Chapter 7 GSM: Pan-European Digital Cellular System. Prof. Jang-Ping Sheu

Transcription

1 Chapter 7 GSM: Pan-European Digital Cellular System Prof. Jang-Ping Sheu

2 Background and Goals GSM (Global System for Mobile Communications) Beginning from 1982 European standard Full roaming in Europe A purely digital system Goals: full international roaming 2003/3/24 J. P. Sheu 2

3 Background and Goals provision for national variations in charging and rates efficient interoperation with ISDN systems signal quality better than or equal to that of existing mobile systems traffic capacity higher than or equal to that of present systems lower cost than existing systems accommodation of non-voice services and portable terminals 2003/3/24 J. P. Sheu 3

4 Architecture Network elements Mobile stations, base stations, and mobile switching center Three databases Home location registers (HLR) Visitor location registers (VLR) Equipment identity registers (EIR) 2003/3/24 J. P. Sheu 4

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6 Architecture In contrast to the original cellular, micro cells are used in GSM A BS separates into two parts: BTS (base transceiver station) and BSC (base station controller) Typically, a BSC controls several BTS To reduce the cost with the greatest possible service extent 2003/3/24 J. P. Sheu 6

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8 Architecture Subscriber identity module (SIM) An important GSM innovation A removable card that stores information, including ID number, abbreviated dialing code, and subscriber s service plan Easy to change telephones 2003/3/24 J. P. Sheu 8

9 Architecture As in the other systems, GSM uses a variety of ID codes GSM Identifiers International Mobile Subscriber Identity (15 digits) Temporary Mobile Subscriber Identity (32 bits) Advantages: Privacy and save BW International Mobile Equipment Identifier (15 digits) 2003/3/24 J. P. Sheu 9

10 Architecture Authentication Key (max = 128 bits) Cipher key (64 bits): Terminal and network use authentication key to compute the cipher key Mobile station classmark including: Version of the GSM standard RF power capability (power levels available) Encryption method Other properties of terminal 2003/3/24 J. P. Sheu 10

11 Architecture Training Sequence (26 bits) help a terminal verify that it receives information from the correct BS rather than another BS using the same physical channel BS Identity Code (6 bits) Location Area Identity (40 bits) including: A mobile country code, network code, and area code 2003/3/24 J. P. Sheu 11

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13 Radio Transmission GSM Spectrum There are two 25 MHz bands separated by 45 MHz Initial GSM systems operate in the upper 10 MHz Physical Channels GSM is a Hybrid FDMA/TDMA system Each GSM band has carriers spaced at 200 khz The frame duration is 120/26 = 4.62 ms Each frame contains 8 time slots There are 25 MHz/200 k Hz = 125 carriers in per direction GSM specifies only 124 carriers (one is used as guard band) 2003/3/24 J. P. Sheu 13

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17 Radio Transmission GSM time interval A hyperframe = 2048 superframe = 3 h 28 m s A superframe = 51 traffic multiframes = 26 control multiframes = 6.12 s A traffic multiframe = 26 frames = 120 ms A control multiframe = 51 frames = ms A frame = 8 time slots = ms A slot = bits = 577 µs A bit = 3.69 µs 2003/3/24 J. P. Sheu 17

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19 Physical Channels Traffic Channels A full-rate traffic channel (TCH/F) occupies one time slot in 24 of 26 frames in every multiframe Traffic channel information travels in frames 0-11 and Control information travels in frames 12 and 25 The SACCH occupies one frame in every traffic multiframe A SACCH associated with a full-rate traffic channel alternatively occupies one slot in frame 12 and one slot in frame /3/24 J. P. Sheu 19

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21 Physical Channels A half-rate traffic channel (TCH/H) occupies a specific time slot in 12 of 26 frames in every multiframe Each carrier can carry up to 16 half-rate traffic channels Eight of these traffic channels have a SACCH in frame 12 and the other eight half-rate channel have a SACCH in frame /3/24 J. P. Sheu 21

22 GSM Bit Stream The contents of a GSM time slot is shown in Fig bits of training sequence serves as a purpose similar to that of the SYNC field in NA-TDMA GSM specifies 8 different training sequences with low mutual cross-correlation Network operators assign different training sequences to nearby cells that use the same carrier The two DATA fields carry either user information or network control information 2003/3/24 J. P. Sheu 22

23 Radio Transmission The FLAG indicates whether the DATA field contains user information or control one The TAIL bits all set to 0 There is also a guard time 0f 30.5 µs The GSM transmission rate is kb/s The modulation scheme in GSM is GMSK a form of frequency shift keying The modulation efficiency of GSM is 1.35 b/s/hz GSM BS turn off its transmitter at the end of each time slot. It resume transmitting after a pause of 30.5µs to send to another terminal in the next time slot The BS turn off its transmitter in unassigned time slots 2003/3/24 J. P. Sheu 23

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25 Slow Frequency Hopping The signal moves from one frequency to another in every frame The purpose of FH is to reduce the transmission impairments Without FH, the entire signal is subject to distortion whenever the assigned carrier is impaired Network operator assigns different hopping patterns to different cells 2003/3/24 J. P. Sheu 25

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27 Radiated Power GSM specifies 5 classes of mobile stations transmitting power, ranging from 20 W (43 dbm) to 0.8 W (29 dbm) Typically, vehicle-mounted terminal is 8 W and portable terminals is 2 W 2003/3/24 J. P. Sheu 27

28 Spectrum Efficiency The reuse factor of N = 3 or 4 The number of physical channel is 124 carriers x 8 channels/carriers = 992 physical channels The efficiency of GSM is E = 992 channels/4 cells/cluster/50 MHz = 4.96 conversation/cell/mhz (N= 4) or The efficiency of GSM is E = 992 channels/3 cells/cluster/50 MHz = 6.61 conversation/cell/mhz (N= 3) 2003/3/24 J. P. Sheu 28

29 Logical Channels Traffic channels (two-way) Broadcast channels (base-to-mobile) Common control channels (base-to-mobile or mobile-to-base) Dedicated control channels (two-way) 2003/3/24 J. P. Sheu 29

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31 Broadcast channels and Common control channels The broadcast channels always occupy time slot 0 The common control channels can also occupy time slots 0 Control Multiframe There are 5 groups of frames, each containing ten frames beginning with a frequency-correction frame and a synchronization frame In the reverse direction, time slot 0 is assigned to random access channels in all 51 frames 2003/3/24 J. P. Sheu 31

32 Figure 7.11 shows the contents of time slot 0 in each of the 51 frames 2003/3/24 J. P. Sheu 32

33 Logical Channels Frequency Correction Channel (FCCH) The FCCH simply transmits 148 0s The FCCH always occupies time slot 0 in a frame of 8 time slots A terminal without a call in progress searches for a FCCH Synchronization Channel (SCH) A BS transmits a SCH in time slot 0 of every frame that follows a frame containing an FCCH The SCH contains a TRAINING sequence The DATA fields contain BS identity code (6 bits) and the present frame number 2003/3/24 J. P. Sheu 33

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35 Logical Channels Broadcast Control Channel (BCCH) BS use the BCCH to transmit the information that terminals need to set up a call, including the control channel configuration and the access protocol The message length is 184 bits and the encoded message is 456 bits occupying 4 time slots 2003/3/24 J. P. Sheu 35

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37 Logical Channels Paging Channel (PCH) and Access Grant Channel (AGCH) The purpose of the AGCH is to direct a terminal to a stand-alone dedicated control channel (SDCCH) Both channels use the same coding scheme as the BCCH They occupy 36 frames of time slot 0 per multiframe 2003/3/24 J. P. Sheu 37

38 Logical Channels Random Access Channel (RACH) GSM terminals send messages on the RACH to originate phone calls, initiate transmissions of short messages, respond to paging messages, and register their locations Terminals with information to transmit use the slotted ALOHA protocol to gain access to the time slot The Ack directs the terminal to a stand-alone dedicated control channel (SDCCH) to be used for further communications 2003/3/24 J. P. Sheu 38

39 Logical Channels The RACH slot includes a 41-bit TRAIN and 36-bit DATA The 36-bit DATA field carries a simple 8-bit message Three of the 8 bits indicate the purpose of the access attempt and the other 5 bits are produced by a random number generator The 5-bit random code is likely (with probability 31/32) to distinguish the successful terminal from the other 2003/3/24 J. P. Sheu 39

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42 Logical Channels Stand-Alone Dedicated Control Channel (SDCCH) SDCCH is a two-way channel assigned to a specific terminal The physical channel used by an SDCCH is a set of four time slots in each 51-frame control multiframe With 114 data bits per time slot, the data rate of the SDCCH is b/s (see eq. 7.7) Each SDCCH has a slow associated control channel The SACCH occupies an average of two time slots per control multiframe (969 b/s) 2003/3/24 J. P. Sheu 42

43 Logical Channels Traffic Channels (TCH) GSM defines two traffic channels, a full-rate channel occupies 24 time slots in every 26-frame and a half-rate channel The bit rate of a full-rate traffic channel is 22,800 b/s SACCH occupies time slots in frames 12 or 25 of each 26-frame traffic multiframe The transmission rate of a traffic SACCH is 950 b/s With 456 bits transmitted per message, a message spans four traffic multiframes, a time interval of 480 ms 2003/3/24 J. P. Sheu 43

44 Logical Channels Fast Associated Control Channel (FACCH) Use the traffic channel to transmit control information, which is an in-band signaling channel Each FACCH message is multiplexed with user information and interleaved over 8 frames. Therefore, the transmission time of an FACCH message is approximately 40 ms 2003/3/24 J. P. Sheu 44

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46 Messages GSM Protocol Layers GSM provides a large number of open interfaces Message Structure All of the signaling message length is 184 bits with the exception of the FCCH, SCH, and RACH 2003/3/24 J. P. Sheu 46

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49 Network Operations Call to a GSM Terminal Terminal uses the frequency correction channel (FCCH) to synchronize its local oscillator It then gains timing information from the SCH The terminal then obtains important information from broadcast control channel (BCCH) After the initialization procedure, the terminal monitors a paging channel (PCH) Eventually, it detects a paging request message and this message cause the terminal to transmit a channel request message on the random access channel (RACH) 2003/3/24 J. P. Sheu 49

50 Network Operations The network response this request by transmitting an immediate assignment message on an access grant channel (AGCH) This message established a stand-alone dedicated control channel (SDCCH) to be used for exchange of mobility management messages and call management messages When terminal moves to SDCCH, it transmits a paging response message to BS The BS then initiates the GSM authentication procedure 2003/3/24 J. P. Sheu 50

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52 Network Operations Authentication and Encryption Procedure The terminal received a 128-bit random number (RAND) from BS Then it applies a GSM encryption algorithm A3 to compute a 32-bit signed response, SRES The inputs of A3 are RAND and secret key Ki The secret key Ki is stored in the subscriber information module (SIM) The terminal applies another encryption algorithm A8 to compute a 64-bit ciphering key Kc from SRES and Ki The network also uses A3 to compute SRES from RAND and Ki 2003/3/24 J. P. Sheu 52

53 Network Operations If the two values of SRES are identical, the network accept the the user as an authorized subscriber To encrypt user information and network control information, the BS and network derive a 114-bit mask to be added (modulo 2) to the two DATA fields The inputs of A5 are the 64-bit ciphering key Kc and the current 22-bit frame number Because A5 uses the frame number to compute the ciphering mask, the mask change from frame to frame 2003/3/24 J. P. Sheu 53

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55 Network Operations To Setup a Call BS transmits a setup message to the terminal The terminal Ack this message with a call confirmed The terminal then send a connect message to the network In response, the network moves the call to a traffic channel by means of an assignment command message Note that, GSM assigns a traffic channel after the mobile subscriber accepts the call 2003/3/24 J. P. Sheu 55

56 Network Operations Location-Based Registration Terminal registers its location when it moves to a new cell Mobile-Assisted Handover When mobile terminal finds a channel quality is better than present one the handover procedures will be executed 2003/3/24 J. P. Sheu 56

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58 Status of GSM GSM operates in 900 MHz, 1800 MHz, and 1900 MHz bands GPRS (generalized packet radio service) with 100 kbits/s data rate Enhanced Data Rate for GSM Evolution (EDGE) with 300 kbits/s data rate Universal Mobile Telecommunication Services (UMTS): 3G telecommunication technology up to 2 Mbits/s data rate using WCDMA or TD/CDMA transceiver 2003/3/24 J. P. Sheu 58