Patent Publication Number: US-2003224775-A1

Title: Radio access communication system capable of preventing packet loss

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a radio access communication system, a radio terminal device and a radio base station for use therein, and a radio access communication method, and particularly, relates to a radio access communication system capable of preventing a packet loss between the radio terminal device and radio base station, a radio access communication method, and a program for use in the radio access communication system.  
       [0003] 2. Description of the Related Art  
       [0004] In general, a radio access communication system of this kind includes a plurality of radio base stations for defining cells, and a radio terminal device such as a mobile telephone or a PDA (Personal Digital Assistants). The radio terminal device includes a cellular system and a wireless LAN system. The cellular system enables communication while moving among a plurality of cells, as in a GSM system, a PDC system or the like. The wireless LAN system is standardized under IEEE 802.11, and is for performing radio communication in an area of the base station mainly with a personal computer as the radio terminal device. In the cellular system and wireless LAN system, data or the like is transmitted/received in the form of a packet.  
       [0005] In any case, it is highly likely that the radio terminal device used in the radio access communication system of this kind would move among the cells or among the areas. Therefore, when the radio terminal device under the communication moves from one cell to another cell, it is necessary for the radio terminal device to switch (handover) from one radio base station under the connection to another radio base station.  
       [0006] Various techniques have been considered and used in association with such handover. For example, in the GSM system and PDC system, a radio channel is divided into time slots, and is synchronized between the radio base station and the radio terminal device. Therefore, the radio terminal device searches for other radio base stations using a time slot other than the allotted time slot, thereby accomplishing significantly shortened handover time. Such an example is disclosed in, for instance, Japanese Patent Publication Laid-open No. 5-73297.  
       [0007] On the other hand, in the wireless LAN standardized under the IEEE 802.11, the radio base station (access point) and a plurality of radio terminal devices obtain a radio channel by random access, and transmit packets asynchronously, unlike in the GSM system and PDC system. Therefore, it is necessary for the radio terminal device to receive all the packets transmitted to the radio channel and judge whether the packets are addressed to the device itself.  
       [0008] In the aforementioned radio access communication system for performing packet transmission asynchronously, when the radio terminal device equipped with only one receiver is used, the following problems are posed. Namely, if this radio terminal device searches for the neighboring radio base stations, a packet loss is caused. Such a packet loss has a great impact especially when data is transmitted/received.  
       [0009] The cause of the packet loss is as follows. It is necessary to tune the receiver to a radio frequency different from the radio frequency used at present, in order for the radio terminal device to search for the neighboring radio base stations. When the radio base station transmits a packet addressed to the radio terminal device during the tuning, the packet loss is caused.  
       [0010] On the other hand, in the radio access communication system for performing packet transmission asynchronously, use of a radio terminal device comprising a plurality of receivers is also proposed. In this case, in the radio terminal device, one receiver is always tuned to one radio frequency, and the other receivers are used to search for the neighboring radio base stations using different radio frequencies. However, the radio terminal device comprising a plurality of receivers causes increased costs, an obstacle to a size reduction, and increased power consumption, presenting a difficulty in an aspect of economical efficiency.  
       SUMMARY OF THE INVENTION  
       [0011] Therefore, an object of the present invention is to provide a radio access communication system for searching for neighboring radio base stations without causing a packet loss even if a radio terminal device equipped with only one transmitter/receiver is used in the radio access communication system for performing packet transmission asynchronously.  
       [0012] Another object of the present invention is to provide a radio access communication method suitable for the above radio access communication system.  
       [0013] Still another object of the present invention is to provide a radio terminal device capable of performing handover without causing a packet loss.  
       [0014] Yet another object of the present invention is to provide a radio base station capable of communicating with the aforementioned radio terminal device.  
       [0015] The present invention is applied to a radio access communication system including a plurality of radio base stations and a radio terminal device connectable to the plurality of radio base stations.  
       [0016] According to a first aspect of the present invention, a radio terminal device includes a judgment unit for judging whether communication conditions have deteriorated. When connected to a first radio base station included in the plurality of radio base stations, the radio terminal device transmits a transmission suppress signal for controlling a communication with the radio terminal device to the first radio base station if the communication conditions with the first radio base station deteriorate, and the radio terminal device searches for the communication conditions with other radio base stations. When receiving the transmission suppress signal, the first radio base station temporarily retains data addressed to the radio terminal device. The radio terminal device can be connected to the other radio base station according to the result of the search.  
       [0017] According to a second aspect of the present invention, each of radio base stations includes a judgment unit for judging whether communication conditions have deteriorated. A first radio base station connected to a radio terminal device, among a plurality of radio base stations, controls a communication with the radio terminal device if the communication conditions with the radio terminal device deteriorate. The first radio base station also transmits a transmission suppress signal to the radio terminal device, and lets the radio terminal device search for the communication conditions with other radio base stations.  
       [0018] According to a third aspect of the present invention, a radio terminal device includes a judgment unit for judging whether communication conditions have deteriorated. When connected to a first radio base station included in a plurality of radio base stations, the radio terminal device transmits a transmission suppress signal for controlling a communication with the radio terminal device to the first radio base station if the communication conditions with the first radio base station deteriorate, and the radio terminal device searches for the communication conditions with other radio base stations. The radio terminal device also transmits a control information requesting signal to detect whether a radio base station exists, when searching for the communication conditions with the other radio base stations.  
       [0019] According to a fourth aspect of the present invention, a radio terminal device connectable with a plurality of radio base stations is provided. The radio terminal device can operate in an operation mode of either a normal mode or a communication suppress mode. The radio terminal device includes an informing unit for informing of communication conditions with a radio base station being connected to among the plurality of radio base stations, and a control unit for changing the operation mode to the communication suppress mode when detecting deterioration of the informed communication conditions. In this case, the control unit transmits a transmission suppress signal for informing the radio base station being connected to of entrance into a power saving mode, or a transmission suppress signal for requesting congestion avoidance of the radio base station being connected to.  
       [0020] According to a fifth aspect of the present invention, a radio base station for communicating with a radio terminal device is provided. The radio base station includes an informing unit for informing of communication conditions with the radio terminal device, and a control unit for instructing the radio terminal device to transmit a transmission suppress signal, when the communication conditions are monitored and deterioration of the communication conditions is detected. In this case, the control unit transmits a transmission suppress signal for switching the radio terminal device into a power saving mode, or a transmission suppress signal for informing of congestion avoidance, when the deterioration of the communication conditions is detected.  
       [0021] According to a sixth aspect of the present invention, a radio access communication method between a plurality of radio base stations and a radio terminal device connectable with the plurality of radio base stations is provided. The present radio access communication method includes the steps of: detecting deterioration of communication conditions between the radio terminal device in a connected state and the radio base stations; and preventing a packet loss by controlling the communication between the radio terminal device in a connected state and the radio base stations and storing packets transmitted and received by the radio terminal device and the radio base stations, when deterioration of the communication conditions is detected.  
       [0022] According to a seventh aspect of the present invention, a radio access communication method between a radio base station and a radio terminal device capable of communicating asynchronously with the radio base station is provided. The present radio access communication method includes the steps of: detecting deterioration of communication conditions in one of the radio terminal device or the radio base station; and transmitting a transmission suppress signal for controlling the communication to the other one of the radio base station or the radio terminal device. In the present radio access communication method, the step of searching for the communication conditions with other radio base stations is executed in the radio terminal device, in connection with transmission and reception of the transmission suppress signal.  
       [0023] According to an eighth aspect of the present invention, a program for a radio terminal device capable of operating in an operation mode of either a normal mode or a communication suppress mode, and connectable with a plurality of radio base stations. The present program includes the steps of: judging which operation mode the radio terminal device is operating in, the normal mode or the communication suppress mode; judging whether communication conditions with a radio base station being connected to have deteriorated, among the plurality of radio base stations; and changing the operation mode to the communication suppress mode when the communication conditions are judged to have deteriorated. In this case, the present program may include the step of searching for radio base stations other than the radio base station being connected to, when changed to the communication suppress mode, or the step of changing the connection to the other radio base stations according to the result of searching.  
       [0024] According to a ninth aspect of the present invention, a program for a radio base station connected to a radio terminal device capable of operating in an operation mode of either a normal mode or a communication suppress mode is provided. The present program includes the steps of: judging the operation mode of the radio terminal device; buffering data for the radio terminal device when the operation mode of the radio terminal device is judged to be the communication suppress mode; and informing the radio terminal device that the data is buffered.  
       [0025] According to a tenth aspect of the present invention, a program for a radio base station connectable with a radio terminal device capable of transmitting a transmission suppress signal is provided. The present program includes the steps of: judging whether the transmission suppress signal from the radio terminal device is received; transmitting a response signal directed to the transmission suppress signal to the radio terminal device, when the transmission suppress signal from the radio terminal device is judged to be received; and switching an operation mode in the radio terminal device to a communication suppress mode. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0026]FIG. 1 is a network constitution view for describing first, second and third embodiments of the present invention;  
     [0027]FIG. 2 is a constitution view of a radio base station in a radio access communication system in accordance with the first to third embodiments of the present invention;  
     [0028]FIG. 3 is a flowchart for describing the operation of the radio base station in the radio access communication system in accordance with the first to third embodiments of the present invention;  
     [0029]FIG. 4 is a constitution view of a radio terminal device in the radio access communication system in accordance with the first to third embodiments of the present invention;  
     [0030]FIG. 5 is a flowchart for describing the operation of the radio terminal device in the radio access communication system in accordance with the first to third embodiments of the present invention;  
     [0031]FIG. 6A to FIG. 6E are sequence diagrams for describing the operation of the radio access communication system in accordance with the first embodiment of the present invention;  
     [0032]FIG. 7A to FIG. 7E are sequence diagrams for describing the operation of the radio access communication system in accordance with the second embodiment of the present invention; and  
     [0033]FIG. 8A to FIG. 8E are sequence diagrams for describing the operation of the radio access communication system in accordance with the third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0034] A radio access communication system in accordance with the present invention will be described. The radio access communication system shown in FIG. 1 illustrates a wireless LAN. Here, the operation of the radio access communication system in accordance with the present invention will be schematically described, on the assumption that it supports a power saving mode.  
     [0035] A radio terminal device comprises one transmitter/receiver, and is capable of operating in the power saving mode. When a communication is not made for a certain period, the radio terminal device informs a radio base station of entrance into the power saving mode with a transmission suppress signal, and enters the power saving mode. The radio terminal device in the power saving mode receives only part of beacons transmitted by the radio base station at regular intervals, and for other periods, the transmitter/receiver of the radio terminal device is in a state where a power source is turned OFF. The beacon is controlling data addressed and broadcast from the radio base station to all its subordinate radio terminal devices.  
     [0036] On the other hand, when receiving data addressed to the radio terminal device operating in the power saving mode, the radio base station once retains the received data. The radio base station also transmits information indicating that the radio base station is retaining the data, including the information in a beacon, so as to inform the radio terminal device in the power saving mode. The radio terminal device that has received this information returns to a normal mode, and informs the radio base station that it has returned to the normal mode, and then normal data transmission/reception is started.  
     [0037] The present invention can also be applied to a radio access communication system that supports a congestion avoiding mode. In this case also, about the same operation as that described above is performed. That is, when the radio base station detects the congestion, or receives congestion information (transmission suppress signal) from the radio terminal device, the radio base station once buffers the data addressed to the radio terminal device. Also when detecting a recovery from the congestion or receiving a transmission request from the radio terminal device, the radio base station returns to a normal operation.  
     [0038] Next, concrete constitution of the radio access communication system in accordance with a first embodiment of the present invention will be described with reference to FIG. 1. The radio access communication system includes a router  70 , a switching hub  10 , radio base stations  20   a  and  20   b,  and a radio terminal device  50 . The radio base stations  20   a  and  20   b  and the router  70  are each connected via the switching hub  10  by Ethernets (registered trademark)  60   a,    60   b  and  60   c.  The router  70  is connected to an Internet network  80 . The radio access communication system to which the present invention is applied may not be connected to the Internet network  80 .  
     [0039] The radio base stations  20   a  and  20   b  can communicate with the radio terminal device  50  through radio channels  30   a  and  30   b  each at different frequencies. It is assumed that the radio base stations and all the radio terminal devices perform channel access to the radio channels by a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) method. Also, it is assumed that the radio terminal device  50  exists in an overlapping area of two cells  40   a  and  40   b,  and is in a situation to be able to communicate with the radio base stations  20   a  and  20   b.    
     [0040] With reference to FIG. 2 to FIG. 5, constitution of the radio terminal device  50  and the radio base station  20   a  (or  20   b ) constituting the radio access communication system in accordance with the present invention and their operational sequences will be described in detail.  
     [0041]FIG. 2 shows the constitution of the radio base station  20   a,  and the radio base station  20   b  also has similar constitution. The radio base station  20   a  is constituted of a first frame transmission/receiving unit  400 , a first MAC control unit  410 , a second frame transmission/receiving unit  420 , a second MAC control unit  430 , and a buffer  440 .  
     [0042] The second frame transmission/receiving unit  420  performs an operation of handing over a frame received from the Ethernet (registered trademark)  60   a  to the second MAC control unit  430  via a second MAC interface  470 . The second frame transmission/receiving unit  420  also performs an operation of transmitting the frame handed over from the second MAC control unit  430  via the second MAC interface  470  to the Ethernet (registered trademark)  60   a.    
     [0043] The second MAC control unit  430  hands over a frame that should be transmitted to the radio channel  30   a  among the frames handed over from the second frame transmission/receiving unit  420 , to the first MAC control unit  410  via a bridge interface  480 . The second MAC control unit  430  also hands over only the frame that should be transmitted to the Ethernet (registered trademark)  60   a  among the frames handed over from the first MAC control unit  410  via the bridge interface  480 , to the second frame transmission/receiving unit  420 .  
     [0044] The first frame transmission/receiving unit  400  hands over a frame received from the radio channel  30   a  to the first MAC control unit  410  via the first MAC interface  460 . The first frame transmission/receiving unit  400  also transmits the frame handed over from the first MAC control unit  410  via the first MAC interface  460  to the radio channel  30   a.  The first frame transmission/receiving unit  400  further informs of a received power level of the received frame together when handing over the frame to the first MAC control unit  410 .  
     [0045] The first MAC control unit  410  has a radio terminal monitoring unit  450  built therein for managing the conditions of the radio terminal device. The first MAC control unit  410  hands over only the frame that should be transmitted to the Ethernet (registered trademark)  60   a  among the frames handed over from the first frame transmission/receiving unit  400 , to the second MAC control unit  430 . The first MAC control unit  410  is also handed over a frame from the second MAC control unit  430  via the bridge interface  480 .  
     [0046] The frame that should be transmitted to the radio channel  30   a  is dealt with in the following manner. When an operation mode is the normal mode, the first MAC control unit  410  hands over the frame handed over from the second MAC control unit  430  to the first frame transmission/receiving unit  400  via the first MAC interface  460 . On the other hand, when the operation mode is the power saving mode, the first MAC control unit  410  hands over the frame handed over from the second MAC control unit  430  to the buffer  440  via a buffer interface  500 .  
     [0047] The first MAC control unit  410  further informs the radio terminal monitoring unit  450  of interruption when the received power level informed by the first frame transmission/receiving unit  400  is a preset threshold value or less, or when a value of a bit/packet error rate based on frame check sequence processing in the first MAC control unit  410  is a prescribed threshold value or less, or when the first MAC control unit  410  receives a transmission suppress signal from the radio terminal device. The radio terminal monitoring unit  450  manages the operation mode (normal mode and power saving mode) of the radio terminal device, and changes the operation mode according to the interruption information from the first MAC control unit  410 .  
     [0048] The operation associated with the operation mode of the first MAC control unit  410  and radio terminal monitoring unit  450  is performed in accordance with a program stored in a memory (not shown) that the first MAC control unit  410  comprises. The operation in accordance with this program can be indicated, for example, in a flowchart shown in FIG. 3.  
     [0049] With reference to FIG. 3, in the first MAC control unit  410 , first, a frame reception is judged whether it is the frame reception from the first frame transmission/receiving unit  400  or the frame reception from the second MAC control unit  430 . As a result of the judgment, in the case of the former, the operation moves to step AP- 1   b,  while in the case of the latter, the operation moves to step AP- 1   a.    
     [0050] Here, when the frame reception is performed from the Ethernet (registered trademark)  60   a  via the second frame transmission/receiving unit  420  and the second MAC control unit  430 , the first MAC control unit  410  checks the operation mode of the radio terminal device  50  in step AP- 1   a.  When the operation mode is the normal mode as a result of the checking in step AP- 1   a,  the first MAC control unit  410  hands over the received frame to the first frame transmission/receiving unit  400 . Then, the first frame transmission/receiving unit  400  transmits the received frame to the radio terminal device  50  (step AP- 2   a ), and the first MAC control unit  410  terminates the processing in accordance with the program.  
     [0051] On the other hand, when the operation mode of the radio terminal device  50  is judged to be the power saving mode in step AP- 1   a,  the received frame received by the first MAC control unit  410  is buffered by the buffer  440  under the control of the program (step AP- 2   b ). Then, the first MAC control unit  410  instructs the first frame transmission/receiving unit  400  on the informing of the frame reception to inform that the first MAC control unit  410  is buffering the frame for the radio terminal device  50 . As a result, the frame reception information is set in the beacon output from the first frame transmission/receiving unit  400  (step AP- 3   b ).  
     [0052] When receiving a frame from the first frame transmission/receiving unit  400 , the first MAC control unit  410  judges whether or not the received frame is the transmission suppress signal (step AP- 1   b ). When the received frame is judged to be the transmission suppress signal in step AP- 1   b,  the first MAC control unit  410  changes the operation mode to the power saving mode under the control of the program (step AP- 2   c ). The first MAC control unit  410  then creates an acknowledgment signal to hand over to the first frame transmission/receiving unit  400  and terminates the processing in accordance with the program (step AP- 3   c ).  
     [0053] On the other hand, when the frame received from the first frame transmission/receiving unit  400  is judged not to be the transmission suppress signal in step AP- 1   b,  the first MAC control unit  410  checks the operation mode of the radio terminal device  50  (step AP- 2   d ). When the operation mode is judged to be the power saving mode in step AP- 2   d,  step AP- 3   d  follows. In step AP- 3   d,  the first MAC control unit  410  changes the operation mode to the normal mode, and then moves to step AP- 4   d  to compare reception characteristics, for example, the received power level with the preset threshold value. When the received power level is not the preset threshold value or less as a result of the comparison, the first MAC control unit  410  terminates the processing in accordance with the program. On the other hand, when the received power level is the preset threshold value or less, the first MAC control unit  410  instructs the first frame transmission/receiving unit  400  to generate a transmission suppress signal. As a result, the transmission suppress signal is transmitted from the first frame transmission/receiving unit  400  (step AP- 5   d ). Then, when the acknowledgement signal in response to the transmission suppress signal is received from the first frame transmission/receiving unit  400 , the operation mode of the first MAC control unit  410  is changed to the power saving mode (step AP- 6   d ).  
     [0054] In addition, when the operation mode is the normal mode as a result of the checking of the operation mode in step AP- 2   d,  step AP- 4   d  follows. In step AP- 4   d,  the above reception characteristics and the threshold value are compared. After that, the same processing as in the case of the power saving mode mentioned above will be performed.  
     [0055] Next, the radio terminal device  50  shown in FIG. 1, for example, comprises the constitution shown in FIG. 4. In FIG. 4, the radio terminal device  50  includes a frame transmission/receiving unit  600 , a MAC control unit  610  and a buffer  620 . The frame transmission/receiving unit  600  hands over a frame received via the radio channel  30   a  to the MAC control unit  610  via a MAC interface  650 . The frame transmission/receiving unit  600  also transmits the frame handed over from the MAC control unit  610  via the MAC interface  650  to the radio channel  30   a.  The frame transmission/receiving unit  600  further informs of a received power level together when handing over the received frame to the MAC control unit  610 .  
     [0056] The MAC control unit  610  has a reception characteristics monitoring unit  630  and a program memory (not shown) inside, and operates according to a program stored in this program memory. That is, the MAC control unit  610  outputs the frame handed over from the frame transmission/receiving unit  600  via the MAC interface  650  to a software interface  660  according to the program. The MAC control unit  610  also processes the frame received via the software interface  660  according to the program. Concretely, when the operation mode of the radio terminal device  50  is the normal mode, the MAC control unit  610  hands over the received frame to the frame transmission/receiving unit  600  via the MAC interface  650 . On the other hand, when the operation mode is the power saving mode, the MAC control unit  610  stores the received frame in the buffer  620  via a buffer interface  640 .  
     [0057] The MAC control unit  610  further informs the reception characteristics monitoring unit  630  of interruption when the received power level informed by the frame transmission/receiving unit  600  is the preset threshold value or less, or when a value of a bit/packet error rate based on the frame check sequence processing in the MAC control unit  610  is the prescribed threshold value or less. The reception characteristics monitoring unit  630  manages the operation mode (normal mode and power saving mode) of the radio terminal device according to the program mentioned above, and changes the operation mode according to the interruption information from the MAC control unit  610 .  
     [0058] The operation of the MAC control unit  610  and the reception characteristics monitoring unit  630  is performed in accordance with the program. This program can be indicated, for example, in a flowchart shown in FIG. 5.  
     [0059] In FIG. 5, first, a frame reception is judged whether it is the frame reception from the software interface  660  or the frame reception from the frame transmission/receiving unit  600 . When it is judged to be the frame reception from the software interface  660 , step STA-a 1  follows, and the operation mode is checked. When the operation mode is judged to be the normal mode in step STA-a 1 , the received frame received via the software interface  660  is sent to the frame transmission/receiving unit  600  under the control of the program. The frame transmission/receiving unit  600  transmits the received frame (step STA-a 2 ).  
     [0060] On the other hand, when the operation mode is judged to be the power saving mode in step STA-a 1 , the processing in accordance with the program will move to step STA-a 3 . The MAC control unit  610  checks whether the radio terminal device  50  is in searching processing for the radio base station in step STA-a 3 . If the radio terminal device  50  is in the power saving mode and not in the searching processing for the radio base station, step STA-a 4  follows. In step STA-a 4 , the operation mode is changed to the normal mode, and then the received frame is sent to the frame transmission/receiving unit  600 . The frame transmission/receiving unit  600  transmits the received frame (step STA-a 5 ).  
     [0061] On the other hand, when the radio terminal device  50  is judged to be in the searching processing for the radio base station in step STA-a 3 , the MAC control unit  610  executes step STA-a 6 , and stores the received frame from the software interface  660  in the buffer  620 . After that, when the searching processing for the radio base station in the radio terminal device  50  terminates (step STA-a 7 ), it is judged whether or not to continue the connection with the radio base station  20   a  being connected to at present, in other words, whether or not to be connected to a new radio base station (step STA-a 8 ). When the connection with the radio base station  20   a  being connected to at present is continued, the MAC control unit  610  sends the buffered frame to the frame transmission/receiving unit  600 . The frame transmission/receiving unit  600  transmits the frame to the radio base station  20   a  (step STA-a 9 ).  
     [0062] When deciding to be connected to the new radio base station as a result of the searching for the radio base station in step STA-a 8 , the MAC control unit  610  performs changing processing of the radio base station (step STA-a 10 ), and then sends the frame buffered in the buffer  620  to the frame transmission/receiving unit  600 . The frame transmission/receiving unit  600  transmits the frame (step STA-a 11 ).  
     [0063] When it is judged to be the frame reception from the frame transmission/receiving unit  600  at the first frame reception, the operation mode of the radio terminal device  50  is judged whether it is the normal mode or the power saving mode in step STA-b 1 . When the operation mode is judged to be the normal mode, the operation of the MAC control unit  610  proceeds to step STA-b 2 . In step STA-b 2 , the received frame is checked whether or not it is the transmission suppress signal.  
     [0064] When the received frame is not the transmission suppress signal, the MAC control unit  610  compares the reception characteristics, for example, a received power level with the preset threshold value in step STA-b 3 . When the received power level is not the preset threshold value or less as a result of the comparison, the processing in accordance with the program is terminated. On the other hand, when the received power level is the preset threshold value or less, the MAC control unit  610  sends the transmission suppress signal to the frame transmission/receiving unit  600 . The frame transmission/receiving unit  600  transmits the transmission suppress signal (step STA-b 4 ). After this, when finishing the reception of the acknowledgment signal in response to the transmission suppress signal via the frame transmission/receiving unit  600  (step STA-b 5 ), the MAC control unit  610  changes the operation mode to the power saving mode (step STA-b 6 ), and starts the searching processing for the new radio base station (step STA-b 7 ). When finishing the searching processing for the radio base station (step STA-b 8 ), the MAC control unit  610  judges whether or not to be connected to the new radio base station (step STA-b 9 ). When the MAC control unit  610  is not connected to the new radio base station as a result of the judgment, the processing in accordance with the program is terminated. On the other hand, when it is judged to be connected to the new radio base station in step STA-b 9 , the MAC control unit  610  changes the radio base station to be connected to (step STA-b 10 ), and terminates the processing.  
     [0065] On the other hand, when the frame received from the frame transmission/receiving unit  600  in the normal mode is judged to be the transmission suppress signal in step STA-b 2 , the MAC control unit  610  instructs the frame transmission/receiving unit  600  to generate an acknowledgment signal. As a result, the acknowledgment signal in response to the transmission suppress signal is transmitted from the frame transmission/receiving unit  600  (step STA-b 11 ). Then, the MAC control unit  610  changes the operation mode of the radio terminal device  50  to the power saving mode (step STA-b 12 ), and starts the searching processing for a new radio base station (step STA-b 13 ). When the searching processing for a new radio base station is terminated in step STA-b 14 , the MAC control unit  610  judges whether or not to be connected to the new radio base station (step STA-b 15 ). When the MAC control unit  610  decides not to be connected to the new radio base station as a result of the judgment, the processing is terminated. In addition, when it is judged to be connected to the new radio base station in step STA-b 15 , the radio base station to be connected to is changed under the control of the program in step STA-b 16 , and the processing is terminated.  
     [0066] Furthermore, when the operation mode of the radio terminal device  50  is judged to be the power saving mode in step STA-b 1  mentioned above, the received frame is judged whether or not it is a beacon (step STA-b 18 ). When the received frame is not a beacon as a result of the judgment, the processing is terminated. On the other hand, when the received frame is a beacon, it is judged whether or not frame reception information addressed to the radio terminal device  50  is set in the beacon (step STA-b 19 ). When the frame addressed to the radio base station itself is received by the radio base station, the radio base station is informed of a state transition to the normal mode, in order to receive the frame addressed to the station itself (step STA-b 20 ).  
     [0067] When the data frame is sent from the radio base station in this state and it is detected that the reception of the data frame has been finished (step STA-b 21 ), it is judged whether or not to be connected to the new radio base station (step STA-b 22 ). When it is not necessary to be connected to the new radio base station as a result of the judgment, the processing is terminated. On the other hand, when it is judged to be connected to the new radio base station, a connection change to the radio base station is made (step STA-b 23 ). When the frame addressed to the radio base station itself is not received by the radio base station in step STA-b 19 , the step STA-b 22  follows, and the same step STA-b 22  and step STA-b 23  as above will be executed.  
     [0068] It is as described above that the aforementioned operation is performed in accordance with the program stored in the program memory of the MAC control unit  610 .  
     [0069] It has been described that the operation of the radio base station and the radio terminal device shown in FIG. 3 and FIG. 5 is executed by the software. However, the operation in FIG. 3 and FIG. 5 can also be accomplished by hardware. It is also possible to accomplish in such a manner that the software and hardware share the processing with each other. When the software and hardware share the processing, it is preferable that the processing requiring high speed may be accomplished by the hardware.  
     [0070] Next, with reference to FIG. 6A to FIG. 6E, an example of operational sequences of the radio base stations  20   a  and  20   b  and the radio terminal device  50  will be described in association with each other. The sequences in the first embodiment indicate a case where a radio signal, that is, a received power level of a packet is used as measured reception characteristics.  
     [0071]FIG. 6A shows a received packet sequence from the Ethernet (registered trademark)  60   a  in the radio base stations  20   a,  and FIG. 6B shows a transmitted packet sequence to the radio channel  30   a  in the radio base station  20   a.  FIG. 6C shows a transmitted packet sequence to the radio channel  30   b  in the radio base station  20   b,  and FIG. 6D shows a transmitted packet sequence to the radio channels  30   a  and  30   b  in the radio terminal device  50 . FIG. 6E shows radio signal reception characteristics in the radio terminal device  50 . In FIG. 6A to FIG. 6E, it is shown that time passes from left to right.  
     [0072] In the radio access communication system in the first embodiment, the two radio channels  30   a  and  30   b  are assumed as the usable radio channels, as described above.  
     [0073] As indicated by arrows  210  and  220  in lower parts of FIG. 6B to FIG. 6C, the radio base stations  20   a  and  20   b  transmit beacons each at definite beacon periods. As apparent from the radio signal reception characteristics in FIG. 6E, the radio terminal device  50  always checks the received power levels of the received packets on the basis of the preset threshold value  320 .  
     [0074] In the states shown in FIG. 6A to FIG. 6E, the radio terminal device  50  is first connected to the radio base station  20   a.  When the radio base station  20   a  receives data packets  100   a  and  100   b  from the Ethernet (registered trademark)  60   a  as shown in FIG. 6A, the radio base station  20   a  transmits a beacon  110   a  as shown in FIG. 6B, and then transfers data packets  120   a  and  120   b  via the radio channel  30   a.    
     [0075] When receiving the data packets  120   a  and  120   b  normally, the radio terminal device  50  transmits each of acknowledgment signals (i.e., normal response signals)  130   a  and  130   b,  as shown in FIG. 6D. When receiving the beacons  110   a  and  110   b  (see FIG. 6B) as shown in FIG. 6E, the radio terminal device  50  also detects received power levels  230   a  and  230   d  of the received beacon, and compares the detected received power levels  230   a  and  230   d  with the preset threshold value  320 . When the obtained received power level  230   d  is detected to be the preset threshold value  320  or less as a result of receiving the beacon  110   b  as shown in FIG. 6E, the radio terminal device  50  transmits a state transition informing packet  150  to the radio base station  20   a  being connected to, as shown in FIG. 6D.  
     [0076] In this example, the radio terminal device  50  and the radio base station  20   a  each transit to the power saving mode by the transmission/reception of the state transition informing packet  150 , and are in a state of controlling communication. Therefore, the state transition informing packet  150  has a function as the transmission suppress signal.  
     [0077] When receiving the state transition informing packet  150  from the radio terminal device  50 , the radio base station  20   a  transmits a response signal, that is, an acknowledgment signal  310 , as shown in FIG. 6B. When receiving the acknowledgment signal  310  from the radio base station  20   a,  the radio terminal device  50  starts a radio base station search  250 , and changes the setting to receive the packet of the radio channel  30   b.  As a result, the radio terminal device  50  receives a beacon  140   b  (see FIG. 6C) transmitted by the radio base station  20   b.    
     [0078] When detecting that a received power level  240   a  of the beacon  140   b  from the radio base station  20   b  is higher than the preset threshold value  320  as shown in FIG. 6E, the radio terminal device  50  decides to switch the connecting end from the radio base station  20   a  to the radio base station  20   b.  Then, the radio terminal device  50  again changes the setting to receive the packet of the radio channel  30   b.    
     [0079] On the other hand, when receiving a data packet  100   c  addressed to the radio terminal device  50  at the time when the radio terminal device  50  is in the power saving mode, the radio base station  20   a  informs the radio terminal device  50  that the radio base station  20   a  is retaining a packet addressed to the radio terminal device  50  in a beacon  110   c  transmitted next. Receiving this information, the radio terminal device  50  returns to the normal mode and transmits a state transition informing packet  160  (see FIG. 6D), and then informs the radio base station  20   a  that the radio terminal device  50  has returned to the normal mode. The radio terminal device  50  receives a data packet  120   c  transmitted by the radio base station  20   a,  and transmits an acknowledgment signal  130   c  (see FIG. 6D) in response to that. From the fact that the data packet is not transmitted to the radio channel  30   a  after transmitting the acknowledgment signal  130   c,  the radio terminal device  50  detects no more data packet to receive, and transmits a connection cancellation requesting packet  170  to the radio base station  20   a.  When receiving the connection cancellation requesting packet  170 , the radio base station  20   a  transmits an acknowledgment signal  180  to the radio terminal device  50 .  
     [0080] When receiving the acknowledgment signal  180  from the radio base station  20   a,  the radio terminal device  50  again changes the setting to receive the packet of the radio channel  30   b,  and transmits a connection requesting packet  190  to the radio base station  20   b.  The radio terminal device  50  receives an acknowledgment signal  200  from the radio base station  20   b,  and completes connection switching processing.  
     [0081] As described above, in the embodiment shown in FIG. 6A to FIG. 6E, the packet received in the power saving mode is once retained in the radio base station  20   a,  and then transmitted from the radio base station  20   a  to the radio terminal device  50  after the cancellation of the power saving mode. In this way, a packet loss can be prevented.  
     [0082] With reference to operational sequence diagrams shown in FIG. 7A to FIG. 7E, the radio access communication system in accordance with a second embodiment of the present invention will be described.  
     [0083]FIG. 7A to FIG. 7E show the operational sequences of the radio terminal device  50 , the radio base stations  20   a  and  20   b  in the radio access communication system in accordance with the second embodiment of the present invention. Similarly to FIG. 6A to FIG. 6D, FIG. 7A shows a received packet sequence of the radio base stations  20   a,  and FIG. 7B shows a transmitted packet sequence to the radio channel  30   a  in the radio base station  20   a.  FIG. 7C shows a transmitted packet sequence to the radio channel  30   b  in the radio base station  20   b,  and FIG. 7D shows a transmitted packet sequence to the radio channels  30   a  and  30   b  in the radio terminal device  50 . In addition, FIG. 7E shows radio signal reception characteristics in the radio base stations  20   a.    
     [0084] In FIG. 7A to FIG. 7E also, time passes from left to right, similarly to FIG. 6A to FIG. 6E. Furthermore, also in the radio access communication system in accordance with the second embodiment, the two radio channels  30   a  and  30   b  are assumed as the usable radio channels. The radio base stations  20   a  and  20   b  transmit beacons each at definite beacon periods  210  and  220 . It will be assumed that the radio base station  20   a  always checks the received power levels of the packets received from the radio terminal device  50 .  
     [0085] First, it is assumed that the radio terminal device  50  is connected to the radio base station  20   a,  and that the data packets  100   a  and  100   b  to the radio terminal device  50  have been given to the radio base station  20   a  from the Ethernet (registered trademark)  60   a.  In this case, when receiving the data packets  100   a  and  100   b,  the radio base station  20   a  transmits the beacon  110   a,  and then transfers the data packets  120   a  and  120   b  to the radio channel  30   a  (see FIG. 7B). When receiving the data packets  120   a  and  120   b  normally, the radio terminal device  50  transmits the acknowledgment signals  130   a  and  130   b  as response signals, as shown in FIG. 7D.  
     [0086] In this example, as shown in FIG. 7E, received power levels  340   a  and  340   b  of the acknowledgment signals  130   a  and  130   b  received in the radio base station  20   a  are compared with the preset threshold value  320 . In this example, when the received power level  340   b  of the acknowledgment signal  130   b  is detected to be the preset threshold value  320  or less, the radio base station  20   a  transmits a state transition informing packet  330 , as shown in FIG. 7B. In this example, the state transition informing packet  330  informs of a transition to the state of the power saving mode. As a result, the state transition informing packet  330  functions as the transmission suppress signal.  
     [0087] When receiving the state transition informing packet  330 , in response to this, the radio terminal device  50  changes the setting to receive the packet of the radio channel  30   b.  In this way, the radio terminal device  50  can receive the beacon  140   b  transmitted by the radio base station  20   b,  and detects the presence of the radio base station  20   b.    
     [0088] Similarly to the case of FIG. 6A to FIG. 6E, the radio terminal device  50  detects that the received power level of the beacon  140   b  is the preset threshold value  320  or more, and decides to switch the connecting end from the radio base station  20   a  to the radio base station  20   b.  After this decision, the radio terminal device  50  again changes the setting to receive the packet of the radio channel  30   a.  In this state, the radio terminal device  50  receives the beacon  110   c  (see FIG. 7C) transmitted by the radio base station  20   a.  When detecting that the radio base station  20   a  is not retaining the packet addressed to the radio terminal device  50  from the beacon  110   c  received from the radio base station  20   a,  the radio terminal device  50  transmits the connection cancellation requesting packet  170  (see FIG. 7D).  
     [0089] When receiving the acknowledgment signal  180 , which is a response to the connection cancellation requesting packet  170 , from the radio base station  20   a,  the radio terminal device  50  again transmits the connection requesting packet  190  of the radio channel  30   b.  When receiving the acknowledgment signal  200  from the radio base station  20   b,  the radio terminal device  50  completes the connection switching processing.  
     [0090] As described above, in the second embodiment, the radio base station monitors the received power level of the acknowledgment signal from the radio terminal device  50 , and the radio terminal device  50  is put in a communication controlled state, in this example, the power saving mode when the received power level becomes the preset threshold value or less, thereby preventing a packet loss.  
     [0091] With reference to FIG. 8A to FIG. 8E showing the operational sequences of the radio base stations  20   a  and  20   b,  the radio access communication system in accordance with a third embodiment of the present invention will be described. FIG. 8A shows a received packet sequence from the Ethernet (registered trademark)  60   a  in the radio base stations  20   a,  and FIG. 8B shows a transmitted packet sequence to the radio channel  30   a  in the radio base station  20   a.  FIG. 8C shows a transmitted packet sequence to the radio channel  30   b  in the radio base station  20   b,  and FIG. 8D shows a transmitted packet sequence to the radio channels  30   a  and  30   b  in the radio terminal device  50 . FIG. 8E shows the radio signal reception characteristics in the radio terminal device  50 , similarly to FIG. 6E.  
     [0092] In FIG. 8A to FIG. 8E also, time passes from left to right. Also in this example, the two radio channels  30   a  and  30   b  are assumed as the usable radio channels. As shown in FIG. 8B and FIG. 8C, the radio base stations  20   a  and  20   b  transmit beacons each at the definite beacon periods  210  and  220 . The radio terminal device  50  always checks the received power levels of the received packets on the basis of the preset threshold value  320 .  
     [0093] First, the radio terminal device  50  is connected to the radio base station  20   a.  When receiving the data packets  100   a  and  100   b  from the Ethernet (registered trademark)  60   a,  the radio terminal device  50  transfers the data packets  120   a  and  120   b  to the radio channel  30   a  after transmitting the beacon  110   a.  The radio terminal device  50  receives the data packets  120   a  and  120   b  normally, and transmits the acknowledgment signals  130   a  and  130   b.    
     [0094] When detecting that the obtained received power level  230   c  is the preset threshold value  320  or less as a result of receiving the beacon  110   b  as shown in FIG. 8E, the radio terminal device  50  transmits the state transition informing packet  150  (see FIG. 8D), and informs the radio base station  20   a  of a transition to the state of the power saving mode. As a response to this, the radio base station  20   a  transmits the acknowledgment signal  310 . The radio terminal device  50 , which has received the acknowledgment signal  310 , starts the radio base station search  250 , and changes the setting to be able to receive the packet of the radio channel  30   b.  The radio terminal device  50  transmits a beacon requesting packet (control information requesting signal)  380  to the radio channel  30   b  (see FIG. 8D). Receiving this, the radio base station  20   b  transmits a beacon  370  (see FIG. 8C).  
     [0095] When detecting that the received power level  240   a  of the received beacon  370  is higher than the preset threshold value  320 , the radio terminal device  50  decides to switch the connecting end from the radio base station  20   a  to the radio base station  20   b.  Then, the radio terminal device  50  again changes the setting to receive the packet of the radio channel  30   a.  When receiving the beacon  110   c  transmitted by the radio base station  20   a  and detecting that the radio base station  20   a  is not retaining the packet addressed to the radio terminal device  50 , the radio terminal device  50  transmits the connection cancellation requesting packet  170  (see FIG. 8D).  
     [0096] In addition, when receiving the acknowledgment signal  180  from the radio base station  20   a,  the radio terminal device  50  again changes the setting to be able to receive the packet of the radio channel  30   b,  and transmits the connection requesting packet  190  to the radio base station  20   b.  Further, when receiving the acknowledgment signal  200  from the radio base station  20   b,  the radio terminal device  50  completes the connection switching processing.  
     [0097] The third embodiment described above is different from the first and second embodiments in that the radio terminal device actively transmits the beacon requesting packet  380  to the neighboring radio base stations when the radio terminal device is in the communication controlled state such as the power saving mode.  
     [0098] As above, the present invention has been described in connection with the three embodiments, and in any of the embodiments, the radio terminal device can prevent the packet loss due to the handover simply by providing a single transmission/receiving device.  
     [0099] It is needless to say that the present invention is not limited to the first to third embodiments, and that various modifications may be aimed within the scope of the present invention. For example, although the power saving mode function for controlling the packet transmission to the radio terminal device is used in the first to third embodiments, it is also possible to use the aforementioned congestion avoiding function for controlling the packet transmission to the radio terminal device. The radio base station and the radio terminal device perform channel access to the same radio channel using the multiaccess control method, however, it is possible to apply the radio channels at different frequencies to an up channel and a down channel.  
     [0100] The case of switching the two radio base stations has been described as an example of switching the connection of the radio terminal device, however, it is also possible to selectively switch three radio base stations or more. The radio terminal device judges a new connection end from the result of the radio base station search, however, it is also possible to inform the radio base station of the result of the radio base station search and leave the judgment to the radio base station.  
     [0101] The received power level is used as a judgmental standard of the reception characteristics, however, it is also possible to apply the bit error rate and packet error rate. In the first to third embodiments, the connection switching processing with the radio base station in the radio terminal device takes a procedure of starting a connection with a new radio base station after canceling the connection with the radio base station being connected to. However, the present invention can also apply a procedure of starting the connection immediately after detecting a new radio base station, and informing the new radio base station of entrance into the power saving mode, before canceling the connection with the former radio base station.  
     [0102] In the first to third embodiments, when changing the connection to a new radio base station, the radio terminal device once again switches the connection to the radio base station being connected to after detecting the new radio base station, in order to receive data buffered by the radio base station being connected to. However, it is also possible not to switch the connection to the radio base station being connected to, and perform connection processing with the new radio base station immediately after detecting the new radio base station, and further make the data, which is buffered by the radio base station formerly connected to, transferred to the new radio base station.  
     [0103] The effects of the present invention is that, in the radio access communication system performing the packet transmission asynchronously, the radio terminal device equipped with only one transmitter/receiver can search for the neighboring radio base stations using different frequencies, without causing a packet loss even while receiving data from the radio base station. This is because the radio terminal device transmits the transmission suppress signal representing the power saving mode or the like to the radio base station, and temporarily stops the transmission of packets addressed to the radio terminal device, and then searches for the neighboring radio base stations.