GSM Global System for Mobiles.

GSM Global System for Mobiles

TOPICS GSM CONCEPTS GSM SYSTEM ARCHITECTURE IDENTITIES USED IN GSM
GSM CHANNELS GSM RADIO LINK MOBILITY MANAGEMENT CALL MANAGEMENT RADIO RESOURCE MANAGEMENT

Telecom Basics Communication Voice and Data Analog and Digital
Circuit Switched and Packet Switched Media - Copper Wire, Co-axial cable, Air, Optical Fibre Networks -PSTN, ISDN, PDN and Mobile Networks

Background to GSM 1G : Advanced Mobile Phone Service (AMPS)
Analog, Circuit Switched, FDMA, FDD 2G : Digital Advanced Mobile Phone Service (D-AMPS) Digital, Circuit Switched, FDMA, FDD 2G : Global System for Mobile (GSM) Digital, Circuit Switched, FDMA and TDMA, FDD 2G : Code Division Multiple Access (CDMA) Digital, Circuit Switched, CDMA, FDD

GSM History

Development of the GSM Standard
1992: Official commercial launch of GSM service in Europe. First Launch in Finland 1982: Groupe Spécial Mobile (GSM) created 1984: Description of GSM features 1993: The GSM-MoU has 62 signatories in 39 countries worldwide. 1985: List of recommendations settled 1995: Specifications of GSM phase 2 are frozen. 1987: Initial MoU (Memorandum of Understanding) aside the drafting of technical specifications was signed by network operators of 13 countries: 1982: CEPT (Conférence Européenne des Administrations des Postes et Télécommunications) decides to establish a "Groupe Spécial Mobile" (the initial origin of the Term GSM) to develop a set of common standards for a future pan-European Cellular Mobile Network. 1984: Establishment of three Working Parties to define and describe GSM features: the radio interface, transmission and signaling protocols, interfaces and network architecture. 1985, 1986: Discussion and adoption of a list of recommendations to be generated by the Group Spécial Mobile. A so-called permanent nucleus is established to continuously coordinate the work, which is intensely supported by industry delegates. Much thought goes into developing a radio transmission prototype. 1999: GSM MoU joins 3GPP (UMTS) GPRS Trials begins 1988: Validation and trials, of the radio interface. 2000: 480M GSM Network operators Worldwide First GPRS Networks roll out 1991: First system trials are demonstrated at the Telecom 91 exhibition. End 2002: 792M GSM Net work Operators Worldwide

APPROVAL SPECIFICATIONS
GSM Specifications 12 SERIES OPERATION AND MAINTENANCE 01 SERIES GENERAL 02 SERIES SERVICE ASPECTS 03 SERIES NETWORK ASPECTS 04 SERIES MS-BSS INTERFACE AND PROTOCOLS 05 SERIES PHYSICAL LAYER ON THE RADIO PATH. 06 SERIES SPEECH CODING SPECIFICATIONS 07 SERIES TERMINAL ADAPTERS FOR MOBILE STATIONS 11 SERIES EQUIPMENT AND TYPE APPROVAL SPECIFICATIONS 10 SERIES SERVICE INTERWORKING 09 SERIES NETWORK INTERWORKING 08 SERIES BSS TO MSC INTERFACES One important question was how far GSM should go in its specification work; that is, to what degree the system had to be specified so as to be identical in all countries, and how much could be left to the operators and suppliers to agree upon. Clearly, without identical air interfaces in all networks, the subscribers were not going to have free roaming between networks. This was considered to be the absolute minimum degree of standardization, and this solution was favored. It might have been seen to be advantageous to specify everything in the system, including the hardware and the mobile station and even other parts of the system. It was agreed however that there would be no attempt to specify the system in such detail. Basically, only the functional interfaces between the major buildings blocks would be specified. This approach had several advantages, perhaps the most important of which is that for each major building block, the principle of functional specifications offers each operator, and thus the customer, the opportunity to purchase whatever make of equipment he wants, thus setting the stage for maximum competition between manufacturers. For instance the fact that an operator has purchased an exchange from a certain supplier does not force him to go on buying equipment from the same supplier. Standardized electrical interfaces as well as protocols were provided for both the fixed network and subscriber equipment. These included standardized rate adaptations compatible with conventional ISDN (Integrated Services Digital Network) definitions.

Increasing GSM Data Rates
10 sec 1 min 10 min 1 hour UMTS E/GPRS ISDN PSTN GSM web photo video clip report GSM phase 2 We can currently use a data terminal attached to an MS to connect to any standard data service provided by the PSTN, ISDN or PDN networks as long as the network accepts a data rate of 9.6 kbps and the Inter-Working Function (IWF) is installed. This includes access to the Web, , fax etc.. Use of these facilities is generally limited due to the speed of the communication. Internet use is expensive and slow due to the limited data rate and the circuit switched nature of the GSM system. The BSS provides two modes: transparent data service, non transparent data service, using RLP protocol between the MS and the IWF. GSM phase 2+ A new service has been standardized in ETSI to reach 14.4 kbps user rate on one TS. This new data rate is the result of a new channel coding on the radio interface. This enhancement is a part of a global strategy aimed at offering higher data rates and called High Speed Circuit Switched Data (HSCSD). HSCSD allows 14.4 kbps in one TS and up to 56 kbps in the future, using multiple TSs. It is however, still a circuit switched system which will supply expensive connections unless the operators pricing schemes are imaginative. It will help those who use data over GSM today and encourage others to use the services but it does involve a capacity penalty for the network. GPRS = General Packet Radio Service HSCSD = High Speed Circuit Switched Data EDGE = Enhanced Data rate for GSM Evolution UMTS = Universal Mobile Telecommunication System Transmission Time

Wireless Data Technology Options
2 M UMTS 1 M packet EDGE throughput kbps GPRS 100 k circuit HSCSD 64 k 14.4 Today, GSM has the capability to handle messages via the Short Message Service SMS and a 14.4 kbps circuit switched data service for data/fax calls. This maximum speed of 14.4 kbps is relatively low compared to wireline modem speeds of 34.4 and even 56 kbps. To enhance the current data capabilities of GSM, operators and infrastructure providers have specified new extensions to GSM phase 2: High Speed Circuit Switched Data (HSCSD) by using several circuit channels. General Packet Radio Service (GPRS) to provide packet radio access to external Packet Data Networks (Internet or X.25 networks). Enhanced Data rate for Gsm Evolution (EDGE), using a new modulation scheme, to allow up to three times higher throughput (for HSCSD and GPRS). Universal Mobile Telecommunication System (UMTS), a new wireless technology but utilizing new infrastructure deployment. These extensions enable: higher data throughput, better spectral efficiency, lower call setup times. 10 k 9.6 Time frame 1 k 1998 1999 2000 2001 2002 GPRS = General Packet Radio Service HSCSD = High Speed Circuit Switched Data EDGE = Enhanced Data rate for GSM Evolution UMTS = Universal Mobile Telecommunication System

Circuit-Switched or Packet-Switched
Circuit mode Packet mode A B C D E F G H The typical internet data traffic is characterized by an ON/OFF model. The user spends a certain amount of time downloading web pages in quick succession followed by indefinite periods of inactivity. During this inactivity the end-user may read the information or think or do something else. The traffic is sporadic and can be characterized as data packets of average size 16 kbytes/s with average intervals of 7 seconds. If a circuit switch connection is used to access the Internet, the bandwidth dedicated for the entire duration of the session is under-utilized. A→F A→F C→G A→F D→H C→G D→H A→F C→G C→G D→H A→F C→G D→H D→H

GSM Concepts - Cellular Structure
1 2 3 4 5 6 7 Cellular Networking technology that breaks geographic area into cells shaped like honey comb Cell is the radio coverage area of one base transceiver station

What are the types in GSM Network?
GSM-900 (Channels 125 operating band 900Mhz carrier spacing 200khz spacing 45Mhz) GSM (Channels 374 spacing 95Mhz) GSM -1900(Used in USA)

Multiple Access Technique
Multiple Access – Achieved by dividing the available radio frequency spectrum, so that multiple users can be given access at the same time. FDMA - Frequency Division Multiple Access ( eg: GSM each Frequency channel is 200KHz) TDMA - Time Division Multiple Access ( eg: GSM each frequency channel is divided into 8 timeslots) CDMA - Code Division Multiple Access (eg: IS95- Each User data is coded with a unique code)

Duplex Technique Duplex - How the up link and Down link of a user is separated FDD - Frequency Division Duplex (eg:In GSM the up link and down link of a user is separated by 45MHz ) TDD - Time Division Duplex (the up link and down link of a user will be at the same frequency but at different Time )

GSM System specifications
Frequency band Uplink MHz Downlink MHz Duplex Frequency Spacing 45MHz Carrier separation KHz Frequency Channels 124 Time Slots /Frame(Full Rate) 8 Voice Coder Bit Rate 13Kbps Modulation GMSK Air transmission rate Kbps Access method FDMA/TDMA Speech Coder RPE-LTP-LPC

Access Techniques Uplink 890 MHz to 915 MHz
Down Link 935 MHz to 960 MHz 25 MHz divided into 125 channels of 200 KHz bandwidth 890.0 890.2 890.4 914.8 915.0 935.0 935.2 935.4 959.8 960.0 UP DOWN

Access Techniques ... Time Division Multiple Access
Each carrier frequency subdivided in time domain into 8 time slots Each mobile transmits data in a frequency, in its particular time slot - Burst period = milli secs. 8 time slots called a TDMA frame. Period is .577 * 8 = milli secs 1 2 3 4 5 6 7 4.616 ms 0.577 ms

GSM in comparison with other Standards
GSM gives mobility without any loss in Audio quality Encryption techniques used gives high security in the air Interface and also use of SIM. Bit Interleaving for high efficiency in Transmission. Variable Power (Power budgeting- extend battery life) Minimum Interference. Features-CCS7 Signaling SMS (Short Message Services) Emergency Calls CELL Broadcast

GSM - Network Structure
MS Um HLR BTS VLR BSC Abis MSC B A H C AuC MS GMSC BTS E F Abis EIR A E MSC PSTN BSC Um BTS X.25 VLR X.25 OMC Server

GSM Network Switching System Base Station System SS AUC External
PSTN & PDN N/W VLR HLR EIR OMC MSC MS Mobile Station BTS Base transceiver System BSC Base Station Controller MSC Mobile Switching Center HLR Home Location Register VLR Visitor Location Register EIR Equipment Identity Register AUC Authentication Center OMC Operation And Maintenance Center Base Station System BSS BSC BTS MS

GSM Architecture VMSC SMSC GSM Air interface B S C AUC HLR PSTN TRAU
Abis interface PSTN TRAU MSC VLR B S C BTS BTS EIR BTS OMCS BTS BTS BTS Network and switching subsystem Mobile Station A interface SS7 / speech OMCR X.25 SS7 Base Station System

Fundamentals 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Downlink (TDMA frame) = 8 TS
960 MHz 124 123 ……. …… 2 1 959.8MHz TS: Time slot GSM utilizes two bands of 25 MHz MHz band is used for uplink while the MHz is used for downlink. The frequency bands are divided into 200 KHz wide channels called ARFCNs (Absolute Radio Frequency Channel Numbers) i.e. there are 125 ARFCNs out of which only 124 are used. Each ARFCN supports 8 users with each user transmitting / receiving on a particular time slot (TS). DOWNLINK Downlink (TDMA frame) = 8 TS 1 2 3 4 5 6 7 200KHz 935.2 Mhz 935 MHz Data burst = bit periods = 576.9s 915 MHz 124 123 ……. …… 2 1 914.8 MHz 45 MHz 1 2 3 4 5 6 7 Delay 200KHz UPLINK Uplink (TDMA frame) 890.2 MHz 890 MHz Therefore 1 TDMA frame = x 8 = 1250 bits and has a duration of s x 8 = ms The technology

Mobile Station (MS) Hand portable unit
Contains Mobile Equipment(ME) and Subscriber Identity Module (SIM)

Mobile Equipment(ME) Frequency and Time Synchronization
Voice encoding and transmission Voice encryption/decryption functions Power measurements of adjacent cells Display of short messages International Mobile Equipment Identifier (IMEI)

SIM Portable Smart Card with memory (ROM-6KB to 16KB-A3/A8 algorithm, RAM- 128KB TO 256KB, EEPROM- 3KB to 8KB ) Static Information International Mobile Subscriber Identity(IMSI) Personal Identification Number (PIN) Authentication Key (Ki) Dynamic Information Temporary Mobile Subscriber Identity(TMSI) Location Area Identity (LAI) Phone memories, billing information Ability to store Short Messages received

Base Transceiver Station (BTS)
Handles the radio interface to the mobile station. Consists of one or more radio terminals for transmission and reception Each Radio terminal represents an RF Channel TRX and MS communicates over Um interface Received data transcoding Voice encryption/decryption Signal processing functions of the radio interface Uplink Radio channel power measurements

Base Station Controller (BSC)
Provides all the control functions and physical links between the MSC and BTS External Interfaces ‘Abis’ interface towards the BTS ‘A’ interface towards the MSC Monitors and controls several BTSs Management of channels on the radio interface Alarm Handling from the external interfaces Performs inter-cell Handover Switching from ‘Abis’ link to the ‘A’ link Interface to OMC for BSS Management

Mobile Switching Center (MSC)
Performs call switching Interface of the cellular network to PSTN Routes calls between PLMN and PSTN Queries HLR when calls come from PSTN to mobile user Inter-BSC Handover Paging Billing

Home Location Register (HLR)
Stores user data of all Subscribers related to the GMSC International Mobile Subscriber Identity(IMSI) Users telephone number (MS ISDN) Subscription information and services VLR address Reference to Authentication center for key (Ki) Referred when call comes from public land network

Visitor Location Register (VLR)
Database that contains Subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR Identity of Mobile Subscriber Copy of subscriber data from HLR Generates and allocates a Temporary Mobile Subscriber Identity(TMSI) Location Area Code Provides necessary data when mobile originates call

Authentication Center (AuC)
Stores Subscriber authentication data called Ki, a copy of which is also stored in in the SIM card Generates security related parameters to authorize a subscriber (SRES-Signed RESponse) Generates unique data pattern called Cipher key (Kc) for user data encryption Provides triplets - RAND, SRES & Kc, to the HLR on request.

EIR (Equipment Identity Register)
EIR is a database that contains a list of all valid mobile station equipment within the network, where each mobile station is identified by its International Mobile Equipment Identity(IMEI). EIR has three databases., White list - For all known,good IMEI’s Black list - For all bad or stolen handsets Grey list - For handsets/IMEI’s that are on observation

Interfaces and Protocols
Um Abis A C B E D F Digital Networks POTS TUP ISUP MAP BSSAP LAPD LAPDm G

GSM Entities and Signaling Architecture

GSM Protocols CM - Connection Management MM - Mobility Management
RR - Radio resource LAPDm - LAPD for mobile LAPD - Link Access Procedure for D channel BTSM - BTS Management Part BSSAP - BSS Application Part (BSC - MSC) DTAP Direct Transfer Application Part (MS - MSC) MAP Mobile Application Part MTP Message Transfer part of SS7 SCCP Signalling Connection Control Part of SS7 TCAP Transaction Capabilities Application Part ISUP - ISDN User Part

Functional Plane of GSM
MS BTS BSC MSC/VLR HLR GMSC CC MM RR Trans MS BTS BSC MSC/ HLR GMSC VLR

TOPICS GSM CONCEPTS GSM SYSTEM ARCHITECTURE IDENTIFIERS USED IN GSM

Subscriber Identity -MSISDN
The MSISDN is a GSM directory number which uniquely identifies a mobile subscription in the Public Switched Telephone Network (PSTN). Calls will be routed from the PSTN and other networks based on the Mobile Subscribers’ MSISDN number. MSISDN= CC + NDC + SN CC= Country Code (91) NDC= National Destination Code(98370) SN= Subscriber Number (12345)

International Mobile Subscriber Identity [IMSI]
Subscriber always identified within the GSM network by the IMSI This is used for all signaling in the PLMN stored in SIM and HLR/VLR The IMSI consists of three different parts MCC = Mobile Country Code(3 Digits) MNC = Mobile Network Code(2 Digits) MSIN = Mobile Station Identification Number(Upto 10 digits)

Temporary Mobile Subscriber Identity [TMSI]
The TMSI is used for the subscriber’s confidentiality. It should be combined with the LAI to uniquely identify a MS. Since the TMSI has only local significance (that is, within the MSC/VLR area), the structure may be chosen by each administration. The TMSI should not consist of more than four octets.

Mobile Station Roaming Number[MSRN]
HLR knows in what Service area the subscriber is located. In order to provide a temporary number to be used for routing, the HLR requests the current MSC/VLR to allocate a Mobile Station Roaming Number(MSRN) to the called subscriber and to return it. At reception of the MSRN, HLR sends it to the MSC, which now can route the call to the VLR where the called subscriber is currently registered.

International Mobile Equipment Identity [IMEI]
The IMEI is used for equipment identification. An IMEI uniquely identifies a mobile station as a piece or assembly of equipment. IMEI = TAC + FAC + SNR + sp TAC= Type Approval Code (6 digits),determined by GSM body FAC= Final Assembly Code (2 digits), identifies themanufacturer SNR= Serial Number (6 digits), uniquely identifying all equipment within each TAC and FAC sp = Spare for future use (1 digit)

Location Area Identity
LAI identifies a location area which is a group of cells.. It is transmitted in the BCCH. When the MS moves into another LA (detected by monitoring LAI transmitted on the BCCH) it must perform a LU. LAI = MCC + MNC + LAC MCC= Mobile Country Code(3 digits), identifies the country MNC= Mobile Network Code(1-2 digits), identifies the GSM-PLMN LAC= Location Area Code, identifies a location area within a GSM PLMN network. The maximum length of LAC is 16 bits,enabling different location areas to be defined in one GSM PLMN.

Channels : differentiating between Physical and Logical channels
Physical channels : The combination of an ARFCN and a time slot defines a physical channel. Logical channels : These are channels specified by GSM which are mapped on physical channels.

Channel concept Physical channel:
One timeslot of a TDMA-frame on one carrier is referred to as a physical channel. There are 8 physical channels per carrier in GSM,channel 0-7(timeslot 0-7) Logical channel: A great variety of information must be transmitted between BTS and the MS,for e.g. user data and control signaling.Depending on the kind of information transmitted we refer to different logical channels.These logical channels are mapped on physical channel.

Logical Channels on Air interface
COMMON CHANNELS DEDICATED CHANNELS BROADCAST CHANNELS COMMON CONTROL CHANNELS DEDICATED CONTROL CHANNELS TRAFFIC CHANNELS FCCH SCH BCCH SDCCH SACCH FACCH PCH RACH AGCH TCH/F TCH/H TCH/EFR

Frequency plan and importance of BCCH
Sectored antennas B5 B6 B4 BPL frequency plan: Broadcast frequencies : 15 Broadcast channels = 48-62 15 Hopping channels = 32-46 B3 B7 B1 B8 B2 B12 MS ( monitoring the broadcast radio B1 in ‘idle mode’ ) B9 B10 B11 F0 F1 F2 F3 F4 F5 F10 F11 F50 F S B ….. I F,S,B exist in time slot 0 of each frame

Broadcast channels BCH
Broadcast Channel-BCH Alloted one ARFCN & is ON all the time in every cell. Present in TS0 and other 7 TS used by TCH. Frequency correction channel-FCCH To make sure this is the BCCH carrier. Allow the MS to synchronize to the frequency. Carries a 142 bit zero sequence and repeats once in every 10 frames on the BCH. Synchronization Channel-SCH This is used by the MS to synchronize to the TDMA frame structure within the particular cell. Listening to the SCH the MS receives the TDMA frame number and also the BSIC ( in the coded part- 39 bits). Repeats once in every 10 frames.

The MS is monitoring the BCCH and has all the decoded information stored on the SIM ( including the LAC) As soon as the mobile is on a TCH it sends the signal strength indication on the corresponding SACCH The BSC monitors the signal strengths and on analysis sends a ‘handoff request’ on FACCH. The handoff process is completed on the FACCH. After the completion of call, the MS starts monitoring the BCCH again. On finding the LAC (stored on SIM) and that decoded from the BCCH to be different , the MS requests a ‘Location Update’ through SDCCH. Back

Broadcast channels BCH ...
The last information the MS must receive in order to receive calls or make calls is some information concerning the cell. This is BCCH. This include the information of Max power allowed in the cell. List of channels in use in the cell. BCCH carriers for the neighboring cells,Location Area Identity etc. BCCH occupies 4 frames (normal bursts) on BCH and repeats once every Multiframe. This is transmitted Downlink point to multipoint. Cell Broadcast Channel - CBCH Used for the Transmission of generally accessible information like Short Message Services(SMS)

What information does Broadcast Control channel (BCCH) contain?
Serves as a Beacon for the Cell Country Code (CC) and the Network Code (NC) Location Area Identity (LAI) List of neighboring cells which should be monitored by MS List of frequencies used in the cell Cell identity Back

Common Control Channels CCCH
Shares TS-0 with BCH on a Multiframe. Random access channel-RACH: Used by Mobile Station for requesting for a channel. When the mobile realizes it is paged it answers by requesting a signaling channel (SDCCH) on RACH. RACH is also used by the MS if it wants to originate a call. Initially MS doesn’t know the path delay (timing advance), hence uses a short burst (with a large guard period = bits). MS sends normal burst only after getting the timing advance info on the SACCH. It is transmitted in Uplink point to point.

Common Control Channels CCCH ..
Access Grant Channel-AGCH On request for a signaling channel by MS the network assigns a signaling channel(SDCCH) through AGCH. AGCH is transmitted on the downlink point to point. Paging Channel-PCH The information on this channel is a paging message including the MS’s identity(IMSI/TMSI).This is transmitted on Downlink, point-to-multipoint.

Dedicated Control Channels-DCCH
Stand alone dedicated control channel(SDCCH) AGCH assigns SDCCH as signaling channel on request by MS.The MS is informed about which frequency(ARFCN) & timeslot to use for traffic. Used for location update, subscriber authentication, ciphering information, equipment validation and assignment of TCH. This is used both sides, up and Downlink point-point.

Dedicated Control Channels-DCCH
Slow associated control channel-SACCH Transmission of radio link signal measurement, power control etc. Average signal strengths(RXLev) and quality of service (RXQual) of the serving base station and of the neighboring cells is sent on SACCH (on uplink). Mobile receives information like what TX power it has to transmit and the timing advance. It is associated with TCH or SDCCH Fast associated control channel-FACCH Used for Hand over commands and during call setup and release. FACCH data is sent over TCH with stealing flag set

Traffic Channels-TCH TCH carries the voice data.
Two blocks of 57 bits contain voice data in the normal burst. One TCH is allocated for every active call. Full rate traffic channel occupies one physical channel(one TS on a carrier) and carries voice data at 13kbps Two half rate (6.5kbps) TCHs can share one physical channel.

GSM Radio Link Speech Coding -Done at Transcoder of BSC and MS
The Linear Predictive Coder uses RPE-LTP(Regular Pulse Excitation- Long Term Prediction) Converts 64kbps voice to 13kbps(260 bits every 20ms) Channel Coding - Done at BTS and MS Uses Convolution Coding and CRC (Cyclic Redundancy Check) Converts 13 kbps to 22.8 kbps (456 bits per 20ms)

GSM Radio Link Bit Interleaving - Done at BTS and MS
Encryption - Done at BTS and MS EX OR data with cipher block, which is generated by applying A5 Algorithm to the Ciphering Key(Kc) Multiplexing - Done at BTS Modulation - Done at BTS and MS GMSK(Gaussian filtered Minimum Shift Keying) Phase change of +90 for 0 and -90 for 1

Speech Coding BP A/D LP D/A 50 132 78 50 3 132 4 378 coded bits 78 1A
ENCODER CHANNEL CODING To modulator Every 20ms 160 samples taken Data rate = 160 * 13/20ms = 104 kbps 1A 1B 2 BAND PASS 300 Hz kHZ Every 125 s value is sampled from analog signal and quantised by 13 bit word Data rate = 13/125*10 -6 = 104 kbps 50 132 78 3 crc bits Four 0 bits for codec 50 3 132 4 Conv coding rate = 1/2 delay = 4 Linear Predictive Coding & Regular Pulse Excitation Analysis 1. Generates 160 filter coeff 2. These blocks sorted in 4 sequence 1,5,9,…37 / 2,6, / 3,7,11…39/8,12,16…40 3. Selects the sequence with most energy So data rate = 104/4 = 26 kbps Long term prediction analysis 1. Previous sequences stored in memory 2. Find out the correlation between the present seq. And previous sequences 3. Select the highest correlation sequence 4. Find a value representing the difference between the two sequences. Reduces data rate = 26 kbps/2 = 13 kbps ie 260 bits in 20ms 378 coded bits 78 456 bits in 20 ms = 22.8 kbps 57 x 8 = 456 1A = Filter Coeff block ampl, LTP params 1B = RPE pointers & pulses 2 = RPE pulse & filter CHANNEL DECODING LP D/A SPEECH DECODER

Interleaving Encoded speech blocks - Diagonal Interleaving Even bits
57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 Even bits 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 Odd bits Bn-4 Bn-3 Bn-2 Bn-1 Bn Bn+1 Bn+2 Bn+3 Encoded control channel blocks - Rectangular Interleaving 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 Even bits 57 57 57 57 57 57 57 57 57 57 57 57 Odd bits 57 57 57 57 57 57 57 57 57 57 57 57 Bn-4 Bn-3 Bn-2 Bn-1 Bn Bn+1 Bn+2 Bn+3 Tb 3 Coded Data 57 F 1 Training Sequence 26 F 1 Coded Data 57 Tb 3 Gp 8.25

Burst The information format transmitted during one timeslot in the TDMA frame is called a burst. Different Types of Bursts Normal Burst Random Access Burst Frequency Correction Burst Synchronization Burst

Normal Burst Tail Bit(T) :Used as Guard Time
bits ms T 3 Coded Data 57 S 1 T. Seq. 26 S 1 Coded Data 57 T 3 GP 8.25 Tail Bit(T) :Used as Guard Time Coded Data :It is the Data part associated with the burst Stealing Flag :This indicates whether the burst is carrying Signaling data (FACCH) or user info (TCH). Training Seq. :This is a fixed bit sequence known both to the BTS & the MS.This takes care of the signal deterioration.

bits ms T 3 Training Sequence 41 Coded Data 36 T 3 GP 68.25 Random Access Burst bits ms T 3 Fixed Bit Sequence 142 T 3 GP 8.25 Freq. Correc. Burst bits ms Coded Data 39 Training Sequence 64 Coded Data 39 T 3 GP 8.25 T 3 Synchronization Burst

Transmission on the radio channels
A timeslot has a duration of .577 m seconds (148 Bits) 8 timeslots(8 x = 4.62 ms) form a TDMA frame If a mobile is assigned one TS it transmits only in this time slot and stays idle for the other 7 with its transmitter off, called bursting The start on the uplink is delayed from downlink by 3 TS periods One TS = duration of bits, and its physical contents is called a burst Downlink BTS > MS 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Uplink MS > BTS Offset

Timing Advance 1 2 3 4 5 6 7 MS1 near MS2 far At BTS

Frames Types On Um Interface
TDMA Frame 8 Time slots (Burst Period) Length is 4.62 ms(8 * 0.577ms) 26-TDMA Multiframe 26 TDMA Frames (24 TCH, SACCH, Idle) 120 ms (26 * 4.62ms) 51-TDMA Multiframe 26 TDMA Frames (FCCH, SCH, BCCH, SDCCH, CCCH) 235.6 ms (51 * 4.62ms)

Frames Types On Um Interface
Super Frame 51* 26 TDMA Frames 6.12 S Hyper Frame 2048 * 51* 26 TDMA Frames 3 Hours, 28 Minutes, 53 Secs and 760 ms

Mobility Management Mobility Management (MM)
Location updating- normal,periodic, IMSI attach Paging Security Management Preventing unauthorized users- authentication Maintaining Privacy of users- ciphering Providing roaming facility MM functionality mainly handled by MS, HLR, MSC/VLR.

Network Attachment Cell Identification
MS scans complete GSM frequency band for highest power Tunes to highest powered frequency and looks for FCCH. Synchronizes in frequency domain Get training sequence from SCH which follows FCCH. Synchronizes in time domain. Accesses BCCH for network id, location area and frequencies of the neighboring cells. Stores a list of 30 BCCH channels

Network Attachment….. PLMN Selection Cell Selection Location Update
Get the operator information from SIM. Cell Selection Selected cell should be a cell of the selected PLMN Signal strength should be above the threshold. Cell should not be barred Location Update Register with the network by means of location updation procedures.

MS Location Update (registration)
MS BTS BSC (G)MSC VLR HLR Action Channel Request (RACH) Channel Assignment (AGCH) Location Update Request (SDCCH) TMSI + old LAI Authentication Request (SDCCH) Authentication Response (SDCCH) Comparison of Authentication params Accept LUP and allocTMSI (SDCCH) Ack of LUP and TMSI (SDCCH) Entry of new area and identity into VLR and HLR Channel Release (SDCCH)

Security - Authentication
MS Ki RAND A3 SRES Auth Result AuC BTS Authentication center provides RAND to Mobile AuC generates SRES using Ki of subscriber and RAND Mobile generates SRES using Ki and RAND Mobile transmits SRES to BTS BTS compares received SRES with one generated by AuC

Security - Ciphering Data sent on air interface ciphered for security
MS Ki RAND A8 Kc Network Um interface A5 Data Ciphered Data sent on air interface ciphered for security A5 and A8 algorithms used to cipher data Ciphering Key is never transmitted on air

Communication Management (CM)
Setup of calls between users on request Routing function i.e. Choice of transmission segments linking users Point to Point Short message services

Mobile Originated Call
Request for Service Authentication Ciphering Equipment Validation Call Setup Handovers Call Release

Mobile Originated Call
MS HLR BTS BSC VLR Authentication response (SDCCH) AuC MS GMSC BTS ACM EIR TCH assigned Assn complete Authentication request (SDCCH) Assigns TCH Req Activate TCH Call set-up forwarded to MSC Connect message MSC Req for dedicated channel for signaling (RACH) BSC PSTN Give SDCCH Allocates SDCCH using the AGCH BTS Ring tone over FACCH Ring tone ceases over FACCH SDCCH released Speech path enabled Called Sub answers Ring alert Sends call set-up request including dialled digits on SDCCH Release SDCCH Call set-up forwarded to BSC 

Mobile Terminated Call
Paging Authentication Ciphering Equipment Validation Call Setup Handovers Call Release

Mobile Terminated Call
MS BTS BSC MSC VLR GMSC HLR PSTN EIR AuC Paging Assignment CMD (=TCH) on SDCCH TMSI Paged on PCH * MS tunes * Assgn CMP * Phone rings Page RESP on SDCCH ( TMSI + LAI) Ch. REQ over RACH Allocate SDCCH over AGCH Query for VLR info Reply (MSRN) Connect traffic Ch.to trunk frees SDCCH Assgn CMP Assign. REQ Paging the area (+TMSI) Page RES Page RES Query VLR for LAC and TMSI Network Alerting Route to MSC Land to Mobile call (MSISDN) Authentication and Ciphering procedure done as seen in Location Updation

Radio Resource Management
Establish maintain and release stable connections between MS and MSC Manage Limited Radio and Terrestrial resources Handover process is the sole responsibility of the RR Layer Functions of RR layer are performed by MS and BSC and partly by MSC

Radio Resource Management
Power Control Hand over Control Discontinuous Transmission Frequency Hopping

Power Control BTS commands MS at different
distances to use different power levels so that the power arriving at the BTS’s Rx is approximately the same for each TS - Reduce interference - Longer battery life

Handover Means to continue a call even a mobile crosses the border of one cell to another Procedure which made the mobile station really roam Handover causes RxLev (Signal strength , uplink or downlink) RxQual (BER on data) O & M intervention Timing Advance Traffic or Load balancing

Handover Types Internal Handover (Intra-BSS)
Within same base station - intra cell Between different base stations - inter cell External Handover (Inter-BSS) Within same MSC -intra MSC Between different MSCs - inter-MSC

Handover Types GMSC MSC BSC BSC C-3 BSC MSC C-4 C-1 C-2 BSC

Intra BSC handover BSC BTS 2 Cell 2 BTS 1 Cell 1 HO performed
HO required Activate TCH(facch) with HoRef# if 1. Check for HO passed 2. Channel avail in new BTS BSC MS tunes into new frequency and TS and sends HO message to new BTS (facch) Acknowledges and alloctes TCH (facch) BTS 2 Periodic Measurement Reports Release TCH Periodic Measurement Reports (SACCH) HO cmd with HoRef# Receives new BTS data(FACCH) Periodic Measurement Reports (SACCH) Cell 2 BTS 1 Cell 1

Discontinuous Transmission
Discontinuous Transmission(DTX) allows the radio transmitter to be switched off most of the time during speech pauses. A Silence Indicator Block is transmitted at 500bps, which generates a comfort noise Down Link interference is decreased. Up link battery is saved

Frequency Hopping Frequency Hopping permits the dynamic switching of radio links from one carrier frequency to another. Base Band Hopping At the BTS each the timeslot is shifted to another transceiver, which is transmitting at the hop frequency. User will be connected to different Transceivers depending on hop sequence. Synthesis Hopping At the BTS transceiver changes the frequencies used. The user will be connected to only one transceiver. Decreases the probability of interference Suppresses the effect of Rayleigh fading

Wireless Data Data Application Time Internet Mobile Network 98 99 2000
Enhanced Data rate for GSM Evolution EDGE is an enhancement of GPRS and CSD technologies. Based on the current GSM technology - same TDMA frame structure, same bandwidth (200 kHz). Uses 8-PSK modulation instead of GMSK. Requires good propagation conditions. Allows upto 48 kbps (EGPRS) and upto 28.8 kbps (ECSD) on every radio channel EDGE helps GSM-Only operators to compete with UMTS. Add-on to GSM network : PCU; Packet Segmentation/re-assembly and scheduling Radio channel access control and management Transmission error detection and retransmission. Power control SGSN: GPRS mobility Encryption Charging GGSN : Interface to the PDN, Internet Max user data rate : 21.4 kbps Dynamic rate adaptation to suit the radio conditions at that time ( 9.05 kbps, 13.4 kbps, 15.6 kbps 21.4 kbps) Universal Mobile Telecommunication Standards Innovative Service Architecture : VHE Concept - providing the us the same look and feel of its personalized services independent of network and terminal. Global Convergence : Fixed/Mobile, Telecom/Datacom, public/private Mobile Multimedia driven market. Wideband bearers - 2GHz band ( 5 MHz per carrier), -max. 2Mbps Wireless Data Gateway Service W @ P F o n e Internet Mobile Network Surf the Internet while on the move Gateway : Adaptation of the information to the mobile Compression of the data Buffering of the information 98 99 2000 2001 GSM DATA HSCSD GPRS EDGE UMTS SIM Toolkit WAP Data Application Time Circuit Switched technology Packet Switched technology Technology for Applications High Speed Circuit Switched Data User Data Rate:14.5kbps Use multiple timeslots (max=8), hence max rate = 115.2kbps. Needs a duplexor in MS for simultaneous Tx and Rx SMS Data: 160 -numeric characters User Data Rate : 9.6kbps One time slot over the air interface

References Wireless and Personal Communication Systems. Vijay.K.Garg and Wilkes Overview of the GSM System and Protocol Architecture, IEEE Comm. Magazine, Moe Rahnema. The GSM System for Mobile Communications- Michel Mouly & Marie-Bernadette Pautet Overview of the GSM Comm- John Scourias.

Thank you

Location Updates Location Updates can be classified into two:
Periodic Location Updates: This occurs as per the timer set by the network operator. If the MS does not perform this update the MSC marks the MS as ‘Detached’ on the VLR. Location Update on a handover: This occurs if during a handover the MS is moved into a new Location Area Code (LAC).

GSM Global System for Mobiles.

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