Chapter 7 GSM: Pan-European Digital Cellular System


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Chapter 7 GSM: Pan-European Digital Cellular System

  1. 1. Chapter 7 GSM: Pan-European Digital Cellular System
  2. 2. Background and Goals <ul><li>GSM (Global System for Mobile Communications) </li></ul><ul><ul><li>Beginning from 1982 </li></ul></ul><ul><ul><li>European standard </li></ul></ul><ul><ul><li>Full roaming in Europe </li></ul></ul><ul><ul><li>A purely digital system </li></ul></ul><ul><li>Goals: </li></ul><ul><ul><li>full international roaming </li></ul></ul><ul><ul><li>provision for national variations in charging and rates </li></ul></ul><ul><ul><li>efficient interoperation with ISDN systems </li></ul></ul>
  3. 3. Background and Goals <ul><ul><li>Signal quality better than or equal to that of existing mobile systems </li></ul></ul><ul><ul><li>traffic capacity higher than or equal to that of present systems </li></ul></ul><ul><ul><li>lower cost than existing systems </li></ul></ul><ul><ul><li>accommodation of non-voice services, and </li></ul></ul><ul><ul><li>accommodation of portable terminals </li></ul></ul>
  4. 4. Architecture <ul><li>Network elements </li></ul><ul><ul><li>Mobile stations, base stations, and mobile switching center </li></ul></ul><ul><li>Three databases </li></ul><ul><ul><li>Home location registers (HLR): for full roaming </li></ul></ul><ul><ul><li>Visitor location registers (VLR): for full roaming </li></ul></ul><ul><ul><li>Equipment identity registers (EIR) </li></ul></ul>
  5. 6. SIM of GSM <ul><li>Subscriber identity module (SIM) </li></ul><ul><ul><li>A removable card that stores subscriber information: </li></ul></ul><ul><ul><ul><li>ID number </li></ul></ul></ul><ul><ul><ul><li>abbreviated dialing code </li></ul></ul></ul><ul><ul><ul><li>subscriber’s service plan </li></ul></ul></ul><ul><ul><li>The SIM is the subscriber’s link to the cellular system. </li></ul></ul><ul><ul><ul><li>By removing the SIM, the phone is disabled (except the emergency call). </li></ul></ul></ul><ul><ul><li>Easy to change to other telephones </li></ul></ul><ul><li>In earlier systems, the subscriber’s information is in a FIXED hardware within a terminal. </li></ul><ul><ul><li>Thus, when changing phones, the service provider gets involved, which is inconvenient. </li></ul></ul>
  6. 7. <ul><li>GSM uses a variety of ID codes, which are exchanged between base stations and handsets. </li></ul><ul><ul><li>TSMI (temporary mobile subscriber ID): a temporary number assigned to a terminal </li></ul></ul><ul><ul><ul><li>used in call management and mobility management </li></ul></ul></ul><ul><ul><ul><li>this adds privacy and security </li></ul></ul></ul><ul><ul><li>Ki: authentication key stored in both SIM and the subscriber’s Home system </li></ul></ul><ul><ul><li>Kc: cipher key computed from Ki by the terminal and the network. </li></ul></ul><ul><ul><li>Mobile Station Classmark: to state the property of the terminal </li></ul></ul><ul><ul><ul><li>GSM version </li></ul></ul></ul><ul><ul><ul><li>RF power capability </li></ul></ul></ul><ul><ul><ul><li>encryption algorithm, etc. </li></ul></ul></ul>
  7. 9. Radio Transmission <ul><li>GSM Spectrum </li></ul><ul><ul><li>There are two 25 MHz bands separated by 45 MHz </li></ul></ul><ul><ul><li>Initial GSM systems operate in the upper 10 MHz </li></ul></ul><ul><ul><ul><li>The lower part can be used by older systems. </li></ul></ul></ul><ul><ul><ul><li>This serves for the purpose of “graceful” transition. </li></ul></ul></ul>
  8. 10. Physical Channels <ul><li>GSM is a Hybrid FDMA/TDMA system </li></ul><ul><li>Each GSM band is partitioned into 125 carriers, each spaced at 200 kHz </li></ul><ul><ul><li>Only 124 carriers are used. </li></ul></ul><ul><ul><li>The remaining one serves as a guarded band between existing service and GSM (see Fig. 7.3). </li></ul></ul><ul><li>Each carrier is framed, and each frame contains 8 time slots. </li></ul><ul><ul><li>The frame duration is 4.62 ms (= 120/26) </li></ul></ul><ul><ul><ul><li>This equals 26 frames with a duration of 120 ms. </li></ul></ul></ul>
  9. 13. <ul><li>Thus, each physical channel is specified by a (carrier, time_slot). </li></ul><ul><li>In order to make it unnecessary for a terminal to transmit and receive simultaneous, time slot i at the downlink is coupled with time slot i+3 at the uplink. </li></ul>
  10. 14. Radio Transmission <ul><li>GSM time interval </li></ul><ul><ul><li>A hyperframe = 2048 superframe </li></ul></ul><ul><ul><li>A superframe = 51 traffic multiframes = 26 control multiframes = 6.12 s </li></ul></ul><ul><ul><li>A traffic multiframe = 26 frames = 120 ms </li></ul></ul><ul><ul><li>A control multiframe = 51 frames = 235.4 ms </li></ul></ul><ul><ul><li>A frame = 8 time slots = 4.615 ms </li></ul></ul><ul><ul><li>A slot = 156.25 bits = 577 µs </li></ul></ul><ul><ul><li>A bit = 3.69 µs </li></ul></ul>
  11. 16. Traffic Channels <ul><li>A traffic multiframe = 26 frames with duration 120ms </li></ul><ul><li>A phone speech is a full-rate traffic channel (TCH/F) occupying one time slot in 24 of the 26 frames. </li></ul><ul><ul><li>Traffic travels in frames 0-11 and 13-24. </li></ul></ul><ul><ul><li>Control information (called SACCH) may travel in frames 12 and 25. </li></ul></ul><ul><ul><ul><li>A SACCH associated with a full-rate traffic channel alternatively occupies one slot in frame 12 and one slot in frame 25 </li></ul></ul></ul><ul><ul><li>Each GSM carrier can convey 8 TCH/F’s. </li></ul></ul>
  12. 18. <ul><li>GSM also supports half-rate traffic channel (TCH/H) : </li></ul><ul><ul><li>It occupies a specific time slot in 12 of the 26 frames. </li></ul></ul><ul><ul><li>Each carrier can carry up to 16 TCH/H channels. </li></ul></ul><ul><ul><li>The SACCH control data is in frames 12 and 25. </li></ul></ul><ul><li>There is also a control multiframe of length 51 frames. </li></ul><ul><ul><li>So a complete coupling of traffic multiframe and control multiframe will form a cycle of 51x26 =1326 superframe, of length 6.12 sec. </li></ul></ul>
  13. 19. GSM Bit Stream
  14. 20. <ul><li>Training sequence: </li></ul><ul><ul><li>for synchronization, to estimate the characteristic of time-varying channel. </li></ul></ul><ul><ul><li>to train an adaptive equilizer </li></ul></ul><ul><li>2 data fields: </li></ul><ul><ul><li>to carry user information or network control information </li></ul></ul><ul><li>FLAG: indicate whether the DATA field contains user information or control one </li></ul><ul><li>The TAIL bits all set to 0 </li></ul><ul><li>There is also a guard time 0f 30.5 µs </li></ul><ul><li>The GSM transmission rate is 270.833 kb/s </li></ul>
  15. 21. Radio Carrier’s Frequency <ul><li>GSM supports two kinds of radio carrier: </li></ul><ul><ul><li>conventional sine wave at a single frequency </li></ul></ul><ul><ul><li>frequency hopping </li></ul></ul><ul><li>Slow Frequency Hopping </li></ul><ul><ul><li>The signal moves from one frequency to another in every frame. </li></ul></ul><ul><ul><li>The purpose of FH is to reduce the transmission impairments. </li></ul></ul><ul><ul><li>Without FH, the entire signal is subject to distortion whenever the assigned carrier is impaired. </li></ul></ul>
  16. 23. Radiated Power <ul><li>GSM specifies 5 classes of mobile stations transmitting power, ranging from 20 W (43 dBm) to 0.8 W (29 dBm) </li></ul><ul><li>Typically, vehicle-mounted terminal is 8 W and portable terminals is 2 W </li></ul>
  17. 24. Efficiency <ul><li>Spectrum Efficiency </li></ul><ul><ul><li>The reuse factor of N = 3 or 4 </li></ul></ul><ul><ul><li>The number of physical channel is 124 carriers x 8 channels/carriers = 992 physical channels </li></ul></ul><ul><ul><li>When N = 3: </li></ul></ul><ul><ul><ul><li>The efficiency of GSM is E = 992 channels/3 cells/cluster/50 MHz = 4.96 conversation/cell/MHz </li></ul></ul></ul><ul><ul><li>When N = 4: </li></ul></ul><ul><ul><ul><li>The efficiency of GSM is E = 992 channels/4 cells/cluster/50 MHz = 4.96 conversation/cell/MHz </li></ul></ul></ul>
  18. 25. Logical Channels <ul><li>Logical channels </li></ul><ul><ul><li>Traffic channels (two-way) </li></ul></ul><ul><ul><li>Signaling Channels: </li></ul></ul><ul><ul><ul><li>Broadcast channels (base-to-mobile) </li></ul></ul></ul><ul><ul><ul><li>Common control channels (base-to-mobile or mobile-to-base): available to ALL terminals </li></ul></ul></ul><ul><ul><ul><li>Dedicated control channels (two-way): available to specific terminals </li></ul></ul></ul>
  19. 27. Broadcast and Common Control Channels <ul><li>Purpose: </li></ul><ul><ul><li>mobile terminal to synchronize with base stations, even without a call in progress </li></ul></ul><ul><ul><li>to set up new calls </li></ul></ul><ul><li>Broadcast channels and Common control channels share the same carrier, in a multiplexing manner. </li></ul><ul><ul><li>The broadcast channels always occupy time slot 0. </li></ul></ul><ul><ul><li>The common control channels can occupy time slots 0, 2, 4, and 6. </li></ul></ul><ul><li>The frames of each channel is determined by their positions in the 51-frame control multiframe. </li></ul>
  20. 28. <ul><li>Time slot 0 in each of the 51 frames in a control multiframe: </li></ul><ul><ul><li>Fig. 7.11 </li></ul></ul><ul><ul><li>There are 5 groups of frames, each containing ten frames </li></ul></ul><ul><ul><ul><li>beginning with a frequency-correction frame (FCCH) </li></ul></ul></ul><ul><ul><ul><li>a synchronization frame (SCH) </li></ul></ul></ul><ul><ul><li>These 5 groups end with an Idle Frame (X) </li></ul></ul><ul><li>In the reverse direction (from Terminal to BS): </li></ul><ul><ul><li>The control multiframe share the similar structure. </li></ul></ul><ul><ul><li>Terminals without a call in progress contend on time slot 0 on a contention basis. </li></ul></ul><ul><ul><li>The rest 7 time slots are typically used by traffic channels. </li></ul></ul><ul><ul><li>The even-number slots can also be used for control. </li></ul></ul>
  21. 30. <ul><li>Frequency Correction Channel (FCCH) </li></ul><ul><ul><li>The FCCH simply transmits 148 0’s. </li></ul></ul><ul><ul><li>A terminal without a call in progress searches for a FCCH. </li></ul></ul>
  22. 31. <ul><li>Synchronization Channel (SCH) </li></ul><ul><ul><li>A BS transmits a SCH in time slot 0 of every frame that follows a frame containing an FCCH. </li></ul></ul><ul><ul><li>The SCH contains a TRAINING sequence. </li></ul></ul><ul><ul><li>The DATA fields contain BS identity code and the present frame number. </li></ul></ul>
  23. 32. <ul><li>Broadcast Control Channel (BCCH) </li></ul><ul><ul><li>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 . </li></ul></ul><ul><ul><li>The message length is 184 bits. </li></ul></ul><ul><ul><ul><li>which is encoded to 224 bits (error-checking) </li></ul></ul></ul><ul><ul><ul><li>and then to 456 bits (1/2 convolution code) occupying 4 time slots. </li></ul></ul></ul>
  24. 33. <ul><li>Paging Channel (PCH) and Access Grant Channel (AGCH) </li></ul><ul><ul><li>PCH: to notify terminals of arriving calls </li></ul></ul><ul><ul><li>AGCH: to direct a terminal to a stand-alone dedicated control channel (SDCCH) </li></ul></ul><ul><ul><li>A terminal is allowed to enter a sleep mode. </li></ul></ul><ul><ul><ul><li>Then it will only monitor the PCH and AGCH frames that are assigned to it for newly arrival calls. </li></ul></ul></ul><ul><ul><li>They together occupy 36 frames per multiframe. </li></ul></ul>
  25. 34. <ul><li>Random Access Channel (RACH) </li></ul><ul><ul><li>Terminals send messages on the RACH to originate phone calls, initiate transmissions of short messages, respond to paging messages, and register their locations. </li></ul></ul><ul><ul><li>Terminals with information to transmit use the slotted ALOHA protocol to gain access to the time slot. </li></ul></ul><ul><ul><li>The Ack directs the terminal to a stand-alone dedicated control channel (SDCCH) to be used for further communications. </li></ul></ul><ul><ul><li>RACH is located in 1 time slot in each frame of the 51-frame control multiframe (in the direction from terminals to base stations). </li></ul></ul>
  26. 36. <ul><ul><li>The 36-bit DATA field simply carries a 8-bit message. </li></ul></ul><ul><ul><ul><li>This message is protected by error-detecting code and error-correcting code. </li></ul></ul></ul><ul><ul><ul><li>3 of the 8 bits indicate the purpose of the access attempt. </li></ul></ul></ul><ul><ul><ul><li>5 of the 8 bits contains a random number. </li></ul></ul></ul>
  27. 37. <ul><li>When there is a collision, this 5-bit random code can serve as a purpose to distinguish the successful terminal from the unsuccessful one (with a probability of 31/32). </li></ul><ul><ul><li>This is based on the “capture capability”, that the base station may hear only part of the RACH. </li></ul></ul><ul><ul><li>Then the random code will increase the probability of success. </li></ul></ul>
  28. 39. <ul><li>Stand-Alone Dedicated Control Channel (SDCCH) </li></ul><ul><ul><li>SDCCH is a two-way channel assigned to a specific terminal. </li></ul></ul><ul><ul><li>The physical channel used by an SDCCH is a set of four time slots in each 51-frame control multiframe. </li></ul></ul><ul><ul><li>With 114 data bits per time slot, the data rate of the SDCCH is 1937.25 b/s </li></ul></ul><ul><ul><li>Each SDCCH has a slow associated control channel called SACCH. </li></ul></ul>
  29. 40. <ul><li>Traffic Channels (TCH) </li></ul><ul><ul><li>two kinds: </li></ul></ul><ul><ul><ul><li>a full-rate channel occupies 24 time slots </li></ul></ul></ul><ul><ul><ul><ul><li>The bit rate of a full-rate traffic channel is 22,800 b/s </li></ul></ul></ul></ul><ul><ul><ul><li>a half-rate channel occupies 12 time slots </li></ul></ul></ul><ul><ul><li>SACCH occupies time slots in frames 12 or 25 of each 26-frame traffic multiframe. </li></ul></ul><ul><ul><ul><li>S means “slow”. </li></ul></ul></ul><ul><ul><ul><li>The transmission rate of a traffic SACCH is 950 b/s </li></ul></ul></ul><ul><li>Fast Associated Control Channel (FACCH) </li></ul><ul><ul><li>If SACCH is too slow, we can use the traffic channel to transmit control information. </li></ul></ul><ul><ul><li>Each FACCH message is multiplexed with user information. </li></ul></ul>
  30. 41. Messages <ul><li>GSM Protocol Layers </li></ul><ul><ul><li>GSM provides a large number of open interfaces </li></ul></ul><ul><li>Message Structure </li></ul><ul><ul><li>All of the signaling message length is 184 bits with the exception of the FCCH, SCH, and RACH </li></ul></ul>
  31. 43. Network Operations (I) <ul><li>Call to a GSM Terminal </li></ul><ul><ul><li>Terminal uses the frequency correction channel (FCCH) to synchronize its local oscillator </li></ul></ul><ul><ul><li>It then gains timing information from the SCH </li></ul></ul><ul><ul><li>The terminal then obtains important information from broadcast control channel (BCCH) </li></ul></ul><ul><ul><li>After the initialization procedure, the terminal monitors a paging channel (PCH) </li></ul></ul><ul><ul><li>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) </li></ul></ul>
  32. 44. <ul><ul><li>The network responds this request by transmitting an immediate assignment message on an access grant channel (AGCH) </li></ul></ul><ul><ul><li>This message established a stand-alone dedicated control channel (SDCCH) to be used for exchange of mobility management messages and call management messages. </li></ul></ul><ul><ul><li>When terminal moves to SDCCH, it transmits a paging response message to BS </li></ul></ul><ul><ul><li>The BS then initiates the GSM authentication procedure </li></ul></ul>
  33. 45. Network Operations (II) <ul><li>Authentication and Encryption Procedure </li></ul><ul><ul><li>The terminal received a 128-bit random number from BS </li></ul></ul><ul><ul><li>Then it applies a GSM encryption algorithm A3 to compute a 32-bit signed response, SRES </li></ul></ul><ul><ul><ul><li>The secret key Ki is stored in the subscriber information module (SIM) </li></ul></ul></ul><ul><ul><li>From SRES and Ki, the terminal applies another encryption algorithm A8 to compute a 64-bit ciphering key Kc. </li></ul></ul><ul><ul><li>The base station also uses the same way to compute these numbers. </li></ul></ul>
  34. 46. <ul><ul><li>If the two values of SRES are identical, the network accept the the user as an authorized subscriber </li></ul></ul><ul><ul><li>To encrypt user information and network control information, the BS and network derive, through an algorithm A5, a 114-bit mask to be added to the two DATA fields. </li></ul></ul><ul><ul><ul><li>The inputs of A5 are the 64-bit ciphering key Kc and the current 22-bit frame number </li></ul></ul></ul><ul><ul><ul><li>Because A5 uses the frame number to compute the ciphering mask, the mask change from frame to frame. </li></ul></ul></ul>
  35. 48. Network Operations (III) <ul><li>To Setup a Call </li></ul><ul><ul><li>BS transmits a setup message to the terminal </li></ul></ul><ul><ul><li>The terminal Ack this message with a call confirmed </li></ul></ul><ul><ul><li>The terminal then send a connect message to the network </li></ul></ul><ul><ul><li>In response, the network moves the call to a traffic channel by means of an assignment command message </li></ul></ul><ul><ul><ul><li>Note that, GSM assigns a traffic channel after the mobile subscriber accepts the call </li></ul></ul></ul>
  36. 50. Network Operations (IV) <ul><li>Location-Based Registration </li></ul><ul><ul><li>Terminal registers its location when it moves to a new cell </li></ul></ul><ul><li>Mobile-Assisted Handover </li></ul><ul><ul><li>When mobile terminal finds a channel quality is better than present one the handover procedures will be executed </li></ul></ul><ul><li>Status of GSM </li></ul><ul><ul><li>GSM operates in 900 MHz, 1800 MHz, and 1900 MHz bands </li></ul></ul><ul><ul><li>New GSM services include a packet data transmission protocol referred to as GPRS (generalized packet radio service) and multiple-full-rate circuit switched services </li></ul></ul>