate TCH TRAFFIC CHANNELS Half Rate TCH TCH/FR TCH/HR TCH/F 9.6 TCH/H4.8 TCH/F 4.8 TCH/H2.4 TCH/F 2.4 Full Rate Traffic Channels Half Rate Traffic Channels Name Type Data Rate NAME Type Data Rate TCH/FR Speech 22.8 kbps TCH/HR Speech 11.4 kbps TCH/F9.6 Data 22.8 kbps TCH/H9.6 Data 11.4 kbps TCH/F4.8 Data 22.8 kbps TCH/H4.8 Data 11.4 kbps TCH/F2.4 Data 22.8 kbps 24 24

GSM CONTROL CHANNELS (CCH) There are three main control channels in the GSM system Broadcast Channel (BCH) Common Control Channel (CCCH) Dedicated Control Channel (DCCH) Each control channel consists of several logical channels having different Control Functions. 25 25

GSM CONTROL CHANNELS (CCH) 26 26

The BCH provides synchronization for all mobiles within the cell. i) BROADCAST CONTROL CHANNEL (BCCH) The BCH provides synchronization for all mobiles within the cell. BCCH is a forward control channel. information such as cell and network identity. The BCCH is transmitted by the BTS at all times. The RF carrier used to transmit the BCCH is referred to as the BCCH carrier. Information on BCCH is monitored by the MS periodically (at least every 30 secs), when switched on & not in a call. The BCCH is transmitted at constant power at all times, and its signal strength is measured by all MS which may seek to use it. 27 27

BCCH Carries the following information (this is only a partial list): BROADCAST CONTROL CHANNEL Cntd…. BCCH Carries the following information (this is only a partial list): Location Area Identity (LAI). List of neighboring cells which should be monitored by the MS. List of frequencies used in the cell. Cell identity. Power control indicator. DTX permitted. Access control (for example, emergency calls, call barring). List of Channels currently in use within a cell. 28 28

ii) FREQUENCY CORRECTION CHANNEL (FCCH) FCCH is a forward control channel. The FCCH allows each subscriber unit to synchronize its internal frequency standard (local oscillator) to exact frequency of the base station 29 29

iii) SYNCHRONIZATION CHANNEL (SCH) It’s also a Downlink Channel. SCH allows each mobile to frame synchronize with the base station. 30 30

CCCH consist of three different channels. 2]COMMON CONTROL CHANNEL (CCCH) The common control channels occupy TS 0 of every GSM frame that is not otherwise used by the BCH. CCCH consist of three different channels. These channels are described in following slides 31 31

i) PAGING CHANNEL (PCH) It exists only on downlink (Forward channel). The paging channel (PCH) provides paging signals to all mobiles. notifies a specific mobile of an incoming call which originates from the PSTN. The PCH transmit the IMSI of the target subscriber, along with a request for acknowledgment from the mobile unit. the PCH is also used to provide cell broadcast ASCII text messages to all subscribers, as part of the SMS feature of GSM 32 32

ii) RANDOM ACCESS CHANNEL (RACH) The RACH is the only reverse link (uplink) channel. MS acknowledges a page from the PCH on RACH. RACH is also used by mobiles to originate a call. At the BTS, every frame (even the idle frame) will accept RACH transmissions from mobiles during TS 0 33 33

AGCH is used by the BS to provide forward link communication. iii) ACCESS GRANT CHANNEL (AGCH). AGCH is used by the BS to provide forward link communication. The AGCH is used by the BS to respond to a RACH sent by a mobile station. It carries data for MS to operate in a particular physical channel The AGCH is the final CCCH message sent by the base station to a subscriber 34 34

iv) CELL BROADCAST CHANNEL (CBCH) CBCH is used to transmit messages to be broadcasted to all MSs within a cell. it is considered a common channel because the messages can be received by all mobiles in the cell. 35 35

3]DEDICATED CONTROL CHANNELS (DCCH) There are three (03) types of Dedicated Control Channels in GSM. like Traffic Channels they are bi-directional. Same format and function in both the forward and reverse links. 36 36

i) STAND-ALONE DEDICATED COTROL CHANNEL (SDCCH) The SDCCH carries signaling data following the connection of the MS with the BTS just before a TCH assignment. The SDCCH ensures the Connection b/w MS and BS during the verification of subscriber unit & allocation of resources for the MS. 37 37

i) STAND-ALONE DEDICATED COTROL CHANNEL (SDCCH) The SDCCH carries signaling data following the connection of the MS with the BTS just before a TCH assignment. The SDCCH ensures the Connection b/w MS and BS during the verification of subscriber unit & allocation of resources for the MS. 38 38

The SDCCH can be thought of as an intermediate and temporary channel. i) STAND-ALONE DEDICATED COTROL CHANNEL (SDCCH)…… The SDCCH is requested from the MS via the RACH and assigned via the AGCH. The SDCCH can be thought of as an intermediate and temporary channel. A SDCCH may also be used for call setup, Authentication location updating SMS point to point e-Fax 39 39

On forward channel -BTS to the mobile station. ii) SLOW-ASSOCITED CONTROL CHANNEL (SACCH) On forward channel -BTS to the mobile station. transmit power level instruction. and specific timing advance instruction. 40 40

On reverse channel SACCH sends ii) SLOW-ASSOCITED CONTROL CHANNEL (SACCH)….. On reverse channel SACCH sends received signal strength (form Serving BTS). quality of the TCH. BCH measurement results (from neighboring cells) The SACCH is transmitted during the 13th frame of every speech dedicated control channel 41 41

iii) FAST-ASSOCIATED CONTROL CHANNEL (FACCH) FACCH carries urgent messages, and contains essentially the same type of information as the SDCCH. A FACCH is assigned whenever a SDCCH has not been dedicated for a particular user and there is an urgent message (e.g. handoff request). The FACCH gains access to a time slot by “stealing” frames from the traffic channel to which it is assigned 42 42

GSM Basic Call Sequence 43 43

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Call Setup Procedure 45 45

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GSM Speech Signal Processing For more detail contact us