Logical Channels:1 VOICE CHANNEL10 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 withalmost Land line qualityBACK
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 ChannelsBCCH = BROADCAST CONTROLCHANNEL- Used for sending information to the mobile likeCGI (Cell Global identity), LAI (Location AreaIdentity), BCCH carriers of the neighboring cells,maximum output power allowed in the cell andother broadcast messages like barred cell.(Downlink only)BACK
CCCH = Common Control ChannelsPCH = PAGING CHANNEL- Used for paging the mobile; Reason could be an incoming callor an incoming short message(Downlink only)RACH = RANDOM ACCESS CHANNEL- Used for responding to the paging (terminating), locationupdating or to make call access (originating) by asking for asignaling channel.(Uplink only)AGCH = ACCESS GRANT CHANNEL- Used to allocate SDCCH to the mobile.(Downlink only)BACK
DCCH = Dedicated Control ChannelsSDCCH = 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)
SACCH = SLOW ASSOCIATED CONTROL CHANNEL- Used for sending information to the mobile like CGI (CellGlobal Identity), LAI (Location Area Identity), BCCH of all theneighbors and TA (Timing Advance)- Send Short Text message to Busy Mobile(Downlink)- Used for sending signal strength & bit error ratemeasurement of the serving cell and signal strength of theBCCHs of the neighboring cells.(Uplink)BACKDCCH = Dedicated Control Channels
CBCH = Cell Broadcast Channel Used for sending short messages to all themobiles within a geographic area. Typical example: traffic congestion in a major road ora major accident in an area. Up to 93 characters canbe sent. If the mobile is in the Idle mode then the shortmessage will be send through the CBCH. If themobile 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. Themessages are not sent straight to the other mobile butis sent to message center operated by the NetworkProvider
If the mobile was switched off or is at outside of thecoverage area, the message is stored in the MessageService Center. The message will be offered to thesubscriber when the mobile is switched on again orhas 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 busythe short message will be sent through the SACCH.BACKWhat is an SMS?
C D T T T T T TC D T T T T T TC D T T T T T TC D A T A T A TS D T T T T T TF 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 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 TS D T T T T T TF 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 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 TTS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS74.615 ms 4.615 ms1 Frame(Downlink - BTS transmit)1 carrier = 200 kHz1 Frame(Uplink - Mobile transmit)1 carrier = 200 kHzR = RACHA = SACCHT = TCHF = FCCHS = SCHB = BCCHC = PCH orAGCHD = SDCCHA = SACCHT = TCH
Downlink….Uplink ……….0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 75432107654321076543210765 76….3 TIMESLOTS3 * 0.577ms = 1.73ms3 TIMESLOTS3 * 0.577ms = 1.73ms3 TIMESLOTS3 * 0.577ms = 1.73msTCH UP-DOWNLINK OFFSETThis means that the mobile does not transmit and receive at the sametime.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
MSISDNMOBILE STATIONS ISDN NUMBERIs the mobile number used in a GSM PLMN(Public Land Mobile Network)MSISDN = Country Code + National DestinationCode + Subscriber numberMaximum length is 15 digitse.g.: 63 + 0918 + 8889999
IMSIINTERNATIONAL MOBILE SUBSCRIBER IDENTITYIs 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 + MobileIdentification Number[ 3 digits ] [ 2 digits ] [ 11 digits ]e.g.: 502 + 19 + 2345451
TMSITEMPORARY MOBILE SUBSCRIBER IDENTITYIs used for the subscribers confidentiality. Since the TMSI hasonly local significance (within MSC/VLR), the structure of theTMSI can be chosen by the Vendor. But the size must be 1/2 ofthe size of IMSI. Each time a mobile request for locationupdating or call setup, MSC/VLR allocates to the IMSI a newTMSI, so the TMSI is used on the signaling path, protecting theIMSI identity. Plus since the TMSI is half the size of IMSI, wecan page twice the amount compared to IMSI.
LAILOCATION AREA IDENTITYIs used to uniquely identify each location area in the GSM PLMN.When the system receives an incoming call it knows in whichlocation area it should page the mobile and does not page theentire network.LAI = MCC + MNC + LAC= Mobile Country Code + Mobile Network Code + LocationArea Code[ 3 digits ] [ 2 digits ] [ 1 to 65536 ] e.x. = 502 + 20 + 60001
CGICELL GLOBAL IDENTITYIs used for cell identification within the GSM networkCGI = MCC + MNC + LAC + CI e.x. = 502 + 20 + 60001 + 50001MobileCountry Code[ 3 digits ]+ MobileNetwork Code[ 2 digits ]LocationArea Code[ 1 to 65536 ]CellIdentity[ 1 to 65536 ]+ +=
BSICBASE STATION IDENTITY CODEIs used to distinguish co channel frequencyused in the neighboring cellBSIC = NCC + BCC= Network Color Code + Base StationColorCode[ 0 to 7 ] [ 0 to 7 ]
SIMSUBSCRIBER IDENTITY MODULEUsed 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 areused for security purposes- BCCH information : List of carrier frequencies to be used for cellselection- 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 bechanged as many times an user wishes including the length of the PINnumber. The user can disable the PIN function but again can beinhibited at subscription time by a authorized person. If an incorrect PINis entered, an indication is given to the user. After 3 consecutiveincorrect entries, the SIM is blocked, even if if the SIM is removed or themobile is switch off and on. If the SIM card is blocked the user cannotaccess the network. The unblocking of the SIM card can only be doneby keying in the PUK (Personal Unlock Key). PUK is 8 digit and is givento the user at subscription time. If an incorrect PUK is entered more than10 times then the PUK will not work anymore and the SIM card willcontinue to be blocked until taken to the mobile vendor service center.
Two Physical Types of SIM:ID-1 SIM - looks like acredit cardPlug in SIM - looks likea small chip is installedsemi permanently inthe mobile equipment
When the mobile is in idle mode it must alwaysbe camped to a BCCH carrier! For the PLMN to know in which location area the mobile is so that itcan page the mobile when an incoming call or short text message isreceived. The mobile can initiate a call by accessing the network on RandomAccess Channel (RACH) of the cell which it camped on. To receive system information from the PLMN like traffic congestionand major accidents.WHY?!THERE ARE THREE REASONS:
An Idle Mobile Does Four Tasks:PLMN SELECTIONCELL SELECTIONCELL RESELECTIONLOCATION UPDATING
PLMN SELECTION If there is no registered PLMN or theregistered PLMN is not available (nocoverage) then the mobile will try to selectanother PLMN eitherBACKWhen the mobile is switched on it will selectthe registered PLMN in the mobile if thereexists one.manuallyautomatically ordepending on its mode.
Automatic ModeThe automatic mode uses a list of PLMNS in an orderof priority. The priority will be : The last network on which you were registered Home PLMN Each PLMN stored in the SIM card in priority order Other PLMN with signal level above -85 dBm inrandom order All other PLMN in decreasing signal strengthBACK
Manual Mode In the manual mode the mobile will try to connect tothe Home PLMN first. If it is unsuccessful then it willprovide a list of available PLMN and ask the user tochoose one. If the second chosen PLMN is notsuccessful then the mobile will make an indication tothe user to select another PLMN. Until the usersselects another network a message “No Access” willbe displayed. If there is no GSM or DCS coverage atall then a message “No Network” will be displayed.BACK
CELL SELECTIONOnce the mobile is switched on and the registeredhome PLMN was selected (e.x. SMART), it will nextsearch for a BCCH frequency list, stored in its memoryor in its SIM card. The list can have up to 32 BCCHfrequencies for the mobile to scan. This reduces the timeof cell selection, compared to scanning the wholefrequency band. If this feature is turned off at the switchthen the mobile has to scan the entire frequency bandfor the strongest BCCH carrier.
CELL SELECTIONThe BCCH frequency list is called BA (BCCH Allocation) listand there are 2 types, ACTIVE AND IDLE. Idle is a list of BCCHused for scanning when the mobile is in an idle mode and Active isa list of BCCH used during mobile busy mode. Why 2 List ???When the mobile is in idle mode it may want to scan a longer list ofBCCH and tune to the strongest whereas when in Active mode thelist of BCCH should be shorter (correspond to defined neighbors) sothat the mobile will scan the short list and get a more accuratesignal strength measurements to achieve better handoverperformance. It is also to reduce the time spend by the mobile todecode the BSIC. Recommendation: ACTIVE MODE LIST SHOULD NOT BEMORE THAN 15 BCC FREQUENCIES.
CELL SELECTIONIf there is no BA list stored in the Mobile or SIM card then themobile will scan all the 124 GSM channels and 374 DCS channeland arrange the frequencies in a DESCENDING order of signalstrength. It will take the mobile 3 to 5 seconds to scan the wholeband. After which it will tune to the strongest frequency. The mobilewill check if this is a BCCH carrier by looking out for the frequencycorrection burst send by the FCCH (Frequency CorrectionChannel). If it is the BCCH carrier than mobile tunes to this carrierto read the SCH (Synchronization Channel) for the BSIC parameter.Next it will read the BCCH for system information like CGI (CellGlobal identity), LAI (Location Area Identity), BCCH carriers of theneighboring cells (BA List), maximum output power allowed in thecell and other broadcast messages like barred cell.
CELL SELECTIONNext the mobile will compare if the selected cell belongsto a forbidden PLMN stored in its SIM card. It will look at the2 digit Network Mobile Code transmitted by the BCCH on theLAI (Location Area Identity). If those 2 digits ware registeredas forbidden in the SIM card then the mobile will not selectthis cell. The mobile then will tune to the second strongestBCCH carrier and subsequently does the same process overand over again until it finds the right cell. Once it finds theright cell it will start using the BA (BCCH allocation) listtransmitted by the BCCH carrier for cell reselection, will bediscussed later.
CELL SELECTION Lets say the chosen PLMN is correct, able to read the FCH, SCHand BCCH and the chosen Cell is accessible (no cell barring),NO!! THERE IS ONE LAST CRITERIA CALLED C1 CELL SELECTIONCRITERION WHICH MUST BE CALCULATED BY THE MOBILE AND IFTHE C1 VALUE IS GREATER THAN 0 THEN THE MOBILE CAN CAMPON THIS CELL OR ELSE THE NEXT CELL WITH C1 > 0 WILL BESELECTED.DOES THIS MEAN THAT THE MOBILE NOW CAN CAMP ONTHIS SITE?
C1-CELL SELECTION CRITERIONC1 = A - Max (B,0)and C1 > 0 for the mobile to camp on this BTS RxLev = Signal strength received by the mobile RxLevAccMin = Minimum Signal level to be received by the mobile from BTS (BCCH)before it could access the BTS MsTxPwrMaxCCH = Maximum Transmit Power allowed to access the BTS (usingRACH) 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 camp on this sitewhere:A = RxLev - RxLevAccMinB = MsTxPwrMaxCCH - P
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 mobileshould be 2dB higher than the mobile sensitivityWHY?!
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 forGSM 900= 3 dB recommended by ETSI and 3 dB recommended byEricsson for DCS 1800Multipath loss = Signal loss from base station due to reflection by buildings, etcbefore reaching mobile. Normally the Multipath loss is around 3 dB but can beovercome by Antenna Diversity which has gain around 3 dB too. (Space diversity = 3dB, 90 degrees polarized diversity = 3 dB, 45 degrees slant polarized diversity = 4.5dB)Interference Margin = Margin allocated to overcome C/I and C/N, therecommended value is 2 dBWhat is the accurate way of setting theRxLevAccessMin parameter ?
RxLevAccessMin = Mobile Sensitivity + Body loss +Multipath loss + Interference MarginRxLevAccessMin = -104 + 3 + 0 + 2 (for GSM 900 withETSI standard)= - 99 dBmRxLevAccessMin = -104 + 5 + 0 + 2 (for GSM 900 withEricsson standard)= - 97 dBmRxLevAccessMin = -104 + 3 + 0 + 2 (for DCS 1800)= - 99 dBmWhat is the accurate way of setting theRxLevAccessMin parameter ?
MsTxPwrMaxCCH (Nokia)CCHPWR (Ericsson) PLC (TACS)= 33dBm(GSM900)= 30dBm(DCS1800)Maximum Transmit Power allowed to access theBTS (using RACH) - mobile is idle= 0 (28dBm)(DCS1800)
MsTxPwr (Nokia)MsTxPwr (Ericsson) PLVM (TACS)= 33dBm(GSM900)= 30dBm(DCS1800)Maximum Transmit Power allowed to use in a BTSduring busy status (using TCH) - mobile is busy= 0 (28dBm)(DCS1800)
Minimum Transmit Power allowed to use in a BTSduring busy status (using TCH) – mobile is busyMsTxPwrMin (Nokia)= 13dBm (GSM900) = 13dBm (DCS1800)
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
An Idle Mobile Does Four Tasks:PLMN SELECTIONCELL SELECTIONCELL RESELECTIONLOCATION UPDATING
After the cell has been successfully selected, the mobile nowwill start reselection tasks. It will continuously make measurementson its neighboring cells (as indicated by the BA list) to initiate cellreselection if necessary. At least 5 measurement samples perneighboring cell are needed. A running average of the receivedsignal level will be maintained for each carrier in the BA list.CELL RESELECTION1) Perform Cell Reselection Measurement First
All system information messages sent on the current BCCH on theserving cell must be read by the mobile every 30 seconds to monitorchanges in cell parameters (ex: MsTxPwrMax). The mobile also has to readthe 6 strongest BCCH every 5 minutes to receive its cell parameters (ex:MsTxPwrMax). The 6 strongest can be seen from the BA list which has theupdated measurement of the 32 BCCH carrier. The neighboring list for thebest 6 neighbors is updated every 60 seconds, which means the mobile hasto measure each neighbor by 10 seconds. The mobile also has to read theBSIC of the 6 strongest BCCH every 30 seconds to confirm that it is stillmonitoring the same cells. If a new BSIC is detected, then the BCCH of thisBSIC will be read to receive the cell parameters.CELL RESELECTION
CELL RESELECTION1) Cell Reselection CriteriaThe mobile will reselect and camp on another cell if any of the following criteria is satisfied: The serving cell is barred. C1 value in the current cell is below 0 for 5 seconds which indicates that the path loss ishigh and the mobile needs to change cell. The Mobile has unsuccessfully tried to access the network as defined by the MAXRET(Ericsson) parameter or MaxNumberRetransmissions (Nokia).MAXRET is the maximum number of retransmission a mobile can do when it is accessingthe system. It is defined per cell.Assuming that one of the criteria above was satisfied then the mobile will select a cell witha better C1.However if the cell belongs to a different location area then the C1 for that cell has toexceed a reselection hysterisis parameter called CRH (Ericsson) or CellReselectHyseteris(Nokia) for the reselection to happen!!
CELL RESELECTION If the mobile is moving in a border area between location areas, it might repeatedly changebetween location areas. Each change requires location updating and cause heavy signaling load andrisk paging 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 theone broadcast 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 2
BASICCOMPARISONMOBILE IDLEWhen the mobile is idle, it listens to the best cell to camp. Mobile decides to choose the cell byitself without the help of BSC. This is done by comparing signal strength of each BCCHfrequency and if found the strongest then it will camp once the C1 > 0 (Cell SelectionCriterion). If after camping to this cell, it finds that a neighboring cell is much better then it willchange to that neighboring cell. If the new cell is in same location area the mobile does nothave to inform the BSC about its new cell but if the new cell is from a different different locationarea then the mobile will perform a location updating to inform the BSC.MOBILE BUSYA mobile is considered busy when there is a call going on (speech, data or fax) or it is in themiddle of a call setup. At this stage the mobile cannot decide by itself whether it is necessaryfor the mobile to handover to a better cell. Only BSC can determine if a mobile has to changeto another cell other than the serving cell. BSC makes the decision based on measurementreports 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.