Communication system

Abstract

To provide a technique for suppressing the generation of unnecessary traffic when a terminal device on a receiving side is incapable of voice calls.SOLUTION: A second terminal device 100b determines whether or not a voice call is possible based on the strength of a signal received from a second base station device 200b, and transmits a determination result to a server 500. The server 500 manages the determination result as a reception status. A first terminal device 100a transmits a call request signal to a server 400. If the reception status indicates that a voice call is possible, the server 400 transmits a calling signal to the second terminal device 100b.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to communication technology, and more particularly to communication systems that provide voice calls. [Background technology]

[0002] The PTT (Push to Talk) function is a function that provides users with the ease of use of a half-duplex radio. A user can call and talk to multiple other users simply by pressing the call button without having to enter a phone number or the like. The other users can hear the voice of the speech output from their mobile phones without having to press the answer button. A service that provides the PTT function via a mobile communication network is the PoC (Push-to-Talk over Cellular) service (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-527942 Summary of the Invention [Problem to be solved by the invention]

[0004] In PoC, a terminal device (PoC terminal) that has a session established with a PoC server receives a voice signal. However, if the terminal device is located near the fringe (in a weak electric field environment), it may fail to receive the voice signal or may be unable to communicate. Taking the LTE (Long Term Evolution) communication system as an example, the terminal device generally receives a reference signal (CRS (Cell-specific Reference Signal)) broadcast from a base station device, measures RSRP (Reference Signal Received Power) or RSRQ (Reference Signal Received Quality), and reports the measurement results to the base station device. The base station device then disconnects the RRC (Radio Resource Control) connection between the terminal device and the radio bearer section according to the measurement results. However, the mobile communication network maintains the EPS (Evolved Packet System) bearer of the core network even if the RRC connection is disconnected. Therefore, when a transmitting terminal device (hereinafter referred to as "first terminal device") attempts to make a call (establish a session for a PoC call) to a receiving terminal device (hereinafter referred to as "second terminal device"), the PoC server attempts to set up a session with the second terminal device. This process is performed even if the second terminal device is located near the fringe and is therefore unable to make a voice call. As a result, unnecessary traffic occurs when the receiving terminal device is unable to make a voice call.

[0005] The present invention has been made in view of the above circumstances, and its purpose is to provide a technique for suppressing the generation of unnecessary traffic when a terminal device on the receiving side is unable to carry out voice calls. [Means for solving the problem]

[0006] In order to solve the above problem, a communication system according to one embodiment of the present invention includes a first terminal device and a second terminal device capable of voice communication, a first base station device capable of wireless communication with the first terminal device, a second base station device capable of wireless communication with the second terminal device, a core network connecting the first base station device and the second base station device and establishing a communication network between the first terminal device and the second terminal device, and a server connected to the core network, managing calls between the first terminal device and the second terminal device, and managing a reception status of signals from the second base station device at the second terminal device. When the second terminal device determines whether or not the voice communication is possible based on the strength of a signal received from the second base station device, the server transmits the determination result to the server. When the server receives the determination result from the second terminal device, the server manages the determination result as a reception status. When the first terminal device starts a call with the second terminal device, the first terminal device transmits a call request signal to the server. If the reception status indicates that the voice communication is possible, the server transmits a call signal to the second terminal device via the core network and the second base station device.

[0007] Another aspect of the present invention is a communication system. The communication system includes a first terminal device and a second terminal device capable of voice communication, a first base station device capable of wireless communication with the first terminal device, a second base station device capable of wireless communication with the second terminal device, a core network connecting the first base station device and the second base station device and establishing a communication network between the first terminal device and the second terminal device, and a server connected to the core network, managing calls between the first terminal device and the second terminal device, and managing a reception status of signals from the second base station device at the second terminal device. The second terminal device measures the strength of a signal received from the second base station device and transmits information about the strength to the server. Upon receiving the strength information from the second terminal device, the server manages the strength information as a reception status. When initiating a call with the second terminal device, the first terminal device transmits a call request signal to the server. If the reception status indicates a strength sufficient for voice communication, the server transmits a call signal to the second terminal device via the core network and the second base station device.

[0008] Another aspect of the present invention provides a communication system comprising a first terminal device and a second terminal device capable of voice calls, a first base station device capable of wireless communication with the first terminal device, a second base station device capable of wireless communication with the second terminal device, a core network connecting the first base station device and the second base station device and establishing a communication network with each of the first terminal device and the second terminal device, a first server connected to the core network and managing calls between the first terminal device and the second terminal device, and a second server connected to the core network and managing the reception status at the second terminal device of signals from the second base station device. When the second terminal device determines whether or not a voice call is possible based on the strength of the signal received from the second base station device, it transmits the determination result to the second server, and when the second server receives the determination result from the second terminal device, it manages the determination result as a reception status, and when the first terminal device starts a call with the second terminal device, it transmits a call request signal to the first server, and when the first server receives the call request signal from the first terminal device, it obtains the reception status from the second server, and if the reception status indicates that a voice call is possible, the first server transmits a call signal to the second terminal device via the core network and the second base station device.

[0009] Any combination of the above components, and any transformation of the present invention into a method, device, system, recording medium, computer program, etc., are also valid aspects of the present invention. [Effects of the Invention]

[0010] According to the present invention, it is possible to suppress the generation of unnecessary traffic when the receiving terminal device is unable to carry out voice communication. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram illustrating a configuration of a communication system according to an embodiment. [Figure 2] FIG. 2 is a diagram illustrating a configuration of a terminal device in FIG. [Figure 3] FIG. 2 is a diagram illustrating a configuration of a first server in FIG. [Figure 4]10 is a flowchart showing a reporting procedure by the second terminal device of FIG. [Figure 5] 10 is a flowchart showing a processing procedure performed by the first server of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] Before describing this embodiment in detail, an overview will be provided. This embodiment relates to a communication system for executing PoC. In the communication system, a first terminal device and a second terminal device are connected to a core network, and an EPS bearer is established in the core network. In addition, a first server, which is the PoC server described above, is connected to the core network. The second terminal device determines whether voice communication is possible based on the strength of a signal received from a base station device, and transmits the determination result to a second server connected to the core network. The second server stores the received determination result as the reception status of the second terminal device.

[0013] In this situation, the first terminal device transmits a call request signal to the first server to request the start of a voice call with the second terminal device. Upon receiving the call request signal, the first server acquires the reception status of the second terminal device from the second server. If the reception status of the second terminal device indicates that the call is possible, the first server transmits a call signal to the second terminal device using the EPS bearer. When the second terminal device responds to the call signal, the voice call between the first terminal device and the second terminal device is established. On the other hand, if the reception status of the second terminal device indicates that the call is impossible, the first server does not transmit the call signal to the second terminal device. In other words, by having the first server determine whether or not to establish a PoC voice call, unnecessary traffic generated from the first server to the second terminal device is reduced.

[0014] 1 shows the configuration of a communication system 1000. The communication system 1000 includes a first terminal device 100a and a second terminal device 100b, which are collectively referred to as terminal devices 100, a first base station device 200a and a second base station device 200b, which are collectively referred to as base station devices 200, a core network 300, a first server 400, and a second server 500. The number of terminal devices 100 and the number of base station devices 200 included in the communication system 1000 are not limited to "2".

[0015] The communication system 1000 is a PoC system that uses a data communication function of a mobile communication system such as LTE (Long Term Evolution) or 5G. The first terminal device 100a is a wireless communication device used by a user (hereinafter also referred to as a "first user") and communicates with the first base station device 200a using a wireless communication method for mobile communication. The second terminal device 100b is a wireless communication device used by a user (hereinafter also referred to as a "second user") and communicates with the second base station device 200b using a wireless communication method for mobile communication. Here, it is assumed that the first terminal device 100a is the terminal device 100 on the transmitting side, the second terminal device 100b is the terminal device 100 on the receiving side, and a voice call is performed between the first terminal device 100a and the second terminal device 100b.

[0016] The first base station device 200a is a base station device 200 capable of wireless communication with the first terminal device 100a, and is also connected to the core network 300. The second base station device 200b is a base station device 200 capable of wireless communication with the second terminal device 100b, and is also connected to the core network 300. Here, as an example, the first terminal device 100a and the second terminal device 100b are connected to different base station devices 200, but the first terminal device 100a and the second terminal device 100b may also be connected to the same base station device 200.

[0017] The core network 300 is a network of a mobile communication system that connects the first base station device 200a and the second base station device 200b. In LTE, the core network 300 is called EPC (Evelved Packet Core) and sets up an EPS bearer between the first terminal device 100a and the second terminal device 100b. In 5G, the core network 300 is called 5GC (5th Generation Core Network) and sets up a PDU (Protocol Data Unit) session between the first terminal device 100a and the second terminal device 100b. Known technologies may be used to set up the EPS bearer and PDU session in the core network 300, so a description thereof will be omitted here. The following description will be given assuming that the communication method is LTE.

[0018] The first server 400 is connected to the core network 300 and manages calls between the first terminal device 100a and the second terminal device 100b. The first server 400 also manages the accounts of the terminal devices 100 and controls PoC voice calls.

[0019] The following describes the process of establishing a voice call by PoC in such a communication system 1000, also with reference to Figures 2 and 3. Figure 2 is a diagram showing the configuration of a terminal device 100. The terminal device 100 includes a speaker 110, a microphone 112, a PTT button 114, a communication unit 116, a control unit 118, and a storage unit 120. The first terminal device 100a and the second terminal device 100b in Figure 1 have the same configuration.

[0020] The communication unit 116 performs wireless communication compatible with the above-mentioned mobile communication system and communicates with the base station device 200. The operation of the communication unit 116 is controlled by the control unit 118. The storage unit 120 stores information used for wireless communication or PoC calls in the communication unit 116. For example, the storage unit 120 stores address information of the terminal device 100, address information of other terminal devices 100 that are destinations, etc. The storage unit 120 also stores a threshold value to be compared with the signal strength. Such a threshold value may be set by the user.

[0021] The PTT button 114 is a button that can be pressed by the user. It can also be said to be an interface operated by the user of the terminal device 100. The PTT button 114 outputs the received operation to the control unit 118. When the PTT button 114 is pressed, the communication unit 116 can transmit an audio signal. On the other hand, when the PTT button 114 is not pressed, the communication unit 116 can receive an audio signal.

[0022] The microphone 112 acquires voice generated by the user when the user presses down the PTT button 114. The microphone 112 converts the acquired voice into an electrical signal and outputs the converted voice (hereinafter referred to as "voice signal") to the control unit 118. When the user does not press down the PTT button, the control unit 118 receives a voice signal from the communication unit 116, converts the voice signal into voice, and outputs the voice from the speaker 110.

[0023] When the second terminal device 100b is powered on, the control unit 118 causes the communication unit 116 to transmit a signal requesting connection to the core network 300 (hereinafter referred to as a "connection request signal"). When the second base station device 200b receives the connection request signal, it transmits the connection request signal to the core network 300. The core network 300 authenticates the second terminal device 100b based on the connection request signal. When the authentication is complete, the core network 300 transmits a signal notifying the completion of authentication (hereinafter referred to as a "completion report signal") to the second terminal device 100b via the second base station device 200b.

[0024] The communication unit 116 of the second terminal device 100b receives a completion report signal from the core network 300. The control unit 118 recognizes the completion of authentication with the core network 300 by receiving the completion report signal at the communication unit 116. Furthermore, the second terminal device 100b performs PoC account authentication with the first server, and the communication unit 116 receives account information from the first server 400. The control unit 118 stores the account information received at the communication unit 116 in the storage unit 120.

[0025] When the communication unit 116 receives a signal, for example, a reference signal, from the second base station device 200b, it measures the signal strength. Specifically, the signal strength may be measured using a Common Reference Signal (CRS) in the case of LTE, or a Channel State Information Reference Signal (CSI-RS) in the case of 5G. The method for measuring the signal strength is not limited to this, and other signals may be used. The control unit 118 compares the strength of the signal received from the communication unit 116 with a threshold value stored in the storage unit 120. If the signal strength is greater than the threshold value, the control unit 118 determines that voice communication is possible. On the other hand, if the signal strength is equal to or less than the threshold value, the control unit 118 determines that voice communication is impossible. The control unit 118 outputs a determination result indicating whether voice communication is possible or impossible to the communication unit 116. The communication unit 116 transmits the determination result to the second server 500 via the second base station device 200b and the core network 300.

[0026] The second server 500 is connected to the core network 300, and receives the determination result from the second terminal device 100b via the second base station device 200b and the core network 300. The second server 500 manages the determination result from the second terminal device 100b. The determination result from the second terminal device 100b can be said to be the reception status at the second terminal device 100b of the signal from the second base station device 200b. The operations described above are also performed in the first terminal device 100a.

[0027] When a first user of the first terminal device 100a wishes to make a call with a second user of the second terminal device 100b, the first user presses the PTT button 114 of the first terminal device 100a. At this time, the second terminal device 100b, which is the destination, is selected by the first user's operation. The control unit 118 accepts a request to start the call by pressing the PTT button 114. Upon accepting the request to start the call, the control unit 118 generates a call request signal. The call request signal includes address information of the first terminal device 100a, account information of the second terminal device 100b, which is the destination, and a request to establish a session for a voice call between the first terminal device 100a and the first server 400. The call request signal is a signal for requesting the start of the call with the second terminal device 100b. The communication unit 116 transmits the call request signal to the first server 400 via the first base station device 200a and the core network 300.

[0028] FIG. 3 shows the configuration of the first server 400. The first server 400 includes a communication unit 416, a control unit 418, and a storage unit 420. The communication unit 416 receives a call request signal from the first terminal device 100a via the first base station device 200a and the core network 300. The control unit 418 establishes a session with the first terminal device 100a based on the address information and account information of the first terminal device 100a in the received call request signal. The control unit 418 also extracts account information of the second terminal device 100b, which is the destination, from the received call request signal. The control unit 418 generates a signal for inquiring about the reception status of the second terminal device 100b (hereinafter referred to as an "inquiry signal") and outputs the inquiry signal to the communication unit 416. The inquiry signal also includes address information of the second terminal device 100b. The communication unit 416 transmits the inquiry signal to the second server 500.

[0029] The second server 500 receives an inquiry signal from the first server 400. The second server 500 acquires address information of the second terminal device 100b included in the inquiry signal. The second server 500 extracts the reception status of the second terminal device 100b corresponding to the acquired address information, and transmits a signal including the reception status of the second terminal device 100b (hereinafter referred to as a "response signal") to the first server 400.

[0030] The communication unit 416 of the first server 400 receives a response signal from the second server 500. The control unit 418 acquires the reception status of the second terminal device 100b included in the response signal. If the reception status indicates that voice communication is possible, the control unit 418 generates a call signal. The call signal includes address information of the first terminal device 100a, which is the sender, and account information of the second terminal device 100b, which is the destination. The communication unit 416 transmits the call signal to the second terminal device 100b via the core network 300 and the second base station device 200b using an EPS bearer with the second terminal device 100b. At this time, the control unit 418 establishes a session with the second terminal device 100b. As a result, a call is performed between the first terminal device 100a and the second terminal device 100b by transmitting and receiving voice signals. This may be achieved using a known technique.

[0031] On the other hand, if the reception status indicates that voice communication is not possible, the control unit 418 does not generate a call signal, and the communication unit 416 does not transmit the call signal to the second terminal apparatus 100b. As a result, no call is made between the first terminal apparatus 100a and the second terminal apparatus 100b. The control unit 418 may also store the voice signal from the first terminal apparatus 100a.

[0032] This configuration can be realized in hardware terms by the CPU, memory, and other LSIs of any computer, and in software terms by programs loaded into memory, but here we depict functional blocks realized by the cooperation of these. Therefore, it will be understood by those skilled in the art that these functional blocks can be realized in various forms using only hardware, only software, or a combination of both.

[0033] The operation of the communication system 1000 configured as described above will be described. FIG. 4 is a flowchart showing a reporting procedure by the second terminal device 100b. The power is turned on (S10). The control unit 118 receives notification of the completion of authentication with the core network 300 via the communication unit 116 (S12). The control unit 118 acquires account information from the first server 400 via the communication unit 116 (S14). The communication unit 116 measures the strength of the signal from the second base station device 200b (S16). If the strength is greater than the threshold (Y in S18), the communication unit 116 transmits a determination result that communication is possible (S20). On the other hand, if the strength is not greater than the threshold (N in S18), the communication unit 116 transmits a determination result that communication is impossible (S22). The determination result is stored in the second server 500 (S24). Next, the process returns to step S16, and steps S16 to S24 are repeated. If the determination result in step S18 is the same as the previous time, sending of the determination result may be omitted.

[0034] In step S18, the determination result may be sent either when the strength of the signal from the second base station device 200b is greater than a threshold or when it is not greater than a threshold. In this case, only the unique ID of the second terminal device 100b may be sent as the determination result. For example, if the unique ID of the second terminal device 100b is sent when the strength of the communication from the second base station device 200b is greater than a threshold, the second server 500 stores a determination result that the second terminal device 100b is capable of communication when the unique ID of the second terminal device 100b is received, and that the second terminal device 100b is not capable of communication when the unique ID of the second terminal device 100b is not received. Furthermore, in a configuration in which the unique ID of the second terminal device 100b is sent when the strength of the signal with the second base station device 200b is not greater than a threshold, the second server 500 stores a determination result that the second terminal device 100b is unable to communicate when the unique ID of the second terminal device 100b is received, and that the second terminal device 100b is able to communicate when the unique ID of the second terminal device 100b is not received. In a configuration in which the determination result is sent when the strength of the signal from the second base station device 200b is greater than a threshold, assuming that the second terminal device 100b and the second base station device 200b are completely unable to communicate, it is desirable to use a configuration in which the determination result is sent when the strength of the communication from the second base station device 200b is greater than a threshold.

[0035] 5 is a flowchart showing a processing procedure performed by the first server 400. The communication unit 416 receives a call request signal from the first terminal device 100a (S50). The control unit 418 establishes a session with the first terminal device 100a (S52). The control unit 418 identifies the second terminal device 100b as the other party (S54). The control unit 418 acquires the reception status of the second terminal device 100b from the second server 500 via the communication unit 416 (S56). If a call is possible (Y in S58), the control unit 418 establishes a session with the second terminal device 100b (S60). As a result, a voice call is established between the first terminal device 100a and the second terminal device 100b (S62). On the other hand, if a call is not possible (N in S58), the control unit 418 stores the voice signal from the first terminal device 100a (S64).

[0036] (Variation) Up until now, each terminal device 100 has reflected whether or not a call is possible, and the determination result has been transmitted to the second server 500. On the other hand, in the modified example, the determination of whether or not a call is possible is made in the first server 400. The communication system 1000, terminal device 100, and first server 400 according to the modified example are of the same type as those in Figures 1 to 3. Here, the differences from the previous examples will be mainly explained.

[0037] When the communication unit 116 of the second terminal device 100b receives a signal, for example, a reference signal (CRS), from the second base station device 200b, it measures the strength of the signal. The control unit 118 generates a signal (hereinafter referred to as "strength information") including the strength of the signal received from the communication unit 116, and outputs the strength information to the communication unit 116. The communication unit 116 transmits the strength information to the second server 500 via the second base station device 200b and the core network 300.

[0038] The second server 500 receives strength information from the second terminal device 100b via the second base station device 200b and the core network 300. The second server 500 manages the strength information from the second terminal device 100b. The strength information from the second terminal device 100b can be said to be the reception status at the second terminal device 100b of the signal from the second base station device 200b. This operation is also performed in the first terminal device 100a.

[0039] The communication unit 416 of the first server 400 receives a call request signal from the first terminal device 100a via the first base station device 200a and the core network 300. The control unit 418 establishes a session with the first terminal device 100a based on the address information and account information of the first terminal device 100a in the received call request signal. The control unit 418 also extracts account information of the second terminal device 100b, which is the destination, from the received call request signal. The control unit 418 generates an inquiry signal to inquire about the reception status of the second terminal device 100b, and outputs the inquiry signal to the communication unit 416. The inquiry signal also includes the address information of the second terminal device 100b. The communication unit 416 transmits the inquiry signal to the second server 500.

[0040] The second server 500 receives the inquiry signal from the first server 400. The second server 500 acquires the address information of the second terminal device 100b included in the inquiry signal. The second server 500 extracts the reception status of the second terminal device 100b corresponding to the acquired address information, and transmits a response signal including the reception status of the second terminal device 100b to the first server 400.

[0041] The communication unit 416 of the first server 400 receives a response signal from the second server 500. The control unit 418 acquires the reception status of the second terminal device 100b included in the response signal, i.e., strength information. The control unit 418 compares the acquired strength information with a threshold value stored in the storage unit 420. If the strength information is greater than the threshold value, i.e., if the reception status indicates a strength sufficient for voice communication, the control unit 418 generates a call signal. The call signal includes address information of the first terminal device 100a, which is the sender, and address information of the second terminal device 100b, which is the destination. The communication unit 416 transmits the call signal to the second terminal device 100b via the core network 300 and the second base station device 200b using an EPS bearer with the second terminal device 100b. At this time, the control unit 418 establishes a session with the second terminal device 100b. As a result, a call is established between the first terminal device 100a and the second terminal device 100b by transmitting and receiving voice signals. This can be achieved by using known techniques.

[0042] On the other hand, if the strength information is equal to or less than the threshold, that is, if the reception conditions indicate a strength that makes voice calls impossible, the control unit 418 does not generate a call signal, and the communication unit 416 does not transmit the call signal to the second terminal apparatus 100b. As a result, no call is made between the first terminal apparatus 100a and the second terminal apparatus 100b. The control unit 418 may also store the voice signal from the first terminal apparatus 100a.

[0043] According to this embodiment, even if an EPS bearer with the receiving-side terminal device 100 is established, if the receiving-side terminal device 100 determines that communication is not possible, the first server 400 does not transmit a call signal to the receiving-side terminal device 100. This makes it possible to suppress the generation of unnecessary traffic when the receiving-side terminal device is unable to make a voice call. Furthermore, even if an EPS bearer with the receiving-side terminal device 100 is established, if the signal strength at the receiving-side terminal device 100 is such that communication is not possible, the first server 400 does not transmit a call signal to the receiving-side terminal device 100. This makes it possible to suppress the generation of unnecessary traffic when the receiving-side terminal device is unable to make a voice call. Furthermore, because the first base station device 200a and the second base station device 200b are the same, the degree of freedom in the configuration can be improved.

[0044] The present invention has been described above based on the embodiments. In the embodiments, an audio signal has been described, but the present invention is not limited to this and may also be applied to information signals such as data signals. These embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components and processing processes, and that such modifications are also within the scope of the present invention.

[0045] In the above embodiment, the first server and the second server are described as separate entities, but the functions of the first server and the second server may be combined into one server.Furthermore, the functions of the first server and the second server may be distributed among multiple servers. [Explanation of symbols]

[0046] 100 terminal device, 110 speaker, 112 microphone, 114 PTT button, 116 communication unit, 118 control unit, 120 memory unit, 200 base station device, 300 core network, 400 first server, 416 communication unit, 418 control unit, 420 memory unit, 500 second server, 1000 communication system.

Claims

1. a first terminal device and a second terminal device capable of voice communication; a first base station device capable of wirelessly communicating with the first terminal device; a second base station device capable of wirelessly communicating with the second terminal device; a core network that connects the first base station device and the second base station device and establishes a communication network between the first terminal device and the second terminal device; a server connected to the core network, managing calls between the first terminal device and the second terminal device, and managing a reception status of a signal from the second base station device at the second terminal device; the second terminal device determines whether or not voice communication is possible based on the strength of the signal received from the second base station device, and transmits a determination result to the server; When the server receives the determination result from the second terminal device, the server manages the determination result as the reception status; the first terminal device transmits a call request signal to the server when starting a call with the second terminal device; the server, when the reception status indicates that voice communication is possible, transmits a call signal to the second terminal device via the core network and the second base station device. Communication system.

2. The second terminal device a storage unit that stores a signal strength threshold; a communication unit that measures the strength of a signal from the second base station device; a control unit that compares the signal strength measured by the communication unit with a signal strength threshold, and determines that voice communication is possible when the signal strength is greater than the threshold, and determines that voice communication is impossible when the signal strength is equal to or less than the threshold; Equipped with The communication system of claim 1 .

3. a first terminal device and a second terminal device capable of voice communication; a first base station device capable of wirelessly communicating with the first terminal device; a second base station device capable of wirelessly communicating with the second terminal device; a core network that connects the first base station device and the second base station device and establishes a communication network between the first terminal device and the second terminal device; a server connected to the core network, managing calls between the first terminal device and the second terminal device, and managing a reception status of a signal from the second base station device at the second terminal device; the second terminal device measures the strength of the signal received from the second base station device and transmits information about the strength to the server; When the server receives the strength information from the second terminal device, the server manages the strength information as the reception status; the first terminal device transmits a call request signal to the server when starting a call with the second terminal device; the server transmits a call signal to the second terminal device via the core network and the second base station device when the reception status indicates a strength sufficient for voice communication. Communication system.

4. a first terminal device and a second terminal device capable of voice communication; a first base station device capable of wirelessly communicating with the first terminal device; a second base station device capable of wirelessly communicating with the second terminal device; a core network that connects the first base station device and the second base station device and establishes a communication network between the first terminal device and the second terminal device; a first server connected to the core network and managing calls between the first terminal device and the second terminal device; a second server connected to the core network and managing a reception status of a signal from the second base station device at the second terminal device; the second terminal device determines whether or not voice communication is possible based on the strength of the signal received from the second base station device, and transmits a determination result to the second server; When the second server receives the determination result from the second terminal device, the second server manages the determination result as the reception status; the first terminal device transmits a call request signal to the first server when starting a call with the second terminal device; the first server, upon receiving the call request signal from the first terminal device, acquires the reception status from the second server; the first server transmits a call signal to the second terminal device via the core network and the second base station device when the reception status indicates that voice communication is possible; Communication system.

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