Core Network: Architecture and Signaling Procedures

The source eNodeB decides on an X2-based handover from the measurement reports of the UE and sends an X2AP Handover Request message to the target eNodeB to prepare resources for the UE. The message carries the Old eNB UE X2AP ID, the Cause, for example "Handover Desirable for Radio Reasons", the Target Cell ID (ECGI), and the GUMMEI. It also carries the UE Context Information, with the MME UE S1AP ID, the UE Security Capabilities, the AS Security Information with the KeNB* and the NCC, the UE-AMBR, the E-RABs To Be Setup List, with the E-RAB ID, the E-RAB Level QoS Parameters, such as the QCI and the ARP, and the S-GW UL GTP Tunnel Endpoint of each E-RAB, and the RRC Context with the RRC HandoverPreparationInformation message (TS 36.423 clause 9.1.1.1, and TS 33.401 clause 7.2.8.4.2).

The target eNodeB performs admission control for the requested E-RABs and returns an X2AP Handover Request Acknowledge message to the source eNodeB. The message carries the Old eNB UE X2AP ID, the New eNB UE X2AP ID, the E-RABs Admitted List, with the E-RAB ID and the GTP tunnel endpoints for data forwarding, the E-RABs Not Admitted List, and the Target eNB To Source eNB Transparent Container. The container holds the RRC HandoverCommand message (TS 36.423 clause 9.1.1.2), which in turn holds the RRCConnectionReconfiguration message that the target eNodeB has prepared for the UE.

The source eNodeB forwards the handover command of the target eNodeB to the UE in an RRC RRCConnectionReconfiguration message over the radio interface. The message carries the mobilityControlInfo field, with the target physical cell identity, the carrier frequency, timer T304, the new Cell Radio Network Temporary Identifier (C-RNTI), and, optionally, a dedicated random access preamble, together with the radioResourceConfigDedicated field and the securityConfigHO field, with the security algorithm configuration and the NCC (TS 36.331 clause 6.2.2). The UE leaves the source cell, derives the new KeNB from the received NCC (TS 33.401 clause 7.2.8.4.4), and accesses the target cell.

To preserve the packet sequence, the source eNodeB sends an X2AP SN Status Transfer message to the target eNodeB. The message carries the Old eNB UE X2AP ID, the New eNB UE X2AP ID, and the E-RABs Subject To Status Transfer List, with the E-RAB ID, the Receive Status Of UL PDCP SDUs, the UL COUNT Value, and the DL COUNT Value of each E-RAB, where a COUNT value consists of the PDCP-SN and the HFN (TS 36.423 clause 9.1.1.4).

After the UE has synchronized to the target cell, it sends an RRC RRCConnectionReconfigurationComplete message to the target eNodeB, carrying the RRC Transaction Identifier (TS 36.331 clause 6.2.2). From this point, the target eNodeB can deliver the forwarded downlink data to the UE and send the uplink data of the UE towards the S-GW.

The target eNodeB informs the MME that the UE has changed cell by sending an S1AP Path Switch Request message over S1-MME (TS 23.401 clause 5.5.1.1.2, step 1b). The message carries the eNB UE S1AP ID, the E-RAB To Be Switched in Downlink List, with the E-RAB ID, the transport layer address, and the GTP-TEID of the target eNodeB for each E-RAB, the Source MME UE S1AP ID, the E-UTRAN CGI, the TAI, and the UE Security Capabilities. For a CSG cell or a hybrid cell, the message also carries the CSG Id, the Cell Access Mode, and the CSG Membership Status (TS 36.413 clause 9.1.5.8).

If the MME relocates the S-GW, the MME sends a GTPv2 Create Session Request message to the target S-GW over S11, one message for each PDN connection whose default bearer the target eNodeB has accepted (TS 23.401 clause 5.5.1.1.3, step 2). The message carries the IMSI, the Sender F-TEID for Control Plane, the PGW S5/S8 Address for Control Plane, the RAT Type, the Serving Network, the User Location Information, and the Bearer Contexts to be created, with the EBI, the S1-U eNodeB F-TEID of the target eNodeB, and the S5/S8-U PGW F-TEID. The Indication Flags IE carries the Operation Indication flag set to 1 (TS 29.274 clause 7.2.1). The target S-GW allocates one S1-U uplink TEID for each bearer.

If the S-GW is relocated, the target S-GW allocates the downlink TEIDs for the traffic from the P-GW and sends a GTPv2 Modify Bearer Request message to the P-GW over S5/S8, one message for each PDN connection (TS 23.401 clause 5.5.1.1.3, step 3). The message carries the Sender F-TEID for Control Plane of the target S-GW, the Bearer Contexts to be modified, with the EBI and the S5/8-U SGW F-TEID, and, when the MME has provided it, the User Location Information (TS 29.274 clause 7.2.7).

If the S-GW is relocated, the P-GW updates its context, switches the downlink path to the target S-GW, and returns a GTPv2 Modify Bearer Response message to the target S-GW. The message carries the Cause set to "Request accepted" (16), the MSISDN, if the P-GW has stored it, the Bearer Contexts modified, with the Charging ID of each bearer, and, when applicable, the Change Reporting Action (TS 29.274 clauses 7.2.8 and 8.4). Immediately after switching the path, the P-GW sends one or more end marker packets on the old path, and the source S-GW forwards these packets to the source eNodeB (TS 23.401 clause 5.5.1.1.3, step 3).

If the S-GW is relocated, the target S-GW returns a GTPv2 Create Session Response message to the MME over S11. The message carries the Cause set to "Request accepted" (16), the Sender F-TEID for Control Plane of the target S-GW, and the Bearer Contexts created, with the EBI, the Cause, and the S1-U SGW F-TEID for the uplink traffic of each bearer (TS 29.274 clause 7.2.2). The MME starts the timer that supervises the release of the source S-GW in step 15 (TS 23.401 clause 5.5.1.1.3, step 4).

If the S-GW isn’t relocated, the MME sends a GTPv2 Modify Bearer Request message to the S-GW that serves the UE over S11, one message for each PDN connection whose default bearer the target eNodeB has accepted (TS 23.401 clause 5.5.1.1.2, step 2). The message carries the Bearer Contexts to be modified, with the EBI and the S1 eNodeB F-TEID of the target eNodeB. It also carries the User Location Information, if the P-GW has requested location change reporting and the location has changed, and the User CSG Information, if the P-GW has requested it and the information has changed. Because neither the MME nor the S-GW changes, the message doesn’t carry the Sender F-TEID for Control Plane (TS 29.274 clause 7.2.7).

If the S-GW isn’t relocated, the S-GW starts sending downlink packets to the target eNodeB and returns a GTPv2 Modify Bearer Response message to the MME. The message carries the Cause set to "Request accepted" (16) and the Bearer Contexts modified, with the EBI, the Cause, and the S1-U SGW F-TEID of each bearer, and, when applicable, the Change Reporting Action (TS 29.274 clause 7.2.8). Immediately after switching the path, the S-GW sends one or more end marker packets on the old path to assist the reordering function in the target eNodeB (TS 23.401 clause 5.5.1.1.2, steps 4 and 5).

The MME confirms the path switch by returning an S1AP Path Switch Request Acknowledge message to the target eNodeB. The message carries the MME UE S1AP ID, the eNB UE S1AP ID, and the Security Context, with one fresh {NH, NCC} pair that the MME computes after it has increased its locally kept NCC by one (TS 33.401 clause 7.2.8.4.2). It carries the UE-AMBR if the value has changed and the E-RAB To Be Switched in Uplink List, with the S-GW transport layer address and GTP-TEID of each E-RAB, if the uplink tunnel endpoints have changed. If some bearers failed in the core network, the message carries the E-RAB To Be Released List (TS 36.413 clauses 8.4.4.2 and 9.1.5.9, and TS 23.401 clause 5.5.1.1.2, step 6).

The target eNodeB informs the source eNodeB that the handover has succeeded by sending an X2AP UE Context Release message, which carries the Old eNB UE X2AP ID and the New eNB UE X2AP ID. The source eNodeB then releases the radio and control-plane resources of the UE. The target eNodeB sends this message only after it has received the Path Switch Request Acknowledge message, and any ongoing data forwarding can continue after the release (TS 36.300 clause 10.1.2.1.1, steps 17 and 18, TS 36.423 clause 8.2.3, and TS 23.401 clause 5.5.1.1.2, step 7).

If the S-GW is relocated, the MME releases the bearers in the source S-GW when the timer of step 10 expires, by sending a GTPv2 Delete Session Request message over S11. The message carries the Indication Flags, with the Operation Indication flag not set, so that the source S-GW doesn’t start a deletion towards the P-GW, and with the Scope Indication flag set to 1, together with the User Location Information. The message carries a Cause only when it also deactivates ISR, with the value "ISR deactivation" (TS 23.401 clause 5.5.1.1.3, step 7, and TS 29.274 clause 7.2.9.1).

The source S-GW releases the bearer resources of the UE and returns a GTPv2 Delete Session Response message to the MME over S11, with the Cause set to "Request accepted" (16) (TS 29.274 clause 7.2.10).

The source eNodeB decides on an S1-based handover and sends an S1AP Handover Required message to the source MME over S1-MME. The message carries the MME UE S1AP ID, the eNB UE S1AP ID, the Handover Type set to "IntraLTE", the Cause, for example "Handover desirable for radio reasons", the Target ID, with the Global eNB ID of the target eNodeB and the selected TAI, the Direct Forwarding Path Availability, and the Source to Target Transparent Container (TS 36.413 clauses 9.1.5.1 and 9.2.1.6). For a handover within E-UTRAN, the container is the Source eNB to Target eNB Transparent Container, with the RRC Container, the E-RABs Information List, which marks the E-RABs that the source eNodeB proposes for downlink forwarding, the Target Cell ID (ECGI), the Subscriber Profile ID for RAT/Frequency priority, and the UE History Information (TS 36.413 clause 9.2.1.7). The container also carries the AS ciphering and integrity algorithms of the source cell (TS 33.401 clause 7.2.8.4.3).

The source MME selects the target MME and sends a GTPv2 Forward Relocation Request message to the target MME over S10 (TS 23.401 clause 5.5.1.2.2, step 3). On receiving the Handover Required message, the source MME has increased its locally kept NCC by one and computed a fresh NH (TS 33.401 clause 7.2.8.4.3). The message carries the IMSI, the Sender’s F-TEID for Control Plane, the Target Identification, the SGW S11/S4 IP Address and TEID for Control Plane, the S1-AP Cause, which holds the cause that the source eNodeB has sent, and the E-UTRAN Transparent Container, which holds the Source to Target Transparent Container. It also carries the MME/SGSN/AMF UE MM Context, with the EPS security context, that is the KASME, its eKSI, and the fresh {NH, NCC} pair, and the UE Network Capability, and the MME/SGSN/AMF UE EPS PDN Connections, with the APN, the PGW S5/S8 IP Address for Control Plane, and the bearer contexts, with the EBI, the Bearer Level QoS, the SGW S1/S4/S12 IP Address and TEID for user plane, and the PGW S5/S8 IP Address and TEID for user plane (TS 29.274 clause 7.3.1, and TS 33.401 clause 7.2.8.4.3).

If the target MME relocates the S-GW, the target MME sends a GTPv2 Create Session Request message to the target S-GW over S11, one message for each PDN connection (TS 23.401 clause 5.5.1.2.2, step 4). The message carries the IMSI, the Sender F-TEID for Control Plane of the target MME, the PGW S5/S8 Address for Control Plane, the RAT Type, the Serving Network, the Indication Flags, and the Bearer Contexts to be created, with the EBI, the Bearer Level QoS, and the S5/S8-U PGW F-TEID (TS 29.274 clause 7.2.1). Unlike the X2-based handover, the message doesn’t carry the User Location Information, which the target MME sends later in the Modify Bearer Request message (TS 29.274 clause 7.2.1, and TS 23.401 clause 5.5.1.2.2, steps 4 and 15).

If the S-GW is relocated, the target S-GW allocates one S1-U uplink TEID for each bearer and returns a GTPv2 Create Session Response message to the target MME over S11. The message carries the Cause set to "Request accepted" (16), the Sender F-TEID for Control Plane of the target S-GW, and the Bearer Contexts created, with the EBI, the Cause, and the S1-U SGW F-TEID of each bearer (TS 29.274 clause 7.2.2).

The target MME sends an S1AP Handover Request message to the target eNodeB to create the UE context there (TS 23.401 clause 5.5.1.2.2, step 5). The message carries the MME UE S1AP ID, the Handover Type set to "IntraLTE", the Cause, the UE-AMBR, the E-RABs To Be Setup List, with the E-RAB ID, the transport layer address and the GTP-TEID of the S-GW for the uplink, and the E-RAB Level QoS Parameters, such as the QCI and the ARP, the Source to Target Transparent Container, the UE Security Capabilities, the Handover Restriction List, if available, and the Security Context, with the {NH, NCC} pair from the source MME (TS 36.413 clause 9.1.5.4, and TS 33.401 clause 7.2.8.4.3).

The target eNodeB performs admission control for the requested E-RABs and returns an S1AP Handover Request Acknowledge message to the target MME. The message carries the MME UE S1AP ID, the eNB UE S1AP ID, the E-RABs Admitted List, with the E-RAB ID, the transport layer address and the GTP-TEID of the target eNodeB for the downlink, and, if forwarding applies, the DL and UL transport layer addresses and GTP-TEIDs for the forwarded data, the E-RABs Failed to Setup List, and the Target to Source Transparent Container (TS 36.413 clause 9.1.5.5). The container holds the RRC handover command for the UE, with the NCC from the received {NH, NCC} pair (TS 33.401 clause 7.2.8.4.3).

The target MME returns a GTPv2 Forward Relocation Response message to the source MME over S10. The message carries the Cause set to "Request accepted" (16), the Sender’s F-TEID for Control Plane of the target MME, the Indication Flags, with the SGW Change Indication set to 1 if the target MME has selected a new S-GW, the List of Set-up Bearers, with the EBI and the eNodeB F-TEID for DL data forwarding or, with indirect forwarding and S-GW relocation, the SGW F-TEID for DL data forwarding, and the E-UTRAN Transparent Container, which holds the Target to Source Transparent Container (TS 29.274 clause 7.3.2).

The source MME sends an S1AP Handover Command message to the source eNodeB. The message carries the MME UE S1AP ID, the eNB UE S1AP ID, the Handover Type, the E-RABs Subject to Forwarding List, with the E-RAB ID and the DL and UL transport layer addresses and GTP-TEIDs for data forwarding, the E-RABs to Release List, and the Target to Source Transparent Container (TS 36.413 clause 9.1.5.2, and TS 23.401 clause 5.5.1.2.2, step 9).

The source eNodeB builds the handover command from the Target to Source Transparent Container and sends it to the UE in an RRC RRCConnectionReconfiguration message (TS 23.401 clause 5.5.1.2.2, step 9a). The message carries the mobilityControlInfo field, with the target physical cell identity, the carrier frequency, timer T304, the new C-RNTI, and, optionally, a dedicated random access preamble, the radioResourceConfigDedicated field, and the securityConfigHO field, with the security algorithm configuration and the NCC (TS 36.331 clause 6.2.2). The UE removes any EPS bearer for which it hasn’t received a radio bearer in the target cell, derives the new KeNB, and accesses the target cell (TS 23.401 clause 5.5.1.2.2, step 9a, and TS 33.401 clause 7.2.8.4.4).

The source eNodeB sends an S1AP eNB Status Transfer message to the source MME, carrying the MME UE S1AP ID, the eNB UE S1AP ID, and the eNB Status Transfer Transparent Container (TS 36.413 clause 9.1.5.13). The container holds the E-RABs Subject to Status Transfer List, with the E-RAB ID, the UL COUNT value, and the DL COUNT value of each E-RAB for which PDCP-SN and HFN status preservation applies (TS 36.413 clause 8.4.6). The source eNodeB can omit the message when none of the E-RABs needs PDCP status preservation (TS 23.401 clause 5.5.1.2.2, step 10).

Because the MME is relocated, the source MME sends the status information to the target MME in a GTPv2 Forward Access Context Notification message over S10 (TS 23.401 clause 5.5.1.2.2, step 10). The message carries the E-UTRAN Transparent Container, which holds the eNB Status Transfer Transparent Container (TS 29.274 clause 7.3.10).

The target MME acknowledges the status information by returning a GTPv2 Forward Access Context Acknowledge message to the source MME, with the Cause set to "Request accepted" (16) (TS 29.274 clause 7.3.11).

The target MME sends the status information to the target eNodeB in an S1AP MME Status Transfer message, carrying the MME UE S1AP ID, the eNB UE S1AP ID, and the eNB Status Transfer Transparent Container (TS 36.413 clause 9.1.5.14).

After the UE has synchronized to the target cell, it sends an RRC RRCConnectionReconfigurationComplete message to the target eNodeB, carrying the RRC Transaction Identifier (TS 36.331 clause 6.2.2). The target eNodeB can then deliver the forwarded downlink data to the UE, and the uplink data of the UE flows to the target S-GW and on to the P-GW (TS 23.401 clause 5.5.1.2.2, step 12).

The target eNodeB informs the target MME that the UE has arrived in the target cell by sending an S1AP Handover Notify message, which carries the MME UE S1AP ID, the eNB UE S1AP ID, the E-UTRAN CGI, and the TAI of the target cell (TS 36.413 clause 9.1.5.7).

The target MME sends a GTPv2 Forward Relocation Complete Notification message to the source MME over S10 (TS 23.401 clause 5.5.1.2.2, step 14). The message has no mandatory IE, and it carries the Indication Flags IE only when one of its flags is set to 1 (TS 29.274 clause 7.3.3).

The source MME returns a GTPv2 Forward Relocation Complete Acknowledge message to the target MME, with the Cause set to "Request accepted" (16) (TS 29.274 clause 7.3.4). The source MME starts a timer that supervises the release of the resources in the source eNodeB and, if the S-GW is relocated, in the source S-GW (TS 23.401 clause 5.5.1.2.2, step 14).

The target MME sends a GTPv2 Modify Bearer Request message to the target S-GW over S11, one message for each PDN connection (TS 23.401 clause 5.5.1.2.2, step 15). When the S-GW isn’t relocated, the target S-GW is the source S-GW. The message carries the Bearer Contexts to be modified, with the EBI and the S1 eNodeB F-TEID of the target eNodeB for the downlink, and, when they apply, the User Location Information, the Serving Network, and the User CSG Information. When the S-GW isn’t relocated, the message also carries the Sender F-TEID for Control Plane of the target MME (TS 29.274 clause 7.2.7).

If the S-GW is relocated, the target S-GW allocates the downlink TEIDs for the traffic from the P-GW and sends a GTPv2 Modify Bearer Request message to the P-GW over S5/S8, one message for each PDN connection. The message carries the Sender F-TEID for Control Plane of the target S-GW, the Bearer Contexts to be modified, with the EBI and the S5/8-U SGW F-TEID, and, when the MME has provided it, the User Location Information (TS 23.401 clause 5.5.1.2.2, step 16, and TS 29.274 clause 7.2.7).

If the S-GW is relocated, the P-GW updates its context, switches the downlink path to the target S-GW, and returns a GTPv2 Modify Bearer Response message to the target S-GW. The message carries the Cause set to "Request accepted" (16), the MSISDN, if the P-GW has stored it, and the Bearer Contexts modified, with the Charging ID of each bearer (TS 29.274 clause 7.2.8). Immediately after switching the path, the P-GW sends one or more end marker packets on the old path, and the source S-GW forwards these packets to the source eNodeB (TS 23.401 clause 5.5.1.2.2, step 16).

The target S-GW returns a GTPv2 Modify Bearer Response message to the target MME over S11. The message carries the Cause set to "Request accepted" (16), the Bearer Contexts modified, with the EBI, the Cause, and the S1-U SGW F-TEID of each bearer, and, when applicable, the Change Reporting Action (TS 29.274 clause 7.2.8). If the S-GW isn’t relocated, the S-GW sends one or more end marker packets on the old path immediately after switching the path (TS 23.401 clause 5.5.1.2.2, step 17).

When the timer of step 17 expires, the source MME sends an S1AP UE Context Release Command message to the source eNodeB, carrying the UE S1AP IDs and a Cause, for example "Successful Handover" (TS 23.401 clause 5.5.1.2.2, step 19, and TS 36.413 clauses 9.1.4.6 and 9.2.1.3).

The source eNodeB releases its resources for the UE and returns an S1AP UE Context Release Complete message to the source MME (TS 23.401 clause 5.5.1.2.2, step 19, and TS 36.413 clause 9.1.4.7).

If the source MME has received the SGW Change Indication in the Forward Relocation Response message, the source MME deletes the bearer resources in the source S-GW when the timer of step 17 expires, by sending a GTPv2 Delete Session Request message over S11. The message carries the Indication Flags, with the Operation Indication flag not set, so that the source S-GW doesn’t start a deletion towards the P-GW, and with the Scope Indication flag set to 1, together with the User Location Information. The message carries a Cause only when it also deactivates ISR, with the value "ISR deactivation" (TS 23.401 clause 5.5.1.2.2, step 19, and TS 29.274 clause 7.2.9.1).

The source S-GW releases the bearer resources of the UE and returns a GTPv2 Delete Session Response message to the source MME, with the Cause set to "Request accepted" (16) (TS 29.274 clause 7.2.10).

The source eNodeB decides on an S1-based handover, for example because it has no X2 connectivity to the target eNodeB, and sends an S1AP Handover Required message to the MME over S1-MME (TS 23.401 clause 5.5.1.2.2, steps 1 and 2). The message carries the MME UE S1AP ID, the eNB UE S1AP ID, the Handover Type set to "IntraLTE", the Cause, for example "Handover desirable for radio reasons", the Target ID, with the Global eNB ID of the target eNodeB and the selected TAI, the Direct Forwarding Path Availability, and the Source to Target Transparent Container (TS 36.413 clauses 9.1.5.1 and 9.2.1.6). The container holds the RRC Container, the E-RABs Information List, the Target Cell ID (ECGI), the Subscriber Profile ID for RAT/Frequency priority, and the UE History Information (TS 36.413 clause 9.2.1.7), and the AS ciphering and integrity algorithms of the source cell (TS 33.401 clause 7.2.8.4.3).

If the MME relocates the S-GW, for example because the target cell is outside the service area of the current S-GW, the MME selects a target S-GW and sends a GTPv2 Create Session Request message to the target S-GW over S11, one message for each PDN connection (TS 23.401 clause 5.5.1.2.2, step 4). The message carries the IMSI, the Sender F-TEID for Control Plane of the MME, the PGW S5/S8 Address for Control Plane, the RAT Type, the Serving Network, the Indication Flags, and the Bearer Contexts to be created, with the EBI, the Bearer Level QoS, and the S5/S8-U PGW F-TEID (TS 29.274 clause 7.2.1). The message doesn’t carry the User Location Information, which the MME sends later in the Modify Bearer Request message (TS 29.274 clause 7.2.1, and TS 23.401 clause 5.5.1.2.2, steps 4 and 15). The S-GW selection function selects an S-GW that serves the location of the UE (TS 23.401 clause 4.3.8.2).

If the S-GW is relocated, the target S-GW allocates one S1-U uplink TEID for each bearer and returns a GTPv2 Create Session Response message to the MME over S11. The message carries the Cause set to "Request accepted" (16), the Sender F-TEID for Control Plane of the target S-GW, and the Bearer Contexts created, with the EBI, the Cause, and the S1-U SGW F-TEID of each bearer (TS 29.274 clause 7.2.2).

On receiving the Handover Required message, the MME has increased its locally kept NCC by one and computed a fresh NH (TS 33.401 clause 7.2.8.4.3). The MME sends an S1AP Handover Request message to the target eNodeB to create the UE context there. The message carries the MME UE S1AP ID, the Handover Type set to "IntraLTE", the Cause, the UE-AMBR, the E-RABs To Be Setup List, with the E-RAB ID, the transport layer address and the GTP-TEID of the S-GW for the uplink, and the E-RAB Level QoS Parameters, such as the QCI and the ARP, the Source to Target Transparent Container, the UE Security Capabilities, the Handover Restriction List, if available, and the Security Context, with the fresh {NH, NCC} pair (TS 36.413 clause 9.1.5.4).

The target eNodeB performs admission control for the requested E-RABs and returns an S1AP Handover Request Acknowledge message to the MME. The message carries the MME UE S1AP ID, the eNB UE S1AP ID, the E-RABs Admitted List, with the E-RAB ID, the transport layer address and the GTP-TEID of the target eNodeB for the downlink, and, if forwarding applies, the DL and UL transport layer addresses and GTP-TEIDs for the forwarded data, the E-RABs Failed to Setup List, and the Target to Source Transparent Container (TS 36.413 clause 9.1.5.5). The container holds the RRC handover command for the UE, with the NCC from the {NH, NCC} pair (TS 33.401 clause 7.2.8.4.3).

If indirect forwarding applies, the MME sends a GTPv2 Create Indirect Data Forwarding Tunnel Request message to the target S-GW, which is the S-GW that serves the UE when the S-GW isn’t relocated (TS 23.401 clause 5.5.1.2.2, steps 6 and 8). The message carries the Bearer Contexts, with the EBI and the eNodeB F-TEID for DL data forwarding of the target eNodeB. It carries the Sender F-TEID for Control Plane only when the S-GW that the MME selects for indirect forwarding isn’t the S-GW that anchors the bearers of the UE (TS 29.274 clause 7.2.18).

If indirect forwarding applies, the S-GW allocates the forwarding tunnels and returns a GTPv2 Create Indirect Data Forwarding Tunnel Response message to the MME. The message carries the Cause, the Bearer Contexts, with the EBI, the Cause, and the S-GW F-TEID for the forwarded data, and the Sender F-TEID for Control Plane if the request carried one (TS 29.274 clause 7.2.19).

If indirect forwarding applies and the S-GW is relocated, the MME also sends a GTPv2 Create Indirect Data Forwarding Tunnel Request message to the source S-GW. The Bearer Contexts of this message carry the SGW F-TEID for DL data forwarding that the target S-GW has allocated in step 7 (TS 23.401 clause 5.5.1.2.2, step 8, and TS 29.274 clause 7.2.18).

The source S-GW allocates the forwarding tunnels and returns a GTPv2 Create Indirect Data Forwarding Tunnel Response message to the MME, with the Cause and the forwarding F-TEIDs of the source S-GW (TS 29.274 clause 7.2.19).

The MME sends an S1AP Handover Command message to the source eNodeB. The message carries the MME UE S1AP ID, the eNB UE S1AP ID, the Handover Type, the E-RABs Subject to Forwarding List, with the E-RAB ID and the DL and UL transport layer addresses and GTP-TEIDs for data forwarding, which belong to the target eNodeB or to the S-GW, the E-RABs to Release List, and the Target to Source Transparent Container (TS 36.413 clause 9.1.5.2, and TS 23.401 clause 5.5.1.2.2, step 9).

The source eNodeB builds the handover command from the Target to Source Transparent Container and sends it to the UE in an RRC RRCConnectionReconfiguration message. The message carries the mobilityControlInfo field, with the target physical cell identity, the carrier frequency, timer T304, the new C-RNTI, and, optionally, a dedicated random access preamble, the radioResourceConfigDedicated field, and the securityConfigHO field, with the security algorithm configuration and the NCC (TS 36.331 clause 6.2.2). The UE derives the new KeNB and accesses the target cell (TS 33.401 clause 7.2.8.4.4).

The source eNodeB sends an S1AP eNB Status Transfer message to the MME, carrying the MME UE S1AP ID, the eNB UE S1AP ID, and the eNB Status Transfer Transparent Container, with the E-RABs Subject to Status Transfer List, that is the E-RAB ID, the UL COUNT value, and the DL COUNT value of each E-RAB for which PDCP-SN and HFN status preservation applies (TS 36.413 clauses 8.4.6 and 9.1.5.13). The source eNodeB can omit the message when none of the E-RABs needs PDCP status preservation (TS 23.401 clause 5.5.1.2.2, step 10).

Because the MME isn’t relocated, the MME sends the status information directly to the target eNodeB in an S1AP MME Status Transfer message, without any S10 signaling. The message carries the MME UE S1AP ID, the eNB UE S1AP ID, and the eNB Status Transfer Transparent Container (TS 23.401 clause 5.5.1.2.2, step 10, and TS 36.413 clause 9.1.5.14).

After the UE has synchronized to the target cell, it sends an RRC RRCConnectionReconfigurationComplete message to the target eNodeB, carrying the RRC Transaction Identifier (TS 36.331 clause 6.2.2).

The target eNodeB informs the MME that the UE has arrived in the target cell by sending an S1AP Handover Notify message, which carries the MME UE S1AP ID, the eNB UE S1AP ID, the E-UTRAN CGI, and the TAI of the target cell (TS 36.413 clause 9.1.5.7). The MME starts a timer that supervises the release of the resources in the source eNodeB and, if the S-GW is relocated, in the source S-GW (TS 23.401 clause 5.5.1.2.2, step 14).

The MME sends a GTPv2 Modify Bearer Request message to the target S-GW over S11, one message for each PDN connection (TS 23.401 clause 5.5.1.2.2, step 15). The message carries the Bearer Contexts to be modified, with the EBI and the S1 eNodeB F-TEID of the target eNodeB for the downlink, and, when they apply, the User Location Information, the Serving Network, and the User CSG Information (TS 29.274 clause 7.2.7).

If the S-GW is relocated, the target S-GW allocates the downlink TEIDs for the traffic from the P-GW and sends a GTPv2 Modify Bearer Request message to the P-GW over S5/S8, one message for each PDN connection. The message carries the Sender F-TEID for Control Plane of the target S-GW, the Bearer Contexts to be modified, with the EBI and the S5/8-U SGW F-TEID, and, when the MME has provided it, the User Location Information (TS 23.401 clause 5.5.1.2.2, step 16, and TS 29.274 clause 7.2.7).

If the S-GW is relocated, the P-GW updates its context, switches the downlink path to the target S-GW, and returns a GTPv2 Modify Bearer Response message to the target S-GW. The message carries the Cause set to "Request accepted" (16), the MSISDN, if the P-GW has stored it, and the Bearer Contexts modified, with the Charging ID of each bearer (TS 29.274 clause 7.2.8). Immediately after switching the path, the P-GW sends one or more end marker packets on the old path, and the source S-GW forwards these packets to the source eNodeB (TS 23.401 clause 5.5.1.2.2, step 16).

The target S-GW returns a GTPv2 Modify Bearer Response message to the MME over S11. The message carries the Cause set to "Request accepted" (16), the Bearer Contexts modified, with the EBI, the Cause, and the S1-U SGW F-TEID of each bearer, and, when applicable, the Change Reporting Action (TS 29.274 clause 7.2.8). If the S-GW isn’t relocated, the S-GW sends one or more end marker packets on the old path immediately after switching the path (TS 23.401 clause 5.5.1.2.2, step 17).

When the timer of step 15 expires, the MME sends an S1AP UE Context Release Command message to the source eNodeB, carrying the UE S1AP IDs and a Cause, for example "Successful Handover" (TS 23.401 clause 5.5.1.2.2, step 19, and TS 36.413 clauses 9.1.4.6 and 9.2.1.3).

The source eNodeB releases its resources for the UE and returns an S1AP UE Context Release Complete message to the MME (TS 23.401 clause 5.5.1.2.2, step 19, and TS 36.413 clause 9.1.4.7).

If the S-GW is relocated, the MME deletes the bearer resources in the source S-GW when the timer of step 15 expires, by sending a GTPv2 Delete Session Request message over S11. The message carries the Indication Flags, with the Operation Indication flag not set, so that the source S-GW doesn’t start a deletion towards the P-GW, and with the Scope Indication flag set to 1, together with the User Location Information. The message carries a Cause only when it also deactivates ISR, with the value "ISR deactivation" (TS 23.401 clause 5.5.1.2.2, step 19, and TS 29.274 clause 7.2.9.1).

The source S-GW releases the bearer resources of the UE and returns a GTPv2 Delete Session Response message to the MME, with the Cause set to "Request accepted" (16) (TS 29.274 clause 7.2.10).