Management device and communication control method

Abstract

This management device responds when a new connection request is received in a control slot in a state in which all call slots are used in a call, the management device comprising: a slot division determination unit that, when no call slots are available and a terminal currently using a slot to be queried among the call slots in use is capable of changing the bit rate, determines the slot to be queried as a dividable slot; and a slot allocation unit that allocates, to the divided slots, each of the currently using terminal and a requesting terminal that has newly requested a connection. When the slot to be queried is being used in a group call, the slot division determination unit determines the slot to be queried as a dividable slot if the number of terminals that have responded that the bit rate can be changed from the currently using terminal meets a prescribed condition.

Description

Management device and communication control method

[0001] The present disclosure relates to a management device and a communication control method.

[0002] A trunking wireless communication system including a plurality of wireless terminal devices and a plurality of repeaters is known. For example, Patent Document 1 discloses a technique that can shorten the time required for the location registration process of a wireless terminal device in a trunking system.

[0003] Japanese Patent Application Laid-Open No. 2015-2449

[0004] In a trunking system, if a terminal requests a new connection and there are no available slots to use as channels, the terminal sends a busy signal to the terminal requesting the new connection, indicating that the connection is not possible, and the connection is rejected. If an available slot could be generated when there are no available slots and assigned to the terminal requesting the new connection, connection rejections could be reduced. However, to generate a new available slot, it is necessary to divide an available slot and assign the new slot to the terminal currently using the slot and the terminal requesting the new connection. If the terminal currently using the slot cannot handle the bit rate required for communication in the newly assigned slot, the call cannot continue. Furthermore, if the terminal requesting the new connection cannot handle the bit rate required for communication in the newly assigned slot, communication cannot be continued. It is necessary to confirm that communication is possible in the newly assigned slot and appropriately assign the slot to the terminal currently using the slot and the terminal requesting the new connection.

[0005] An object of the present disclosure is to provide a management device and a communication control method that can appropriately allocate slots to terminals.

[0006] The management device of the present disclosure has a plurality of slots, and responds when a new connection request is received via a control slot when all of the call slots are in use for calls. The management device includes: a communication unit that communicates with terminals using the call slots and the control slot; a slot division determination unit that, when there are no available call slots and a new connection request is received via the control slot, selects a slot to be queried from among the call slots in use based on predetermined conditions, inquires of a terminal in use that is using the selected slot to be queried about whether or not the bit rate can be changed, and, when the terminal in use responds that the bit rate can be changed, designates the queried slot as a divisible slot; a slot division control unit that instructs the terminal in use to change the bit rate and divides the divisible slot; and a slot allocation unit that assigns the terminal in use and a terminal that has made a new connection request to the divided slots, respectively. When the queried slot is being used for a group call, the slot division determination unit designates the queried slot as the divisible slot when the number of terminals in use that have responded that the bit rate can be changed satisfies a predetermined condition.

[0007] A communication control method disclosed herein has a plurality of slots, and is configured to respond when a new connection request is received via a control slot when all of the call slots are in use for calls. The communication control method includes the steps of: communicating with a terminal using the call slot and the control slot; when there are no available call slots and a new connection request is received via the control slot, selecting a slot to be queried from among the call slots in use based on predetermined conditions, inquiring of a terminal in use that is using the selected call slot as to whether or not the bit rate can be changed, and if the terminal in use responds that the bit rate can be changed, designating the queried slot as a divisible slot; instructing the terminal in use to change the bit rate and dividing the divisible slot; and assigning the terminal in use and a terminal that has made a new connection request to the divided slots, respectively; and, if the queried slot is being used for a group call, designating the queried slot as the divisible slot if the number of terminals in use that have responded that the bit rate can be changed satisfies a predetermined condition.

[0008] According to the present disclosure, slots can be appropriately allocated to terminals.

[0009] FIG. 1 is a diagram for explaining a communication system according to a first embodiment. FIG. 2 is a diagram for explaining a configuration example of a communication system according to the first embodiment. FIG. 3 is a block diagram for explaining a configuration example of a management device according to the first embodiment. FIG. 4 is a block diagram for explaining a configuration example of a terminal according to the first embodiment. FIG. 5 is a flowchart showing processing details of a management device according to the first embodiment. FIG. 6 is a flowchart showing processing details of a terminal according to the first embodiment. FIG. 7 is a flowchart showing the flow of slot division determination processing according to a first example of the first embodiment. FIG. 8 is a flowchart showing the flow of slot division determination processing according to a second example of the first embodiment. FIG. 9 is a flowchart showing the flow of slot division determination processing according to a third example of the first embodiment. FIG. 10 is a flowchart showing the flow of slot division determination processing according to a fourth example of the first embodiment. FIG. 11 is a flowchart showing the flow of slot division determination processing according to a fifth example of the first embodiment. FIG. 12 is a flowchart showing the flow of slot division determination processing according to a sixth example of the first embodiment. FIG. 13 is a flowchart showing the flow of slot division determination processing according to a seventh example of the first embodiment. FIG. 14 is a diagram for explaining a communication system according to a second embodiment. FIG. 15 is a block diagram showing an example of the configuration of a management device according to the second embodiment. Fig. 16 is a diagram for explaining a method for dividing a slot according to the second embodiment. Fig. 17 is a flowchart showing the flow of processing by the management device according to the second embodiment. Fig. 18 is a diagram for explaining a method for restoring a divided slot according to the second embodiment.

[0010] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Note that the present disclosure is not limited to these embodiments, and in the following embodiments, the same components are designated by the same reference numerals, and redundant description will be omitted.

[0011] [Overview] An overview of the communication system according to the first embodiment will be described with reference to Fig. 1. Fig. 1 is a diagram for explaining the communication system according to the first embodiment.

[0012] 1 shows an overview of a trunking system, for example, in the field of a land mobile radio (LMR) system. The trunking system 1 includes, for example, a controller 2, a repeater 3, a repeater 4, a repeater 5, a repeater 6, and a terminal 14. Repeater 3 is assumed to be a repeater for a control channel. Repeaters 4 to 6 are assumed to be repeaters for communication channels.

[0013] The controller 2 controls the repeaters 3 to 6 to manage the terminal 14. The controller 2 detects the status of each slot of the repeaters 4 to 6 for the communication channel to determine whether new communication is possible.

[0014] The repeater 4 has two communication channels, a channel CH1 and a channel CH2. The channel CH1 has a slot SL1. The channel CH2 has a slot SL2.

[0015] The repeater 5 has two communication channels, channel CH3 and channel CH4. Channel CH3 has slot SL3. Channel CH4 has slot SL4.

[0016] Repeater 6 has two communication channels, channel CH5 and channel CH6. Channel CH5 has slot SL5. Channel CH6 has slot SL6.

[0017] When the controller 2 detects the terminal 14 that has made a new communication connection request through the repeater 3, the controller 2 identifies a repeater that has an empty slot among the repeaters 4 to 6. The controller 2 causes the repeater with an empty slot to manage the terminal 14 that has made the new communication connection request.

[0018] Repeater 4 manages the channels in slots SL1 and SL2. Repeater 4 notifies the controller 2 of the status of slots SL1 and SL2. Repeater 5 manages the channels in slots SL3 and SL4. Repeater 5 notifies the controller 2 of the status of slots SL3 and SL4. Repeater 6 manages the channels in slots SL5 and SL6. Repeater 6 notifies the controller 2 of the status of slots SL5 and SL6.

[0019] In response to a request from the controller 2, the repeaters 4 to 6 allocate slots to be used by the terminal 14, connect to the terminal 14, and manage the communication of the terminal 14.

[0020] In a normal trunking system, the controller 2 detects the terminal 14 using the repeater 3, and if it determines that there are no available slots between the repeaters 4 and 6, it sends a disconnection signal to the terminal 14. In other words, if there are no available slots between the repeaters 4 and 6, the terminal 14 cannot communicate.

[0021] The communication system disclosed herein is a trunking system capable of using a TDMA (Time Division Multiple Access) method depending on the connection status of terminals. Specifically, the communication system disclosed herein controls the audio bit rate during communication to be reduced to a level that does not impede communication even when a channel is occupied, and divides the active channel into active slots and empty slots. The communication system disclosed herein can then increase the number of simultaneous communications by allocating new communications to the empty slots.

[0022] [First embodiment] (Communication system) A configuration example of a communication system according to the first embodiment will be described with reference to Fig. 2. Fig. 2 is a diagram showing a configuration example of a communication system according to the first embodiment.

[0023] 2, the communication system 10 includes a management device 12 and a plurality of terminals 14. The management device 12 and the plurality of terminals 14 are communicatively connected via a wireless network N. The communication system 10 is a communication system in which the management device 12 manages group communication among the plurality of terminals 14.

[0024] (Management Device) An example of the configuration of the management device according to the first embodiment will be described with reference to Fig. 3. Fig. 3 is a block diagram showing an example of the configuration of the management device according to the first embodiment.

[0025] The management device 12 includes a communication unit 20, a storage unit 22, a control unit 24, and a slot management unit 26. The management device 12 is realized by an information processing device such as a personal computer, for example.

[0026] 1, and repeaters 3 to 6. The control unit 24 realizes the controller 2 and repeater 3 for the control channel. The slot management unit 26 realizes repeaters 4 to 6 for the communication channel.

[0027] The communication unit 20 is a communication interface that communicates between the management device 12 and an external device. The communication unit 20 executes communication between, for example, the management device 12 and the terminal 14. The communication unit 20 relays communication between, for example, the terminals 14. Communication of control information between the management device 12 and the terminal 14 may be performed using a control channel of the communication unit 20. Control slots are assigned to the control channel, and communication of control information is performed using these control slots. This control information may be a disconnection signal such as a busy signal, an ACK / NACK signal that responds to a request, a connection request signal from a terminal attempting a new connection request (described below), an inquiry signal inquiring about whether a bit rate can be changed, a bit rate change request signal, or the like. Calls between the terminals 14 may be performed via call slots assigned as communication channels of the communication unit 20 for calls. The communication unit 20 may be implemented, for example, by a business radio, a wireless LAN (Local Area Network), Wi-Fi (registered trademark), or the like.

[0028] The memory unit 22 stores various types of information. The memory unit 22 stores information such as the calculation contents of the control unit 24 and the slot management unit 26, programs, etc. The memory unit 22 includes, for example, at least one of a random access memory (RAM), a main storage device such as a read-only memory (ROM), and an external storage device such as a hard disk drive (HDD). The memory unit 22 may store connection destination information for one-to-one individual communication and group information for group communication. This connection destination information and group information are used to identify and communicate with terminals that will be communication partners, and therefore may include identification information that identifies each terminal. This connection destination information and group information may include information on frequencies and slots assigned for communication with each terminal that will be communication partners, or may be stored in association with each other. This connection destination information and group information may include information for identifying the communication destination, such as an IP address, when the respective communication terminals are connected via the Internet, or may be stored in association with each other. This connection destination information and group information may be stored in advance in the storage unit 22 in order to set which other terminals 14 the terminal 14 will connect to, or may be newly stored in the storage unit 22 when connection destination information or group information to be communicated is transmitted from a terminal 14 that has made a connection request to the management device 12.

[0029] The control unit 24 and the slot management unit 26 control each unit of the management device 12. The control unit 24 and the slot management unit 26 include, for example, an information processing device such as a CPU (Central Processing Unit) or an MPU (Micro Processing Unit), and a storage device such as a RAM or ROM. The control unit 24 and the slot management unit 26 execute a program that controls the operation of the management device 12 according to the present disclosure. The control unit 24 and the slot management unit 26 may be implemented by an integrated circuit such as an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array). The control unit 24 may be implemented by a combination of hardware and software.

[0030] The control unit 24 includes a connection request terminal detection unit 30 .

[0031] The connection request terminal detection unit 30 receives a connection request signal indicating a connection request from the terminal 14 via the communication unit 20. The connection request terminal detection unit 30 identifies the terminal 14 that has made the connection request based on the received connection request signal.

[0032] The slot management unit 26 manages slots used by the terminal 14 for communication. Slots are classified into control slots used as control channels and call slots used for calls. Hereinafter, call slots may be simply referred to as slots. The slot management unit 26 checks whether there are any empty slots among the call slots it manages. If there is a connection request from the terminal 14 and there is an empty slot among the call slots it manages, the slot management unit 26 allocates the identified empty slot to the communication of the terminal that made the connection request. If there is a connection request from the terminal 14 and there is no empty slot among the call slots it manages, the slot management unit 26 identifies an in-use slot to be divided (hereinafter, a slot to be divided) from among slots currently being used for calls (hereinafter, an in-use slot). If the slot management unit 26 determines that the slot to be divided can be divided, it performs a division process on the slot to be divided and allocates the empty slot after division to the communication of the terminal 14 that made the connection request.

[0033] The slot management unit 26 includes an empty slot detection unit 40 , a slot division determination unit 42 , a slot division control unit 44 , and a slot allocation unit 46 .

[0034] The empty slot detection unit 40 detects whether or not there is an empty slot among the plurality of call slots managed by the slot management unit 26 .

[0035] When there are no empty slots among the call slots managed by the slot management unit 26, the slot division determination unit 42 identifies an in-use slot to be queried (hereinafter, referred to as the queried slot). The terminal 14 using the queried slot is sometimes referred to as the in-use terminal. The slot division determination unit 42 inquires of the in-use terminal using the queried slot about whether the bit rate can be changed. Based on a response from the in-use terminal that the bit rate can be changed, the slot division determination unit 42 identifies the in-use slot used by the in-use terminal as a divisible slot. Here, if the communication using the queried slot is a group call, the in-use terminals using the queried slot are the transmitting terminal 14 and at least two receiving terminals 14. If the communication using the in-use slot is a normal one-to-one individual call, the in-use terminals are the transmitting terminal 14 and the receiving terminal 14.

[0036] The slot division determination unit 42 may designate the queried slot as a divisible slot if the number of terminals among the terminals in use that have responded that the bit rate can be changed satisfies a predetermined condition. This predetermined condition may be, for example, a condition that the number of terminals that have responded that the bit rate can be changed from the terminals in use is a predetermined number. This predetermined condition may also be, for example, a condition that the number of terminals that have responded that the bit rate can be changed from the terminals in use is set to vary depending on the number of terminals in use. For example, a predetermined percentage may be set, and the number of terminals that have responded that the bit rate can be changed from the terminals in use may be a predetermined percentage of the number of terminals in use. When the queried slot is being used for a group call consisting of multiple terminals including a transmitting terminal 14 and at least two receiving terminals 14, the slot division determination unit 42 queries the multiple terminals 14, including the transmitting terminal 14 and at least two receiving terminals 14, that are participating in the group call using the busy slot, as the using terminals, regarding whether the bit rate can be changed. If the transmitting terminal and at least two receiving terminals among the terminals in use respond that the slot can be changed, the slot division determination unit 42 determines that the slot is a divisible slot. A slot that can be divided is also called a divisible slot.

[0037] The slot division control unit 44 transmits a bit rate change request signal indicating an instruction to change the bit rate to a terminal currently using the divisible slot via the communication unit 20. The slot division control unit 44 divides the divisible slot while transmitting the bit rate change request signal to the terminal 14.

[0038] The slot allocation unit 46 allocates the divided slots to the terminal in use and the terminal 14 that has made the connection request. The terminal 14 is allocated a divisible slot, and is then ready to communicate.

[0039] (Terminal Device) An example of the configuration of a terminal according to the first embodiment will be described with reference to Fig. 4. Fig. 4 is a block diagram showing an example of the configuration of a terminal according to the first embodiment.

[0040] 4, the terminal 14 includes an input unit 60, an audio input unit 62, a display unit 64, an audio output unit 66, a communication unit 68, a storage unit 70, and a control unit 72. The terminal 14 is, for example, but is not limited to, a commercial wireless communication device, a mobile phone, a smartphone, a tablet terminal, or the like.

[0041] The input unit 60 accepts various operations for the terminal 14. The input unit 60 is realized by various input devices such as a switch, a button, or a touch panel.

[0042] The voice input unit 62 detects the voice of the user using the terminal 14. The voice input unit 62 converts the detected voice into a voice signal. The voice input unit 62 is realized by a microphone.

[0043] The display unit 64 displays various types of information and is realized by a display including, for example, a liquid crystal display or an organic EL (Electro-Luminescence) display.

[0044] The audio output unit 66 is a speaker that outputs various sounds, such as the voice of the other party.

[0045] The communication unit 68 is a communication interface that performs communication between the terminal 14 and an external device. The communication unit 68 performs communication between the terminal 14 and the management device 12, for example.

[0046] The storage unit 70 stores various types of information, such as the contents of calculations performed by the control unit 72 and information such as programs. The storage unit 70 includes at least one of, for example, a RAM, a main storage device such as a ROM, and an external storage device such as an HDD.

[0047] The control unit 72 controls each unit of the terminal 14. The control unit 72 includes, for example, an information processing device such as a CPU or an MPU, and a storage device such as a RAM or a ROM. The control unit 72 executes a program that controls the operation of the terminal 14 according to the present disclosure. The control unit 72 may be realized by an integrated circuit such as an ASIC or an FPGA. The control unit 72 may be realized by a combination of hardware and software.

[0048] The control unit 72 includes a communication request unit 80 , a bit rate control unit 82 , and a slot control unit 84 .

[0049] When the terminal 14 attempts to communicate, the communication request unit 80 transmits a connection request signal indicating a connection request to the management device 12 via the communication unit 68 .

[0050] The bit rate control unit 82 changes the bit rate of a communication signal transmitted by the terminal 14. The bit rate control unit 82 changes, for example, the bit rate of an audio signal transmitted by the terminal 14. When the bit rate control unit 82 receives a bit rate change request signal from the management device 12, it changes the bit rate of the audio signal based on the received bit rate change request signal. When the bit rate control unit 82 has changed the bit rate, it transmits a response signal indicating this to the management device 12.

[0051] The slot control unit 84 connects to the slot assigned by the management device 12. The slot control unit 84 connects to the slot assigned by the management device 12 and executes communication.

[0052] (Processing of Management Apparatus) The processing content of the management apparatus according to the first embodiment will be described with reference to Fig. 5. Fig. 5 is a flowchart showing the processing content of the management apparatus according to the first embodiment.

[0053] The connection request terminal detection unit 30 detects a connection request from the terminal 14 (step S10). Specifically, when the connection request terminal detection unit 30 receives a connection request signal from the terminal 14, it detects the connection request from the terminal 14. Then, the process proceeds to step S12.

[0054] The empty slot detection unit 40 checks the usage status of the call slots (step S12), and then the process proceeds to step S14.

[0055] The empty slot detector 40 determines whether or not there is an empty slot in the call slots (step S14). If it is determined that there is an empty slot in the call slots (step S14; Yes), the process proceeds to step S16. If it is determined that there is not an empty slot in the call slots (step S14; No), the process proceeds to step S18.

[0056] If the answer to step S14 is Yes, the slot allocation unit 46 allocates an empty slot to the terminal 14 that has made the connection request (step S16), and then the process of FIG. 5 is terminated.

[0057] If step S14 returns No, the slot management unit 26 executes a slot division determination process to determine whether the in-use slot can be divided (step S18). Because there are no available slots, the slot management unit 26 identifies a queried slot from among the in-use slots currently in use. The slot management unit 26 then identifies the terminal 14 using the identified queried slot. To identify the terminal 14 using the identified queried slot, the slot management unit 26 may inquire of the slot allocation unit 46 about the allocation status of the terminal 14 to the slot to identify the terminal 14. The storage unit 22 may also store destination information for one-to-one individual calls or group information for group calls assigned to the in-use slot. In this case, when identifying the terminal 14 using the queried slot, the slot management unit 26 may identify the terminal 14 by reading the destination information and group information for calls using the queried slot from the storage unit 22 of the management device. The slot division determination unit 42, for example, inquires of the identified terminal 14 about whether the bit rate can be changed, and determines whether slot division is possible based on the response from the terminal 14. The slot division determination process will be described in detail later. Then, the process proceeds to step S20.

[0058] The slot division determination unit 42 determines whether or not slot division of a busy slot is possible (step S20). If it is determined that slot division of a busy slot is possible (step S20; Yes), the process proceeds to step S22. If it is not determined that slot division of a busy slot is possible (step S20; No), the process proceeds to step S28.

[0059] If the answer to step S20 is Yes, the slot division control unit 44 divides the slot to be divided (step S22), and then the process proceeds to step S24.

[0060] The slot division control unit 44 transmits a bit rate change request signal to the terminal 14 that has made the connection request (step S24), and then the process proceeds to step S26.

[0061] The slot allocation unit 46 sends a slot movement command to the terminal 14 that has made the connection request, and allocates the slot to be divided to the empty slot (step S26). This enables the terminal 14 to communicate. Then, the processing of FIG. 5 ends.

[0062] If the answer to step S20 is No, the empty slot detector 40 determines whether there are other occupied slots (step S28). If it is determined that there are other occupied slots (step S28; Yes), the process proceeds to step S30. If it is not determined that there are other occupied slots (step S28; No), the process proceeds to step S32.

[0063] If the answer to step S28 is Yes, the empty slot detector 40 identifies other occupied slots (step S30), and then the process proceeds to step S18 to repeat the process.

[0064] If the determination in step S28 is No, the control unit 24 transmits a disconnection signal to the terminal 14 that made the connection request (step S32), thereby disconnecting communication between the management device 12 and the terminal 14. Then, the process in FIG. 5 ends.

[0065] (Processing of Terminal) Processing of the terminal according to the first embodiment will be described with reference to Fig. 6. Fig. 6 is a flowchart showing processing of the terminal according to the first embodiment.

[0066] The communication request unit 80 transmits a connection request signal to the management device 12 (step S40), and then the process proceeds to step S42.

[0067] The control unit 72 determines whether a call slot has been allocated by the management device 12 (step S42). If it is determined that a call slot has been allocated (step S42; Yes), the process proceeds to step S54. If it is determined that a call slot has not been allocated (step S42; No), the process proceeds to step S44.

[0068] If the determination in step S42 is No, the control unit 72 determines whether or not a bit rate change request signal has been received from the management device 12 (step S44). If it is determined that a bit rate change request signal has been received (step S44; Yes), the control unit 72 proceeds to step S48. If it is determined that a bit rate change request signal has not been received (step S44; No), the control unit 72 proceeds to step S46.

[0069] If the determination in step S44 is No, the control unit 72 determines whether connection is not possible (step S46). The control unit 72 determines that connection is not possible, for example, when a disconnection signal is received from the management device 12, or when a slot is not allocated or a bit rate change request signal is not received for a predetermined time or longer after transmitting a connection request signal. If it is determined that connection is not possible (step S46; Yes), the processing in FIG. 6 is terminated. If it is not determined that connection is not possible (step S46; No), the processing returns to step S44 and is repeated.

[0070] If the answer to step S44 is Yes, the bit rate control unit 82 changes the bit rate when transmitting the audio signal (step S48), and then the process proceeds to step S50.

[0071] The bit rate control unit 82 transmits a response signal indicating that the bit rate has been changed to the management device 12 (step S50), and then the process proceeds to step S52.

[0072] The slot control unit 84 receives a slot movement command from the management device 12 (step S52), which allocates a slot to the terminal 14. Then, the process proceeds to step S54.

[0073] If the determination in step S42 is Yes, or after step S52, the control unit 72 starts communication (step S54), and then ends the processing in FIG.

[0074] (Slot Division Determination Process) Next, the slot division determination process will be described. In the slot division determination process, a busy slot that satisfies a predetermined condition based on the number of terminals 14 using the busy slot is determined to be a queried slot, and processing is performed. Here, the predetermined condition for determining a slot as a queried slot may include, for example, when selecting a queried slot from among busy slots, lowering the priority of busy slots used in group calls. In other words, when selecting a queried slot, the slot division determination unit may select the queried slot by lowering the selection priority of call slots used in group calls. Furthermore, the predetermined condition for determining a slot as a queried slot may include, for example, when selecting a queried slot from among busy slots, if all busy slots are group calls, selecting the queried slot in order of the number of busy terminals belonging to the group calls. Furthermore, the predetermined condition for determining a slot as a queried slot may include, for example, when selecting a queried slot from among busy slots, selecting the queried slot in order of the number of busy slots belonging to the group calls.

[0075] (First Example of Slot Division Determination Processing) The slot division determination processing according to the first example of the first embodiment will be described with reference to Fig. 7. Fig. 7 is a flowchart showing the flow of the slot division determination processing according to the first example of the first embodiment.

[0076] The slot division determining unit 42 sets a slot to be queried from among the slots in use (step S60), and then proceeds to step S62.

[0077] The slot division determination unit 42 determines whether the communication using the target slot is a group call including the transmitting terminal 14 and multiple receiving terminals 14 (step S62). Here, the slot division determination unit 42 may determine whether the communication using the target slot is a group call including the transmitting terminal 14 and multiple receiving terminals 14, for example, by obtaining information about the terminal 14 assigned to the queried slot from the slot allocation unit 46. If it is determined that the communication is a group call including the transmitting terminal 14 and multiple receiving terminals 14 (step S62; Yes), the process proceeds to step S64. If it is not determined that the communication is a group call including the transmitting terminal 14 and multiple receiving terminals 14 (step S62; No), the process proceeds to step S72.

[0078] If step S62 returns Yes, the slot division determination unit 42 identifies the terminal 14 using the queried slot (step S64). Here, the slot division determination unit 42 may, for example, acquire information about the terminal 14 assigned to the queried slot from the slot allocation unit 46 to identify the transmitting terminal 14 and multiple receiving terminals 14. In this case, the slot allocation unit 46 identifies which terminal 14 is assigned to the queried slot and notifies the slot division determination unit 42 of the information about the terminal 14 assigned to the queried slot. Furthermore, when determining whether the call is a group call, the slot division determination unit 42 may read from the storage unit 22 the connection destination information and group information for the call using the queried slot to identify the terminals 14 participating in the call. Then, the process proceeds to step S66.

[0079] The slot division determining unit 42 transmits a bit rate change request signal to the terminal 14 (step S66), and then the process proceeds to step S68.

[0080] The slot division determination unit 42 determines whether or not there has been a response from the transmitting terminal 14 and at least two receiving terminals 14 that the bit rate can be changed (step S68). If it is determined that there has been a response from the transmitting terminal 14 and at least two receiving terminals 14 that the bit rate can be changed (step S68; Yes), the process proceeds to step S70. If it is not determined that there has been a response from the transmitting terminal 14 and at least two receiving terminals 14 that the bit rate can be changed (step S68; No), the process proceeds to step S72.

[0081] If the answer to step S68 is Yes, the slot division determining unit 42 determines that the slot can be divided (step S70), and then ends the processing of FIG.

[0082] If the determination is No in step S62 or No in step S68, the slot division determination unit 42 determines whether or not there are other slots in use (step S72). If it is determined that there are other slots in use (step S72; Yes), the process returns to step S60 and repeats. If it is not determined that there are other slots in use (step S72; No), the process proceeds to step S74.

[0083] If the determination in step S72 is No, the slot division determination unit 42 determines that the slot cannot be divided (step S74), and then ends the processing in FIG.

[0084] (Second Example of Slot Division Determination Processing) A slot division determination processing according to a second example of the first embodiment will be described with reference to Fig. 8. Fig. 8 is a flowchart showing the flow of the slot division determination processing according to the second example of the first embodiment.

[0085] The processes from step S80 to step S86 and step S90 to step S94 are the same as the processes from step S60 to step S66 and step S70 to step S74 shown in FIG. 7, respectively, and therefore will not be described here.

[0086] After step S86, the slot division determination unit 42 determines whether the number of terminals that responded that the bit rate can be changed from the active terminals satisfies a predetermined condition. Here, the predetermined condition is whether a predetermined number of terminals 14, including the transmitting terminal 14 and at least two receiving terminals 14, have responded that the bit rate can be changed (step S88). The predetermined number may be all terminals participating in the group call, or any arbitrary number. If it is determined that a predetermined number of terminals 14, including the transmitting terminal 14 and at least two receiving terminals 14, have responded that the bit rate can be changed (step S88; Yes), the process proceeds to step S90. If it is not determined that a predetermined number of terminals 14, including the transmitting terminal 14 and at least two receiving terminals 14, have responded that the bit rate can be changed (step S88; No), the process proceeds to step S92.

[0087] (Third Example of Slot Division Determination Processing) The slot division determination processing according to the third example of the first embodiment will be described with reference to Fig. 9. Fig. 9 is a flowchart showing the flow of the slot division determination processing according to the third example of the first embodiment.

[0088] The processes from step S100 to step S106 and step S110 to step S114 are the same as the processes from step S60 to step S66 and step S70 to step S74 shown in FIG. 7, respectively, and therefore will not be described here.

[0089] After step S106, the slot division determination unit 42 determines whether or not a predetermined number of terminals 14, including the transmitting terminal 14 and at least two receiving terminals 14, have responded that the bit rate can be changed within a predetermined time period (step S108). The predetermined time period is, for example, within one second, but is not limited to this. The predetermined time period may be, for example, from one to two seconds. If it is determined that a predetermined number of terminals 14, including the transmitting terminal 14 and at least two receiving terminals 14, have responded that the bit rate can be changed within the predetermined time period (step S108; Yes), the slot division determination unit 42 proceeds to step S110. If it is not determined that a predetermined number of terminals 14, including the transmitting terminal 14 and at least two receiving terminals 14, have responded that the bit rate can be changed within the predetermined time period (step S108; No), the slot division determination unit 42 proceeds to step S112.

[0090] (Fourth Example of Slot Division Determination Processing) The slot division determination processing according to the fourth example of the first embodiment will be described with reference to Fig. 10. Fig. 10 is a flowchart showing the flow of the slot division determination processing according to the fourth example of the first embodiment.

[0091] The processes from step S120 to step S126 and step S130 to step S134 are the same as the processes from step S60 to step S66 and step S70 to step S74 shown in FIG. 7, respectively, and therefore will not be described here.

[0092] After step S126, the slot division determination unit 42 determines whether or not a terminal 14 with a priority higher than or equal to a predetermined level has responded that the bit rate can be changed (step S128). The priority may be set arbitrarily in advance. For example, a terminal 14 used by a specific user may be set to have a higher priority. For example, a terminal 14 used by a user with a higher position may be set to have a higher priority. If it is determined that a terminal 14 with a priority higher than or equal to a predetermined level has responded that the bit rate can be changed (step S128; Yes), the process proceeds to step S130. If it is not determined that a terminal 14 with a priority higher than or equal to a predetermined level has responded that the bit rate can be changed (step S128; No), the process proceeds to step S132.

[0093] (Fifth Example of Slot Division Determination Processing) The slot division determination processing according to the fifth example of the first embodiment will be described with reference to Fig. 11. Fig. 11 is a flowchart showing the flow of the slot division determination processing according to the fifth example of the first embodiment.

[0094] The process of step S140 is the same as the process of step S60 shown in Fig. 7, and therefore its description will be omitted. The processes of steps S144 and S148 are the same as the processes of steps S64 and S68 shown in Fig. 7, and therefore their description will be omitted. The processes of steps S150 to S154 are the same as the processes of steps S70 to S74 shown in Fig. 7, and therefore their description will be omitted.

[0095] After step S140, the slot division determination unit 42 determines whether the communication using the target slot is a call between two terminals 14, the transmitting terminal 14 and the receiving terminal 14 (step S142). If it is determined that the communication is a call between two terminals 14, the transmitting terminal 14 and the receiving terminal 14 (step S142; Yes), the process proceeds to step S144. If it is not determined that the communication is a call between two terminals 14, the transmitting terminal 14 and the receiving terminal 14 (step S142; No), the process proceeds to step S152.

[0096] (Sixth Example of Slot Division Determination Processing) A slot division determination processing according to a sixth example of the first embodiment will be described with reference to Fig. 12. Fig. 12 is a flowchart showing the flow of the slot division determination processing according to the sixth example of the first embodiment.

[0097] The processes of steps S160 to S166 and steps S170 to S174 are the same as the processes of steps S140 to S146 and steps S150 to S154 shown in Fig. 11, respectively, and therefore will not be described. The process of step S168 is the same as the process of step S88 shown in Fig. 8, and therefore will not be described.

[0098] That is, the sixth example of the slot division determination process is a combination of the second example of the slot division determination process and the fifth example of the slot division determination process.

[0099] (Seventh Example of Slot Division Determination Processing) The slot division determination processing according to the seventh example of the first embodiment will be described with reference to Fig. 13. Fig. 13 is a flowchart showing the flow of the slot division determination processing according to the seventh example of the first embodiment.

[0100] The processes of steps S180 to S186 and steps S190 to S194 are the same as the processes of steps S140 to S146 and steps S150 to S154 shown in Fig. 11, respectively, and therefore will not be described. The process of step S168 is the same as the process of step S108 shown in Fig. 9, and therefore will not be described.

[0101] That is, the seventh example of the slot division process determination method is a combination of the third example of the slot division process and the fifth example of the slot division process.

[0102] As described above, in the first embodiment, when a new connection request is received from a terminal while a slot is being used in group communication, each terminal is inquired as to whether or not the bit rate can be changed. Then, in the first embodiment, when a specific terminal responds that the bit rate can be changed, the slot is divided and the divided slots are assigned to each terminal. In this way, the first embodiment can appropriately divide the slot and realize uninterrupted group communication.

[0103] Second Embodiment An overview of a communication system according to a second embodiment will be described with reference to Fig. 14. Fig. 14 is a diagram for explaining a communication system according to the second embodiment.

[0104] A trunking system 1A according to the second embodiment includes, for example, a controller 2, a repeater 3, a repeater 4A, a repeater 5A, a repeater 6A, and a terminal 14.

[0105] The repeater 4A has two communication channels, channel CH1 and channel CH2. Channel CH1 has slots SL1A and SL1B. Channel CH2 has slots SL2A and SL2B.

[0106] Repeater 5A has two communication channels, channel CH3 and channel CH4. Channel CH3 has slots SL3A and SL3B. Channel CH4 has slots SL4A and SL4B.

[0107] Repeater 6A has two communication channels, channel CH5 and channel CH6. Channel CH5 has slots SL5A and SL5B. Channel CH6 has slots SL6A and SL6B.

[0108] That is, in the second embodiment, each communication channel from repeater 4A to repeater 6A has two slots. Note that the number of slots each communication channel has is not limited to two. This communication channel is used for calls, and the slots that the communication channel has are call slots.

[0109] In the second embodiment, it is determined whether the number of calls can be increased based on the status of slots managed by each of a plurality of repeaters and responses from the terminal 14 .

[0110] (Management Device) An example of the configuration of a management device according to the second embodiment will be described with reference to Fig. 15. Fig. 15 is a block diagram showing an example of the configuration of a management device according to the second embodiment.

[0111] 15, the management device 12A differs from the management device 12 shown in FIG. 3 in that a control unit 24A includes an extension determination unit 32 and an execution control unit 34.

[0112] The extension determination unit 32 determines whether or not the number of calls can be extended based on the status of the slots managed by each of the multiple repeaters and responses from the terminals 14 .

[0113] When the extension determination unit 32 determines that the number of calls should be extended, the execution control unit 34 executes call extension processing.

[0114] A method of dividing a slot according to the second embodiment will be described with reference to Fig. 16. Fig. 16 is a diagram for explaining a method of dividing a slot according to the second embodiment.

[0115] It is assumed that the repeater 4 is performing a first communication using slot SL1 for channel CH1, and a second communication using slot SL2 for channel CH2.

[0116] It is assumed that the repeater 5 is conducting a third communication using slot SL3 for channel CH3, and a fourth communication using slot SL4 for channel CH4.

[0117] It is assumed that the repeater 6 is performing a fifth communication using slot SL5 for channel CH5, and a sixth communication using slot SL6 for channel CH6.

[0118] Assume that the connection request terminal detection unit 30 detects a new seventh communication when all slots of each repeater are in use. In this case, the slot division determination unit 42 determines whether any of slots SL1 to SL6 of repeaters 4 to 6 can be divided. For example, if the slot division determination unit 42 determines that slot SL1 of repeater 4 can be divided, the slot division control unit 44 divides slot SL1 into slot SL1A and slot SL1B. The slot allocation unit 46 assigns the seventh communication to slot SL1B divided by the slot division control unit 44. In this case, the first communication is performed using slot SL1A. Note that, although the example shown in FIG. 16 illustrates an example in which one slot is divided into two slots, the present disclosure is not limited to this. In the present disclosure, one slot may be divided into three or more slots. Since the bit rate decreases as the number of divisions of one slot increases, the number of divisions of a slot may be determined within a range that maintains acceptable communication quality.

[0119] (Processing of Management Apparatus) Processing of the management apparatus according to the second embodiment will be described with reference to Fig. 17. Fig. 17 is a flowchart showing the flow of processing of the management apparatus according to the second embodiment.

[0120] The process of step S200 is the same as the process of step S10 shown in FIG. 5, and therefore a description thereof will be omitted.

[0121] The empty slot detection unit 40 checks the usage status of the call slots of each repeater (step S202), and then proceeds to step S204.

[0122] The empty slot detection unit 40 determines whether or not a repeater has an empty slot in the call slot (step S204). If it is determined that a repeater has an empty slot in the call slot (step S204; Yes), the process proceeds to step S206. If it is not determined that a repeater has an empty slot in the call slot (step S204; No), the process proceeds to step S208.

[0123] If the answer to step S204 is Yes, the slot management unit 26A switches the repeater that manages the call to a repeater that has an empty slot (step S206), and then ends the processing of FIG.

[0124] If the answer to step S204 is No, the slot management unit 26A selects a repeater to request an increase in the number of calls (step S208), and then the process proceeds to step S208.

[0125] The slot management unit 26A transmits a call number expansion command to the selected repeater to divide the slot (step S210), and then the process proceeds to step S212.

[0126] The slot management unit 26A executes a slot division determination process to determine whether the plot in use can be divided in the repeater that transmitted the call number expansion request command (step S212). The details of the slot division determination process are the same as those in the first embodiment, and therefore will not be described here.

[0127] The slot division determination unit 42 determines whether or not the number of calls can be increased by dividing the slots in use (step S214). If it is determined that the number of calls can be increased (step S214; Yes), the process proceeds to step S216. If it is not determined that the number of calls can be increased (step S214; No), the process proceeds to step S224.

[0128] If the answer to step S214 is Yes, the slot management unit 26A sets the terminal determined to be able to expand the number of calls as the repeater that manages the terminal 14 that has made the new connection request (step S216).Then, the process proceeds to step S218.

[0129] The slot division control unit 44 transmits a call number extension request execution command to the repeater that manages the terminal 14 that made the connection request, causing the repeater to divide the slot (step S218), and then the process proceeds to step S220.

[0130] The slot division control unit 44 transmits a bit rate change request signal to the terminal 14 that has made the connection request (step S220), and then the process proceeds to step S222.

[0131] The slot allocation unit 46 transmits a slot movement command to the terminal 14 that has made the connection request, and allocates the divided empty slot (step S222). This enables the terminal 14 to communicate. Then, the processing of FIG. 17 ends.

[0132] If the result of step S214 is No, the empty slot detector 40 determines whether or not there is another repeater with an empty slot (step S224). If it is determined that there is another repeater with an empty slot (step S224; Yes), the process proceeds to step S228. If it is not determined that there is another repeater with an empty slot (step S224; No), the process proceeds to step S226.

[0133] If the determination in step S224 is No, the control unit 24A transmits a disconnection signal to the terminal 14 that made the connection request (step S226), thereby disconnecting communication between the management device 12 and the terminal 14. Then, the processing in FIG. 17 ends.

[0134] If the answer to step S224 is Yes, the slot management unit 26A selects the next repeater to which the request for an increase in the number of calls is to be made (step S228), and then the process proceeds to step S210.

[0135] A method for restoring a divided slot to its original state according to the second embodiment will be described with reference to Fig. 18. Fig. 18 is a diagram for explaining a method for restoring a divided slot to its original state according to the second embodiment.

[0136] Assume that a seventh communication is taking place in slot SL1B of repeater 4. When controller 2 detects the seventh communication, it transmits to repeater 4 a call number expansion request command to change the number of slots on channel CH1 to one.

[0137] The repeater 2 transmits a call number extension request command to the terminal 14 that is performing the first communication in the slot SL1A, to change the number of slots to one.

[0138] If the terminal 14 performing the first communication can change to the original communication method and the original audio bit rate, it transmits an ACK to the controller 2 .

[0139] When the controller 2 receives an ACK from the terminal 14 that is performing the first communication, it transmits a call number expansion execution command to change the number of slots on channel CH1 of the repeater 4 to one.

[0140] When the repeater 4 receives the call number expansion execution command, it transmits a bit rate change request signal to the terminal 14 performing the first communication, instructing it to return the audio bit rate to the normal bit rate. The repeater 4 combines slot SL1A and slot SL1B into one slot SL1. The repeater 4 assigns the first communication to slot SL1. The repeater 4 transmits a slot movement command to the terminal 14 performing the first communication to move to slot SL1. The repeater 4 then changes its control state from call number expansion mode to normal mode.

[0141] When the terminal 14 performing the first communication receives the bit rate change request signal, it returns the audio bit rate to the normal bit rate. When the terminal 14 performing the first communication receives the slot movement command, it moves to slot SL1 and starts communication.

[0142] As described above, in the second embodiment, when a slot is being used for group communication and a new connection request is received from a terminal, the repeaters determine whether there is an empty slot. In the second embodiment, if there is a repeater with an empty slot, the repeater with the empty slot manages the terminal's communication. In the second embodiment, if there is no repeater with an empty slot, an arbitrary repeater is selected and the selected repeater performs slot division determination processing. In this way, the second embodiment can appropriately divide slots and realize uninterrupted group communication.

[0143] [Third Embodiment] When a slot is allocated in response to a connection request from a terminal 14 that has newly made a connection request, a terminal 14 that is the communication partner of the terminal 14 that has made the new connection request (hereinafter also referred to as a partner terminal) will communicate using the slot allocated to the terminal 14 that has newly made the connection request. If the slot allocated to the terminal 14 that has made the new connection request is a divided slot, in order to communicate using the divided slot, the partner terminal also needs to be able to communicate at the divided bit rate.

[0144] Therefore, when a connection request is received from a terminal 14 that has newly made a connection request and there are no available slots, the slot division determination unit 42 checks whether connection destination information or group information including the terminal 14 that has newly made the connection request is stored in the storage unit 22. If connection destination information or group information including the terminal 14 that has newly made the connection request is stored in the storage unit 22, the slot division determination unit 42 identifies the remote terminal from the stored connection destination information or group information. The slot division determination unit 42 inquires of the identified remote terminal whether it is possible to change the bit rate to be used in the divided slot. If there are no available slots and the number of identified remote terminals that have responded to the identified remote terminal that they can change the bit rate satisfies a predetermined condition, the slot division determination unit 42 may proceed to the slot division determination process. In other words, as a preprocessing step of the slot division determination process, the identified remote terminal may be inquired of whether it is possible to change the bit rate. Here, this predetermined condition may be, for example, a condition that the number of terminals that have responded to the identified remote terminal that they can change the bit rate is a predetermined number. Furthermore, this predetermined condition may be, for example, the number of terminals that have responded from the remote terminal that they can change the bit rate, which is set to vary depending on the number of identified remote terminals. For example, a predetermined percentage may be set, and the number of terminals that have responded from the remote terminal that they can change the bit rate may be set as the condition.

[0145] When the third embodiment is to be added to the processing of the first embodiment and executed, the processing may be modified as follows. For example, in the processing of the first embodiment shown in FIG. 5 , if a process of inquiring of a specified remote terminal as to whether or not a bit rate change is possible is performed as preprocessing for the slot division determination processing of the third embodiment, the following process may be added. Specifically, when it is determined that there is no available slot in FIG. 5 (step S14; No), before proceeding to step S18, the slot division determination unit 42 first checks whether connection destination information or group information including the terminal 14 that has newly issued a connection request is stored in the storage unit 22. If connection destination information or group information including the terminal 14 that has newly issued a connection request is stored in the storage unit 22, the slot division determination unit 42 identifies the remote terminal from the stored connection destination information or group information. Thereafter, the slot division determination unit 42 inquires of the specified remote terminal as to whether or not a change to the bit rate to be used in the divided slot is possible. If the number of identified remote terminals that have responded that they can change the bit rate from the identified remote terminal satisfies a predetermined condition, the process may proceed to the next slot division determination process (step S18). If the number of identified remote terminals that have responded that they can change the bit rate from the identified remote terminal does not satisfy the predetermined condition, communication with the remote terminal will not be possible even if a divided slot is assigned, so a disconnection signal is sent to terminal 14 that has newly made a connection request (step S32), and the process ends.

[0146] When the third embodiment is to be added to the processing of the second embodiment, the processing may be modified as follows. For example, in the processing of the second embodiment shown in FIG. 17 , if a process of inquiring of a specified remote terminal as to whether or not a bit rate change is possible is performed as preprocessing of the slot division determination processing of the third embodiment, the following process may be added. Specifically, when it is determined that there is no remote terminal with an available slot in FIG. 17 (No in step S204), before proceeding to step S208, the slot division determination unit 42 first checks whether connection destination information or group information including the terminal 14 that has newly issued a connection request is stored in the storage unit 22. If connection destination information or group information including the terminal 14 that has newly issued a connection request is stored in the storage unit 22, the slot division determination unit 42 identifies the remote terminal from the stored connection destination information or group information. Then, the slot division determination unit 42 inquires of the specified remote terminal as to whether or not a change to the bit rate to be used in the divided slot is possible. If the number of terminals among the identified remote terminals that have responded that they can change the bit rate from the identified remote terminal satisfies a predetermined condition, a leaver with an available slot is selected (step S208), a call number extension request command is transmitted (step S210), and the process proceeds to the next slot division determination process (step S18).If the number of terminals among the identified remote terminals that have responded that they can change the bit rate from the identified remote terminal does not satisfy the predetermined condition, communication with the remote terminal is not possible even if a divided slot is assigned, so a disconnection signal is transmitted to the terminal 14 that newly issued the connection request (step S226), and the process ends.

[0147] [Variation of the Third Embodiment] In the third embodiment, the slot division determination unit 42 identifies a remote terminal from connection destination information or group information stored in the storage unit 22, including the terminal 14 that has newly made a connection request, and inquires of the identified remote terminal whether it is possible to change the bit rate to be used in the divided slot. As a variation, instead of storing information indicating whether the remote terminal is able to change the bit rate in the storage unit 22 and inquiring of the remote terminal whether it is possible to change the bit rate, the slot division determination unit 42 may refer to the information stored in the storage unit 22 indicating whether the bit rate for each identified remote terminal is able to change, and, if the number of remote terminals for which the bit rate is able to be changed is identified from the referred information, proceed to slot division determination processing if a predetermined condition is satisfied.

[0148] As described above, in the third embodiment and its modified examples, when a connection request is received from a terminal 14 that has newly made a connection request and there are no available slots, the remote terminal is identified from the destination information or group information including the terminal 14 that made the new connection request, and the identified remote terminal is inquired about whether it is possible to change the bit rate to be used in the divided slots. If there are no available slots and the number of identified remote terminals that have responded that they can change the bit rate satisfies a predetermined condition, or if the number of remote terminals whose bit rate can be changed is identified and satisfies the predetermined condition, slot division determination processing is performed. As a result, even when a divided slot is assigned, it is possible to appropriately divide the slots and realize uninterrupted group calls after confirming in advance that the terminal 14 that made the new connection request can communicate with the remote terminal.

[0149] The components of each device shown in the figure are conceptual functional components and do not necessarily have to be physically configured as shown. In other words, the specific form of distribution and integration of each device is not limited to that shown, and all or part of the devices can be functionally or physically distributed and integrated in any unit depending on various loads, usage conditions, etc. Note that this distribution and integration configuration may also be performed dynamically.

[0150] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the contents of these embodiments. Furthermore, the above-described components include those that can be easily imagined by a person skilled in the art, those that are substantially the same, and those that are within the so-called equivalent range. Furthermore, the above-described components can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the components can be made without departing from the spirit of the above-described embodiments.

[0151] The management device and communication control method of the present disclosure can be used in a trunking system.

[0152] REFERENCE SIGNS LIST 1 Trunking system 2 Controller 3, 4, 5, 6 Repeater 10 Communication system 12, 12A Management device 14 Terminal 20, 68 Communication unit 22, 70 Storage unit 24, 24A, 72 Control unit 26, 26A Slot management unit 30 Connection request terminal detection unit 32 Extension determination unit 34 Execution control unit 40 Empty slot detection unit 42 Slot division determination unit 44 Slot division control unit 46 Slot allocation unit 60 Input unit 62 Audio input unit 64 Display unit 66 Audio output unit 80 Communication request unit 82 Bit rate control unit 84 Slot control unit

Claims

1. A management device having a plurality of slots, which responds when a new connection request is received via a control slot when all of the call slots are in use for a call, comprising: a communication unit which communicates with terminals using the call slots and the control slot; a slot division determination unit which, when there are no free call slots and a new connection request is received via the control slot, selects a slot to be queried from among the call slots in use based on a predetermined condition, inquires of a terminal in use that is using the selected slot to be queried as to whether or not the bit rate can be changed, and, when the terminal in use responds that the bit rate can be changed, designates the queried slot as a divisible slot; a slot division control unit which instructs the terminal in use to change the bit rate and divides the divisible slot; and a slot allocation unit which assigns the terminal in use and a requesting terminal that has made a new connection request to the divided slots, respectively; and when the queried slot is being used for a group call, the slot division determination unit designates the queried slot as the divisible slot when the number of terminals in use that have responded that the bit rate can be changed satisfies a predetermined condition.

2. The management device according to claim 1, wherein the slot division determination unit selects the inquiry target slot based on the number of terminals currently using the call slot.

3. The management device according to claim 2, wherein the slot division determination unit lowers the priority of selection of call slots used in group calls when selecting the slot to be queried.

4. A management device as claimed in any one of claims 1 to 3, wherein the slot division determination unit determines the queried slot to be the divisible slot if the number of using terminals that have responded that the bit rate can be changed satisfies a specified condition within a specified time.

5. A management device as described in claim 1, further comprising a memory unit for storing destination information or group information used to identify a terminal to communicate with, wherein when there are no available call slots and a new connection request is received in a control slot, the slot division determination unit identifies a partner terminal communicating with the terminal that has made the new connection request from the destination information or group information stored in the memory unit and inquires as to whether the bit rate can be changed, and if the partner terminal responds that the bit rate can be changed, selects a slot to be queried from among call slots in use based on predetermined conditions, inquires of a busy terminal using the selected slot to be queried as to whether the bit rate can be changed, and if the busy terminal responds that the bit rate can be changed, sets the queried slot as a splittable slot.

6. A communication control method having a plurality of slots, which responds when a new connection request is received via a control slot when all of the call slots are in use for a call, comprising: a step of communicating with a terminal using the call slot and the control slot; a step of selecting a slot to be queried from among the call slots in use based on a predetermined condition when there are no free call slots and a new connection request is received via the control slot, inquiring of a terminal in use using the selected call slot as to whether or not the bit rate can be changed, and setting the queried slot as a divisible slot when the terminal in use responds that the bit rate can be changed; a step of instructing the terminal in use to change the bit rate and dividing the divisible slot; a step of respectively allocating the divided slots to the terminal in use and the requesting terminal that has made the new connection request; and a step of setting the queried slot as the divisible slot when the number of terminals in use that have responded that the bit rate can be changed satisfies a predetermined condition when the queried slot is being used for a group call.

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