KR100838199B1 - Method for data transfer in handover between RASs in WiBro system - Google Patents

Abstract

The present invention relates to a data transmission method for handover between base stations.

When the mobile station performs handover between base stations in the mobile communication system, the gateway transmits to the mobile terminal by using the Internet Group Managemet Protocol (IGMP) to the serving base station currently serving the mobile terminal and the target base station to be handed over. The data traffic to be transmitted is transmitted by a bi-casting method.

Therefore, when the mobile station performs the handover between the base stations, the gateway transmits data traffic by the bicasting method, thereby efficiently using the resources of the user plane connection interval between the gateway and the base station, and that is, downlink data traffic Therefore, the loss of data can be minimized.

Handover, Data Pathing, Base Station, Data Loss, User Plane

Description

Method for data transfer in base station handover {Method for data transfer in handover between RASs in WiBro system}

1 is a structural diagram of a general mobile communication system.

2 is a protocol structural diagram of a general mobile communication system.

3 is an exemplary diagram illustrating a data connection structure between a general gateway and a base station.

4 is an exemplary diagram illustrating a general inter-base station handover.

5 and 6 are packet structure diagrams for the IGMP according to an embodiment of the present invention.

7 is an exemplary diagram for defining an IGMP message according to an embodiment of the present invention.

8 is a flowchart illustrating a data transmission procedure during handover between base stations according to an embodiment of the present invention.

9 is a flowchart illustrating an operation procedure of a gateway for data transfer in a base station handover procedure according to an embodiment of the present invention.

10 is an exemplary diagram illustrating SAE bearer context information according to an embodiment of the present invention.

The present invention relates to a handover, and more particularly, to a method for transmitting data during handover between base stations.

In general, in the next generation mobile communication system (3GPP), when the mobile terminal performs a handover between the base stations, the gateway is a user plane connection interval between the gateway and the base station used to provide a flexible service to the base station (hereinafter referred to as "S1 interface"). To transmit data traffic. However, when data traffic is transmitted from the gateway to the base station using the S1 interface, a bicasting transmission method of transmitting data traffic to both a serving base station providing a service to the mobile terminal and a target base station to which the mobile terminal is to be handed over is used. There is overhead in casting.

In addition, when a handover occurs between a plurality of base stations, the mobile terminal has a long forwarding path, and thus a packet transmission delay occurs until the target base station targeted for handover reorders the packets. In order to prevent such a packet transmission delay, a method of switching in a gateway has emerged, but there is a problem in that a method for reducing packet loss such as buffering of the gateway must be additionally considered.

Accordingly, when a mobile terminal performs a handover between base stations in a mobile communication system, it provides a data transmission method that can minimize the loss of data to be transmitted to the mobile terminal.

In another aspect of the present invention, there is provided a method for transmitting data during base station handover in a mobile communication system.

Receiving a handover request message including target base station information from a serving base station, and forwarding a handover request message including multicast group information to the target base station based on the handover request message; Receive a message informing that the target base station is joined to the multicast group, where the message includes updated multicast group information including the target base station information and receives data traffic to the serving base station and the target base station. Transmitting each; And receiving a handover completion message from the target base station indicating that handover from the serving base station to the target base station included in the updated multicast group information is completed, and transmitting data traffic to the mobile terminal through the target base station. Transmitting.

In another aspect of the present invention, a method for transmitting data during base station handover in a method for transmitting data during base station handover in a mobile communication system includes:

The target base station receiving a handover request message including multicast group information from the gateway; The message indicating that the target base station joins the multicast group including the mobile terminal and the serving base station based on the handover request message, wherein the message includes the multicast group information updated to include the target base station information. Sending-to the gateway; The target base station receiving data traffic from the gateway and buffering the received data traffic until the mobile terminal completes a handover; Receiving, by the target base station, a base station context data message from the serving base station, and performing a synchronization procedure and a wireless section setting procedure with the mobile terminal; And when the target base station receives the handover completion message from the mobile terminal, transmitting the buffered data traffic to the mobile terminal based on the base station context data message.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention. Like parts are designated by like reference numerals throughout the specification.

In addition, when a part is said to "include" a certain component, which means that it may further include other components, except to exclude other components unless otherwise stated.

1 is a structural diagram of a general mobile communication system.

The system structure diagram shown in FIG. 1 illustrates the structure of an IP-based next generation mobile communication system of a third generation (3GE) system, which is a field of a mobile communication system, as an embodiment. Here, the IP-based next generation mobile communication system refers to a system using a wireless access method such as orthogonal frequency division multiplexing (OFDM). The third generation system is largely connected to a user equipment (UE) 10, a mobile base station (eNB), or an "base station" 30, and a gateway (aGW) 40 interconnected. It is.

The mobile terminal 10 may be further classified into a mobile terminal (MT) that is in charge of a wireless access function and a protocol function and a terminal equipment (TE) 20 that is responsible for a service function. It is in charge of wireless connection and protocol functions. The gateway may be divided into an MME (Mobility Management Entity) in charge of protocol and control function and a UPE (User Plane Entity) in charge of data traffic processing function.

Next, a structure of a protocol operating in each node of the mobile communication system mentioned in FIG. 1 will be described with reference to FIG. 2. The protocol structure of FIG. 2 will be described by dividing the protocol stack of the control plane as shown in FIG. 2A at the top and the protocol stack of the user plane shown in FIG. 2B at the bottom.

2 is a protocol structural diagram of a general mobile communication system.

In particular, FIG. 2A illustrates a protocol stack of a control plane among protocol structures of a general mobile communication system, and FIG. 2B illustrates a user plane protocol of a mobile communication system among protocol structures of a general mobile communication system. It is a figure which shows a stack.

 As shown in FIG. 2A, the protocol stack of the control plane may be divided into a protocol stack structure of the mobile terminal 10, a protocol stack structure of the base station 30, and a protocol stack structure of the gateway 40. First, the protocol stack of the mobile terminal 10 includes a physical (PHY), a media access control (MAC), a radio link control (RLC), a radio resource control (RRC), a packet mobility management (PMM), and a session management (SM). Doing.

In addition, the protocol stack of the base station 30 is PHY, L1 (Layer 1), L2 (Layer 2), MAC, IP, RLC. Stream Control Transmission Protocol (SCTP), RRC, Radio Access Network Application Part (RANAP) and Relay (Relay), and the protocol stack of the gateway 40 is L1, L2, IP, SCTP, RANAP, PMM and SM. It is included. Such a control plane is generally a standardized UMTS protocol stack and can be understood by those of ordinary skill in the art to which the present invention pertains.

As shown in FIG. 2B, a protocol stack of a user plane may be divided into a protocol stack structure of a mobile terminal 10, a protocol stack structure of a base station, and a protocol stack structure of a gateway, similarly to the control plane. Here, the user plane protocol is already known, and a detailed description thereof will be omitted in embodiments of the present invention.

In the method of transmitting data traffic during base station handover described in the embodiment of the present invention, the data traffic through evolved GPRS Tunneling Protocol (eGTP) which is a user plane protocol between the gateway and the base station in the protocol structure of the mobile communication system of FIG. This is done through the transfer. Before describing data traffic transmission through eGTP according to an embodiment of the present invention, a data connection structure of a general gateway 40 and a base station 30 will be described with reference to FIG. 3.

3 is an exemplary diagram illustrating a data connection structure between a general gateway and a base station.

In general, the user plane access point between the gateway and the base station is defined as S1, and S1 is responsible for transmitting up / down data traffic to the serving base station where the mobile terminal receives the IP-based packet from the upper layer. . 3 is a diagram illustrating the S1, and illustrates a structure for the S1 interface.

Data traffic received from the base station 30 through the S1 interface is transmitted to the mobile terminal using a radio bearer (RB), which is preset, for this purpose, the gateway must manage a SAE bearer context (System Architecture Evolution Bearer Context). do.

Next, a handover between base stations during handover of a mobile communication system will be described with reference to FIG. 4. Herein, the third generation system of the mobile communication system will be described as an example, and is not necessarily limited to the third generation system.

4 is an exemplary diagram illustrating handover between base stations in a general mobile communication system.

As shown in FIG. 4, a handover of a mobile communication system generally refers to a process of allowing a mobile terminal 10 to move while maintaining a smooth call between base stations 31 and 32. That is, the handover situation between the base stations 31 and 32 means that the mobile terminal 10 moves from the serving base station 31 linked to the gateway 40 to the target base station 32 so that handover occurs.

When a handover occurs, the determination of the handover is performed by the serving base station 31 by receiving a periodic measurement report of the mobile terminal 10, and the target base station 32 and the handover based on the received information. Is determined. In the current standardization of 3GPP, data traffic transmission method using S1 interface is proposed as follows in order to minimize loss of data traffic during service and provide flexible service during handover between mobile communication systems.

1) Bi-casting at the gateway

By-casting in the gateway is a method of bicasting data traffic by connecting an S1 connection from the gateway to the serving base station and the target base station when the mobile terminal performs the handover between the base stations. By using this method, since there is no data traffic forwarding between the serving base station and the target base station, the load due to forwarding is reduced. However, resource consumption between S1 is severe, and an overhead for data traffic transmission occurs simultaneously at the gateway to the serving base station and the target base station.

2) Forwarding of data traffic from serving base station to target base station

Forwarding of data traffic from the serving base station to the target base station is a method of establishing a tunnel between the serving base station and the target base station and forwarding the data traffic through the tunnel. This method consumes less resources than bicasting between S1 connections. However, the cost of reordering the packets received by the target base station is high, and the forwarding path becomes long when a handover occurs between a plurality of base stations. In addition, a packet transmission delay occurs due to the rearrangement of packets received by the target base station.

3) Switching at the Gateway

Switching at the gateway is a method of switching the S1 path at the gateway when a handover occurs. This method does not have packet forwarding, but a method for reducing packet loss such as buffering of the gateway should be additionally considered. In addition, a packet transmission delay occurs due to the buffering.

An embodiment of the present invention proposes a method for minimizing data traffic loss and reducing the complexity of an operation of a base station when performing handover between base stations. To this end, when the mobile terminal described with reference to FIG. 4 performs handover between base stations, data traffic between the gateway and the base station is transmitted using a bicasting method.

In other words, in the general scheme, after the handover is determined, the gateway sets up respective unicast tunnels for the serving base station and the target base station. However, in the embodiment of the present invention, the gateway establishes a multicast group for the serving base station and the target base station, and transmits data traffic by a bicasting method to the serving base station and the target base station included in the multicast group.

In addition, using the eGTP protocol shown in the user plane of Figure 2 performs a data connection between the gateway and the base station through the S1 interface, since the eGTP protocol is in the form of UDP / IP, IP-level multicast is eGTP It does not affect the protocol. However, only transport (Transport) can be transmitted by multicast to efficiently use S1 resources.

In addition, in the IP-based mobile communication system currently being standardized, the transport must support IPv4 and IPv6, so for this purpose, the Internet Group Management Protocol (IGMP) proposed by the IETF, an Internet standardization organization, can be used. Under such a definition, a packet structure for IGMP according to an embodiment of the present invention will be described with reference to FIGS. 5 and 6.

5 is a packet structure diagram for IGMP according to an embodiment of the present invention. 5 illustrates an example of a packet structure based on IPv4.

As shown in FIG. 5, an IGMP message has a packet structure consisting of fields of type, max resp. Time, checksum, and group address.

In this case, the type field serves as a delimiter for whether the multicast router sends a query or a response sent by the host. That is, the query message may be represented as 1 and the report message may be represented as 2 by defining the type of the message. The maximum response time field is used only for inquiry and specifies the maximum time allowed in 1/10 units when sending a response.

The checksum field is a simple mathematical calculation used to identify errors in header information and is a checksum for an ICMP message. The group address field means an IP address of class D (that is, a group to support multicast communication, which is one-to-many communication). Generally, the group address is set to 0 during the inquiry process, and the group address in the reporting process is Indicate the group address to be reported. In the IPv4 as described above, the IGMP message structure is a packet structure provided in RFC2236, and thus its detailed description will be omitted.

IPv6 uses the same IGMP message structure as IPv4, and, unlike IPv4, includes more code fields and multicast address fields. This is the same as FIG. 6 illustrated using IPv6 as an example of a packet structure for IGMP according to an embodiment of the present invention.

That is, the code field is a delimiter for determining the function of the packet is written, it can be known whether the message is broadcast based on the code field. The multicast address field stores an address of a multicast group including a mobile terminal and a serving base station for transmitting and receiving data traffic.

The procedure of the IGMP protocol using the packet structure for IGMP is largely a procedure for transferring information of a multicast group, joining a specific multicast group, and leaving a specific multicast group. You can share it. The definition of the message for these procedures is shown in FIG.

7 is an exemplary diagram for defining an IGMP message according to an embodiment of the present invention.

As shown in FIG. 7, the IGMP message includes IGMP membership report information for joining a target base station to a specific multicast group, IGMP escape group information for leaving a specific multicast group, and query information for transmitting multicast group information. It includes.

Next, a procedure of transmitting data traffic using bicasting during handover between base stations using the IGMP message mentioned in FIG. 7 will be described in detail with reference to FIG. 8.

8 is a flowchart illustrating a data traffic transmission procedure during handover between base stations according to an embodiment of the present invention.

As shown in FIG. 8, the mobile terminal 100 periodically performs a measurement procedure S100 in association with a serving base station. Through the measurement procedure, the mobile terminal 100 can estimate the signal strength of the pilot channel of the serving base station 210, and whether the serving base station 210 needs handover according to the estimation result reported from the mobile terminal 100. Can be determined. That is, when there is a mobile terminal 100 representing the strength of the estimated pilot channel signal strength or more, the serving base station 210 determines that handover is required for the mobile terminal 100.

When handover is required for the mobile terminal 100 based on the measurement information collected in step S100, the serving base station 210 determines the target base station 220 to be the handover target (S110) to the target base station 220 Perform a handover of. At the same time, the serving base station 210 transmits a handover request message to the gateway 300 to perform the handover of the mobile terminal 100 to the target base station 220 determined as the handover target (S120). The gateway 300 receiving the request message performs a handover preparation procedure of the mobile terminal 100. At this time, the handover preparation procedure means steps S130 to S165 to be described below.

Upon receiving the handover request message transmitted by the serving base station 210, the gateway 300 transmits the same resource to the serving base station 210 and the target base station 220 during the handover. The multicast group information, which is preset for 100, is included in the handover request message and transmitted to the target base station 220 (S130). In this case, as shown in the IGMP message structure of FIG. 7, the multicast group information is transmitted to the target base station 220 in the query information so that the gateway 300 transmits the information of the multicast group. That is, by transmitting the multicast group information including the mobile terminal 100 information, the serving base station 210 information, and the like, which are currently interworked with each other through the gateway 300, to the target base station 220, the mobile terminal 100 later. When performing the handover, the target base station 220 can transmit the packet to the handovered mobile terminal 100.

The target base station 220 receiving the handover request message transmits the IGMP membership report information in the case of IPv4 and the IGMP report message in the case of IPv6 (S140). In this case, the message transmission procedure is performed to join the target base station 220 to which the handover is to be performed at the gateway 300 to the existing multicast group. The IGMP report message includes the multicast group information generated by updating the mobile station 100 information, the serving base station 210 information, and the target base station 220 information by the target base station 220.

The target base station 220 joining the base station to the existing multicast group through the S140 procedure transmits the handover response message to the gateway 300 as a response message of the handover request message (S150). When such a procedure is completed, the serving base station 210 and the target base station 220 joined to the multicast group may receive the same traffic from the gateway 300 through eGTP (S160 and S165). That is, the gateway 300 bicasts the same traffic and transmits the same traffic to the serving base station 210 and the target base station 220, respectively, and the serving base station 210 and the target base station 220 receive the traffic.

The serving base station 210 transmits the received traffic to the mobile terminal 100, but the target base station 220 buffers the received traffic because the mobile terminal 100 is still in a state before handover. In this case, the gateway 300 transmitting traffic to the serving base station 210 and the target base station 220 by bicasting prevents a packet loss while the mobile terminal 100 performs a handover to prevent data loss. To minimize this.

As described above, when the handover preparation procedure (S130 ˜ S165) is completed, the gateway 300 transmits a handover command message to the serving base station 210 in the wired section (S180) to indicate that the preparation procedure is completed. Notify me. The serving base station 210 notified that the preparation procedure is completed transmits a handover command message to the mobile terminal 100 so as to perform handover (S190).

The serving base station 210 receiving the handover command message transmits context information of the mobile terminal 100 to be handed over to the target base station 220 (S200). At this time, the serving base station 210 transmits the base station context data message including the sequence number of the eGTP packet transmitted to the wireless section to the target base station 220. Here, the context data information includes information necessary for transmitting data traffic through the serving base station 210 (for example, the sequence number of the SDU transmitted by the serving base station 210) and the mobile terminal 100 / RAN context ( For example, identifier, QoS profile, SAE bearer information, etc. related to the mobile terminal 100 may be included.

The target base station 220 receiving the base station context data message transmits the base station context confirmation message to the serving base station 210 in response to receiving the corresponding message (S210).

When the context information transmission procedure of the mobile terminal 100 is completed, the mobile terminal 100 and the target base station 220 establishes a wireless section synchronization procedure and a wireless section setup through a layer 1 / layer 2 signaling setup procedure. Perform the procedure (S220). Here, the L1 / L2 signaling setup procedure is a procedure related to L1 / L2, such as synchronization of the PHY layer and random access / scheduling of the MAC layer, and is well known. Will be omitted.

When the wireless section synchronization procedure and the wireless section setting procedure are completed through step S220, the mobile terminal 100 transmits a handover complete message to the target base station 220 (S230), and the target base station 220 receives the received hand. The over complete message is transmitted to the gateway 300 (S240). As the gateway 300 receives the handover complete message, the gateway 300 recognizes that the mobile terminal 100 has completed the handover to the new base station, that is, the target base station 220.

Next, when the mobile terminal 100 completes the handover to the target base station 220, the target base station 220 sequentially transmits the traffic buffered by bicasting to the mobile terminal 100. At this time, the traffic is transmitted to the mobile terminal 100 using the sequence number of the eGTP packet received from the serving base station 210 in step S200. In addition, when the transmission of the buffered traffic is completed, the traffic received from the gateway 300 is transmitted to the mobile terminal 100 (S250).

After the step S220 is completed, the serving base station 210 transmits an IGMP Leave Group (IPv4) or Done message to the gateway 300, through which the serving base station 210 leaves the preset multicast group (S260). In FIG. 8, the step S260 is performed after the step S250, but in practice, the step S260 in which the serving base station 210 leaves the multicast group is performed in parallel while the step S220 is performed. Thereafter, the serving base station 210 performs an L1 / L2 signaling procedure for releasing to release the set radio section (S270). At this time, the procedure is already known, the detailed description thereof will be omitted in the embodiment of the present invention.

Next, a gateway operation procedure for delivering data traffic in a gateway in the handover procedure described with reference to FIG. 8 will be described with reference to FIG. 9.

9 is a flowchart illustrating an operation procedure of a gateway for data traffic delivery in a base station handover procedure according to an embodiment of the present invention. The operation procedure of the gateway will be described by dividing into an initial session start procedure (FIG. 9A), a handover procedure (FIG. 9B), and a session release procedure (FIG. 9C).

First, as shown in FIG. 9A, in the idle state S310, the gateway determines whether a session is started (S311). If it is determined that the session has not started, the gateway continuously maintains the idle state (S310). In this case, the idle state refers to a state in which a session for the mobile terminal does not exist. However, when the session is started, the gateway creates and manages a multicast group for S1 connection of the corresponding session (S312), and sets up the SAE bearer accordingly (S313) so that the session is on (S314).

As described above, a procedure in the case where handover occurs due to the movement of the mobile terminal while the session is on will be described with reference to FIG.

As illustrated in (b) of FIG. 9, when the gateway detects the movement of the mobile terminal (S321), the gateway determines whether a handover has occurred (S322). If the mobile terminal moves to a state where no handover occurs, the session is continuously kept in the on state (S320).

However, if a handover occurs, the gateway manages the updated multicast group table by the target base station through the joining procedure of the multicast group (S323), and transmits data traffic to the serving base station and the target base station by bicasting, respectively. (S324). Thereafter, the gateway determines whether the handover of the mobile terminal is completed (S325), and when the handover is completed, the gateway updates and manages the multicast group table generated by the serving base station leaving the multicast group (S326). do. That is, the multicast group table managed by the gateway in step S323 includes the information of the mobile terminal, the information of the serving base station and the information of the target base station, but the information of the mobile terminal and the target base station is included in the multicast group table managed in step S326. Contains information.

As shown in (c) of FIG. 9, it is determined whether data session transmission is completed between the mobile terminal and the serving / target base station (S331), and the session maintaining the on state (S330) is terminated. In step S332, the SAE bearer configured for S1 access of the corresponding session is released. Thereafter, the gateway deletes the multicast group managed by the gateway (S333).

Here, the SAE bearer context information for the SAE bearer established and released at the gateway when describing the procedures of FIGS. 9A to 9C is shown in FIG. 10.

10 is an exemplary diagram illustrating SAE bearer context information according to an embodiment of the present invention.

As shown in FIG. 10, since the present invention performs the S1 data access function using multicast, the SAE bearer context includes a multicast IP address, IP addresses of serving base stations and target base stations, and tunnel identifiers used in eGTP. Tunnel identifier) and QoS profile describing service characteristics of S1.

Here, a program for realizing a function corresponding to the configuration of the above-described embodiment of the present invention or a recording medium on which the program is recorded is also included in the scope of the present invention.

Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements of those skilled in the art using the basic concepts of the present invention defined in the following claims are also provided. It belongs to the scope of rights.

According to the above-described embodiment, when the mobile terminal performs handover between base stations, the gateway bicasts data traffic to the target base station to which the mobile terminal is to be handed over, thereby efficiently saving resources of the user plane connection interval between the gateway and the base station. It is available.

In addition, by transmitting data traffic using bicasting during handover between base stations, it is possible to minimize the loss of data transmitted in downlink.

In addition, when the base station transmits the data traffic to the mobile terminal through the wireless section, since the data re-ordering procedure is omitted, the load of the base station can be reduced and the structure of the base station can be easily implemented.

Claims (16)

In the method for transmitting data in the handover between base stations in a mobile communication system, Receiving a handover request message including target base station information from a serving base station, and forwarding a handover request message including multicast group information to the target base station based on the handover request message; Receive a message informing that the target base station is joined to the multicast group, where the message includes updated multicast group information including the target base station information and receives data traffic to the serving base station and the target base station. Transmitting each; And Receives a handover complete message from the target base station indicating that handover from the serving base station to the target base station included in the updated multicast group information is completed, and transmits data traffic to the mobile terminal through the target base station. Steps to Data transmission method comprising a. The method of claim 1, Transmitting data traffic to the serving base station and the target base station, respectively; The gateway transmits the same data traffic to the serving base station and the target base station in a bi-casting manner, respectively. The method of claim 2, The data traffic transmitted to the target base station is buffered at the target base station until the mobile terminal completes the handover from the serving base station to the target base station. The method of claim 1, Before forwarding the handover request message to the target base station, Determining whether a session for transmitting data traffic to the mobile terminal is connected; When the session is connected, generating the multicast group including the mobile terminal information and the serving base station information; And Establishing a system architecture evolution (SAE) bearer for transmitting data traffic transmitted to the serving base station to the mobile terminal; Data transmission method further comprising. The method according to claim 1 or 2, Transmitting a handover command message to the serving base station, informing that the handover preparation procedure is completed; Data transmission method further comprising. The method according to claim 1 or 2, When the handover to the target base station is completed, receiving a multicast group leave message from the serving base station indicating that the multicast group has left the multicast group; And Updating the multicast group information when the multicast group leave message is received; Data transmission method further comprising. The method of claim 6, The multicast group information includes the mobile terminal information, the serving base station information and connection information between the mobile terminal and the serving base station. The method according to any one of claims 1 to 4, The data traffic transmitted to the serving base station and the target base station is transmitted in a data traffic method using evolved GPRS Tunneling Protocol (eGTP), which is a user plane protocol. The method of claim 8, The SAE bearer includes an IP address of the multicast group, an IP address of a serving base station, an IP address of a target base station, a tunnel identifier used in the eGTP, and QoS profile information. In the method for transmitting data in the handover between base stations in a mobile communication system, The target base station receiving a handover request message including multicast group information from the gateway; The message indicating that the target base station joins the multicast group including the mobile terminal and the serving base station based on the handover request message, wherein the message includes the multicast group information updated to include the target base station information. Sending-to the gateway; The target base station receiving data traffic from the gateway and buffering the received data traffic until the mobile terminal completes a handover; Receiving, by the target base station, a base station context data message from the serving base station, and performing a synchronization procedure and a wireless section setting procedure with the mobile terminal; And When the target base station receives a handover complete message from the mobile terminal, transmitting the buffered data traffic to the mobile terminal based on the base station context data message Data transmission method comprising a. The method of claim 10, The multicast group information includes the mobile terminal information and the serving base station information, and is preset for data transmission between the mobile terminal and the serving base station. The method of claim 10, Determining, by the serving base station, the handover of the mobile terminal and transmitting a handover request message including information of a target base station to a gateway; And The serving base station receiving a handover command message from the gateway informing that the handover preparation procedure of the mobile terminal has been performed, and transmitting the base station context data message to a target base station; Data transmission method further comprising. The method of claim 12, The station context data message includes a sequence number of traffic last transmitted by the serving base station to the mobile terminal. The method of claim 10, And while the synchronization procedure between the mobile terminal and the target base station is performed, the serving base station performs a procedure of leaving the multicast group. The method of claim 14, And when the serving base station deviates from the multicast group, the serving base station performs a procedure of releasing a resource of a radio section established with the mobile terminal. The method of claim 10, The data traffic received from the gateway receives the same data traffic as the data traffic transmitted to the serving base station in a bicasting manner.

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