BASIC GSM

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Full rate => Used for speech at 13 Kbits/s
or sending data at 9.6 Kbits/s

Half rate => Used for speech at 6.5 Kbits/s
or sending data at 4.8 Kbits/s

Enhanced Full rate => Used for speech at 13 Kbits/s
or sending data at 9.6 Kbits/s but
with almost Land line quality
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)

BCCH = BROADCAST CONTROL CHANNEL
=> 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)
PCH = PAGING CHANNEL
=> Used for paging the Mobile. (Downlink only)
Reason could be an incoming call or an incoming Short Message.

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)

Published in: Education, Technology, Business

BASIC GSM

  1. 1. TACS GSM 900 DCS 1800Uplink 890 - 905 MHz 890 - 915 MHz 1710 - 1785 MHzDownlink 935 - 950 MHz 935 - 960 MHz 1805 - 1880 MHzDuplex Distance 45 MHz 45 MHz 95 MHzCarrier Separation 25 kHz 200 kHz 200 kHzNumber of Channels 15 Mhz / 25 kHz = 600 25 MHz / 200 kHz = 124 75 MHz / 200 kHz = 374Channels defined in Switch 1, 2, ….599, 600 1, 2, ….123, 124 512 , 513 ….884, 885Access Method FDMA TDMA TDMACOMPARISON
  2. 2. TACS GSM 900/DCS 1800
  3. 3. 3 BroadcastChannels1) FCCH2) SCH3) BCCH3 Common ControlChannels1) PCH2) RACH3) AGCH3 Dedicated ControlChannels1) SDCCH2) SACCH3) FACCHTCHCBCHLOGICAL CHANNELS
  4. 4. Full rate => Used for speech at 13 Kbits/sor sending data at 9.6 Kbits/sHalf rate => Used for speech at 6.5 Kbits/sor sending data at 4.8 Kbits/sEnhanced Full rate => Used for speech at 13 Kbits/sor sending data at 9.6 Kbits/s butwith almost Land line qualityTCH = TRAFFIC CHANNEL
  5. 5. 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)BCCH = BROADCAST CONTROL CHANNEL=> Used for sending information to the mobile likeCGI (Cell Global identity), LAI (Location Area Identity),BCCH carriers of the neighboring cells,maximum output power allowed in the cell and otherbroadcast messages like barred cell. (Downlink only)BROADCAST CHANNELS
  6. 6. PCH = PAGING CHANNEL=> Used for paging the Mobile. (Downlink only)Reason could be an incoming call or an incoming Short Message.RACH = RANDOM ACCESS CHANNEL=> Used for responding to the paging (terminating), Location updatingor 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)COMMON CONTROL CHANNELS
  7. 7. SDCCH = STAND ALONE DEDICATED CONTROL CHANNEL=> Used for allocating voice channel (TCH) to the mobile (call setup) andLocation updating.=> Send Short Text message to Idle Mobile(Uplink & Downlink)SACCH = SLOW ASSOCIATED CONTROL CHANNEL=> Used for sending information to the mobile likeCGI (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 theserving cell and signal strength of the BCCHs of the neighboring cells.(Uplink)FACCH = FAST ASSOCIATED CONTROL CHANNEL=> Used for handover. (Uplink & Downlink)DEDICATED CONTROL CHANNELS
  8. 8. => SMS messages are short TEXT messages up to 160 characters in length that youcan send or receive. The messages are not sent straight to the other mobile but issent to message centre operated by the Network provider.=> If the mobile was switched off or is at outside of the coverage area,the message is stored in the Message Service Center. The messagewill be offered to the subscriber when the mobile is switched on againor has reentered the coverage area again.=> If the mobile is in the Idle mode the short message will be send throughthe SDCCH. If the mobile is Busy the short message will send throughthe SACCH.CBCH = CELL BROADCAST CHANNEL=> Used for sending short messages to all the mobiles within a geographic area.Typical example is Traffic congestion in a major road or a major accidentin an area. Up to 93 characters can be sent.=> If the mobile is in the Idle mode then the short message will be send throughthe CBCH. If the mobile is Busy, it will not be sent.NOT TO BE CONFUSED WITH SMS !!!!!!!!(SHORT MESSAGE SERVICE)
  9. 9. TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7200 kHz4.615 ms0.577 msFCCH,SCH,BCCHPCH,RACH,AGCHSDCCH,SACCHCBCHTCH,SACCHFACCHTCH,SACCHFACCHTCH,SACCHFACCHTCH,SACCHFACCHTCH,SACCHFACCHTCH,SACCHFACCH1 TDMA FRAME
  10. 10. F D T T T T T TS D T T T T T TB D T T T T T TB D T T T T T TB D T T T T T TB D T T T T T TC D T T T T T TC D T T T T T TC D T T T T T TC D T T T T T TF D T T T T T TS D T T T T T TC D A T A T A TC D T T T T T TC D T T T T T TC D T T T T T TTS0 TS1 TS2 TS3 TS4 TS5 TS6 TS74.615 ms........................1 Frame(Downlink - BTS transmit)1 carrier = 200 kHzF = FCCHS = SCHB = BCCHC = PCH orAGCHD = SDCCHA = SACCHT = TCHR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D T T T T T TR D A I A I A IR D T T T T T TR D T T T T T TR D T T T T T TTS0 TS1 TS2 TS3 TS4 TS5 TS6 TS74.615 ms........................1 Frame(Uplink - Mobile transmit)1 carrier = 200 kHzR = RACHA = SACCHT = TCH
  11. 11. 3 TIMESLOT3 * 0.577ms = 1.73msDownlinkUplinkTCH UP-DOWNLINK OFFSET3 TIMESLOT3 * 0.577ms = 1.73ms3 TIMESLOT3 * 0.577ms = 1.73ms0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 70 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 75 6 7….…..…...…..This 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 framesin TS 1 : All the Logical Channels will repeat itself after 102 framesin TS 2 to 7 : All the Logical Channels will repeat itself after 26 frames
  12. 12. MOBILE STATIONS ISDN NUMBER (MSISDN)=> Is the mobile number used in a GSM PLMN (Public Land Mobile Network)MSISDN = Country Code + National Destination Code + Subscriber numbere.x. 63 + 0918 + 8889999Maximum length is 15 digits.INTERNATIONAL MOBILE SUBSCRIBER IDENTITY (IMSI)=> Is the subscriber number used over radio path for all signaling in the GSM PLMN.This number is stored in SIM (Subscriber Identity Module), HLR (Home Location Register,and VLR (Visitor Location Register).IMSI = MCC + MNC + MSIN= Mobile Country Code + Mobile Network Code + Mobile Identification Number[ 3 digit ] [ 2 digit ] [ 11 digit ]e.x. 502 + 19 + 2345451TEMPORARY MOBILE SUBSCRIBER IDENTITY (TMSI)=> Is used for the subscribers confidentiality. Since the TMSI has only 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 locationupdating or call setup, MSC/VLR allocates to the IMSI a new TMSI, so the TMSIis used on the signaling path, protecting the IMSI identity. Plus since the TMSI is halfthe size of IMSI, we can page twice the amount compared to IMSI.
  13. 13. LOCATION AREA IDENTITY (LAI)=> Is used to uniquely identify each location area in the GSM PLMN. When the systemreceives an incoming call it knows in which location area it should page the mobileand does not page the entire network.LAI = MCC + MNC + LACMobile Country Code + Mobile Network Code + Location Area Code[ 3 digit ] [ 2 digit ] [ 1 to 65 536 ]e.x. = 502 + 20 + 60001CELL GLOBAL IDENTITY (CGI)=> Is used for cell identification within the GSM network.LAI = MCC + MNC + LAC + CIMobile Country Code + Mobile Network Code + Location Area Code + Cell Identity[ 3 digit ] [ 2 digit ] [ 1 to 65 536 ] [ 1 to 65 536 ]e.x. = 502 + 20 + 60001 + 50001BASE STATION IDENTITY CODE (BSIC)=> Is used to distinguish co channel Frequency used in the neighboring cell.BSIC = NCC + BCCNetwork Color Code + Base Station Color Code[ 1 to 7 ] [ 1 to 7 ]
  14. 14. SUBSCRIBER IDENTITY MODULE (SIM)SIM is 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 managementThe PIN number consist of 4 to 8 digit and it is loaded by the service activator an subscription time. Afterwardsthe 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 authorizedperson. If an incorrect PIN is entered, an indication is given to the user. After 3 consecutive entriesthe 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 bedone by keying in the PUK (Personal Unlock Key). PUK is 8 digit and is given to the user at subscriptiontime. If an incorrect PUK is entered more than 10 times then the PUK will not work anymore and theSIM card will continue to be blocked until taken to the mobile vendor service center.Two physical types of SIM are specified :ID - 1 SIM - Looks like a Credit cardPlug in SIM - Look like a small chip is installed semi permanent in the mobile equipment.
  15. 15. • PLMN SELECTION• CELL SELECTION• CELL RESELECTION• LOCATION UPDATINGAN IDLE MOBILE DOES 4 TASKS:When the mobile is in idle mode it must alwaysbe camped to a BCCH carrier. Why ???There are 3 reasons:1) For the PLMN to know in which location area the mobile is so that it canpage the mobile when an incoming call or Short Text Message is received.2) The Mobile can initiate a call by accessing the network on Random AccessChannel (RACH) of the cell which it camped on.3) To receive system information from the PLMN like Traffic congestion andmajor Accidents.
  16. 16. When the mobile is switched on it will select the registered PLMN in the mobile if there exist one. Ifthere is no registered PLMN or the registered PLMN is not available (no coverage) then the mobilewill try to select another PLMN either automatically or manually depending on it mode.AUTOMATIC MODEThe automatic mode uses a list of PLMNS in an order of priority. The priority will be :1)The last network on which you were registered.2) Home PLMN.3) Each PLMN stored in the SIM card in priority order.4) Other PLMN with signal level above -85 dBm in random order.5) All other PLMN in decreasing signal strength.MANUAL MODEIn the manual mode the mobile will try to connect to the Home PLMN first. If it is unsuccessful then itwill provide a list of available PLMN and ask the user to choose one. If the second chosen PLMN is notsuccessful then the mobile will make an indication to the user to select another PLMN. Until the usersselects another network a message “No access” will be displayed. If there is no GSM or DCS coverage atall then a message “No Network” will be displayed.PLMN SELECTION
  17. 17. Once the mobile is switched on and the registered home PLMN was selected (e.x. SMART), it will nextsearch for a BCCH frequency list, stored in its memory or in its SIM card. The list can have up to32 BCCH frequencies for the mobile to scan. This reduces the time of cell selection, compared toscanning the whole frequency band. If this feature is turned off at the switch then the mobile has to scanthe entire frequency band for the strongest BCCH carrier.The BCCH frequency list is called BA (BCCH Allocation) list and there are 2 types, Active and Idle.Idle is a list of BCCH used for scanning when the mobile is in an idle mode and Active is a list of BCCHused during mobile busy mode. Why 2 List ???When the mobile is in idle mode it may want to scan a longer list of BCCH and tune to the strongestwhereas when in Active mode the list of BCCH should be shorter (correspond to defined neighbors) sothat the mobile will scan the short list and get a more accurate signal strength measurements to achievebetter handover performance. It is also to reduce the time spend by the mobile to decode the BSIC.Recommendation : ACTIVE MODE LIST SHOULD NOT BE MORE THAN 15 BCCHFREQUENCIES.If there is no BA list stored in the Mobile or SIM card then the mobile will scan all the 124 GSMchannels and 374 DCS channel and arrange the frequencies in a DESCENDING order of signalstrength. It will take the mobile 3 to 5 seconds to scan the whole band. After which it will tune to thestrongest frequency. The mobile will check if this is a BCCH carrier by looking out for the frequencycorrection burst send by the FCCH (Frequency Correction Channel). If it is the BCCH carrier thanmobile tunes to this carrier to read the SCH (Synchronization Channel) for the BSIC parameter.Next it will read the BCCH for system information like CGI (Cell Global identity), LAI (LocationArea Identity), BCCH carriers of the neighboring cells (BA List), maximum output powerallowed in the cell and other broadcast messages like barred cell.[Continues …]CELL SELECTION
  18. 18. Next the mobile will compare if the selected cell belongs to a forbidden PLMN stored in its SIM card. Itwill look at the 2 digit Network Mobile Code transmitted by the BCCH on the LAI (Location AreaIdentity). If those 2 digits ware registered as forbidden in the SIM card then the mobile will not selectthis cell. The mobile then will tune to the second strongest BCCH carrier and subsequently does thesame process over and over again until it finds the right cell. Once it finds the right cell it will start usingthe BA (BCCH allocation) list transmitted by the BCCH carrier for cell reselection, will be discussedlater.Lets say the chosen PLMN is correct, able to read the FCH, SCH and BCCH and the chosen Cell isaccessible (no cell barring), DOES THIS MEAN THAT THE MOBILE NOW CAN CAMP ONTHIS SITE ??? NO !!!!!!!!!!!THERE IS ONE LAST CRITERIA CALLED C1 CELL SELECTIONCRITERION WHICH MUST BE CALCULATED BY THE MOBILE ANDIF THE C1 VALUE IS GREATER THAN 0 THEN THE MOBILECAN CAMP ON THIS CELL OR ELSE THE NEXT CELL WITH C1 > 0WILL BE SELECTEDCELL SELECTION
  19. 19. C1 = A - Max (B,0)and C1 > 0 for the mobile to camp on this BTSwhere:A = RxLev - RxLevAccMinB = MsTxPwrMaxCCH - PC1-CELL SELECTION CRITERIONRxLev = Signal strength received by the mobileRxLevAccMin = Minimum Signal level to be received by the mobilefrom BTS (BCCH) before it could access the BTSMsTxPwrMaxCCH = Maximum Transmit Power allowed to access theBTS (using RACH)P = Mobile Class powerC1 = (RxLev - RxLevAccMin) - Max(MsTxPwrMaxCCH - P, 0)Ex : C1 = ( -80 - (-100) ) - Max (33 - 33, 0)= -80 + 100 - Max (0, 0)= 20=> C1 > 0 so mobile will camp on this siteShortCut : If RxLevel > RxLevAccMin then Mobile can campon this site
  20. 20. Minimum Signal level that must be received by the mobilefrom BTS (BCCH) before it could access the BTSRxLevAccessMin (Nokia)ACCMIN (Ericsson)SSACC (TACS) (Uplink)= -102 (GSM900) = -100 (DCS1800)General rule : The signal received by the mobile should be2 dB higher than the mobile sensitivity
  21. 21. What is the accurate way of setting the RxLevAccessMin parameter ?RxLevAccessMin = Mobile Sensitivity + Body loss + Multipath loss + Interference MarginMobile Sensitivity = -104 for GSM900 and -102 for DCS1800Body loss = 3 dB recommended by ETSI and 5 dB recommended by Ericsson for GSM 900= 3 dB recommended by ETSI and 3 dB recommended by Ericsson for DCS 1800Multipath loss = Signal loss from base station due to reflection by buildings, etc before reaching mobile.Normally the Multipath loss is around 3 dB but can be overcome by Antenna Diversity which has gainaround 3 dB too. (Space diversity = 3 dB, 90 degrees polarized diversity = 3 dB,45 degrees slant polarized diversity = 4.5 dB)Interference Margin = Margin allocated to overcome C/I and C/N, the recommended value is 2 dBRxLevAccessMin = Mobile Sensitivity + Body loss + Multipath loss + Interference MarginRxLevAccessMin = -104 + 3 + 0 + 2 (for GSM 900 with ETSI standard)= - 99 dBmRxLevAccessMin = -104 + 5 + 0 + 2 (for GSM 900 with Ericsson standard)= - 97 dBmRxLevAccessMin = -104 + 3 + 0 + 2 (for DCS 1800)= - 99 dBm
  22. 22. TACS GSM 900 DCS 1800Class 1 10 Watt (40dBm) 20 Watt (43dBm) 1 Watt (30dBm)Class 2 4 Watt (36dBm) 8 Watt (39dBm) 0.25 Watt (24dBm)Class 3 1 Watt (30dBm) 5 Watt (37dBm) 4 Watt (36dBm)Class 4 0.6 Watt (28dBm) 2 Watt (33dBm) *Class 5 * 0.8 Watt (29dBm) *TACS GSM 900 DCS 1800Mobile Sensitivity -113 dBm -104 dBm -102 dBmBTS Sensitivity -116 dBm -107 dBm -106 dBm
  23. 23. Maximum Transmit Power allowed to access theBTS (using RACH) - Mobile is IdleMsTxPwrMaxCCH(Nokia)CCHPWR (Ericsson)PLC (TACS)= 30 dBm (DCS1800) = 0 (28 dBm) (TACS)= 33 dBm (GSM900)Maximum Transmit Power allowed to use in a BTSduring busy status (Using TCH) - Mobile is BusyMsTxPwrMax(Nokia)MsTxPwr (Ericsson)PLVM (TACS)= 30 dBm (DCS1800) = 0 (28 dBm) (TACS)= 33 dBm (GSM900)Minimum Transmit Power allowed to use in a BTSduring busy status (Using TCH) - Mobile is BusyMsTxPwrMin(Nokia)= 13 dBm (DCS1800)= 13 dBm (GSM900)
  24. 24. Switch on the MobileFor 2 to 3 seconds the Mobile will scan all the 124channels in GSM900 and 374 channels in DCS1800The mobile will synchronize to this carrierand read the BCCH info like LAI, CGITune to the secondstrongest channelMobile will compare the signal strengthof the 124 channels and tune to the strongestMobile will check if it is a BCCH carrier ?Does the BCCH belong to the wanted PLMN,E.x: Smart, Globe, Islacom ?Is the Cell Barred from accessing ?Is C1 >0 ?Camp on this site !!!YesYesYesNoYesNoNoNo
  25. 25. CELL RESELECTIONAfter the cell has been successfully selected, the mobile now will start reselection tasks. It will continuouslymake measurements on its neighboring cells (as indicated by the BA list) to initiate cell reselection ifnecessary. At least 5 measurement sample per neighboring cell is needed. A running average of thereceived signal level will be maintained for each carrier in the BA list.All system information messages sent on the current BCCH on the serving cell must be read by the mobileevery 30 seconds to monitor changes in cell parameters (ex: MsTxPwrMax). The mobile also has to read the6 strongest BCCH every 5 minutes to receive its cell parameters (ex: MsTxPwrMax). The 6 strongest can beseen from the BA list which has the updated measurement of the 32 BCCH carrier. The neighboring list forthe best 6 neighbors is updated every 60 seconds, which means the mobile has to measure each neighbor by10 seconds. The mobile also has to read the BSIC of the 6 strongest BCCH every 30 seconds to confirm thatit is still monitoring the same cells. If a new BSIC is detected, then the BCCH of this BSIC will be read toreceive the cell parameters.-Every 30 secs Every 5 minuteEvery 30 secsBSIC BCCHServing cellSix neighbors[Continues …]1) Perform Cell reselection measurement first
  26. 26. The mobile will reselect and camp on another cell if any of the following criteria is satisfied :a) The serving cell is barred.b) C1 value in the current cell is below 0 for 5 seconds which indicates that the path loss is high and themobile needs to change cell.c) The Mobile has unsuccessfully tried to access the network as defined by the MAXRET (Ericsson) parameteror MaxNumberRetransmissions (Nokia).MAXRET is the maximum number of retransmission a mobile can do when it is accessing the systemIt is defined per cell.CELL RESELECTION2) Cell reselection Criteria :Assuming that one of the criteria above was satisfied then the mobilewill select a cell with a better C1.However if the cell belongs to a different location area then the C1 forthat cell has to exceed a reselection hysterisis parameter called CRH(Ericsson) or CellReselectHyseteris (Nokia) for the reselection to happen !![Continues …]
  27. 27. If the mobile is moving in a border area between location areas, it might repeatedly change betweenlocation areas. Each change requires location updating and cause heavy signaling load and riskpaging message being lost. To prevent this, a cell reselect hysteresis parameter CRH is used.The cell in a different location area will only be selected if the C1 of that cell is higher than the C1 ofthe current serving cell by the value of the Reselect Hysteresis.Since the Value of CRH maybe different for each cell, the CRH used for comparison will be the onebroadcast by the serving cell. If the value is set very low then the mobile will Ping-Pong betweenlocation areas which will increase signaling load. If the value is set very high the mobile may camp inthe wrong cell too long.Recommendations : Set the value to 6C1 = 1CRH = 4C1 = 6Serving cell Target cellLA 1 LA 2CELL RESELECTION
  28. 28. MOBILE IDLEWhen the mobile is idle, it listens to the best cell to camp. Mobile decides to choose the cell by itselfwithout the help of BSC. This is done by comparing signal strength of each BCCH frequency and iffound the strongest then it will camp once the C1 > 0 (Cell Selection Criterion). If after camping to thiscell, it finds that a neighboring cell is much better then it will change to that neighboring cell. If the newcell is in same location area the mobile does not have to inform the BSC about its new cell but if the newcell is from a different different location area then the mobile will perform a location updating to informthe BSC.BASIC COMPARISONMOBILE BUSYA mobile is considered busy when there is a call going on (speech, data or fax) or it is in the middle ofa call setup. At this stage the mobile cannot decide by itself whether it is necessary for the mobile tohandover to a better cell. Only BSC can determine if a mobile has to change to another cell other than theserving cell. BSC makes the decision based on measurement reports sent by both Mobile and BTS.This decision making is called locating. In a busy state, mobiles can receive Short Text Message (SMS)but cannot receive Cell Broadcast Messages.

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