It may be worthwhile brushing up the LTE/SAE Interfaces and Architecture before proceeding.
1) Overview of Handover Operation
With EPC, continuous communication is possible, even while the terminal switches from one type of radio access system to another.
Specifically, in order to achieve the internal network path switching required to change radio access systems, the S-GW provides a mobility management anchor function for handover between 3GPP radio access systems, and the P-GW provides the function for handover between 3GPP and non-3GPP radio access systems. In this way, the IP address does not change when the terminal switches radio access systems, and communications can continue after handover.
In handover between the 3GPP radio access systems, LTE and 3G, handover preparation is done before changing systems, including tasks such as securing resources on the target radio access system, through cooperation between the radio access systems (Figure 3 (a)(A)). Then, when the actual switch occurs, only the network path needs to be switched, reducing handover processing time (Fig.3 (a)(B)). Also, loss of data packets that arrive at the pre-switch access point during handover can be avoided using a data forwarding function (Fig.3 (b)).
In this way, through interaction between radio access systems, fast handover without packet loss is possible, even between radio access systems such as LTE and 3G which cannot be used simultaneously.
2) Handover Preparation Procedure (Fig.3 (a)(A))
The handover preparation procedure for switching radio access from LTE to 3G is shown in Figure 4.
Step (1):The terminal sends a radio quality report containing the handover candidate base-stations and other information to the eNodeB. The eNodeB decides whether handover shall be performed based on the information in the report, identifies the base station and RNC to switch to, and begins handover preparation.
Steps (2) to (3): The eNodeB sends a handover required to the MME, sending the RNC identifier and transmission control information for the target radio access system. The MME identifies the SGSN connected to the target RNC based on the received RNC identifier and sends the communication control and other information it received from the eNodeB to the SGSN in a forward relocation request signal. The information required to configure the communications path between the S-GW and SGSN, which is used for data transmission after the MME has completed the handover, is sent at the same time.
Steps (4) to (5): The SGSN forwards the relocation request to the RNC, notifying it of the communications control information transmitted from the eNodeB. The RNC performs the required radio configuration processing based on the received information and sends a relocation response to the SGSN. Note that through this process, a 3G radio access bearer is prepared between the SGSN and RNC.
Step (6): The SGSN sends a forward relocation response to the MME in order to notify it that relocation procedure has completed. This signal also includes data issued by the SSGN and required to configure a communications path from the S-GW to the SGSN, to be used for data forwarding.
Steps (7) to (8): The MME sends a create indirect data forwarding tunnel request to the S-GW, informing it of the information issued by the SSGN that it just received. From the information that the S-GW receives, it establishes a communications path from the S-GW to the SGSN for data forwarding and sends a create indirect data forwarding tunnel response to the MME.
Through this handover preparation, target 3G radio-access resources are readied, the radio access bearer between the SGSN and RNC is configured, and the data forwarding path from the
S-GW to the SGSN configuration is completed.
3) Handover Procedure for Radio Access System Switching (Fig. 3(a)(B)):
The handover process after switching radio access system is shown in Figure 5.
Steps (1) to (2): When the handover preparation described in Fig.4 is completed, the MME sends a handover command to the eNodeB. When it receives this signal, the eNodeB sends a handover from LTE command for the terminal to switch radio systems. Note that when the eNodeB receives the handover command from the MME, it begins forwarding data packets received from the S-GW. Thereafter, packets for the terminal that arrive at the S-GW are forwarded to the terminal by the path: S-GW, eNodeB, S-GW, SGSN, RNC.
Steps (3) to (6): The terminal switches to 3G and when the radio link configuration is completed, notification that it has connected to the 3G radio access system is sent over each of the links through to the MME: from terminal to RNC, from RNC to SGSN, and from SGSN to MME. This way, the MME can perform Step (10) described below to release the eNodeB resources after a set period of time has elapsed.
Step (7): The MME sends a forward relocation complete acknowledgement to the SGSN. A set period of time after receiving this signal, the SGSN releases the resources related to data forwarding.
Step (8): The SGSN sends a modify bearer request to the S-GW to change from the communications path before the handover, between the S-GW and eNodeB, to one between the S-GW and SGSN. This signal contains information elements required to configure the path from S-GW to SGSN, including those issued by the SGSN. When the S-GW receives this signal, it configures a communications path from the S-GW to the SGSN. In this way, the communications path becomes: S-GW, SGSN, RNC, terminal; and data transmission to the target 3G radio access system begins.
Note that after this point, data forwarding is no longer needed, so the S-GW sends a packet to the eNodeB with an “End Marker” attached, and when the eNodeB receives this packet, it releases its resources related to data forwarding.
Steps (9) to (10): The S-GW sends a modify bearer response to the SGSN, indicating that handover procedure has completed. The MME also releases eNodeB resources that are no longer needed.
Through this handover procedure, data is forwarded during the handover, the switch of radio access bearer is completed, and the communications path from the P-GW to the terminal is updated.
In the examples above, we described the handover procedure between 3GPP radio access systems in which the S-GW did not change, but handovers with S-GW relocation are also possible. In these cases, the P-GW provides the anchor function for path switching, as with switches to non-3GPP access systems.
Anchor function: A function which switches the communications path according to the area where the terminal is located, and forwards packets for the terminal to that area.
Relocation: Switching communications equipment such as area switches during communication.