Patent Publication Number: US-2020296636-A1

Title: Cell handover method and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application No. PCT/CN2017/109349, filed on Nov. 3, 2017, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This application relates to the field of mobile communications, and in particular, to a cell handover method and apparatus. 
     BACKGROUND 
     Currently, in a communication process, a terminal selects a cell for access, and performs data exchange with a wireless communications network by using the accessed cell. When the terminal moves from coverage of the cell accessed by the terminal to coverage of another cell, the terminal needs to perform a cell handover to access the another cell. 
     As shown in  FIG. 1 , a current communications system  10  includes a terminal  11 , a source evolved NodeB (evolutional Node B, eNB)  12 , a target eNB  13 , a core network, and an application server  18  (Application Server). The terminal  11  communicates with the source eNB  12  by using a second interface, and/or the terminal  11  communicates with the target eNB  13  by using the second interface. The source eNB  12  and the target eNB  13  are any two neighboring base stations in the communications system  10 , and the source eNB  12  is connected to the target eNB  13  by using an X2 interface. Each eNB is connected to the core network by using an S1 interface, and the core network is also referred to as an evolved packet core (Evolved Packet Core, EPC). The EPC mainly includes the following key logical network elements: a mobility management entity  14  (Mobility Management Entity, MME), a source serving gateway  15  (Serving Gateway, SGW), a target SGW  16 , a packet data network gateway  17  (Packet Data Network Gateway, PGW), and a home subscriber server (Home Subscriber Server, HSS) (which is not shown in the figure). Both the source eNB  12  and the target eNB  13  are connected to the MME  14  by using an S1-C interface, the source eNB  12  is connected to the source SGW  15  by using an S1-U interface, and the target eNB  12  is connected to the target SGW  16  by using the S1-U interface. The MME  14  is connected to the source SGW  15  by using an S11 interface, and the MME  14  is connected to the target SGW  16  by using the S11 interface. Both the source SGW  15  and the target SGW  16  are connected to the PGW  17  by using an S5/S8 interface, and the PGW  17  is connected to the application server  18  on the Internet. 
     Based on the communications system  10  provided above, the related art provides a cell handover method, including: sending, by a terminal  11 , a measurement report to a source eNB  12  to which a source cell accessed by the terminal  11  belongs; selecting, by the source eNB  12 , a target cell for the terminal  11  when determining, based on the measurement report, that a cell handover needs to be performed, sending a handover request message to a target eNB  13  to which the target cell belongs, receiving a handover request acknowledgment message sent by the target eNB  13 , and sending a radio resource control protocol (Radio Resource Control, RRC) connection reconfiguration message to the terminal  11 , where the RRC connection reconfiguration message carries an identifier of the target cell; and receiving, by the terminal  11 , the RRC connection reconfiguration message, disconnecting from the source eNB  12 , establishing a connection to the target eNB  13  based on the identifier of the target cell, and accessing the target cell, so that the terminal  11  is handed over from the source cell to the target cell. In a process in which the terminal  11  establishes the connection to the target eNB  13 , the terminal  11  is connected to a PGW  17  through signaling interaction among the target eNB  13 , an MME  14 , an HHS, a target SGW  16 , and the PGW  17 , and then accesses an application server  18  on the Internet by creating a packet data network (Packet Data Network, PDN) connection by using the PGW  17 , so that the application server  18  provides a background service for an application in the terminal  11 . 
     In the cell handover method that is based on the communications system  10 , because the PGW  17  is used as a service anchor, if the PGW  17  is deployed at a location relatively far away from the target eNB  13 , a relatively long service delay is caused. 
     SUMMARY 
     To resolve a problem that a current cell handover method causes a relatively long service delay, this application provides a cell handover method and apparatus. The technical solutions are as follows: 
     This application provides a communications system including a source wireless gateway, a target wireless gateway, and a forwarding device, so that in a cell handover process based on the communications system, uplink data and downlink data of a terminal before and after a handover do not need to rely on a specific network element as a service anchor, and can be directly output by using the forwarding device, to shorten a service delay. 
     According to a first aspect, a cell handover method is provided. 
     The method is applied to a communications system. The system includes at least a source wireless gateway, a target wireless gateway, and a forwarding device. The source wireless gateway and the target wireless gateway are connected to the forwarding device by using a first interface, and the forwarding device is connected to an application server by using the first interface. The method includes:
         receiving, by the source wireless gateway, a measurement report (Measurement Report) sent by a terminal, where the source wireless gateway is a gateway corresponding to a source cell;   determining, by the source wireless gateway, the target wireless gateway based on the measurement report;   sending, by the source wireless gateway, a handover request (Handover Request) message to the target wireless gateway; and   sending by the source wireless gateway, a handover instruction (Handover Command) message to the terminal when receiving a handover request acknowledgment message (Handover Request Acknowledge) sent by the target wireless gateway.       

     In the implementation, the communications system includes the source wireless gateway, the target wireless gateway, and the forwarding device, both the source wireless gateway and the target wireless gateway are connected to the forwarding device by using the first interface, and the forwarding device is connected to the application server by using the first interface, so that before and after the cell handover of the terminal, uplink data and downlink data of the terminal do not need to rely on a specific network element as a service anchor, but can be directly output by using the forwarding device. This avoids a problem in the related art that time consumption is serious because both uplink data and downlink data of a terminal need to be output by using a PGW in a unified manner, thereby shortening a service delay. 
     In the implementation, the networking structure of the communications system is further simplified into three network elements: the source wireless gateway, the target wireless gateway, and the forwarding device respectively. This avoids a problem in the related art that signaling overheads between network elements are relatively high because there are many network elements (for example, a source eNB, a target eNB, an MME, a source SGW, a target SGW, and a PGW) in the communications system during cell handover, so that only three gateway network elements need to perform signaling interaction during cell handover, thereby greatly reducing signaling overheads between the network elements in the communications system during cell handover. 
     In a possible implementation of the first aspect, after the sending, by the source wireless gateway, a handover instruction message to the terminal, the method further includes:
         establishing, by the source wireless gateway, an X2 extended channel with the target wireless gateway, where the X2 extended channel is configured to transmit a context transfer message of the terminal (UE Context Transfer) to the target wireless gateway.       

     In the implementation, the X2 extended channel is established between the source wireless gateway and the target wireless gateway, so that the context transfer message of the terminal can be directly transmitted to the target wireless gateway through the X2 extended channel. This avoids a case in which the MME can obtain the context transfer message of the terminal only through interaction between a plurality of pieces of signaling in the related art, thereby reducing signaling overheads. 
     In another possible implementation of the first aspect, after the sending, by the source wireless gateway, a handover instruction message to the terminal when receiving a handover request acknowledgment message sent by the target wireless gateway, the method further includes:
         receiving, by the source wireless gateway, a release resource message sent by the target wireless gateway, where the release resource message is sent after the target wireless gateway sends an update request to a target port of the forwarding device, and the update request is used to instruct the forwarding device to update a port identifier corresponding to a target address to a target port identifier of the target port; and   releasing, by the source wireless gateway a radio bearer between the terminal and the source wireless gateway based on the release resource message.       

     In the implementation, after the source wireless gateway sends the handover instruction message to the terminal, the source wireless gateway receives the release resource message sent by the target wireless gateway, and releases the radio bearer between the source wireless gateway and the terminal based on the release resource message, so that the radio bearer between the terminal and the source wireless gateway is released only after the radio bearer between the terminal and the target wireless gateway is established, thereby ensuring that session continuity of the terminal is not interrupted. 
     In the implementation, the target wireless gateway further sends the update request to the target port of the forwarding device, where the update request is used to instruct the forwarding device to update the port identifier corresponding to the target address to the target port identifier of the target port, so that the forwarding device can switch a path corresponding to the downlink data of the terminal from the source wireless gateway to the target wireless gateway, to further enable the downlink data delivered by the application server no longer to be forwarded to the target wireless gateway by using the source wireless gateway, but to be directly sent by the switch to the target wireless gateway, thereby further shortening a time length during cell handover. 
     In another possible implementation of the first aspect, the forwarding device is a switch, and the update request is a gratuitous address resolution protocol (Address Resolution Protocol, ARP) packet;
         or   the forwarding device is a router, and the update request is a link state (Link State, LS) update packet.       

     In the implementation, when the forwarding device is the switch, the update request sent by the target wireless gateway to the target port of the forwarding device is the ARP packet; or when the forwarding device is the router, the update request sent by the target wireless gateway to the target port of the forwarding device is the LS update packet, so that the cell handover manner is applicable to both a forwarding device (which is the switch in this case) operating in an L2 networking architecture of the communications system and a forwarding device (which is the router in this case) operating in an L3 networking architecture. 
     According to a second aspect, a cell handover method is provided. The method is applied to a communications system. The system includes at least a source wireless gateway, a target wireless gateway, and a forwarding device. Both the source wireless gateway and the target wireless gateway are connected to the forwarding device by using a first interface, and the forwarding device is connected to an application server by using the first interface. The method includes:
         receiving, by the target wireless gateway, a handover request message sent by the source wireless gateway, where the source wireless gateway is a gateway corresponding to a source cell, and the target wireless gateway is a gateway corresponding to a target cell;   sending, by the target wireless gateway, a handover request acknowledgment message to the source wireless gateway; and   receiving, by the target wireless gateway, a handover acknowledgment message (Handover Confirm) sent by a terminal.       

     In a possible implementation of the second aspect, after the receiving, by the target wireless gateway, a handover request message sent by the source wireless gateway, the method further includes:
         establishing, by the target wireless gateway, an X2 extended channel with the source wireless gateway, where the X2 extended channel is configured to transmit a context transfer message of the terminal.       

     In another possible implementation of the second aspect, after the establishing, by the target wireless gateway, an X2 extended channel with the source wireless gateway, the method further includes:
         receiving and buffering, by the target wireless gateway, uplink data sent by the source wireless gateway through the X2 extended channel; and   after the receiving, by the target wireless gateway, a handover acknowledgment message sent by a terminal, the method further includes:   transmitting, by the target wireless gateway, the buffered uplink data to the application server by using the forwarding device.       

     In another possible implementation of the second aspect, after the receiving, by the target wireless gateway, a handover acknowledgment message sent by a terminal, the method further includes:
         sending, by the target wireless gateway, an update request to a target port of the forwarding device, where the update request carries a target address, the target address is an address used when the application server sends downlink data to the terminal, and the update request is used to instruct the forwarding device to update a port identifier corresponding to the target address to a target port identifier of the target port;   receiving, by the target wireless gateway, the downlink data forwarded by the forwarding device through the target port; and   transmitting, by the target wireless gateway, the downlink data to the terminal corresponding to the target address.       

     In another possible implementation of the second aspect, the forwarding device is a switch, and the update request is a gratuitous address resolution protocol ARP packet; or the forwarding device is a router, and the update request is a link state LS update packet. 
     According to a third aspect, a communications system provided. The communications system includes at least a source wireless gateway, a target wireless gateway, and a forwarding device;
         both the source wireless gateway and the target wireless gateway are connected to the forwarding device by using a first interface; and the source wireless gateway is configured to communicate with a terminal by using a second interface, and the target wireless gateway is configured to communicate with the terminal by using the second interface; and   the forwarding device is connected to an application server by using the first interface, where   the terminal is configured to hand over from a source cell to a target cell, where the source wireless gateway is a gateway corresponding to the source cell, and the target wireless gateway is a gateway corresponding to the target cell.       

     In a possible implementation of the third aspect, the source wireless gateway is connected to the target wireless gateway by using an X2 extended channel, and the X2 extended channel is configured to transmit a context transfer message of the terminal. 
     In another possible implementation of the third aspect, when the forwarding device operates at a data link layer, the forwarding device is a switch. 
     In another possible implementation of the third aspect, when the forwarding device operates at a network layer, the forwarding device is a router. 
     According to a fourth aspect, a source wireless gateway is provided. The source wireless gateway is the source wireless gateway provided in any one of the third aspect or the possible implementations of the third aspect. The source wireless gateway includes: a receiving unit, a processing unit, and a sending unit, where
         the receiving unit is configured to receive a measurement report sent by a terminal; and the source wireless gateway is a gateway corresponding to a source cell;   the processing unit is configured to determine a target wireless gateway based on the measurement report;   the sending unit is configured to send a handover request message to the target wireless gateway; and   the sending unit is further configured to send a handover instruction message to the terminal when a handover request acknowledgment message sent by the target wireless gateway is received.       

     In a possible implementation of the fourth aspect, the processing unit is further configured to establish an X2 extended channel with the target wireless gateway, where the X2 extended channel is configured to transmit a context transfer message of the terminal. 
     In another possible implementation of the fourth aspect, the receiving unit is further configured to receive a release resource message sent by the target wireless gateway, where the release resource message is sent after the target wireless gateway sends an update request to a target port of a forwarding device, and the update request is used to instruct the forwarding device to update a port identifier corresponding to a target address to a target port identifier of the target port; and
         the processing unit is further configured to release a radio bearer between the terminal and the source wireless gateway based on the release resource message.       

     In another possible implementation of the fourth aspect,
         the forwarding device is a switch, and the update request is an ARP packet;   or   the forwarding device is a router, and the update request is an LS update packet.       

     According to a fifth aspect, a target wireless gateway is provided. The target wireless gateway is the target wireless gateway provided in any one of the third aspect or the possible implementations of the third aspect. The target wireless gateway includes: a receiving unit, a processing unit, and a sending unit, where
         the receiving unit is configured to receive a handover request message sent by a source wireless gateway, wherein the source wireless gateway is a gateway corresponding to a source cell, and the target wireless gateway is a gateway corresponding to a target cell;   the sending unit is configured to send a handover request acknowledgment message to the source wireless gateway, and   the receiving unit is further configured to receive a handover acknowledgment message sent by a terminal.       

     In a possible implementation of the fifth aspect, the processing unit is configured to establish an X2 extended channel with the source wireless gateway, where the X2 extended channel is configured to transmit a context transfer message of the terminal. 
     In another possible implementation of the fifth aspect, the receiving unit is further configured to receive and buffer, by the target wireless gateway, uplink data sent by the source wireless gateway through the X2 extended channel; and
         the sending unit is further configured to transmit, by the target wireless gateway, the buffered uplink data to an application server by using a forwarding device.       

     In another possible implementation of the fifth aspect, the sending unit is further configured to send an update request to a target port of a forwarding device, where the update request carries a target address, the target address is an address used when an application server sends downlink data to the terminal, and the update request is used to instruct the forwarding device to update a port identifier corresponding to the target address to a target port identifier of the target port;
         the receiving unit is further configured to receive the downlink data forwarded by the forwarding device through the target port; and   the sending unit is further configured to transmit the downlink data to the terminal corresponding to the target address.       

     In another possible implementation of the fifth aspect,
         the forwarding device is a switch, and the update request is an ARP packet;   or   the forwarding device is a router, and the update request is an LS update packet.       

     According to a sixth aspect, a source wireless gateway is provided. The source wireless gateway is the source wireless gateway provided in any one of the third aspect or the possible implementations of the third aspect. The source wireless gateway includes: a processor and a transmitter and a receiver that are connected to the processor, where
         the transmitter and the receiver are configured to be controlled by the processor, and the processor is configured to implement the cell handover method provided in any one of the first aspect or the possible implementations of the first aspect.       

     According to a seventh aspect, a target wireless gateway is provided. The target wireless gateway is the target wireless gateway provided in any one of the third aspect or the possible implementations of the third aspect. The target wireless gateway includes: a processor and a transmitter and a receiver that are connected to the processor, where
         the transmitter and the receiver are configured to be controlled by the processor, and the processor is configured to implement the cell handover method provided in any one of the second aspect or the possible implementations of the second aspect.       

     According to an eighth aspect, a source wireless gateway is provided. The source wireless gateway includes a chip and at least one network interface. The chip includes a programmable logic circuit and/or a program instruction. When the chip is nm, the chip is configured to implement the cell handover method provided in any one of the first aspect or the optional implementations of the first aspect. 
     According to a ninth aspect, a target wireless gateway is provided. The target wireless gateway includes a chip and at least one network interface. The chip includes a programmable logic circuit and/or a program instruction. When the chip is run, the chip is configured to implement the cell handover method provided in any one of the second aspect or the optional implementations of the second aspect. 
     According to a tenth aspect, a communications apparatus is provided. The apparatus includes units or measures configured or used to perform the described steps in any one of the first aspect or the possible implementations of the first aspect. 
     According to an eleventh aspect, a communications apparatus is provided. The apparatus includes units or measures configured or used to perform the described steps in any one of the second aspect or the possible implementations of the second aspect. 
     According to a twelfth aspect, a communications apparatus is provided. The apparatus includes a processing element and a storage element, where the storage element is configured to store a program, and when the program is invoked by the processing element, the processing element is configured to perform the cell handover method provided in any one of the first aspect or the possible implementations of the first aspect. 
     According to a thirteenth aspect, a communications apparatus is provided. The apparatus includes a processing element and a storage element, where the storage element is configured to store a program, and when the program is invoked by the processing element, the processing element is configured to perform the cell handover method provided in any one of the second aspect or the possible implementations of the second aspect. 
     According to a fourteenth aspect, a computer-readable storage medium is provided. The computer-readable storage medium stores at least one instruction, and the at least one instruction is loaded and executed by a processor, to implement the cell handover method provided in any one of the first aspect or the optional implementations of the first aspect. 
     According to a fifteenth aspect, a computer-readable storage medium is provided. The computer-readable storage medium stores at least one instruction, and the at least one instruction is loaded and executed by a processor, to implement the cell handover method provided in any one of the second aspect or the optional implementations of the second aspect. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a network architecture diagram of a communications system  10  according to a related art; 
         FIG. 2  is a network architecture diagram of a communications system  20  according to an embodiment of this application; 
         FIG. 3  is a flowchart of a cell handover method according to an embodiment of this application; 
         FIG. 4  is a flowchart of a cell handover method according to another embodiment of this application; 
         FIG. 5  is a structural block diagram of a source wireless gateway according to an embodiment of this application; 
         FIG. 6  is a structural block diagram of a target wireless gateway according to an embodiment of this application; 
         FIG. 7  is a block diagram of a source wireless gateway according to another embodiment of this application; and 
         FIG. 8  is a block diagram of a target wireless gateway according to another embodiment of this application. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     To make the objectives, technical solutions, and advantages of this application clearer, the following further describes in detail the implementations of this application with reference to the accompanying drawings. 
     For ease of understanding, the following explains terms in the embodiments of this application. 
     Source cell: also referred to as a serving cell (Serving Cell), which is a cell that establishes a radio resource control (Radio Resource Control, RRC) connection to a terminal and provides a service for the terminal. 
     Target cell: also referred to as a neighboring cell (Neighbor Cell, NCell) or an adjacent cell, which is another cell different from a source cell currently accessed by the terminal. When the terminal is in an RRC connected mode, the terminal measures the neighboring cell, and reports a measurement report to a source wireless gateway. The source wireless gateway instructs, based on the measurement report, the terminal to perform cell handover between the source cell and the target cell. 
       FIG. 2  is a network architecture diagram of a communications system  20  according to an embodiment of this application. The communications system  20  includes a terminal  21 , a source wireless gateway  22 , a target wireless gateway  23 , a forwarding device  24 , and an application server  25 . 
     The terminal  21  is a device that performs data communication with one or more wireless gateways (including the source wireless gateway  22  and the target wireless gateway  23 ). Optionally, an application program is run on the terminal  21 . The terminal  21  may be a mobile terminal device, for example, the terminal  21  is a mobile phone (or referred to as a “cellular” phone) or a computer having a mobile terminal device. For example, the terminal  21  is a portable, pocket-sized, handheld, computer built-in, or vehicle-mounted mobile apparatus. When different communication protocols are used, the terminal  21  in this embodiment of this application may be referred to as a subscriber unit (Subscriber Unit), a subscriber station (Subscriber Station), a mobile station (Mobile Station), a mobile console (Mobile), a remote station (Remote Station), an access point (Access Point), a remote terminal device (Remote Terminal), an access terminal device (Access Terminal), a user apparatus (User Terminal), a user agent (User Agent), a terminal device (User Device), or user equipment (User Equipment, UE). Optionally, the terminal  21  may alternatively be a relay (Relay) device, but this embodiment is not limited thereto. 
     The terminal  21  is configured to hand over from a source cell to a target cell, where the source cell is a cell that provides a service for the source wireless gateway  22 , and the target cell is a cell that provides a service for the target wireless gateway  23 . The source cell is a cell in which the terminal  21  is located before the handover, the target cell is a cell in which the terminal  21  is located after the handover, and the source cell and the target cell are any two neighboring cells in the communications system  20 . 
     The terminal  21  communicates with the source wireless gateway  22  by using a second interface, and/or the terminal  21  communicates with the target wireless gateway  23  by using the second interface. Optionally, when the communications system is a long term evolution (Long Term Evolution, LTE) system, the second interface is an air interface (Air Interface). 
     The source wireless gateway  22  and the target wireless gateway  23  are any two neighboring wireless gateways in the communications system  20 . The source wireless gateway  22  may be configured to perform all radio-related functions in the source cell, and the target wireless gateway  23  may be configured to perform all radio-related functions in the target cell. 
     Optionally, the source wireless gateway  22  is connected to the target wireless gateway  23  through an X2 extended interface, an X2 extended channel is established between the source wireless gateway  22  and the target wireless gateway  23  by using the X2 extended interface, and the X2 extended channel is configured to transmit a context transfer message of the terminal. 
     Optionally, each wireless gateway (including the source wireless gateway and the target wireless gateway above) may be configured to complete processing of a signaling plane function, and the processing includes but is not limited to the following several types: authentication for access of the terminal  21 , receiving and sending a message of an access stratum (Access Stratum, AS) and/or a message of a non-access stratum (Non-Access Stratum, NAS), and allocating a network resource to the terminal, where the network resource includes an Internet protocol address (English: Internet Protocol Address, IP) and a quality of service (Quality of Service, QoS) parameter that correspond to the terminal. Each wireless gateway is further configured to connect to/release a bearer of the terminal  21 . 
     Optionally, each wireless gateway may have at least one function of a network element such as an eNB, an MME, an HSS, an SGW, or a PGW in a related art. In the related art, a function of the MME includes at least one of authentication for access of the terminal  21 , a handover of the terminal  21 , and mobility management and user context and bearer management of the terminal  21  in an idle state; a function of the HSS includes storing subscription information of a user; a function of the SGW includes completing routing and forwarding of packet data, and providing at least one of lawful interception-related functions; and a function of the PGW includes at least one of functions such as assignment of a user address, execution of policy control and a charging rule, and lawful interception. Both the source wireless gateway  22  and the target wireless gateway  23  are connected to the forwarding device  24  by using a first interface. Optionally, when the communications system is an LTE system, the first interface is an SGi interface. 
     The forwarding device  24  is configured to: receive uplink data of a terminal sent by the source wireless gateway  22  or the target wireless gateway  23 , and send the uplink data of the terminal to the application server  25 . 
     The forwarding device  24  is further configured to: receive downlink data of a terminal sent by the application server  25 , determine a target address corresponding to the downlink data, and forward the downlink data to a corresponding wireless gateway based on the target address, so that the wireless gateway sends the downlink data to the terminal  21 . 
     Optionally, the forwarding device  24  stores a correspondence between the target address and a port identifier. The forwarding, by the forwarding device  24 , the downlink data to a corresponding wireless gateway based on the target address includes: after determining the target address, searching, by the forwarding device, for the part identifier corresponding to the target address, and sending the downlink data of the terminal to the corresponding wireless gateway through a port corresponding to the port identifier. 
     Optionally, the forwarding device  24  includes one of a switch and a router. 
     In a possible implementation, when the forwarding device  24  operates in an L2 networking architecture of the communications system, that is, when the forwarding device  24  operates at a data link layer, the forwarding device  24  is a switch. 
     Because the communications system does not naturally support the L2 networking architecture, both the source wireless gateway  22  and the target wireless gateway  23  have functions of a two-port layer 2 network device, where the two-port layer 2 network device includes a layer 2 tunnel and a bridge. 
     For example, the target wireless gateway  23  is used as an example. The target wireless gateway  23  establishes a generic routing encapsulation (Generic Routing Encapsulation. GRE) tunnel with the terminal  21 , and encapsulates transmitted data by using the GRE tunnel. For another example, the target wireless gateway  23  establishes a bridge with the terminal  21 , and forwards, through the bridge, the data based on a MAC address carried in the transmitted data. 
     In another possible implementation, when the forwarding device  24  operates in an L3 networking architecture of the communications system, that is, when the forwarding device  24  operates at a network layer, the forwarding device  24  is a router. 
     Optionally, when the forwarding device  24  is a router, both the source wireless gateway  22  and the target wireless gateway  23  pre-establish an open shortest path first (Open Shortest Path First, OSPF) neighbor relationship with the router. 
     For example, the target wireless gateway  23  is used as an example. The target wireless gateway  23  sends a hello packet to the terminal  21 , to establish bidirectional communication between the target wireless gateway  23  and the terminal  21 . After the bidirectional communication is established, the target wireless gateway  23  exchanges a series of packets with the terminal  21  to achieve database synchronization, thereby forming an OSPF neighbor relationship. 
     Optionally, the forwarding device  24  is deployed near the source wireless gateway  22  and the target wireless gateway  23 , that is, a distance between the forwarding device  24  and the source wireless gateway  22  is less than a predetermined threshold, and a distance between the forwarding device  24  and the target wireless gateway  23  is less than a predetermined threshold. 
     The forwarding device  24  is connected to the application server  25  by using the first interface. Optionally, the application server  25  is configured to provide a background service for an application program in the terminal  21 . 
     In conclusion, this embodiment of this application provides the communications system including the source wireless gateway  22 , the target wireless gateway  23 , and the forwarding device  24 , both the source wireless gateway  22  and the target wireless gateway  23  are connected to the forwarding device  24  by using the first interface, and the forwarding device  24  is connected to the application server  25  by using the first interface, so that before and after the cell handover of the terminal, uplink data and downlink data of the terminal do not need to rely on a specific network element, that is, a PGW, as a service anchor, but some functions corresponding to the PGW are integrated into the wireless gateway, and are directly output by using the forwarding device connected to the wireless gateway. That is, the uplink data is sent to the application server by using the target wireless gateway and the forwarding device, and the downlink data is sent to the terminal by using the forwarding device and the target wireless gateway. This avoids a problem in the related art that time consumption is serious because both uplink data and downlink data of a terminal need to be output by using a PGW in a unified manner, thereby shortening a service delay. 
     It should be noted that the technical solution of this application may be applied to an LTE system, may be applied to a 5th generation mobile communications technology (5th-Generation, 5G) system, or may be a 5G next-generation mobile communications technology system. This is not limited in this embodiment of this application. An example in which the communications system is an LTE system is used for description below. 
     It should be further noted that when the communications system uses a 5G system or a 5G next-generation mobile communications technology system, the network elements may have different names in the 5G system or the 5G next-generation mobile communications technology system, but have same or similar functions. This is not limited in the embodiments of this application. 
       FIG. 3  is a flowchart of a cell handover method according to an embodiment of this application. This embodiment is described by using an example in which the method is applied to the communications system that is shown in  FIG. 2  and that includes the source wireless gateway  22 , the target wireless gateway  23 , and the forwarding device  24 . The method includes: 
     Step  301 . A source wireless gateway receives a measurement report sent by a terminal. 
     The measurement report includes signal quality of cells measured by the terminal at a current location. 
     The source wireless gateway delivers an RRC connection configuration message to the terminal, where the RRC connection configuration message carries a measurement configuration message, and the measurement configuration message is used to configure a measurement policy used when the terminal performs cell measurement. Correspondingly, the terminal receives the RRC connection configuration message sent by the source wireless gateway, obtains the measurement configuration message from the RRC connection configuration message, performs measurement based on the measurement configuration message, generates the measurement report, and sends the measurement report to the source wireless gateway. Correspondingly, the source wireless gateway receives the measurement report sent by the terminal. 
     Optionally, the measurement report includes signal quality of cells measured at a location of the terminal. 
     Step  302 . The source wireless gateway determines a target wireless gateway based on the measurement report. 
     The source wireless gateway determines, based on the measurement report of the terminal, whether the terminal needs to perform a cell handover, where the cell handover is a handover of the terminal from a source cell to a target cell. The source wireless gateway is a gateway corresponding to the source cell, and the target wireless gateway is a gateway corresponding to the target cell. 
     Optionally, when the measurement report indicates that signal quality of a first cell is poorer than a first condition, and signal quality of a second cell is better than a second condition, the source wireless gateway determines that the terminal needs to be handed over from the source cell to the target cell, that is, determines to perform a cell handover on the terminal. 
     Step  303 . The source wireless gateway sends a handover request message to the target wireless gateway. 
     After determining the target wireless gateway for the terminal, the source wireless gateway sends the handover request message to the target wireless gateway, where the handover request message is used to query the target wireless gateway about whether to allow a cell handover of the terminal. 
     Step  304 . The target wireless gateway receives the handover request message sent by the source wireless gateway. 
     Correspondingly, the target wireless gateway receives the handover request message sent by the source wireless gateway, and determines whether to allow the cell handover of the terminal. 
     If determining not to allow the cell handover of the terminal, the target wireless gateway sends a first handover failure message to the source wireless gateway, where the first handover failure message is used to indicate that the target wireless gateway does not allow the cell handover of the terminal. Optionally, the first acknowledgment failure message is also referred to as a handover preparation failure (Handover Preparation Failure) message. 
     If allowing the cell handover of the terminal, the target wireless gateway reserves a corresponding resource for the terminal based on the handover request message, and performs step  305 . 
     For example, the reserved resource includes at least one of a resource used to establish a radio bearer between the terminal and the target wireless gateway, a cell radio network temporary identifier (Cell Radio-Network Temporary Identifier, C-RNTI), a dedicated random access pilot, and the like. 
     Step  305 . The target wireless gateway sends a handover request acknowledgment message to the source wireless gateway. 
     After reserving the resource, the target wireless gateway sends a handover request acknowledgment message to the source wireless gateway, where the handover request acknowledgment message is used to indicate that the target wireless gateway allows the cell handover of the terminal and reserves the corresponding resource for the terminal. Correspondingly, the source wireless gateway receives the handover request acknowledgment message sent by the target wireless gateway. 
     Optionally, the handover request acknowledgment message carries a dedicated access pilot allocated by the target wireless gateway to the terminal. 
     To avoid a case in which an MME can obtain a context transfer message of a terminal only through interaction between a plurality of pieces of signaling in a related art, after receiving the handover request acknowledgment message sent by the target wireless gateway, the source wireless gateway establishes an X2 extended channel with the target wireless gateway by using an X2 extended interface, or after sending the handover request acknowledgment message to the source wireless gateway, the target wireless gateway establishes an X2 extended channel with the source wireless gateway by using an X2 extended interface. The X2 extended channel is configured to transmit the context transfer message of the terminal. 
     Optionally, the source wireless gateway sends the context transfer message of the terminal to the target wireless gateway through the X2 extended channel. 
     The context transfer message is used to perform encryption and integrity protection on transmitted data. Optionally, the context transfer message carries a context message and a security capability parameter of the terminal. For example, the context message includes a radio access bearer (Evolved Radio Access Bearer, E-RAB) message and a historical message of the terminal, and the historical message of the terminal includes a radio resource management (Radio Resource Management, RRM) message of the terminal and/or a related message of a cell accessed by the terminal. The security capability parameter includes a negotiated security algorithm, a generated key, and the like. 
     The X2 extended channel is further used to transmit a sequence number status transfer message. Optionally, the source wireless gateway sends the sequence number status transfer message to the target wireless gateway through the X2 extended channel. 
     Optionally, to ensure lossless data transmission in a cell handover process, the source wireless gateway sends the sequence number status transfer (SN Status Transfer) message to the target wireless gateway, where the sequence number status transfer message carries a sequence number (SN, Sequence Number). Correspondingly, the target wireless gateway receives the sequence number status transfer message sent by the source wireless gateway, and the target wireless gateway obtains the sequence number from the sequence number status transfer message, and determines a data transmission sequence based on the obtained sequence number during cell handover. 
     Optionally, the sequence number status transfer message is further used to indicate a receiving state of an uplink packet data convergence protocol (Packet Data Convergence Protocol, PDCP), and a sending state of a downlink PDCP. 
     The source wireless gateway sends buffered service data of the terminal to the target wireless gateway through the X2 extended channel where the service data includes uplink data and/or downlink data of the terminal buffered in the source wireless gateway. Correspondingly, the target wireless gateway receives the service data of the terminal sent by the source wireless gateway. 
     When an uplink between the terminal and an application server is established by using the target wireless gateway, the target wireless gateway transmits buffered uplink data to the application server by using a forwarding device. When a downlink between the terminal and the application server is established by using the target wireless gateway, the target wireless gateway transmits buffered downlink data to the terminal. However, in this case, because a radio bearer between the terminal and the target wireless gateway has not been established, the target wireless gateway first buffers the received service data of the terminal. 
     Step  306 . The source wireless gateway sends a handover instruction message to the terminal when receiving the handover request acknowledgment message sent by the target wireless gateway. 
     When the source wireless gateway sends the handover instruction message to the terminal, correspondingly, the terminal receives the handover instruction message, where the handover instruction message is used to instruct the terminal to be handed over from the source cell to the target cell. The terminal determines, based on the handover instruction message, that a cell handover needs to be performed. 
     That the terminal determines, based on the handover instruction message, that a cell handover needs to be performed includes but is not limited to the following two possible implementations: 
     In a possible implementation, the handover instruction message carries a cell identifier of the target cell, and the terminal determines, based on the identifier of the target cell carried in the handover instruction message, the target wireless gateway corresponding to the target cell, and establishes a radio bearer with the target wireless gateway. 
     In another possible implementation, the handover instruction message carries a cell identifier of the target cell and a dedicated access pilot. After determining the target wireless gateway corresponding to the target cell, the terminal establishes a radio bearer with the target wireless gateway by using the dedicated access pilot. 
     A process of establishing the radio bearer between the terminal and the target wireless gateway includes: detaching, by the terminal, from the source wireless gateway, establishing synchronization with the target wireless gateway in a random access process, and sending, by the terminal, a handover acknowledgment message to the target wireless gateway, where the handover acknowledgment message is used to indicate that the terminal has been successfully handed over from the source cell to the target cell. 
     The detachment means removing a radio bearer between the terminal and an attachment point. The attachment point means a station associated with the terminal and providing a service for the terminal. In this embodiment, the attachment point is a source wireless gateway with which the terminal is registered. 
     Step  307 . The target wireless gateway receives the handover acknowledgment message sent by the terminal. 
     The terminal sends the handover acknowledgment message to the target wireless gateway. Correspondingly, the target wireless gateway receives the handover acknowledgment message, where the handover acknowledgment message is used to indicate that the terminal has been successfully handed over from the source cell to the target cell. 
     At this point, the radio bearer between the terminal and the target wireless gateway is established. 
     In conclusion, according to this embodiment of this application, the communications system includes the source wireless gateway, the target wireless gateway, and the forwarding device, both the source wireless gateway and the target wireless gateway are connected to the forwarding device by using the first interface, and the forwarding device is connected to the application server by using the first interface, so that before and after the cell handover of the terminal uplink data and downlink data of the terminal do not need to rely on a specific network element, that is, a PGW, as a service anchor, but some functions corresponding to the PGW are integrated into the wireless gateway, and are directly output by using the forwarding device connected to the wireless gateway. That is, the uplink data is sent to the application server by using the target wireless gateway and the forwarding device, and the downlink data is sent to the terminal by using the forwarding device and the target wireless gateway. This avoids a problem in the related art that time consumption is serious because both uplink data and downlink data of a terminal need to be output by using a PGW in a unified manner, thereby shortening a service delay. 
     In this embodiment of this application, the networking structure of the communications system is further simplified into three network elements: the source wireless gateway, the target wireless gateway and the forwarding device respectively. This avoids a problem in the related art that signaling overheads between network elements are relatively high because there are many network elements (for example, a source eNB, a target eNB, an MME, a source SGW, a target SGW, and a PGW) in the communications system during cell handover, so that only three gateway network elements need to perform signaling interaction during cell handover, thereby greatly reducing signaling overheads between the network elements in the communications system during cell handover. 
     In this embodiment of this application, the X2 extended channel is further established between the source wireless gateway and the target wireless gateway, so that the context transfer message of the terminal can be directly transmitted to the target wireless gateway through the X2 extended channel. This avoids a case in which the MME can obtain the context transfer message of the terminal only through interaction between a plurality of pieces of signaling in the related art, thereby reducing signaling overheads. 
     It should be noted that when the terminal establishes the radio bearer with the target wireless gateway, it indicates that the uplink between the terminal and the application server is established by using the target wireless gateway. When the terminal sends the uplink data of the terminal to the target wireless gateway, the target wireless gateway receives and sends the uplink data of the terminal to the forwarding device. Correspondingly, the forwarding device receives and sends the uplink data of the terminal to the application server. 
     However, in this case, the downlink data sent by the application server to the terminal still needs to pass through the source wireless gateway, and then is forwarded by the source wireless gateway to the target wireless gateway. Therefore, to enable the downlink data delivered by the application server no longer to be forwarded to the target wireless gateway by using the source wireless gateway, but to be directly sent by a switch to the target wireless gateway, refer to a downlink handover process in a cell handover method provided in  FIG. 4 . 
     Based on the cell handover method provided in  FIG. 3 , referring to  FIG. 4 , after step  307 , the following several steps are further included. 
     Step  401 . The target wireless gateway sends an update request to a target port of the forwarding device. 
     Correspondingly, the forwarding device receives, through the target port, the update request sent by the target wireless gateway. 
     The update request carries a target address, where the target address is an address used when the application server sends the downlink data to the terminal. The update request is used to instruct the forwarding device to update a port identifier corresponding to the target address to a target port identifier of the target port. 
     Step  402 . The forwarding device performs path switching based on the update request. 
     The path switching is switching a path corresponding to the downlink data of the terminal from the source wireless gateway to the target wireless gateway. 
     Optionally, the performing, by the forwarding device, path switching includes: determining, by the forwarding device when receiving, through the target port, the update request sent by the target wireless gateway, the target port identifier corresponding to the target port, obtaining the target address carried in the update request, and updating the port identifier corresponding to the target address to the target port identifier. 
     Optionally, the forwarding device stores a correspondence between the target address and the port identifier, where the port identifier is used to indicate a port between the forwarding device and a wireless gateway that is accessed by the terminal; and the forwarding device is configured to forward data from the application server through the port. 
     For example, the correspondence between the target address and the port identifier in the forwarding device is shown in Table 1. In Table 1, the forwarding device stores three target addresses: a target address “T01”, a target address “T02”, and a target address “T03” respectively. A port identifier corresponding to the target address “T01” is “P02”, a port identifier corresponding to the target address “T02” is “P00”, and a port identifier corresponding to the target address “T03” is “P01”. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Target address 
                 Port identifier 
               
               
                   
                   
               
             
            
               
                   
                 T01 
                 P02 
               
               
                   
                 T02 
                 P00 
               
               
                   
                 T03 
                 P01 
               
               
                   
                   
               
            
           
         
       
     
     Based on the correspondence provided in Table 1, in an example, when receiving, through the target port  5 , an update request that carries a target address “T01”, the forwarding device determines a target port identifier “P05” corresponding to the target port  5 , and updates a port identifier “P02” corresponding to the target address “T01” to the target port identifier “P05”. 
     When the cell handover manner is applied to an L2 networking architecture of a communications system, the forwarding device is a switch. When the cell handover manner is applied to an L3 networking architecture of a communications system, the forwarding device is a router. In a possible implementation, the forwarding device is a switch, the update request is an ARP packet, and the target address is a media access control (Media Access Control, MAC) address. 
     In the implementation, the switch stores a MAC address table, where the MAC address table includes a correspondence between a MAC address of the terminal and a port identifier. For example, the correspondence between the MAC address and the port identifier in the switch is shown in Table 2. In Table 2, the switch stores two MAC addresses: a MAC address “MAC01” and a MAC address “MAC02” respectively. A port identifier corresponding to the MAC address “MAC01” is “P00”, and a port identifier corresponding to the MAC address “MAC02” is “P01”. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 MAC address 
                 Port identifier 
               
               
                   
                   
               
             
            
               
                   
                 MAC01 
                 P00 
               
               
                   
                 MAC02 
                 P01 
               
               
                   
                   
               
            
           
         
       
     
     In another possible implementation, the forwarding device is a router, the update request is an LS update packet, and the target address is an IP address. 
     In the implementation, the router stores a routing table, where the routing table includes a correspondence between an IP address of the terminal and a port identifier. For example, the correspondence between the IP address and the port identifier in the router is shown in Table 3. In Table 3, the router stores two IP addresses: an IP address “IP01” and an IP address “IP02” respectively. A port identifier corresponding to the IP address “IP01” is “P00”, and a port identifier corresponding to the IP address “IP02” is “P01”. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 IP address 
                 Port identifier 
               
               
                   
                   
               
             
            
               
                   
                 IP01 
                 P00 
               
               
                   
                 IP02 
                 P01 
               
               
                   
                   
               
            
           
         
       
     
     Step  403 . The application server sends the downlink data of the terminal to the forwarding device. 
     Correspondingly, the forwarding device receives the downlink data of the terminal that is sent by the application server. 
     Step  404 . The forwarding device sends the downlink data of the terminal to the target wireless gateway through the target port. 
     When receiving the downlink data of the terminal from the application server, the forwarding device determines the target address corresponding to the downlink data, searches the forwarding device for the port identifier corresponding to the target address, and sends the downlink data of the terminal to the target wireless gateway through the port corresponding to the port identifier. 
     Step  405 . The target wireless gateway receives and buffers the downlink data of the terminal that is forwarded by the forwarding device through the target port. 
     When the target wireless gateway receives the downlink data sent by the forwarding device, because it is considered that buffered downlink data forwarded by the source wireless gateway to the target wireless gateway may not be completely forwarded, that is, in this case, the source wireless gateway still buffers downlink data of the terminal, the target wireless gateway also buffers the downlink data from the forwarding device. 
     Step  406 . The source wireless gateway sends an end marker message to the target wireless gateway. 
     Optionally, when clearing the buffered service data of the terminal the source wireless gateway sends an end marker (End Marker) message to the target wireless gateway, where the end marker message is used to indicate that the service data of the terminal buffered in the source wireless gateway is cleared. Correspondingly, the target wireless gateway receives the end marker message sent by the source wireless gateway. 
     Step  407 . The target wireless gateway transmits the downlink data of the terminal to the terminal corresponding to the target address. 
     The target wireless gateway sends the buffered downlink data to the terminal when receiving the end marker message. 
     It should be noted that, after the target wireless gateway receives the end marker message, when receiving the downlink data forwarded by the forwarding device through the target port, the target wireless gateway directly sends the downlink data to the terminal corresponding to the target address, and does not need to buffer the downlink data. 
     Step  408 . The target wireless gateway sends a release resource message to the source wireless gateway. 
     To ensure that session continuity of the terminal is not interrupted, after sending the update request to the target port of the forwarding device, the target wireless gateway sends the release resource message to the source wireless gateway. 
     The release resource (Release Resource) message is used to instruct the source wireless gateway to release a radio bearer between the source wireless gateway and the terminal. 
     The target wireless gateway sends the release resource message to the source wireless gateway through the X2 extended channel. Correspondingly, the source wireless gateway receives the release resource message sent by the target wireless gateway. 
     Step  409 . The source wireless gateway receives the release resource message sent by the target wireless gateway. 
     The release resource message is sent after the target wireless gateway sends the update request to the target port of the forwarding device. 
     Step  410 . The source wireless gateway releases the radio bearer between the source wireless gateway and the terminal based on the release resource message. 
     The source wireless gateway releases the radio bearer between the source wireless gateway and the terminal based on the release resource message, where the radio bearer includes an air interface resource used by the terminal. At this point, the third process in the cell handover process is completely performed. 
     In conclusion, according to this embodiment of this application, after the source wireless gateway sends the handover instruction message to the terminal, the source wireless gateway receives the release resource message sent by the target wireless gateway, and releases the radio bearer between the source wireless gateway and the terminal based on the release resource message, so that the radio bearer between the terminal and the source wireless gateway is released only after the radio bearer between the terminal and the target wireless gateway is established, thereby ensuring that session continuity of the terminal is not interrupted. 
     According to this embodiment of this application, when the forwarding device is the switch, the update request sent by the target wireless gateway to the target port of the forwarding device is the ARP packet; or when the forwarding device is the router, the update request sent by the target wireless gateway to the target port of the forwarding device is the LS update packet, so that the cell handover manner is applicable to both a forwarding device (which is the switch in this case) operating in an L2 networking architecture of the communications system and a forwarding device (which is the router in this case) operating in an L3 networking architecture. 
     According to this embodiment of this application, the target wireless gateway further sends the update request to the target port of the forwarding device, where the update request is used to instruct the forwarding device to update the port identifier corresponding to the target address to the target port identifier of the target port. In one aspect, the forwarding device is enabled to switch a path corresponding to the downlink data of the terminal from the source wireless gateway to the target wireless gateway, to further enable the downlink data delivered by the application server no longer to be forwarded to the target wireless gateway by using the source wireless gateway, but to be directly sent by the switch to the target wireless gateway, thereby further shortening a time length during cell handover, and improving cell handover efficiency. In another aspect, because the forwarding device stores the correspondence between the target address and the port identifier, the forwarding device updates the part identifier corresponding to the target address when receiving the update request, so that the target address of the terminal can remain unchanged during cell handover. 
     The following is an apparatus embodiment of this application. For a part that is not described in detail in the apparatus embodiment, refer to the details in the foregoing method embodiment. 
       FIG. 5  is a structural block diagram of a source wireless gateway according to an embodiment of this application. The source wireless gateway includes a processor  510 , a memory  520 , and a communications interface  530 . 
     The processor  510  is separately connected to the memory  520  and the communications interface  530  by using a bus. 
     The communications interface  530  is configured to implement communication with another network element. 
     The processor  510  includes one or more processing cores. The processor  510  runs an operating system or an application program module, to implement functions of the source wireless gateway in the foregoing method embodiments. 
     Optionally, the memory  520  may store an operating system  522  and an application program module  524  required by at least one function. Optionally, the application program module  524  includes a receiving unit  524   a , a processing unit  524   b , and a sending unit  524   c . The receiving unit  524   a  is configured to implement a step related to receiving. The processing unit  524   b  is configured to implement a step related to calculation or processing. The sending unit  524   c  is configured to implement a step related to sending. 
     In addition, the memory  520  may be implemented by any type of volatile or non-volatile storage device or a combination thereof for example, a static random access memory (SRAM), an electrically erasable programmable read-only memory (EEPROM), an erasable programmable read-only memory (EPROM), or a programmable read-only memory (PROM), a read-only memory (ROM), a magnetic memory, a flash memory, a magnetic disk, or an optical disc. 
     A person skilled in the art may understand that the structure shown in  FIG. 5  does not constitute a limitation on the source wireless gateway, and the source wireless gateway may include more or fewer components than those shown in the figure, or combine some components, or have different component arrangements. 
       FIG. 6  is a structural block diagram of a target wireless gateway according to an embodiment of this application. The target wireless gateway includes a processor  610 , a memory  620 , and a communications interface  630 . 
     The processor  610  is separately connected to the memory  620  and the communications interface  630  by using a bus. 
     The communications interface  630  is configured to implement communication with another network element, for example, when communicating with a terminal the communications interface  630  is an air interface, or when communicating with a forwarding device, the communications interface  630  is an SGi interface. 
     The processor  610  includes one or more processing cores. The processor  610  runs an operating system or an application program module, to implement functions of the target wireless gateway in the foregoing method embodiments. 
     Optionally, the memory  620  may store an operating system  622  and an application program module  624  required by at least one function. Optionally, the application program module  624  includes a receiving unit  624   a , a processing unit  624   b , and a sending unit  624   c . The receiving unit  624   a  is configured to implement a step related to receiving. The processing unit  624   b  is configured to implement a step related to calculation or processing. The sending unit  624   c  is configured to implement a step related to sending. 
     In addition, the memory  620  may be implemented by any type of volatile or non-volatile storage device or a combination thereof, for example, a static random access memory (SRAM), an electrically erasable programmable read-only memory (EEPROM), an erasable programmable read-only memory (EPROM), or a programmable read-only memory (PROM), a read-only memory (ROM), a magnetic memory, a flash memory, a magnetic disk, or an optical disc. 
     A person skilled in the art may understand that the structure shown in  FIG. 6  does not constitute a limitation on the target wireless gateway, and the target wireless gateway may include more or fewer components than those shown in the figure, or combine some components, or have different component arrangements. 
       FIG. 7  is a block diagram of a source wireless gateway according to another embodiment of this application. The source wireless gateway includes a receiving unit  710 , a processing unit  720 , and a sending unit  730 . 
     The receiving unit  710  is configured to implement receiving steps such as step  301 , step  405 , and step  409  in the embodiment in  FIG. 3  or the embodiment in  FIG. 4 , and other implicit receiving steps performed by the source wireless gateway. 
     The processing unit  720  is configured to implement processing steps such as step  302  and step  410  in the embodiment in  FIG. 3  or the embodiment in  FIG. 4 , and other implicit processing steps performed by the source wireless gateway. 
     The sending unit  730  is configured to implement sending steps such as step  303 , step  306 , and step  407  in the embodiment in  FIG. 3  or the embodiment in  FIG. 4 , and other implicit sending steps performed by the source wireless gateway. 
     The receiving unit  710  may be implemented by the processor  510  in  FIG. 5  by executing the receiving module  524   a  in the memory  520 . The processing unit  720  may be implemented by the processor  510  in  FIG. 5  by executing the processing module  524   b  in the memory  520 . The sending unit  730  may be implemented by the processor  510  in  FIG. 5  by executing the sending module  524   c  in the memory  520 . 
       FIG. 8  is a block diagram of a target wireless gateway according to another embodiment of this application. The target wireless gateway includes a receiving unit  810 , a processing unit  820 , and a sending unit  830 . 
     The receiving unit  810  is configured to implement receiving steps such as step  304  and step  307  in the embodiment in  FIG. 3  or the embodiment in  FIG. 4 , and other implicit receiving steps performed by the target wireless gateway. 
     The processing unit  820  is configured to implement processing steps that are performed by the target wireless gateway and that are implicit in the embodiment in  FIG. 3  or the embodiment in  FIG. 4 . 
     The sending unit  830  is configured to implement sending steps such as step  305 , step  401 , step  404 , and step  408  in the embodiment in  FIG. 3  or the embodiment in  FIG. 4 , and other implicit sending steps performed by the target wireless gateway. 
     The receiving unit  810  may be implemented by the processor  610  in  FIG. 6  by executing the receiving module  624   a  in the memory  620 . The processing unit  820  may be implemented by the processor  610  in  FIG. 6  by executing the processing module  624   b  in the memory  620 . The sending unit  830  may be implemented by the processor  610  in  FIG. 6  by executing the sending module  624   c  in the memory  620 . 
     It should be noted that an embodiment of this application further provides a source wireless gateway. The source wireless gateway includes a chip and at least one network interface. The chip includes a programmable logic circuit and/or a program instruction. When the chip is run, the chip is configured to implement the cell handover method provided in the foregoing method embodiments. 
     An embodiment of this application further provides a target wireless gateway. The target wireless gateway includes a chip and at least one network interface. The chip includes a programmable logic circuit and/or a program instruction. When the chip is nm, the chip is configured to implement the cell handover method provided in the foregoing method embodiments. 
     An embodiment of this application further provides a computer-readable storage medium. The computer-readable storage medium stores at least one instruction, and the at least one instruction is loaded and executed by a processor to implement the cell handover method provided in the foregoing method embodiments. 
     An embodiment of this application further provides a computer-readable storage medium. The computer-readable storage medium stores at least one instruction, and the at least one instruction is loaded and executed by a processor to implement the cell handover method provided in the foregoing method embodiments. 
     A person of ordinary skill in the art may understand that all or some of the steps of the embodiments may be implemented by hardware or a program instructing related hardware. The program may be stored in a computer-readable storage medium. The storage medium may be a read-only memory, a magnetic disk, or an optical disc. 
     The foregoing descriptions are merely example embodiments of this application, but are not intended to limit this application. Any modification, equivalent replacement, or improvement made without departing from the spirit and principle of this application should fall within the protection scope of this application.