Abstract:
A communication method in a communication system which includes a master apparatus and plural slave apparatuses including at least first, second and third slave apparatuses, that are connected to communication medium and are associated with plural cyclic slots including at least first and second slots for triggering data transmission after a latency time, wherein data transmissions of the first and the second slave apparatuses are triggered by the first slot; and data transmission of the third slave apparatus is triggered by the second slot, the method including: sensing the communication medium by the master apparatus; and modifying, by the master apparatus, the first slave apparatus so as to trigger the data transmission of the first slave apparatus by a third slot which differs from the first and the second slots when the data transmission of the first slave apparatus triggered by the first slot is sensed in the sensing process.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a communication method, a communication apparatus, an integrated circuit, and a circuit module, which allows plural communication apparatuses sharing a communication medium to communicate with each other. 
         [0003]    2. Background Art 
         [0004]    In communication systems in which plural communication apparatuses sharing a communication medium communicate with each other, such as a power-line communication and a wireless LAN (Local Area Network), since the plural communication apparatuses communicate with each other through a single communication medium, it is necessary to avoid conflicts of accesses from the plural communication apparatuses. 
         [0005]    One method thereof is a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) method. In this method, communication apparatuses monitor the usage of a communication medium, transmit data when the other communication apparatuses do not use the communication medium, and start the transmission in a random wait time when sensing that the other communication apparatuses do not use the communication medium. Hereinafter, the random wait time is referred to as a back-off time. 
         [0006]      FIG. 25  shows an example of times when transmitting data using the CSMA/CA method. When communication apparatus A transmits packet PA 1  at time t 1 , communication apparatus B connected to the same transmission line (network) senses packet PA 1  at time t 2  in a carrier sensing time Ct and recognizes that the transmission line is used. When communication apparatus B intends to transmit data through the transmission line, communication apparatus B waits until packet PA 1  from communication apparatus A is ended. 
         [0007]    When sensing that packet PA 1  from communication apparatus A is ended at time t 3 , communication apparatus B waits for a time period (random back-off time) corresponding to a random number times a unit back-off time (slot time) St. Specifically, when the communication apparatus counts the slot times St and the count value reaches a random value generated at that time, the communication apparatus transmits packet PB 1 . In the example shown in  FIG. 25 , the random value is 8 and packet PB 1  is transmitted at time t 4  with the lapse of time St×8. The slot time St is set to be equal to or greater than the carrier sensing time Ct. Accordingly, when another communication apparatus having the smaller random value starts the transmission, it is possible to reliably sense the start of transmission from another communication apparatus before counting the slots. 
         [0008]    In the example shown in  FIG. 25 , since no carrier is sensed until counting the random value 8, communication apparatus B can transmit packet PB 1 . However, when another communication apparatus having the smaller random value than 8 exists, the communication apparatus senses a carrier before time t 4 , stops the counting, and waits until a packet from another communication apparatus is ended. The method of generating the random value is not fixed necessarily, and the maximum value of the random value to be generated may be changed depending on priority. For example, when the maximum value is increased at the time of re-transmission (returns to the original maximum value with the success of the re-transmission) or when the maximum value is decreased (returns to the original maximum value with the success of the transmission) by sensing the carrier of another communication apparatus during the wait and stopping the counting, it is possible to avoid the continuation of a state where one communication apparatus cannot communicate. The maximum value of the random value to be generated may be changed depending on the traffic condition of the transmission line. 
         [0009]    In this way, by employing the CSMA/CA method, when plural communication apparatuses intend to communicate through the same transmission line, the transmission can be started in the random back-off time, thereby reducing the occurrence of collision. However, when the upper limit is of the random value is fixed, the collision can easily occur with the increase of the number of accessing devices (the number of communication apparatuses connected to the network) and thus it is not possible to avoid the deterioration in performance. When the random value is increased with the increase of the number of connected devices, the probability of collision is reduced but the average value of the back-off time is increased, thereby deteriorating the performance. Since the access is performed in the random back-off time, it is not possible to define the maximum delay. 
         [0010]    In another method of avoiding the conflicts of accesses from plural communication apparatuses, an access right is set every slot (for example, see Patent Document 1). In this method, specific Individual ID numbers are assigned to all the communication apparatuses connected to a transmission line (network) (which is generally performed by a master terminal managing the communication of the network), all the nodes (communication apparatuses serving as a connection point to the network are hereinafter referred to as nodes) count suggestive slot number every constant time (slot time St), and a transmission right is acquired when the Individual ID number assigned to the node and is equal to the slot number. In this case, the slot time St is set to be equal to or greater than the carrier sensing time Ct. 
         [0011]      FIG. 26  shows an example of a timing when transmitting data using the slot access method. In the example shown in  FIG. 26 , the number of nodes connected to a network is 7 (for example, 7 communication apparatuses are connected) and Individual ID numbers 0 to 6 are assigned to the nodes, respectively. Accordingly, the slot number returns to 0 after counting 6. 
         [0012]    Here, when the node having the Individual ID number of 0 transmits packet P 01  at time t 1  and packet P 01  is ended at time t 2 , the nodes start the counting to count the slot number every slot time St. Since the packet (that is, the slot number of 0) of the node having the node number of 0 is being transmitted before time t 2 , the slot number is set to 1 at time t 2  and the nodes count the slot number every slot time St. At this time, when only the node having the Individual ID number of 4 intends to transmit data, the slot number of the node having the Individual ID number of 4 is counted as 4 and the transmission right is given to the node having the node number of 4 to transmit packet P 41  at time t 3  after the slot time St. 
         [0013]    Similarly, when packet P 41  is ended at time t 4 , the node having the node number of 3 transmits packet P 31  at time t 5  by counting the slot number. When packet P 31  is ended at time t 6 , the node having the node number of 4 transmits packet P 42  at time t 7  by counting the slot number. 
         [0014]    In this way, since the communication apparatus (node) having the transmission right is set every slot, the communication apparatuses can transmit data without any collision of data. However, when the number of connected communication apparatuses increases (when the number of set nodes increases), the overhead increases, thereby deteriorating the performance. Since the transmission right is given to the nodes every circulation of the slot number, it is not possible to set the priority of transmission. 
         [0015]    [Patent Document 1] JP-A-2004-320422 
       SUMMARY 
       [0016]    The invention is contrived to solve the above-mentioned problems. An object of the invention is to provide a communication method, a communication apparatus, an integrated circuit, and a circuit module, which can prevent the occurrence of collision to prevent the deterioration in performance even when the number of connected communication apparatuses increases and which can define the maximum delay of transmission to set the priority. 
         [0017]    According to an aspect of the invention, there is provided a communication method in a communication system which includes a master apparatus and plural slave apparatuses including at least first, second and third slave apparatuses, that are connected to communication medium and are associated with plural cyclic slots including at least first and second slots for triggering data transmission after a latency time, wherein data transmissions of the first and the second slave apparatuses are triggered by the first slot; and data transmission of the third slave apparatus is triggered by the second slot, the method comprising: sensing the communication medium by the master apparatus; and modifying, by the master apparatus, the first slave apparatus so as to trigger the data transmission of the first slave apparatus by a third slot which differs from the first and the second slots when the data transmission of the first slave apparatus triggered by the first slot is sensed in the sensing process. 
         [0018]    According to the above-mentioned invention, since the time for exclusive transmission is assigned to only a specific communication apparatus and the other communication apparatuses transmit data at a time to be used in common, an idle period can be reduced. Accordingly, even when the number of connected communication apparatuses increases, it is possible to prevent the occurrence of collision without deteriorating performance. The Individual ID numbers assigned to the communication apparatuses, particularly, the slave communication apparatuses, are arbitrary and is not fixed to the communication apparatuses. It is preferable that a fixed Individual ID number is assigned to the master terminal, but the Individual ID number may not be assigned fixedly when the master terminal only performs the management. 
         [0019]    According to another aspect of the invention, there is provided a communication apparatus connected via communication medium to plural slave apparatuses including at least first, second and third slave apparatuses, which are associated with plural cyclic slots including at least first and second slots for triggering data transmission after a latency time, wherein data transmissions of the first and the second slave apparatuses are triggered by the first slot; and data transmission of the third slave apparatus is triggered by the second slot, the communication apparatus comprising: a communication unit which communicates with the first, second and third slave apparatuses; and a control unit which senses the communication medium, wherein the control unit modifies the first slave apparatus so as to trigger the data transmission of the first slave apparatus by a third slot which differs from the first and the second slots when the control unit senses the data transmission of the first slave apparatus triggered by the first slot; wherein the control unit previously assigns the third slave apparatus with a first unique identifier associated with the second slot; and wherein the control unit assigns the first slave apparatus with a second unique identifier associated with the third slot so that the first slave apparatus is modified to trigger the data transmission by the third slot. 
         [0020]    According to another aspect of the invention, there is provided a communication apparatus connected via communication medium to a master apparatus and plural slave apparatuses including at least first and second slave apparatuses, which are associated with plural cyclic slots including at least first and second slots for triggering data transmission after a latency time, wherein data transmission of the second slave apparatus is triggered by the first slot; and data transmission of the third slave apparatus is triggered by the second slot, the communication apparatus comprising: a communication unit which communicates with the master apparatus and the first and second slave apparatuses; and a control unit which triggers data transmission by the first slot to transmit data to the communication medium, wherein when the control unit receives, from the master apparatus, a notification that the communication apparatus is to be modified so that the data transmission triggered by a third slot which differs from the first and the second slots, and a unique identifier associated with the third slot, the control unit modifies to trigger the data transmission by the third slot. 
         [0021]    According to another aspect of the invention, there is provided an integrated circuit, comprising: a communication unit which is connected to a communication medium via a coupler for serving as an interface connected via communication medium to plural slave apparatuses including at least first, second and third slave apparatuses, which are associated with plural cyclic slots including at least first and second slots for triggering data transmission after a latency time, wherein data transmissions of the first and the second slave apparatuses are triggered by the first slot; and data transmission of the third slave apparatus is triggered by the second slot, the communication unit communicating with the first, second and third slave apparatuses; and a control unit which senses the communication medium, wherein the control unit modifies the first slave apparatus so as to trigger the data transmission of the first slave apparatus by a third slot which differs from the first and the second slots when the control unit senses the data transmission of the first slave apparatus triggered by the first slot; wherein the control unit previously assigns the third slave apparatus with a first unique identifier associated with the second slot; and wherein the control unit assigns the first slave apparatus with a second unique identifier associated with the third slot so that the first slave apparatus is modified to trigger the data transmission by the third slot. 
         [0022]    According to another aspect of the invention, there is provided a circuit module comprising: a coupler for serving as an interface connected via communication medium to plural slave apparatuses including at least first, second and third slave apparatuses, which are associated with plural cyclic slots including at least first and second slots for triggering data transmission after a latency time, wherein data transmissions of the first and the second slave apparatuses are triggered by the first slot; and data transmission of the third slave apparatus is triggered by the second slot; a communication unit which is connected via the coupler to the communication medium and communicates with the first, second and third slave apparatuses; and a control unit which senses the communication medium, wherein the control unit modifies the first slave apparatus so as to trigger the data transmission of the first slave apparatus by a third slot which differs from the first and the second slots when the control unit senses the data transmission of the first slave apparatus triggered by the first slot; wherein the control unit previously assigns the third slave apparatus with a first unique identifier associated with the second slot; and wherein the control unit assigns the first slave apparatus with a second unique identifier associated with the third slot so that the first slave apparatus is modified to trigger the data transmission by the third slot. 
         [0023]    As can be apparently seen from the above description, according to the invention, it is possible to provide a communication method, a communication apparatus, an integrated circuit, and a circuit module, which can prevent the occurrence of collision to prevent the deterioration in performance even when the number of connected communication apparatuses increases and which can define the maximum delay of transmission to set the priority. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein like reference numerals designate like or corresponding parts throughout the several views, and wherein: 
           [0025]      FIG. 1  is a diagram schematically illustrating an example of a power-line communication system used to embody a communication method and a communication system according to an embodiment of the invention; 
           [0026]      FIGS. 2A ,  2 B and  2 C are diagrams illustrating an appearance of a PLC modem according to an embodiment of the invention; 
           [0027]      FIG. 3  is a diagram illustrating an example of the hardware of the PLC modem according to the embodiment of the invention; 
           [0028]      FIG. 4  is a diagram illustrating an example of a digital signal process in the PLC modem according to the embodiment of the invention; 
           [0029]      FIG. 5  is a diagram illustrating an example of a data transmission time in a power-line communication system according to the embodiment of the invention; 
           [0030]      FIG. 6  is a diagram illustrating an example of the data transmission time in the power-line communication system according to the embodiment of the invention; 
           [0031]      FIG. 7  is a diagram illustrating an example of a master terminal management table in the power-line communication system according to the embodiment of the invention; 
           [0032]      FIG. 8  is a diagram schematically illustrating a configuration of a buffer provided every priority of data in the PLC modem according to the embodiment of the invention; 
           [0033]      FIG. 9  is a diagram illustrating a queue search period of transmission data and a slot generating time according to the embodiment of the invention; 
           [0034]      FIG. 10  is a diagram illustrating an example of a random back-off in the power-line communication system according to the embodiment of the invention; 
           [0035]      FIGS. 11A and 11B  are diagrams illustrating another example of the random back-off in the power-line communication system according to the embodiment of the invention. 
           [0036]      FIGS. 12A and 12B  are diagrams schematically illustrating structures of a data frame and a control frame, respectively, in the power-line communication system according to the embodiment of the invention. 
           [0037]      FIG. 13  is a diagram illustrating an example of an assigned information format in a beacon according to the embodiment of the invention; 
           [0038]      FIGS. 14A ,  14 B and  14 C are diagrams illustrating an individual ID number assigning sequence in the power-line communication system according to the embodiment of the invention; 
           [0039]      FIG. 15  is a diagram illustrating a common time when all the PLC modems can transmit data in the power-line communication system according to the embodiment of the invention; 
           [0040]      FIGS. 16A ,  16 B and  16 C are diagrams illustrating an individual ID number deleting sequence in the power-line communication system according to the embodiment of the invention. 
           [0041]      FIG. 17  is a diagram schematically illustrating an operation flow of a PLC modem serving as a master terminal in the power-line communication system according to the embodiment of the invention; 
           [0042]      FIG. 18  is a diagram illustrating an example of a slave terminal management table in the power-line communication system according to the embodiment of the invention; 
           [0043]      FIG. 19  is a diagram schematically illustrating an operation flow of a PLC modem serving as a slave terminal in the power-line communication system according to the embodiment of the invention; 
           [0044]      FIG. 20  is a diagram illustrating a transmission and reception flow of a PLC modem serving as a master terminal in the power-line communication system according to the embodiment of the invention; 
           [0045]      FIG. 21  is a diagram illustrating a transmission and reception flow of a PLC modem serving as a master terminal in the power-line communication system according to the embodiment of the invention; 
           [0046]      FIG. 22  is a diagram illustrating an example of a congestion back-off in the power-line communication system according to the embodiment of the invention; 
           [0047]      FIG. 23  is a diagram illustrating a multi segment structure having plural PLC modems as another example of the power-line communication system according to the embodiment of the invention; 
           [0048]      FIG. 24  is a diagram illustrating an example of a random back-off in the multi segment structure shown in  FIG. 23 ; 
           [0049]      FIG. 25  is a diagram illustrating an example of a time when transmitting data using a CSMA method; and 
           [0050]      FIG. 26  is a diagram illustrating an example of a time when transmitting data using a slot access method. 
       
    
    
     DETAILED DESCRIPTION 
       [0051]    Hereinafter, embodiments of the invention will be described with reference to the accompanying drawings. 
         [0052]      FIG. 1  is a diagram schematically illustrating an example of a power-line communication system for embodying a communication method and a communication system according to the invention. The power-line communication system shown in  FIG. 1  includes plural PLC (Power Line Communication) modems  100 M,  100 T 1 ,  100 T 2 ,  100 T 3 , and  10 T 4  connected to a power line  900 . Five PLC modems are shown in  FIG. 1 , but the number is of connected modems is arbitrary. The PLC modem  100 M serves as a master terminal and manages connection states (link states) of the PLC modems  100 T 1 , . . . ,  100 T 4  serving as a slave terminal. 
         [0053]    In the following description, the master terminal and the specific slave terminals are described as the PLC modems  100 M,  100 T 1 ,  100 T 2 ,  100 T 3 , and  100 T 4  and a general slave terminal is described as the PLC modem  100 T. 
         [0054]    The PLC modem not limited to the master terminal and the slave terminals is described simply as the PLC modem  100 . 
         [0055]    The power line  900  is shown as one line in  FIG. 1 , but two or more lines. The PLC modem  100  is connected to the lines. 
         [0056]    As described in detail later, the PLC modem  100  includes a LAN modular jack such as RJ45. A television (TV)  51 , a personal computer (PC)  52 , an IP phone  53 , a video recorder  54 , and a broadband router  55  are connected to the modular jack. The broadband router  55  is connected to the Internet. The power-line communication system is an example of a communication system for embodying the communication method according to the invention and may be another communication system such as a wireless LAN. 
         [0057]      FIG. 2  is a diagram illustrating an appearance of the PLC modem  100 , where  FIG. 2A  is an external perspective view illustrating the front side,  FIG. 2B  is a front view, and  FIG. 2C  is a rear view. The PLC modem  100  shown in  FIG. 2  includes a chassis  101  and as shown in  FIGS. 2A and 2B , and a display unit  105  including LEDs (Light Emitting Diodes)  105 A,  105 B, and  105 C is disposed in the front surface of the chassis  101 . As shown in  FIG. 2C , a power source connector  102 , a LAN modular jack  103  such as a RJ45, and a switch  104  for switching an operation mode, etc. are disposed in the rear surface of the chassis  101 . The power cable (not shown in  FIG. 2 ) is connected to the power source connector  102  and a LAN cable (not shown in  FIG. 2 ) is connected to the modular jack  103 . The PLC modem  100  may further include a Dsub (D-Subminiature) connector for connection to a Dsub cable. 
         [0058]      FIG. 3  is a block diagram illustrating an example of the hardware of the PLC modem  100 . The PLC modem  100  includes a circuit module  200  and a switching power source  300  as shown in  FIG. 3 . The switching power source  300  serves to supply various voltages (for example, +1.2 V, +3.3 V, and +12 V) to the circuit module  200  and includes, for example, a switching transformer and DC-DC converter (which are not shown). 
         [0059]    The circuit module  200  includes a main IC (Integrated Circuit)  210 , an AFE IC (Analog Front End Integrated Circuit)  220 , an Ethernet PHY IC (Physical Layer Integrated Circuit)  230 , a memory  240 , a low-pass filter (LPF)  251 , a driver IC  252 , a band-pass filter (BPF)  260 , and a coupler  270 . The switching power source  300  and the coupler  270  are connected to the power connector  102  and are connected to the power line  900  through a power cable  600 , a power plug  400 , and a socket  500 . The main IC  210  serves as a control circuit for performing the power line communication. 
         [0060]    The main IC  210  includes a CPU (Central Processing Unit)  211 , a PLC MAC (Power Line Communication Media Access Control layer) block  212  and a PLC PHY (Power Line Communication Physical layer) block  213 . The CPU  211  includes a 32-bit RISC (Reduced Instruction Set Computer) processor. The PLC MAC block  212  serves to manage a MAC layer of a transmission/reception signal and the PLC PHY block  213  serves to manage a PHY layer of the transmission/reception signal. The AFE IC  220  includes a DA converter (DAC)  221 , an AD converter (ADC)  222 , and a variable gain amplifier (VGA)  223 . The coupler  270  includes a coil transformer  271  and coupling capacitors  272   a  and  272   b . The CPU  211  controls the operations of the PLC MAC block  212  and the PLC PHY block  213  by the use of data stored in the memory  211  and also controls the entire operations of the PLC modem  100 . 
         [0061]    The communication using the PLC modem  100  is schematically performed as follows. Data input from the modular jack  103  ser sent to the main IC  210  through the Ethernet PHY IC  230  and a digital transmission signal is generated by performing a digital signal process. The generated digital transmission signal is converted into an analog signal by the DA converter (DAC)  221  of the AFE IC  220  and is output to the power line  900  through the low-pass filter  251 , the driver IC  252 , the coupler  270 , the power connector  102 , the power cable  600 , the power plug  400 , and the socket  500 . 
         [0062]    The signal received from the power line  900  is sent to the band-pass filter  260  through the coupler  270 , the gain thereof is adjusted by the variable gain amplifier (VGA)  223  of the AFE IC  220 , and then the resultant signal is converted into a digital signal by the AD converter (ADC)  222 . The converted digital signal is sent to the main IC  210  and is converted into digital data by performing the digital signal process thereon. The converted digital data are output from the modular jack  103  through the Ethernet PHY IC  230 . 
         [0063]    An example of the digital signal process performed by the main IC  210  is described now. The PLC modem  100  performs a multi carrier communication using plural sub-carriers in an OFDM (Orthogonal Frequency Division Multiplexing) method, etc. The digital process of converting transmission data into an OFDM transmission signal and converting the OFDM reception signal into reception data is performed mainly by the PLC PHY block  213 . 
         [0064]      FIG. 4  is a functional block diagram illustrating an example of the digital signal process performed by the PLC PHY block  213 , where an OFDM transmission using a wavelet transform is performed. As shown in  FIG. 4 , the PLC PHY block  213  serves as a conversion controller  10 , a symbol mapper  11 , a serial-parallel converter (S/P converter)  12 , an inverse wavelet transformer  13 , and wavelet transformer  14 , a parallel-serial converter (P/S converter)  15 , and a demapper  16 . 
         [0065]    The symbol mapper  14  serves to convert bit data to be transmitted into symbol data and to perform a symbol mapping (for example, PAM modulation) on the basis of the symbol data. The S/P converter  12  serves t convert mapped serial data into parallel data. The inverse wavelet transformer  16  serves to inverse-wavelet transform parallel data into data in a time axis and to generate a series of sample values indicating transmission symbols. The data are sent to the DA converter (DAC)  221  of the AFE IC  220 . 
         [0066]    The wavelet transformer  14  serves to discrete-wavelet transform the received digital data (a series of sample values sampled at the same sample rate as transmission) obtained from the AD converter (ADC)  222  of the AFE IC  220  into a value on a frequency axis. The P/S converter  15  serves to convert parallel data on the frequency axis into serial data. The demapper  16  serves to acquire reception data by calculating amplitudes of sub carriers and determining the reception signal. 
         [0067]    In transmitting data in the power-line communication system shown in  FIG. 1 , as shown in  FIG. 5 , when the PLC modem  100  as a transmission source transmits data frames Pt 1  to Pt 4  and Pt 0  and the PLC modem  100  as a transmission destination normally receives the frames, it transmits acknowledgement frames At 1  to At 4  and At 0 . Information for managing the communication between the PLC modems is transmitted with a constant interval as notification frames (control frame; beacon) B 1 , B 2 , B 3 , . . . from the PLC modem  100 M. Here, Pt 0  and At 0  are a data frame and an acknowledgement frame which are transmitted and received at a common time to be described later. 
         [0068]    In the example shown in  FIG. 5 , the data frames Pt 1  to Pt 4  and Pt 0  from the plural communication apparatuses are transmitted without collision, but the PLC modem  100  make an access at the times shown in  FIG. 6  so as to transmit data without collision.  FIG. 6  shows an example of transmission times of data from the PLC modem  100 . In the example shown in  FIG. 6 ,  10  PLC modems of the PLC modem  100 M (master terminal  100 M) and the PLC modems  100 T 1  to  100 T 9  are connected to the power line  900 , but only the transmission times of three PLC modems  100 T 1  to  100 T 3  are shown in  FIG. 6 . The process of controlling the transmission times is performed by the main IC  210  shown in  FIG. 3 . 
         [0069]      FIG. 7  shows a schematic configuration of a terminal (PLC modem) management table of the PLC modem  100 M. In the terminal management table of the master terminal, assigned (effective) Individual ID numbers and counter values indicating the latest transmission time of the corresponding PLC modem are stored to correspond to management numbers (where the management numbers are 0, T 1 , T 2 , . . . to correspond to the reference numerals and the MCA addresses are described) of the PLC modems connected to the power line. The counter values are described later. The management numbers of the PLC modems may be a simple number or only a MAC address. Alternatively, both may be stored. The terminal management table is stored in a memory (for example, the memory  240  shown in  FIG. 3 ) of the PLC modems. The Individual ID numbers serves to allow only the assigned PLC modem to transmit data at the specific time corresponding to the respective Individual ID numbers. 
         [0070]    The total number of individual ID numbers is predetermined regardless of the number of connected PLC modems  100  and the greater number of Individual ID numbers than the predetermined number are not assigned. In this example, four Individual ID numbers in maximum are assigned. For example, the Individual ID number=1 is fixedly assigned to the PLC modem  100 M as the master terminal and the other Individual ID numbers are assigned to the other PLC modems T 1  to T 9  under the control of the PLC modem  100 M. The Individual ID number fixedly assigned is not limited to 1, but may be a different value. The Individual ID number may not be fixedly assigned to the master terminal. At the time shown in  FIG. 7 , the Individual ID numbers of 2, 3, and 4 are assigned to the PLC modem  100 T 2 , the PLC modem  100 T 1 , and the PLC modem  100 T 3 , respectively. The Individual ID numbers may be described simply as an ID number. As described later, in order to specify the time when all the PLC modems can transmit data, the common ID number may be set to the Individual ID number of 0 and the transmittable time corresponding to the respective IDs (the Individual ID numbers and the common ID number are comprehensively called access ID numbers) may be described as a slot. The common ID number is not limited to 0, but for example, the Individual ID number fixedly assigned to the master terminal may be set to 0 and the common ID number may be set to 1. 
         [0071]    When the PLC modem returns an acknowledgement frame, the acknowledgement frame may include information on the access ID numbers. By allowing the acknowledgement frame to include the access ID numbers, it is possible to notify the transmission times to the PLC modems (also referred to as hidden terminals) which cannot directly receive the data. 
         [0072]    In  FIG. 6 , when the PLC modem  100 T 1  transmits packet PT 11  at time t 1 , the PLC modems  100 T 2  and  100 T 3  senses packet PT 11  at time t 2  after the carrier sensing time (for example, 25 μsec) Ct. Accordingly, it is possible to sense that the transmission line is in use (although not shown in  FIG. 6 , the other PLC modems  100 T 4  to  100 T 9  connected to the power line to constitute the network can also sense it). When the PLC modems  100 T 2  and  100 T 3  intend to transmit data through the transmission line, the PLC modems  100 T 2  and  100 T 3  wait until packet PT 11  from the PLC modem  100 T 1  is ended. In this way, when transmitting data, the PLC modem  100  checks the communication state of the transmission line and checks whether the transmission line is in use. 
         [0073]      FIG. 8  schematically shows a configuration of a buffer provided every priority of data in the PLC modem.  FIG. 9  shows queue search times of transmission data and slot generating times. For example, as shown in  FIG. 1 , the television  51 , it is assumed that the personal computer  52 , and the IP phone  53  transmit and receive different types of data and the plural types of data are relayed to the broadband router  55  through the PLC modem  100 M. In this case, as shown in  FIG. 8 , the PLC modem  100 M receives data having different priorities (priorities 0 to 7), such as voice data, video data, and control data, from the broadband router  55 . The data input to the PLC modem are distributed to the queues of the priorities and stored in the buffers corresponding to the queues. At the time of outputting the data, the queue search is performed to secure the QoS and the data are taken and output in the order in which the priorities decrease. The priorities of the data are determined by an application treating the data. For example, the application of a real-time voice system such as an IP phone sets the priority to be high so as to prevent the delay. A web access and the like set the priority to be low, because the influence of the delay is small. 
         [0074]    At the time of outputting data, the queue search period is provided after the data frame shown in  FIG. 9  or after the acknowledgement frame (Ack) corresponding to the data frame. The queue of the transmission data is searched out for the queue search period and the data are selected and output sequentially from the highest priority. Accordingly, even when data having high priority  7  is input just before the queue search period, the data of priority  7  can be output prior, thereby enhancing the precision in priority control. After the queue search period, slots are generated as slot times and the counting for the back-off described later is started. 
         [0075]    Referring to  FIG. 6  again, when the end of packet PT 11  is sensed at time t 3 , the PLC modems  100  start the counting and count the slot number every slot time St (for example, 20 μsec). The PLC modems wait for the transmission until the slot corresponding to the Individual ID number assigned thereto after a random time (referred to as a time (a random back-off time) corresponding to the random number times the slot time St (hereinafter, referred to as random back-off)). In this example, the random back-off of the PLC modem  100 T 2  is “4” and the random back-off of the PLC modem  100 T 3  is “2.” 
         [0076]      FIG. 10  shows an example of the random back-off. In  FIG. 10 , a sum of an offset (fixed portion) and a random (random portion) is an actual back-off value. In this case, the offset is decreased as the priority increases. When the priorities based on the types of voice data, video data, and control data are 3, the fixed portion is 4 and the random portions are 0 to 3. Accordingly, the values of 4, 5, 6, and 7 are randomly generated. The random values are calculated at time t 3  by the PLC modems  100 . 
         [0077]    Since packet PT 11  is a packet having the Individual ID number of 3, the slot number is set to “4” at time t 3  and is cyclically counted. Since the Individual ID number of the PLC modem  100 T 3  is “4” but the random back-off time (described as BO in  FIG. 6 ) does not pass, it is not determined as a transmittable time. Since it is sensed at time t 4  that the slot having the slot number of 2 passes and the random back-off time passes, the PLC modem  100 T 2  having the Individual ID number of 2 transmits packet PT 21 . Packet PT 21  is sensed at time t 5  after the carrier sensing time Ct, the slot number is not counted. Accordingly, the packets based on the other access ID numbers are not transmitted and thus the collision of the packets does not occur. 
         [0078]    When the end of packet PT 21  is sensed at time t 6 , the slot number is set to “3” and the counting thereof is started. Since it is sensed at time t 7  after the random back-off time that the slot having the slot number of 4 passes, packet PT 31  is transmitted. It is not essential to wait for the random back-off time and it may be omitted. The random back-off time may be changed depending on the number of priorities or assigned Individual ID numbers and a condition of interference with another network. 
         [0079]      FIG. 11  shows another example of the random back-off. In the table shown in  FIG. 11 , the random back-off may be calculated from matrices of a difference (priority difference) between the assigned priority and the maximum priority and the number of active IDs (the number of terminals to which the master terminal assigns the Individual ID number). As the back-off value of the random back-off, a fixed portion of the offset shown in  FIG. 11A  and a random portion shown in  FIG. 11B  are calculated. Here, the random portion indicates the upper limit and is a random number from 0 to the upper limit. In this case, as the priority difference is smaller, the back-off value is smaller. The back-off value is changed depending on the number of active IDs, and the back-off value is set smaller as the number of active IDs. Accordingly, when the priority difference is small, the back-off time can be decreased, thereby reducing the loss due to the back-off. When the priority difference is great, the back-off time can be increased, thereby enhancing the precision in priority control. When the number of active IDs is small, the back-off time can be decreased to suppress the unnecessary back-off, thereby realizing the priority control with the minimum back-off time. In the example shown in  FIG. 11 , similarly to the example shown in  FIG. 10 , the random back-off may be calculated using the priority instead of the priority difference. The back-off value may be not changed but fixed on the basis of the number of active IDs. 
         [0080]    As described above, since the upper limit of the number of assigned Individual ID numbers is set to a predetermined number, the slot period is not greater than a predetermined time, thereby advance the transmission time from the PLC modem  100  intending to transmit data. In general, since all the PLC modems  100  hardly intend to simultaneously transmit data, there is no problem even when the Individual ID numbers are assigned to only some PLC modems  100 . Although the transmission times of the PLC modems  100 T 1  to  100 T 3  to which the Individual ID numbers are assigned are shown in  FIG. 6 , the transmission of data is performed with the slot having the slot number  0  when the PLC modems  100  to which the Individual ID numbers are not assigned among the other PLC modems  100  intend to transmit data. 
         [0081]    The information of the master terminal management table shown in FIG.  7  includes the Individual ID numbers assigned to the slave terminals, the identification information (for example, MAC addresses) for identifying the communication apparatuses to which the Individual ID numbers are assigned, and the counter values counted every predetermined time from the start of the corresponding communication apparatus and indicating the time when data are finally transmitted from the communication apparatus. For example, the counter value is counted every 1 μsec. 
         [0082]    The control frame is notified by allowing the PLC modem  100 M to transmit the control frame to the other PLC modems  100 T 1  to  100 T 9 . The structure of the control frame is shown in  FIG. 12A . In the control frame, the control information containing the Individual ID number assignment information is added to data subsequent to a preamble and a header. The Individual ID number assignment information includes the newly assigned Individual ID numbers, the identification information for identifying the communication apparatuses to which the access information is assigned, and the number of effective Individual ID numbers among the Individual ID numbers, thereby reducing the notification data. As described later, when the assigned Individual ID number is deleted, the assigned communication apparatus notifies as the control information the deleted Individual ID number, the identification information for identifying the communication apparatus to which the Individual ID number is assigned, and the number of effective Individual ID numbers among the Individual ID numbers. 
         [0083]      FIG. 13  is a diagram illustrating an example of an assigned information format in a beacon. The beacon as the notification frame may include the assigned information format for notifying the assigned information shown in  FIG. 13 . With the assigned information format, information on the maximum priority corresponding to the Individual ID numbers or the common ID number of the entire communication apparatuses and the MAC addresses and the number of active IDs can be periodically transmitted. Here, the assigned information format includes an update counter, a maximum priority (M.P.: Max Priority), the number of active IDs (A.N.: Active Num), and MAC addresses (MAC addr) arranged in the order of Individual ID numbers. Tag, Length, and the like for indicating other information are also provided. With the number of active IDs, the slave PLC modems can sense the slot period. The slot period is obtained by adding 1 to the number of active IDs (adding the assigned period to the common ID number). The maximum priority is the value of the maximum priority sensed by the master terminal in the previous beacon period. The update counter is a counter value counted up when the ID assignment information is updated. The MAC address includes 15 MAC addresses (from ID=1 to ID=15) in the order of assigned Individual ID numbers. Here, the MAC address is NULL (=0) when the Individual ID number is not assigned. 
         [0084]    The slave PLC modem compares the update counter stored in the memory register with the update counter of the beacon, checks the assigned information of the Individual ID numbers when it is updated, and acquires the number of active IDs and the maximum priority every time. When the Individual ID number is not assigned to the slave PLC modem as the check result at the time of reception, the slave PLC modem opens the Individual ID number at once. Only the changed Individual ID number is notified in notifying the assigned information using the above-mentioned control frame, but all the Individual ID numbers (or all the MAC addresses) are notified in notifying the assigned information using the beacon. By using the beacon, it is not necessary to confirm the reception of information individually using the control frame, thereby enhance the efficiency. By always notifying the newest information, it is possible to early return to the original state even in an abnormal state where the information is different between the terminals. The notification of the assigned information using the beacon may be performed with a predetermined period, not every beacon period. Instead of the MAC addresses, any information may be used so long as it is information for identifying the terminals. 
         [0085]    The data frame transmitted from the PLC modem  100  includes information indicating the slot (a transmission time corresponding to the Individual ID number or a transmission time corresponding to the common ID number) at which the data frame is transmitted. An example of the structure of the data frame is shown in  FIG. 12B . In the data frame, data to be transmitted is included subsequent to the preamble and the header. For example, the Individual ID number or the common ID number, priority information, and the like are added to the header of the data frame. Accordingly, the transmission line can simply sense the slot number of the packet to be transmitted. The portion to which the Individual ID number or the common ID number is added is not limited to the header, but may be a data portion and is limited to a portion (portion of which the modulation method and the encryption are known and in which the reception data can be demodulated by another communication apparatus) which can skim the PLC modems  100 T other than the destination of the data frame. Accordingly, since it can be sensed whether data can be transmitted from the destination to the transmission source or a communication apparatus other than the destination with common modulation method and encryption, the master terminal can sense the slave terminal communicating with it and the type of data without acquiring information on the communication directly from the source or the destination, thereby managing the priority of the communication data. 
         [0086]    Next, the Individual ID number assigning process and the assigned Individual ID number deleting process of the master PLC modem  100 M will be described. 
         [0087]      FIG. 14  shows the schematic sequence of the Individual ID number assigning process. It is assumed that the slave PLC modem  100 Tx intends to transmit data to another slave PLC modem  100 Ty. Then, since the Individual ID number is not assigned to the slave PLC modem  100 Tx, the transmission of data is started at the common time when all the PLC modems can transmit data (see S 11 ). At this time, the common ID number “0” is added to the header of the data frame. 
         [0088]      FIG. 15  shows the common time when all the PLC modems can transmit data.  FIG. 15  shows an example where the random back-off is 5. The communication apparatus with the common ID (Individual ID number=0) transmits data of PT 01  at time t 12  which is a time of the Individual ID number of “0” after time  11  when the random back-off time has passed in the slot after the time t 3 . 
         [0089]    The data frame is also sensed by the master PLC modem  100 M (see S 12 ). In this way, the master PLC modem  100 M monitors the entire communication and assigns the Individual ID number to the slave PLC modem  100 Tx having transmitted the data frame when sensing the communication using the common ID number. The assignment is performed on the smaller number of PLC modems than the PLC modems  100  connected to the power line and is unconditionally performed when the smaller number of Individual ID numbers than a predetermined number are assigned. When the predetermined number of Individual ID numbers are assigned already, the assignment may be performed after the assigned Individual ID number is deleted under the below-mentioned condition, or the Individual ID number used in transmitting the data frame may be deleted and the Individual ID number may be assigned. 
         [0090]    When the Individual ID number is assigned, as shown in  FIG. 14B , the notification frame (control frame, beacon, and the like) is notified from the master PLC modem  100 M (see S 30 ). At this time, the transmission of data from the slave PLC modem  100 Tx to the slave PLC modem  100 Ty is performed at the time of the common ID number (see S 21 ). The control frame includes the newly assigned Individual ID number, the identification information for identifying the communication apparatus to which the individual access information, and the number of effective individual ID numbers among the individual ID numbers. When the control frame is notified from the master PLC modem  100 M, the slave PLC modem  100 Tx sets the notified individual ID number and the transmission of the data frame thereafter is performed at the time corresponding to the notified individual ID number (see S 41 ). 
         [0091]    Like the control frame, a copy of the access ID number of the previously received data frame may be stored in a special frame which is transmitted when the communication medium is secured using a method (for example, the beacon region, the VoIP (Voice over Internet Protocol) Reservation, the TDMA (Time Division Multiple Access), and the like) other than the above-mentioned method. In this case, the side receiving the frame does not check the overlapping of the access ID included in the frame with its individual ID number. Accordingly, the time using the effective individual ID number is maintained and thus the unfair assignment of the individual ID number is prevented. That is, the access ID number of the frame operating in this method before entering the secured region is handed over. 
         [0092]      FIG. 16  shows a schematic sequence of the process of deleting the individual ID number assigned to the slave PLC modem  100 T. Here, as shown in  FIG. 16A , it is assumed that the individual ID number of “2” is assigned to the slave PLC modem  100 Tx, the individual ID number of “4” is assigned to the slave PLC modem  100 Ty, and the access ID period is “5” (a state where five individual ID numbers including the common ID are assigned). By subtracting the counter value at the times of the final transmission corresponding to the individual ID numbers in the terminal management table managed by the master terminal from the counter value indicating the current time, the counter value corresponding to the time from the time of final transmission to the current time is obtained. 
         [0093]    In this state, when the slave PLC modem  100 Tx does not transmit data for a predetermined time (β), the slave PLC modem  100 Tx senses the fact and deletes the individual ID number of “2”. Specifically, the Individual ID number stored in the memory (for example, the memory  240  shown in  FIG. 3 ) in the slave PLC modem  100 Tx is deleted and is rewritten into the common ID number of “0” (see  FIG. 16B ). 
         [0094]    When the transmission of data is not performed for a predetermined time (α) (β&gt;α), the master PLC modem  100 M monitoring the entire communication can sense the fact and thus deletes the individual ID number of “2” set for the PLC modem  100 Tx in the terminal management table. Since the deleted individual ID number is smaller than the maximum value “4” of the individual ID number corresponding to the access ID period=5, the individual ID number “2” is assigned to the PLC modem  100 T to which the individual ID number of “4” is assigned and the access ID period is decreased by 1 so as to be the access ID period=4. 
         [0095]    It is notified to all the slave PLC modems  100 T using the notification frame S 50  that the PLC modem  100 T to which the individual ID number “2” is assigned is changed and that the individual ID number of the changed PLC modem  100 T, the identification information for identifying the communication apparatus to which the individual access information is assigned, and the is number of effective individual ID numbers “4” among the individual ID numbers are changed (see  FIG. 16B ). As a result, the set state shown in  FIG. 16C  is obtained. 
         [0096]    Next, the operation of the PLC modems  100  constituting the power-line communication system shown in  FIG. 1  will be described with reference to  FIGS. 17 to 21 .  FIG. 17  is a diagram illustrating an operation flow of the PLC modem serving as a master terminal,  FIG. 19  is a diagram illustrating an operation flow of the PLC modem serving as a slave terminal, and  FIGS. 20 and 21  are diagrams illustrating a flow of the transmission/reception operation of the PLC modem serving as a slave terminal. 
         [0097]    The operation flow of the master PLC modem will be described now. 
         [0098]    In step S 101  of  FIG. 17 , the PLC modem  100 M determines whether a data frame is received. When a data frame is received, the PLC modem searches the terminal management table for the management number of the PLC modem as a transmission source of the data frame (step S 102 ). When the management number of the PLC modem as the transmission source is not registered in the terminal management table (determination of step S 103 ), the process of step S 111  is performed. Here, the registration means authentication in which the MAC addresses are exchanged in advance to allow the master terminal and the slave terminal to communicate with each other. The master terminal monitors all the data frames and senses the data frame in any one of a communication between the master terminal and the slave terminal and a communication between the slave terminals. 
         [0099]    When the management number is registered (it is generally determined that it is registered in step S 103 ), the counter value is recorded as the time stamp for receiving the data frame in the terminal management table in step S 104 . Then, it is determined whether the access ID number of the data frame is the common ID number (that is, ID=0) (step S 105 ). When it is not the common ID number, the process of step S 111  is performed. 
         [0100]    When the access ID number is the common ID number, it means that the individual ID number is not assigned and thus the individual ID number assigning process is performed. Prior to the process, it is determined in step S 106  whether the individual ID number is assigned already to the PLC modem  100 T. This determination step is to prevent the assignment of a new individual ID number when the individual ID number is assigned already but the control frame (notification frame) is not transmitted and the PLC modem  100 T is not sensed yet. When the individual ID number is assigned already, the control frame for notifying the assignment of the assigned individual ID number is prepared in step S 110  and the control frame is transmitted in step S 109 . 
         [0101]    When it is determined in step S 106  that the individual ID number is not assigned to the PLC modem  100 T, it is determined in step S 107  whether a new individual ID number can be assigned. When the predetermined number of Individual ID numbers are already assigned, a new individual ID number cannot be assigned and thus the process of step S 111  is performed. When a new individual ID number can be assigned, the assignment process is performed, the control frame for notifying the result is prepared, and the terminal management table is updated again (step S 108 ). The prepared control frame is transmitted in step S 109 . 
         [0102]    When the data frame is not received (the determination of step S 101 ), when the PLC modem  100 T as the transmission source is not registered in the terminal management table (determination of step S 103 ), and when the access ID number of the received data frame is not the common ID number (determination of step S 105 ), the counter values corresponding to all the PLC modem and stored as the time stamp in the terminal management table are checked in step S 111 . That is, for all the registered PLC modems, by subtracting the counter value indicating the time of final transmission corresponding to the respective individual ID numbers in the terminal management table managed by the master terminal from the counter value indicating the current time, the counter value corresponding to the time from the time of final transmission to the current time is calculated to check the passed time. 
         [0103]    When the passed time from the time of final transmission is equal to or greater than the predetermined time (α) (determination of step S 112 ), the individual ID number of the PLC modem (terminal) to which the individual ID number is assigned is deleted in step S 113 . In step S 114 , the control frame for notifying the individual ID number assignment state after the deletion is prepared and the terminal management table is updated. The prepared control frame is transmitted (step S 109 ). 
         [0104]    When there is no PLC modem to which the individual ID number of which the passed time from the time of final transmission is equal to or greater than the predetermined time (α) is assigned (determination of step S 112 ), the control frame is transmitted in step S 109 . In this case, the control frame does not include information on the assignment of the individual ID number. 
         [0105]    Since the master PLC modem  100 M performs such an operation, the individual ID number is set for the PLC modem  100 T having transmitted data using the common ID number. When data are not transmitted for the predetermined time (α), the assigned individual ID number is deleted. Then, when the assignment of the individual ID number is changed, the details are notified to all the PLC modems  100 T using the control frame. 
         [0106]    The operation flow of the slave PLC modem will be described now. 
         [0107]      FIG. 18  shows a schematic configuration of the terminal management table of the slave PLC modem. The slave terminal copies and stores the assigned information (access IDs such as individual ID numbers, MAC addresses, and the like) in the beacon transmitted from the master terminal in the management table shown in  FIG. 18 . The counter value associated with the access ID when the slave communication apparatus transmits data finally is stored along with the terminal assignment information. The slave terminal may store only its terminal assignment information and the counter value at the time of final transmission. 
         [0108]    In step S 201  shown in  FIG. 19 , the slave PLC modem  100 T determines whether the assignment of the individual ID number is ended, and sets the individual ID number as the access ID number for determining the time for transmitting the data frame when the assignment is ended (step S 203 ). When the acquisition of the individual ID number is not ended, the common ID number (ID=0) is set as the access ID number for determining the time for transmitting the data frame (step S 202 ). Then, the frame transmitting and receiving process is performed (step S 204 ). The details of the frame transmitting and receiving process will be described later. 
         [0109]    When the frame transmitting and receiving process is ended, the counter value corresponding to the time from the time of final transmission to the current time is calculated by subtracting the counter value at the time of final transmission of the PLC modem  100 T as the transmission time stamp from the counter value indicating the current time, and then it is determined whether the predetermined time (β) passes (step S 205 ). When the predetermined time (β) passes, the acquired individual ID number is deleted (step S 206 ) and the process of step S 207  is performed. When the predetermined time (β) does not pass, the process of step S 207  is directly performed. 
         [0110]    In step S 207 , it is determined whether the control frame (including the beacon) is received from the master PLC modem  100 M and the procedure is ended when the control frame is not received. When the control frame is received, the number of effective individual ID numbers among the individual ID numbers included in the control frame is acquired (step S 208 ) and it is determined whether the individual ID number is assigned to itself (step S 209 ). When the individual ID number is assigned to itself, the individual ID number is acquired and the procedure is ended (step S 210 ). 
         [0111]    When the individual ID number is not assigned (determination of step S 209 ), it is determined whether the value of the stored individual ID number is greater than the access ID period (step S 211 ). When it is greater than the access ID period, it is determined that the stored individual ID number is erroneous, the individual ID number is deleted, and the procedure is ended (step S 212 ). 
         [0112]    The details of the frame transmitting and receiving process of step S 204  will be described now with reference to  FIGS. 20 and 21 . In step S 301  shown in  FIG. 20 , it is determined whether a data frame is received. When a data frame is not received, the process of step S 307  is performed. When a data frame is received, it is determined whether the access ID number of the data frame is its own individual ID number (step S 302 ). When the access ID number is its own individual ID number, an error can be determined and thus the individual ID number is deleted in step S 303 . 
         [0113]    When the access ID number of the data frame is not its own individual ID number and when the individual ID number is deleted, it is determined in step S 304  whether the destination of the received data frame is itself. When the destination of the data frame is not itself, the process of step S 307  is performed. 
         [0114]    When the destination of the received data frame is itself, the access ID number of the data frame is set in the acknowledgement frame in step S 305  and the acknowledgement frame is transmitted (step S 306 ). Then, in step S 307 , it is determined whether data to be transmitted remains. When data to be transmitted do not remain, the procedure is ended. When data to be transmitted remains, it is determined in step S 308  whether the back-off expires. When the back-off does not expire, the procedure is ended. This wait time is called a congestion back-off. 
         [0115]    The congestion back-off is to avoid a continuous collision of transmitted frames (particularly, the easy occurrence of collision in access using the common ID number).  FIG. 22  shows an example of the congestion back-off. As shown in  FIG. 22 , the number of continuous collisions (the collisions are sensed on the basis of the fact that the acknowledgement frame cannot be received) changes the range of values of the random back-off. 
         [0116]    In the congestion control, when the salve terminal transmits a data frame but senses that the acknowledgement frame does not return from the transmission destination, the master PLC modem or the slave PLC modem generates a random number from 0 to the upper limit to set the back-off. When the common ID number is not used, the first upper limit starts from 0 (no back-off). When the common ID number is used and unicast data of which the acknowledgement frame returns is transmitted, the first upper limit starts from 7. On the other hand, when the common ID number is used and multi-cast data or broad-cast data of which the acknowledgement frame does not return, a value of 31 is used as the first upper limit. When the common ID number is used, a value other than 0 is used as the first upper limit. 7 or 31 as the upper limit is only an example. When the number of repeating collisions is 1 or more, the same upper limit is set regardless of the type of the ID. Then, the back-off value is increased depending on the number of repeating collisions. When the PLC modems simultaneously using the common ID number exist, it is possible to reduce the collisions by the use of the congestion back-off. Particularly, since the multi-cast data or the broad-cast data has no acknowledgement frame, the congestion cannot be sensed and the number of collisions is maintained in 0. When the back-off is set to 0, the repeating collisions occur. However, by setting the great back-off value, it is possible to reduce the number of repeating collisions. 
         [0117]    When it is determined in step S 308  that the collision back-off expires, the random back-off value is calculated in step S 309  and it is determined whether data transmitted from another PLC modem exist (step S 310 ). When data are transmitted from another PLC modem  100 , the procedure is ended. 
         [0118]    The random back-off value is used to set a random number of transmission times depending on the priority by giving different back-off values depending on the priority set by the PLC modem  100 . 
         [0119]    When data are not transmitted from another PLC modem  100 , the expiration of the random back-off is waited for (step S 311 ). When the random back-off expires, the process of step S 312  is performed. Then, when the transmission of data from another PLC modem  100  is sensed, the procedure is ended. When the transmission of data from another PLC modem  100  is not sensed, the sensing of its own slot (transmittable time determined by the set individual ID or the common ID) is waited for (step S 313 ). 
         [0120]    When its own slot is sensed in step S 313 , the data frame is transmitted using the slot (step S 314 ). Then, the reception of an acknowledgement frame is waited for (step S 315 ). When the acknowledgement frame is received, the number of continuous collisions is set to 0 (step S 316 ), the congestion back-off is set (step S 318 ), the counter value indicating the transmission time is recorded (step S 319 ), and the procedure is ended. 
         [0121]    When the acknowledgement frame is not sensed, the number of continuous collisions is increased, the congestion back-off is set (step S 318 ), the counter value indicating the transmission time is recorded (step S 319 ), and the procedure is ended. 
         [0122]      FIG. 23  shows another example of the power-line communication system and is a diagram illustrating a multi segment structure including plural PLC communication systems. Here, a PLC communication system  1  in which a master terminal  100 M 1 , a slave terminal  100 T 3 , and a slave terminal  100 T 4  can communicate with each other and a PLC communication system  2  in which a master terminal  100 M 2  and a slave terminal  100 T 2  can communicate with each other are connected to the same power line  900 . The PLC communication system  1  is a PC system using a personal computer  52  and IP phone  53  and the PLC communication system  2  is a TV system using a television  51  and video recorder  54 . The multi segment structure means a structure in which plural systems are located adjacent to each other such as a structure in which plural PLC communication systems are connected to the same power line. 
         [0123]    In the multi-segment structure, the random back-off is set as shown in  FIG. 24 . In this case, the back-off values of the fixed portion and the random portion are determined using only the priority and the number of active IDs or the priority difference is not used. That is, with reference to the fixed portion and the random portion corresponding to its own priority, the number of slots for which it is waited as the random back-off is calculated by generating a random number from 0 to the value of the random portion and adding the generated random number to the fixed value of the fixed portion. Since the master terminals of the PLC communication systems can manage only the corresponding systems. Accordingly, when the back-off is set to be optimal for the corresponding system, a deviation is generated between the networks and the collision frequently occurs. As a result, the priority control using the priority cannot be embodied. Therefore, by using the above-mentioned random back-off, it is possible to embody the priority control even when plural networks are mixed. 
         [0124]    According to the invention, even when the number of connected communication apparatuses increases, it is possible to prevent the occurrence of collision to prevent the deterioration in performance and to define the maximum delay of transmission. Accordingly, the invention is useful for a communication method, a communication system, and a communication apparatus therefor, which can set the priority. 
         [0125]    This application is based upon and claims the benefit of priority of Japanese Patent Applications No. 2007-147259 filed on Jun. 1, 2007 and No. 2008-34332 filed on Feb. 15, 2008, the contents of which are incorporated herein by reference in its entirety.