Abstract:
Provided is an optical access system comprising: an optical line terminal connected to another network; a plurality of optical network units, each connected to a user terminal; and at least one of an optical switching unit and an optical splitter, which is installed between the optical line terminal and the plurality of optical network units. The optical line terminal allocates a length of time to a discovery phase for detecting the plurality of optical network units, and a length of time to data transmission phases for transferring data from the plurality of optical network units; and changes a ratio of the length of time of the discovery phase to the length of time of the data transmission phases so that the length of time of the discovery phase is shortened in the case where a number of the optical network units that are registered in the optical line terminal increase.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese patent application JP 2008-274085 filed on Oct. 24, 2008, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     This invention relates to an optical access system and, more particularly, to a technique of executing discovery of optical network units quickly. 
     The recent propagation of Fiber To The Home (FTTH) which uses an optical fiber has increased the speed of access networks. A representative example of FTTH is a passive optical network (PON) system. 
     A PON system has, as illustrated in  FIG. 2 , a plurality of (N) optical network units (ONU)  20 , which communicate with user terminals  10 , and an optical line terminal (OLT)  40 , which communicates with a backbone network  60  via a gateway  50 . The plurality of ONUs  20  and the OLT  40  are connected to each other via an optical splitter  80 , which is a passive device requiring no power feeding. The PON system can thus implement an inexpensive access system  70 . 
     For instance, IEEE 802.3ah standardizes Ethernet-PONs (EPONs) in which data is transferred between an OLT and at least one ONU in conformity to the Ethernet (registered trademark; hereinafter, Ethernet®). Also, the standardization of 10G-EPONs in which the transmission speed is raised to 10 Gbps is in progress (see IEEE 802.3av). 
     Discovery processing executed in the optical access system as illustrated in  FIG. 2  is processing in which a shared optical communication path is used to detect the ONU  20  that has not been registered, the detected ONU  20  is registered, and the communication distance (Round Trip Time (RTT)) between the ONU  20  and the OLT  40  is measured. The discovery processing allows the optical splitter or optical switch to control communication sessions such that overlapping is avoided. The OLT  40  ultimately discriminates a registered ONU  20  from an unregistered ONU  20  by its logical link ID (LLID). The sequence of a discovery procedure in conventional EPONs and 10G-PONs is illustrated in  FIG. 4 . 
     In the discovery processing, the ONUs  20  have not been registered and the OLT  40  first uses a Discovery GATE message SIG 20  to check the presence of the ONUs  20  that do not have assigned LLIDs. The Discovery GATE message SIG 20  is distinguished from a normal GATE message by setting “1” to a discovery flag in the message. An identifier defined for broadcast is used as the LLID and a multicast address is used as the destination MAC address. 
     The Discovery GATE message SIG 20  sent from the OLT  40  travels through the optical splitter  80  and reaches every ONU  20  that is connected to the optical splitter  80 . Receiving the Discovery GATE message SIG 20 , the unregistered ONUs  20  to which LLIDs have not been assigned each send a REGISTER_REQ message SIG 30  in order to request the OLT  40  to execute registration. 
     The plurality of REGISTER_REQ messages SIG 30  have to be prevented from bumping into one another in the section between the optical splitter  80  and the OLT  40 , but the collision cannot be avoided completely. To lower the chance of collision, each unregistered ONU  20  sends the REGISTER_REQ message SIG 30  at a time point T 3 , which is reached after a random time period elapses since a transmission start time point T 2  written in the Discovery GATE message SIG 20 . 
     When the REGISTER_REQ message SIG 30  is received within a time period defined as a discovery window, the OLT  40  obtains the MAC address of the ONU  20  from which this REGISTER_REQ message SIG 30  has been sent, and manages the association relation between the obtained MAC address of the ONU  20  and an LLID. The OLT  40  also starts processing for assigning the LLID to this ONU  20 . 
     The OLT  40  notifies the assigned LLID to the ONU  20  by sending a REGISTER message SIG 40  in which the MAC address of this ONU  20  is set as the destination MAC address and the LLID is written. The ONU  20  that has this destination MAC address receives the REGISTER message SIG 40  and obtains the assigned LLID. From then on, the assigned LLID is contained in the preamble of a frame sent from the ONU  20 , to thereby enable the OLT  40  to identify the source ONU  20 . Also, the LLID contained in the preamble of a frame that is sent from the OLT  40  enables the ONU  20  to determine whether the frame is destined to itself. 
     Thereafter, in order to measure the round trip time RTT between the OLT  40  and the ONU  20 , the OLT  40  sends a GATE message SIG 50  in which the ONU  20  is specified by its LLID, a multicast address is set as the destination MAC address, and “0” is set to the discovery flag. 
     The ONU  20  that has the specified LLID receives the GATE message SIG 50  and extracts time information (time stamp) T 6  and a transmission start time point (grant start time) T 7  from the GATE message SIG 50 . The time stamp T 6  is set to a clock of the ONU  20 . When the set clock hits the grant start time T 7 , the ONU  20  sends a REGISTER_ACK message SIG 60  to the OLT  40 . 
     The OLT  40  receives the REGISTER_ACK message SIG 60  at a time point T 8  by its own clock. From T 8  and from T 7  contained in the received REGISTER_ACK message SIG 60 , the OLT  40  calculates the round trip time RTT between the OLT  40  and the ONU  20  (RTT=T 8 −T 7 ). 
     Through the sequence described above, the registration (LLID assignment) and measurement of the communication distance RTT are finished for one ONU  20 . When there are a plurality of unregistered ONUs, one Discovery GATE message SIG 20 , a plurality of REGISTER_REQ messages SIG 30 , a plurality of REGISTER messages SIG 40 , a plurality of GATE messages SIG 50 , and a plurality of REGISTER_ACK messages SIG 60  are exchanged in a single discovery sequence. A plurality of ONUs are registered by exchanging these messages repeatedly. 
     How frequently the discovery processing is executed is not regulated by IEEE 802.3ah and IEEE 802.3av, and varies from practice to practice. Commonly, as illustrated in  FIG. 3 , a given period of time is sectioned into N phases one of which serves as a discovery phase, with remaining N-1 phases serving as data transmission phases, and processing of the given period of time is repeated. 
     SUMMARY OF THE INVENTION 
     Next, how long it takes to complete discovery for all the ONUs  20  is examined. In the case where the distance between each ONU  20  and the OLT  40  is 20 km, RTT between the OLT  40  and the ONU  20  is about 200 microseconds. When the length of time of a single discovery phase is 1 millisecond and a plurality of ONUs  20  send the REGISTER_REQ messages SIG 30  within a discovery window  550  in response to the Discovery GATE message SIG 20  of a discovery handshake illustrated in  FIG. 4 , the time left to exchange the remaining messages, namely, the REGISTER messages SIG 40 , the GATE messages SIG 50 , and the REGISTER_ACK messages SIG 60 , is about 800 microseconds, which is only long enough to register four ONUs  20  at most. When there are thirty-two ONUs  20  to handle and the discovery phase  500  of  FIG. 3  is executed once in two hundred and fifty phases, in other words, once for every two hundred and forty-nine data transmission phases  501 , the discovery of the last ONU  20  is completed at 1.751 seconds (250 milliseconds×(32/4−1) times+1 millisecond) from the start of the discovery of the first ONU  20 . This result is shown in a field for Case  1  in a table  901  of  FIG. 7 . 
     An even longer time is required before the discovery of the last ONU  20  is completed in the case where the optical access system has many ONUs  20  and the distance between the OLT  40  and the ONUs  20  is long. For example, in the case where there are a hundred and twenty-eight ONUs  20  and the distance between the OLT  40  and each ONU  20  is 40 km, RTT between the OLT  40  and the ONU  20  is about 400 microseconds. When the length of time of a single discovery phase is 1 millisecond in the discovery handshake of  FIG. 4 , only one ONU  20  is registered within one discovery phase. Accordingly, the discovery of the last ONU  20  is completed at 31.751 seconds (250 milliseconds×127 times+1 millisecond) from the start of the discovery of the first ONU  20 . This result is shown in a field for Case  4  in the table  901  of  FIG. 7 . 
     In  FIG. 7 , the distance between the OLT  40  and the ONUs  20  has two different values, 20 km and 40 km, and the number of the ONUs  20  managed by the OLT  40  has two different values,  32  and  128 .  FIG. 7  illustrates, in addition to the table  901  in which the discovery phase is executed once in two hundred and fifty phases as described above, a table  902  in which the discovery phase is executed once in a hundred phases, a table  903  in which the discovery phase is executed once in ten phases, and a table  904  in which the discovery phase is executed once in five phases. 
     An optical access system in which the number of the ONUs  20  managed by the OLT  40  is large and the distance between the OLT  40  and the ONUs  20  is long scores high in terms of practicality but has a problem in that it takes long to complete discovery for all the ONUs  20  as described above. Discovery could be completed for all the ONUs  20  in a short period of time by increasing the frequency of executing a discovery phase as shown in the tables  902 ,  903 , and  904  of  FIG. 7 . However, it would give rise to another problem of the relative reduction in data transmission phase ratio and the resultant lowering in the efficiency of data transmission as illustrated in  FIG. 6 . 
     As countermeasures, methods focusing attention on the discovery window within the discovery phase have been proposed in which the window width of the discovery window or the cycle of generating the discovery window is dynamically changed (see JP 2004-201099 A, for example). 
     It is therefore an object of this invention to finish discovery for all ONUs in an optical access system within a given period of time (or to cut short the time required to complete discovery for all ONUs) while preventing the efficiency of data transmission in data transmission phases from dropping. 
     A representative aspect of this invention is as follows. That is, there is provided an optical access system comprising: an optical line terminal which is connected to another network; a plurality of optical network units, each of which is connected to a user terminal; and at least one of an optical switching unit and an optical splitter, which is installed between the optical line terminal and the plurality of optical network units. The optical line terminal allocates a length of time to a discovery phase for detecting the plurality of optical network units, and a length of time to data transmission phases for transferring data from the plurality of optical network units; and changes a ratio of the length of time of the discovery phase to the length of time of the data transmission phases so that the length of time of the discovery phase is shortened in the case where a number of the optical network units that are registered in the optical line terminal increase. 
     According to an embodiment of this invention, discovery can be completed for all ONUs within a given period of time (or the time required to complete discovery for all ONUs can be cut short) and, at the same time, the efficiency of data transmission is prevented from dropping. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein: 
         FIG. 1  is a timing chart illustrating an example of changing a frequency of executing a discovery phase according to a first embodiment of this invention; 
         FIG. 2  is a diagram illustrating an example of a configuration of an optical network system according to the first embodiment of this invention; 
         FIG. 3  is a timing chart illustrating a data transmission phase according to the first embodiment of this invention; 
         FIG. 4  is a sequence diagram illustrating a discovery handshake of a conventional EPON system; 
         FIG. 5  is a flow chart illustrating a processing of changing the frequency of executing the discovery phase according to the first embodiment of this invention; 
         FIG. 6  is an explanatory diagram illustrating a relation between a discovery phase frequency and data transmission efficiency of the conventional EPON system; 
         FIG. 7  is a an explanatory diagram illustrating a relation between the discovery phase frequency, a distance between an OLT and an ONU, and a time of discovery completion of the conventional EPON system; 
         FIG. 8  is an explanatory diagram illustrating tables for determining a threshold for the frequency of executing the discovery phase according to the first embodiment of this invention; 
         FIG. 9  is a diagram illustrating an example of a configuration of an ONU according to the first embodiment of this invention; 
         FIG. 10  is a block diagram illustrating an example of a configuration of an OLT according to the first embodiment of this invention; 
         FIG. 11  is a block diagram illustrating an example of a configuration of an optical splitter according to the first embodiment of this invention; 
         FIG. 12  is a timing chart illustrating an example of changing a length of time of executing a discovery phase according to a second embodiment of this invention; 
         FIG. 13  is a flow chart illustrating a processing for changing a length of time of executing the discovery phase according to the second embodiment of this invention; 
         FIG. 14  is a diagram illustrating another example of a configuration of an optical network system according to the first embodiment of this invention; 
         FIG. 15  is an explanatory diagram illustrating tables for determining a threshold for a frequency of the discovery phase according to the second embodiment of this invention; 
         FIG. 16  is a block diagram illustrating a configuration example of an optical switch of a third embodiment of this invention; 
         FIG. 17  is a flow chart illustrating a processing for changing a length of time of executing a discovery phase according to a fourth embodiment of this invention; and 
         FIG. 18  is a flow chart for illustrating a processing for changing a time between the discovery phases according to the fourth embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An outline of an embodiment of this invention is given first. 
     An optical access system according to the embodiment of this invention has a plurality of optical network units (ONUs)  20 , which communicate with user terminals, an optical line terminal (OLT)  40 , which communicates with a backbone network  60  via a gateway  50 , and an optical splitter  80  (or an optical switching unit (OSW  30 )), which connects the plurality of ONUs  20  and the OLT  40 . Different time intervals between discovery phases are set for different numbers of the registered ONUs  20 . The OLT  40  sets a max time  510  of all ONUs discovery completion. When the max time  510  of all ONUs discovery completion elapses, the OLT  40  chooses a time interval between discovery phases that is suitable for the number of the ONUs  20  that have finished registration by that point, and executes discovery processing at the chosen time interval. The time interval between discovery phases is set small in the case where the number of the registered ONUs  20  is small with respect to the maximum number of the ONUs  20  managed by the OLT  40 . In the case where the number of the registered ONUs  20  is large, on the other hand, the time interval between discovery phases is set large. 
     In an optical access system according to another embodiment of this invention, different lengths of time of a single discovery phase are set for different numbers of the registered ONUs  20 . The OLT  40  sets the max time  510  of all ONUs discovery completion. When the max time  510  of all ONUs discovery completion elapses, the OLT  40  chooses a time pf a single discovery phase that is suitable for the number of the ONUs  20  that have finished registration by that point, and executes discovery processing at the chosen time. The time of the single discovery phase is set long in the case where the number of the registered ONUs  20  is small with respect to the maximum number of the ONUs  20  managed by the OLT  40 . In the case where the number of the registered ONUs  20  is large, on the other hand, the time of the single discovery phase is set short. 
     First Embodiment 
     Described in a first embodiment of this invention is a method of cutting short the time required to complete discovery for all ONUs by dynamically changing the time interval between discovery phases (discovery phase frequency). 
       FIG. 2  is a diagram illustrating a configuration example of an optical network system according to the first embodiment of this invention. 
     An optical access system  70  has the optical network units (ONUs)  20 , the optical splitter  80 , and the optical line terminal (OLT)  40 . 
     The optical access system  70  has as many optical network units (ONUs)  20  as the number of users, and the plurality of ONUs  20  are respectively connected to user terminals  10  to communicate with the user terminals  10 . The optical line terminal (OLT)  40  is connected to the gateway  50  to communicate with the backbone network  60  via the gateway  50 . The plurality of ONUs  20  and the OLT  40  are connected to each other via the optical splitter  80 , which is a passive device requiring no power feeding. 
       FIG. 9  is a diagram illustrating a configuration example of the ONUs  20  of the first embodiment. 
     The ONUs  20  of the first embodiment have the same configuration as that of conventional ONUs. Each ONU  20  has a wavelength multiplexer, demultiplexer  200 , an E/O converter  201 , an O/E converter  211 , a PHY/MAC receiver logic circuit  212 , a PHY/MAC transmitter logic circuit  202 , an MPCP control logic circuit  220 , a terminal side PHY/MAC logic circuit  230 , and a terminal interface  240 . 
     The wavelength multiplexer, demultiplexer  200  receives an optical signal from the optical splitter  80  and sends an optical signal to the optical splitter  80 . The O/E converter  211  converts the received optical signal into an electric signal. The PHY/MAC receiver logic circuit  212  controls frames received from the optical splitter  80 . The PHY/MAC transmitter logic circuit  202  controls frames to be sent to the optical splitter  80 . The MPCP control logic circuit  220  is a logic circuit for communicating with the OLT  40 . The terminal side PHY/MAC logic circuit  230  controls frames sent and received on the user terminal side. The terminal interface  240  is an interface between the ONU  20  and its connected user terminal  10 . 
       FIG. 10  is a block diagram illustrating a configuration example of the OLT  40  of the first embodiment. 
     The OLT  40  has a wavelength multiplexer, demultiplexer  400 , an E/O converter  401 , an O/E converter  411 , a PHY/MAC receiver logic circuit  412 , a PHY/MAC transmitter logic circuit  402 , an MPCP control logic circuit  420 , a gateway side PHY/MAC logic circuit  430 , and a gateway interface  440 . 
     The wavelength multiplexer, demultiplexer  400  receives an optical signal from the optical splitter  80  and sends an optical signal to the optical splitter  80 . The O/E converter  411  converts the received optical signal into an electric signal. The PHY/MAC receiver logic circuit  412  controls frames received from the optical splitter  80 . The PHY/MAC transmitter logic circuit  402  controls frames to be sent to the optical splitter  80 . The MPCP control logic circuit  420  controls the plurality of ONUs  20  with the use of multipoint control protocol (MPCP) frames. The gateway side PHY/MAC logic circuit  430  controls frames sent and received on the gateway side. The gateway interface  440  is an interface between the OLT  40  and the gateway  50 . 
     The OLT  40  of this embodiment has the same hardware configuration as that of conventional OLTs, except that the MPCP control logic circuit  420  executes discovery processing that differs from prior art. 
       FIG. 11  is a block diagram illustrating a configuration example of the optical splitter  80  of the first embodiment. 
     The optical splitter  80  of the first embodiment has the same configuration as that of conventional optical splitters. The optical splitter has a wavelength multiplexer, demultiplexer  361 , a wavelength multiplexer, demultiplexer  360 , a downstream optical splitter  311 , and an upstream optical splitter  312 . 
     The wavelength multiplexer, demultiplexer  361  receives an optical signal from the OLT  40  and sends an optical signal to the OLT  40 . The wavelength multiplexer, demultiplexer  360  receives optical signals from the ONUs  20  and sends optical signals to the ONUs  20 . The downstream optical splitter  311  distributes optical signals from the OLT  40  to the respective ONUs  20 . The upstream optical splitter  312  integrates optical signals from the ONUs  20  into one to be sent to the OLT  40 . 
       FIG. 1  is a timing chart illustrating an example of changing the frequency of executing a discovery phase according to the first embodiment. 
     In the example of  FIG. 1 , two different thresholds A and B are used as a threshold for the number of the registered ONUs  20  managed by the OLT  40 . The threshold B is larger than the threshold A. A constant length of time is set to every discovery phase  500 , and three different lengths of time (short, medium, long) are used as a data transmission phases interval between the discovery phases  500 . Discovery phases and data transmission phases that are executed within a length of time required to complete discovery for all the ONUs  20  are grouped together into one communication set. In  FIG. 1 , this length of time required for one communication set is the “max time  510  of all ONUs discovery completion.” 
     In a communication set K, which is a first communication set  511 - 1 , when the number of the ONUs  20  that have been registered at the start of this set is equal to or smaller than the threshold A, the length of time between two discovery phases (i.e., length of data transmission phases) is set to a “short length” (small interval). In other words, a discovery phase is executed frequently in order to finish registering all the ONUs  20  at an early point. More discovery phases mean lower efficiency of data transmission. However, with only a small number of ONUs  20  registered, a satisfactory level of data transmission efficiency is accomplished. By the time the max time  510  of all ONUs discovery completion elapses, registration is completed for all the ONUs  20  that have managed to send the REGISTER_REQ messages SIG 30  inside the discovery window  550  of  FIG. 4  without running into each other. 
     At the end of the communication set K, there may be the ONU  20  to which the OLT  40  has failed to respond, or the ONU  20  whose registration has been canceled in the middle of the communication, for some reason. The discovery processing is repeated in the subsequent communication sets for the ONUs  20  that have failed to be registered in the communication set K  511 - 1 . 
     When the number of the registered ONUs  20  managed by the OLT  40  is larger than the threshold A and equal to or smaller than the threshold B, in a communication set K+1, which is the next communication set denoted by  511 - 2 , the length of time between two discovery phases is set to a “medium length” (medium interval). In the communication set K+1  511 - 2 , where the number of the registered ONUs  20  is larger than in the communication set K  511 - 1 , the unregistered ONUs  20  can be registered within the same max time  510  of all ONUs discovery completion through fewer discovery phases. Furthermore, with the data transmission phase set longer, the ONUs  20  that have already been registered are improved in the efficiency of data transmission. 
     When the number of the registered ONUs  20  managed by the OLT  40  is larger than the threshold B, in a communication set K+2, which is the next communication set denoted by  511 - 3 , the length of time between two discovery phases is set to a “long length” (large interval). In the communication set K+2  511 - 3 , where the number of the registered ONUs  20  is larger than in the communication set K  511 - 1  and the communication set K+1  511 - 2 , the unregistered ONUs  20  can be registered within the same max time  510  of all ONUs discovery completion through even fewer discovery phases. Furthermore, with the data transmission phase set longer, the ONUs  20  that have already been registered are improved in the efficiency of data transmission. 
       FIG. 5  is a flow chart for the processing of changing the frequency of executing a discovery phase which is illustrated in  FIG. 1 . The processing of  FIG. 5  is executed by the MPCP control logic circuit  420  of the OLT  40 . 
     First, a threshold that indicates the ratio of a discovery phase to data transmission phases is set (S 600 ) and then the first communication set is started. In the first communication set, a discovery phase is executed first and a counter K is initialized to zero (S 601 ). The counter K counts how many data transmission phases are executed in one communication set (the length of time of data transmission phases). 
     Next, a data transmission phase is executed and the value of the counter K is increased by 1 (S 602 ). Thereafter, whether or not the value of the counter K has exceeded the threshold is determined (S 603 ). When the value of the counter K is not over the threshold, the MPCP control logic circuit  420  returns to Step S 602  to repeat a data transmission phase. When the value of the counter K is over the threshold, on the other hand, whether or not the length of time elapsed since the start of the current communication set has exceeded the max time  510  of all ONUs discovery completion is determined (S 604 ). 
     When it is found as a result that the elapsed time of the communication set has not exceeded the max time  510  of all ONUs discovery completion, the MPCP control logic circuit  420  returns to Step S 601  to execute a discovery phase. When the elapsed time of the communication set exceeds the max time  510  of all ONUs discovery completion, the number of the ONUs  20  that have been registered by that point is referred to in order to update the threshold (S 605 ). The MPCP control logic circuit  420  then returns to Step S 601  to execute a discovery phase. 
       FIG. 8  is an explanatory diagram of tables for determining a threshold for the discovery phase ratio (discovery phase ratio information), and is used in Step S 605  of the processing of  FIG. 5 . 
     In a table  911  of  FIG. 8 , the max time  510  of all ONUs discovery completion is about 1.5 seconds, the maximum number of the ONUs  20  managed by the OLT  40  is  128 , and the discovery phase ratio is updated each time the number of the registered ONUs  20  increases by 16. The table  911  also shows the efficiency of data transmission for each discovery phase ratio. 
     For example, when the number of the registered ONUs  20  is 0 or more and less than 16, the discovery phase ratio is 1/12, which means that the discovery phase is executed once in twelve phases while the data transmission phase is executed eleven times. As can be seen in the table  911 , a desirable relation is obtained in which the discovery phase frequency in a communication set is lowered and the efficiency of data transmission is improved as the number of the registered ONUs  20  increases. 
     A table  912  and a table  913  show cases where the max time  510  of all ONUs discovery completion is set to about 2.5 seconds and about 3.5 seconds, respectively. It is understood from the tables  912  and  913  that increasing the max time  510  of all ONUs discovery completion lowers the discovery phase frequency and improves the efficiency of data transmission. 
     In the tables  911 ,  912 , and  913 , the number of the registered ONUs  20  is classified into eight different ranges to set a discovery phase ratio for each of the eight ranges as a discovery threshold. Instead, more discovery thresholds may be used. For example, when the maximum number of the ONUs  20  managed by the OLT  40  is  128 , a hundred and twenty-eight discovery thresholds may be used. Discovery phase ratios corresponding to discovery thresholds may be prepared in advance, or may be calculated each time. 
     An example of a formula of this calculation is shown in Mathematical Expression (1). The premise of the calculation by Expression (1) is that every ONU  20  is placed far enough from the OLT  40  and that only one ONU  20  is registered in a single discovery phase.
 
DISCOVERY PHASE THRESHOLD=ROUNDDOWN ((MAXIMUM ALL ONU DISCOVERY COMPLETION TIME−PHASE TIME)/(MAXIMUM ONU COUNT−REGISTERED ONU COUNT)*PHASE TIME))  (1)
 
     In Expression (1), “discovery phase threshold” represents how many phases in total are executed to execute the discovery phase once, including the one discovery phase, and “RUNDOWN” means that the fraction is rounded down. 
     The first embodiment describes an optical access system that has the optical splitter  80  as illustrated in  FIG. 2 . However, this invention is also applicable to an optical access system as the one illustrated in  FIG. 14  in which an active optical switch (OSW)  30  is introduced in place of the optical splitter  80 . 
     As has been described, according to the first embodiment of this invention, where the OLT  40  sets a maximum time for completing discovery for all the ONUs  20  and dynamically changes the discovery phase ratio to suite the number of the ONUs  20  that have been registered, discovery can be completed for all the ONUs  20  within a given period of time, or at least the time required to complete discovery for all the ONUs  20  can be cut short, and the efficiency of data transmission is prevented from dropping. 
     Second Embodiment 
     A second embodiment of this invention is described next which deals with a method of cutting short the time required to complete discovery for all the ONUs  20  by dynamically changing the length of a single discovery phase, instead of changing the time interval between discovery phases. 
     An optical access system of the second embodiment has the same configuration as that of the optical access system of the first embodiment which is equipped with the optical splitter  80  as described above with reference to  FIG. 2 . The second embodiment is also applicable to the optical access system of  FIG. 14  which has the active optical switch  30  instead of the optical splitter  80 . 
       FIG. 12  is a timing chart illustrating an example of changing the length of time of executing a discovery phase according to the second embodiment. 
     In the example of  FIG. 12 , two different thresholds A and B are used as a threshold for the number of the registered ONUs  20  managed by the OLT  40 . The threshold B is larger than the threshold A. A constant length of time is set to a data transmission phase between two discovery phases  500 , and three different lengths of time (long, medium, short) are used as a time of a single discovery phase. Discovery phases and data transmission phases that are executed within a length of time required to complete discovery for all the ONUs  20  are grouped together into one communication set. In  FIG. 12 , this length of time required for one communication set is the “max time  510  of all ONUs discovery completion.” 
     In a communication set K, which is a first communication set  521 - 1 , when the number of the ONUs  20  that have been registered at the start of this set is equal to or smaller than the threshold A, the time of the single discovery phase is set to a “long length” (large interval). In other words, the time of the single discovery phase is set to be long in order to finish registering all the ONUs  20  at an early point. Longer time of the single discovery phase means lower efficiency of data transmission. However, with only a small number of ONUs  20  registered, a satisfactory level of data transmission efficiency is accomplished. By the time the max time  510  of all ONUs discovery completion elapses, registration is completed for all the ONUs  20  that have managed to send the REGISTER_REQ messages SIG 30  inside the discovery window  550  of  FIG. 4  without running into each other. 
     At the end of the communication set K, there may be the ONU  20  to which the OLT  40  has failed to respond, or the ONU  20  whose registration has been canceled in the middle of the communication, for some reason. The discovery processing is repeated in the subsequent communication sets for the ONUs  20  that have failed to be registered in the communication set K  521 - 1 . 
     When the number of the registered ONUs  20  managed by the OLT is larger than the threshold A and equal to or smaller than the threshold B, in a communication set K+1, which is the next communication set denoted by  521 - 2 , the time of the single discovery phase is set to a “medium length” (medium interval). In the communication set K+1  521 - 2 , where the number of the registered ONUs  20  is larger than in the communication set K  521 - 1 , the unregistered ONUs  20  can be registered within the same max time  510  of all ONUs discovery completion through a shorter time of the single discovery phase. Furthermore, with the data transmission phase set longer, the ONUs  20  that have already been registered are improved in the efficiency of data transmission. 
     When the number of the registered ONUs  20  managed by the OLT is larger than the threshold B, in a communication set K+2, which is the next communication set denoted by  521 - 3 , the time of the single discovery phases is set to a “short length” (small interval). In the communication set K+2  521 - 3 , where the number of the registered ONUs  20  is larger than in the communication set K  521 - 1  and the communication set K+1  521 - 2 , the unregistered ONUs  20  can be registered within the same max time  510  of all ONUs discovery completion through an even shorter time of the single discovery phase. Furthermore, with the data transmission phase set longer, the ONUs  20  that have already been registered are improved in the efficiency of data transmission. 
       FIG. 13  is a flow chart for the processing of changing the length of time of executing a discovery phase which is illustrated in  FIG. 12 . The processing of  FIG. 12  is executed by the MPCP control logic circuit  420  of the OLT  40 . 
     First, a time T of a single discovery phase is set to an initial value (S 610 ) and then the first communication set is started. In the first communication set, a discovery phase is executed first and the counter K is initialized to zero (S 611 ). The MPCP control logic circuit  420  then stands by until the run time of the discovery phase exceeds the time T (S 612 ). 
     When the time T has elapsed, a data transmission phase is executed and the value of the counter K is increased by 1 (S 613 ). Thereafter, whether or not the value of the counter K has exceeded a predetermined value is determined (S 614 ). When the value of the counter K is not over the predetermined value, the MPCP control logic circuit  420  repeats a data transmission phase (S 613 ). When the value of the counter K is over the predetermined value, on the other hand, whether or not the length of elapsed time has exceeded the max time  510  of all ONUs discovery completion is determined (S 615 ). 
     When it is found as a result that the elapsed time of the communication set has not exceeded the max time  510  of all ONUs discovery completion, the MPCP control logic circuit  420  returns to Step S 611  to execute a discovery phase. When the elapsed time of the communication set exceeds the max time  510  of all ONUs discovery completion, the number of the ONUs  20  that have been registered by that point is referred to in order to update the time T of the single discovery phase (S 616 ). The MPCP control logic circuit  420  then executes a discovery phase (S 610 ). 
       FIG. 15  is an explanatory diagram of tables for determining a threshold for the discovery phase ratio (discovery time information), and is used in Step S 616  of the processing of  FIG. 12 . 
       FIG. 15  illustrates in a table  921 , a table  922 , and a table  923  relations among the number of the registered ONUs  20 , the time of the single discovery phase time, and the efficiency of data transmission when the time required to complete discovery is about 1.5 seconds, about 2.5 seconds, and about 3.5 seconds, respectively. In the tables  921 ,  922 , and  923 , the length of time of the data transmission phase  501  illustrated in  FIG. 3  is 1 millisecond and the discovery phase  500  having a specified length is executed once in two hundred and fifty phases. In other words, the frequency of executing a discovery phase is once in two hundred and fifty times. The distance between the OLT and the ONUs is 40 km, and the number of ONUs that the OLT accommodates is  128 . Of a discovery phase, a period from the transmission of the Discovery GATE message SIG 20  to the end of the discovery window  550  is 500 milliseconds, and a group of signals necessary to register a single ONU, specifically, the REGISTER message SIG 40 , the GATE message SIG 50 , and the REGISTER_ACK message SIG 60 , is exchanged in 450 milliseconds. 
     As can be seen in the tables  921 ,  922 , and  923  of  FIG. 15 , a specified length of time to complete discovery is met while setting the time of the single discovery phase time long when the number of the registered ONUs  20  is small and setting the time of the single discovery phase time short when the number of the registered ONUs  20  is large. 
     In the tables  921 ,  922 , and  923 , the number of the registered ONUs  20  is classified into eight different ranges to set time T of the single discovery phase for each of the eight ranges as a discovery threshold. Instead, more discovery thresholds may be used. For example, when the maximum number of the ONUs  20  managed by the OLT  40  is  128 , a hundred and twenty-eight discovery thresholds may be used. The time T of the single discovery phase corresponding to the discovery threshold may be prepared in advance, or may be calculated each time. 
     An example of a formula of this calculation is shown in Mathematical Expressions (2) and (3). The time of the single discovery phase time T that satisfies Expressions (2) and (3) both needs to be obtained.
 
TIME OF SINGLE DISCOVERY PHASE T=ROUNDDOWN ((MAXIMUM ALL ONU DISCOVERY COMPLETION TIME/REPETITION COUNT)−DATA TRANSFER PHASE COUNT IN COMMUNICATION SET*PHASE TIME)  (2)
 
TIME OF SINGLE DISCOVERY PHASE T=RTT+DW+((MAXIMUM ONU COUNT)−(REGISTERED ONU COUNT))/REPETITION COUNT*RTT  (3)
 
     In Expression (2), “ROUNDDOWN” means that the fraction is rounded down, and “repetition count” is an integer that determines how many times a discovery phase is to be executed within the max time for all ONUs discovery completion. This integer equals to the number of communication sets contained in the max time for all ONUs discovery completion. 
     In Expression (3), “RTT” represents the length of communication from the OLT to the ONU and back, “DW” represents the length of the discovery window, and “repetition count” is the same as the repetition count of Expression (2). A small repetition count makes the time of the single discovery phase time a very long period of time during which no data transfer phase is executed, and can deteriorate the data transmission response. On the other hand, a large repetition count may make the time of the single discovery phase time too short to execute discovery even once. Therefore, an appropriate repetition count within the permissible zone should be selected. 
     As has been described, according to the second embodiment of this invention, where the OLT  40  sets a maximum time for completing discovery for all the ONUs  20  and dynamically changes the time of the single discovery phase time to suite the number of the ONUs  20  that have been registered, discovery can be completed for all the ONUs  20  within a given period of time, or at least the time required to complete discovery for all the ONUs  20  can be cut short, and the efficiency of data transmission is prevented from dropping. 
     Third Embodiment 
     A third embodiment of this invention is described next which deals with a method of cutting short the time required to complete discovery for all the ONUs  20  that have not been registered in the OLT  40  and that are requesting connection in the optical access system of  FIG. 14 , where the optical switch  30  detects optical signals from the ONUs  20 . The description in the third embodiment focuses on differences from the first and second embodiments described above. 
       FIG. 16  is a block diagram illustrating a configuration example of the optical switch  30  of the third embodiment. 
     The optical switch  30  of the third embodiment is characterized by having a power monitor  313  for detecting the presence or absence of an optical signal in the upstream optical switch  312 . 
     The optical switch  30  has the wavelength multiplexer, demultiplexer  360 , the wavelength multiplexer, demultiplexer  361 , an optical splitter  340 , the downstream optical switch  311 , the upstream optical switch  312 , a 2-in 1-out optical switch  350 , the power monitor  313 , an O/E converter  341 , a PHY/MAC logic circuit  342  for a switch, an E/O converter  343 , and an optical switch driver  320 . 
     The optical switch  30  has a plurality of wavelength multiplexer, demultiplexers  360  ( 360 - 1  to  360 -N) to receive optical signals from the ONUs  20  and sent optical signals to the ONUs  20 . The wavelength multiplexer, demultiplexer  361  receives an optical signal from the OLT  40  and sends an optical signal to the OLT  40 . The optical splitter  340  separates signal for downstream optical communication into a signal destined to the ONU  20  and a signal destined to the OSW  30 . A 2-in 1-out optical switch may be used in place of the optical splitter  340  in order to avoid impairing an optical signal. The downstream optical switch  311  makes a switch between downstream optical communication paths. The upstream optical switch  312  makes a switch between upstream optical communication paths. The power monitor  313  detects the presence or absence of an optical signal input to the upstream optical switch  312 . 
     The O/E converter  341  converts an optical signal that is created in the optical splitter  340  by separating a whole optical signal into an electric signal. The PHY/MAC logic circuit  342  for a switch reads frame information out of an electric signal (MPCP frame) converted from an optical signal, and controls frames sent and received by the optical switch. A port management logic circuit  343  manages the relation between each port of the OSW  30  and an LLID. The PHY/MAC logic circuit  342  for a switch controls frames sent and received by the optical switch. The E/O converter  343  converts an electric signal into an optical signal, and sends the optical signal. The optical switch driver  320  controls the downstream optical switch  311  and the upstream optical switch  312 . 
     In the third embodiment, the configurations of the OLT  40  and the ONUs  20  are the same as in the first and second embodiments described above, except that the MPCP control logic circuit  420  of the OLT  40  and the MPCP control logic circuit  220  of each ONU  20  execute control different from the one in the first and second embodiments. Described below is the difference in control method. 
     First, in the case where the ONU  20  that requests the OLT  40  to establish a connection has not been registered in the OLT  40 , the ONU  20  activates a light emitting device (laser) to send a specific signal to the optical switch  30  continuously. The operation of the laser is controlled by the MPCP control logic circuit  220  illustrated in  FIG. 9 . The specific signal sent by the laser is sufficient if the optical switch  30  is notified of the fact that the laser is turned on (in short, it is sufficient if the signal is continuous light), and does not need to have a value that is meaningful as a frame or a signal of a specific bit size. For example, a value “1” may be transmitted continuously. 
     Unlike the optical splitter  80 , the upstream optical switch  312  of the optical switch  30  controls such that one ONU  20  is connected to the OLT  40  at a time. Therefore, a specific signal (laser light) issued from the unregistered ONU  20  does not hinder communication for data transmission that is held by the other ONUs  20 . 
     Next, the OLT  40  requests the optical switch  30  to report the number of the ONUs  20  that have not been registered in the OLT  40  at the start of a discovery phase and that are requesting connection. 
     The power monitor  313  of the optical switch  30  detects the ONUs  20  whose lasers are turned on. In response to the request for the number of unregistered ONUs, the optical switch  30  reports the number of the detected ONUs  20  (connection request count Nc) to the OLT  40 . When the maximum number of ONUs managed by the OLT is given as Nm, the number of the registered ONUs  20  is expressed as Nm−Nc. 
     Thereafter, the time interval between discovery phases (the frequency of executing a discovery phase) is dynamically changed as in the first embodiment. Alternatively, the time of the single discovery phase time is changed dynamically as in the second embodiment. 
     According to the third embodiment, the OLT sets a maximum time to complete discovery for all ONUs and the ratio of a discovery phase to the total number of phases is dynamically changed, or the time of the single discovery phase is dynamically changed, to suite the number of the registered ONUs  20  (Nm−Nc). In either method, discovery can be completed for all the ONUs  20  within a given period of time, or at least the time required to complete discovery for all the ONUs  20  can be cut short, and the efficiency of data transmission is prevented from dropping. 
     Fourth Embodiment 
     A fourth embodiment of this invention is described next which deals with a method of dynamically changing the frequency of executing a discovery phase or the time of the single discovery phase time without setting a maximum time to complete discovery for all ONUs. Both methods can be implemented by slightly modifying the first embodiment and the second embodiment. In the fourth embodiment, differences from the first and second embodiments are described with reference to flow charts. 
       FIG. 17  is a flow chart for processing of dynamically changing the time interval between discovery phases (the frequency of executing a discovery phase), without setting a maximum time to complete discovery for all ONUs, according to the fourth embodiment. 
     First, a threshold that indicates the ratio of a discovery phase to data transmission phases is set (S 600 ) and then the first communication set is started. In the first communication set, a discovery phase is executed first and the counter K is initialized to zero (S 601 ). 
     Next, a data transmission phase is executed and the value of the counter K is increased by 1 (S 602 ). Thereafter, whether or not the value of the counter K has exceeded the threshold is determined (S 603 ). When the value of the counter K is not over the threshold, the MPCP control logic circuit  420  returns to Step S 602  to repeat a data transmission phase. When the value of the counter K is over the threshold, on the other hand, the number of the ONUs  20  that have been registered by that point is referred to in order to update the threshold (S 605 ). The MPCP control logic circuit  420  then returns to Step S 601  to execute a discovery phase. 
     In short, the processing of the fourth embodiment is the processing of the first embodiment (the flow chart of  FIG. 5 ) minus Step S 604 . 
     A method of dynamically changing the time of the single discovery phase time without setting a maximum time to complete discovery for all ONUs is described next. 
       FIG. 18  is a flow chart for processing of changing the time between discovery phases, without setting a maximum time to complete discovery for all ONUs, according to the fourth embodiment. 
     First, the time T of the single discovery phase time is set to an initial value (S 610 ) and then the first communication set is started. In the first communication set, a discovery phase is executed first and the counter K is initialized to zero (S 611 ). The MPCP control logic circuit  420  then stands by until the run time of the discovery phase exceeds the time T (S 612 ). 
     When the time T elapses, a data transmission phase is executed and the value of the counter K is increased by 1 (S 613 ). Thereafter, whether or not the value of the counter K has exceeded a given value is determined (S 614 ). When the value of the counter K is not over the given value, a data transmission phase (S 613 ) is repeated. When the value of the counter K is over the given value, the number of the ONUs  20  that have been registered by that point is referred to in order to update the time T of the single discovery phase time (S 616 ). Thereafter, a discovery phase is executed (S 610 ). 
     In short, the processing of the fourth embodiment is the processing of the second embodiment (the flow chart of  FIG. 13 ) minus Step S 614 . 
     In the fourth embodiment, too, the discovery phase ratio and the time T of the single discovery phase time may be prepared in advance, or may be calculated each time as in the first and second embodiments described above. In either case, each time the next discovery phase is executed, the frequency of executing a discovery phase or the length of time of a discovery phase is updated based on the number of the registered ONUs  20 , regardless of the maximum time to complete discovery for all the ONUs  20 . As a result, the time required to complete discovery for all the ONUs  20  can be cut short. 
     The discovery speed-up methods of the embodiments of this invention described above are applicable to any optical access system that uses the optical splitter  80  or the optical switch  30 . 
     A detailed description has been given on an optical access system according to embodiments of this invention. However, the description given above is merely about a mode of carrying out this invention, and various modifications can be made without departing from the technical concept and technical scope of this invention.