The IDLE state is defined as the state when both RRC state (i.e., eNB and UE signaling connection state) and ECM state (i.e., NAS signaling connection state) are IDLE while the EMM state maintained by the MME is still REGISTERED. This indicates that the UE is logically attached to the LTE network but the physical connection and the corresponding resources have been released from the UE up to the SGW (i.e., DRB and S1 bearer). The UE may fall into the IDLE mode when there hasn't been any activities for a while between the UE and the LTE network or when it is regarded the UE connection is lost.
In order to transfer any data to the UE in the IDLE mode, the MME has to wake up the UE in the first place so the UE can reconnect to the network. While reconnecting to the network, the UE and the network shall recover required resources and connections over RRC, S1AP and S1 interfaces. Once the EPS bearer is recovered, the data buffered in the SGW can be transferred to the UE. The following shows the conceptual sequence of the data transfer to the UE in the IDLE mode.
I. Network Initiated Service Request
The following flow shows the detailed procedure of how the application payload is delivered to the UE in the IDLE mode. As a precondition, the UE stays in the IDLE mode and the payload (i.e., SIP Message) has arrived at SGW.
[1] Upon receiving the data when there is no available S1 bearer at the moment as the UE is in IDLE mode, the SGW sends the Downlink Data Notification (DDN) to the MME to request for paging the UE. The DDN includes the EPS Bearer ID (EBI) stored in the EPS bearer context of the bearer on which the downlink data packet was received over S5 interface.
[2] The MME acknowledges the request by sending Downlink Data Notification Acknowledge.
[3] The MME initiates the paging procedures by sending PAGING message to each eNB that serves cells belonging to the tracking area in which UE is registered.
The following shows paging messages distributed across multiple eNBs that belong to the target TA List.
NOTE As the paging procedure itself can cause heavy traffic in the air, the MME may need to have an optimized paging scheme to minimize the side-effect. After having been awaken, the UE performs RRC connection establishment procedure with the eNB from which the paging was sent.
[4] Once the RRC connection is established, the UE requests the network to establish NAS Signaling connection, radio connection and S1 bearers by sending Service Request towards the MME. The Service Request is delivered wrapped in RRC Connection Complete and Initial UE Message on RRC and S1AP interfaces. The RRC state transits from the RRC-Idle to RRC-Connected.
[5] The MME requests to establish the E-RAB connection by sending the Initial Context Setup Request to the eNB.
In this example, there are three E-RABs to be setup which is the list of non-GBR bearers. Each E-RAB information contains the E-RAB ID, ARP and the SGW GTP-U TEID.
Upon receiving the Initial Context Setup Request, the eNB executes the E-RAB configuration and creates UE context based on the received parameters. In the meantime, the MME may perform the security setup procedure with the UE for integrity protection and ciphering of signaling data.
[6-7] The eNB requests the UE to establish the DRB by sending RRC Connection Reconfiguration. Once the DRB is established successfully, the UE responds with the RRC Connection Reconfiguration Complete to the eNB.
[8] After the DRB is successfully established, the eNB responds with the Initial Context Setup Response to the MME. The ECM state transits from ECM-Idle to ECM-Connected.
[9] The MME requests the SGW to resume the suspended S1 bearer by sending the Modify Bearer Request. If there were multiple APNs to which the UE was connected before falling into IDLE state, there can be multiple Modify Bearer Request sent, one for each APN.
The Bearer Context IE contains the EBI and the eNB GTP-U TEID with which the SGW will establish the S1 bearer towards the eNB. In this example, there are two EBIs one for the default EPS bearer and the other one for dedicated EPS bearer.
Once the S1 bearer is recovered, the SGW starts sending buffered data towards the UE. This procedure happens seamlessly without necessarily user interaction.
II. UE Initiated Service Request
In the same way, the UE may initiate the service request procedure in the IDLE to perform the VoLTE call initiation or location update procedures in the IDLE mode.
[1-10] Same as [4-10] in the figure 2. During this procedure, the RRC connection and NAS signaling connection is re-established. The UE context is recovered in MME. The EPS bearer including DRB and the S1 bearer is also re-established.
[11-12] The UE may trigger the TAU procedure by sending Tracking Area Update (TAU) request following the criteria as defined in TS23.401. The TAU contains the UE's current Tracking Area Identifier (TAI) and the ECGI. The last visited TAI is included if the UE has a valid TAI of the last visited tracking area and used by the MME to make a good list of TAI(i.e., TAL) for the UE. The TAL is contained in the TAU Accept.
Please refer to "VoLTE: Tracking Area Update and Combined Attach" for the detail.
Once the EPS bearer is recovered, the UE is able to initiate a voice call setup procedure by sending SIP INVITE. If there is no service data flow for a while, the UE and the network may again fall into the IDLE state.
In order to optimize the transactions between the UE and the network, the eNB shall be optimized as to criteria based on which the UE state falls into the IDLE mode. Furthermore the MME shall also provide the optimized scheme for paging schedules considering user experiences. If the paging scheme (i.e., timer values) is too loose, it will aggravate user experiences. If it is too frequent, it will cause heavy traffic in the air.
REFERENCES
[1] 3GPP TS29.274, "3GPP Evolved Packet System (EPS); Evolved General Packet Radio Services (GPRS) Tunneling Protocol for Control Plane (GTPv2-C); stage 3", v13.0.0
[2] 3GPP TS36.413, "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP)", v12.3.0
[3] Netmanias, "LTE EMM and ECM States", Sep 2013
Updated: Sep 2015
In order to transfer any data to the UE in the IDLE mode, the MME has to wake up the UE in the first place so the UE can reconnect to the network. While reconnecting to the network, the UE and the network shall recover required resources and connections over RRC, S1AP and S1 interfaces. Once the EPS bearer is recovered, the data buffered in the SGW can be transferred to the UE. The following shows the conceptual sequence of the data transfer to the UE in the IDLE mode.
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| Fig 1. Conceptual diagram of network initiated service request |
I. Network Initiated Service Request
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| Fig 2. VoLTE call flow - network initiated service request |
[1] Upon receiving the data when there is no available S1 bearer at the moment as the UE is in IDLE mode, the SGW sends the Downlink Data Notification (DDN) to the MME to request for paging the UE. The DDN includes the EPS Bearer ID (EBI) stored in the EPS bearer context of the bearer on which the downlink data packet was received over S5 interface.
[2] The MME acknowledges the request by sending Downlink Data Notification Acknowledge.
[3] The MME initiates the paging procedures by sending PAGING message to each eNB that serves cells belonging to the tracking area in which UE is registered.
The following shows paging messages distributed across multiple eNBs that belong to the target TA List.
NOTE As the paging procedure itself can cause heavy traffic in the air, the MME may need to have an optimized paging scheme to minimize the side-effect. After having been awaken, the UE performs RRC connection establishment procedure with the eNB from which the paging was sent.
[4] Once the RRC connection is established, the UE requests the network to establish NAS Signaling connection, radio connection and S1 bearers by sending Service Request towards the MME. The Service Request is delivered wrapped in RRC Connection Complete and Initial UE Message on RRC and S1AP interfaces. The RRC state transits from the RRC-Idle to RRC-Connected.
[5] The MME requests to establish the E-RAB connection by sending the Initial Context Setup Request to the eNB.
- UE-AMBR indicates the maximum aggregated bit rate for non-GBR bearers for the concerned UE.
- E-RAB To Be Setup Item includes the E-RAB information for each EPS bearers to be setup.
In this example, there are three E-RABs to be setup which is the list of non-GBR bearers. Each E-RAB information contains the E-RAB ID, ARP and the SGW GTP-U TEID.
Upon receiving the Initial Context Setup Request, the eNB executes the E-RAB configuration and creates UE context based on the received parameters. In the meantime, the MME may perform the security setup procedure with the UE for integrity protection and ciphering of signaling data.
[6-7] The eNB requests the UE to establish the DRB by sending RRC Connection Reconfiguration. Once the DRB is established successfully, the UE responds with the RRC Connection Reconfiguration Complete to the eNB.
[8] After the DRB is successfully established, the eNB responds with the Initial Context Setup Response to the MME. The ECM state transits from ECM-Idle to ECM-Connected.
- E-RAB Setup List is a list of E-RABs that has been successfully established, which contains each E-RAB ID and eNB GTP-U TEID.
- E-RAB Failed Setup List is a list of E-RABs that has failed to establish.
[9] The MME requests the SGW to resume the suspended S1 bearer by sending the Modify Bearer Request. If there were multiple APNs to which the UE was connected before falling into IDLE state, there can be multiple Modify Bearer Request sent, one for each APN.
The Bearer Context IE contains the EBI and the eNB GTP-U TEID with which the SGW will establish the S1 bearer towards the eNB. In this example, there are two EBIs one for the default EPS bearer and the other one for dedicated EPS bearer.
[10] The SGW responds with the Modify EPS bearer response containing the resulting cause value for each requested EBI. The Bearer Context IE contains the EBIs and the SGW GTP-U TEID which is the same value that was contained in the Initial Context Setup Request at step #5.
Once the S1 bearer is recovered, the SGW starts sending buffered data towards the UE. This procedure happens seamlessly without necessarily user interaction.
II. UE Initiated Service Request
In the same way, the UE may initiate the service request procedure in the IDLE to perform the VoLTE call initiation or location update procedures in the IDLE mode.
[1-10] Same as [4-10] in the figure 2. During this procedure, the RRC connection and NAS signaling connection is re-established. The UE context is recovered in MME. The EPS bearer including DRB and the S1 bearer is also re-established.
[11-12] The UE may trigger the TAU procedure by sending Tracking Area Update (TAU) request following the criteria as defined in TS23.401. The TAU contains the UE's current Tracking Area Identifier (TAI) and the ECGI. The last visited TAI is included if the UE has a valid TAI of the last visited tracking area and used by the MME to make a good list of TAI(i.e., TAL) for the UE. The TAL is contained in the TAU Accept.
Please refer to "VoLTE: Tracking Area Update and Combined Attach" for the detail.
Once the EPS bearer is recovered, the UE is able to initiate a voice call setup procedure by sending SIP INVITE. If there is no service data flow for a while, the UE and the network may again fall into the IDLE state.
***
Red Mouse
REFERENCES
[1] 3GPP TS29.274, "3GPP Evolved Packet System (EPS); Evolved General Packet Radio Services (GPRS) Tunneling Protocol for Control Plane (GTPv2-C); stage 3", v13.0.0
[2] 3GPP TS36.413, "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP)", v12.3.0
[3] Netmanias, "LTE EMM and ECM States", Sep 2013
Updated: Sep 2015
Hi Hong.. Great blog for everyone.. Sincere Thanks to you
ReplyDeleteCould you please explain detail the VoWiFi concepts , CS termination and CS Origination via IMS.
Sahring the references for VoWiFi to sarat.dece445@gmail.com also no problem.
Thank you in advance
Thank you so much, Hong. Great writeup.
ReplyDeleteGreat hongjoo. Nice Blogs and easy to understand.
ReplyDeleteExpects such technical IMS and EPC Blogs in future also.
WHat would happen if the UE moves in idle mode to another enodeb on which volte service is not activated under the same TA. How enodeb and UE would respond in case of such scenario for MO and MT calls. Thanks in advance for your inputs.
ReplyDeletethank you so much for the detailed explanation
ReplyDeleteafter tracking Area update(TAU)in LTE ,will UE initiate VOLTE registration again ,If UE is VOLTE subscription
ReplyDelete