basic gsm principles

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BASIC GSM PRINCIPLES

COMPARISONTACS Uplink 890 - 905 MHz Downlink 935 - 950 MHz Duplex Distance 45 MHz Carrier Separation 25 kHz Number of Channels 15 Mhz / 25 kHz = 600 Channels defined in Switch 1, 2, .599, 600 Access Method FDMA GSM 900 890 - 915 MHz 935 - 960 MHz 45 MHz 200 kHz 25 MHz / 200 kHz = 124 1, 2, .123, 124 TDMA DCS 1800 1710 - 1785 MHz 1805 - 1880 MHz 95 MHz 200 kHz 75 MHz / 200 kHz = 374 512 , 513 .884, 885 TDMA

TACS

GSM 900 DCS 1800

1 Voice Channel1 Control Channel

1 Voice Channel10 Control Channels

Logical Channels:1 VOICE CHANNEL 10 CONTROL CHANNELS

TCH = Traffic Channel Full rate - Used for speech at 13 kbits/s orsending data at 9.6 kbits/s

Half rate - Used for speech at 6.5 kbits/s orsending data at 4.8 kbits/s

Enhanced Full rate - Used for speech at 13kbits/s or sending data at 9.6 kbits/s but with almost Land line qualityBACK

DCCHCCCH CBCH

NEXT

BCH

BCH = Broadcast Channels

FCCH = FREQUENCY CORRECTION CHANNEL- To tell the Mobile that this is the BCCH carrier - To able the Mobile to synchronize to the frequency (Downlink only)

SCH = SYNCHRONISATION CHANNEL- Used for sending BSIC (Base station Identity Code) - Give TDMA frame number to the Mobile. (Downlink only)

BCH = Broadcast Channels

BCCH = BROADCAST CONTROLCHANNEL - Used for sending information to the mobile like CGI (Cell Global identity), LAI (Location Area Identity), BCCH carriers of the neighboring cells, maximum output power allowed in the cell and other broadcast messages like barred cell. (Downlink only)BACK

CCCH = Common Control ChannelsPCH = PAGING CHANNEL - Used for paging the mobile; Reason could be an incoming call or an incoming short message (Downlink only) RACH = RANDOM ACCESS CHANNEL - Used for responding to the paging (terminating), location updating or to make call access (originating) by asking for a signaling channel. (Uplink only) AGCH = ACCESS GRANT CHANNEL - Used to allocate SDCCH to the mobile. (Downlink only) BACK

DCCH = Dedicated Control Channels

SDCCH = STAND ALONE DEDICATED CONTROLCHANNEL - Used for allocating voice channel (TCH) to the mobile (call setup) and Location updating. - Send Short Text message to Idle Mobile (Uplink & Downlink)

FACCH = FAST ASSOCIATED CONTROL CHANNEL- Used for handover (Uplink & Downlink)

DCCH = Dedicated Control ChannelsSACCH = SLOW ASSOCIATED CONTROL CHANNEL- Used for sending information to the mobile like CGI (Cell Global Identity), LAI (Location Area Identity), BCCH of all the neighbors and TA (Timing Advance) - Send Short Text message to Busy Mobile (Downlink) - Used for sending signal strength & bit error rate measurement of the serving cell and signal strength of the BCCHs of the neighboring cells. (Uplink)BACK

CBCH = Cell Broadcast Channel Used for sending short messages to all the mobiles within a geographic area. Typical example: traffic congestion in a major road or a major accident in an area. Up to 93 characters can be sent. If the mobile is in the Idle mode then the short message will be send through the CBCH. If the mobile is Busy, it will not be sent. NOT TO BE CONFUSED WITH SMS

What is an SMS? Short Message Service SMS messages are short TEXT messages up to 160characters in length that you can send or receive. The messages are not sent straight to the other mobile but is sent to message center operated by the Network Provider

What is an SMS? If the mobile was switched off or is at outside of thecoverage area, the message is stored in the Message Service Center. The message will be offered to the subscriber when the mobile is switched on again or has reentered the coverage area again.

If the mobile is in the idle mode the short messagewill be send through the SDCCH. If the mobile is busy the short message will be sent through the SACCH.BACK

1 TDMA FRAME 200 kHz 4.615 ms 0.577 msTS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7

FCCH, SCH, BCCH PCH, RACH, AGCH

SDCCH, SACCH, CBCH

TCH, SACCH, FACCH

TCH, TCH, SACCH, SACCH, FACCH FACCH

TCH, SACCH, FACCH

TCH, SACCH, FACCH

TCH, SACCH, FACCH

F S B B B B C C C C F S C C C C

D D D D D D D D D D D D D D D D

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T T T T T

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T T T T T

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T T T T T

R R R R R R R R R R R R R R R R

D D D D D D D D D D D D D D D D

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T I T T T

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T I T T T

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T I T T T

4.615 msTS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7

4.615 msTS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7

F = FCCH S = SCH B = BCCH C = PCH or AGCH D = SDCCH A = SACCH T = TCH

F S B B B B C C C C F S C C C C

D D D D D D D D D D D D D D D D

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T T T T T

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T T T T T

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T T T T T

R = RACH A = SACCH T = TCH

R R R R R R R R R R R R R R R R

D D D D D D D D D D D D D D D D

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T I T T T

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T I T T T

T T T T T T T T T T T T A T T T

T T T T T T T T T T T T I T T T

1 Frame (Downlink - BTS transmit) 1 carrier = 200 kHz

1 Frame (Uplink - Mobile transmit)

1 carrier = 200 kHz

Downlink . 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

.

Uplink

5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 73 TIMESLOTS 3 * 0.577ms = 1.73ms 3 TIMESLOTS 3 * 0.577ms = 1.73ms

.

3 TIMESLOTS 3 * 0.577ms = 1.73ms

TCH UP-DOWNLINK OFFSETThis means that the mobile does not transmit and receive at the same time. Also note that : in TS 0 : All the Logical Channels will repeat itself after 51 frames in TS 1 : All the Logical Channels will repeat itself after 102 frames in TS 2 to 7 : All the Logical Channels will repeat itself after 26 frames

MSISDNMOBILE STATIONS ISDN NUMBER Is the mobile number used in a GSM PLMN(Public Land Mobile Network)

MSISDN = Country Code + NationalCode + Subscriber number

Destination

Maximum length is 15 digits e.g.: 63 + 0918 + 8889999

IMSIINTERNATIONAL MOBILE SUBSCRIBER IDENTITY Is the subscriber number used over radio path for all signaling in theGSM PLMN.

This number is stored in SIM (Subscriber Identity Module), HLR (HomeLocation Register, and VLR (Visitor Location Register)

IMSI = MCC + MNC + MSIN= Mobile Country Code + Mobile Network Code + Mobile Identification Number [ 3 digits ] [ 2 digits ] [ 11 digits ]

e.g.:

502 + 19 + 2345451

TMSITEMPORARY MOBILE SUBSCRIBER IDENTITY Is used for the subscriber's confidentiality. Since the TMSI hasonly local significance (within MSC/VLR), the structure of the TMSI can be chosen by the Vendor. But the size must be 1/2 of the size of IMSI. Each time a mobile request for location updating or call setup, MSC/VLR allocates to the IMSI a new TMSI, so the TMSI is used on the signaling path, protecting the IMSI identity. Plus since the TMSI is half the size of IMSI, we can page twice the amount compared to IMSI.

LAILOCATION AREA IDENTITY Is used to uniquely identify each location area in the GSM PLMN.When the system receives an incoming call it knows in which location area it should page the mobile and does not page the entire network.

LAI = MCC + MNC + LAC= Mobile Country Code + Mobile Network Code + Location Area Code [ 3 digits ] [ 2 digits ] [ 1 to 65536 ]

e.x. =

502 + 20 + 60001

CGICELL GLOBAL IDENTITYIs used for cell identification within the GSM network CGI = MCC + MNC + LAC + CICountry Code [ 3 digits ]

= Mobile

+

Cell Mobile + Location + Identity Network Code Area Code [ 2 digits ] [ 1 to 65536 ] [ 1 to 65536 ]

e.x. = 502 + 20 + 60001 + 50001

BSICBASE STATION IDENTITY CODE Is used to distinguish co channel frequencyused in the neighboring cell

BSIC = NCC + BCC= Network Color Code + Base Station Color Code [ 0 to 7 ] [ 0 to 7 ]

SIMSUBSCRIBER IDENTITY MODULE Used to provide storage on subscriber related information as following :- IMSI (International Mobile Subscriber Identity) - Temporary network data like TMSI, LAI, Location update status - Subscriber Authentication Key (Ki) and Ciphering Key (Kc) which are used for security purposes - BCCH information : List of carrier frequencies to be used for cell selection - Forbidden PLMN - Language preference - PIN number (Personal Identification Number) and PIN error counter - PUK number (Personal Unlock Key) and PUK error counter

Pin Management The PIN number consist of 4 to 8 digits and it is loaded by the serviceactivator an subscription time. Afterwards the PIN number can be changed as many times an user wishes including the length of the PIN number. The user can disable the PIN function but again can be inhibited at subscription time by a authorized person. If an incorrect PIN is entered, an indication is given to the user. After 3 consecutive incorrect entries, the SIM is blocked, even if if the SIM is removed or the mobile is switch off and on. If the SIM card is blocked the user cannot access the network. The unblocking of the SIM card can only be done by keying in the PUK (Personal Unlock Key). PUK is 8 digit and is given to the user at subscription time.