Monday, 9 November 2015

E-UTRAN to UTRAN CSFB Based On Blind Redirection Signaling Analysis

1 Overview

CSFB function is only available in case E-UTRAN coverage is overlapped by either UTRAN coverage or GRAN coverage. CSFB is a procedure in essence that a CS service (including emergency calls is set up in CS domain of target network .When the CSFB capable UE serviced by E-UTRAN as a caller triggering the mobile originating (MO) CS service or as a called triggering the mobile terminating (MT) CS service, the CS service will be fallback to target UTRAN or GRAN CS domain to set up.
This document mainly introduces the signaling processes of CS fallback based on blind redirection from E-UTRAN to UTRAN network. The CSFB process based on blind redirection is below:
Receiving the CSFB indicator in UE context setup process, UE will choose the highest priority system of “Target System of Idle UE for CS Fallback” parameter as the target system. Receiving the CSFB indicator in UE context modification process, UE will choose the highest priority system of “Target System of Connected UE for CS Fallback” parameter as the target system. Then UE choose the highest priority frequency of target system and redirect to the cell of this frequency to setup CS service.
We need finish all the configuration of CSFB through configuration reference at first. Configure “Redirection Algorithm for Coverage” as “open” and “CSFB Method of UTRAN” as “Redirection” of path “Radio Parameter/LTE FDD/E-UTRAN FDD Cell”. Configure the “RAT priority”, “UTRAN-Frequency Number” and “UTRAN-Frequency Measurement Configuration” (“Band Indicator for FDD DL Frequency”, “UTRAN FDD Frequency”, “UTRAN Frequency Redirection Priority” and “UTRAN Frequency CSFB Priority”) of path “Radio Parameter/LTE FDD/E-UTRAN FDD Cell/Measurement Parameter”. Configure “Switch for CS Fallback Based Measurement” as “no”. Configure “Target System of Idle UE for CS Fallback” and “Target System of Connected UE for CS Fallback”.
 Networking Environment
 Problem Cause Analysis
2  CSFB Protocol Process
This process is mainly referred ts_123272v101100p of EPC protocol R10 version.

2.1 MO Call In Connected Mode

UE setups CS service in the RRC Connected mode. The signaling process is below:  

  1a.The UE sends an Extended Service Request for mobile originating CS fallback to the MME. Extended Service Request message is encapsulated in RRC and S1-AP messages. eNodeB transfer this message to MME through NAS message.
1b.The MME sends an S1-AP UE Context Modification Request (CS Fallback Indicator, LAI) message to eNodeB. This message indicates to the eNodeB that the UE should be moved to UTRAN/GRAN. The registered PLMN for CS domain is identified by the PLMN ID included in the LAI, which is allocated by the MME.
1c.The eNodeB shall reply with S1-AP UE Context Modification Response message.
3c.If the UE and network support "RRC connection release with redirection and Multi Cell System Information to GRAN/UTRAN". The eNodeB can trigger RRC connection release with redirection to GRAN or UTRAN and include one or more physical cell identities and their associated System Information.
4.The eNodeB sends an S1-AP UE Context Release Request message to the MME. If the target cell is GRAN and either the target cell or the UE does not support DTM the message includes an indication that the UE is not available for the PS service.
5. The MME releases the UE Context in the eNodeB as well as all eNodeB related information in the S-GW as specified in TS 23.401 [2].

2.2 MO Call In Idle Mode

Mobile Originating call in idle Mode procedure is specified by reusing the Mobile Originating Call in Active mode procedures as specified in clauses 6.2 and 6.3 with Extended Service Request for mobile originating CS fallback to the MME where the messages S1-AP UE Context Modification Request and Response are replaced by S1-AP Initial UE Context Request and Response. The LAI is included in the S1-AP Initial UE Context Request message and sent to the eNodeB. The UE is transited to ECM-CONNECTED mode by following the applicable procedures specified in TS 23.401 [2].

2.3 MT Call In Connected Mode

UE receives CS service in the RRC Connected mode. The signaling process is below:  
 1a.MME shall send the CS Service Notification to the UE. The eNodeB forwards the paging message to the UE.
1b.UE sends an Extended Service Request (Reject or Accept) message to the MME for mobile terminating CS fallback. The Extended Service Request message is encapsulated in RRC and S1-AP messages. The UE may decide to reject CSFB based on Caller Line Identification.
1d.MME sends an S1-AP UE Context Modification Request (CS Fallback Indicator, LAI) message to eNodeB. This message indicates to the eNodeB that the UE should be moved to UTRAN/GRAN. The registered PLMN for CS domain is identified by the PLMN ID included in the LAI, which is allocated by the MME.
1e.The eNodeB shall reply with S1-AP UE Context Modification Response message.
3c.If the UE and network support "RRC connection release with redirection and Multi Cell System Information to GRAN/UTRAN". The eNodeB can trigger RRC connection release with redirection to GRAN or UTRAN and include one or more physical cell identities and their associated System Information.
4.The eNodeB sends an S1-AP UE Context Release Request message to the MME. If the target cell is GRAN and either the target cell or the UE does not support DTM the message includes an indication that the UE is not available for the PS service.
5. The MME releases the UE Context in the eNodeB as well as all eNodeB related information in the S-GW as specified in TS 23.401 [2].
 
 2.4 MT Call In Idle Mode
UE receives CS service in the RRC Idle mode. The signaling process is below:


 
 

7a.The UE establishes an RRC connection and sends an “Extended Service Request”(encapsulated in RRC and S1-AP messages) for mobile terminating CS fallback to MME.
7b.MME sends S1-AP Initial UE Context Setup (UE capabilities, CS Fallback Indicator, LAI and other parameters specified in TS 23.401 [2]) to indicate the eNodeB to move the UE to UTRAN/GRAN. The registered PLMN for CS domain is identified by the PLMN ID included in the LAI, which is allocated by the MME.
7c.The eNodeB shall reply with S1-AP Initial UE Context Setup Response message.
8a.The information flow shall continue as described in clause "MT call in Connected Mode" from step 2.
 3 CSFB Test Case
 
3.1 MO Call In Connected Mode
UE setups CS service in the RRC Connected mode.
The signaling of OMC is below. Focus on the UE CONTEXT MODIFICATION REQUESTUE CONTEXT RELEASE REQUEST and RRC Connection Release.

 
UE sends Extend Service Request to MME. The signaling in eNodeB OMC is “UL Information Transfer” from UE to eNodeB and UPLINK NAS TRANSPORT from eNodeB to MME. Extended Service Request message is encapsulated in UPLINK NAS TRANSPORT messages. The detail is below containing Service Type: Mobile originating CS fallback and TMSI.

 
The MME sends an S1-AP UE Context Modification Request (CS Fallback Indicator, LAI) message to eNodeB. This message indicates to the eNodeB that the UE should be moved to UTRAN/GRAN. The detail of UE Context Modification Request is below containing registered LAI and CS Fallback Indicator. The registered LAI contains PLMN identity and LAC of UTRAN system. The CS Fallback Indicator is 0 indicating CS fallback required. Sometimes this message also contain UE aggregate maximum bit rate, but it is without any meaning in CSFB process. The version V3.10.20P02B03 didn’t support this field in this message in CSFB process.

 
eNodeB replies UE CONTEXT MODIFICATION RESPONSE to MME finishing the context modification process. Then UE sends UE CONTEXT RELEASE REQUEST to MME requesting release UE context and the caused IE is “CS Fallback triggered”.

 
MME sends UE CONTEXT RELEASE COMMAND to eNodeB after receiving UE CONTEXT RELEASE REQUEST to release UE context and the caused IE is “CS Fallback triggered”.

 
  After UE context release process, eNodeB sends RRC Connection Release to UE. This message indicates UE the frequency (ARFCN) of redirecting system. The redirectedcarrierInfo “UTRAN FDD=10638” indicates the target system is FDD UTRAN and the ARFCN is 10638 corresponding 2127.6MHz.

 
The CSFB process of E-UTRAN system has finished here and UE redirected to UTRAN system setting up CS service while keeping the PS service. If you need to ensure that the process accuracy,  you should check UE signaling focusing on UE whether redirect to the target system.
 
3.2 MO Call In Idle Mode
UE setups CS service in the RRC Idle mode.
The signaling of OMC is below. The no. 1 signaling in the process should be in no. 4 and no. 14, should be in no. 13 in screenshot. Focus on the RRC Connection Request, INITIAL CONTEXT SETUP REQUESTUE CONTEXT RELEASE REQUEST and RRC Connection Release message.

 
eNodeB sends UE CONTEXT RELEASE REQUEST to MME to release the UE context and the caused IE is S1AP user inactivity. Then UE moves to idle mode.

 
The UE initiates RRC connection setup process when sets up CS service. The caused IE in RRC Connection Request message is “mo data” below.

 
  The extend service request is encapsulated in RRC connection setup process, eNodeB forward it to MME through initial UE message. The detail decoding is below containing Service Type: Mobile originating CS fallback and TMSI information.

 
eNodeB receives the INITIAL CONTEXT SETUP REQUEST from MME containing CS fallback indicator and registered LAI. The value of CS fallback indicator is S1AP CS fallback required. The registered LAI contains PLMN identity and LAC of UTRAN system.

 
UE sends UE CONTEXT RELEASE REQUEST after UE context setup to release context. The caused IE is “CS Fallback triggered”.

 
After UE context release process, eNodeB sends RRC Connection Release to UE. This message indicates UE the frequency of redirecting system. The redirectedcarrierInfo “UTRAN FDD=10638” indicates the target system is FDD UTRAN and the ARFCN (Absolute Radio Frequency Channel Number) is 10638 corresponding 2127.6MHz.

 
  

3.3 MT Call In Connected Mode

UE receives CS service in the RRC Connected mode.
There is one more paging process than connected mode. The detail is below. Focus on UE CONTEXT MODIFICATION REQUEST, UE CONTEXT RELEASE REQUEST and RRC Connection Release message.
 
eNodeB transferred NAS message(DLInformationTransfer) after received DOWNLINK NAS TRANSPORT from MME. The detail of NAS message is CS service notification and the decoding is below.  
  UE sends Extend Service Request to MME. The signaling in eNodeB OMC is “UL Information Transfer” from UE to eNodeB and UPLINK NAS TRANSPORT from eNodeB to MME. Extended Service Request message is encapsulated in UPLINK NAS TRANSPORT messages. The detail is below containing Service Type: Mobile originating CS fallback, Mobile identity type: TMSI, CSFB response: CS fallback accepted by the UE.

 
The MME sends an S1-AP UE Context Modification Request (CS Fallback Indicator, LAI) message to eNodeB. This message indicates to the eNodeB that the UE should be moved to UTRAN/GRAN. The detail of UE Context Modification Request is below containing registered LAI and CS Fallback Indicator. The registered LAI contains PLMN identity and LAC of UTRAN system. The CS Fallback Indicator is 0 indicating CS fallback required.  
  eNodeB replies UE CONTEXT MODIFICATION RESPONSE to MME finishing the context modification process. Then UE sends UE CONTEXT RELEASE REQUEST to MME requesting release UE context and the caused IE is “CS Fallback triggered”.

 
  MME sends UE CONTEXT RELEASE COMMAND to eNodeB to release UE context. The caused IE is CS Fallback triggered.

 
After UE context release process, eNodeB sends RRC Connection Release to UE. This message indicates UE the frequency of redirecting system. The redirectedcarrierInfo “UTRAN FDD=10638” indicates the target system is FDD UTRAN and the ARFCN (Absolute Radio Frequency Channel Number) is 10638 corresponding 2127.6MHz.

 
 
 

3.4 MT Call In Idle Mode

UE receives CS service in the RRC Idle mode.
The signaling of OMC is below. The no. 8 signaling in the process should be in no. 6 in screenshot. Focus on the RRC Connection Request, INITIAL CONTEXT SETUP REQUESTUE CONTEXT RELEASE REQUEST and RRC Connection Release message.  
eNodeB sends UE CONTEXT RELEASE REQUEST to MME to release the UE context and the caused IE is S1AP user inactivity. Then UE moves to idle mode.

 
  The UE initiates RRC connection setup process when receives CS service. The caused IE in RRC Connection Request message is “mt access” below.

 
The extend service request is encapsulated in RRC connection setup process, eNodeB forward it to MME through initial UE message. The detail is below containing Service Type: Mobile originating CS fallback, Mobile identity type: TMSI, CSFB response: CS fallback accepted by the UE.

 
eNodeB receives the INITIAL CONTEXT SETUP REQUEST from MME containing CS fallback indicator and registered LAI. The value of CS fallback indicator is S1AP CS fallback required. The registered LAI contains PLMN identity and LAC of UTRAN system.

 
UE sends UE CONTEXT RELEASE REQUEST after UE context setup to release context. The caused IE is “CS Fallback triggered”.

 
After UE context release process, eNodeB sends RRC Connection Release to UE. This message indicates UE the frequency of redirecting system. The redirectedcarrierInfo “UTRAN FDD=10638” indicates the target system is FDD UTRAN and the ARFCN (Absolute Radio Frequency Channel Number) is 10638 corresponding 2127.6MHz.

 

4  Summary

This document mainly introduces the signaling processes of CS fallback based on blind redirection from E-UTRAN to UTRAN network. We can reference the 3GPP of EPC R10 ts_123272v101100p when facing the problems in the testing process.
The signaling of E-UTRAN to GRAN CSFB based on Blind Redirection is similar with E-UTRAN to UTRAN. The difference is that the caused IE is “UE not available for PS service” in UE CONTEXT RELEASE process because of UE not support CS and PS service in GRAN and that the frequency contains starting ARFCN, following ARFCN and band indicator in RRC release process depending on frequency characteristics of the GRAN network.

Budi Prasetyo

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