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Cellular Mobile Communications-III Introduction to GSM

Cellular Mobile Communications-III Introduction to GSM. Dr. Nasir D. Gohar. AN INTRODUCTION TO GSM. Second Generation Cellular Mobile Radio Systems-Digital Cellular Telephony: Main Advantages of Digital Cellular Networks Higher Spectrum Efficiency Speech Encoding

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Cellular Mobile Communications-III Introduction to GSM

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  1. Cellular MobileCommunications-IIIIntroduction to GSM Dr. Nasir D. Gohar

  2. AN INTRODUCTION TO GSM • Second Generation Cellular Mobile Radio Systems-Digital Cellular Telephony: • Main Advantages of Digital Cellular Networks • Higher Spectrum Efficiency • Speech Encoding • Better Access Technologies: TDMA and CDMA • Higher System Capacity • Aggressive Frequency Reuse • Low Infra-Structure and Terminal Cost • Application of VLSI/VVLSI Chips • Many Traffic Channels per Carrier Channel [Transceivers are shared among several users) • Low S/I Ratio (SIR) Allowing Smarter Equipment • Better Integration with Digital PSTN [ISDN] • New Services such as Data Communications, Fax, and SMS • Better Privacy due to Encryption NDG Notes

  3. AN INTRODUCTION TO GSM • Digital Cellular Standards • Why So Many Standards? NDG Notes

  4. AN INTRODUCTION TO GSM • Digital Cellular Standards NDG Notes

  5. AN INTRODUCTION TO GSM • Overview of GSM Milestones • 1982GSM Group, set up by CEPT, started Investigation to reserve a Spectrum in 900 MHz band for Pan-European Cellular Digital Telephony • 1986 Two 25 MHz frequency Blocks [890-915 MHz, 935-960 MHz] reserved by EC, July 01, 1991 set as Introduction date. Choice of Digital and Narrow band [Approx. 200 KHz] TDMA made. • 198713 Operators from 12 European Countries signed MOU to Support GSM • 1989ETSI established which took over GSM from CEPT • 1992Introduction of first Commercial GSM Network • 199451 Commercial Networks Established • 1996More Than 20 Million Subscribers in 191 Networks, More than Double of all Subscribers of other Digital Systems [D-AMPS, PDC, IS-95 CDMA] • 2006More than 2 billion subscribers (30% of world population and 82% of cellular market) NDG Notes

  6. AN INTRODUCTION TO GSM • Implementation Phases of GSM • Phase-1: Most Essential Services [Voice, Data, SMS] Implemented. Mostly all Current GSM Systems belong to this Phase. • Phase-2: Half-Rate Speech Encoding, Doubling the System Capacity, Several Encryption Algorithms, Conference Call Facility[ up to 5 Participants] , Call Waiting Facility, Information on Call Charges, Calling or Called Part etc., Cell-Broadcast [Point to Multi-Point SMS]. Many of the Phase-2 Facilities have already been Implemented by some Manufacturers. • Phase-2 + GPRS[General Packet radio Service] for Packet Switched Data Transmission to Support LAN and Internet Traffic. [Considerably Delayed due to Lack of Interest on part of Network Operators due to Large Investment required for Infra-Structure and MS Modifications] • GSM 3G….. NDG Notes

  7. AN INTRODUCTION TO GSM • 2G EVOLUTION TO 3G NDG Notes

  8. AN INTRODUCTION TO GSM • GSM EVOLUTION NDG Notes

  9. AN INTRODUCTION TO GSM • GSM SYSTEM ARCHITECTURE • MS • SIM Card • EIN/ESN • Base Station Subsystem (BSS) • Several Dozens of BTS under one BSC • Each BTS has 3-5 Carrier Channels • Network &Switching Subsystem • MTX [up to 1 M users] • Call set up , all Other Functions • GMTX: An Interface to Other Networks • HLR: Keeps Record of System’s Own Users • VLR: Keeps Temporary Record of Visitors / Roamers [SID, ST,SVCES] • Au C/EIR: Manages the Sub. Authentication and Encryption Data NDG Notes

  10. AN INTRODUCTION TO GSM • GSM SYSTEM ARCHITECTURE-2 • Management Subsystem [Operation Subsystem (OSS)] • Directly or Indirectly Connected to all Other Subsystems • Fault Management • System Configuration • Performance Management • Cost Management • Security Management NDG Notes

  11. AN INTRODUCTION TO GSM • GSM SYSTEM ARCHITECTURE-3 • GSM Interfaces • Radio Interface: Describes Data Interchange between MS and BSS • Abis Interface: Describes Data Communications between BTS and BSC, allows Various Manufacturers Equipment • A Interface: Describes Data Interchange between BSS and NSS NDG Notes

  12. AN INTRODUCTION TO GSM • LOGICAL CHANNELS NDG Notes

  13. AN INTRODUCTION TO GSM • Control Channels • Control channels fall into three categories: • Broadcast:: BCCH, FCCH, SCH • One way, from base to mobile • Common Control: RACH, AGCH, PCH • One way, some from base to mobile and some from mobile to the base • Dedicated: SDCCH, SACCG, FACCH • Two-way, stand-alone or embedded in the traffic channels • All signaling channels share one carrier in a cell • the dedicated control channels may be transmitted on traffic carriers NDG Notes

  14. AN INTRODUCTION TO GSM • Broadcast Channels • Frequency Correction Channel (FCCH) • Carries information for frequency correction • Synchronization Channel (SCH) • Carries information for frame synchronization and for identification of the BTS • Broadcast Control Channel (BCCH) • Broadcasts general information on the BTS • Broadcasts cell-specific information, e.g. Control channel organization, frequency hopping sequences, cell identification, etc. NDG Notes

  15. AN INTRODUCTION TO GSM • Common Control Channels • Paging Channel (PCH) - downlink only • for paging purposes • Random Access Channel (RACH) - uplink only • used by any MS to request allocation of a signaling channel (SDCCH) • a slotted Aloha protocol is used, so collisions among MSs may happen • Access Grant Channel (AGCH) - downlink only • used to allocate a SDCCH or a TCH • Notification Channel (NCH) - downlink only • notify MS of voice group and voice broadcast call (ASCI feature) NDG Notes

  16. AN INTRODUCTION TO GSM • Dedicated Control Channels • Stand Alone Dedicated Control Channel (SDCCH) • used for call setup (authentication, signaling,, traffic channel assignment), location updates and SMS • Slow Associated Control Channel (SACCH) • always coupled with a SDCCH or TCH • for communicating measurement data and control parameters • Fast Associated Control Channel (FACCH) • to respond to increased signaling demand, e.g. during handover • bandwidth (bit slots) are stolen from the associated TCH (traffic data are preempted) NDG Notes

  17. AN INTRODUCTION TO GSM • Traffic Channels • GSM support two types of traffic channels • full rate (TCH/F): 22.8 kbps • half rate (TCH/H): 11.4 kbps • Mapping to physical channel • full rate traffic channel - 1 timeslot • half rate traffic channel - 1 timeslot in alternating frames • Full rate channel may carry • 13 kbps speech or data at 2.4, 4.8 or 9.6 kbps • Half rate channel may carry • 6.5 kbps speech or data at 2.4 or 9.6 kbps NDG Notes

  18. AN INTRODUCTION TO GSM • Channel Usage & MS Terminating Calls NDG Notes

  19. AN INTRODUCTION TO GSM • GSM Call Setup Procedure • Locating the Subscriber • HLR keeps record of the MSC area which the Subscriber last registered • VLR keeps record of the Location Area[LA] in which Subscriber last registered • Location Area: A Group of Neighboring Cells having the same LAC • LAC: Each Cell in the Area Broadcasts this Code • Each Mobile itself periodically registers itself with MSC • Paging and Random Access Procedure [RAP] • GMSC interrogates HLR for MSC the Subscriber last registered • Call is Switched to that MSC • VLR tells about the LA the user last registered • MSC arranges a Paging in all the Cells in the LA NDG Notes

  20. AN INTRODUCTION TO GSM • GSM Call Setup Procedure-2 • Paging and Random Access Procedure[RAP]-Cont’d • The Mobiles listen to the Paging Channels for their Number • If received a Paging Message, the Mobile starts a RAP • It sends a Random Number [RN] + Brief Guide to describe the Purpose • The System [MSC] responds to it by repeating the RN and providing the decided Channel • The Mobile listens to this RN, if it is the same, it gets hold of the allocated Channel • Reconfirmation is done by the System by sending again the Mobile Number [in case some other Mobile has also sent the same RN at the same time] • If the Mobile hears its own number, it responds to the System and thus it gets connected to the incoming Call, • If it was a wrong number [other than its own number] it must leave that Channel. • In case, a Mobile listens no same RN from the System, it may repeat RAP at some time later randomly. NDG Notes

  21. AN INTRODUCTION TO GSM • CALL PROCESSING & MOBILE TERMINATING CALLS NDG Notes

  22. AN INTRODUCTION TO GSM • CALL PROCESSING & MOBILE ORIGINATING NDG Notes

  23. AN INTRODUCTION TO GSM • GSM Handover and Routing • Handover • The Mobile keeps updating MSC about the RSS levels from the neighboring Cells. • MSC, while looking at the quality of the existing link, decides whether a Handoff/ Handover is necessary or not. • If yes, it asks the Candidate Cell to prepare the Channel [ GSM uses Fixed Channel Assignment] • When Channel is allocated, it tells the Mobile to move to that Channel • When Mobile has moved, it deactivates the old Channel • Routing • GMSC interrogates HLR to locate the Subscriber by telling the MSC where the Mobile last registered. • PROBLEM: Call originated from PSTN in CANADA for a US Mobile roaming in CANADA will result in two International Calls “TROMBONE PROBLEM” ???? [Don’t Worry, Be Happy, Other People will Take Care of this Problem] NDG Notes

  24. AN INTRODUCTION TO GSM • Intra-BSC • Old and new BTSs are controlled by the same BSC • The MSC is not involved • Intra-MSC • Old and new BTSs are attached to different BSCs • The BSCs are attached to the same MSC • Inter-MSC • Handover to a new MSC • Serving MSC becomes anchor MSC • IMT (Inter Machine Trunk) is required • HANDOVER TYPES NDG Notes

  25. AN INTRODUCTION TO GSM • GSM Numbering Scheme and Spectrum Efficiency • GSM Numbering Scheme • Each GSM Mobile has at least three Numbers: • MSISDN [Mobile Station Integrated Services Digital Network] stored in SIM Card • SIM Card holds Subscriber ID[MSISDN, IMSI], some Extra Memory to store phone numbers and Encryption Algorithms • IMSI[International Mobile Subscriber Identity] not known to the User, HLR does translation between MSISDN to IMSI and vice versa. • ESN or IMEI:Permanently stored/wired in the Mobile Station • Spectrum Efficiency • TDMA, 200 KHz Channel BW, 8-Time Slots per Carrier Channel • Radio Spectrum = 2 x 25 MHz bands can support 125[124] Duplex Carrier Channels and 1000 Traffic Channels [1000 Simultaneous Calls]. • A typical GSM System Cluster Size = 12, so each Cell have Approx. 10 Carrier Channels and a Capacity of 80 Simultaneous Call [Traffic Channels] • In case of half-rate Coder, Spectrum Efficiency will be Doubled. NDG Notes

  26. AN INTRODUCTION TO GSM • Technical Realization of GSM • GSM Speech Communications • User Speech is Digitized using Adaptive PCM • Speech Encoding using Linear Predictive Coding [LPC] • User data flow = 13 kbps [ 6.5 kbps in case of half-rate coding] • Adding an overhead of 9.8 kbps[for error detection, error correction, and synchronization], we get an over all user data flow of 22.8 kbps. • This user data flow is subdivided into short data blocks each of 456 bits, which is divided into 8 sub-blocks, each of 57 bits, 2 such sub-blocks are used to make a burst [to fit into a time slot of 0.58 ms]. See frame structure. • These bursts from a user are interleaved over 8 time slots spread over 8 frames. • TDMA/FDMA Mechanism • Each Carrier Channels carries data burst of eight (8) users, each in its allocated time slot in the frame. • Time slots are transmitted on several Channels [Carrier Channel or Frequency] • Each Carrier Channel transmits the data of 8 users at 271 kbps. NDG Notes

  27. AN INTRODUCTION TO GSM • Technical Realization of GSM [ Cont’d] • GSM Speech Communications-2 • GSM Carrier Frequencies are numbered 1-124 Channels[Duplex] Forward Channel = 935.2 MHz + (n-1) * 200 KHz Reverse Channel = 890.2 MHz + (n-1) * 200 KHz • Due to slightly more than 200 KHz bandwidth of the modulated signal, Consecutive Frequencies are not used in the same System • Also, 1 and 124 Channels are generally not used. • The Spectrum of 124 Channels is generally not allotted to one Service Provider. • Separation between two Duplex paired Channels is 45 MHz. • Each user’s bursts are transmitted every 4.6ms apart. NDG Notes

  28. AN INTRODUCTION TO GSM • SPEECH PROCESSING NDG Notes

  29. AN INTRODUCTION TO GSM • SPEECH CODING • SPEECH ENCODER NDG Notes

  30. AN INTRODUCTION TO GSM • SPEECH DATA PROCESSING NDG Notes

  31. AN INTRODUCTION TO GSM • CHANNEL ENCODING NDG Notes

  32. AN INTRODUCTION TO GSM • INTERLEAVING NDG Notes

  33. AN INTRODUCTION TO GSM • VOICE TRANSMISSION PATH NDG Notes

  34. AN INTRODUCTION TO GSM • Technical Realization of GSM [ Cont’d] • GSM Framing Structure NDG Notes

  35. AN INTRODUCTION TO GSM • TIME-SLOT STRUCTURE NDG Notes

  36. AN INTRODUCTION TO GSM • FRAME HIERARCHY NDG Notes

  37. AN INTRODUCTION TO GSM • BURST NDG Notes

  38. AN INTRODUCTION TO GSM • TYPES OF BURST • Five different types of bursts • Normal burst • Traffic and control payload • Frequency correction burst • All zeroes sequence • Synchronization burst • A special fixed sequence • Random access burst • Extended guard period of 68.25 bitts (252 μs) • Dummy burst NDG Notes

  39. AN INTRODUCTION TO GSM • BURST STRUCTURES NDG Notes

  40. AN INTRODUCTION TO GSM • Technical Realization of GSM [ Cont’d] • GSM Modulation Scheme • Gaussian Minimum Shift Keying [GMSK] (For Detail see Ch-5 of the Text Book) • No Speech, No Transmission • Saves Energy in MS, Reduces Ave. Interference • Comfort Noise added for the Listener’s Pleasure. • GSM MS Power Classes • Class 1: 20 W Not (yet) Available • Class 2: 8 W Car Phone • Class 3: 5 W - • Class 4: 2 W Normal Pocket Phone • Class 5: 0.8 W Limited Coverage Phone [Urban Areas Only] • Maximum Cell Size: Depends on Max. Permitted Delay, 35 km. • Maximum Mobile Speed: 250 km/hr NDG Notes

  41. AN INTRODUCTION TO GSM • Technical Realization of GSM [ Cont’d] • GSM SMS • Individual Messages: • up to 160 Characters long can be sent and received by MS • SMS uses Signaling Channel, thus, it can be received during current Communication Session • Max. rate 600 bps • Cell Broadcast: • up to 93 Character long message can be sent to all users in a given area [Cells]. • Uses Communication Channel, so, can’t be received during current Communication Session • Neither Addressed Nor Encrypted • Applications of SMS: .Network Operator Messages, Third Party Messages, Public Interest Information Messages • SMS Cost: Differs from Operator to Operator and depends on type of Application NDG Notes

  42. AN INTRODUCTION TO GSM • Technical Realization of GSM [ Cont’d] • GSM Security Aspects • GSM provides security about the identity of MS • Guards against Eavesdropping • Implementation • User Authentication [A3 Algorithm]: 128 bit Secret Number, Ki, assigned to each User, stored in SIM Card as well as AuC, is used to create SRES using Ki and 128 bit RAND number from the network, if SRES of MS matches with SRES of the network, the MS is accepted. NDG Notes

  43. AN INTRODUCTION TO GSM • Data Encryption [A8+A5 Algorithm]: • Frame # + (RAND+Ki(Alg-A8))(Alg-A5) -> 144 bit Code Train  144 bit user Data Train -> [Network] Frame # + (RAND+Ki(Alg-A8))(Alg-A5) ->Original Message NDG Notes

  44. AN INTRODUCTION TO GSM • Technical Realization of GSM [Cont’d] • GSM Inter-Connection with Other Networks: • PSTN/ISDN for Voice • GSM Speech data is Digital and Compressed • Converted into audio of 3.1 KHz BW • PSTN/ISDN for Data • For Data Interconnection, Modem Pools are used in GSM Network. Many Telephone Modems at GMSC/IWF supporting all the important telephone modem standards V.21[0.3 kbps, AS], V.22[1.2 kbps, AS and S], V.32 [4.8 kbps, S] etc. • PSDN (Basic PAD or Dedicated PAD) for Data • 300 bps to 9600 bps on AS/S links NDG Notes

  45. NEW DEVELOPMENTS IN GSM • Frequency Shortage and Extended GSM Band • In some European countries, up to half GSM band is occupied by Interim Systems; NMT, ETACS etc. • Additional frequencies adjacent to GSM band is recommended by ERO[1996], new terminals for this extended band are not available in bulk • New Encoding Techniques • Half-Rate Speech Encoding • Instead of 13 kbps only 6.5 kbps will be required for the same speech quality • Standardized in early 1995, but, only few manufacturers have implemented • lack of interest on part of system suppliers • fear of investment decrease in network expansion [uCell] • Enhanced Full Rate [EFR]:Developed by GSM and DCS-1800 Suppliers • Same 13 kbps speed, but, considerable improvement in speech quality • More Immunity to transmission errors NDG Notes

  46. NEW DEVELOPMENTS IN GSM • Any Cellular Phone may have many Speech Coders, including FR/HR/EFR, it must be able to switch, and at least support FR for roaming purpose. • Data Communications over GSM • General Packet Radio Service[GPRS] offers packet-switched data communications suitable for LAN and Internet Applications[PVC] • Requires GPRS enabled Handsets and Changes at BSS[addition of Packet Control Unit, PCU] • Option of Upgrading of BTS to support Enhanced Data rate for GSM Evolution, EDGE [8-PSK system] • Multi-band Terminals • Cellular phones to support more than frequency bands [GSM and DSC-1800 or GSM and PCS-1900 or GSM/DCS/PCS or many more combinations] • GSM Moving into 3G Mode • GSM extends itself into 3G as Universal Mobile Telecomm. Services [UMTS] • Radio Interface will use WCDMA technology [UMTS Terrestrial Radio Access, UTRA] in two different modes: FDD [two different frequencies for uplink and downlink] or TDD[same frequency for both uplink and downlink but time-shared] NDG Notes

  47. The Market Share of GSM Suppliers [1/97] • Networking & Switching Subsystems [NSS] • Ericsson [48 %], Siemens [21 %], Nokia [14 %], and Alcatel [10%] = 93 % • Others (Lucent, Motorola, Nortel, etc] = 7% • Basestation Sub-System [BSS] • Nokia [22%], Motorola [13%], Alcatel [10%] and Ericsson [7%] = 52% • Others [Italtel, Lucent, Matra, Philips etc] = 48% • GSM / DCS-1800 / PCS-1900 Mobile Terminals • Ericsson [25 %], Nokia [24%], MOTOROLA [20 %], SIEMENS [9%] =78 % • Others [Alcatel, Panasonic, Nortel, etc.] = 22 % • Due to Licensing Problems, Unfair and Restricted Competition in GSM Markets NDG Notes

  48. GSM and Health Problems • GSM Terminals Cause Interference with • Hearing Aid Devices and • Pacemakers [Instruments used to regulate the heart functionality of Heart Patients], and • Some Research show that GSM phones cause Brain Tumors * • Inherent in GSM TDMA setup to generate strongly pulsating transmission signals [Continuously Tx is switched on and off that generates LF signals 217, 434, 651 Hz] • * Extensive Research required to prove the validity of GSM Cellular Phones being the cause of Brain Tumors or Cancer. NDG Notes

  49. GSM Derivative Systems:DCS-1800 & PCS-1900 • Digital Cellular System[DCS]-1800 • Originally started [in 1990] as a separate system, but, later on became just a GSM variant • Main modifications were made only in Air Interface • Developed [by ETSI] particularly for densely populated urban areas • 1.7 - 2.3 GHZ band [ 2 x 75 MHz spectrum, 1710- 1785 + 1805-1888 MHz] • Duplex separation is 95 MHz, Channel BW is 200 KHz, 374 duplex channels • Much smaller cells [cells within a building], lower power BTS and MS as compared to GSM • handoff problems are much cleverly settled using hierarchical Cell structure • Max Cell size 8 km with Class 1 MS [1W] • Max Cell size with Class II[0.25W] even smaller [0.5 -4/5 km] • International as well as National roaming is possible [Country to Country, Network to Network, and Intra-Network] • Half-rate speech coding is possible/EFR is more likely to prevail. NDG Notes

  50. GSM Derivative Systems:DCS-1800 & PCS-1900 • Personal Communication Services [PCS]-1900 • A GSM variant to adapt to US Market • Frequency band 1900 MHz [1850-1890, 1930-1970 MHz] • 2 x 40 MHz bands with Duplex Separation of 80 MHz • Channel BW is 200 KHz, 200 Duplex Channels • TDMA 8 time-slots • EFR speech encoding is getting more Interest from US Service Providers NDG Notes

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