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Chapter 7 GSM: Pan-European Digital Cellular System

Chapter 7 GSM: Pan-European Digital Cellular System. 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

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Chapter 7 GSM: Pan-European Digital Cellular System

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  1. Chapter 7 GSM: Pan-European Digital Cellular System

  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 • provision for national variations in charging and rates • efficient interoperation with ISDN systems

  3. Background and Goals • 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 • accommodation of portable terminals

  4. Architecture • Network elements • Mobile stations, base stations, and mobile switching center • Three databases • Home location registers (HLR): for full roaming • Visitor location registers (VLR): for full roaming • Equipment identity registers (EIR)

  5. SIM of GSM • Subscriber identity module (SIM) • A removable card that stores subscriber information: • ID number • abbreviated dialing code • subscriber’s service plan • The SIM is the subscriber’s link to the cellular system. • By removing the SIM, the phone is disabled (except the emergency call). • Easy to change to other telephones • In earlier systems, the subscriber’s information is in a FIXED hardware within a terminal. • Thus, when changing phones, the service provider gets involved, which is inconvenient.

  6. GSM uses a variety of ID codes, which are exchanged between base stations and handsets. • TSMI (temporary mobile subscriber ID): a temporary number assigned to a terminal • used in call management and mobility management • this adds privacy and security • Ki: authentication key stored in both SIM and the subscriber’s Home system • Kc: cipher key computed from Ki by the terminal and the network. • Mobile Station Classmark: to state the property of the terminal • GSM version • RF power capability • encryption algorithm, etc.

  7. Radio Transmission • GSM Spectrum • There are two 25 MHz bands separated by 45 MHz • Initial GSM systems operate in the upper 10 MHz • The lower part can be used by older systems. • This serves for the purpose of “graceful” transition.

  8. Physical Channels • GSM is a Hybrid FDMA/TDMA system • Each GSM band is partitioned into 125 carriers, each spaced at 200 kHz • Only 124 carriers are used. • The remaining one serves as a guarded band between existing service and GSM (see Fig. 7.3). • Each carrier is framed, and each frame contains 8 time slots. • The frame duration is 4.62 ms (= 120/26) • This equals 26 frames with a duration of 120 ms.

  9. Thus, each physical channel is specified by a (carrier, time_slot). • 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.

  10. Radio Transmission • GSM time interval • A hyperframe = 2048 superframe • A superframe = 51 traffic multiframes = 26 control multiframes = 6.12 s • A traffic multiframe = 26 frames = 120 ms • A control multiframe = 51 frames = 235.4 ms • A frame = 8 time slots = 4.615 ms • A slot = 156.25 bits = 577 µs • A bit = 3.69 µs

  11. Traffic Channels • A traffic multiframe = 26 frames with duration 120ms • A phone speech is a full-rate traffic channel (TCH/F) occupying one time slot in 24 of the 26 frames. • Traffic travels in frames 0-11 and 13-24. • Control information (called SACCH) may travel in frames 12 and 25. • A SACCH associated with a full-rate traffic channel alternatively occupies one slot in frame 12 and one slot in frame 25 • Each GSM carrier can convey 8 TCH/F’s.

  12. GSM also supports half-rate traffic channel (TCH/H): • It occupies a specific time slot in 12 of the 26 frames. • Each carrier can carry up to 16 TCH/H channels. • The SACCH control data is in frames 12 and 25. • There is also a control multiframe of length 51 frames. • So a complete coupling of traffic multiframe and control multiframe will form a cycle of 51x26 =1326 superframe, of length 6.12 sec.

  13. GSM Bit Stream

  14. Training sequence: • for synchronization, to estimate the characteristic of time-varying channel. • to train an adaptive equilizer • 2 data fields: • to carry user information or network control information • FLAG: indicate 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 270.833 kb/s

  15. Radio Carrier’s Frequency • GSM supports two kinds of radio carrier: • conventional sine wave at a single frequency • frequency hopping • 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.

  16. 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

  17. Efficiency • 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 • When N = 3: • The efficiency of GSM is E = 992 channels/3 cells/cluster/50 MHz = 4.96 conversation/cell/MHz • When N = 4: • The efficiency of GSM is E = 992 channels/4 cells/cluster/50 MHz = 4.96 conversation/cell/MHz

  18. Logical Channels • Logical channels • Traffic channels (two-way) • Signaling Channels: • Broadcast channels (base-to-mobile) • Common control channels (base-to-mobile or mobile-to-base): available to ALL terminals • Dedicated control channels (two-way): available to specific terminals

  19. Broadcast and Common Control Channels • Purpose: • mobile terminal to synchronize with base stations, even without a call in progress • to set up new calls • Broadcast channels and Common control channels share the same carrier, in a multiplexing manner. • The broadcast channels always occupy time slot 0. • The common control channels can occupy time slots 0, 2, 4, and 6. • The frames of each channel is determined by their positions in the 51-frame control multiframe.

  20. Time slot 0 in each of the 51 frames in a control multiframe: • Fig. 7.11 • There are 5 groups of frames, each containing ten frames • beginning with a frequency-correction frame (FCCH) • a synchronization frame (SCH) • These 5 groups end with an Idle Frame (X) • In the reverse direction (from Terminal to BS): • The control multiframe share the similar structure. • Terminals without a call in progress contend on time slot 0 on a contention basis. • The rest 7 time slots are typically used by traffic channels. • The even-number slots can also be used for control.

  21. Frequency Correction Channel (FCCH) • The FCCH simply transmits 148 0’s. • A terminal without a call in progress searches for a FCCH.

  22. 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 and the present frame number.

  23. 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. • which is encoded to 224 bits (error-checking) • and then to 456 bits (1/2 convolution code) occupying 4 time slots.

  24. Paging Channel (PCH) and Access Grant Channel (AGCH) • PCH: to notify terminals of arriving calls • AGCH: to direct a terminal to a stand-alone dedicated control channel (SDCCH) • A terminal is allowed to enter a sleep mode. • Then it will only monitor the PCH and AGCH frames that are assigned to it for newly arrival calls. • They together occupy 36 frames per multiframe.

  25. Random Access Channel (RACH) • 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. • RACH is located in 1 time slot in each frame of the 51-frame control multiframe (in the direction from terminals to base stations).

  26. The 36-bit DATA field simply carries a 8-bit message. • This message is protected by error-detecting code and error-correcting code. • 3 of the 8 bits indicate the purpose of the access attempt. • 5 of the 8 bits contains a random number.

  27. 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). • This is based on the “capture capability”, that the base station may hear only part of the RACH. • Then the random code will increase the probability of success.

  28. 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 1937.25 b/s • Each SDCCH has a slow associated control channel called SACCH.

  29. Traffic Channels (TCH) • two kinds: • a full-rate channel occupies 24 time slots • The bit rate of a full-rate traffic channel is 22,800 b/s • a half-rate channel occupies 12 time slots • SACCH occupies time slots in frames 12 or 25 of each 26-frame traffic multiframe. • S means “slow”. • The transmission rate of a traffic SACCH is 950 b/s • Fast Associated Control Channel (FACCH) • If SACCH is too slow, we can use the traffic channel to transmit control information. • Each FACCH message is multiplexed with user information.

  30. 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

  31. Network Operations (I) • 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)

  32. The network responds 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

  33. Network Operations (II) • Authentication and Encryption Procedure • The terminal received a 128-bit random number from BS • Then it applies a GSM encryption algorithm A3 to compute a 32-bit signed response, SRES • The secret key Ki is stored in the subscriber information module (SIM) • From SRES and Ki, the terminal applies another encryption algorithm A8 to compute a 64-bit ciphering key Kc. • The base station also uses the same way to compute these numbers.

  34. 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, through an algorithm A5, a 114-bit mask to be added 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.

  35. Network Operations (III) • 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

  36. Network Operations (IV) • 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 • Status of GSM • GSM operates in 900 MHz, 1800 MHz, and 1900 MHz bands • New GSM services include a packet data transmission protocol referred to as GPRS (generalized packet radio service) and multiple-full-rate circuit switched services

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