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GSM concepts_Slide with frame structure.ppt
- 1. Global System forMobile Communications
- 2. vYV lsaVj 2 •Understand theevolution and history of GSM. Learn about GSM network architecture. Study GSM interfaces and protocols. Explore GSM logical and physical channels. Understand GSM services and applications. Overview of GSM security features. Content
- 3. vYV lsaVj 3 •GSM standsfor Global System for Mobile Communications. Developed by ETSI in the 1980s. Standard for 2G digital cellular networks. Widely used in Europe, Asia, and globally. Offers voice, SMS, and limited data services. Introduction to GSM
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- 5. 5 Interfaces in GSM •Um:MS ↔ BTS (Air interface) •Abis: BTS ↔ BSC •A: BSC ↔ MSC •Others: MAP, B, C, D, E interfaces
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- 8. 8 GSM Channel Types LogicalChannels: •Traffic Channels (TCH): Voice and data. •Control Channels: • Broadcast Channels (BCCH) • Common Control Channels (PCH, RACH, AGCH) • Dedicated Control Channels (SDCCH, SACCH, FACCH) Physical Channels: •Use TDMA (8 timeslots/frame)
- 9. 9 Broadcast Control Channels usedto broadcast synchronization and general network information to all the MS within a cell. •It has three types: 1. FREQUENCY CORRECTION CHANNEL (FCCH): •Used for the frequency correction / synchronization of a MS oscillator frequency with Base station Frequency. •Sends Frequency Bursts every 10 seconds. 2. SYNCHRONISATION CHANNEL (SCH): •Syncs the MS internal clock with the BTS clock. •Sends Synchronization Bursts every 10 frames. 3. BROADCAST CONTROL CHANNEL (BCH): •Point-to-multipoint (BTS → MS) channel. •Broadcasts: •Cell identity •Network identity •Channel availability •Congestion info •Parameters used by MS to access BTS
- 10. 10 Common Control Channels tocarry paging and access requests between BTS and all MS. •It has three types: 1. PCH – Paging Channel (BTS → MS) •Broadcasts paging signals from BTS to all MS during Mobile Terminated Call (MTC). •Notifies a particular MS of incoming call and transmits the IMSI of the target subscriber. •Also used to send broadcast text messages to all MSs. 2. RACH – Random Access Ch (MS → BTS) •Used by MS to send access request in Mobile Originated Call (MOC). • Updates MS location in VLR. 3. AGCH – Access Granted Channel (BTS → MS) •Sends instructions from BTS to MS to allocate a specific physical channel (Time slot or ARFCN).
- 11. GSM Specifications GSM 900 Mobileto BS (UP-LINK) - 890 to 915 MHz BS to Mobile (DOWN -LINK) - 935 to 960 MHz Bandwidth - 25 MHz GSM 1800 ( DCS ) : Mobile to Cell (UP-LINK) - 1710 to 1785 MHz Cell to Mobile (DOWN -LINK) - 1805 to 1880 MHz Bandwidth - 75 MHz RF Spectrum : 11
- 12. GSM Specifications Carrier Separation- 200 kHz No. of RF Carriers - 124 Access Method - TDMA/FDMA Modulation Method - GMSK Transmission Rate - 270.833 Kbps Speech Coding - Full rate 13 Kbps Half rate 6.5 Kbps Duplex Distance - 45 MHz 12
- 13. GSM - MULTIPLEACCESS • GSM uses both FDMA & TDMA Freq Mhz. 890.2 1 890.4 2 890.6 3 890.8 4 891.0 5 6 914.8 124 • FDMA Access along Frequency axis • Each RF carrier 200khz apart • Total 124 RF Channels available. One or more carrier assigned to each base station ……... 13
- 14. • Absolute RadioFreq Carrier Number (ARFCN) 1 and 124 not used until it is co-ordinated with Non -GSM operators in adjacent freq. bands. • Thus for practical purposes only 122 RF Carriers are available. F up-link (n) = 890.2 +0.2* ( n-1 ) MHz F down-link (n) = 935.2 +0.2* ( n-1 ) MHz • Frequency for any ARFCN ( n) can be calculated from : Here 124. GSM - MULTIPLE ACCESS 14
- 15. GSM FDMA 25 MHz25 MHz Mobile to Base 0 1 2 890.2 890.4 890.6 (MHz) Base to Mobile 0 1 2 935.2 935.4 935.6 200 kHz 45MHz Channel layout and frequency bands of operation 890 935 960 915 200 kHz 15
- 16. Digital Voice Transmission -In GSM speech coding a block of 20 ms is encoded in one set of 260 bits. - This calculates as 50X 260 = 13 kbps. Thus GSM speech coder produces a bit rate of 13 kbps per subscriber. - This provides speech quality which is acceptable for mobile telephony and comparable with wire-line PSTN phones. Speech Coding 16
- 17. 01100011000111110011100 Speech Code 20 ms SpeechSignal Speech Coding Parameters like tone, length of tone, pitch are transmitted Sampling=50 times/sec of 260 bits each overall bit rate= 50x260x8 subs=104kbps 17
- 18. Channel Coding Block coder 50 Veryimportant bits 132 Important bits 78 Not so important bits 1:2 Convolutional Coder 456 4 Tail bits 53 bits 378 bits 3 parity bits 260 bits Detection & correction of errors 18
- 19. GSM Digital VoiceTransmission - It uses 260 bits from speech coding as input and outputs 456 encoded bits. - In one burst one block of 57 bits from one sample and another block from another sample are sent together. - These 456 bits for every 20 ms of speech are interleaved forming eight blocks of 57 bits each. Channel Coding Interleaving 19
- 20. GSM Digital VoiceTransmission To counteract the problems encountered in radio path: Burst Formatting - Additional bits as training sequence added to basic speech/data. - Total of 136 bits added, bringing overall total to 592 bits. - Each TS of TDMA frame is 0.577 ms long and during this time 156.25 bits are transmitted. - One burst contains only 148 bits. Rest of the space, 8.25 bits time, is empty and is called Guard Period ( GP ). - GP enables MS/BTS to “ramp up” and “ ramp down”. 20
- 21. 8.25 3 57 1 26 1 57 3 1 2 34 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 D D D D D D D D D D D D D D D D 456 bits Sample 1 456 bits Sample 2 Normal Burst Stream of Time Slots Interleaving & Burst Formatting 1st Sample of 20 ms speech 2nd Sample of 20 ms speech 21
- 22. GSM: Speech toRadio waves Speech Coding Channel Coding Interleaving Burst formatting Ciphering Burst formatting Deciphering Analog Modulation 200kHz BW Speech Decoding Analog 200kHz BW Demodulation De-interleaving Channel Decoding 23
- 23. vYV lsaVj Equalization, Diversityand Channel Coding EQUALIZATION: Compensate intersymbol interference (ISI) created by multipath within the time dispersive channels. DIVERSITY : It is used to compensate for fading channel impairment and is usually implemented by using two or more receiving antennas, CHANNEL CODING: it improves the small scale link performance by adding redundant data bits in the transmitted message so that if an instantaneous fade occur in the channel the data may still be recovered at the receiver.(ex: Block codes, Convolution code and turbo codes) – Block Codes : Block codes are forward error correction (FEC) codes that enable a limited numbers of error to be detected without retransmission.( Channel Coding) – INTERLEAVER: Function of interleaver to spread these bit out in time so that if there is a deep fade or noise bursts the important bits from a block of source data do not corrupted at the same time.( Time Diversity) 24
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- 25. CM CC SMS SS NormalBurst 3 T 57 Encrypted 1 S 26 Training 1 S 57 Encrypted 3 T 8.25 GP CM CC SMS SS 3 T 142 Fixed Bits 3 T 8.25 GP FCCH Burst 26
- 26. 8 7 65 4 3 2 1 FRAME OF 8 TIME SLOTS FRAME REPETITION PHYSICAL CHANNELS 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 1 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3 4 4 4 4 4 6 6 6 6 6 7 7 7 7 7 8 8 8 8 8 5 5 5 5 5 PHYSICAL CHANNELS 1 2 4 5 6 7 8 3 27
- 27. GSM-- TDMA STRUCTURE •TDMA 8 Time Slots / RF Channel • Time slot duration 0.577m sec or 15 / 26 m sec • Frame 8 Burst Periods ( Time Slots) = 8 15/26 = 4.615 m sec • Multi Frame Traffic 26 4.615 = 120 msec Control 51 4.615 = 235.365 m sec • Super Frame 51 Traffic Multi frames 26 Control Multi frames • Hyper Frame 2048 Super Frames = 3 28 52.76 hr min sec 28
- 28. GSM Radio Interface- CYCLES 2047 0 Hyperframe = 2048 Superframes 3 Hours 28 Minutes 53 Seconds and 760 milliseconds 50 0 25 0 Superframe = 26× 51 multiframes 6.12 Seconds 51 Multiframe Approx 235 mS 26 Multiframe 120 mS 25 24 2 1 0 50 49 48 1 0 7 6 5 4 3 2 1 0 TDMA frame 4.615 mS 29 f1, f2, f3, f4, f5, f6, There is the 26- multiframe, which contains 26 TDMA frames with a duration of 120 ms and which carries only traffic channels and the associated control channels. The other variant is the 51-multiframe, which contains 51 TDMA frames with a duration of 235.8 ms and which carries signaling data exclusively. Each superframe consists of twenty-six 51-multiframes or fifty-one 26-multiframes.
- 29. Organisation of Speech& Data 24 25 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 BP 7 BP 6 BP 5 BP 4 BP 3 BP 2 BP 1 BP 0 8.25 3 57 1 26 1 57 3 Frames 0-11 : TCH Frames 12 : SACCH Frames 13-24 : TCH Frames 25 : Unused 26 – frame multiframe Duration: 120 ms TDMA frame Duration: 60/13 ms =4.615 ms Tail Tail bits bits Data bits Data bits Stealing Stealing bit bit Training Training sequence sequence Stealing Stealing bit bit Data bits Data bits Tail Tail bits bits Guard Guard bits bits Normal burst Normal burst Duration 15/26 Duration 15/26 ms ms 30
- 30. GSM LOGICAL CHANNELS • USERINFORMATION( TRAFFIC) • SIGNALLING INFORMATION (CONTROL) 31
- 31. GSM CONTOL CHHANELS OVERLOGICAL CHANNELS THREE TYPES OF CONTROL CHANNELS • Intended to carry signaling and synchronization • Broadcast control channel BCCH • Common control channel CCCH • Dedicated control channel DCCH 32
- 32. Logical Channels Logical Channels Traffic Channels Control Channels TCH/F TCH/H Common Control Channels Dedicated Control Channels Broadcast Channels SDCCH (Stand alonededicated Ch) SACCH ( Slow Associated Ch) FACCH ( Fast associated Ch) RACH (Random access CH) PCH (Paging CH) AGCH (Access Grant CH) FCCH (FREQ CORREC CHL) SCH (SYNC CHL) BCCH (BROADCAST 33
- 33. 34 Function GSM LogicalChannels LTE Logical Channels Broadcast Info FCCH, SCH, BCCH BCH (Broadcast Channel), PCH (Paging Channel) Random Access RACH (Random Access Channel) PRACH (Physical Random Access Channel) Paging PCH (Paging Channel) PCH (Paging Channel) Access Grant AGCH (Access Grant Channel) - (Part of RAR via PDCCH in LTE) Call Setup Signaling SDCCH, SACCH, FACCH DCCH (Dedicated Control Channel) Traffic (Voice/Data) TCH/F, TCH/H (Full and Half Rate Traffic Channels) DTCH (Dedicated Traffic Channel), MTCH (Multicast Traffic Channel) Physical Mapping Logical mapped to Physical and Logical Channels Logical → Transport Channels → Physical Channels Architecture Level Channels directly map to physical layer (simpler structure) Layered (Logical → Transport → Physical); allows flexibility and higher efficiency GSM vs. LTE Logical Channels Comparison
- 34. GSM THREE TYPES OFCONTROL CHANNELS Dedicated control channel DCCH P - P For Registration SDCCH ,authentication SACCH & Handover FACCH Broadcast control channel BCCH P- MP For Freq Correction FCCH For Syncronisation SCH BCCH Common control channel CCCH For ACCESS Management PCH RACH AGCH 35
- 35. Logical Channels Configuration CombinedTime Slot • BCCH & 9 Blocks of CCCH. Non Combined Time Slot • BCCH, SDCCH/4 & 3 Blocks of CCCH. SDCCH not on TS0 • SDCCH/8. 36
- 36. Multiplexing of BCHsand CCCHs onTS0 0 4 9 14 19 F S B B B B C0 C0 C0 C0 F S C1 C1 C1 C1 C2 C2 C2 C2 20 24 29 34 39 F S C3 C3 C3 C3 C4 C4 C4 C4 F S C5 C5 C5 C5 C6 C6 C6 C6 40 44 49 F S C7 C7 C7 C7 C8 C8 C8 C8 I Non-Combined Time Slot 0
- 37. Multiplexing of BCHsand SDCCH onTS0 0 5 9 15 19 F S B B B B C0 C0 C0 C0 F S C1 C1 C1 C1 C2 C2 C2 C2 20 25 31 35 39 F SD0 1 1 1 F S D2 D2 D2D 2 D3 D3 D3 D3 0 0 0 1 40 45 49 F S A 0 A0 A0 A0 A2 A2 A2 A2 I D D D D D D D Combined Time Slot 0 FCCH ,SCH,BCCH,PCH/AGCH,IDLE
- 38. Multiplexing of SDCCHsand SACCHs on TS2 0 4 9 14 19 D5 D5 D5 D5 D6 D6 D6 D6 D7 D7 D7 D7 A0 A0 A0 A0 A1 A1 A1 A1 20 24 29 34 39 40 44 49 A2 A2 A2 A2 A3 A3 A3 A3 I I I D0 D0 D0 D0 D1 D1 D1 D1 D2 D2 D2 D2 D3 D3 D3 D3 D4 D4 D4 D4
- 39. OPERATIONAL CONCEPTS • Subscribersare not allocated dedicated channels • TCH Allocated to users only when needed • Hence IDLE MODE & DEDICATED MODE • DEDICATED MODE -- When a full Bi -directional P to P CHL has been allocated during an established call • IDLE MODE MODE -- When MS is powered on (active) without being in dedicated mode 40
- 40. OPERATIONAL CONCEPTS IDLE MODE --When MS is powered on (active) without being in dedicated mode • MS stays continuously in touch with BS • Listens to transmissions from BS to intercept Paging Messages ( for incoming calls) • Monitors Radio Environment in order to evaluate Chl Quality & choose the most suitable BS • Listens to BS to avail short message broadcast service 41
- 41. OPERATIONAL CONCEPTS ACCESS PROCEDURE --Access to system ( switch over from IDLE to DEDICATED Mode) • MS indicates to BS that it needs a connection • BS accepts the request & indicates which traffic CHL it may use • For above purpose specific transmission is done over “ Common Channels” 42
- 42. OPERATIONAL CONCEPTS MOBILE O/GCall • MS sends access over RACH • System allocates SDCCH through AGCH • Lastly TCH is assigned through SDCCH when a conversation can start • Set up information exchanged over SDCCH ( Authentication , Measurement Reports, Power Control) 43
- 43. OPERATIONAL CONCEPTS MOBILE I/CCall • Paging to MS through PCH since MS is monitoring PAGCH • MS responds by sending a page response over RACH • Set up information exchanged over SDCCH ( Authentication, Call set-up messages , Power Control) • As a result system allocates SDCCH to MS over AGCH • Lastly TCH is allocated to mobile over SDCCH . Mobile starts conversation. 44
- 44. Other Salient FeaturesOf GSM RF INTERFACE: GSM – RF Interface - Control of Transmitted Power. - Timing Advance. - Discontinuous Transmission. - Diversity. - Frequency Hopping. 45
- 45. Discontinuous Transmission (DTX) •Speech activity only 40% of time. • Needs Voice activity detection. • Determination of voice threshold vis-à-vis noise. • Annoying clicks/inefficient DTX. • Generation of Comfort Noise at receiver to avoid the feeling of the set being dead. 46
- 46. Timing Alignment - Largedistance between BTS and MS causes the problem. - Each MS on call is allocated a timeslot on TDMA frame. - The problem occurs when the information transmitted by MS does not reach BTS on allocated timeslot. 0 1 2 3 4 5 6 7 BT TDMA Frame A –on TS3 B –on TS2 TS3 TS2 47
- 47. Timing Advance ( Tocounteract problem of Time Alignment ) -- MS instructed to do its transmission certain bit-times earlier or later – to reach its timeslot at BTS in right time. -- In GSM systems maximum 63 bit-times can be used. -- This limits the GSM cell size to 35 Km radius. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Time Start Sending 48
- 48. Antenna Diversity - Mountingtwo receiver antenna physically separated a distance. - At 900 MHz with antenna spacing of 5-6 m we get 3 db gain. Space Diversity - Probability of both of them being affected by a deep fading dip at same time is low. No Diversity Antenna Diversity Tx Rx Rx (A) Rx ( B) Tx Polarization Diversity - - Dual polarized antenna – vertical and horizontal arrays. 49
- 49. FREQUENCY HOPPING • Changeof frequency after every frame in a pre-determined manner • SFH improves performance in multi-path fading • Decreases required C/I • Mandatory for MS when requested by BS • FCCH ,SCH ,BCCH are not hopped • Algorithm : Cyclic or pseudorandom • Provides interference diversity
- 50. SUMMARY • Radio Interface •Frequency Bands & Specifications • Multiple Access Method FDMA & TDMA • FDMA /TDMA Frame Representation • Logical Channels Traffic & Control • Operational Concepts • Other Salient Features of RF I/F- DTX, Time Alignment Diversity, Fr. Hopping Power Control. 51
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Editor's Notes
- #29 In a GSM system, every TDMA frame is assigned a fixed number, which repeats itself in a time period of 3 hours, 28 minutes, 53 seconds, and 760 milliseconds.