Section: 5.3.4B.3 Mobile Terminated Data Transport in Control Plane CIoT EPS Optimisation with P-GW connectivity
Content: Figure 5.3.4B.3-1: MT Data transport in NAS PDUs 0. The UE is EPS attached and in ECM-Idle mode. 1. When the S-GW receives a downlink data packet/control signalling for a UE, if the S- GW context data indicates no downlink user plane TEID towards the MME), it buffers the downlink data packet and identifies which MME is serving that UE. If that MME has requested the Serving GW to throttle downlink low priority traffic and if the downlink data packet is received on a low priority bearer to be throttled (see clause 4.3.7.4.1a), the S-GW drops the downlink data. The steps below are not executed. If that MME has requested the S-GW to delay sending the Downlink Data Notification (see clause 5.3.4.2 on "Handling of abnormal conditions in UE triggered Service Request"), the Serving GW buffers the downlink data and waits until the timer expires before continuing with step 2. If the DL-TEID and MME address for that UE is received before the expiry of the timer, the timer shall be cancelled and the Mobile Terminated Data transport procedure is progressed from step 11 as Downlink data are sent to the UE. If the Serving GW receives additional downlink data packets/control signalling for this UE before the expiry of the timer, the Serving GW does not restart this timer. 2. If the Serving GW is buffering data in step 1, the Serving GW sends a Downlink Data Notification message (ARP, EPS Bearer ID) to the MME for which it has control plane connectivity for the given UE. The ARP and EPS Bearer ID are always set in Downlink Data Notification. The MME responds to the S-GW with a Downlink Data Notification Ack message. An MME detects that the UE is in a power saving state (e.g. Power Saving Mode) and cannot be reached by paging at the time of receiving Downlink data notification, shall invoke extended buffering depending on operator configuration, except for cases described in next paragraphs. The MME derives the expected time before radio bearers can be established to the UE. The MME then indicates Downlink Buffering Requested to the Serving GW in the Downlink Data Notification Ack message and includes a Downlink Buffering Duration time and optionally a Downlink Buffering Suggested Packet Count. The MME stores a new value for the Downlink Data Buffer Expiration Time in the MM context for the UE based on the Downlink Buffering Duration time and skips the remaining steps of this procedure. The Downlink Data Buffer Expiration Time is used for UEs using power saving state and indicates that there are buffered data in the Serving GW and that the user plane setup procedure is needed when the UE makes signalling with the network. When the Downlink Data Buffer Expiration Time has expired, the MME considers no Downlink data to be buffered and no indications of Buffered Downlink Data Waiting are sent during context transfers at TAU procedures. If there is an "Availability after DDN Failure" monitoring event configured for the UE in the MME, the MME does not invoke extended buffering. Instead, the MME sets the Notify-on-available-after-DDN-failure flag to remember to send an "Availability after DDN Failure" notification when the UE becomes available. If there is a "UE Reachability" monitoring event configured for the UE in the MME, the MME does not invoke extended buffering. NOTE 1: When "Availability after DDN failure" and "UE reachability" monitoring events are used for a UE, the application server is assumed to send data only when the UE is reachable, hence no extended buffering is needed. If there are multiple application servers, the event notifications and extended buffering may be needed simultaneously. It is assumed this is handled through additional information based on SLA as described in the next paragraph. The MME may use additional information based on a SLA with the MTC user for when to invoke extended buffering, e.g. only invoke it for a certain APN, do not invoke it for certain subscribers, invoke extended buffering in conjunction with "Availability after DDN failure" and "UE reachability" monitoring events, etc. A Serving GW that receives a Downlink Buffering Requested indication in a Downlink Data Notification Ack message stores a new value for the Downlink Data Buffer Expiration Time based on the Downlink Buffering Duration time and does not send any additional Downlink Data Notification if subsequent downlink data packets are received in the Serving GW before the buffer time Downlink Data Buffer Expiration Time has expired for the UE. If the Serving GW, while waiting for the user plane to be established, is triggered to send a second Downlink Data Notification for a bearer with higher priority (i.e. ARP priority level) than that of the bearer for which the first Downlink Data Notification was sent, the S-GW sends a new Downlink Data Notification message indicating the higher priority to the MME. If the Serving GW receives additional downlink data packets for a bearer with same or lower priority than the first Downlink Data Notification was sent for or if the Serving GW has sent the second Downlink Data Notification message indicating the higher priority and receives additional downlink data packets for this UE, the Serving GW buffers these downlink data packets and does not send a new Downlink Data Notification. If the Serving GW, while waiting for the user plane to be established, receives a Modify Bearer Request message from an MME other than the one it sent a Downlink Data Notification message to, the Serving GW re-sends the Downlink Data Notification message but only to the new MME from which it received the Modify Bearer Request message. If the MME holds stored Paging Restriction Information (see clause 4.3.33.6) for the UE that restricts the Downlink Data from causing paging, the MME sends Downlink Data Notification Ack message with an indication that the Downlink Data Notification message has been temporarily rejected. Upon reception of a Downlink Data Notification Ack message with an indication that the Downlink Data Notification message has been temporarily rejected and if the Downlink Data Notification is triggered by the arrival of downlink data packets at the Serving GW, the Serving GW may start a locally configured guard timer and buffers all downlink user packets received to the given UE and waits for a Modify Bearer Request message to come. Upon reception of a Modify Bearer Request message, the Serving GW re-sends the Downlink Data Notification message but only to the new MME from which it received the Modify Bearer Request message. Otherwise the Serving GW releases buffered downlink user packets upon expiry of the guard timer or upon receiving the Delete Session Request message from MME. If the S11-U is already established (buffering is in the MME), step 2 is not executed and step 11 is immediately executed. Steps 7,8,9,10 are executed only if conditions are met when the NAS control plane service request is received at step 6, as outlined below in the respective clauses. An MME detecting that the UE is in a power saving state (e.g. Power Saving Mode) and cannot be reached by paging at the time of receiving Downlink data, shall start extended buffering depending on operator configuration, except for cases described in next paragraphs. The MME derives the expected time before radio bearers can be established to the UE, stores a new value for the Downlink Data Buffer Expiration Time in the MM context for the UE and skips the remaining steps of this procedure. When the Downlink Data Buffer Expiration Time has expired, the MME considers no Downlink data to be buffered. Also for the case of buffering in the MME the "Availability after DDN Failure" monitoring event can be configured for the UE, even though the actual DDN is not received and the Downlink data is received. The "UE Reachability" monitoring event can also be configured. The extended buffering can also be configured as per what is described above in this step of the procedure for the case of buffering in S-GW. 3. If the UE is registered in the MME and considered reachable, the MME sends Paging message(s) as specified in step 3a of clause 5.3.4.3. 4. If eNodeBs receive paging messages from the MME, the UE is paged by the eNodeBs as specified in step 4a of clause 5.3.4.3. 5. As the UE is in the ECM-IDLE state, upon reception of paging indication, the UE sends Control Plane Service Request NAS message (as defined in TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46]) over RRC Connection request and S1-AP initial message. The Control Plane Service Request NAS message, when Control Plane CIoT EPS Optimisation applies, does not trigger Data radio bearer establishment by the MME and the MME can immediately send Downlink Data it receives using a NAS PDU to the eNodeB. The MME supervises the paging procedure with a timer. If the MME receives no response from the UE to the Paging Request message, it may repeat the paging according to any applicable paging strategy described in step 3. If the Multi-USIM UE in ECM-IDLE state, upon reception of paging indication in E-UTRAN access and if the UE decides not to accept the paging, the UE attempts to send a Reject Paging Indication in the Control Plane Service Request NAS message (as defined in TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46]) over RRC Connection request and S1-AP initial message. If the Control Plane Service Request NAS message includes a Reject Paging Indication, then: - If the Service Request message includes Paging Restriction Information, the MME may accept or reject the Paging Restriction Information requested by the UE based on operator policy. If the MME rejects the Paging Restriction Information, the MME removes any stored Paging Restriction Information from the UE context and discards the UE requested Paging Restriction Information. If the MME accepts the Paging Restriction Information from the UE, the MME stores the Paging Restriction Information from the UE in the UE context. The MME informs the UE about the acceptance/rejection of the requested Paging Restriction Information in the Service Accept message. If no Paging Restriction Information is provided, no paging restrictions apply; - no Downlink or Uplink Data is sent (steps 7-20 are skipped); - the MME triggers the S1 release procedure in step 21. 5b. In the NB-IoT case, the eNodeB, based on configuration, may retrieve the EPS negotiated QoS profile from the MME, if not previously retrieved. The MME Code within the S-TMSI in the RRCConnectionRequest message is used to identify the MME. In the case of network sharing, the MME Codes shall be unique within the area of overlapping MME pools of the participating operators. The eNodeB may apply prioritisation between requests from different UEs before triggering step 6 and throughout the RRC connection. The eNodeB may retrieve additional parameters (e.g., UE Radio Capabilities - see TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36]). 6. If the MME receives no response from the UE after this paging repetition procedure, it shall use the Downlink Data Notification Reject message to notify the Serving GW about the paging failure (or, equivalently, if the buffering is in the MME, the MME simply discards data for the UE locally), unless the MME is aware of an ongoing MM procedure that prevents the UE from responding, i.e. the MME received a Context Request message indicating that the UE performs TAU with another MME. When a Downlink Data Notification Reject message is received, the Serving GW deletes the buffered packet(s). The Serving GW may invoke the procedure PDN GW Pause of Charging (clause 5.3.6A) if UE is in ECM IDLE and the PDN GW has enabled "PDN charging pause" feature. If buffering is in the MME, Pause Charging is triggered by the MME via a Release Access Bearer Request to the S-GW(not shown in Figure 5.3.4B.3-1) including a "Abnormal Release of Radio Link" cause, which releases the S11-U. NOTE 2: The Serving GW (or MME, in the case of buffering in the MME) may initiate the procedure P-GW Pause of Charging at any time before step 5 if the UE is in ECM IDLE and the P-GW has indicated that the feature is enabled for this PDN. See clause 5.3.6A.0. To assist Location Services, the eNodeB indicates the UE's Coverage Level to the MME. The MME performs (and the UE responds to) any EMM or ESM procedures if necessary, e.g. the security related procedures. Steps 7 to 11 can continue in parallel to this, however, steps 12 and 13 shall await completion of all the EMM and ESM procedures. 7. If the S11-U is not established, the MME sends a Modify Bearer Request message (MME address, MME TEID DL, Delay Downlink Packet Notification Request, RAT Type, LTE-M RAT type reporting to PGW flag) for each PDN connection to the Serving GW. The Serving GW is now able to transmit downlink data towards the UE. The usage of the Delay Downlink Packet Notification Request Information Element is specified in clause 5.3.4.2 with reference to the UE initiated service request procedure, but it equally applies in this case. The MME shall indicate S11-U tunnelling of NAS user data and send its own S11-U IP address and MME DL TEID for DL data forwarding by the SGW. Also, regardless of whether the S11-U was already established: - If the P-GW requested UE's location and/or User CSG information and the UE's location and/or User CSG information has changed, the MME shall send the Modify Bearer Request message and also includes the User Location Information IE and/or User CSG Information IE in this message. - If the Serving Network IE has changed compared to the last reported Serving Network IE then the MME shall send the Modify Bearer Request message and also includes the Serving Network IE in this message. - If the UE Time Zone has changed compared to the last reported UE Time Zone then the MME shall send the Modify Bearer Request message and include the UE Time Zone IE in this message. If the RAT type currently used is NB-IOT this shall be reported as different from other -E-UTRA flavours. If the UE is using the LTE-M RAT type and the PDN GW expects the LTE-M RAT type reporting as specified in clause 5.11.5, the MME also includes the LTE-M RAT type reporting to PGW flag to indicate to the Serving GW to forward the LTE-M RAT type to the PDN GW. 8. If the RAT Type has changed compared to the last reported RAT Type or if the UE's Location and/or Info IEs and/or UE Time Zone and Serving Network id are present in step 7, the Serving GW shall send the Modify Bearer Request message (RAT Type) to the P-GW. User Location Information IE and/or User CSG Information IE and/or Serving Network IE and/or UE Time Zone are also included if they are present in step 7. If LTE-M RAT type and the LTE-M RAT type reporting to PGW flag were received at step 7, the Serving GW shall include the LTE-M RAT type in the Modify Bearer Request message to the PGW. Otherwise the Serving GW includes RAT type WB-E-UTRAN. If the Modify Bearer Request message is not sent because of above reasons and the PDN GW charging is paused, then the S-GW shall send a Modify Bearer Request message with PDN Charging Pause Stop Indication to inform the PDN GW that the charging is no longer paused. Other IEs are not included in this message. 9. The PDN GW sends the Modify Bearer Response to the Serving GW. 10. If a Modify Bearer Request message was sent at step 7, the Serving GW shall return a Modify Bearer Response (Serving GW address and TEID for uplink traffic) to the MME as a response to a Modify Bearer Request message. The Serving GW address for S11-U User Plane and Serving GW TEID are used by the MME to forward UL data to the SGW. 11. Buffered (if S11-U was not established) Downlink data is sent by the S-GW to the MME. 12-13. The MME encrypts and integrity protects Downlink data and sends it to the eNodeB using a NAS PDU carried by a Downlink S1-AP message. If the configuration in the MME indicates that the eNodeB supports acknowledgements of downlink NAS data PDUs and if acknowledgements of downlink NAS data PDUs are enabled in the subscription information for the UE, the MME indicates in the Downlink S1-AP message that acknowledgment is requested from the eNodeB. For IP PDN type PDN connections configured to support Header Compression, the MME shall apply header compression before encapsulating data into the NAS message. Alternatively and if the MME decides that S1-U bearers need to be established in the case that the UE and MME accept User Plane EPS Optimisation or S1-U data transfer, steps 4-12 from clause 5.3.4.1 are followed. If the UE is accessing via an NB-IoT cell, or if it is accessing via an WB-E-UTRAN cell and is capable of CE mode B, to determine the NAS PDU retransmission strategy the MME should take into account the transmission delay of the NAS PDU and the CE mode B Restricted parameter stored in the MME's MM context and, if applicable, the CE mode, i.e. set the NAS timers long enough according to the worst transmission delay (see TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46]). 14. The NAS PDU with data is delivered to the UE via a Downlink RRC message. This is taken by the UE as implicit acknowledgment of the Service Request message sent in step 5. If header compression was applied, to the PDN, the UE shall perform header decompression to rebuild the IP header. 15. The eNodeB sends a NAS Delivery indication to the MME if requested. If the eNodeB reports an unsuccessful delivery with an S1-AP NAS Non Delivery Indication, the MME should wait for some time until the UE has potentially changed cell and re-established contact with the MME, by which MME should resend the Downlink S1-AP message to the eNodeB, otherwise the MME reports an unsuccessful delivery to the SCEF in the case of T6a procedure (see TS 23.682[ Architecture enhancements to facilitate communications with packet data networks and applications ]  [74], clause 5.13.3). If the eNodeB reports a successful delivery with an S1-AP NAS Delivery Indication and if the Downlink data was received over the T6a interface, the MME should respond to the SCEF (see TS 23.682[ Architecture enhancements to facilitate communications with packet data networks and applications ]  [74], clause 5.13.3). If the eNodeB does not support S1-AP NAS delivery indications, the MME indicates a cause code 'Success Unacknowledged Delivery' to the SCEF otherwise 'Success Acknowledged Delivery', for the SCEF to know if reliable delivery was possible or not. 16. While the RRC connection is still up, further Uplink and Downlink data can be transferred using NAS PDUs. In step 17 an Uplink data transfer is shown using an Uplink RRC message encapsulating a NAS PDU with data. At any time the UE has no user plane bearers established, the UE may provide a Release Assistance Information with Uplink data in the NAS PDU. For IP PDN type PDN connections configured to support Header Compression, the UE shall apply header compression before encapsulating it into the NAS message. 17. The NAS PDU with data is send to the MME in a Uplink S1-AP message. To assist Location Services, the eNodeB may indicate, if changed, the UE's Coverage Level to the MME. If the Release Assistance Information is received from the UE it overrides the Traffic Profile (see TS 23.682[ Architecture enhancements to facilitate communications with packet data networks and applications ]  [74]) and the MME does not send the Traffic Profile to the eNodeB. 18. The data is checked for integrity and decrypted. If header compression was applied to the PDN, the MME shall perform header decompression to rebuild the IP header. 19. The MME sends Uplink data to the PDN GW via the S-GW and executes any action related to the presence of Release Assistance Information as follows: - for the case where the Release Assistance Information indicates there is no downlink data to follow the uplink data then unless the MME is aware of pending MT traffic, and unless S1-U bearers exist, the MME immediately releases the connection and therefore step 21 is executed. - for the case where the Release Assistance Information indicates that downlink data will follow the uplink transmission then unless the MME is aware of additional pending MT traffic and unless S1-U bearers exist, the MME sends a S1 UE Context Release Command to the eNodeB immediately after the S1-AP message including the Downlink data encapsulated in NAS PDU. 20. If no NAS activity exists for a while the eNodeB detects inactivity and executes step 21. 21. The eNodeB starts an eNodeB initiated S1 release procedure according to clause 5.3.5 or Connection Suspend Procedure defined in clause 5.3.4A. The UE and the MME shall store the ROHC configuration and context for the uplink/downlink data transmission when entering ECM_CONNECTED state next time.