Abstract:
The present disclosure relates to a pre-5 th -Generation (5G) or 5G communication system to be provided for supporting higher data rates Beyond 4 th -Generation (4G) communication system such as Long Term Evolution (LTE). A method for preventing a loss of data packets of a transmission end is provided. The method includes performing a path switching operation, receiving, from a reception end, a switching status report that includes information related to the data packets that were not received by the reception end, and retransmitting the data packets that were not received by the reception end, where the path switching operation is an operation of switching a data transmission path from a first communication system to a second communication system or from the second communication system to the first communication system, and where the first communication system is a fourth generation (4G) communication system, and the second communication system is a fifth generation (5G) communication system that uses a millimeter-wave (mm-wave) band.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed on Feb. 4, 2016 in the Korean Intellectual Property Office and assigned Serial number 10-2016-0014453, and of a Korean patent application filed on Mar. 29, 2016 in the Korean Intellectual Property Office and assigned Serial number 10-2016-0037989, the entire disclosure of each of which is hereby incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to the function of a fifth generation (5G) mobile communication system. More particularly, the present disclosure relates to communication devices that support two or more general communication systems. 
       BACKGROUND 
       [0003]    In order to meet the wireless data traffic demand that is on an increasing trend after commercialization of fourth generation (4G) communication system, efforts for developing improved fifth generation (5G) communication system or pre-5G communication system have been made. For this reason, the 5G communication system or pre-5G communication system has been called beyond 4G network communication system or post long term evolution (LTE) system. 
         [0004]    In order to achieve high data rate, implementation of 5G communication system in a millimeter wave (mmwave) band (e.g., like 60 GHz band) has been considered. In order to mitigate a radio wave path loss and to increase a radio wave transmission distance in the mmwave band, technologies of beam-forming, massive multiple input and multiple output (MIMO), full dimension MIMO (FD-MIMO), analog beam-forming, and large scale antenna for the 5G communication system have been discussed. 
         [0005]    Further, for system network improvement in the 5G communication system, technology developments have been made for an evolved small cell, improved small cell, cloud radio access network (cloud RAN), ultra-dense network, device to device communication (D2D), wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), and reception interference cancellation. 
         [0006]    In addition, hybrid frequency shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and sliding window superposition coding (SWSC), which correspond to advanced coding modulation (ACM) system, and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA), which correspond to advanced connection technology, have been developed in the 5G system. 
         [0007]    The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure. 
       SUMMARY 
       [0008]    In a primary deployment scenario of a fifth generation (5G) mobile communication network, however, a scenario that is based on interlocking with the existing fourth generation (4G) communication system is included. In this case, if it is difficult to correct a 4G communication terminal modem of a 4G communication device, a method and an apparatus for preventing a loss of data packets and reordering the sequence thereof are required to switch or split data paths between communication devices of different generations. 
         [0009]    Aspects of the present disclosure are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present disclosure is to provide an apparatus and method for preventing a loss of data packets and reordering the sequence thereof to switch or split data paths between communication devices of different generations. 
         [0010]    In accordance with an aspect of the present disclosure, a method for preventing a loss of data packets of a transmission end is provided. The method includes performing a path switching operation, receiving, from a reception end, a switching status report that includes information related to the data packets that were not received by the reception end, and retransmitting the data packets that were not received by the reception end, wherein the path switching operation is an operation of switching a data transmission path from a first communication system to a second communication system or from the second communication system to the first communication system, and wherein the first communication system is a fourth generation (4G) communication system, and the second communication system is a fifth generation (5G) communication system that uses a millimeter-wave (mm-wave) band. 
         [0011]    In accordance with another aspect of the present disclosure, a method for preventing a loss of data packets of a reception end is provided. The method includes performing a path switching operation, transmitting, to a transmission end, a switching status report that includes information related to the data packets that were not received by the reception end, and re-receiving the data packets that were not received, wherein the path switching operation is an operation of switching a data transmission path from a first communication system to a second communication system or from the second communication system to the first communication system, and wherein the first communication system is a fourth generation (4G) communication system, and the second communication system is a fifth generation (5G) communication system that uses a millimeter-wave (mm-wave) band. 
         [0012]    In accordance with still another aspect of the present disclosure, a transmission end apparatus for preventing a loss of data packets is provided. The transmission end apparatus includes a transceiver configured to transmit and receive signals with a reception end apparatus and at least one processor configured to perform a path switching operation, receive, from the reception end apparatus, a switching status report that includes information related to the data packets that were not received by the reception end apparatus, and retransmit the data packets that were not received by the reception end apparatus, wherein the path switching operation is an operation of switching a data transmission path from a first communication system to a second communication system or from the second communication system to the first communication system, and wherein the first communication system is a fourth generation (4G) communication system, and the second communication system is a fifth generation (5G) communication system that uses a millimeter-wave (mm-wave) band. 
         [0013]    In accordance with yet still another aspect of the present disclosure, a reception end apparatus for preventing a loss of data packets is provided. The reception end apparatus includes a transceiver configured to transmit and receive signals with a transmission end apparatus and at least one processor configured to perform a path switching operation, transmit, to the transmission end apparatus, a switching status report that includes information related to the data packets that were not received by the reception end apparatus, and re-receive the data packets that were not received, wherein the path switching operation is an operation of switching a data transmission path from a first communication system to a second communication system or from the second communication system to the first communication system, and wherein the first communication system is a fourth generation (4G) communication system, and the second communication system is a fifth generation (5G) communication system that uses a millimeter-wave (mm-wave) band. 
         [0014]    Here, as an example, if the data packets are downlink data packets, the transmission end is a base station and the reception end is a terminal, and if the data packets are uplink data packets, the transmission end is the terminal and the reception end is the base station. 
         [0015]    According to the various embodiments of the present disclosure, it is possible to perform lossless data transmission even in the case of data path switching and data path splitting in an environment in which two different kinds of communication systems coexist. 
         [0016]    Other aspects, advantages, and salient features of the disclosure will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses various embodiments of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
           [0018]      FIG. 1  is a diagram illustrating devices that constitute a scenario according to an embodiment of the present disclosure; 
           [0019]      FIG. 2  is a diagram illustrating an example of downlink traffic in a scenario according to an embodiment of the present disclosure; 
           [0020]      FIG. 3  is a diagram illustrating a simple protocol stack of a base station and a terminal that constitute according to an embodiment of the present disclosure; 
           [0021]      FIG. 4  is a diagram illustrating an example of a detailed device configuration that can carry out according to an embodiment of the present disclosure; 
           [0022]      FIG. 5  is a diagram illustrating an example of a detailed device configuration that can carry out according to an embodiment of the present disclosure; 
           [0023]      FIG. 6  is a diagram illustrating an example of a detailed device configuration that can carry out according to an embodiment of the present disclosure; 
           [0024]      FIG. 7  is a diagram illustrating an example of a detailed device configuration that can carry out according to an embodiment of the present disclosure; 
           [0025]      FIG. 8  is a diagram illustrating an example of a method for preventing a loss of downlink data when data path switching is performed from a fifth generation (5G) link to a fourth generation (4G) link according to an embodiment of the present disclosure; 
           [0026]      FIG. 9  is a diagram illustrating an example of a method for preventing a loss of uplink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure; 
           [0027]      FIG. 10  is a diagram illustrating an example of a method for preventing a loss of downlink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure; 
           [0028]      FIG. 11  is a diagram illustrating another example of a method for preventing a loss of downlink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure; 
           [0029]      FIG. 12  is a diagram illustrating an example of a method for preventing a loss of uplink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure; 
           [0030]      FIG. 13  is a diagram illustrating another example of a method for preventing a loss of downlink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure; 
           [0031]      FIG. 14  is a diagram illustrating another example of a method for preventing a loss of uplink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure; 
           [0032]      FIG. 15  is a diagram illustrating an example of a switching (SWI)/splitting (SPL) header structure in the case where SWI/SPL has a sequence number (SN) according to an embodiment of the present disclosure; 
           [0033]      FIG. 16  is a diagram illustrating an example of a method for preventing a loss of downlink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure; 
           [0034]      FIG. 17  is a diagram illustrating an example of a method for preventing a loss of uplink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure; 
           [0035]      FIG. 18  is a diagram illustrating an example of a method for preventing a loss of downlink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure; 
           [0036]      FIG. 19  is a diagram illustrating an example of a method for preventing a loss of uplink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure; 
           [0037]      FIG. 20  is a diagram illustrating an example of a method for preventing a loss of downlink data when data path splitting, through which data is transmitted to both a 5G link and a 4G link, is performed according to an embodiment of the present disclosure; 
           [0038]      FIG. 21  is a diagram illustrating an example of a method for preventing a loss of uplink data when data path splitting, through which data is transmitted to both a 5G link and a 4G link, is performed according to an embodiment of the present disclosure; 
           [0039]      FIG. 22  is a block diagram illustrating a structure of a base station  100  according to an embodiment of the present disclosure; 
           [0040]      FIG. 23  is a block diagram illustrating a structure of a terminal  110  according to an embodiment of the present disclosure; 
           [0041]      FIG. 24  is a diagram illustrating an example of a method for preventing a loss of downlink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure; 
           [0042]      FIG. 25  is a diagram illustrating an example of a method for preventing a loss of downlink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure; 
           [0043]      FIG. 26  is a diagram illustrating an example of a method for preventing a loss of uplink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure; and 
           [0044]      FIG. 27  is a diagram illustrating an example of a method for preventing a loss of uplink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure. 
       
    
    
       [0045]    Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures. 
       DETAILED DESCRIPTION 
       [0046]    The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness. 
         [0047]    The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the present disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the present disclosure as defined by the appended claims and their equivalents. 
         [0048]    It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
         [0049]    Further, in describing various embodiments of the present disclosure in detail, although the main objects would be a fourth generation (4G) mobile communication system and a fifth generation (5G) mobile communication system, the main subject of the present disclosure can be applied to other communication systems adopting similar technical backgrounds and channel types with slight modifications within a range that does not greatly deviate from the scope of the present disclosure, and this would be possible according to the judgment of a person skilled in the art to which the present disclosure pertains. 
         [0050]    The aspects and features of the present disclosure and methods for achieving the aspects and features will be apparent by referring to the various embodiments to be described in detail with reference to the accompanying drawings. However, the present disclosure is not limited to the various embodiments disclosed hereinafter, but can be implemented in diverse forms. The matters defined in the description, such as the detailed construction and elements, are nothing but specific details provided to assist those of ordinary skill in the art in a comprehensive understanding of the disclosure, and the present disclosure is only defined within the scope of the appended claims. In the entire description of the present disclosure, the same drawing reference numerals are used for the same elements across various figures. 
         [0051]    As technology in the related art, there exist 4G mobile communication technology and 5G mobile communication technology. The 4G mobile communication technology is technology capable of operating in a frequency band that is equal to or lower than 6 GHz on the basis of orthogonal frequency-division multiple access (OFDMA) and single carrier FDMA (SC-FDMA). The 5G communication technology in the related art may be divided into “Above 6 GHz” technology that operates in a millimeter-wave (mm-wave) band that is equal to or higher than 6 GHz and “Below 6 GHz” technology that operates in an operation frequency band of the 4G mobile communication that is equal to or lower than 6 GHz. 
         [0052]    A main deployment scenario of a 5G mobile communication network includes a scenario that is based on interlocking with the existing 4G communication system. In this case, since it is difficult to correct the 4G technology standard and related devices, it is preferred to minimize correction of the existing 4G communication system. In such a situation, if it is difficult to correct a 4G communication device, and in particular, a 4G communication terminal modem thereof, it becomes difficult to prevent a loss of data packets and to perform sequence redeployment in switching or splitting data paths between communication devices having different generations. 
         [0053]    According to the present disclosure, it is possible to achieve lossless data transmission in the case where a data communication path is switched between different systems in a scenario where the 5G communication system and the 4G communication system coexist or data packets are split and transmitted to different systems (e.g., in the case where a part of data packets is transmitted through the 5G communication technology and the remainder thereof is transmitted through the 4G communication technology). The present disclosure may be equally applied to perform the lossless data transmission in the case where the data communication path is switched between the different systems in the scenario where the 5G communication system and the 4G communication system coexist in the upper concept, or the data packets are split and transmitted to the different systems. 
         [0054]      FIG. 1  is a diagram illustrating devices that constitute a scenario according to an embodiment of the present disclosure. 
         [0055]    Referring to  FIG. 1 , the present disclosure relates to mobile communications or cellular communications, and assumes an environment in which one or more base stations (eNBs) for the mobile communications and one or more terminals (UEs) are provided. Further, the present disclosure assumes base stations and terminals that support communications using two different kinds of communication systems. For example, base stations  100  and terminals  110  that respectively support the 4G mobile communication technology and the 5G mobile communication technology may be exemplified. As described above, the 5G mobile communication technology may be one of the “Above 6 GHz” technology and the “Below 6 GHz” technology. In the description of the present disclosure, for convenience in understanding, the 4G and 5G communication systems will be described, but essentially, there is no limit in communication technology used in the present disclosure. 
         [0056]    In the environment where the two kinds of communication technologies coexist as described above, it may be difficult to exchange internal information between communication devices that support different communication systems due to several technical limitations including backward compatibility. As an example, it may be difficult to transfer lately used sequence number (SN) information to the coexisting 5G network at a specific time of a specific radio bearer (RB) in a 4G long term evolution (LTE) communication system. In order to transfer such internal information between the communication devices, it is required that an interface exists between communication technologies in the communication devices, and for this, correction of the communication device is required. 
         [0057]    In the present disclosure, it is assumed that the communication devices in the base station can exchange the internal information between the different communication technologies through correction of the base station device. However, it is assumed that the terminal cannot exchange the internal information between the communication technologies. As an example, a terminal that supports both the 4G communication network and the 5G communication network cannot transfer the internal information including packet data convergence protocol (PDCP) SN of the 4G communication device to the 5G communication device of the terminal. In the same manner, the internal information including the PDCP SN of the 5G communication device cannot be transferred to the 4G communication device. 
         [0058]      FIG. 2  is a diagram illustrating an example of downlink traffic in a scenario according to an embodiment of the present disclosure. 
         [0059]    Referring to  FIG. 2 , downlink traffic that is transmitted from a base station to a terminal may be transferred to the terminal through transmission of the downlink traffic to a 4G terminal device  230  of the terminal through a 4G base station device  220  in the base station. This is called a 4G link  200 . Further, downlink traffic that is transmitted from the base station to the terminal may be transferred to the terminal through transmission of the downlink traffic to a 5G terminal device  232  of the terminal through a 5G base station device  222  in the base station according to the present disclosure. This is called a 5G link  210 . According to various embodiments, the base station and the terminal that have both the 4G communication device and the 5G communication device may be respectively called a 5G base station and a 5G terminal in the wide concept. Further, such a 5G base station and a 5G terminal may be called 5G communication devices in the upper concept. 
         [0060]    Communications may be performed using only one of the 4G link and the 5G link at the specific time according to support capability of the base station or the terminal or a protocol of the communication network. In this case, a situation, in which the communications are performed through the 4G link, and then the communication link is switched from the 4G link to the 5G link, or a situation, in which the communications are performed through the 5G link, and then the communication link is switched from the 5G link to the 4G link on the contrary, may be considered. Such a situation is called “data path switching”. Further, the communications may be simultaneously performed using both the 4G link and the 5G link according to the support capability of the base station or the terminal or the protocol of the communication network. Such a situation is called “data path splitting”. 
         [0061]    The present disclosure proposes a method for securing lossless data transmission in the data path switching and data path splitting scenario. As previously assumed, the 4G communication device and the 5G communication device of the terminal cannot mutually exchange the internal information, such as PDCP SN. However, the base station can know and control the internal information between the 5G and 4G base station communication devices through the protocol and the device correction. It is not required to perform the internal information exchange and control between the communication systems with respect to all kinds of information and settings, but the internal information exchange and control may be minimally performed only with respect to those that are required to operate the communication network. 
         [0062]    Further, in the case of uplink traffic, explanation may be made in the same manner as the downlink traffic except for a different point that the direction of traffic is changed from the terminal to the base station. 
         [0063]      FIG. 3  is a diagram illustrating a simple protocol stack of a base station and a terminal that constitute according to an embodiment of the present disclosure. 
         [0064]    Referring to  FIG. 3 , an environment is assumed, in which an LTE that is a kind of 4G mobile communication technology and 5G communication technology coexist.  FIG. 3  illustrates one LTE-dedicated RB  300 , an LTE RB  310  connected through 5G switching (SWI)/splitting (SPL) sublayer or function (hereinafter referred to as “SWI/SPL”), and a 5G RB  320  connected through 5G SWI/SPL sublayer or function. The LTE RB  310  and the 5G RB  320  that are connected through the SWI/SPL may be one of a data RB (DRB) and a signaling RB (SRB). The SWI/SPL may be operated in the form of a sublayer or function, and may perform one or more roles of path switching and path spitting. The SWI/SPL of a reception end transmits data packets that have come through the LTE or 5G path to an upper layer to match the transmission sequence. 
         [0065]    Although the LTE RB  310  and the 5G RB  320  that are branched from the SWI/SPL according to the setting and technology definition may be called one 5G RB, the LTE RB  310  is recognized only as an LTE RB, such as the LTE-dedicated RB  300 , from the side of an LTE communication device, and the upper connection thereof becomes only the SWI/SPL. In the wide concept, an LTE protocol stack and a 5G protocol stack, which interlock with each other according to the above embodiment, may be called a 5G protocol stack. 
         [0066]    However, the scope of the present disclosure is not limited in a manner that the SWI/SPL layer or function should be surely located at an upper end of the 5G stack, and the contents to be described in the present disclosure are applied as they are in the case where the SWI/SPL layer or function is located at one portion of an upper end of the LTE RB or the 5G RB. In addition, the contents to be described in the present disclosure can be applied as they are even in the case where the SWI/SPL layer  330  or function is located at a third position other than the upper end of the LTE RB or the 5G RB. In  FIG. 3 , a two-layer structure that is defined by the 3rd generation partnership project (3GPP) of PDCP ( 340 ,  342 ,  344 ), radio link control (RLC) ( 350 ,  352 ,  354 ), and medium access control (MAC) ( 360 ,  362 ) is assumed. However, the present disclosure is not limited even in other layer structures so far as it has functions that are required in the present disclosure. For convenience, in the present disclosure, it is described that both the 4G and the 5G have the two-layer structure of the 3GPP. 
         [0067]      FIG. 4  is a diagram illustrating an example of a detailed device configuration that can carry out according to an embodiment of the present disclosure. 
         [0068]    Referring to  FIG. 4 , a 5G communication device includes a 4G modem  400 , a 5G modem  404 , and a 5G application processor (AP)  402 . However, the 5G communication device may be configured to include other devices including an antenna module. In the present disclosure, devices of which explanation is not necessary will be omitted, but this does not mean that the other devices are not included in the 5G communication device. 
         [0069]    In the 5G communication device in the wide meaning, an LTE modem that is composed of one or more SRBs and one or more DRBs may exist in the 4G modem  400 . A 4G SRB  410  is connected to a radio resource control (RRC) sublayer (hereinafter referred to as “RRC”)  420  that controls the 4G link, and serves to transfer data traffic that is generated in an upper layer. A plurality of DRBs may exist, and according to 5G coexistence scenario, parts of DRBs may independently operate as 4G, while other DRBs may be connected to a 5G RRC  430  or a SWI/SPL  432 . The 5G RRC  430  serves to transfer a control message of a 5G communication system. For stable transmission, the control message that is transmitted by the 5G RRC  430  may be encapsulated and transmitted using a 4G DRB  418 . The SWI/SPL  432  performs path switching or splitting of 5G data as described above with reference to  FIG. 3 . The SWI/SPL  432  may transmit the arriving data packets to a 4G DRB or 5G DRB  416  that is previously connected thereto. In the embodiment of  FIG. 4 , it is assumed that the 5G RRC and the 5G SWI/SPL are located at the AP  402  of the 5G communication device, and it is exemplified that one 5G RB (SRB or DRB) corresponds to one 4G DRB. 
         [0070]      FIG. 5  is a diagram illustrating another example of a detailed device configuration that can carry out according to an embodiment of the present disclosure. 
         [0071]    The configuration of  FIG. 5  is generally similar to the configuration of  FIG. 4 , and a 5G RRC  510  and a SWI/SPL  512  are located in a 5G modem  502 . In this case, it may be required for the 5G modem  502  controls a 4G modem  500 . According to an implementation method, a direct interface may not exist between the 4G modem  500  and the 5G modem  502 , and even in this case, the same logical flow is presented, in which a 4G DRB is connected to the 5G RRC  510  or the SWI/SPL  512 . 
         [0072]      FIG. 6  is a diagram illustrating still another example of a detailed device configuration that can carry out according to an embodiment of the present disclosure. 
         [0073]    Referring to  FIG. 6 , a 5G communication device includes a 4G modem  600 , a 5G modem  604 , and a 5G AP  602 . The configuration of  FIG. 6  is generally similar to the configuration of  FIG. 4 , and a scenario in which 5G data traffic that is connected from a 5G RRC  610  and a SWI/SPL  612  is multiplexed (or mixed) in one 4G DRB  620  is assumed. In this case, a D/C multiplexing (hereinafter referred to as “D/C Mux”) function  614  is added, and if necessary, a control message that is transmitted from a 5G RRC  610  and a header type divider that divides data may be added. Such a header may be configured as a SWI/SPL header. In this embodiment, the D/C Mux function may be located in the 5G AP  602 . 
         [0074]      FIG. 7  is a diagram illustrating yet still another example of a detailed device configuration that can carry out according to an embodiment of the present disclosure. 
         [0075]    Referring to  FIG. 7 , a 5G communication device includes a 4G modem  700  and a 5G modem  702 . The configuration of  FIG. 7  is generally similar to the configuration of  FIG. 5 , and a scenario in which a control message and 5G data traffic that are connected from a 5G RRC  710  and a SWI/SPL  712  are multiplexed in one 4G DRB  720  is assumed. In this case, a D/C Mux function is added, and if necessary, a control message that is transmitted from a 5G RRC  710  and a header type divider that divides data may be added. Such a header may be configured as a SWI/SPL header. In this embodiment, the D/C Mux function may be located in the 5G modem  702 . 
         [0076]    Hereinafter, a detailed operation according to the present disclosure will be described. 
         [0077]      FIG. 8  is a diagram illustrating an example of a method for preventing a loss of downlink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure. 
         [0078]    Referring to  FIG. 8 , a SWI/SPL performs only a switching function, and is hereinafter described as SWI. Further,  FIG. 8  illustrates a method for SWIs  802  and  812  to prevent a data loss using PDCP SN information of a 5G communication system and a 4G communication system without any separate header. In  FIG. 8  and the subsequent figures, a circular point that is located on an arrow therein means that a message or data is transmitted via a layer having the circular point. 
         [0079]    Since a 5G link is first used for data communications, user data, which may be mixedly used with a data packet, a service data unit (SDU), or a packet data unit (PDU), is transmitted to a 5G SWI  802  of a terminal  110  through transmission of the user data from a 5G SWI  812  of a base station  100  to a 5G PDCP  804  of the terminal and a related lower layer through a 5G PDCP sublayer (hereinafter referred to as “PDCP”)  810  and a related lower layer (S 820 ). Thereafter, at a specific time, the terminal  110  and the base station  100  perform a switching procedure of a data path from a 5G link to a 4G link (S 825 ). The path switching operation may be defined in several methods, and in the present disclosure, the above-described operation is not limited. In the following operation according to the present disclosure, a message or an information element (IE) in the switching operation may be included in a message in the path switching operation. 
         [0080]    After the path switching operation, the base station  100  may request a PDCP status report of the 5G DRB from the terminal  110 . In this embodiment, it is assumed that such a report in the form of an IE of the 5G RRC that corresponds to PDCP status report triggering is transmitted from a 5G RRC  814  of the base station to a 5G RRC  800  of the terminal through a 4G PDCP  808  of the base station and a 4G PDCP  806  of the terminal (S 830 ). The PDCP status report triggering IE is transferred from the 5G RRC  800  to the 5G PDCP  806  (S 835 ). 
         [0081]    Thereafter, the 5G PDCP  806  prepares a PDCP status report till then, and transmits the prepared PDCP status report to the 5G RRC  800  and an 5G PDCP  804  (S 840 ). The PDCP status report information is generated on the basis of the 5G PDCP SN that is received at the PDCP status report generation time. Thereafter, the terminal  110  transmits a switching status report message to a 5G RRC  814  of the base station  100  through the 5G RRC  800  (S 845 ). In this embodiment, it is assumed that the switching status report is included in an RRC message. In this case, if the 5G PDCP  804  of the terminal  110  transmits information related to the PDCP status report to the 5G RRC  800 , the 5G RRC  800  may include the same information in the switching status report to be transmitted. However, such an internal operation of the terminal may differ according to the implementation thereof. 
         [0082]    Since the switching status report is included in the 5G RRC message to be transmitted, it is transmitted via the 4G PDCPs  806  and  808  of the terminal and the base station for the stable transmission thereof. However, as described above, since the 4G PDCP transmits the control message that is transmitted from the 5G RRC to the DRB, the contents of the control message cannot be known, and thus are processed as data. Thereafter, the 5G RRC  814  transfers the PDCP status report in the received switching status report to the 5G SWI  812  (S 850 ). 
         [0083]    Since the 5G SWI  812  can know the non-received user data through the contents of the 5G PDCP status report that is included in the switching status report, it forwards the data that is not received in the terminal to the sublayer of the 4G PDCP  808  (S 855 ), and then transmits 5G data packets after the path switching operation to the 4G PDCP  808  in order to transmit the 5G data packets to the terminal  110  (S 865 ). The 4G PDCP  808  transmits the data to the 4G PDCP  804  of the terminal  110  through giving of new PDCP SNs (4G PDCP SNs) to the data packets in the reception sequence through the 5G SWI  812  (S 860  and S 865 ), and the 4G PDCP  806  of the terminal  110  transfers the data to the 5G SWI  802  that is an upper layer to match the sequence of the 4G PDCP SNs of the data (S 860  and S 865 ). In this case, the 5G SWI  802  may order the data sequence through insertion of the PDCP data (which may be mixedly used with the SDU or data packets) that is transferred from the 4G PDCP  806  (S 860 ) in sequence into the non-received user data that is included in the 5G PDCP status report. After restoring all the non-received SDUs that are included in the PDCP status report, the 5G SWI  802  of the terminal  110  may transfer the data upward. This function is called reordering (S 870 ). 
         [0084]    In the present disclosure, it is not required that the 5G PDCP SN that was previously given to the data essentially coincides with a 4G PDCP SN that is newly given to the data, but it is important that non-received SDUs that are interpreted on the basis of the 5G PDCP status report using the 5G PDCP SN sequentially correspond to the newly given 4G PDCP SNs. Based on this, the data reordering can be performed in the 5G SWI  802  of the terminal. 
         [0085]    As described above, in the whole description of the present disclosure, the 5G SWI is not necessarily required to be located at an upper end of the 5G protocol stack, but may be located on the outside. Actually, as illustrated in  FIGS. 4 and 6 , the SWI may be located in the 5G AP or in other places. 
         [0086]      FIG. 9  is a diagram illustrating an example of a method for preventing a loss of uplink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure. 
         [0087]    Referring to  FIG. 9 , a SWI/SPL performs only a switching function, and is hereinafter described as SWI. Further,  FIG. 9  illustrates a method for SWIs  902  and  912  to prevent a data loss using PDCP SN information of a 5G communication system and a 4G communication system without any separate header. 
         [0088]    Since a 5G link is first used for data communications, user data is transmitted to a 5G SWI  912  of a base station  100  through transmission of the user data from a 5G SWI  902  of a terminal  110  to a 5G PDCP  910  of the base station  100  and a related lower layer through a 5G PDCP  904  and a related lower layer (S 920 ). Thereafter, at a specific time, the terminal  110  and the base station  100  perform a switching operation of a data path from a 5G link to a 4G link (S 925 ). 
         [0089]    In this case, a 5G PDCP sublayer  910  of the base station  100  that is a data reception end prepares a PDCP status report and transfers the prepared PDCP status report to the 5G SWI  912  and a 5G RRC  914  (S 930 ). The PDCP status report is in the form of a switching status report of a 5G RRC message, and is transmitted from a 5G RRC  914  of the base station  100  to a 5G RRC  900  of the terminal  100  (S 935 ). In this case, the RRC message information includes the same information as the PDCP status report information. If this message is transferred to a layer of the 5G SWI  902  of the terminal  110  (S 940 ), the 5G SWI  902  forwards non-received SDUs of the PDCP status report to a 4G PDCP  906  of the terminal (S 945 ), and transmits them to the 5G SWI  912  through the 4G PDCP  910  of the base station  100  (S 950 ). Thereafter, the user data is continuously transmitted (S 955 ). 
         [0090]    In the same manner as the case of  FIG. 8 , the terminal sequentially transmits the non-received SDUs that are indicated in the PDCP status report (switching status report) that is prepared using the 5G PDCP SN through the 4G PDCP using the 4G PDCP SN, and the 5G SWI of the base station sequentially puts the PDCP SDUs that are received through the 4G PDCP in the existing SDU places to complete the reordering (S 960 ). 
         [0091]      FIG. 10  is a diagram illustrating an example of a method for preventing a loss of downlink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure. 
         [0092]    Referring to  FIG. 10 , a SWI/SPL performs only a switching function, and is hereinafter described as SWI. Further,  FIG. 10  illustrates a method for SWls  1002  and  1012  to prevent a data loss using PDCP SN information of a 5G communication system and a 4G communication system without any separate header. 
         [0093]    Since a 4G link is first used for data communications, user data is transmitted to the 5G SWI  1002  of a terminal  110  through transmission of the user data from the 5G SWI  1012  of a base station  100  to a 4G PDCP  1006  of the terminal and a related lower layer through a 4G PDCP  1008  and a related lower layer (S 1020 ). Thereafter, at a specific time, the terminal  110  and the base station  100  perform switching of a data path from a 4G link to a 5G link (S 1025 ). The path switching operation may be defined in several methods, and in the present disclosure, the path switching operation is not limited to the above-described operation. In the following operation according to the present disclosure, a message or an IE in the switching operation may be included. 
         [0094]    After the path switching operation, the 4G PDCP  1006  of the terminal  110  cannot directly transmit the status report information to a 5G PDCP  1004 , and thus transmits the status report to the base station. For this, the 4G RRC  1008  of the base station  100  transmits an RRCConnectionReconfiguration message that is one of RRC messages to the 4G RRC  1006  through setting of a StatusReportRequired field of the RRCConnectionReconfiguration message to an ON state (S 1030 ), and the 4G RRC  1006  transmits an RRCConnectionReconfigurationComplete message to the 4G RRC  1008  (S 1035 ). Thereafter, the 4G PDCP  1006  transmits the PDCP status report to a 4G PDCP  1010  of the base station (S 1040 ). The 4G PDCP  1010  of the base station transmits this information to a 5G RRC  1014  and the 5G PDCP  1010  (S 1045  and S 1050 ), and the 5G RRC  1014  that has received the PDCP status report transmits a switching status report message to the 5G RRC  1000  of the terminal  110  (S 1055 ). Thereafter, the 5G RRC  1000  transfers the switching status report or a PDCP status report provided therein to the 5G PDCP  1004  and the 5G SWI  1002  (S 1060 ). 
         [0095]    Then, the 4G PDCP  1008  of the base station  100  forwards non-received PDCP SDUs (or PDCP protocol data units (PDUs)) based on the status report to the 5G PDCP  1010  (or 5G SWI  1012 ) (in  FIG. 10 , it is indicated as the 5G PDCP) (S 1065 ), and first transmits the forwarded data to the terminal  110  through the 5G PDCP  1010  (S 1070 ). Thereafter, the data is sequentially transmitted to the terminal (S 1075 ). In this case, the data is transmitted from the 5G PDCP  1010  of the base station  100  to the 5G PDCP  1004  of the terminal  110 , and the data that is transmitted to the terminal is transferred to the 5G SWI  1002  to perform the reordering on the basis of the status report information (S 1080 ). 
         [0096]      FIG. 11  is a diagram illustrating another example of a method for preventing a loss of downlink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure. 
         [0097]    Referring to  FIG. 11 , a SWI/SPL performs only a switching function, and is hereinafter described as SWI. Further,  FIG. 11  illustrates a method for SWIs  1102  and  1112  to prevent a data loss without any separate header. 
         [0098]    Since a 4G link is first used for data communications, user data is transmitted to the 5G SWI  1102  of a terminal  110  through transmission of the user data from the 5G SWI  1112  of a base station  100  to a 4G PDCP  1106  of the terminal  110  and a related lower layer through a 4G PDCP  1108  and a related lower layer (S 1120 ). Thereafter, at a specific time, the terminal  110  and the base station  100  perform switching of a data path from a 4G link to a 5G link (S 1125 ). The path switching operation may be defined in several methods, and in the present disclosure, the path switching operation is not limited to the above-described operation. 
         [0099]    In  FIG. 11 , after the path switching operation is completed, a timer A  1130  operates. The length of the timer A may be predetermined or may be settable. Until the timer A expires, the terminal  110  receives only data that is transferred to the 5G SWI  1102  of the terminal  110  through 4G PDCPs  1106  and  1108  of the base station and the terminal (S 1135 ), and sequentially transmits the received data upward. After the timer A expires, the terminal  110  may sequentially transmit the data that is transmitted to the 5G SWI  1102  through the 5G PDCPs  1104  and  1110  of the base station and the terminal upward (S 1140 ). 
         [0100]    This is because, since the data before the path switching operation has been transmitted using the 4G link, it is expected that the data that is transmitted through the 4G PDCPs  1106  and  1108  of the base station and the terminal precedes the data that is transmitted through the 5G PDCPs  1104  and  1110  of the base station and the terminal. For the above-described operation, the length of the timer A may be properly set. 
         [0101]      FIG. 12  is a diagram illustrating another example of a method for preventing a loss of uplink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure. 
         [0102]    Referring to  FIG. 12 , a SWI/SPL performs only a switching function, and is hereinafter described as SWI. Further,  FIG. 12  illustrates a method for a SWI to prevent a data loss without any separate header. 
         [0103]    Since a 4G link is first used for data communications, user data is transmitted to a 5G SWI  1212  of a base station  100  through transmission of the user data from a 5G SWI  1202  of a terminal  110  to a 4G PDCP  1208  of the base station  100  and a related lower layer through a 4G PDCP  1206  and a related lower layer (S 1220 ). Thereafter, at a specific time, the terminal  110  and the base station  100  perform switching of a data path from a 4G link to a 5G link (S 1225 ). The path switching operation may be defined in several methods, and in the present disclosure, the path switching operation is not limited to the above-described operation. 
         [0104]    In  FIG. 12 , after the path switching operation is completed, a timer A  1230  operates. The length of the timer A may be predetermined or may be settable. Until the timer A expires, the base station  100  receives only data that is transmitted to the 5G SWI  1212  of the base station  100  through the 4G PDCPs  1206  and  1208  of the base station and the terminal (S 1235 ), and sequentially transmits the received data upward. After the timer A expires, the terminal  110  may sequentially transmit the data that is transmitted to the 5G SWI  1212  through the 5G PDCPs  1204  and  1210  of the base station and the terminal upward (S 1240 ). 
         [0105]      FIG. 13  is a diagram illustrating another example of a method for preventing a loss of downlink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure. 
         [0106]    Referring to  FIG. 13 , a SWI/SPL performs only a switching function, and is hereinafter described as SWI. Further,  FIG. 13  illustrates a method for a SWI to prevent a data loss without any separate header. 
         [0107]    Since a 5G link is first used for data communications, user data is transmitted to a 5G SWI  1302  of a terminal  110  through transmission of the user data from a 5G SWI  1312  of a base station  100  to a 5G PDCP  1304  of the terminal  110  and a related lower layer through a 5G PDCP  1310  and a related lower layer (S 1320 ). Thereafter, at a specific time, the terminal  110  and the base station  100  perform switching of a data path from a 5G link to a 4G link (S 1325 ). The path switching operation may be defined in several methods, and in the present disclosure, the path switching operation is not limited to the above-described operation. 
         [0108]    In an embodiment of  FIG. 13 , it is assumed that even at a time when the path switching operation is completed, data that is in a 5G PDCP buffer of the base station can be transmitted to the 5G PDCP  1304  of the terminal in all. After transmitting all the data stored in the 5G PDCP buffer thereof, the base station transfers the data to the terminal through 4G PDCPs  1306  and  1308  of the base station and the terminal, which are the switched paths. 
         [0109]    In an embodiment of  FIG. 13 , after the path switching operation is completed, a timer A operates. The length of the timer A may be predetermined or may be settable. Until the timer A expires, the terminal  110  receives only data that is transmitted to the 5G SWI  1312  of the base station  100  through the 5G SWI  1302  through the 5G PDCPs  1304  and  1310  of the base station and the terminal (S 1335 ), and sequentially transmits the received data upward. After the timer A expires, the 5G SWI  1302  of the terminal may sequentially transmit the data that is transmitted through the 4G PDCPs  1306  and  1308  of the base station and the terminal upward (S 1325 ). 
         [0110]      FIG. 14  is a diagram illustrating another example of a method for preventing a loss of uplink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure. 
         [0111]    Referring to  FIG. 14 , a SWI/SPL performs only a switching function, and is hereinafter described as SWI. Further,  FIG. 14  illustrates a method for a SWI to prevent a data loss without any separate header. 
         [0112]    Since a 5G link is first used for data communications, user data is transmitted to a 5G SWI  1412  of a base station  100  through transmission of the user data from a 5G SWI  1402  of a terminal  110  to a 5G PDCP  1410  of the base station  100  and a related lower layer through a 5G PDCP  1404  and a related lower layer (S 1420 ). Thereafter, at a specific time, the terminal  110  and the base station  100  perform switching of a data path from a 5G link to a 4G link (S 1425 ). The path switching operation may be defined in several methods, and in the present disclosure, the path switching operation is not limited to the above-described operation. 
         [0113]    In  FIG. 14 , it is assumed that even at a time when the path switching operation is completed, data that is in a 5G PDCP buffer of the terminal  110  can be transmitted to the 5G PDCP  1410  of the base station  100  in all. After transmitting all the data stored in the 5G PDCP buffer thereof, the terminal transfers the data to the base station through 4G PDCPs  1406  and  1408  of the base station and the terminal, which are the switched paths. 
         [0114]    In an embodiment of  FIG. 14 , after the path switching operation is completed, a timer A operates (S 1430 ). The length of the timer A may be predetermined or may be settable. Until the timer A expires, the base station  100  receives only data that is transmitted to the 5G SWI  1412  through the 5G PDCPs  1404  and  1410  of the base station and the terminal (S 1435 ), and sequentially transmits the received data upward. After the timer A expires, the 5G SWI  1412  of the base station may sequentially transmit the data that is transmitted through the 4G PDCPs  1406  and  1408  of the base station and the terminal upward (S 1440 ). 
         [0115]      FIG. 15  is a diagram illustrating an example of a SWI/SPL header structure in the case where SWI/SPL has a SN according to an embodiment of the present disclosure. 
         [0116]    Referring to  FIG. 15 , a SWI/SPL SN  1510  has a length of 23 bits, and a D/C field  1500  is arranged with a length of one bit to cope with a case where the D/C Mux function as illustrated in  FIGS. 6 and 7  is added. However, the length of the SN or the existence/nonexistence of the D/C field may differ depending on the detailed operation according to the present disclosure. 
         [0117]      FIG. 16  is a diagram illustrating still another example of a method for preventing a loss of downlink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure. 
         [0118]    Referring to  FIG. 16 , a SWI/SPL performs only a switching function, and is hereinafter described as SWI. Further,  FIG. 16  illustrates a method for preventing a data loss using SN information of a header that includes the SN of the function as described above with reference to  FIG. 15 . 
         [0119]    Since a 5G link is first used for data communications, user data is transmitted to a 5G SWI  1602  of a terminal  110  through transmission of the user data from a 5G SWI  1612  of a base station  100  to a 5G PDCP  1604  of the terminal  110  and a related lower layer through a 5G PDCP  1608  and a related lower layer (S 1620 ). In this case, the 5G SWI  1612  of the base station  100  transmits the data through addition of an SWI header thereto, and the 5G SWI  1602  of the terminal  110  transmits SDUs upward after removal of the SWI header. Thereafter, at a specific time, the terminal  110  and the base station  100  perform switching of a data path from a 5G link to a 4G link (S 1625 ). 
         [0120]    In this case, a 5G RRC  1614  of the base station may generate switching status report triggering. The switching status report triggering is encapsulated into a 5G RRC message to be transmitted from the base station  100  to the terminal  110  via 4G PDCPs  1608  and  1606  of the base station and the terminal (S 1630 ). A switching status report triggering message is transferred from a 5G RRC  1600  of the terminal  110  to the 5G SWI  1602  (S 1635 ). Such a switching status report triggering procedure may be included in the path switching operation (S 1625 ) or may be omitted. 
         [0121]    The terminal  110  transmits a switching status report to the base station  100  after the path switching process or the switching status report triggering. In this case, the 5G SWI  1602  of the terminal transmits information related to non-received SDUs to the 5G RRC  1600  of the terminal (S 1640 ), the 5G RRC  1600  of the terminal transmits a switching status report to the 5G RRC  1614  of the base station (S 1645 ), and the 5G RRC  1614  of the base station transmits the switching status report or information related to the non-received SDUs that is included in the switching status report to the 5G SWI  1612  (S 1650 ). The type of the switching status report may be the type that is similar to the type of the PDCP status report or may be the type of the RRC message. In  FIG. 16 , the switching status report is described as a message that is encapsulated into the 5G RRC message. Further, such switching status report information may be based on the SWI SN. 
         [0122]    The 5G SWI  1612  of the base station that has received the switching status report information that is transferred thereto transmits the non-received data (SDUs) to the 4G PDCP  1608  of the base station on the basis of this information (S 1655 ). The 4G PDCP  1608  transmits the data that is received from the 5G SWI  1612  to the 5G SWI  1602  through the 4G PDCP  1606  of the terminal (S 1660 ), and the 5G SWI  1602  may perform a reordering procedure on the basis of the SN that is included in the SWI header (S 1670 ). 
         [0123]      FIG. 17  is a diagram illustrating still another example of a method for preventing a loss of uplink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure. 
         [0124]    Referring to  FIG. 17 , a SWI/SPL performs only a switching function, and is hereinafter described as SWI. Further,  FIG. 17  illustrates a method for preventing a data loss using SN information of a header that includes the SN of the function as described above with reference to  FIG. 15 . 
         [0125]    Since a 5G link is first used for data communications, user data is transmitted to a 5G SWI  1712  of a base station  100  through transmission of the user data from a 5G SWI  1702  of a terminal  110  to a 5G PDCP  1710  of the base station  100  and a related lower layer through a 5G PDCP  1704  and a related lower layer (S 1720 ). In this case, the 5G SWI  1702  of the terminal  110  transmits the data through addition of an SWI header thereto, and the 5G SWI  1712  of the base station  100  sends SDUs upward after removal of the SWI header. Thereafter, at a specific time, the terminal  110  and the base station  100  perform switching of a data path from a 5G link to a 4G link (S 1725 ). 
         [0126]    The base station  100  transmits a switching status report to the terminal  110  after the path switching operation. Specifically, the 5G SWI  1712  of the base station transmits information related to non-received SDUs to the 5G RRC  1714  (S 1730 ), the 5G RRC  1714  of the base station transmits a switching status report to the 5G RRC  1700  of the terminal (S 1735 ), and the 5G RRC  1700  of the terminal transmits the switching status report or information related to the non-received SDUs that is included in the switching status report to the 5G SWI  1702  (S 1740 ). The type of the switching status report may be the type that is similar to the type of the PDCP status report or may be the type of the RRC message. In  FIG. 17 , the switching status report is described as a message that is encapsulated into the 5G RRC message. Further, such switching status report information may be based on the SWI SN. 
         [0127]    The 5G SWI  1702  of the terminal that has received the switching status report information that is transferred thereto forwards the non-received SDUs to the 4G PDCP  1706  on the basis of this information (S 1745 ). The 4G PDCP  1706  transmits the data that is transferred from the 5G SWI  1702  to the 5G SWI  1712  through the 4G PDCP  1708  of the base station (S 1750 ), and the 5G SWI  1712  may perform a reordering procedure on the basis of the SWI SN (S 1760 ). 
         [0128]      FIG. 18  is a diagram illustrating still another example of a method for preventing a loss of downlink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure. 
         [0129]    Referring to  FIG. 18 , a SWI/SPL performs only a switching function, and is hereinafter described as SWI. Further,  FIG. 18  illustrates a method for preventing a data loss using SN information of a header that includes the SN of the function as described above with reference to  FIG. 15 . 
         [0130]    Since a 4G link is first used for data communications, user data is transmitted to a 5G SWI  1802  of a terminal  110  through transmission of the user data from a 5G SWI  1812  of a base station  100  to a 4G PDCP  1806  of the terminal  110  and a related lower layer through a 4G PDCP  1808  and a related lower layer (S 1820 ). In this case, the 5G SWI  1812  of the base station  100  transmits the data through addition of an SWI header thereto, and the 5G SWI  1802  of the terminal  110  sends SDUs upward after removal of the SWI header. Thereafter, at a specific time, the terminal  110  and the base station  100  perform switching of a data path from a 4G link to a 5G link (S 1825 ). 
         [0131]    In this case, a 5G RRC  1814  of the base station may generate switching status report triggering. The switching status report triggering is encapsulated into a 5G RRC message to be transmitted from the base station  100  to the terminal  110  via 4G PDCPs  1806  and  1808  of the base station and the terminal (S 1830 ). A switching status report triggering message is transferred from a 5G RRC  1800  of the terminal  110  to the 5G SWI  1802  (S 1835 ). Such a switching status report triggering procedure may be included in the path switching operation (S 1825 ) or may be omitted. 
         [0132]    The terminal  110  transmits a switching status report to the base station  100  after the path switching process or the switching status report triggering. In this case, the 5G SWI  1802  of the terminal transmits information related to non-received SDUs to the 5G RRC  1800  of the terminal (S 1840 ), the 5G RRC  1800  of the terminal transmits a switching status report to the 5G RRC  1814  of the base station (S 1845 ), and the 5G RRC  1814  of the base station transmits the switching status report or information related to the non-received SDUs that is included in the switching status report to the 5G SWI  1812  (S 1850 ). The type of the switching status report may be the type that is similar to the type of the PDCP status report or may be the type of the RRC message. In  FIG. 18 , the switching status report is described as a message that is encapsulated into the 5G RRC message. Further, such switching status report information may be based on the SWI SN. 
         [0133]    The 5G SWI  1812  of the base station that has received the switching status report information that is transferred thereto transmits the non-received data (SDUs) to the 5G PDCP  1810  of the base station on the basis of this information (S 1855 ). The 5G PDCP  1810  of the base station transmits the data that is transmitted from the 5G SWI  1812  of the base station to the 5G SWI  1802  of the terminal through the 5G PDCP  1804  of the terminal (S 1860 ). Thereafter, the 5G SWI  1802  of the terminal may perform a reordering procedure on the basis of the SWI SN (S 1870 ). 
         [0134]      FIG. 19  is a diagram illustrating still another example of a method for preventing a loss of uplink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure. 
         [0135]    Referring to  FIG. 19 , a SWI/SPL performs only a switching function, and is hereinafter described as SWI. Further,  FIG. 19  illustrates a method for preventing a data loss using SN information of a header that includes the SN of the function as described above with reference to  FIG. 15 . 
         [0136]    Since a 4G link is first used for data communications, user data is transmitted to a 5G SWI  1912  of a base station  100  through transmission of the user data from a 5G SWI  1902  of a terminal  110  to a 4G PDCP  1908  of the base station  100  and a related lower layer through a 4G PDCP  1906  and a related lower layer (S 1920 ). In this case, the 5G SWI  1902  of the terminal  110  transmits the data (SDUs) through addition of an SWI header thereto, and the 5G SWI  1912  of the base station  100  sends SDUs upward after removal of the SWI header. Thereafter, at a specific time, the terminal  110  and the base station  100  perform switching of a data path from a 4G link to a 5G link (S 1925 ). 
         [0137]    The base station  100  transmits a switching status report to the terminal  110  after the path switching operation. Specifically, the 5G SWI  1912  of the base station transmits information related to non-received SDUs to the 5G RRC  1914  (S 1930 ), the 5G RRC  1914  of the base station transmits a switching status report to the 5G RRC  1900  of the terminal (S 1935 ), and the 5G RRC  1900  of the terminal transmits the switching status report or information related to the non-received SDUs that is included in the switching status report to the 5G SWI  1902  (S 1940 ). The type of the switching status report may be the type that is similar to the type of the PDCP status report or may be the type of the RRC message. In  FIG. 19 , the switching status report is described as a message that is encapsulated into the 5G RRC message. Further, such switching status report information may be based on the SWI SN. 
         [0138]    The 5G SWI  1902  of the terminal that has received the switching status report information that is transferred thereto forwards the non-received SDUs to the 5G PDCP  1910  on the basis of this information (S 1945 ). The 5G PDCP  1904  transmits the data that is transferred from the 5G SWI  1902  to the 5G SWI  1912  through the 5G PDCP  1910  of the base station (S 1950 ), and the 5G SWI may perform a reordering procedure on the basis of the SWI SN (S 1960 ). 
         [0139]      FIG. 20  is a diagram illustrating an example of a method for preventing a loss of downlink data when data path splitting, through which data is transmitted to both a 5G link and a 4G link, is performed according to an embodiment of the present disclosure. 
         [0140]    Referring to  FIG. 20 , a SWI/SPL performs only a splitting function, and is hereinafter described as SPL. If at least one link of a path that is split and transmitted is not valid anymore according to circumstances, a terminal (UE)  110  and a base station (eNB)  100  may operate to perform path switching like the above-described various embodiments.  FIG. 20  illustrates a method for preventing a data loss using SN information of a header that includes the SN of the function as described above with reference to  FIG. 15 . 
         [0141]    In  FIG. 20 , a path for transmitting user data from a 5G SPL  2012  of the base station  100  to a 5G SPL  2002  of the terminal  110  through a 5G PDCP  2010  of the base station and a 5G PDCP  2004  of the terminal and a path for transmitting user data from a 5G SPL  2010  of the base station to the 5G SPL  2002  of the terminal through a 4G PDCP  2008  of the base station and a 4G PDCP  2006  of the terminal coexist (S 2020 ). In this case, the 5G SPL  2002  of the terminal performs a reordering procedure for ordering the sequence of data input through different links using the SPL SN (S 2025 ). Accordingly, in a splitting scenario, unlike the path switching, it is always required to perform the reordering procedure. 
         [0142]    If a specific condition is satisfied, the 5G SPL  2002  of the terminal may notify the 5G SPL  2012  of the base station of information on non-received SDUs through transmission of a switching status report to the 5G SPL  2012  of the base station. The triggering condition of the switching status report may be designated by the base station through information in a 5G RRC message, or may be operated through implementation of the terminal. It is also possible to operate a timer for the triggering or to periodically transmit the triggering. Specifically, the 5G SPL  2002  of the terminal triggers the switching status report (S 2030 ), information on the non-received data (SDUs) is transferred from the 5G SPL  2002  to a 5G RRC  2000  (S 2035 ), and the 5G RRC  2000  of the terminal transmits the switching status report to a 5G RRC  2014  of the base station (S 2040 ). Thereafter, the 5G RRC  2014  of the base station transfers the switching status report or information on the non-received SDUs in the switching status report to the 5G SPL  2012  to perform the switching status report. 
         [0143]    If the switching status report that is generated in any method is received, the base station  100  may retransmit the non-received SDUs of the terminal  110  from the 5G SPL  2012  to the 5G SPL  2002  of the terminal (S 2050 ). Based on the retransmitted SDUs, the terminal may send the data upward through ordering of the SDU sequence. 
         [0144]      FIG. 21  is a diagram illustrating an example of a method for preventing a loss of uplink data when data path splitting, through which data is transmitted to both a 5G link and a 4G link, is performed according to an embodiment of the present disclosure. 
         [0145]    Referring to  FIG. 21 , a SWI/SPL performs only a splitting function, and is hereinafter described as SPL. If at least one link of a path that is split and transmitted is not valid anymore according to circumstances, a terminal (UE)  110  and a base station (eNB)  100  may operate to perform path switching like the above-described various embodiments.  FIG. 21  illustrates a method for preventing a data loss using SN information of a header that includes the SN of the function as described above with reference to  FIG. 15 . 
         [0146]    In  FIG. 21 , a path for transmitting user data from a 5G SPL  2102  of the terminal  110  to a 5G SPL  2112  of the base station  100  through a 5G PDCP  2104  of the terminal and a 5G PDCP  2110  of the base station and a path for transmitting user data from the 5G SPL  2102  of the terminal to the 5G SPL  2112  of the base station through a 4G PDCP  2106  of the terminal and a 4G PDCP  2108  of the base station coexist (S 2120 ). In this case, the 5G SPL  2112  of the base station performs a reordering procedure for ordering the sequence of data input through different links using the SPL SN (S 2125 ). Accordingly, in a splitting scenario, unlike the path switching, it is always required to perform the reordering procedure. 
         [0147]    The 5G SPL  2112  of the base station may notify the 5G SPL  20102  of the terminal of information on non-received SDUs through transmission of a switching status report to the 5G SPL  2102  of the terminal. The triggering condition of the switching status report may be operated under the determination of the base station or under a predefined condition. It is also possible to operate a timer for the triggering or to periodically transmit the triggering. Specifically, the 5G SPL  2112  of the base station triggers the switching status report (S 2130 ), information on the non-received data (SDUs) is transferred from the 5G SPL  2112  to a 5G RRC  2114  (S 2135 ), and the 5G RRC  2114  of the base station transmits the switching status report to a 5G RRC  2100  of the terminal (S 2140 ). Thereafter, the 5G RRC  2100  of the terminal transfers the switching status report or information on the non-received SDUs in the switching status report to the 5G SPL  2102  to perform the switching status report. 
         [0148]    If the switching status report that is generated in any method is received, the terminal  110  may retransmit the non-received SDUs of the terminal  110  from the 5G SPL  2102  to the 5G SPL  2112  of the base station. Based on the retransmitted SDUs, the base station may send the data upward through ordering of the SDU sequence. 
         [0149]      FIG. 22  is a block diagram illustrating the structure of a base station  100  according to an embodiment of the present disclosure. 
         [0150]    Referring to  FIG. 22 , the base station  100  may be composed of a transceiver unit  2210  and a control unit  2220 . The transceiver unit  2210  may transmit and receive signals with a terminal  110 , and such signals may include a message for a path switching operation, a switching status report, switching status report triggering, and data. The control unit  2220  may operate to carry out the various embodiments as described in  FIGS. 8 to 12, and 14 to 21 . As an example, referring to  FIG. 8 , the control unit  2220  may control a 5G RRC  814  to transmit PDCP status report triggering (RRC IE) to the terminal via a 4G PDCP  808 , and may control the 5G RRC  814  to transfer a PDCP status report to a 5G SWI  812 . Further, the control unit  2220  may control the 5G SWI  812  to transfer data that is not received by the terminal to the 4G PDCP  808 , and may control the 4G PDCP  808  to transmit the data to the terminal. 
         [0151]      FIG. 23  is a block diagram illustrating the structure of a terminal  110  according to an embodiment of the present disclosure. 
         [0152]    Referring to  FIG. 23 , the terminal  110  may be composed of a transceiver unit  2310  and a control unit  2320 . The transceiver unit  2310  may transmit and receive signals with a base station  100 , and such signals may include a message for a path switching operation, a switching status report, switching status report triggering, and data. The control unit  2320  may operate to carry out the various embodiments as described in  FIGS. 8 to 12, and 14 to 21 . As an example, referring to  FIG. 8 , the control unit  2320  may receive a PDCP status report triggering message (RRC IE) via a 4G PDCP  806 , and may control a 5G RRC  800  to transfer the PDCP status report triggering message to a 5G PDCP  804 . Further, the control unit  2320  may control the 5G PDCP  804  to generate and transfer a PDCP status report to the 5G RRC  800  and a 5G SWI  802 , may control the 5G RRC  800  to transmit a switching status report (RRC message) to the base station via a 4G PDCP  806 , and may control the 4G PDCP  804  to receive the data. 
         [0153]      FIG. 24  is a diagram illustrating still another example of a method for preventing a loss of downlink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure. 
         [0154]    Referring to  FIG. 24 , a 5G layer  2400  in a terminal (UE)  110  may mean a 5G RRC, 5G PDCP, or 5G SWI. A 4G layer  2402  may mean a 4G RRC or 4G PDCP. Further, a 5G layer  2406  in a base station (eNB)  100  may mean a 5G RRC, 5G PDCP, or 5G SWI. A 4G layer  2404  may mean a 4G RRC or 4G PDCP. 
         [0155]    Since a 4G link is first used for data communications, user data is transmitted from the base station to the terminal through a 4G link. Thereafter, if it is decided to switch a data path from the 4G link to a 5G link, the 5G RRC  2406  of the base station transmits an RRCConnectionReconfiguration message to the 5G RRC  2400  of the terminal (S 2410 ). If the RRCConnectionReconfiguration message is transmitted, the 5G SWI of the base station does not transmit downlink data to the 4G PDCP  2404  anymore. Even in this case, since it is estimated that the 4G link is yet reliable, the user data that remains in a buffer of the 4G PDCP  2404  of the base station may be transmitted to the terminal using the 4G link (S 2420 ). Thereafter, the 5G RRC  2400  of the terminal transmits an RRCConnectionReconfigurationComplete message to the 5G RRC  2406  of the base station (S 2430 ). Such a 5G RRCConnectionReconfiguration process may correspond to the path switching process of  FIGS. 8 and 11 . 
         [0156]    Thereafter, if the base station senses that all user data has been transmitted through the 4G link through an RLC STATUS PDU (e.g., a PDCP buffer is empty and ˜˜(NACK) has not been transmitted from the terminal), the 4G layer  2404  transmits an indication to the 5G SWI (5G PDCP)  2406  (S 2440 ), and the base station starts to transmit the data to the 5G PDCP  2406 . In this case, the terminal receives the user data through the 5G link, and thus in-sequence transmission can be secured. 
         [0157]    Thereafter, the 5G layers  2400  and  2406  of the base station and the terminal perform 5G DRB setup process for performing a random access (S 2450 ) for the 5G link, and transmit the downlink data using the 5G link after the DRB setup (S 2460 ). 
         [0158]    In this case, it is assumed that the SWI of the reception side (terminal) does not perform the switching function, but receives the user data from the 4G and 5G links to forward the received user data to the AP. This method enables lossless switching to be performed without changing the structure and the function of the terminal. In the case of uplink data transmission under the above assumption, an LTE chipset is not changed, and thus the indication from the 4G PDCP  2402  to the 5G SWI  2400  in the terminal becomes impossible. 
         [0159]      FIG. 25  is a diagram illustrating still another example of a method for preventing a loss of downlink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure. 
         [0160]    Referring to  FIG. 25 , a 5G layer  2500  in a terminal (UE)  110  may mean a 5G RRC, 5G PDCP, or 5G SWI. A 4G layer  2502  may mean a 4G RRC or 4G PDCP. Further, a 5G layer  2506  in a base station (eNB)  100  may mean a 5G RRC, 5G PDCP, or 5G SWI. A 4G layer  2504  may mean a 4G RRC or 4G PDCP. 
         [0161]    Since a 5G link is first used for data communications, user data is transmitted from the base station to the terminal through a 5G link. Thereafter, due to a poor quality of the 5G link, data path switching from the 5G link to a 4G link may be triggered. In this case, since the 5G link is unable to be used at the switching time, data forwarding and retransmission may be supported for lossless transmission. The user data that is transmitted through a 5G DRB is transferred to the 5G PDCP  2506  (S 2510 ). 
         [0162]    Thereafter, the 5G DRB is released, and the switching procedure is triggered through a 5G RRCConnectionReconfiguration message that is transmitted from the 5G RRC  2506  of the base station to the 5G RRC  2500  of the terminal (S 2520 ). If the 5G RRCConnectionReconfiguration message is received, the terminal discards out-of-sequence PDCP SDUs. Thereafter, the 5G RRC  2500  of the terminal transmits a 5G RRCConnectionReconfigurationComplete message to the 5G RRC  2506  of the base station (S 2530 ). Thereafter, for lossless transmission, the 5G PDCP  2506  of the base station forwards the user data after a data packet that has received the last ACK to the 4G PDCP  2504  (S 2540 ). The 4G PDCP  2504  gives a 4G PDCP SN to the data packet and transmits PDCP PDUs. Thereafter, the base station transmits the user data using the 4G link (S 2550 ). In this case, the lossless transmission can be secured, but redundant data transmission may occur. 
         [0163]    In the case of applying the above-described method to the uplink transmission, data forwarding from the 5G PDCP  2500  to the 4G PDCP  2502  in the terminal becomes necessary. However, in the case of the current LTE chipset, since an interface that can transmit data from the 5G PDCP to the 4G PDCP does not exist, it may be difficult to apply the above-described method to the uplink transmission without changing the structure and the function of the current terminal. 
         [0164]      FIG. 26  is a diagram illustrating still another example of a method for preventing a loss of uplink data when data path switching is performed from a 4G link to a 5G link according to an embodiment of the present disclosure. 
         [0165]    The method that is disclosed in  FIG. 26  is the same as the method for preventing a loss of downlink data of  FIG. 24  except for the point that a transmission end and a reception end have been changed to each other. In order to perform the method of  FIG. 26 , it should be assumed that an indication from a 4G PDCP  2602  to a 5G SWI  2600  in a terminal is possible. 
         [0166]      FIG. 27  is a diagram illustrating still another example of a method for preventing a loss of uplink data when data path switching is performed from a 5G link to a 4G link according to an embodiment of the present disclosure. 
         [0167]    The method that is disclosed in  FIG. 27  is the same as the method for preventing a loss of downlink data of  FIG. 25  except for the point that a transmission end and a reception end have been changed to each other. In order to perform the method of  FIG. 27 , since data forwarding from a 5G PDCP  2700  to a 4G PDCP  2702  in a terminal is necessary, an interface for the data forwarding operation should exist, and thus it is required to change the current terminal. 
         [0168]    According to an embodiment of the present disclosure, it is possible to perform lossless data transmission even in the case of data path switching and data path splitting in an environment in which two different kinds of communication systems coexist. 
         [0169]    While the present disclosure has been shown and described with reference to various embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the appended claims and their equivalents.