Abstract:
In a system including a parent station and a plurality of child stations connected only to the parent station, which is unable to detect a response among the plurality of child stations, the parent station includes a transmission permission sequential allocation mechanism for giving transmission permission sequentially and a first timer for monitoring a first period which is a response detection time period. The child station includes a transmission permission detection mechanism for detecting that the identifier sent by the parent station is directed to the own station. Each of the child stations transmits data in a band period when the child station is permitted to communicate. When the parent station does not receive a response from the child station permitted to communicate within the first period, the parent station permits a next child station to communicate.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a transmission equipment including an interface for transmitting ETHERNET (trade mark) packet data in LAN (Local Area Network), etc. as a user network interface (UNI). Particularly, this invention relates to a transmission equipment for transmitting variable length ETHERNET packet data in a PON (Passive Optical Network).  
           [0003]    2. Description of the related Art  
           [0004]    A PON system according to the related art is explained.  
           [0005]    [0005]FIG. 12 shows a block configuration chart for explaining an optical communication equipment, its communication method and a system in which its control program is written disclosed in Japanese Unexamined Published Patent Application Hei 11-98151.  
           [0006]    In FIG. 12, the optical communication equipment includes a station equipment  111  and a subscriber equipment  113 . The station equipment  111  is connected to an external work station  112 , and the subscriber equipment  113  is connected to a terminal  114 .  
           [0007]    The station equipment  111  has a function for changing band (band period or time band or time period) allocations dynamically. By this function, an unutilized band is shared among a plurality of subscribers who are members of band unassurance service instead of band assurance service for communications with the station equipment  111  in the PON system.  
           [0008]    In this configuration, the subscriber equipment  113  includes a PDS (Passive Double Star) terminator  131  for terminating an interface with the station equipment  111 , a terminal interface terminator  133  for terminating an interface with the terminal for the band unassurance service in a user side, a memory  132  for storing packet data which are converted between the PDS terminator  131  and the terminal interface terminator  133 , and a memory controller  134  for measuring a stored data amount in the memory  132  and sending a request for transmission to the PDS terminator  131  when it is necessary to send a signal to the station equipment through the PDS terminator  131 .  
           [0009]    Meanwhile, the station equipment  111  includes a PDS terminator  121  for terminating an interface with the subscriber equipment  113 , a controller  125  for terminating an interface with the external work station  112 , which sends control information, and for distributing necessary control information to each of function blocks in the equipment, a subscriber information storing unit  124  for storing information on a subscriber who has applied to use a shared band (band period) in the control information received from the controller  125 , a packet analyzer  123  for receiving the information on the subscriber from the subscriber information storing unit  124  and sending subscriber ID (Identification Data) and a polling instruction to the PDS terminator  121 , and a time division switch (TSW)  122 .  
           [0010]    Operations of this system are explained.  
           [0011]    The packet analyzer  123  sends the subscriber ID and the polling instruction to the PDS terminator  121 , and receives a transmission request  151  and the subscriber ID from the concerning subscriber equipment  113 . When the packet analyzer  123  receives the transmission request from the subscriber, the packet analyzer  123  sends a transmission permission signal  152  to the concerning subscriber equipment  113  queuing to transmit to the station equipment in the shared band period. Then, the packet analyzer  123  receives a packet signal returned from the subscriber equipment  113 .  
           [0012]    The packet analyzer  123  analyzes a packet length signal written in a packet overhead (LLC (Logical Link Control) part) in a first packet signal received from the subscriber equipment  113 , and determines a time period to be occupied by the concerning subscriber equipment  113  based on the packet length. Then, the packet analyzer  123  sends the transmission permission signal  152  to the subscriber equipment  113  indicating the time period to be occupied, and receives a packet signal from the subscriber equipment  113 .  
           [0013]    The PON system according to the related art is configured as above stated. When a parent station allocates bands (band periods) for transmission directed to the station terminal to a plurality of child stations, the parent station sends subscriber ID information and polling instruction information to an arbitrary child station, and receives transmission request information and subscriber ID information from the concerning subscriber equipment. Further, when the parent station receives the transmission request information, the parent station sends the transmission permission signal to the concerning subscriber equipment, and receives the packet signal from the concerning subscriber equipment sequentially. Further, the packet analyzer in the parent station analyzes the packet length signal written in the packet overhead (LLC part) in the first packet signal received from the subscriber equipment, and determines the time period to be occupied by the concerning subscriber equipment based on the packet length. Then, the parent station sends the transmission permission signal indicating the time period to be occupied, and receives the packet signal from the subscriber equipment. As stated, complex processing is necessary in the system according to the related art.  
         SUMMARY OF THE INVENTION  
         [0014]    It is one of objects of this invention to solve the above-stated problem. The complex processing of controlling the transmission sequentially by polling becomes unnecessary. According to this invention, a system and a method for controlling the allocations to the child stations in a short time can be realized in a simple configuration.  
           [0015]    According to an aspect of this invention, a packet communication system has a parent station and a plurality of child stations connected only to the parent station, which is unable to detect a response among each other, and transmits data in a band period. The packet communication system includes a first timer for monitoring a first period which is a response detection period in the parent station, and a transmission allocation circuit for setting an identifier of a child station permitted to transmit during the first period and informing the child station permitted, and for setting a next identifier of a next child station unless receiving a response in the parent station.  
           [0016]    According to another aspect of this invention, a packet communication method in a system having a parent station and a plurality of child stations connected only to the parent station, which is unable to detect a response among each other includes allocating transmission by setting an identifier of a child station permitted to transmit and informing the child station permitted sequentially by the parent station, monitoring a response of the child station for a first period by a first timer after permitting the child station to transmit, and updating the child station permitted by permitting a next child station unless receiving a response from the child station permitted to communicate within the first period.  
           [0017]    According to another aspect of this invention, a computer-readable storage medium having a computer program for a communication method in a system having a parent station and a plurality of child stations connected only to the parent station, which is unable to detect a response among each other, includes a code segment for allocating transmission by setting an identifier of a child station permitted to transmit and informing the child station permitted sequentially by the parent station, a code segment for monitoring a response of the child station for a first period by a first timer after permitting the child station to transmit, a code segment for updating the child station permitted by permitting a next child station unless receiving a response from the child station permitted to communicate within the first period, a code segment for monitoring a response of the next child station for a second period by a second timer in the parent station, and a code segment for updating the child station permitted unless receiving a response from the child station permitted to communicate within the second period.  
           [0018]    Further features and applications of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.  
           [0019]    Other objects features, and advantages of the invention will be apparent from the following description when taken in conjunction with the accompany drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 shows a configuration chart of a PON system in Embodiment 1 of this invention;  
         [0021]    [0021]FIG. 2 illustrates a frame signal directed to the subscriber equipment in communications between the parent station and the child station in Embodiment 1;  
         [0022]    [0022]FIG. 3 illustrates a frame signal directed to the station equipment in the communications between the parent station and the child station in Embodiment 1;  
         [0023]    [0023]FIG. 4 illustrates details of the frame signal directed to the subscriber equipment illustrated in FIG. 2;  
         [0024]    [0024]FIG. 5 illustrates details of the frame signal directed to a station equipment illustrated in FIG. 3;  
         [0025]    [0025]FIG. 6 shows a flow chart for explaining transmission permission allocation algorithm of a transmission permission sequential allocation controller of the parent station in Embodiment 1;  
         [0026]    [0026]FIG. 7 illustrates an example of a transmission allocation table in Embodiment 1;  
         [0027]    [0027]FIG. 8 shows a configuration chart of the PON system in Embodiment 2 of this invention;  
         [0028]    [0028]FIG. 9 illustrates an example of the transmission allocation table in Embodiment 4;  
         [0029]    [0029]FIG. 10 illustrates another example of the transmission allocation table in Embodiment 4;  
         [0030]    [0030]FIG. 11 illustrates another example of the transmission allocation table in Embodiment 4; and  
         [0031]    [0031]FIG. 12 shows an optical communication system according to the related art. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    Embodiment 1.  
         [0033]    A system configuration and operations in Embodiment 1 of this invention are explained.  
         [0034]    [0034]FIG. 1 shows a function configuration chart of the PON system which manages variable length ETHERNET packet for explaining the operations of this invention.  
         [0035]    In FIG. 1, a parent station  1 , a child station  2 , a star coupler  3  for connecting one parent station to a plurality of child stations by optical fibers, and an optical fiber  4  are illustrated.  
         [0036]    In the parent station  1 , an O/E·E/O (Optical/Electrical·Electrical/Optical) circuit  5  for converting an output electric signal to an optical signal and converting an input optical signal to an electric signal, a burst receipt circuit  17  for receiving a burst signal and interpolating a no signal period, an ETHERNET PHY (ETHERNET Physical Layer) processing circuit  18  for terminating a physical layer of an ETHERNET signal, an OAM (Operation Administration and Maintenance) packet/frame generation circuit  6  for generating a frame signal including an OAM packet directed to the subscriber equipment, an ETHERNET packet termination circuit  7  for terminating a packet signal sent from the child station, and a signal detection circuit  8  for detecting an OAM packet signal received from the child station are illustrated. Further, a transmission allocation circuit  9  permits an arbitrary child station to transmit the packet directed to the station equipment by setting an identifier (child station_ID) for specifying the child station in an area for transmission permission information in the OAM packet directed to the subscriber equipment.  
         [0037]    In the child station  2 , an O/E·E/O circuit  10  for converting an optical signal to an electric signal and converting an electric signal to an optical signal, an ETHERNET PHY processing circuit  16  for terminating the physical layer of the ETHERNET signal, an OAM packet/frame termination circuit  11  for terminating a frame signal including an OAM packet directed to the subscriber equipment, a transmission permission detection circuit  12  for comparing the identifier (child station_ID) for specifying the child station set in the area for the transmission permission information in the OAM packet directed to the subscriber equipment with an identifier (child station_ID) maintained by the own station, an overhead information detection circuit  13  for detecting overhead information in the OAM packet directed to the subscriber equipment to be attached to a beginning of the packet directed to the station equipment, a packet·buffer  14  for storing the ETHERNET packet signal directed to the station equipment input from an outside of the child station, and a packet generation/transmission circuit  15  for attaching an overhead to the packet signal and the OAM packet directed to the station equipment based on a packet transmission permission signal, and generating a packet signal with a header directed to the station equipment are illustrated.  
         [0038]    [0038]FIG. 2 shows a configuration chart for explaining a configuration of a frame directed to the subscriber equipment in the optical fiber  4  between the parent station  1  and the child station  2  in the PON system according to this embodiment.  
         [0039]    In FIG. 2, the frame directed to the subscriber equipment  41  repeated cyclically, an OAM packet  42  for indicating a beginning of the frame directed to the subscriber equipment, and an area for a user ETHERNET packet  43  are illustrated.  
         [0040]    [0040]FIG. 3 shows an example of an arrangement of a signal directed to the station equipment in the same optical fiber  4 .  
         [0041]    In the optical fiber  4 , the packet directed to the station equipment and the frame directed to the subscriber equipment are transmitted alternately. In FIG. 3, an overhead  45  attached to a beginning of a packet signal sent from each of the child stations, an OAM packet directed to the station equipment #n  46  included in the packet signal, an area for a user ETHERNET packet directed to the station equipment #n  47  included in the packet signal, an area for a user ETHERNET packet directed to the station equipment #i  48  included in the packet signal, and an area for a user ETHERNET packet directed to the station equipment #j  49  included in the packet signal are illustrated.  
         [0042]    [0042]FIG. 4 shows a detailed configuration of the frame signal directed to the subscriber equipment illustrated in FIG. 2.  
         [0043]    In FIG. 4, a packet signal directed to the subscriber equipment  51  is illustrated. Particularly, the OAM packet  42  includes an ETHERNET packet header  52  included in the packet signal directed to the subscriber equipment, a synchronous pattern  53  included in the packet signal, packet identification information  54  included in the packet signal, transmission permission information (child station_ID)  55  included in the packet signal, overhead information  56  included in the packet signal, an area for a message  57  included in the packet signal, a CRC (Cyclic Redundancy Check) operation result  58  for the area for the message, and FCS (Frame Check Sequence) information  59  applied in a time period between a previous OAM packet and a present OAM packet. Further, the area for the user ETHERNET packet  43  includes an idle signal  60  interpolated between the packet signals, a first user ETHERNET packet  61 , a second user ETHERNET packet  62 , a third user ETHERNET packet  63 , etc.  
         [0044]    [0044]FIG. 5 shows a more detailed configuration of the packet signal directed to the station equipment illustrated in FIG. 3.  
         [0045]    In FIG. 5, a packet signal directed to the station equipment  71  includes the OAM packet directed to the station equipment and the area for the user ETHERNET packet. An overhead  72  is attached to a beginning of the packet signal directed to the station equipment. The OAM packet  46  included in the packet signal directed to the station equipment includes an ETHERNET packet header  73 , packet identification information  74 , an area for the message  75 , a CRC operation result  76  for the area for the message, and FCS information  77  applied in a time period between a previous packet and a present packet. Further, the area for the user ETHERNET packet  47  includes an idle signal  78  interpolated between the packet signals, a fourth user ETHERNET packet  79 , a fifth user ETHERNET packet  80 , etc.  
         [0046]    [0046]FIG. 6 shows a flow chart of allocation algorithm for setting the transmission permission for the plurality of child stations  2  by the parent station  1  in Embodiment 1.  
         [0047]    In FIG. 6, a first timer and a second timer are important, but they are not illustrated in FIG. 1. The timers are provided in the transmission allocation circuit  9 , for example. A transmission allocation table 9c is also prepared in the transmission allocation circuit  9 .  
         [0048]    In FIG. 6, in Step  1  (hereinafter, abbreviated as S 1 , etc.), for permitting the n-th child station  2  registered in the parent station to transmit, an identifier (child station ID) corresponding to the n-th child station is specified in an area for indicating a transmission permission information in the OAM packet directed to the subscriber equipment. In S 2 , the first timer, i.e., the first timer setting a receipt period for detecting the packet directed to the station equipment, is started at the same time with the processing in S 1 . In S 3 , the parent station waits if the packet signal directed to the station equipment is received from the n-th child station permitted to transmit by the parent station. In S 4 , a judgement is made if the first timer started in S 2  has ended. In S 5 , if the parent station  1  has received the packet signal directed to the station equipment from the n-th child station  2  permitted to transmit by the parent station  1 , the first timer for setting the receipt period is stopped, and a second timer, i.e., second timer for controlling a transmission permission period, is started. In S 6 , a judgement is made if the second timer started in S 5  has ended. In S 7 , it is detected that the packet signal directed to the station equipment has been received from the n-th child station permitted to transmit by the parent station before the second timer started in S 5  ends. In S 8 , the packet signal directed to the station equipment has been received from the n-th child station  2  permitted to transmit, and the second timer for controlling the transmission permission period is stopped. In S 9 , the parent station refers to the transmission allocation table 9c, and changes the child station permitted to transmit from the n-th child station to another child station.  
         [0049]    With reference to FIG. 6, operations by the parent station  1  of setting the transmission permission sequentially for the plurality of child stations  2  are explained.  
         [0050]    The parent station  1  sets an identifier (child station_ID) for specifying the child station in the area for the transmission permission information in the OAM packet directed to the subscriber equipment for permitting the n-th child station among the plurality of registered child stations (S 1 ). At the same time, the parent station starts the first timer, i.e., first timer setting the period for detecting a response packet (S 2 ). A timer value of the first timer is a period set arbitrarily in considering a delay in a loop of a transmission line between the parent station  1  and the child station  2  in PON.  
         [0051]    The n-th child station  2  detects the identifier (child station_ID) in the area for the transmission permission information in the OAM packet directed to the subscriber equipment, and compares the identifier with the identifier maintained by the own station. If the identifiers are matched, it is judged that the transmission is permitted, and the packet signal directed to the station equipment is sent. The parent station waits if the packet signal directed to the station equipment is received from the n-th child station before the first timer detects the end of the period (S 3 ). When the parent station detects that the first timer has ended before the packet signal is received from the child station, the parent station changes the child station permitted to transmit to a next child station (S 9 ).  
         [0052]    When the parent station receives the packet signal from the child station permitted to transmit before the first timer ends, the parent station stops the first timer for continuing to permit the child station to transmit. At the same time, the second timer, i.e., the second timer for controlling the transmission permission period, is started (S 5 ). A timer value for the second timer is set to limit a transmission permission period for a respective child station. For assuring each of the child stations of a respective band (band period) for transmission directed to the station equipment, the parent station can set the value arbitrarily in considering a total number of the child stations and a content of service of each of the child stations  2  registered in the parent station  1 . While the parent station is receiving the packet signal from the child station permitted to permit, the parent station monitors if the second timer ends (S 6 ) and if the packet signal has been received from the child station (S 7 ). When the parent station detects that the second timer has ended while the packet signal is being received from the child station permitted to transmit, it is judged that a maximum time period allocated to the child station is exceeded. In that case, for terminating the transmission permission for the n-th child station, the child station permitted to transmit is changed from the n-th child station to a next child station (S 9 ).  
         [0053]    When it is detected that the packet signal has been received from the child station permitted to transmit, the second timer is stopped (S 8 ). Then, the child station permitted to transmit is changed from the n-th child station to another child station (S 9 ). For specifying the child station permitted to transmit, a child station data transmission allocation table as illustrated in FIG. 7 is prepared, and a transmission right is allocated sequentially.  
         [0054]    Overhead information is explained more in detailed.  
         [0055]    In the PON system configured as illustrated in FIG. 1, the OAM packet of the packet signal directed to the subscriber equipment  51  as illustrated in FIG. 4 includes an area for indicating the overhead information  56 . Further, at a beginning of the packet signal  71  directed to the station equipment as illustrated in FIG. 5, there is an area for the overhead  72 .  
         [0056]    For stabilizing operations of an optical receiver of the parent station, a certain length of an area for the overhead must be specified in the overhead information of the OAM packet in FIG. 4, and in the overhead attached to the beginning of the packet directed to the station equipment in FIG. 5. When the optical receiver of the parent station receives the packet signal directed to the station equipment from the child station, the optical receiver controls a gain of the optical receiver at first as a level of the optical signal differs due to a difference in a length of an optical fiber from the parent station to each of the child stations. After the gain is stabilized, the packet signal directed to the station equipment is regenerated as an electric signal. In a transition time period till the gain of the optical receiver is stabilized, the beginning of the packet signal directed to the station equipment is not regenerated, or an uncertain signal is generated. Therefore, an overhead corresponding to the time period till the gain of the optical receiver is stabilized is provided at the beginning of the packet signal directed to the station equipment.  
         [0057]    The parent station  1  provides an arbitrary header length information directed to the subscriber equipment for attaching to the beginning of the packet signal directed to the station equipment in the OAM packet, and the child station  2  sends the packet signal directed to the station equipment with a header of a length corresponding to the header length information. Further, also when time for stabilizing the gain of the optical receiver in the parent station differs, the header information corresponding to the time is changed and provided in the OAM packet such as OAM packet  42  directed to the subscriber equipment. Accordingly, the packet signal directed to the station equipment with a header in an arbitrary length can be sent to the parent station from the child station  2 .  
         [0058]    As stated, according to this invention, the parent station sets the identifier and sends the permission for communication to the child station permitted to communicate in a first period. If there is no response within the time period, the parent station permits the next child station to communicate.  
         [0059]    Therefore, in simple controlling, the parent station can send the permission for communication to the child stations sequentially and communicate with the child stations.  
         [0060]    Embodiment 2.  
         [0061]    A system in which one parent station controls a plurality of child stations which have various communication speeds is explained.  
         [0062]    [0062]FIG. 8 shows a configuration of the parent station in the packet communication system for managing the various communication speeds in this embodiment.  
         [0063]    In FIG. 8, a parent station  81 , a respective speed buffer  84  for storing the ETHERNET packet signal from an interface which has a various communication speed, and an OAM packet/frame generation circuit  85  for generating a respective speed OAM signal directed to the subscriber equipment by managing a various speed are illustrated. The parent station  81  reads the ETHERNET packet signal in a constant signal speed from the respective speed buffer  84  for storing the packet signal directed to the subscriber equipment based on the respective signal transmission speed. Then, an identifier (child station ID) for specifying the child station is set in the area for the transmission permission information in the OAM packet in the packet signal directed to the subscriber equipment for permitting a child station. In this case, the child station has an arbitrary communication speed among registered child stations which have various communication speeds. Then, the packet signal directed to the subscriber equipment including the OAM packet with the overhead information and the user ETHERNET packet signal is sent to the child station. The child station extracts the transmission permission information in the OAM packet. When the child station confirms that the information is matched, the child station sends the packet signal directed to the station equipment to the parent station.  
         [0064]    For assuring the child stations which have various communication speeds of bands (band periods) or communications in a determined amount, the parent station  1  sets the second timer and specifies information in the area for indicating the transmission permission information in considering the communication speeds.  
         [0065]    As stated, since the parent station includes the respective speed buffer  84  for storing the packet signal directed to the subscriber equipment based on a respective signal transmission speed, a packet communication system including a plurality of child stations which have various signal transmission speeds can be realized in this embodiment. Accordingly, work, e.g., moving the network, etc. due to an increase in a transmission capacity of the child station becomes unnecessary.  
         [0066]    Further, the time period for communicating with the child station can be determined respectively based on a various communication speed in communications with an outside of the system. Therefore, even if there is a plurality of child stations which have various communication speeds in the system, the system configuration can be simplified.  
         [0067]    Embodiment 3.  
         [0068]    In the PON system explained in each of the above-stated embodiments, child station_ID is used to identify the child station. As illustrated in FIG. 4, the child station is identified by setting child station_ID in the user ETHERNET packet as communication permission information. By corresponding VLANtag (Virtual LAN tag), MAC (Medium Access Control) address, IP (Internet Protocol) address, and MPLS Multi Protocol Label Switching) label to child station_ID as identification information, the ETHERNET signal and the network can be harmonized.  
         [0069]    As an actual configuration, a function for corresponding a destination address in a user packet to the MAC address, the IP address, the MPLS label, and ID of the child station is provided and a correspondence table is prepared to store the relationship in the OAM packet/frame generation circuit  6  in the parent station  1  and the overhead information detection circuit  13  in the child station. The packet is the ETHERNET packet in VLAN (Virtual LAN). When a packet in which the DA (destination address) and VID (identifier of the above VLAN) is described in a frame with a tag is received, the parent station specifies the child station based on the DA and the VID in the VLAN stored in the table. A SA (source address) in the user packet received from the child station is corresponded to the identifier of the child station in the parent station, and stored. For transmission to the child station, the DA in the packet is compared with the SA stored, and the child station to which the transmission is directed is specified.  
         [0070]    Further, since information in the user packet is used for identifying the child station, it is easy to connect to an upper network layer.  
         [0071]    Embodiment 4.  
         [0072]    A configuration for allocating a transmission period or a band period to a child station which has high priority transmission data or transmission data in a large amount is explained.  
         [0073]    In the PON system explained in Embodiment 1, the transmission right is allocated to the child station sequentially based on the child station data transmission allocation table as illustrated in FIG. 7. However, as illustrated in FIG. 9, by providing a minimum band assurance area and a best effort area in the child station data transmission allocation table, a minimum transmission band period can be assured for the signal directed to the station equipment sent from each of the child stations.  
         [0074]    Further, in the best effort area, a transmission right is allocated again only to a child station which sent a user packet when the transmission right was allocated to the child station before. Accordingly, it is possible to give the priority to the allocation of the band period to the child station which wants to transmit the data in a large amount. Hence, the transmission band period for the child station can be utilized more effectively.  
         [0075]    Allocation of the band period to each of the child stations which have various service grades is explained.  
         [0076]    For example, when the child stations have various service grades, an equal service grade area and a high speed service grade area are provided in the child station data transmission allocation table as illustrated in FIG. 10.  
         [0077]    Accordingly, the transmission band period can be allocated to the signal directed to the station equipment individually for each of the child stations.  
         [0078]    Similarly, when the child station maintains a high priority packet and a low priority packet, a high priority packet transmission area and a high and low priority packet transmission area (both packet transmission area) are provided in the child station data transmission allocation table. The priority is given to the transmission of the high priority packet. Transmission of the low priority packet is permitted after the high priority packet has been sent. Accordingly, it is possible to control the priority in the transmission of the signal from the child station. By controlling the priority in this way, emergency data can be sent in a time band in which a particular child station is not specified for sending emergency data, e.g., power down of an electric source, etc. which must be sent urgently besides regular communications. Further, by controlling the priority in this way, it is possible to send the emergency data by providing the time band for emergency communications for each of the child stations operating. By controlling the priority, it is possible to give the priority to the emergency data, e.g., power down of the electric source, etc. which must be sent urgently. Further, the areas of the table are determined and updated regularly based on actual usage conditions.  
         [0079]    Further, the band period is allocated to the transmission based on the transmission allocation table. Therefore, it is possible to control the allocation, e.g., band assurance, priority allocation, respective speed allocation, allocation based on the actual usage condition, etc. to the child stations flexibly.  
         [0080]    Embodiment 5.  
         [0081]    In each of the above embodiments, a specialized hardware element is provided for configuring the system. However, since micro processors and control program technique have been developed and improved, it is not difficult to configure a similar element by using a firmware stored in ROM (Read Only Memory) in which the control program is loaded and combining with a general micro processor.  
         [0082]    In this embodiment, a flow of operations illustrated in FIG. 6 is stored in a computer-readable medium as a firmware or program, which is not illustrated. It is also possible that the general computer reads from the stored medium for configuring characteristic elements illustrated in FIG. 1 in the computer and performing the operations illustrated in FIG. 6.  
         [0083]    In all the above embodiments, the PON system was explained. However, this invention is not limited to the PON system. This invention can be applied to any transmission method for performing packet communications. Further, the transmission line is not limited to the optical fiber. The transmission can be a private line, etc. as far as the packet can be sent.  
         [0084]    In this specifications, “store” and “indicate” means storing in the storing medium.  
         [0085]    Having thus described several particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is limited only as defined in the following claims and the equivalents thereto.