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Semelhante a GSM FRAME STRUCTURE.pptx
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GSM FRAME STRUCTURE.pptx
- 1. GSM Frame structure, VoLTE Call Procedure & WiFi- Off Loading Presnted by- Rohit Soni
- 2. GSM - RADIO INTERFACE IN THIS PRESENTATION • Frequency Bands & Specifications • Multiple Access Method: FDMA & TDMA • FDMA /TDMA Frame Representation • Logical Channels: Traffic & Control
- 3. Communication - Mobile Telephone Exchange Subscriber Line (2W) Inter-Exchange Junction Mobile Switching Centre (MSC) BSC BTS MS
- 4. GSM Uplink & Downlink • Frequency Bands GSM 900 Mhz DCS 1800 MHz B T S
- 5. GSM - MULTIPLE ACCESS • 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 ……...
- 6. 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
- 7. 01100011000111110011100 Speech Code 20 ms Speech Signal Speech Coding Parameters like tone, length of tone, pitch are transmitted Sampling=50 times/sec instead of 8000 of 260 bits each overall bit rate= 50x260x8 subs=104kbps
- 8. GSM Digital Voice Transmission - 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
- 9. GSM Digital Voice Transmission 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”.
- 10. 8.25 3 57 1 26 1 57 3 1 2 3 4 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
- 11. 4 GSM SPEECH CHANNEL INTERLEAVING ( Diagram ) 1 2 3 5 6 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 4 456 bits 5 456 bits 6 456 bits Full rate encoded speech blocks from a single conversation Bursts TDMA Frames Frame 1 Frame 2 Frame 3 Frame 4 INTERLEAVING
- 12. FREQUENCY CORRECTION BURST 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 142 bits 3 3 FRAME1(4.615ms) FRAME2 Fixed Data Tail Bits Tail Bits Guard Period Guard Period 0.546ms 0.577ms Carries FCCH channel. Made up of 142 consecutive zeros. Enables MS to correct its local oscillator locking it to that of the BTS. CHANNEL CONCEPT
- 13. SYNCHRONISATION BURST 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 39 bits 3 3 FRAME1(4.615ms) FRAME2 Synchronisation Sequence Tail Bits Tail Bits Guard Period Guard Period 0.546ms 0.577ms 64 bits 39 bits Encrypted Bits Encrypted Bits Carries SCH channel. Enables MS to synchronise its timings with the BTS. Contains BSIC and TDMA Frame number. CHANNEL CONCEPT
- 14. DUMMY BURST 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 57 bits 57 bits 26 bits 3 3 FRAME1(4.615ms) FRAME2 Training sequence Data Data Tail Bits Tail Bits Flag Bit Flag Bit Guard Period Guard Period 0.546ms 0.577ms Transmitted on the unused timeslots of the BCCH carrier in the downlink. CHANNEL CONCEPT
- 15. ACCESS BURST 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 41 bits 68.25 bits 8 FRAME1(4.615ms) FRAME2 Tail Bits Tail Bits Guard Period 0.577ms 36 bits Synchronisation Sequence Encrypted Bits 3 Carries RACH. Has a bigger guard period since it is used during initial access and the MS does not know how far it is actually from the BTS. CHANNEL CONCEPT
- 16. NORMAL BURST 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 57 bits 57 bits 26 bits 3 3 FRAME1(4.615ms) FRAME2 Training sequence Data Data Tail Bits Tail Bits Flag Bit Flag Bit Guard Period Guard Period 0.546ms 0.577ms Carries traffic channel and control channels BCCH, PCH, AGCH, SDCCH, SACCH and FACCH. CHANNEL CONCEPT
- 17. 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
- 18. 0 1 0 1 2 2045 2046 2047 1 Hyperframe = 2048 superframes = 2,715,648 TDMA frames 3h 28min 53s 760ms 1 Superframe = 1326 TDMAframes = 51(26 fr) 0r 26(51 fr) multiframes 1 2 3 49 48 47 50 0 1 24 25 0 1 2 23 24 25 0 48 1 2 49 50 2 3 4 5 6 7 6.12s 0 235.38ms 120ms Control 51 - Frame Multiframe Traffic 26 - Frame Multiframe 4.615ms TDMA Frame HYPERFRAME AND SUPERFRAME STRUCTURE CHANNEL CONCEPT
- 19. 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
- 20. 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 bits Data bits Stealing bit Training sequence Stealing bit Data bits Tail bits Guard bits Normal burst Duration 15/26 ms
- 21. Physical channel - Each timeslot on a carrier is referred to as a physical channel. Per carrier there are 8 physical channels. Logical channel - Variety of information is transmitted between the MS and BTS. There are different logical channels depending on the information sent. The logical channels are of two types Traffic channel Control channel Downlink Uplink CHANNELS CHANNEL CONCEPT
- 23. BCH Channels BCCH( Broadcast Control Channel ) Downlink only Broadcasts general information of the serving cell called System Information BCCH is transmitted on timeslot zero of BCCH carrier Read only by idle mobile at least once every 30 secs. SCH( Synchronisation Channel ) Downlink only Carries information for frame synchronisation. Contains TDMA frame number and BSIC. FCCH( Frequency Correction Channel ) Downlink only. Enables MS to synchronise to the frequency. Also helps mobiles of the ncells to locate TS 0 of BCCH carrier. CHANNEL CONCEPT
- 24. CCCH Channels RACH( Random Access Channel ) Uplink only Used by the MS to access the Network. AGCH( Access Grant Channel ) Downlink only Used by the network to assign a signalling channel upon successfull decoding of access bursts. PCH( Paging Channel ) Downlink only. Used by the Network to contact the MS. CHANNEL CONCEPT
- 25. DCCH Channels SDCCH( Standalone Dedicated Control Channel ) Uplink and Downlink Used for call setup, location update and SMS. SACCH( Slow Associated Control Channel ) Used on Uplink and Downlink only in dedicated mode. Uplink SACCH messages - Measurement reports. Downlink SACCH messages - control info. FACCH( Fast Associated Control Channel ) Uplink and Downlink. Associated with TCH only. Is used to send fast messages like handover messages. Works by stealing traffic bursts. CHANNEL CONCEPT
- 26. Logical Channels Configuration Non Combined Mode • TS0 = BCCH and CCCH/9. Combined Mode • TS0 = BCCH, SDCCH/4 + SACCH/2 and CCCH/3. • SDCCH not on TS0 • TS2 = SDCCH/8 + SACCH/4.
- 27. T 15 T 5 T 9 T 10 T 11 S 12 T 13 T 14 T 6 T 7 T 8 T 0 T 1 T 2 T 3 T 4 T 16 T 17 T 18 T 19 T 20 T 21 T 22 T 23 T 24 I 25 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 120 msec 4.615 msec 26 FRAME MULTIFRAME STRUCTURE Logical Channels Configuration
- 28. Multiplexing of BCHs and 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
- 29. Multiplexing of BCHs and 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 S D0 1 1 1 F S D2 D2 D2D 2 D3 D3 D3 D3 0 0 0 1 40 45 49 F S A0 A0 A0 A0 A2 A2 A2 A2 I D D D D D D D Combined Time Slot 0
- 30. Multiplexing of SDCCHs and 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
- 31. SUMMARY • Radio Interface • Frequency Bands & Specifications • Multiple Access Method FDMA & TDMA • FDMA /TDMA Frame Representation • Logical Channels Traffic & Control
Notas do Editor
- Air interface or U m –interface The Air Interface is the interface between the BTS (Base Transceiver Station) and the MS (Mobile Station).
- In India there are 2 frequency band Known as gsm 900 and DCS 1800 band.
- Each RF carrier 200khz apart Total 124 RF Channels available. One or more carrier assigned to each base station
- Speech coding is an application of data compression of digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic data compression algorithms to represent the resulting modeled parameters in a compact bitstream.
- The GSM digital speech compression process works by grouping the digital audio signals into 20 msec speech frames. These speech frames are analyzed and characterized (e.g. volume, pitch) by the speech coder. The speech coder removes redundancy in the digital signal (such as silence periods) and characterizes digital patterns that can be made by the human voice using code book tables. The code book table codes are transmitted instead of the original digitized audio signal. This results in the transmission of a 13 kbps compressed digital audio instead of the 64 kbps digitized audio signal.
- To further protect against burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time slot bursts. Since each time slot bursts can carry two 57 bit blocks, each burst carries traffic from two different speech samples.
- The GSM burst, or transmission can fulfil a variety of functions. Some GSM bursts are used for carrying data while others are used for control information. As a result of this a number of different types of GSM burst are defined. Normal burst uplink and downlink Synchronisation burst downlink Frequency correction burst downlink Random Access (Shortened Burst) uplink
- After channel coding, the extracted 456 bits are distributed into 8 groups with each group containing 57 bits. That is the first interleaving, also called internal interleaving as shown in the above diagram. Through the first interleaving, the successive messages in the groups are dispersed. One burst carries two segments of voice information composed of 57 bits. Obviously, if the two groups of 57 bits information from the first interleaving of a successive 20ms voice blocks are inserted to the same burst sequence, the loss of the burst will lead to loss 25% bits in the 20ms voice block. Therefore, one more interleaving is needed between two voice blocks, which is called the inter-block interleaving or second interleaving.
- GSM frequency correction burst With the information in the burst all set to zeros, the burst essentially consists of a constant frequency carrier with no phase alteration. 3 tail bits: Again, these tail bits at the start of the GSM burst give time for the transmitter to ramp up its power. 142 bits all set to zero: 3 tail bits Again these are to enable the transmitter power to ramp down. 8.25 bits guard time: to act as a guard interval.
- GSM synchronisation burst The purpose of this form of GSM burst is to provide synchronisation for the mobiles on the network. 3 tail bits: Again, these tail bits at the start of the GSM burst give time for the transmitter to ramp up its power 39 bits of information: 64 bits of a Long Training Sequence: 39 bits Information: 3 tail bits Again these are to enable the transmitter power to ramp down. 8.25 bits guard time: to act as a guard interval.
- This GSM burst is used for the standard communications between the basestation and the mobile, and typically transfers the digitised voice data. The structure of the normal GSM burst is exactly defined and follows a common format. It contains data that provides a number of different functions: 3 tail bits: These tail bits at the start of the GSM burst give time for the transmitter to ramp up its power 57 data bits: This block of data is used to carry information, and most often contains the digitised voice data although on occasions it may be replaced with signalling information in the form of the Fast Associated Control CHannel (FACCH). The type of data is indicated by the flag that follows the data field 1 bit flag: This bit within the GSM burst indicates the type of data in the previous field. 26 bits training sequence: This training sequence is used as a timing reference and for equalisation. There is a total of eight different bit sequences that may be used, each 26 bits long. The same sequence is used in each GSM slot, but nearby base stations using the same radio frequency channels will use different ones, and this enables the mobile to differentiate between the various cells using the same frequency. 1 bit flag Again this flag indicates the type of data in the data field. 57 data bits Again, this block of data within the GSM burst is used for carrying data. 3 tail bits These final bits within the GSM burst are used to enable the transmitter power to ramp down. They are often called final tail bits, or just tail bits. 8.25 bits guard time At the end of the GSM burst there is a guard period. This is introduced to prevent transmitted bursts from different mobiles overlapping. As a result of their differing distances from the base station.
- GSM random access burst This form of GSM burst used when accessing the network and it is shortened in terms of the data carried, having a much longer guard period. This GSM burst structure is used to ensure that it fits in the time slot regardless of any severe timing problems that may exist. Once the mobile has accessed the network and timing has been aligned, then there is no requirement for the long guard period. 7 tail bits: The increased number of tail bits is included to provide additional margin when accessing the network. 41 training bits: 36 data bits: 3 tail bits Again these are to enable the transmitter power to ramp down. 69.25 bits guard time: The additional guard time, filling the remaining time of the GSM burst provides for large timing differences.
- 3 tail bits: These tail bits at the start of the GSM burst give time for the transmitter to ramp up its power 57 data bits: This block of data is used to carry information, and most often contains the digitised voice data although on occasions it may be replaced with signalling information in the form of the Fast Associated Control CHannel (FACCH). The type of data is indicated by the flag that follows the data field 1 bit flag: This bit within the GSM burst indicates the type of data in the previous field. 26 bits training sequence: This training sequence is used as a timing reference and for equalisation. There is a total of eight different bit sequences that may be used, each 26 bits long. The same sequence is used in each GSM slot, but nearby base stations using the same radio frequency channels will use different ones, and this enables the mobile to differentiate between the various cells using the same frequency. 1 bit flag Again this flag indicates the type of data in the data field. 57 data bits Again, this block of data within the GSM burst is used for carrying data. 3 tail bits These final bits within the GSM burst are used to enable the transmitter power to ramp down. They are often called final tail bits, or just tail bits. 8.25 bits guard time At the end of the GSM burst there is a guard period. This is introduced to prevent transmitted bursts from different mobiles overlapping. As a result of their differing distances from the base station.
- The GSM frame structure is designated as hyperframe, superframe, multiframe and frame. The minimum unit being frame (or TDMA frame) is made of 8 time slots. One GSM hyperframe composed of 2048 superframes. Each GSM superframe composed of multiframes (either 26 or 51 as described below). Each GSM multiframe composed of frames (either 51 or 26 based on multiframe type). Each frame composed of 8 time slots.
- The GSM frames are grouped together to form multiframes and in this way it is possible to establish a time schedule for their operation and the network can be synchronised. There are several GSM multiframe structures: Traffic multiframe: The Traffic Channel frames are organised into multiframes consisting of 26 bursts and taking 120 ms. In a traffic multiframe, 24 bursts are used for traffic. These are numbered 0 to 11 and 13 to 24. One of the remaining bursts is then used to accommodate the SACCH, the remaining frame remaining free. The actual position used alternates between position 12 and 25. Control multiframe: the Control Channel multiframe that comprises 51 bursts and occupies 235.4 ms. This always occurs on the beacon frequency in time slot zero and it may also occur within slots 2, 4 and 6 of the beacon frequency as well. This multiframe is subdivided into logical channels which are time-scheduled. These logical channels and functions include the following:
- BCCH( Broadcast Control Channel ) Broadcasts general information of the serving cell called System Information SCH( Synchronisation Channel ) Carries information for frame synchronisation. Contains TDMA frame number and BSIC. FCCH( Frequency Correction Channel ) Enables MS to synchronise to the frequency. RACH( Random Access Channel ) Used by the MS to access the Network. AGCH( Access Grant Channel ) Used by the network to assign a signalling channel upon successfull decoding of access bursts. PCH( Paging Channel ) Used by the Network to contact the MS. SDCCH( Standalone Dedicated Control Channel ) Used for call setup, location update and SMS. SACCH( Slow Associated Control Channel ) Uplink SACCH messages - Measurement reports. Downlink SACCH messages - control info. FACCH( Fast Associated Control Channel ) Associated with TCH only. Is used to send fast messages like handover messages.
- The BCCH is a point-to-multipoint unidirectional control channel from the fixed subsystem to MS that is intended to broadcast a variety of information to MSs, including information necessary for the MS to register in the system. BCCH has 51 bursts. BCCH is dedicated to slot1 and repeats after every 51 bursts. synchronization channel(SCH), which is used to provide TDMA frame oriented synchronization data to a MS. When a mobile recovers both FCCH and SCH signals, the synchronization is said to be complete. SCH repeats for every 51 frames. SCH carries information for the frame synchronization (TDMA frame number of the MS and the identification of BTS ) .This is also required for the correct operation of the mobile. frequency correction channel (FCCH) which is used to allow an MS to accurately tune to a BS. The FCCH carries information for the frequency correction of MS downlink. It is required for the correct operation of radio system. This is also a point-to multipoint communication. This allows an MS to accurately tune to a BS.
- random access channel (RACH) which is used by MS to request of an SDCCH either as a page response from MS or call origination/ registration from the MS. This is uplink channel and operates in point-point mode(MS to BTS).This uses slotted ALOHA protocol. This causes a possibility of contention. If the mobiles request through this channel is not answered with in a specified time the MS assumes that a collision has occurred and repeats the request. Mobile must allow a random delay before re-initiating the request to avoid repeated collision. access grant channel(AGCH) which is a downlink channel used to assign a MS to a specific SDCCH or a TCH. AGCH operates in point-to-point mode. A combined paging and access grant channel is designated as PAGCH. -- paging channel(PCH), which is used to search(page) the MS in the downlink direction ,
- Standalone DCCH (SDCCH) is used for system signaling during idle periods and call setup before allocating a TCH , for example MS registration, authentication and location updates through this channel. When a TCH is assigned to MS this channel is released. SACCH is data channel carrying information such as measurement reports from the mobile of received signal strength for a serving cell as well as the adjacent cells. This is necessary channel for the assisted over hand over function. FACCH works in a stealing mode. This means that if suddenly during a speech transmission it is necessary to exchange signaling information with the system at a rate much higher than the SACCH can handle
- MS on dedicated mode on a TCH uses a 26-frame multiframe structure. Frame 0-11 and 13-24 used to carry traffic. Frame 12 used as SACCH to carry control information from and to MS to BTS. Frame 25 is idle and is used by mobile to decode the BSIC of neighbor cells.
- This is known as noncombined mode . In noncombined case, there are nine CCCH blocks mentioned as CCCH(0) to CCCH(8). If the System information messages (SIs) are more and can not be occupied in BCCH block then first CCCH block i.e. CCCH(0) can also be used for BCCH and SIs are transmitted on the same.
- In commbined mode configuration, position of FCCH+SCH+BCCH is not changed, but CCCH capacity is reduced from 9 blocks to mere 3 blocks. This 6 blocks are used by 4 blocks of SDCCH and 2 blocks of SACCH.