GSM Call Flows

Intro: GSM Call Flows

Interfaces

• The A-interface lies between the MSC and the BSS (Base Station Subsystem). The A-interface consists of both user plane (PCM links) and control plane (signaling).

• The C/D-interface between the MSC and the HLR is used by the GMSC (Gateway MSC) to interrogate the HLR in order to obtain routing information for a call to a subscriber. Signaling on this interface uses the MAP over TCAP protocols (MAP/C).

• The D-interface between the VLR and HLR is used to exchange data related to the location of an MS and the management of the subscriber. Signalling on this interface uses the MAP over TCAP protocols (MAP/D).

• The E-interface between two MSCs is used during inter-MSC handover. Signalling on this interface uses the MAP over TCAP protocols (MAP/E).

• When the BSS consists of a BSC (Base Station Controller) and one or more BTSs (Base Transceiver Stations), the Abis-interface between the BSC and BTS is used to support the services offered to the GSM users and subscribers.

• When the BSS consists of a BSC (Base Station Controller) and one or more BTSs (Base Transceiver Stations), the Abis-interface between the BSC and BTS is used to support the services offered to the GSM users and subscribers.

• The Um-interface is the air interface that is used for exchanges between an MS and a BSS.

Um – DTAP (Direct Transfer Application Part )Abis – DTAPA -- BSSAP (Base Station System Application Part)

CL3I -- Complete Layer 3 Information

OSI layer Model vs SS7 protocol

Types of Registration• Normal location updating: used to update the LA (Location Area) of an

MS when the MS changes LA.

• Periodic Updating: used to periodically inform the network that the MS is still available and thus prevents the MS from accidentally becoming detached from the system.

• IMSI Attach: used to indicate the IMSI as active in the network; it is the counterpart of the IMSI detach procedure.

• IMSI Detach: An MS may invoke the IMSI detach procedure if the MS is deactivated (e.g. powered down) or if the SIM (Subscriber Identity Module) is detached from the MS.

MT Exchange Property – PURGECONTROL – MGADP;

IMSI -- International Mobile Subscriber IdentityTMSI --Temporary Mobile Subscriber Identity

Location Updating

Location Updating

GSM Authentication & Ciphering• The purpose of authentication is to protect the network against

unauthorized use.

GSM Authentication Parameters• Some of the Authentication parameters that are used in GSM network are as

follows :-• AXE parameter:• 1) AUTHENTICATE :Determines whether authentication is performed in the

MSC/VLR Server. • 2) SELAUTH in GSMMMSC parameter set determines whether selective

authentication is used in GSM MSC/VLR Server.

• Exchange Settings:• 1) IMEIFETCHCIPH : Ciphering used to fetch IMEI • 2) CIPHER : if ciphering is performed in the MSC/VLR Server • 3) AUTHENTICLAI : Authentication is not performed in LU with change of LAI

IMSI Attach

IMSI Detach

Call to MS

Pre Analysis B no.

B-no. Pre analysis & Analysis• <pnbsp:bo=all;• PRE-ANALYSIS OF B-NUMBER INFORMATION

• OPERATING AREA• BO NAPI BNT RESULT• 0 0 OBA=1• 0 1 OBA=0• 4 1 OBA=4• 4 2 OBA=4• ……………………………...• 31 1 OBA=31• 31 2 OBA=30• 31 4 OBA=31• ………………………….. • 401 4 OBA=401• 500 15 OBA=31

• END• <anbsp:b=32-09820; (OBA from Pre analysis + B no.)• B-NUMBER ANALYSIS DATA• OPERATING AREA• B-NUMBER MISCELL F/N ROUTE CHARGE L A• 32-0 CC=2• 32-09 RC=906 CC=2 L=4-8• M=0-00• D=4-2• TRD=100• END

SCCP Analysis • <anrsp:rc=906;• ROUTING CASE DATA• OPERATING AREA• RC CCH BR ROUTING SP DATA• 906 NO TMR-0• &-3• &-4 P01=1 R=MSC96SO MM1 COT EST SI ESS ESR• 0 0 1 1• BNT RN SPR OLI OLI2• 4 00 0• ISC FCP D• 0

• <exrop:r=MSC96SO;• ROUTE DATA• R ROUTE PARAMETERS• MSC96SO DETY=UPD1 TTRANS=1 FNC=3 • R=MSC96SI• ST=6 SI=ISUP4 SP=3-600

• <c7gsp:TT=0,NP=1,NA=4,NS=9198203;• CCITT7 GLOBAL TITLE SERIES DATA

• OPERATING

• TT NP NA NS GTRC • 0 1 4 9198203 2• END ……contd

SCCP Analysis Contd…• <c7gcp:gtrc=2;• CCITT7 GLOBAL TITLE ROUTING CASE DATA

• OPERATING

• GTRC PSP PTERM PINTER PSSN SSP STERM SINTER SSSN LSH • 2 3-320 YES NO 3-1000 NO YES NO

• <c7rsp:dest=3-320;• CCITT7 MTP ROUTING DATA

• DEST SPID DST PRIO LSHB LS SPID RST• 3-320 HLR2 ACC 1 3-320 HLR2 WO• 2 3-600 GMSC6 SB• <c7ltp:ls=3-320;• CCITT7 LINK SET STATE

• LS SPID SLC STATE INHIBST FCODE INFO• 3-320 HLR2 00 ACTIVE• 01 ACTIVE• 02 ACTIVE• 03 ACTIVE• 04 ACTIVE• 05 ACTIVE• 06 ACTIVE• 07 ACTIVE• 08 ACTIVE• 09 ACTIVE• 10 ACTIVE• 11 ACTIVE• 12 ACTIVE• 13 ACTIVE• 14 ACTIVE• 15 ACTIVE• END

Routing Analysis

SCCP Analysis

SCCP Routing in HLR

HLR data • <hgsdp:msisdn=919820315708,all;• HLR SUBSCRIBER DATA

• SUBSCRIBER IDENTITY• MSISDN IMSI STATE AUTHD• 919820315708 404201145038948 CONNECTED AVAILABLE

• NAM• 1

• PERMANENT SUBSCRIBER DATA• SUD• CAT-10 DBSG-1 PRBT-1 BS26-1• TS11-1 TS21-1 TS22-1 OFA-1• …………………………………………….• AMSISDN BS BC• NONE

• SUPPLEMENTARY SERVICE DATA• BSG• TS10• SS STATUS FNUM TIME• CAW ACTIVE-OP• CFU NOT ACTIVE• CFNRY NOT ACTIVE• ………………………………………………..• LOCATION DATA• VLR ADDRESS MSRN MSC NUMBER LMSID• 4-919820906441 919820906441

• PACKET DATA PROTOCOL CONTEXT DATA• NONE

• END• <

HLR SCCP routing • <c7gsp:TT=0,NP=1,NA=4,NS=919820906441;

• CCITT7 GLOBAL TITLE SERIES DATA

• OPERATING

• TT NP NA NS GTRC • 0 1 4 919820906441 39• END

• <c7gcp:gtrc=39;

• CCITT7 GLOBAL TITLE ROUTING CASE DATA

• OPERATING

• GTRC PSP PTERM PINTER PSSN SSP STERM SINTER SSSN LSH • 39 3-900 YES NO NO• END

• <

MSRN (mobile station roaming number)

Call from MS

B no. & RC analysis

B-no. Analysis & VLR data• <mgslp:imsi=404201145038948,all;• MT MOBILE SUBSCRIBER LIST• SUBSCRIBER IDENTITY• IMSI MSTAT MSISDN CAT• 404201145038948 REG 919820315708 10

• SUBSCRIBER NON SUPPLEMENTARY SERVICE DATA• BASIC SERVICE DATA• DCDA9600 SPCH SMS

• SUBSCRIBER SUPPLEMENTARY SERVICE DATA• ………………………………………………..

• <mgisp:imsis=404201145038948;• MT IMSI NUMBER SERIES ANALYSIS DATA• OPERATING TABLE• IMSIS M NA ANRES

• 40420114 5-919820 4 OBA-30• BO-33• OWNMS• NATMS• NRRG-0• PLMN-0• ERIS-0• MAPVER-2• INOPER-1• CBA-19• CBAZ-146• • END

Call Delivery in GMSC

Call Delivery in HLR

MT - SMS

Unsuccessful MT SMS

MO - SMS

MO SMS

• SC – Service Center Add –> Global Title –> SCCP translation.

• Almost 8 SC can be stored in MS

• The short message is transferred to the MS on a Standalone Dedicated Control CHannel (SDCCH) if the MS is idle, and on the Slow Associated Control CHannel SACCH if the mobile is active during a call.