Chapter 7 GSM: Pan-European Digital Cellular System

Prof. Jang-Ping Sheu

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Background and Goals

n GSM (Global System for Mobile Communications)n Beginning from 1982n European standardn Full roaming in Europen A purely digital system

n Goals:n full international roaming

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Background and Goals

n provision for national variations in charging and rates

n efficient interoperation with ISDN systemsn signal quality better than or equal to that of

existing mobile systemsn traffic capacity higher than or equal to that of

present systemsn lower cost than existing systemsn accommodation of non-voice services and portable

terminals

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Architecture

n Network elementsn Mobile stations, base stations, and mobile

switching center

n Three databasesn Home location registers (HLR)n Visitor location registers (VLR)n Equipment identity registers (EIR)

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Architecture

n In contrast to the original cellular, micro cells are used in GSM

n A BS separates into two parts: BTS (base transceiver station) and BSC (base station controller)

n Typically, a BSC controls several BTSn To reduce the cost with the greatest possible

service extent

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Architecture

n Subscriber identity module (SIM)n An important GSM innovationn A removable card that stores information,

including ID number, abbreviated dialing code, and subscriber’s service plan

n Easy to change telephones

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Architecture

n As in the other systems, GSM uses a variety of ID codes

n GSM Identifiersn International Mobile Subscriber Identity (15 digits)n Temporary Mobile Subscriber Identity (32 bits)

n Advantages: Privacy and save BWn International Mobile Equipment Identifier (15 digits)

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Architecture

n Authentication Key (max = 128 bits)n Cipher key (64 bits):

n Terminal and network use authentication key to compute the cipher key

n Mobile station classmark including:n Version of the GSM standardn RF power capability (power levels available)n Encryption methodn Other properties of terminal

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Architecture

n Training Sequence (26 bits)n help a terminal verify that it receives

information from the correct BS rather than another BS using the same physical channel

n BS Identity Code (6 bits)n Location Area Identity (40 bits) including:

n A mobile country code, network code, and area code

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Radio Transmission

n GSM Spectrumn There are two 25 MHz bands separated by 45 MHzn Initial GSM systems operate in the upper 10 MHz

n Physical Channelsn GSM is a Hybrid FDMA/TDMA systemn Each GSM band has carriers spaced at 200 kHzn The frame duration is 120/26 = 4.62 msn Each frame contains 8 time slotsn There are 25 MHz/200 k Hz = 125 carriers in per

directionn GSM specifies only 124 carriers (one is used as guard

band)

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Radio Transmission

n GSM time intervaln A hyperframe = 2048 superframe = 3 h 28 m

53.76 sn A superframe = 51 traffic multiframes = 26

control multiframes = 6.12 sn A traffic multiframe = 26 frames = 120 msn A control multiframe = 51 frames = 235.4 msn A frame = 8 time slots = 4.615 msn A slot = 156.25 bits = 577 µsn A bit = 3.69 µs

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Physical Channels

n Traffic Channelsn A full-rate traffic channel (TCH/F) occupies one

time slot in 24 of 26 frames in every multiframen Traffic channel information travels in frames 0-11

and 13-24 n Control information travels in frames 12 and 25n The SACCH occupies one frame in every traffic

multiframe n A SACCH associated with a full-rate traffic channel

alternatively occupies one slot in frame 12 and one slot in frame 25

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Physical Channels

n A half-rate traffic channel (TCH/H) occupies a specific time slot in 12 of 26 frames in every multiframe

n Each carrier can carry up to 16 half-rate traffic channels

n Eight of these traffic channels have a SACCH in frame 12 and the other eight half-rate channel have a SACCH in frame 25

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GSM Bit Stream

n The contents of a GSM time slot is shown in Fig. 7.8n 26 bits of training sequence serves as a purpose

similar to that of the SYNC field in NA-TDMAn GSM specifies 8 different training sequences with

low mutual cross-correlationn Network operators assign different training

sequences to nearby cells that use the same carrier

n The two DATA fields carry either user information or network control information

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Radio Transmission

n The FLAG indicates whether the DATA field contains user information or control one

n The TAIL bits all set to 0n There is also a guard time 0f 30.5 µsn The GSM transmission rate is 270.833 kb/sn The modulation scheme in GSM is GMSK a form of

frequency shift keyingn The modulation efficiency of GSM is 1.35 b/s/Hzn 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

n The BS turn off its transmitter in unassigned time slots

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Slow Frequency Hopping

n The signal moves from one frequency to another in every frame

n The purpose of FH is to reduce the transmission impairments

n Without FH, the entire signal is subject to distortion whenever the assigned carrier is impaired

n Network operator assigns different hopping patterns to different cells

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Radiated Power

n GSM specifies 5 classes of mobile stations transmitting power, ranging from 20 W (43 dBm) to 0.8 W (29 dBm)

n Typically, vehicle-mounted terminal is 8 W and portable terminals is 2 W

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Spectrum Efficiency

n The reuse factor of N = 3 or 4n The number of physical channel is 124 carriers x 8

channels/carriers = 992 physical channelsn The efficiency of GSM is E = 992 channels/4

cells/cluster/50 MHz = 4.96 conversation/cell/MHz (N= 4) or

n The efficiency of GSM is E = 992 channels/3 cells/cluster/50 MHz = 6.61 conversation/cell/MHz (N= 3)

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Logical Channels

n Traffic channels (two-way)n Broadcast channels (base-to-mobile)n Common control channels (base-to-mobile or

mobile-to-base)n Dedicated control channels (two-way)

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Broadcast channels and Common control channels

n The broadcast channels always occupy time slot 0

n The common control channels can also occupy time slots 0

n Control Multiframen There are 5 groups of frames, each containing ten

frames beginning with a frequency-correction frame and a synchronization frame

n In the reverse direction, time slot 0 is assigned to random access channels in all 51 frames

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Figure 7.11 shows the contents of time slot 0 in each of the 51 frames

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Logical Channels

n Frequency Correction Channel (FCCH)n The FCCH simply transmits 148 0sn The FCCH always occupies time slot 0 in a frame

of 8 time slotsn A terminal without a call in progress searches for a

FCCH

n Synchronization Channel (SCH)n A BS transmits a SCH in time slot 0 of every frame

that follows a frame containing an FCCHn The SCH contains a TRAINING sequencen The DATA fields contain BS identity code (6 bits)

and the present frame number

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Logical Channels

n Broadcast Control Channel (BCCH)n 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

n The message length is 184 bits and the encoded message is 456 bits occupying 4 time slots

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Logical Channels

n Paging Channel (PCH) and Access Grant Channel (AGCH)n The purpose of the AGCH is to direct a terminal to

a stand-alone dedicated control channel (SDCCH)n Both channels use the same coding scheme as the

BCCHn They occupy 36 frames of time slot 0 per

multiframe

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Logical Channels

n Random Access Channel (RACH)n GSM terminals send messages on the RACH to

originate phone calls, initiate transmissions of short messages, respond to paging messages, and register their locations

n Terminals with information to transmit use the slotted ALOHA protocol to gain access to the time slot

n The Ack directs the terminal to a stand-alone dedicated control channel (SDCCH) to be used for further communications

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Logical Channels

n The RACH slot includes a 41-bit TRAIN and 36-bit DATA

n The 36-bit DATA field carries a simple 8-bit message

n Three of the 8 bits indicate the purpose of the access attempt and the other 5 bits are produced by a random number generator

n The 5-bit random code is likely (with probability 31/32) to distinguish the successful terminal from the other

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Logical Channels

n Stand-Alone Dedicated Control Channel (SDCCH)n SDCCH is a two-way channel assigned to a specific

terminaln The physical channel used by an SDCCH is a set of

four time slots in each 51-frame control multiframen With 114 data bits per time slot, the data rate of the

SDCCH is 1937.25 b/s (see eq. 7.7)n Each SDCCH has a slow associated control channeln The SACCH occupies an average of two time slots

per control multiframe (969 b/s)

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Logical Channels

n Traffic Channels (TCH)n GSM defines two traffic channels, a full-rate

channel occupies 24 time slots in every 26-frame and a half-rate channel

n The bit rate of a full-rate traffic channel is 22,800 b/s

n SACCH occupies time slots in frames 12 or 25 of each 26-frame traffic multiframe

n The transmission rate of a traffic SACCH is 950 b/sn With 456 bits transmitted per message, a message

spans four traffic multiframes, a time interval of 480 ms

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Logical Channels

n Fast Associated Control Channel (FACCH)n Use the traffic channel to transmit control

information, which is an in-band signaling channeln 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

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Messages

n GSM Protocol Layersn GSM provides a large number of open interfaces

n Message Structuren All of the signaling message length is 184 bits with

the exception of the FCCH, SCH, and RACH

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Network Operations

n Call to a GSM Terminaln Terminal uses the frequency correction channel

(FCCH) to synchronize its local oscillatorn It then gains timing information from the SCHn The terminal then obtains important information

from broadcast control channel (BCCH)n After the initialization procedure, the terminal

monitors a paging channel (PCH)n 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)

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Network Operations

n The network response this request by transmitting an immediate assignment message on an access grant channel (AGCH)

n This message established a stand-alone dedicated control channel (SDCCH) to be used for exchange of mobility management messages and call management messages

n When terminal moves to SDCCH, it transmits a paging response message to BS

n The BS then initiates the GSM authentication procedure

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Network Operations

n Authentication and Encryption Proceduren The terminal received a 128-bit random number

(RAND) from BSn Then it applies a GSM encryption algorithm A3

to compute a 32-bit signed response, SRES n The inputs of A3 are RAND and secret key Ki

n The secret key Ki is stored in the subscriber information module (SIM)

n The terminal applies another encryption algorithm A8 to compute a 64-bit ciphering key Kc from SRES and Ki

n The network also uses A3 to compute SRES from RAND and Ki

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Network Operations

n If the two values of SRES are identical, the network accept the the user as an authorized subscriber

n 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

n The inputs of A5 are the 64-bit ciphering key Kc and the current 22-bit frame number

n Because A5 uses the frame number to compute the ciphering mask, the mask change from frame to frame

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Network Operations

n To Setup a Calln BS transmits a setup message to the terminaln The terminal Ack this message with a call

confirmedn The terminal then send a connect message to the

networkn In response, the network moves the call to a

traffic channel by means of an assignment command message

n Note that, GSM assigns a traffic channel after the mobile subscriber accepts the call

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Network Operations

n Location-Based Registrationn Terminal registers its location when it moves to a

new cell

n Mobile-Assisted Handovern When mobile terminal finds a channel quality is

better than present one the handover procedures will be executed

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Status of GSM

n GSM operates in 900 MHz, 1800 MHz, and 1900 MHz bands

n GPRS (generalized packet radio service) with 100 kbits/s data rate

n Enhanced Data Rate for GSM Evolution (EDGE) with 300 kbits/s data rate

n Universal Mobile Telecommunication Services (UMTS): 3G telecommunication technology up to 2 Mbits/s data rate using WCDMA or TD/CDMA transceiver