GSM Call Setup (MOC&MTC)Signaling and KPIsPresented by: Nauman AnwarDate 31-12-2009

Contents of PresentationMobile Originated Call Flow SignalingMobile Terminated Call Flow SignalingKPIs related to MOC & MTCConclusions

Mobile Originated Call Flow Diagram

Explanation of MOC SignalingStep 1 MS sends a CHANNEL REQUEST message to BTS on the access channel (RACH) of Um interface. The message contains the cause value "MOC", but that value is not completely accurate, because it is indicated both in the mobile originating call establishment procedureand IMSI detach procedure.Step 2 BTS sends a CHANNEL REQUIRED message to BSC.Step 3 Upon receipt of the CHANNEL REQUIRED message, BSC allocates a signaling channel and sends a CHANNEL ACTIVATION message to BTS.Step 4 If the channel type is correct, upon receipt of the CHANNEL ACTIVATION message, BTSopens the power amplifier on the specified channel, and sends a CHANNEL ACTIVATIONACKNOWLEDGE message to BSC.Step 5 BSC sends an IMMEDIATE ASSIGNMENT COMMAND message to MS via BTS. Themessage is sent on AGCH on Um interface.Step 6 MS sends an SABM frame on SDCCH to BTS to access the network.Step 7 BTS returns a UA frame on SDCCH for acknowledgement.Step 8 BTS sends an ESTABLISHMENT INDICATION message to BSC (This message containsthe accurate causes for MS's access. For example, different cause values are indicated in themobile originating call establishment procedure and IMSI detach procedure). This messagecontains the contents of the CM SERVICE REQUEST message.

Step 9 BSC establishes the SCCP link connection on A interface, and sends a CM SERVICEREQUEST message to MSC.Step 10 MSC returns a message to BSC to acknowledge the link connection.Step 11 MSC sends a CM SERVICE ACCEPTED message to MS. The message is sent on SDCCHon Um interface.Step 12 The calling MS sends a SETUP message on SDCCH.Step 13 MSC sends a CALL PROCEEDING message to the calling MS. The message is sent onSDCCH on Um interface.Step 14 MSC sends an ASSIGNMENT REQUEST message to BSC, which contains the CICallocated to A interface.Step 15 BSC allocates a TCH, and sends a CHANNEL ACTIVATION message to BTS.Step 16 If the channel type is correct, upon receipt of the CHANNEL ACTIVATION message, BTSopens the power amplifier on the specified channel, starts to receive the uplink informationand sends a CHANNEL ACTIVATION ACKNOWLEDGE message to BSC.Step 17 BSC sends an ASSIGNMENT COMMAND message to MS via BTS on SDCCH.Step 18 MS sends an SABM frame to BTS, to access the network on FACCH indicated in theASSIGNMENT COMMAND message.Step 19 BTS sends a UA frame for acknowledgement on FACCH.

Step 20 BTS sends an ESTABLISHMENT INDICATION message to BSC.Step 21 After accessing the TCH, MS sends an ASSIGNMENT COMPLETE message to BSC onFACCH.Step 22 After the radio traffic channel and terrestrial circuit are both successfully connected, BSCsends an ASSIGNMENT COMPLETE message to MSC, and regards this call in session state.Step 23 MSC sends an ALTERING message to the calling MS. The calling MS will hear the ring back.The message is sent on FACCH on Um interface.Step 24 MSC sends a CONNECT message to MS. The message is sent on FACCH on Um interface.Step 25 The calling MS returns a CONNECT ACKNOWLEDGE message on FACCH to MSC.Step 26 The calling MS and called MS enters the session state.Step 27 After the conversation is over, the calling MS hangs up and sends a DISCONNECT messageon FACCH.Step 28 MSC sends a RELEASE message to MS. The message is sent on FACCH on Um interface.Step 29 MS returns a RELEASE COMPLETE message. The message is sent on FACCH on Uminterface.Step 30 MSC sends a CLEAR COMMAND message to BSC. Upon receipt of the message, BSCinitiates the release procedure.

Step 31 BSC sends a CHANNEL RELEASE message to MS through BTS, The message is sent onFACCH on Um interface.Step 32 MS sends DISC frame on FACCH.Step 33 BTS returns UA frame on FACCH.

Mobile Terminated Call Flow

Explanation of MTC SignalingWhen the paged MS is located in the service area of the MSC, the MSC sends a Pagingmessage to the BSC. The MSC can directly send a Paging message to the BSC on the Ainterface. It can also be transmitted by the SGSN, then forwarded to the PCU through theGb interface, and then sent to the BSC through the Pb interface. This message contains alist of paging cells (optional) and the information about TMSI and IMSI.The BSC sends a Paging Command message to the BTS. The message contains thenumber of the occupied timeslots and the number of the paging sub-channel where thepaging group belongs.Upon receipt of the Paging Command message from the BSC, the BTS sends a PagingRequest message to the paged MS on the paging sub-channel where the paging groupbelongs. The message contains the IMSI or TMSI of the paged MS.After decoding the Paging message, if the MS finds itself the paged MS, it sends a ChannelRequest message to the BTS so that the Initial Channel Assignment procedure can betriggered.For other messages, refer to "Mobile Originated Call EstablishmentProcedure."

Very Early and Late AssignmentVery Early AssignmentThe difference between early assignment and very early assignment is that the TCH allocated during the immediate assignment serves as a signaling channel. Therefore, no TCH needs to be reallocated during the assignment procedure. Instead, the TCH allocated in the immediate assignment procedure is adjusted to a speech channel through the Mode Modify message.Very early assignment generally takes place if there is no idle SDCCH for allocation during immediate assignment. However, there are idle TCHs and immediate assignment of TCHs is allowed in the BSC data configuration.Late AssignmentThe difference between early assignment and late assignment is that late assignment takes place after the Alerting indication is sent.The advantage of late assignment is that it can save the seizure time of speech channels.The disadvantage of late assignment is that if the subsequent assignment is unsuccessful, the called MS can only hear the ring but cannot establish a connection. Then, the MS complains about it. Therefore, in actual application, late assignment is generally not used. Instead, early assignment is used.

KPIsThe Main KPI related to Call setup is CSSR.The Formula being used in PTML for CSSR is given as follows:(Immediate Assignment Success Rate)*(1-SDCCH Call Drop rate)*(Assignment Success Rate)*100In recent Past Location update procedure has been excluded from immediate assignments contributed in CSSR formula. Immediate Assignment Success Rate = (Call setup indications (Circuit Service)-Call setup indications(SDCCH)(Location Update))/(Channel Requests (Circuit Service)-Channel Request(Location Updating)). This formula is for BSC6000.Immediate Assignment Success Rate= (Successful Immediate Assignments Successful immediate assignments for Location Update)/(Immediate Assignment Requests- Attempted Channel Seizures for Location update). This formula is for BSC32.Immediate assignment request is pegged when there is channel required from MS both in MOC and MTC.Immediate assignment success is pegged when there is Establish Indication from BTS to BSC in immediate Assignment Procedure.SDCCH Drop rate= SDCCH Call Drops/Successful SDCCH seizuresAssignment Success Rate = Successful TCH seizures for Call/Attempted TCH Seizures for Call.Assignment Request is pegged when there is Assignment request from MSC to BSC.Assignment Success is pegged when there is Assignment Complete MSG from MS to BSC.

Detail of Counters (CSSR)Detail and description of Counters related to CSSR is given in BSS help file.For BSC 6000 ,path for counters is :BSC Counter reference/Call Measurement/Immediate Assignment Measurement Per CellBSC Counter reference/Call Measurement/Assignment Measurement Per CellBSC Counter reference/Call Measurement/KPI Measurement Per CellBSC Counter reference/Call Measurement/SDCCH Call Drop Measurement Per CellBSC Counter reference/Call Measurement/Channel Activation Measurement Per CellBSC Counter reference/Call Measurement/Channel Seizures Measurement Per TRX

For BSC 32, path for counters is:BSC Traffic Statistics System help/Traffic Statistics Items/Cell Measurement FunctionBSC Traffic Statistics System help/Traffic Statistics Items/Channel Measurement FunctionBSC Traffic Statistics System help/Traffic Statistics Items/Cell Measurement Function (3)/Other Handover Measurement Function

ConclusionsThe Complete understanding of Call Setup Flow Signaling is necessary for proper RF Optimization.Thorough knowledge of Counters is very helpful in diagnosing the major causes degrading CSSR.In CSSR still IMSI attach/detach , LCS and SMS are included because in Huawei we can not exclude them from Immediate assignment attempts for MOC.

Thank You !

*SDCCH holding rate has been increased after increase of Random Access Error Threshold.*