Gsm air interface


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GSM system working

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  • Gsm air interface

    1. 1. Radio Transmission Techniques FDMA TDMA CDMAChannels Physical channels Logical channels
    2. 2. FrequencyChannel Time
    3. 3. Frequency Time SlotChannel Time 0 1 2 3 4 5 6 7
    4. 4. Frequency Code Time Code 1 Code 2 Code 3
    5. 5. ChannelsPhysical Channels Associated with frequency bands, time slots, codes Physical channels transfer bits from one network element to anotherLogical Channels Distinguished by the nature of carried information and the way to assemble bits into data units Three types  one-to-one: traffic channels between a BTS and a MS  one-to-many: synchronization signals from BTS to MSs in a cell  many-to-one: from MSs to the same BTS
    6. 6. Air Interface Layers Layer 3 Messages Messages Messages Logical Layer 2 Logical Channels Packets Channels Radio Layer 1 Radio Transmission Bits Transmission Terminal Base Station
    7. 7. GSM Frame Structure Superframe ... 51 Multiframe 6.12 sec 120 msec ... 26 Frames 4.615 msec 0 7 8 Multiframe Coded data Midamble0.57692 msec 3 57 1 26 1 57 3 8.25156.25 bits Tail bit Stealing Flag Guard period
    8. 8. Frame Types • There are two types of multiframe – 26 TDMA-frame multiframe is used to carry TCH, SACCH and FACCH – 51 TDMA-frame multiframe is used to carry BCCH, CCH, SDCCH and SACCH
    9. 9. Burst and Frames• The information contained in one time slot on the TDMA frame is call a burst.• Five types of burst – Normal Burst (NB) – Frequency Correction Burst (FB) – Synchronization Burst (SB) – Access Burst (AB) – Dummy Burst
    10. 10. Traffic TCH/F: Full-rate Traffic Channel Two-waychannels TCH/H: Half-rate Traffic Channel (TCH) FCCH: Frequency correction BCH SCH: Synchronization BCCH: Broadcast control Base-to-Signaling PCH: Paging mobile CCCHchannel AGCH: Access grant RACH: Random access SDCCH: Stand-alone dedicated control DCCH SACCH: Slow associated control Two-way FACCH: Fast associated control
    11. 11. Logical Channels• Control Channels – Broadcast Channels (BCH) – Common Control Channels (CCCH) – Dedicated Control Channels (DCCH)• Traffic Channels (TCH) – Full Rate (TCH/F) – Half Rate (TCH/H)
    12. 12. UP / Down-LinkDown-link: the transmission path from Base Station to Mobile StationUp-link: the transmission path from Mobile Station to Base Station
    13. 13. Control Channels• Broadcast Channels (BCH) – Frequency Correction Channel (FCCH) – Synchronization Channel (SCH) – Broadcast Control Channel (BCCH)• Common Control Channels (CCCH) – Paging Channel (PCH) – Random Access Channel (RACH) – Access Grant Channel (AGCH)• Dedicated Control Channels (DCCH) – Stand alone Dedicated Control Channel (SDCCH) – Cell Broadcast Channel (CBCH) – Slow Associated Control Channel (SACCH) – Fast Associated Control Channel (FACCH)
    14. 14. Broadcast Channels (BCH)To help the MH (Mobile Handset) to turn to a BTS to listen for the cell information  to start roaming, waiting for calls to arrive, making calls Because BTSs are not synchronized with each other, every time a MH decides to camp to another cell, its FCCH, SCH, and BCCH must be read.
    15. 15. Frequency Correction Channel (FCCH)• Provide MH with the frequency reference of the system – To enable the Mobile Handset (MH) to synchronize with the frequency• Transmission properties – Transmit on the down-link – Point to multi-point.
    16. 16. Synchronization Channel (SCH)• MH synchronize with the structure within the locative cell – MH can receive information from the proper time slots on the TDMA structure• To ensure a GSM BTS is chose – The Base Station Identity Code (BSIC) can only be decoded by a GSM BTS• Transmission properties – Transmit on down-link – Point to multi-point.
    17. 17. Broadcast Control Channel (BCCH)• BTS broadcast cell information to MH – LAI ( Location Area Identity), to start roaming, waiting for calls to arrive, making calls – maximum output power allowed in the cell – information about BCCH carriers for the neighboring cells • MH will perform measurement to BTS• Transmission properties – Transmit on down-link – Point to multi-point
    18. 18. Common Control Channels (CCCH)CCCH support the establishment of a dedicated communication path (dedicated channel) between the MH and the BTSThree types of CCCH Paging Channel (PCH) Random Access Channel (RACH) Access Grant Channel (AGCH)
    19. 19. Paging Channel (PCH)• Used by BTS to page particular MH in the cell – MH actively listen to PCH to check contact info within certain time – Contact could be incoming call or short message• Contact info on PCH include – IMSI (MH’s identity number), or – TMSI (temporary number)• Transmission properties – Transmit on down-link – point to point
    20. 20. Random Access Channel (RACH)• Used by MH to request a dedicated channel for call setup – Shared by any MH attempts to access the network – Channel request message contains the reason for the access attempt• Transmission properties – Transmit on up-link – Point to pint.
    21. 21. Access Grant Channel (AGCH)• The network assigns a signaling channel via AGCH – A Stand alone Dedicated Control Channel (SDCCH) is assigned• Transmission properties – Transmit on down-link – Point to point
    22. 22. Dedicated Control Channels(DCCH) DCCH are used for transferring nonuser information between the network and the MH Messages on DCCH Including  channel maintenance  mobility management  radio resource management Four kinds of DCCH Stand alone Dedicated Control Channel (SDCCH) Cell Broadcast Channel (CBCH) Slow Associated Control Channel (SACCH) Fast Associated Control Channel (FACCH)
    23. 23. Stand alone Dedicated Control Channel (SDCCH)• Transfer signaling information between the BTS and the MH• Typically used for location updating prior to use of a traffic channel• Transmission properties – Bidirectional channel, transmit on both up and down-link – Point to point.
    24. 24. Cell Broadcast Channel (CBCH)• To carry Short Message Service Cell Broadcast (SMSCB)• Use the same physical channel as SDCCH• Transmission properties – Transmit on down-link – Point to multi-point
    25. 25. Slow Associated Control Channel (SACCH)• Carries control and measurement parameters along with routine data necessary to maintain a radio link between the MH and the BTS – On the uplink, MS sends averaged measurements (signal strength and quality) of current and neighboring BCCH – On downlink, MS receives information about transmitting power to use and an instruction with time advance/retard• Transmission properties – Bidirection channel, transmit on both up and down link – Point to point
    26. 26. Fast Associated Control Channel (FACCH)• An FACCH is used over a TCH where it steals time slots from a TCH – a 20 ms segment of speech is stolen to carry handover signaling information• Appears on demand
    27. 27. Traffic Channels (TCH)• TCH transport user information (speech/data)• TCH are bidirectional dedicated channels between the network and the MH
    28. 28. Burstinformation contained in one time slot is a The burst Five types of burst Normal Burst (NB)  To carry information on traffic and control channels Frequency Correction Burst (FB)  To synchronize the frequency of the mobile Synchronization Burst (SB)  To synchronize the frames of the mobile Access Burst (AB)  For random and handover access Dummy Burst  For padding the frame