Patent Description:
To meet the increasing demand for wireless data traffic since the deployment of <NUM> communication systems, efforts have been made to develop an improved <NUM> or pre-<NUM> communication system. Therefore, the <NUM> or pre-<NUM> communication system is also called a "beyond <NUM> network" communication system or a "post LTE System.

Implementation of the <NUM> communication system in ultrahigh frequency (mmWave) bands, e.g., <NUM> bands, is being considered in order to accomplish higher data rates. To mitigate a path loss of the radio waves and increase the transmission distance on the radio waves in the ultrahigh frequency bands, beamforming, massive multiple-input multiple-output (massive MIMO), full dimensional MIMO (FD-MIMO), array antenna, analog beamforming, and large scale antenna techniques are being discussed for <NUM> communication systems.

In addition, in <NUM> communication systems, development for system network improvement is under way based on evolved small cells, advanced small cells, cloud radio access networks (cloud RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), reception-end interference cancellation, and the like.

In addition, in the <NUM> system, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC), as advanced coding modulation (ACM) systems, and filter bank multi carrier (FBMC), non-orthogonal multiple access(NOMA), and sparse code multiple access (SCMA), as advanced access technologies, have been developed.

On the other hand, in the case of performing cooperative communication through interworking between the previous <NUM> and <NUM>, an interworking system with dependency between respective networks has been performed as a non-standalone mode.

In such a case, there have been problems that separate interface and processing are necessary between <NUM> and <NUM> base stations.

A publication of <NPL> discloses that a network supports NGx interface to transfer UE MM (and SM) context between the two core networks. After the transfer, the MM (and SM) context is deleted (or marked inactive) in the source network.

The disclosure has been made in order to solve the above-described problems, and the disclosure provide a method and an apparatus for performing interworking without dependency between a <NUM> network and a <NUM> network.

In an aspect of the disclosure in order to solve the above-described problems, a method by a terminal for performing communication in a wireless communication system as defined in the appended claims.

In another aspect of the disclosure, a communication method by a gateway in a wireless communication system as defined in the appended claims.

In still another aspect of the disclosure, a terminal for performing communication in a wireless communication system as defined in the appended claims.

In still another aspect of the disclosure, a gateway device for performing communication in a wireless communication system as defined in the appended claims.

According to embodiments of the disclosure, an interworking system without dependency between a legacy <NUM> network and a <NUM> network is provided, and through this, <NUM> service launch and development can be performed quickly and conveniently. Further, if UE requires a PDN connection having the same access point name (APN) (e.g., Intemet APN) for a user data service with respect to <NUM> and <NUM>, a <NUM> GW can allocate the same IP address to the UE and it can support a seamless service between <NUM> and <NUM> through session binding between <NUM> and <NUM>. Further, in a mobile environment, stability of a <NUM> radio link using mmWave may be lowered, and even in such an environment, a <NUM> fallback can be quickly performed in order to secure stability of the service.

Hereinafter, preferred embodiments of the disclosure will be described in detail with reference to the accompanying drawings. In the accompanying drawings, it is to be noted that the same drawing reference numerals are used for the same elements. Further, detailed explanation of known functions and configurations that may obscure the subject matter of the disclosure will be omitted. The presently claimed invention concerns the embodiments of <FIG> and <FIG> while embodiments unrelated to <FIG> and <FIG> are presented for illustration purposes only.

In explaining the embodiments of the disclosure, explanation of technical contents that are well known in the art to which the disclosure pertains and are not directly related to the disclosure will be omitted. This is to transfer the subject matter of the disclosure more clearly without obscuring the same through omission of unnecessary explanations.

For the same reason, in the accompanying drawings, sizes and relative sizes of some constituent elements may be exaggerated, omitted, or briefly illustrated. Further, sizes of the respective constituent elements do not completely reflect the actual sizes thereof. In the drawings, the same drawing reference numerals are used for the same or corresponding elements across various figures.

The aspects and features of the disclosure and methods for achieving the aspects and features will be apparent by referring to the embodiments to be described in detail with reference to the accompanying drawings. However, the disclosure is not limited to the embodiments disclosed hereinafter, and it can be implemented in diverse forms. The matters defined in the description, such as the detailed construction and elements, are only specific details provided to assist those of ordinary skill in the art in a comprehensive understanding of the disclosure, and the disclosure is only defined within the scope of the appended claims. In the entire description of the disclosure, the same drawing reference numerals are used for the same elements across various figures.

In this case, it will be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by computer program instructions.

Also, each block of the flowchart illustrations may represent a module, segment, or portion of code, which includes one or more executable instructions for implementing the specified logical function(s).

In this case, the term "~unit", as used in an embodiment, means, but is not limited to, a software or hardware component, such as FPGA or ASIC, which performs certain tasks. However, "~unit" is not meant to be limited to software or hardware. The term "~unit" may advantageously be configured to reside on the addressable storage medium and configured to execute on one or more processors. Thus, "~unit" may include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. The functionality provided for in the components and "~units" may be combined into fewer components and "~units" or further separated into additional components and "-units". Further, the components and "~units" may be implemented to operate one or more CPUs in a device or a security multimedia card.

Embodiments of the disclosure described hereinafter relate to interworking without dependency between a first wireless communication system and a second wireless communication system. In an embodiment of the disclosure described hereinafter, the first wireless communication system is exemplified as <NUM>, and the second wireless communication system is exemplified as <NUM>, but the wireless communication systems are not limited thereto.

In this case, <NUM> may mean long term evolution (LTE), and <NUM> may means a communication network implemented in ultrahigh frequency (mmWave) bands, e.g., <NUM> bands.

The existing <NUM> base station, core name, and interface name have been quoted as <NUM> base station, core name, and interface name, respectively, but in the <NUM> and post-<NUM> standards, the <NUM> base station, the core name, and the interface name may be changed although the basic functions thereof are the same.

In the case of using an interworking system with dependency between a <NUM> network (hereinafter, interchangeably used with <NUM>) and a <NUM> network (hereinafter, interchangeably used with <NUM>), the following problems occur. It is necessary to correct and develop separate interface and procedure with respect to the existing <NUM> base station. This may cause a risk that the commercially operating <NUM> base station should be corrected and developed. Further, according to the existing technology, network based switching (in particular, it may be base station based switching) is performed, and according to the structure therefor, it is difficult to achieve a fallback to <NUM> through a quick detection when a radio link failure (RLF) occurs in a <NUM> radio link. The disclosure has been made to solve the above-described problems.

<FIG> is a diagram illustrating the concept of interworking between networks based on a standalone mode according to an embodiment of the disclosure.

The disclosure introduces an interworking system without dependency between a legacy <NUM> network and a <NUM> network. Specifically, user equipment (UE, which can be used interchangeably with terminal or user device) <NUM> independently performs non-access stratum (NAS) operations, such as registration, attach, service request, and mobility, in <NUM> and <NUM> networks. Further, the UE supports an inter-RAT switching function between <NUM> and <NUM> through a common APN at a core level of a network. Through this, if service names (e.g., Intemet APN <NUM>) requested respectively using <NUM> and <NUM> are the same, it is possible to transmit user data through <NUM> or <NUM>.

Further, if the UE requires PDN connection having the same APN (e.g., Intemet APN <NUM>) with respect to <NUM> and <NUM>, a <NUM> gateway (GW) <NUM> performs the same IP address allocation and session binding between <NUM> and <NUM> with respect to the UE to support a seamless service between <NUM> and <NUM>.

Further, stability of a <NUM> radio link using mmWave may be lowered in a mobile environment, and even in such an environment, <NUM> fallback can be quickly performed in order to secure the stability of services. As a possible method, the UE capable of determining the radio state most quickly performs link switch decision between <NUM> and <NUM>, and in order to match switching synchronization with an anchor in the <NUM> GW (hereinafter, interchangeably used with GW), the UE transmits a signaling message (e.g., link switch request) to the anchor in the GW. Further, in order to quickly perform a <NUM> fallback, the GW may maintain a <NUM> connection by transmitting a keep alive message to the UE through the <NUM> connection even in a state where a <NUM> connection is in use. In this case, the link switch request message and the keep alive message may be transmitted and received through generation and utilization of the PDN connection (hereinafter, interchangeably used with the signaling APN) for separate signaling APN <NUM> and <NUM> so that there is no implementation impact in the legacy <NUM> network.

In the disclosure, the packet data network (PDN) means an independent network (e.g., <NUM> network) in which a server providing the services is located, and the access point name (APN) is the name of an access point managed by the network and it indicates the name (character string) of the corresponding PDN. Based on the name of the access point, the corresponding PDN for data transmission and reception is determined.

The signaling APN disclosed in the disclosure may be understood as an identifier for generating a PDN connection for signaling, and the UE, <NUM> GW, and <NUM> and <NUM> HSS may pre-store predefined signaling APN therein. Further, the signaling APN may be determined between the UE and the <NUM> GW during an initial access. The UE and the <NUM> GW may transmit and receive the link switch request message, the keep alive message, and the <NUM>/<NUM> link start marker packet through the PDN connection for signaling, and this may called a signaling APN bearer. In the disclosure, because the signaling APN is generated and used to transmit various kinds of signaling messages, it is possible to transmit the signaling messages between the UE and the <NUM> GW without changing the standard technology in the related art.

<FIG> is a diagram illustrating the basic configuration of architecture for interworking between a <NUM> network and a <NUM> network according to an embodiment of the disclosure.

The basic configuration of the <NUM>-<NUM> interworking architecture is as follow. A <NUM> core <NUM> includes a mobility management entity (MME) <NUM> based on <NUM> standards and a home subscriber server (HSS) <NUM> based on <NUM> standards. A <NUM> core <NUM> includes an HSS <NUM> based on <NUM> standards, an MME <NUM> based on <NUM> standards, and a <NUM> GW <NUM> supporting <NUM>-<NUM> interworking. A <NUM> evolved Node B (eNB, that is interchangeably used with a base station) <NUM> may be an eNB based on <NUM> standards, a <NUM> Node-B <NUM> may be a <NUM> Node-B based on <NUM> standards, and UE <NUM> may support a RAT operation based on <NUM> and <NUM> standards and <NUM>-<NUM> interworking.

The main features of such a <NUM>-<NUM> interworking architecture are as follows. First, UE support <NUM> and <NUM>. Second, the <NUM> HSS operates HSS separately so as to make dual registration possible, or operates as common <NUM> and <NUM> HSS. Third, if there is a paging procedure, the <NUM> MME supports a mobility function, whereas if not, the <NUM> MME does not support the mobility function. The <NUM> GW additionally supports an anchor function in addition to <NUM> serving gateway (SGW)/PDN gateway (PGW) functions. The anchor function means dual attach to <NUM> and <NUM> networks, common IP allocation, session binding based on the allocated IP, bearer ID, and TEID, and switching between <NUM> and <NUM>. Fourth, <NUM> and <NUM> cores share the <NUM> GW ( according to circumstances, the <NUM> SGW can be separately configured and operated rather than being integrated in the <NUM> GW). Fifth, the <NUM> MME, the <NUM> HSS, and the <NUM> eNB follow the <NUM> standards and they have no changed items.

The base operation of the <NUM>-<NUM> interworking using the architecture as described above is as follows. The UE performs registration/attach independently of <NUM> and <NUM>. If the UE enters into a <NUM> coverage, the <NUM> connection is generated, and user traffic is transmitted through the <NUM> network. If the UE gets out of the <NUM> coverage, the <NUM> connection is released, and the connection falls back to the <NUM> network. In this case, the user traffic is transmitted to <NUM>. The UE determines link switching between <NUM> and <NUM> based on the radio condition. The <NUM> GW supports the link switching, and the UE generates the signaling APN for <NUM> and <NUM> networks in order to match synchronization of the link switching between the UE and the <NUM> GW.

According to the disclosure, the operation mode of the <NUM>-<NUM> interworking may be divided into two kinds as follows.

First, if a mobile originated (MO) or mobile terminated (MT) data service is generated, a dependent <NUM>-<NUM> RRC state mode is a mode in which existence/nonexistence of <NUM> is determined always after <NUM> is connected. That is, the <NUM> connection is not triggered in a state where the <NUM> connection is not maintained. If normal data transmission/reception is not possible due to radio problems in <NUM> during the operation in the corresponding mode, quick fallback to <NUM> is possible because the <NUM> connection is maintained. In addition, if <NUM> connection triggering is necessary in accordance with a <NUM> connection triggering condition, the terminal attempts the <NUM> connection after turning on a <NUM> modem, and thus power saving effects can be obtained in comparison with cell searching through always turning on the <NUM> modem.

Second, an independent <NUM>-<NUM> RRC state mode is a mode in which <NUM> and <NUM> connections are independently performed. That is, if the MO or MT data service is generated, the <NUM> connection is generated in the case where <NUM> is available, whereas the <NUM> connection is generated in the case where only <NUM> is available. In contrast with the dependent <NUM>-<NUM> RRC state mode, the UE may receive the data service through the <NUM> connection without the <NUM> connection, and if fast <NUM> fallback is necessary to cope with the <NUM> radio problem occurrence, the UE can also generate the <NUM> connection if needed.

Hereinafter, with reference to <FIG> and <FIG>, the dependent and independent <NUM>-<NUM> RRC state modes are dividedly described in accordance with respective circumstances.

<FIG> is a diagram illustrating a detailed operation scenario in accordance with a situation of a dependent <NUM>-<NUM> RRC state mode.

The reference numeral "<NUM>" denotes a <NUM> idle and <NUM> idle state, and in this case, a <NUM> operation scenario during <NUM> zone detection is as follows. The UE may determine whether it exists in a <NUM> zone through a specific public land mobile network (PLMN) or a tracking area (TA) broadcasted from a <NUM> base station, and if it is determined that the UE attaches to a <NUM> network regardless of a specific area, or the UE exists in the <NUM> zone, the UE generates a <NUM> signaling APN. In this case, if the UE gets out of the <NUM> zone, the <NUM> signaling APN may be maintained or may be released if necessary. Further, the UE performs a connection setup through <NUM> if the MO or MT user data is generated.

The reference numeral "<NUM>" denotes a <NUM> connected and <NUM> idle state, and a switching process from an initial <NUM> to a <NUM> is as follows. If the UE satisfies a <NUM> connection triggering condition, the UE activates a <NUM> modem, and it periodically checks <NUM> availability. The <NUM> connection triggering condition may be whether the UE has recognized that a <NUM> zone exists, whether a data bearer exists, whether to transmit user data, and/or whether the state is a <NUM> RRC connected state, and the terminal attempts the <NUM> connection if one or more conditions are satisfied (e.g., a state where existence of the <NUM> zone is recognized, or a state where existence of the <NUM> zone is recognized and the data bearer exists may be used as the <NUM> connection triggering condition). If <NUM> is available, the UE performs <NUM> RRC connection setup (this may include an attach or a service request). At the same time, if the <NUM> signaling APN is not generated (e.g., during an initial attach), the UE also generates the <NUM> signaling APN. If the <NUM> RRC connection setup is completed, the UE transmits, to the GW, a <NUM> link start marker packet for requesting switching from <NUM> to <NUM> through the <NUM> signaling APN bearer. The GW having received the packet transmits and receives data through a <NUM> link.

The reference numeral "<NUM>" denotes a <NUM> connected and <NUM> connected state, and a method for maintaining a <NUM> connection in a <NUM> active state is as follows. The terminal transmits a <NUM> link start marker to the GW, and the GW can be aware of whether to transmit a keep alive packet non-explicitly through reception of the corresponding message itself or explicitly through a fast fallback flag in the corresponding message. The GW may receive the <NUM> link start marker transmitted by the UE, perform switching to <NUM>, and then periodically transmit the keep alive packet (this may be a dummy packet) to the terminal through the <NUM> signaling APN bearer (however, the keep alive packet transmission period is made to be shorter than an inactivity timer of the <NUM> base station). This is effective in always preparing the <NUM> network in order to quickly perform the <NUM> fallback. In contrast, if the GW receives the <NUM> link start marker, it interrupts transmission of the keep alive packet.

The reference numeral "<NUM>" denotes a state where an RLF is generated in a <NUM> connected and <NUM> connected state, and in this case, operations to be performed are as follows. If a <NUM> radio problem is decided, or a <NUM> RRC connection is released, the UE performs switching from <NUM> to <NUM>, and it transmit a <NUM> link start marker packet for requesting switching from <NUM> to <NUM> to the GW through the <NUM> signaling APN bearer. The GW having received the corresponding packet transmits and receives data through the <NUM> link. If the <NUM> base station first releases the RRC connection in a state where the UE does not recognize the same, the GW may buffer or discard the data until the UE requests switching from <NUM> to <NUM>, and may wait for reception of the <NUM> link start marker.

Further, in the <NUM> state, the UE may perform switching from <NUM> to <NUM>, and if <NUM> is available after a specific time (i.e., <NUM> retry waiting time), the UE may reattempt the <NUM> connection setup. If the reattempt has succeeded, the <NUM> state is changed to the <NUM> state. This function is possible through setting of a <NUM> retry waiting timer, and the reason why the specific time is set is to prevent a ping-pong phenomenon between <NUM> and <NUM>. If necessary, the timer function may be turned off. The <NUM> retry waiting timer may start at a time when the <NUM> radio problem is detected while a data service is performed. If recovery of the <NUM> connection has failed to be in a <NUM> disconnected state after the timer expiration, but the existing or another <NUM> cell is available, the UE attempts <NUM> reconnection, that is, the UE performs switching from <NUM> to <NUM> through the <NUM> connection setup, and it transmits the <NUM> link start marker packet for requesting switching from <NUM> to <NUM> to the GW through the <NUM> signaling APN bearer. In particular, if the <NUM> link is recovered and <NUM> returns in a normal connected state at a time when the <NUM> retry waiting timer expires, the UE may be directly switched to the normal recovered <NUM> without reattempting a separate <NUM> connection setup.

However, the above-described series of processes including the <NUM> cell searching, reconnection attempt, and switching may be performed only in the case where the <NUM> connection triggering condition is satisfied.

<FIG> is a diagram illustrating a detailed operation scenario in accordance with a situation of an independent <NUM>-<NUM> RRC state mode.

The reference numeral "<NUM>" denotes a <NUM> idle and <NUM> idle state, and in this case, the UE operates as follows. The UE determines whether it exists in a <NUM> zone through a specific PLMN or a tracking area (TA) broadcasted from a <NUM> base station. If it is determined that the UE attaches to a <NUM> network regardless of a specific area, or the UE exists in the <NUM> zone, the UE may generate a <NUM> signaling APN. Even if the UE gets out of the <NUM> zone, the <NUM> signaling APN may be maintained or may be released if necessary.

If a <NUM> cell is available in the <NUM> zone and the <NUM> signaling APN is not generated (e.g., during an initial <NUM> attach), the UE may generate the <NUM> signaling APN. If a MO/MT data service is generated, the <NUM> network is not available, and only the <NUM> network is available, the UE is changed from the <NUM> attach or idle state to an active state. If the MO/MT data service is generated, and the <NUM> network is available, the UE is changed from the <NUM> attach or <NUM> idle state to the active state.

The reference numeral "<NUM>" denotes a <NUM> connected and <NUM> idle state, and in this state, a switching operation from an initial <NUM> to a <NUM> is as follows. If the UE is located in a <NUM> zone, and a <NUM> connection triggering condition (e.g., in the case of using a user data service) is satisfied, the UE checks <NUM> availability. If <NUM> is available, the UE performs a <NUM> RRC connection setup (this may include an attach or a service request). At the same time, if the <NUM> signaling APN is not generated (e.g., during an initial <NUM> attach), the UE may also generate the <NUM> signaling APN. If the <NUM> RRC connection setup is completed, the UE transmits, to the GW, a <NUM> link start marker packet for requesting switching from <NUM> to <NUM> through the <NUM> signaling APN bearer. The GW having received the packet transmits and receives user data with the UE through a <NUM> link.

The reference numeral "<NUM>" denotes a <NUM> idle and <NUM> connected state, and in this state, the GW transmits and receives the data with the UE through the <NUM> link, but it does not maintain the <NUM> connection when <NUM> is active. In this case, during the <NUM> fallback due to the <NUM> radio problem, latency for changing to a <NUM> paging process and an idle to active state may be additionally consumed. If it is determined that fast <NUM> fallback is necessary, the UE may request the <NUM> GW to transmit a keep alive packet, and through this, the UE may move to a <NUM> connected and <NUM> connected state (<NUM>).

The reference numeral "<NUM>" denotes a <NUM> connected and <NUM> connected state, and an operation of maintaining the <NUM> connection performed in this state when <NUM> is active is the same as the <NUM> case in the dependent <NUM>-<NUM> RRC state mode.

The reference numeral "<NUM>" denotes a state where a <NUM> RLF is generated in a <NUM> connected and <NUM> connected state, and an operation performed in this case is the same as the <NUM> operation in the dependent <NUM>-<NUM> RRC state mode.

Further, in the mode, recovery switching from <NUM> to <NUM> is performed in the same manner as the dependent <NUM>-<NUM> RRC state mode.

For clear description, although the contents of the RRC state movement between <NUM> and <NUM> have been described on the assumption that <NUM> RLF has occurred, the same operation may be performed even in the case where the <NUM> RLF has occurred.

Hereinafter, a detailed call flow of <NUM>-<NUM> interworking will be described.

First, the detailed call flow in a dependent <NUM>-<NUM> RRC state mode is as follows.

<FIG> is a diagram illustrating a call flow of initial <NUM> attach (including idle to active and handover (HO) to <NUM> zone operations) and initial <NUM> attach (including idle to active) in a dependent <NUM>-<NUM> RRC state mode.

UE <NUM> determines whether the current location is in a <NUM> zone through the PLMN or the tracking area (TA) based on information broadcasted from LTE (this may be a <NUM> network), and if the current location is in the <NUM> zone, the UE <NUM> performs <NUM> attach, idle to active, or HO to <NUM> zone operation (S520). In the case of an initial <NUM> attach, an APN through a <NUM> link is generated (S522). An example of the APN may be the Intemet APN, and this means an APN that is switched between <NUM> and <NUM> for data services.

Thereafter, the UE <NUM> determines that it is currently in the <NUM> zone based on the specific PLMN or TA, and in the case of the <NUM> attach in the <NUM> zone, in the case of the <NUM> attach regardless of the specific area, or in the case of handover coming into the <NUM> zone, the UE <NUM> generates a <NUM> signaling APN (S524). The signaling APN is an APN for <NUM>-<NUM> switching control. Thereafter, if the UE <NUM> has not been switched to <NUM>, it performs link switching to <NUM> (S526). If the UE <NUM> has performed the link switching to <NUM>, it transmits a link start marker packet to a GW through a <NUM> signaling APN bearer to transfer information indicating that the link switching has been performed (S528). A <NUM> GW <NUM> perform the link switching to <NUM> after receiving the link start marker packet (S530). Further, if the GW transmits a keep alive packet to the <NUM> signaling APN bearer after the link switching to <NUM>, it interrupts transmission of the keep alive packet (S532). Thereafter, the UE <NUM> and the <NUM> GW <NUM> transmit and receive data through <NUM> until a <NUM> link is generated (S534).

If a <NUM> connection triggering condition (e.g., <NUM> RRC connected) is satisfied in a <NUM> zone, the UE <NUM> performs <NUM> modem-on and <NUM> searching operation (S536). If an available <NUM> is discovered, the UE performs <NUM> attach or <NUM> idle to active operation (S538). If the operation is an initial <NUM> attach, an APN through the <NUM> link is generated (S540). The APN may be the Intemet APN, and the Intemet APN is an APN being switched between <NUM> and <NUM> for the data services. Further, in the case of the initial <NUM> attach, a <NUM> signaling APN is generated (S542). The signaling APN is an APN for the <NUM>-<NUM> switching control. The UE <NUM> performs switching to a <NUM> link after the <NUM> attach or idle to active procedure is completed (S544). Thereafter, the UE transmits the link start marker packet to the <NUM> GW <NUM> through the <NUM> signaling APN bearer to transfer the information indicating that the link switching to <NUM> has been performed (S546). The <NUM> GW <NUM> performs link switching from <NUM> to <NUM> during reception of the <NUM> link start marker packet (S548), and then starts transmission of the keep alive packet to the UE <NUM> in order to maintain the LTE RRC connection for the purpose of fast <NUM> fallback (S550). Thereafter, data transmission/reception through <NUM> is performed (S552), and the <NUM> GW <NUM> periodically transmits the keep alive packet to the UE <NUM> through the <NUM> signaling APN bearer in order to maintain the LTE RRC connection (S554).

<FIG> is a diagram illustrating fast <NUM> fallback and <NUM> recovery procedures when a <NUM> radio problem occurs in a dependent <NUM>-<NUM> RRC state mode.

The UE <NUM> is currently in a <NUM> and <NUM> RRC connected state (S600). In this case, the <NUM> GW <NUM> periodically transmits the keep alive packet to the UE <NUM> through the <NUM> signaling APN bearer in order to maintain the LTE RRC connection for fast <NUM> fallback (S605). In this case, the data transmission/reception through <NUM> is being performed (S610).

In this case, the UE <NUM> detects the <NUM> radio problem (S615). Thereafter, the UE <NUM> starts a <NUM> retry waiting timer after detecting the <NUM> radio problem (S620). Thereafter, the UE <NUM> performs switching decision and link switching to <NUM> (S625). The UE <NUM> transmits the <NUM> link start marker packet to the <NUM> GW <NUM> through the <NUM> signaling APN in order to transfer the link switching information from <NUM> to <NUM> (S630). When the link start marker packet is received, the <NUM> GW <NUM> performs link switching from <NUM> to <NUM> (S635). Further, if the <NUM> link start marker packet is received, the <NUM> GW <NUM> interrupts transmission of the keep alive packet through <NUM> (S645).

If the <NUM> connection is recovered at a time when the <NUM> retry waiting timer expires, the UE <NUM> and the <NUM> GW <NUM> perform the link switching process to <NUM> (S650). The link switching process to <NUM> is the same as the operation from S544 to S554.

If the <NUM> recovery has failed, <NUM> is in an inactivity state, or <NUM> is switched to an idle state due to the expiration of the <NUM> RLF timer at the time when the <NUM> retry waiting timer expires, a <NUM> Node-B <NUM> perform a <NUM> RRC release procedure if necessary (S655). Thereafter, if the <NUM> connection triggering condition (e.g., <NUM> RRC connected) is satisfied in the <NUM> zone, the UE <NUM> starts <NUM> searching (S660). When the UE <NUM> discovers an available <NUM>, a <NUM> idle to active procedure is performed (S665). Thereafter, the UE <NUM> and the <NUM> GW <NUM> performs link switching to <NUM> (S670). The link switching process to <NUM> is the same as the operation from S544 to S554.

<FIG> is a diagram illustrating a call flow in <NUM> and <NUM> idle states in the case of no traffic in a dependent <NUM>-<NUM> RRC state mode.

The UE <NUM> and the <NUM> GW <NUM> perform data transmission/reception through <NUM> (S700). In this case, the <NUM> GW <NUM> periodically transmits the keep alive packet to the UE <NUM> through the <NUM> signaling APN bearer in order to maintain the LTE RRC connection for fast <NUM> fallback during the data transmission/reception through <NUM> (S705).

In this case, if there is not user data, a <NUM> inactivity timer starts in a <NUM> NB <NUM>. If the inactivity timer expires in the <NUM> NB <NUM>, a <NUM> RRC release procedure is performed (S715). If the <NUM> connection triggering condition is not satisfied (e.g., if the user data does not exist), the UE <NUM> turns off a <NUM> modem (S720), and it performs link switching to <NUM> (S725). The UE <NUM> transmits a <NUM> link start marker packet to the <NUM> GW <NUM> through the <NUM> signaling APN bearer in order to transfer link switching information to <NUM> (S730). When the <NUM> link start marker packet is received, the <NUM> GW <NUM> performs link switching to <NUM> (S735). The <NUM> GW <NUM> interrupts transmission of the keep alive packet through <NUM> after performing the link switching to <NUM> (S740). Thereafter, data transmission/reception through <NUM> may be performed, or if the <NUM> connection triggering condition is satisfied, a <NUM> connection and switching process may be generated (S745).

Thereafter, if there is not the <NUM> link start marker packet or the user data, an eNB <NUM> starts the <NUM> inactivity timer (S750). If the inactivity timer expires, the eNB <NUM> performs the <NUM> RRC release procedure (S755).

Hereinafter, the detailed call flow in an independent <NUM>-<NUM> RRC state mode is as follows.

<FIG> is a diagram illustrating a call flow of initial <NUM> attach (including idle to active and HO to <NUM> zone) and initial <NUM> attach (including change to an active state) in an independent <NUM>-<NUM> RRC state mode.

If a MO/MT data service is generated, the <NUM> network is not available, and only the <NUM> network is available, the UE <NUM> performs <NUM> attach or idle to active operation (S800). In the case of an initial <NUM> attach, an APN through a <NUM> link is generated (S805). The APN may be the Intemet APN, and it is an APN being switched between <NUM> and <NUM> for data services.

The UE <NUM> determines the <NUM> zone based on the specific PLMN or TA, and in the case of the <NUM> attach or HO In in the <NUM> zone, a <NUM> signaling APN is generated (S810). Further, in the case of an LTE attach regardless of a specific area, the <NUM> signaling APN may be basically generated. The signaling APN is an APN for <NUM>-<NUM> switching control. If switching to <NUM> is not performed, the UE <NUM> performs link switching to <NUM> (S815). In the case of a general initial attach, such an operation may not be necessary. After performing the link switching to <NUM>, the UE <NUM> transmits the <NUM> link start marker packet to the <NUM> GW <NUM> through the <NUM> signaling APN bearer in order to transfer information on link switching to <NUM> (S820). The <NUM> GW <NUM> performs the link switching to <NUM> when the <NUM> link start marker packet is received (S830). If the <NUM> GW <NUM> is transmitting the keep alive packet to <NUM> after performing the link switching to <NUM>, it interrupts transmission of the keep alive packet (S835). The data is transmitted and received through <NUM> until the generation of the <NUM> link (S840).

If the MO/MT data service is generated, the <NUM> network is available, the UE <NUM> performs a <NUM> attach or <NUM> idle to active procedure (S845). In the case of an initial <NUM> attach, an APN through a <NUM> link is generated (S850). The APN may be the Intemet APN, and it is an APN being switched between <NUM> and <NUM> for data services. In the case of the initial <NUM> attach, the <NUM> signaling APN is generated (S860). The signaling APN is an APN for <NUM>-<NUM> switching control.

If switching to <NUM> is not performed, the UE <NUM> performs switching to the <NUM> link (S860). The UE <NUM> transmits the link start marker packet to the <NUM> GW <NUM> through the <NUM> signaling APN bearer in order to transfer information on link switching to <NUM> (S865). The <NUM> GW <NUM> performs the link switching to <NUM> when the <NUM> link start marker packet is received (S870). If fast <NUM> fallback is necessary after the link switching to <NUM> is performed, the <NUM> GW <NUM> starts transmission of the keep alive packet in order to maintain the LTE RRC connection (S875). Thereafter, data transmission/reception through <NUM> is performed (S880). Thereafter, the <NUM> GW <NUM> periodically transmits the keep alive packet to the UE <NUM> through the <NUM> signaling APN bearer in order to maintain the LTE RRC connection (S885).

<FIG> is a diagram illustrating fast <NUM> fallback and <NUM> recovery processes when a <NUM> radio problem occurs in an independent <NUM>-<NUM> RRC state mode.

<NUM> is currently in an RRC connected state, and data transmission/reception is being performed (S900). In this case, if it is determined that <NUM> fast fallback is necessary, UE <NUM> and a <NUM> network perform <NUM> attach or active (paging) procedure as a fast <NUM> fallback advance preparation operation in the case where <NUM> is in a disconnected state (S905). The <NUM> GW <NUM> starts transmission of a keep alive packet through a <NUM> signaling APN bearer (S910), and it periodically transmits the keep alive packet through the <NUM> signaling APN bearer in order to maintain the LTE RRC connection (S915). In this case, data transmission/reception through <NUM> is performed (S920).

If the UE <NUM> detects a <NUM> radio problem (S925), the UE <NUM> starts a <NUM> retry waiting timer (S930). The UE <NUM> performs switching decision and link switching to <NUM> (S935). The UE <NUM> transmits a <NUM> link start marker packet to the <NUM> GW <NUM> through the <NUM> signaling APN in order to transfer information on link switching to <NUM> (S940). The <NUM> GW <NUM> performs link switching to <NUM> when the <NUM> link start marker packet is received (S945), and it interrupts transmission of the keep alive packet through <NUM> (S950). Thereafter, data transmission/reception through <NUM> is performed (S955).

If the <NUM> connection is recovered at a time when the <NUM> retry waiting timer expires, link switching to <NUM> is performed (S960).

If the <NUM> recovery has failed at the time when the <NUM> retry waiting time expires, in the case of <NUM> inactivity, or if the state is switched to an idle state due to the expiration of the <NUM> RLF timer, a <NUM> Node-B <NUM> performs <NUM> RRC release procedure if necessary (S965). If a <NUM> connection triggering condition (e.g., <NUM> RRC connected) is satisfied in a <NUM> zone, the UE <NUM> starts <NUM> searching (S970). If an available <NUM> is discovered, the UE <NUM> performs <NUM> idle to active procedure (S975). Thereafter, the link switching to the <NUM> process is performed (S980). The link switching to the <NUM> process is the same as the operation from S544 to S554.

<FIG> is a diagram illustrating a call flow in <NUM> and <NUM> idle states in the case of no traffic in an independent <NUM>-<NUM> RRC state mode.

Data transmission/reception through <NUM> is currently performed (S1000). The <NUM> GW <NUM> may periodically transmit the keep alive packet to the UE <NUM> through the <NUM> signaling APN bearer in order to maintain the LTE RRC connection for fast <NUM> fallback during the data transmission/reception through <NUM> (S1005).

If there is not user data, an inactivity timer starts in a <NUM> NB <NUM>. In this case, if the inactivity timer expires, a <NUM> RRC release procedure is performed (S1015).

The UE <NUM> performs link switching to <NUM> after the <NUM> RRC release (S1020), and it transmits a <NUM> link start marker packet to the <NUM> GW <NUM> through the <NUM> signaling APN bearer in order to transfer information on link switching to <NUM> (S1025). When the <NUM> link start marker packet is received, the <NUM> GW <NUM> performs link switching to <NUM> (S1030), and in this case, it interrupts transmission of the keep alive packet through <NUM> (S1035). Thereafter, data transmission/reception through <NUM> is performed (S1040).

Thereafter, if there is not the <NUM> link start marker packet or the user data, the <NUM> NB <NUM> starts the <NUM> inactivity timer (S1045). If the inactivity timer expires, the <NUM> NB <NUM> performs the <NUM> RRC release procedure (S1050).

Hereinafter, a terminal operation in a dependent <NUM>-<NUM> RRC state mode will be described.

<FIG> is a flowchart illustrating <NUM> and <NUM> initial attach and active procedures in a dependent <NUM>-<NUM> RRC state mode of a terminal.

At operation <NUM>, a terminal performs an LTE attach procedure, Intemet APN and <NUM> signaling APN generation through <NUM>, or a <NUM> idle to active procedure. At operation <NUM>, the terminal performs switching to a <NUM> link, and it transmits a <NUM> link start marker packet to the <NUM> GW. If path setup to <NUM> is immediately performed during an initial access, operation <NUM> can be omitted. At operation <NUM>, a data service through <NUM> is performed between the terminal and a <NUM> base station.

At operation <NUM>, the terminal determines whether a <NUM> connection triggering condition is satisfied. The following items may be considered as the <NUM> connection triggering condition. First, in the case of recognizing a <NUM> zone through matching to a specific PLMN or TA, the terminal determines whether to perform <NUM> connection triggering and switching. Second, in the case of a <NUM> RRC connected state, the terminal determines whether to perform the <NUM> connection triggering and switching. Third, in the case where a basic data bearer through <NUM> or a specific data bearer (e.g., Intemet APN) exists, the terminal determines whether to perform the <NUM> connection triggering and switching. Fourth, in the case where a basic user data through <NUM> or a specific user data (e.g., data through the Intemet APN) is transmitted and received, the terminal determines whether to perform the <NUM> connection triggering and switching. If one or more of the <NUM> connection triggering conditions are satisfied, the terminal performs5G connection attempt operation (e.g., a case where the existence of the <NUM> zone is recognized or a case where the data bearer exists with recognition of the existence of the <NUM> zone may be used as a specific <NUM> connection triggering condition). If the above-described condition is satisfied, the terminal turns on a <NUM> modem based on the <NUM> connection triggering condition, and through this, the terminal may perform the <NUM> cell searching. If an available <NUM> cell exists, the terminal may perform the <NUM> connection and switching operations. In contrast, if the condition is not satisfied, the terminal may perform <NUM> switching operation, and in this case, the terminal may interrupt the <NUM> cell searching, and it may turn off the <NUM> modem.

If the <NUM> connection triggering condition is satisfied at operation <NUM>, the terminal turns on the <NUM> modem and starts <NUM> cell searching at operation <NUM>. At operation <NUM>, the terminal determines whether an available <NUM> is discovered, and if the available <NUM> is not discovered, the terminal continuously performs a data service through <NUM> (<NUM>), whereas if the available <NUM> is discovered, the terminal performs <NUM> attach and Intemet APN and <NUM> signaling APN generation through <NUM>, or <NUM> idle to active procedure at operation <NUM>. Thereafter, at operation <NUM>, the terminal performs switching to a <NUM> link and transmits a <NUM> link start marker packet to the <NUM> GW, and at operation <NUM>, the terminal performs the data service through <NUM>.

If the <NUM> connection triggering condition is not satisfied at operation <NUM>, the terminal returns to operation <NUM> and continuously performs the data service through <NUM>.

<FIG> is a flowchart illustrating fast <NUM> fallback and <NUM> recovery procedures when a <NUM> radio problem occurs in a dependent <NUM>-<NUM> RRC state mode of a terminal.

At operation <NUM>, the data service through <NUM> is performed. In this case, at operation <NUM>, the terminal determines whether the occurrence of the <NUM> radio problem is detected. If it is necessary to prevent a ping-pong phenomenon between <NUM> and <NUM> at operation <NUM>, the terminal starts the <NUM> retry waiting timer to adjust a re-access time of the <NUM> Node-B. If the <NUM> radio problem does not occur, the terminal returns to operation <NUM>.

At operation <NUM> after operation <NUM>, the terminal performs switching to a <NUM> link and it transmits a <NUM> link start marker packet to the <NUM> GW. The data service through <NUM> is performed until the <NUM> retry waiting timer expires (<NUM>). Thereafter, the <NUM> retry waiting timer expires (<NUM>), and the terminal determines whether the <NUM> connection has been recovered (<NUM>). If so, the terminal performs switching to the <NUM> link and it transmits the <NUM> link start marker packet to the <NUM> GW (<NUM>). Thereafter, the terminal returns again to operation <NUM> to perform the data service through <NUM>.

If it is determined that the <NUM> connection recovery has failed at operation <NUM>, the terminal starts the <NUM> cell searching (<NUM>), and it determines whether an available <NUM> is discovered (<NUM>). If the available <NUM> is not discovered, the terminal continuously performs the data service through <NUM>, and it performs the <NUM> cell searching (<NUM>). If the available <NUM> is discovered, the terminal performs <NUM> idle to active procedure (<NUM>). Thereafter, the terminal performs switching to the <NUM> link and it transmits the <NUM> link start marker packet to the <NUM> GW. Thereafter, the terminal returns to operation <NUM> to perform the data service through <NUM>.

<FIG> is a flowchart illustrating a procedure in the case of no traffic with respect to <NUM> in a dependent <NUM>-<NUM> RRC state mode of a terminal.

At operation <NUM>, a data service through <NUM> is performed. If user traffic exists, the data service is continuously performed, whereas if the user traffic does not exist, the <NUM> base station starts an inactivity timer, and the terminal determines whether the inactivity timer in the <NUM> base station expires at operation <NUM>. If the inactivity timer expires, the <NUM> active to idle procedure (i.e., <NUM> RRC connection release process) is performed at operation <NUM>. Thereafter, if the <NUM> connection triggering condition is not satisfied and the existing <NUM> modem-on and searching state is maintained, <NUM> modem-off may be performed (<NUM>). If the inactivity timer does not expire, the terminal returns to operation <NUM>.

At operation <NUM>, a data service through <NUM> is performed. If user traffic exists, the data service is continuously performed, whereas if the user traffic does not exist, the <NUM> base station starts an inactivity timer, and the terminal determines whether the inactivity timer expires at operation <NUM>. If the inactivity timer expires, the <NUM> active to idle procedure (i.e., <NUM> RRC connection release process) is performed (<NUM>). If the inactivity timer does not expire, the terminal returns to operation <NUM>.

If the <NUM> connection triggering condition is not satisfied after operation <NUM>, <NUM> modem-off may be performed (<NUM>), and the terminal performs switching to a <NUM> link and it transmits a <NUM> link start marker packet to the <NUM> GW (<NUM>). Thereafter, if necessary, the terminal performs the data service through <NUM> (<NUM>), and if the user traffic exists, the data service is continuously performed, whereas if the <NUM> connection triggering condition is satisfied, <NUM> connection and switching may be performed. If the user traffic does not exist, the <NUM> base station starts the inactivity timer. At operation <NUM>, the terminal determines whether the inactivity timer in the <NUM> base station expires. If the inactivity timer expires, the terminal performs <NUM> active to idle procedure (i.e., <NUM> RRC connection release) (<NUM>). If the inactivity timer does not expire, the terminal returns to operation <NUM>.

<FIG> is a flowchart illustrating <NUM> and <NUM> initial attach and active procedures of a terminal in an independent <NUM>-<NUM> RRC state mode.

At operation <NUM>, MO/MT data service is generated, and at operation <NUM>, the terminal identifies whether only the <NUM> network is available. If only the <NUM> network is available, the terminal, at operation <NUM>, performs LTE attach procedure and generation of the Intemet APN through <NUM> and <NUM> signaling APN, or <NUM> idle to active procedure. Thereafter, at operation <NUM>, the terminal performs switching to a <NUM> link, and it transmits a <NUM> link start marker packet to the <NUM> GW. If path setup to <NUM> is immediately performed during an initial access, operation <NUM> can be omitted. Thereafter, the terminal performs a data service through <NUM> with the <NUM> base station (<NUM>). Thereafter, the terminal determines whether the <NUM> network is available (<NUM>), and if the <NUM> network is available, the terminal performs <NUM> attach and it generates the Intemet APN through <NUM> and <NUM> signaling APN, or it performs <NUM> idle to active procedure (<NUM>). If the <NUM> network is not available, the terminal, at operation <NUM>, continuously performs the data service. At operation <NUM> after operation <NUM>, the terminal performs switching to the <NUM> link, and it transmits the <NUM> link start marker packet to the <NUM> GW. Thereafter, the terminal performs the data service through the <NUM> connection.

At operation <NUM>, if only the <NUM> network is not available (i.e., if the <NUM> network is available), the terminal performs operation <NUM>.

If the <NUM> radio problem occurs in the independent <NUM>-<NUM> RRC state mode, fast <NUM> fallback and <NUM> recovery procedures are performed in the same manner as those as illustrated in <FIG>.

The procedure in the case of no traffic with respect to <NUM> in the dependent <NUM>-<NUM> RRC state mode is the same as that as illustrated in <FIG>.

Next, the operation of the <NUM> GW (hereinafter, GW) in the dependent <NUM>-<NUM> RRC state mode will be described.

<FIG> is a flowchart illustrating <NUM> and <NUM> initial attach and active procedures of a GW in a dependent <NUM>-<NUM> RRC state mode.

At operation <NUM>, the GW performs an LTE attach procedure according to a request from the terminal, generation of the Intemet APN through <NUM> and <NUM> signaling APN, or an active procedure in the case where <NUM> is in an idle state. At operation <NUM>, the GW receives the <NUM> link start marker packet from the terminal. Thereafter, the GW performs switching to the <NUM> link (<NUM>). If path setup to <NUM> is immediately performed during an initial access, operations <NUM> and <NUM> can be omitted. Thereafter, at operation <NUM>, if the GW pre-transmits a keep alive packet on a <NUM> path, it interrupts transmission of the keep alive packet. Thereafter, at operation <NUM>, the GW performs the data service through <NUM>.

Thereafter, at operation <NUM>, if the <NUM> attach has not been performed, the GW performs the <NUM> attach procedure according to the request from the terminal, generates Intemet APN through <NUM> and <NUM> signaling APN, and performs allocation of the same IP as that of <NUM> and <NUM>-<NUM> session binding in the performing process. Further, if <NUM> is in an idle state, an active procedure is performed. In other cases (e.g., if <NUM> is in a pre-connected state), operation <NUM> can be omitted. Thereafter, at operation <NUM>, the GW identifies whether the <NUM> link start marker packet transmitted by the terminal is received. If the GW receives the <NUM> link start marker packet, it performs switching to the <NUM> link (<NUM>). In order to maintain the <NUM> RRC in a connected state for the purpose of the fast <NUM> fallback, the GW periodically transmits the keep alive packet to the terminal through the <NUM> path (<NUM>). Thereafter, the GW performs the data service through <NUM>. At operation <NUM>, if the GW does not receive the <NUM> link start marker packet, it returns to operation <NUM> to perform the data service using <NUM>.

At operation <NUM> after operation <NUM>, the GW determines whether the <NUM> link start marker packet is received, and if the packet is received, the GW returns to operation <NUM>, whereas if the packet is not received, the GW returns to operation <NUM>.

<FIG> is a flowchart illustrating fast <NUM> fallback and <NUM> recovery procedures in the case of a <NUM> radio problem of a GW in a dependent <NUM>-<NUM> RRC state mode.

At operation <NUM>, the GW performs the data service through <NUM>. In this case, the GW, at operation <NUM>, identifies whether the <NUM> link start marker packet is received, and if the packet is received, the GW performs switching to the <NUM> link (<NUM>), whereas if the packet is not received, the GW returns to operation <NUM>.

If the GW pre-transmits the keep alive packet to the <NUM> path after the switching to the <NUM> link is performed at operation <NUM>, the GW interrupts transmission of the keep alive packet. Thereafter, the GW perform the data service through <NUM> with the terminal.

Thereafter, if <NUM> is in an idle state, the GW performs an active procedure according to the request from the terminal (<NUM>). If <NUM> is already in the active state, the above operation can be omitted. At operation <NUM>, the GW identifies whether the <NUM> link start marker packet is received, and if the packet is received, the GW performs switching to the <NUM> link (<NUM>). If fast <NUM> fallback is necessary, the GW periodically transmits the keep alive packet to the <NUM> path in order to maintain the <NUM> RRC in a connected state (<NUM>), and thereafter, the GW returns to operation <NUM> to perform the data service through <NUM>.

If the <NUM> link start marker packet is not received at operation <NUM>, the GW returns to operation <NUM> to perform the data service using the <NUM> connection.

<FIG> is a flowchart illustrating <NUM> and <NUM> idle procedures in the case of no traffic of a GW in a dependent <NUM>-<NUM> RRC state mode. In <FIG>, the GW is performing a data service through <NUM>.

At operation <NUM>, the GW performs the data service through <NUM>. If user traffic exists, the GW continuously performs the data service, whereas if the user traffic does not exist, the <NUM> base station starts an inactivity timer. The GW determines whether the inactivity timer expires (<NUM>), and if the inactivity timer in the <NUM> base station expires due to the continuous nonexistence of the user traffic, the GW performs <NUM> active to idle procedure (<NUM> S1 connection release) at operation <NUM>. If the inactivity timer does not expire, the GW returns to operation <NUM> to perform the data service through <NUM>.

<FIG> is another flowchart illustrating <NUM> and <NUM> idle procedures in the case of no traffic of a GW in a dependent <NUM>-<NUM> RRC state mode. In <FIG>, the GW is performing a data service through <NUM>.

At operation <NUM>, the GW performs the data service through <NUM>. If user traffic exists, the GW continuously performs the data service, whereas if the user traffic does not exist, the <NUM> base station starts an inactivity timer. The GW determines whether the inactivity timer expires (<NUM>), and if the inactivity timer in the <NUM> base station expires due to the continuous nonexistence of the user traffic, the GW performs <NUM> active to idle procedure (<NUM> S1 connection release) at operation <NUM>. Thereafter, the GW receives the <NUM> link start marker packet from the terminal and it performs switching to the <NUM> link (<NUM>). Thereafter, if necessary, the GW performs the data service through <NUM> (<NUM>).

At operation <NUM>, if the user traffic exists and the inactivity timer does not expire, the GW returns to operation <NUM> to continuously perform the data service.

After operation <NUM>, if the user traffic exists, the GW continuously performs the data service using <NUM>, whereas if the user traffic does not exist, the <NUM> base station starts the inactivity timer. The GW determines whether the inactivity timer expires (<NUM>), and if the inactivity timer in the <NUM> base station expires due to the continuous nonexistence of the user traffic, the GW performs <NUM> active to idle procedure (<NUM> S1 connection release) at operation <NUM>.

At operation <NUM>, if the user traffic exists and the inactivity timer does not expire, the GW return to operation <NUM> to continuously perform the data service.

Next, the operation of the <NUM> GW (hereinafter, GW) in an independent <NUM>-<NUM> RRC state mode will be described.

<FIG> is a flowchart illustrating <NUM> and <NUM> initial attach and active procedures of a GW in an independent <NUM>-<NUM> RRC state mode.

At operation <NUM>, the GW performs the <NUM> attach procedure according to the request from the terminal, generates Internet APN through <NUM> and <NUM> signaling APN, and performs allocation of the same IP as that of <NUM> and <NUM>-<NUM> session binding if the existing <NUM> connection exists in the performing process. Further, if <NUM> is in an idle state, the GW performs an active procedure and moves to operation <NUM>. Further, at operation <NUM>, the GW performs the LTE attach procedure according to the request from the terminal, generates the Intemet APN through <NUM> and <NUM> signaling APN, and performs allocation of the same IP and <NUM>-<NUM> session binding if the existing <NUM> connection exists in the performing process, or the GW performs the active procedure and then it move to operation <NUM> if <NUM> is in an idle state.

At operation <NUM>, the GW identifies whether the <NUM> link start marker packet is received from the terminal, and if the <NUM> link start marker packet is received, the GW performs switching to the <NUM> link (<NUM>). If the path setup to <NUM> is immediately performed during an initial access, operations <NUM> and <NUM> can be omitted. If it is determined that fast <NUM> fallback is necessary, the GW periodically transmits the keep alive packet to the <NUM> path in order to maintain the <NUM> RRC in a connected state (<NUM>). Thereafter, the GW performs the data service through <NUM> (<NUM>).

Thereafter, at operation <NUM>, the GW identifies whether the <NUM> link start marker packet is received from the terminal, and if the packet is received, the GW perform the switching to the <NUM> link (<NUM>). If the path setup to <NUM> is immediately performed during the initial access, operations <NUM> and <NUM> can also be omitted. If the keep alive packet is pre-transmitted to the <NUM> path after operation <NUM>, the GW interrupts transmission of the keep alive packet.

Thereafter, the GW performs the data service through <NUM> (<NUM>).

If the <NUM> link start marker packet is not received from the terminal, the GW returns to operation <NUM>.

The operation of the GW in fast <NUM> fallback and <NUM> recovery procedures in the case where the <NUM> radio problem occurs in an independent <NUM>-<NUM> RRC state mode is the same as that as illustrated in <FIG>.

The operation of the GW in <NUM> and <NUM> idle procedures in the case of no traffic in an independent <NUM>-<NUM> RRC state mode is the same as that as illustrated in <FIG> and <FIG>.

Hereinafter, the definition and format of a message used in the <NUM>-<NUM> interworking architecture will be described.

First, the definition and format of a link start marker packet message will be described.

The message is transmitted from the UE to the <NUM> GW through a <NUM> eNB, or it is transmitted from the UE to the <NUM> GW through a <NUM> Node-B. The message is for the UE to request switching notification and GW switching from the GW after the <NUM>-<NUM> switching operation.

<FIG> is a diagram illustrating a format of a link start marker packet message.

The message includes at least one of an IP header <NUM>, a UDP header <NUM>, a control message type indicator <NUM>, a fast fallback control flag <NUM>, and APN information <NUM>. The control message type indicator <NUM> [<NUM> byte] may be a <NUM> start marker packet (0x00) or a <NUM> start marker packet (0x01), and the fast fallback control flag <NUM> [<NUM> byte] may be fast fallback (keep alive packet non-transmission) non-applied (0x00) or fast fallback (keep alive packet transmission) applied (0x01). Further, the APN information <NUM> for discriminating the APN in multiple PDN connection (APN network identifier and APN operator identifier) [variable size] may be, for example, network ID.

Second, the definition and format of a keep alive packet message will be described.

The message is transmitted from the <NUM> GW to the UE through the <NUM> eNB, and it is periodically transmitted to the corresponding network in order to maintain the RRC of an available network (e.g., <NUM>) that can be switched for fast fallback in a connected state.

<FIG> is a diagram illustrating a keep alive packet message.

The message includes at least one of an IP header <NUM>, a UDP header <NUM>, and a control message type indicator <NUM>. The control message type indicator <NUM> [<NUM> byte] may be a keep alive packet (0x03).

Hereinafter, another embodiment of a signaling message according to the disclosure will be described.

First, a <NUM> start marker message will be described. The <NUM> start marker message is transmitted from the UE to the <NUM> GW through the <NUM> eNB, and the message ID may be 0x00 <NUM><NUM><NUM>. This message is for the UE to perform switching notification to the <NUM> GW after the switching operation from <NUM> to <NUM>, and for the <NUM> GW to request switching from <NUM> to <NUM>, and it may be repeatedly transmitted with a specific period and the specific number of transmissions for reliable reception during transmission to the <NUM> GW. As an example, the message may be repeatedly transmitted <NUM> times for each <NUM>.

Second, a <NUM> start marker message will be described. The <NUM> start marker message is transmitted from the UE to the <NUM> GW through the <NUM> NB, and the message ID may be 0x00 <NUM><NUM><NUM>. This message is for the UE to perform switching notification to the <NUM> GW after the switching operation from <NUM> to <NUM>, and for the <NUM> GW to request switching from <NUM> to <NUM>. The message may be repeatedly transmitted with a specific period and the specific number of transmissions for reliable reception during transmission to the <NUM> GW. As an example, the message may be repeatedly transmitted <NUM> times for each <NUM>.

Third, a <NUM> start complete message will be described. This message is transmitted from the <NUM> GW to the UE through the <NUM> eNB, and the message ID may be 0x00 <NUM><NUM> A4. Basically, the UE and the <NUM> GW can perform the switching immediately after a <NUM> start marker is transmitted and received, but if it is selectively necessary, the <NUM> GW may transmit an identification response message indicating that the switching has been normally performed with respect to the switching operation from <NUM> to <NUM> being requested from the UE through the <NUM> start marker. The corresponding message may be repeatedly transmitted with a specific period and the specific number of transmissions for reliable reception during transmission to the UE. As an example, the message may be repeatedly transmitted <NUM> times for each <NUM>.

Fourth, a <NUM> start complete message will be described. The <NUM> start complete message is transmitted from the <NUM> GW to the UE through the <NUM> NB, and the message ID may be 0x00 <NUM><NUM> A5. Basically, the UE and the <NUM> GW can perform the switching immediately after the <NUM> start marker is transmitted and received, but if it is selectively necessary, the <NUM> GW may transmit an identification response message indicating that the switching has been normally performed with respect to the switching operation from <NUM> to <NUM> being requested from the UE through the <NUM> start marker. The corresponding message may be repeatedly transmitted with a specific period and the specific number of transmissions for reliable reception during transmission to the UE. As an example, the message may be repeatedly transmitted <NUM> times for each <NUM>.

Fifth, a keep alive message will be described. The keep alive message is transmitted from the <NUM> GW to the UE through the <NUM> eNB, or from the <NUM> GW to the UE through the <NUM> NB, and the message ID may be 0x00 <NUM><NUM>0F. The message is periodically transmitted to the corresponding network in order to maintain (keep alive) the RRC connection of an available network (e.g., <NUM>) that can be switched for fast fallback.

<FIG> is a diagram illustrating a basic message format that can be applied to various signaling messages according to an embodiment of the disclosure.

With reference to <FIG>, the basic message format is composed of an IP header <NUM>, a UDP header <NUM>, a reserved ID (8bytes) <NUM>, a transaction IC (<NUM> bytes) <NUM>, a message ID (<NUM> bytes) <NUM>, fast fallback related information (<NUM> byte) <NUM>, APN information (<NUM> bytes) <NUM>, UE information (<NUM> bytes) <NUM>, and a payload. The lengths of the above-described fields are exemplary, and they can be changed.

The reserved ID <NUM> is composed of <NUM> bytes that can be set to "<NUM>" in all. The transaction ID is composed of <NUM> bytes, and the ID value may start from "<NUM>", and in particular, the ID value can start from "<NUM>", and it can be increased by <NUM> whenever the link change occurs. Further, in the case of repeatedly transmitting the same message for reliable transmission, the same transaction ID may be used.

The message ID is a field indicating the attribute of the message with <NUM> bytes, and a <NUM> start marker may be 0x00 <NUM><NUM> A4, and a <NUM> start marker may be 0x00 <NUM><NUM><NUM>. Further, a <NUM> start complete may be 0x00 <NUM><NUM> A4, a <NUM> start complete may be 0x00 <NUM><NUM> A5, and keep alive may be 0x00 <NUM><NUM>0F.

The fast fallback field may be selectively included, and it is a flag for requesting the <NUM> GW to transmit a keep alive packet if fast fallback is necessary during transmission of the <NUM> link start marker. If it is desired not to transmit the keep alive packet, "0x00" may be set, whereas if it is desired to transmit the keep alive packet, "0x01" may be set.

The APN information may be selectively included, and it is information for discriminating the APN in the multiple PDN connection. UE information may be selectively included, and it is information for discriminating the <NUM> UE.

The payload may be filled by "F" up to the last byte.

<FIG> is a diagram illustrating an internal structure of a terminal according to an embodiment of the disclosure.

As illustrated in <FIG>, a terminal according to an embodiment of the disclosure may include a transceiver <NUM> and a controller <NUM>.

The transceiver <NUM> may transmit and receive signals with a <NUM> or <NUM> base station.

The controller <NUM> may control a signal flow between respective blocks so that the terminal according to an embodiment of the disclosure can operate. As an example, the controller <NUM> may independently perform access procedures with respect to a first wireless network and a second wireless network. Further, in accordance with the wireless link states of the first and second wireless networks, the controller <NUM> may control to perform communication for the same service through the first or second wireless network.

Further, the controller <NUM> may control to detect entry to a coverage of the second wireless network during performing of the first wireless network, and to transmit a second wireless network link start marker packet to a node through a second wireless network signaling access point name (APN) generated between the terminal and the node of the second wireless network.

Further, while performing communication through the second wireless network, the controller <NUM> may control to receive a packet for maintaining a connection to the first wireless network from the node.

The above-described function of the controller <NUM> is exemplary, and it is to be noted that the controller can be configured to perform all functions described in the description.

<FIG> is a diagram illustrating an internal structure of a gateway device according to an embodiment of the disclosure.

As illustrated in <FIG>, a gateway device according to an embodiment of the disclosure may include a transceiver <NUM> and a controller <NUM>.

The transceiver <NUM> may transmit and receive signals with certain nodes of the wireless communication system.

The controller <NUM> may control a signal flow between respective blocks so that the gateway device according to an embodiment of the disclosure can operate.

As an example, the controller <NUM> may control to configure an APN for providing the same service to a certain terminal through a first wireless communication network or a second wireless communication network, and to provide the service to the terminal through the APN based on a wireless link state between the terminal and the first wireless communication network or a wireless link state between the terminal and the second wireless communication network.

Further, the controller <NUM> may control to configure a first wireless communication network signaling between the terminal and the first wireless communication network if an access request through the first wireless communication network is received from the terminal, and to configure a second wireless communication network signaling APN between the terminal and the second wireless communication network if an access request through the second wireless communication network is received from the terminal.

Further, while performing communication through the second wireless network, the controller <NUM> may control to transmit a packet for maintaining a connection between the terminal and the first wireless communication network to the terminal.

The interworking system without dependency between the legacy <NUM> network and the <NUM> network described in the disclosure has the following effects. First, <NUM> service launch and development can be performed quickly and conveniently without an impact to the <NUM> base station operated in the existing commercial network through the interworking system proposed in the disclosure. Second, it is possible to support a seamless service between <NUM> and <NUM> through the same IP address allocation with respect to one service in <NUM> and <NUM> cores. Third, the stability of a <NUM> radio link (mmWave) may be lowered in a mobile environment, but in order to secure the stability of the service even in such an environment, the UE that can determine the radio state most quickly performs fast link switch decision between <NUM> and <NUM>, and thus the <NUM> fallback can be quickly performed.

Claim 1:
A method of a terminal (<NUM>) in a wireless communication system, the method comprising:
communicating (S534, <NUM>) with a gateway (<NUM>) based on a first packet data network, PDN, connection, related to a first communication system; further characterized by
establishing (S538, <NUM>) a bearer with the gateway on a second communication system while the terminal is in a radio resource control, RRC, connection state on the first communication system;
establishing (S540, <NUM>) a second PDN connection related to the second communication system;
performing switching (S544, <NUM>) from the first communication system to the second communication system;
transmitting (S546, <NUM>) a link start marker packet to the gateway (<NUM>) based on the second PDN connection; and
transmitting and receiving (S552, <NUM>) data using the gateway (<NUM>) based on the second PDN connection.