SRVCC Call Flow Scenario

Q.How to verify SRVCC related elements in SRVCC call flow message

(a)Initial Context Setup

Direction: MME ->eNB

In case of the establishment of an E-RAB the EPC must be prepared to receive user data before the INITIAL CONTEXT SETUP RESPONSE message has been received by the MME.

The INITIAL CONTEXT SETUP REQUEST message may contain

– Message Type
– MME UE S1AP ID
– eNB UE S1AP ID
– UE Aggregate Maximum Bit Rate
– E-RAB to Be Setup List
– UE Security Capabilities
– Security Key
– the Trace Activation IE.
– the Handover Restriction List IE, which may contain roaming,
  area or  access restrictions.
– the UE Radio Capability IE.
– the Subscriber Profile ID for RAT/Frequency priority IE.
– the CS Fallback Indicator IE.
– the SRVCC Operation Possible IE.
– the CSG Membership Status IE.
– the Registered LAI IE.
– the GUMMEI ID IE, which indicates the MME serving the UE
– the MME UE S1AP ID 2 IE, which indicates the MME UE S1AP ID assigned by the MME,
– the Management Based MDT Allowed IE.

Upon receipt of the INITIAL CONTEXT SETUP REQUEST message the eNB shall store the received SRVCC Operation Possible in the UE context and use it
 – MME includes a “SRVCC operation possible” indication in the S1 AP  Initial Context Setup Request, meaning that both UE and MME are  SRVCC-capable

SRVCC OperationPossible

This element indicates that both UE and MME are SRVCC-capable.E-UTRAN behaviour on receipt of this IE is specified in TS23.216[9].

IE/GroupName	Presence	Range	IEtypeandreference	Semanticsdescription
SRVCC operationpossible	M		ENUMERATED (Possible,…)

(b)Measurement Report from UE

(c)Handover Required

The source eNB RRC initiates the handover preparation by sending the HANDOVER REQUIRED message to the serving MME.
When the source eNB sends the HANDOVER REQUIRED message, it shall start the timer TS1RELOCprep. 

The Handover Required may contain
– Message Type
– MME UE S1AP ID
– eNB UE S1AP ID
– Handover Type
– Cause
– Target ID
– Direct Forwarding Path Availability
– SRVCC HO Indication
– Source to Target Transparent Container
– Source to Target Transparent Container Secondary
– MS Classmark 2
– MS Classmark 3
– CSG Id
– Cell Access Mode
– PS Service Not Available

SRVCC HO Indication

This information element is set by the source eNB to provide an indication that E-RAB may be subjected to hando via SRVCC means.

IE/Group Name	Presence	Range	IE type andreference	Semantics description
SRVCC HO Indication	M		ENUMERATED (PSandCS,CS only,…)

The SRVCC HO Indication IE is set according to the target cell capability and UE capability. In case the target system is GERAN without DTM support or the UE is without DTM support, the source eNB shall indicate “CS only” in the SRVCC HO Indication IE and “PS service not available” in PS Service Not Available IE.

• In case the target system is either GERAN with DTM but without DTM HO support and the UE is supporting DTM or the target system is UTRAN without PS HO support, the source eNB shall indicate “CS only” in the SRVCC HO Indication IE. Otherwise, the source eNB shall indicate “PS and CS” in the SRVCC HO Indication IE.
• In case of inter-system handover from E-UTRAN, the source eNB shall indicate in the Target ID IE, in case the target system is UTRAN, the Target RNC-ID of the RNC (including the Routing Area Code only in case the UTRAN PS domain is involved),
• in case the target system is GERAN the Cell Global Identity (including the Routing Area Code only in case the GERAN PS domain is involved) of the cell in the target system.

Case:1 In case the SRVCC operation is performed, the SRVCC HO Indication IE in the HANDOVER REQUIRED message indicates that handover shall be prepared only for CS domain, and if

 – the target system is GERAN, then the MME

– shall encode the information in the Target to Source Transparent Container IE within the HANDOVER COMMAND message according to the definition of the Layer 3 Information IE as specified in TS 48.008[23], and
– shall not include the Target to Source Transparent Container Secondary IE in the HANDOVER COMMAND message;

– the target system is UTRAN, then the MME

– shall encode the information in the Target to Source Transparent Container IE within the HANDOVER COMMAND message according to the definition of the Target RNC to Source RNC Transparent Container IE as specified in TS 25.413 [19], and
– shall not include the Target to Source Transparent Container Secondary IE in the HANDOVER COMMAND message.

Case:2 In case the SRVCC operation is performed, the SRVCC HO Indication IE in the HANDOVER REQUIRED message indicates that handover shall be prepared for PS and CS domain,

– the target system is GERAN with DTM HO support, and if

– the Handover Preparation procedure has succeeded in the CS and PS domain, then the MME
– shall encode the information in the Target to Source Transparent Container IE within the HANDOVER COMMAND message according to the definition of the Layer 3 Information IE as specified in TS 48.008[23], and
– shall include the Target to Source Transparent Container Secondary IE in the HANDOVER COMMAND message and encode information in the it according to the definition of the Target BSS to Source BSS Transparent Container IE as specified in TS 48.018 [18];
– the Handover Preparation procedure has succeeded in the CS domain only, then the MME
– shall encode the information in the Target to Source Transparent Container IE within the HANDOVER COMMAND message according to the definition of the Layer 3 Information IE as specified in TS 48.008 [23], and
– shall not include the Target to Source Transparent Container Secondary IE in the HANDOVER COMMAND message;

– the target system is UTRAN, then the Handover Preparation procedure shall be considered successful if the

– Handover Preparation procedure has succeeded in the CS domain, and the MME
– shall encode the information in the Target to Source Transparent Container IE within the HANDOVER COMMAND message according to the definition of the Target RNC to Source RNC Transparent Container IE as specified in TS 25.413 [19], and
– shall not include the Target to Source Transparent Container Secondary IE in the HANDOVER COMMAND message.