Patent Publication Number: US-2013236170-A1

Title: Communication apparatus

Description:
FIELD 
     The present invention relates to a communication apparatus that constitutes a point to multi-point communication system. 
     BACKGROUND 
     As an example of a point to multi-point communication system, a PON (Passive Optical Network) has been known. In the PON system, interactive communication is performed based on a logical link established between a station-side device and a subscriber device. 
     In an upstream data transfer in the PON system, TDMA (Time Division Multiple Access) is used, and a burst optical signal is transmitted from each logical link. 
     The station-side device executes bandwidth allocation control (an instruction of a transmission start time and a transmission duration time of a signal), so that burst optical signals transmitted for each logical link from a subscriber device under control thereof do not collide with each other. The cycle of executing the bandwidth allocation control is referred to as “bandwidth update cycle”. In the bandwidth allocation control, the station-side device determines a subscriber device permitted to perform a transmission within the bandwidth update cycle and a time zone (a transmission start time and a transmission duration time) in which the transmission is permitted. Each subscriber device is notified of a determination result and transmits an upstream burst optical signal according to the determination result. 
     A plurality of logical links can be set between the station-side device and a single subscriber device, and each logical link is given an LLID (Logical Link ID) by the station-side device and identified. In this case, at the time of determining a subscriber device permitted to perform a transmission within the bandwidth update cycle by executing the bandwidth allocation control, the station-side device determines the transmission start time and the transmission duration time for each LLID, that is, for each logical link. 
     In the transmission of the burst optical signal, an overhead (PON-OH) time including a laser rise time (Lon), a laser fall time (Loff), and a clock synchronization time (SyncTime) and a time for transmitting control information (a header) given to user data are ensured, other than the time for transmitting the user data. The control information includes information such as the LLID. 
     Furthermore, there is another PON system that reduces power consumption by providing a period during which power feeding is stopped for a user logic that does not need to be operated due to such a reason that there is no transmission of upstream data or the like. 
     A conventional PON system is disclosed in Patent Literatures 1 and 2. In the PON system described in Patent Literature 1, effective use of an upstream bandwidth is realized by collectively transmitting data frames, which have been transmitted individually in the past. Specifically, when there is a subscriber device that has established a plurality of logical links with the station-side device, the subscriber device connects data frames to be transmitted by each logical link, and transmits these frames as one frame (a burst optical signal), so that the PON-OH (the laser rise time (Lon), the laser fall time (Loff), and the clock synchronization time (SyncTime)) at the time of switching the logical links is not required. 
     Further, in the PON system described in Patent Literature 2, the subscriber device monitors a communication status between the station-side device and the subscriber device for each communication path, and stops power supply to an optical interface corresponding to a communication path in which communication is not currently performed, thereby realizing power saving. 
     CITATION LIST 
     Patent Literatures 
     
         
         Patent Literature 1: Japanese Patent Application Laid-open No. 2007-116587 
         Patent Literature 2: Japanese Patent Application Laid-open No. 2007-89027 
       
    
     SUMMARY 
     Technical Problem 
     However, the conventional PON system described above has the following problems. 
     According to the technique described in Patent Literature 1, while the bandwidth use efficiency can be improved, the technique focuses on a reduction of the PON-OH, and a reduction of time for transmitting control information included in each data frame is not taken into consideration. That is, the control information including the LLID and the like needs to be transmitted in a data frame of each logical link, and improvement of the bandwidth use efficiency cannot be deemed as sufficient. 
     According to the technique described in Patent Literature 2, while power saving of the subscriber device is possible, when a plurality of logical links are implemented on the subscriber device, the subscriber device operates at a different time for each logical link, and thus the power feeding stop period becomes short and the power saving effect is degraded. 
     The present invention has been achieved in view of the above problems, and an object of the present invention is to provide a communication apparatus that can effectively use a bandwidth of a transfer path and can realize a transfer with a lower delay. Another object of the present invention is to provide a communication apparatus that can realize a reduction of power consumption. 
     Solution to Problem 
     There is provided a communication apparatus on a subscriber side that, in a point to multi-point communication system, transmits upstream data transmitted from a lower-level device to a station-side device based on a data transmission start time and a transmission duration time notified from the station-side device, the communication apparatus comprising: a data accumulation unit that accumulates upstream data to be transmitted to the station-side device; and a logical-link control unit that determines number of links to be used, which is number of logical links to be used for an upstream data transmission in a next bandwidth update cycle, based on an accumulated data amount in the data accumulation unit. 
     Advantageous Effects of Invention 
     The communication apparatus according to the present invention determines the number of logical links to be used in the transmission of upstream data based on an accumulated amount of the upstream data. Accordingly, an overhead and a bandwidth for transmitting control information required for each logical link can be reduced appropriately, thereby enabling to use the bandwidth effectively. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration example of a communication system according to a first embodiment. 
         FIG. 2  is a flowchart of a characteristic operation of a subscriber device according to the first embodiment. 
         FIG. 3  depicts an upstream data transmitting operation performed by the subscriber device according to the first embodiment. 
         FIG. 4  is an explanatory diagram of effects of the first embodiment. 
         FIG. 5  is an explanatory diagram of effects of the first embodiment. 
         FIG. 6  is a flowchart of a characteristic operation of a subscriber device according to a second embodiment. 
         FIG. 7  is an explanatory diagram of effects of the second embodiment. 
         FIG. 8  is an explanatory diagram of effects of the second embodiment. 
         FIG. 9  is a flowchart of a characteristic operation of a subscriber device according to a third embodiment. 
         FIG. 10  is a flowchart of an upstream data transmitting operation performed by a subscriber device according to a fourth embodiment. 
         FIG. 11  is a flowchart of an upstream data transmitting operation performed by the subscriber device according to the fourth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments of a communication apparatus according to the present invention will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments. 
     First Embodiment 
       FIG. 1  is a configuration example of a point to multi-point communication system (hereinafter, simply “communication system”) according to a first embodiment. As shown in  FIG. 1 , the communication system according to the present embodiment includes a plurality of subscriber devices  1 , a splitter  2 , and a station-side device  3 , and the subscriber devices  1  is connected to the station-side device  3  via the splitter  2  to constitute a PON system. The station-side device  3  is connected to an upper-level device  101 , and a lower-level device  102  is connected to the subscriber device  1 . 
     The subscriber devices  1  have mutually equivalent configuration, and respectively include a UNI unit  11 , an upstream buffer unit  12 , a frame multiplexing unit  13 , an optical transmission unit  14 , a frame control unit  16 , a frame separation unit  17 , an optical reception unit  18 , and a logical-link control unit  15 . 
     In the subscriber device  1 , the UNI unit  11  receives data from the lower-level device  102 , and performs a transmission of downstream data to the lower-level device  102 . The UNI unit  11  acts as data reception unit. The upstream buffer unit  12  as a data accumulation unit temporarily accumulates data received from the lower-level device  102 . The upstream buffer unit  12  includes a plurality of queues. The logical-link control unit  15  monitors a buffer accumulation amount in the upstream buffer unit  12 , and controls the number of logical links according to the monitoring result of the buffer accumulation amount. The frame control unit  16  processes a control frame to be transmitted and received to and from the station-side device  3  according to logical link information notified from the logical-link control unit  15  and the buffer accumulation amount. The frame multiplexing unit  13  reads upstream data (an upstream user frame) stored in the upstream buffer unit  12 , and multiplexes the read upstream user frame with the control frame received from the frame control unit  16 . The optical transmission unit  14  converts an electric signal output from the frame multiplexing unit  13  to an optical signal, and transmits the optical signal to a transfer path. The optical reception unit  18  converts an optical signal transmitted from the station-side device  3  to an electric signal. The frame separation unit  17  separates various frames included in the electric signal received from the optical reception unit  18  into a downstream user frame and a control frame. The frame separation unit  17  outputs the downstream user frame to the UNI unit  11  and outputs the control frame to the frame control unit  16 . The frame multiplexing unit  13  and the optical transmission unit  14  constitute a data transmission unit. 
     An operation when the station-side device  3  and the subscriber device  1  perform a data transfer in the communication system according to the present embodiment is explained next. An upstream data transfer operation of transmitting data from the subscriber device  1  toward the station-side device  3  is explained below. It is assumed here that a plurality of logical links have been established between the station-side device  3  and the subscriber device  1 . 
     The upstream user frame, which is an upstream data signal transmitted from the lower-level device  102 , is received by the UNI unit  11  of the subscriber device  1 , and stored in the upstream buffer unit  12  in which a bandwidth to be used is set for each queue. The logical-link control unit  15  monitors an accumulated data amount in a buffer area (in each queue) of the upstream buffer unit  12 , and determines the number of logical links according to the accumulated data amount in the upstream buffer unit  12  (hereinafter, “buffer accumulation amount”), to notify the frame control unit  16  of the buffer accumulation amount and the determined number of logical links. The number of logical links is the number of logical links to be used for the upstream data transmission in the bandwidth update cycle. 
     The frame control unit  16  generates a control frame based on a content notified from the logical-link control unit  15  (the buffer accumulation amount and the number of logical links). The generated control frame includes a REPORT frame for notifying an upstream transmission data amount (corresponding to the buffer accumulation amount) to be used for the bandwidth control in the station-side device  3 . An upstream bandwidth is allocated by the station-side device  3  by notifying the buffer accumulation amount in the REPORT frame, and the allocation result is notified in a GATE frame. The frame control unit  16  analyzes the control frame received from the frame separation unit  17  to perform a process depending on the content of the control frame. For example, when the control frame is the GATE frame, the frame control unit  16  notifies the frame multiplexing unit  13  of a transmission start time and a transmission duration time for each logical link indicated by the content. 
     The frame multiplexing unit  13  multiplexes the control frame received from the frame control unit  16  (for example, the REPORT frame) with the upstream data accumulated in the upstream buffer unit  12  (an upstream user frame) and outputs the multiplexed data to the optical transmission unit  14 , while taking it into consideration that an upstream burst optical signal is transmitted from the optical transmission unit  14  in a time zone indicated by a GATE frame analysis result acquired by the frame control unit  16  (a period specified by the transmission start time and the transmission duration time). The optical transmission unit  14  converts an electric signal output from the frame multiplexing unit  13  to an optical signal and transmits the optical signal to the station-side device  3 . 
     Subsequently, a characteristic operation of the communication apparatus according to the present embodiment is explained with reference to  FIGS. 2 and 3 . 
       FIG. 2  is a flowchart of a characteristic operation of the subscriber device  1  according to the first embodiment, which specifically depicts an example of an operation of changing the number of logical links. 
     As shown in  FIG. 2 , in the operation of changing the number of logical links, the logical-link control unit  15  compares the total accumulated data amount in the buffer, which is to be transmitted by all the logical links currently used in the subscriber device  1 , that is, the accumulated data amount in the upstream buffer unit  12  (the buffer accumulation amount) with the allocated bandwidth (Step S 11 ). The allocated bandwidth is an upstream bandwidth indicated in the GATE frame received as a response to the REPORT frame transmitted last time by the subscriber device  1  (an upstream bandwidth allocated by the station-side device  3 ). 
     As a result of a comparison at Step S 11 , when the buffer accumulation amount is more than the allocated bandwidth (YES at Step S 11 ), and the number of logical links currently used, that is, the number of logical links used in the data transmission in the current bandwidth update cycle is larger than one (YES at Step S 12 ), the logical-link control unit  15  determines to decrease the number of logical links to be used in the data transmission in the next bandwidth update cycle (Step S 13 ). For example, a number obtained by subtracting 1 from the number of logical links currently used is designated as the number of logical links to be used in the next data transmission. When the number of logical links currently used is equal to or more than three, two or more can be subtracted. The number to be decreased can be dynamically changed according to a difference between the buffer accumulation amount and the allocated bandwidth. On the other hand, when the number of logical links currently used is one (NO at Step S 12 ), the logical-link control unit  15  determines to maintain the number of logical links to be used in the data transmission in the next bandwidth update cycle (the operation of changing the number of logical links is finished without performing Step S 13 ). When the buffer accumulation amount is equal to or less than the allocated bandwidth (NO at Step S 11 ), the logical-link control unit  15  determines to maintain the number of logical links currently used, and to use the same number of logical links as in the current transmission in the next data transmission, or to increase the number of logical links to be used in the next data transmission (Step S 14 ). At this time, it is desired to confirm first whether the buffer accumulation amount does not become larger than the allocated bandwidth even if the number of logical links to be used is increased, and to increase the number of logical links to be used, if the buffer accumulation amount does not become larger than the allocated bandwidth. The number to be increased can be one, or two or more. Furthermore, the number to be increased can be dynamically changed according to the difference between the allocated bandwidth and the buffer accumulation amount. The upper limit of the number of logical links to be used becomes the number of logical links being established between the subscriber device  1  and the station-side device  3 . 
     As a result of performing the operation shown in  FIG. 2 , when it is determined to decrease the number of logical links to be used in the next data transmission, the frame control unit  16  of the subscriber device  1  sets a smaller number of LLIDs than the LLIDs set in the REPORT frame transmitted last time and notifies the buffer accumulation amount (an accumulated data amount in the upstream buffer unit  12 ), in the next transmission of the REPORT frame to the station-side device  3 . At this time, for example, when the number of logical links to be used is two, the LLIDs of these logical links are set in the REPORT frame, and the accumulated data amount in the upstream buffer unit  12  is distributed to each LLID and set. As an example, when it is assumed that the LLIDs of the logical links to be used are respectively LLID# 1  and LLID# 2 , and upstream data having a data amount of X 1 , X 2 , and X 3  are respectively accumulated in three queues in the upstream buffer unit  12 , for example, the accumulated data amount of the LLID# 1  is set to X 1 +X 2 , and the accumulated data amount of the LLID# 2  is set to X 3 . 
     On the other hand, when it is determined to increase the number of logical links to be used in the next data transmission, the frame control unit  16  of the subscriber device  1  sets a larger number of LLIDs than the LLIDs set in the REPORT frame transmitted last time and notifies the buffer accumulation amount in the next transmission of the REPORT frame to the station-side device  3 . At this time, for example, when the number of logical links to be used is one, the frame control unit  16  sets only the LLID of this logical link in the REPORT frame, and sets the accumulated data amount in the upstream buffer unit  12  (the total value of the data amounts accumulated in the respective queues in the upstream buffer unit  12 ). When the number of logical links to be used is two or more, the frame control unit  16  sets the LLIDs of the respective logical links to be used in the REPORT frame, and distributes and sets the accumulated data amount in the upstream buffer unit  12  to each LLID, in the same manner as “when it is determined to decrease the number of logical links”. 
     When a data transmission is newly started, specifically, when data is received from the lower-level device  102  in such a state where a bandwidth for the upstream data transmission has not been allocated for a while because the accumulated data amount in the upstream buffer unit  12  is 0, because the buffer accumulation amount cannot be compared with the allocated bandwidth, the subscriber device  1  uses the number of logical links determined based on only an accumulated state of data in the upstream buffer unit  12 . That is, the LLID and the buffer accumulation amount are set in the REPORT frame for requesting an allocation of an upstream bandwidth in the same control as the conventional general control. 
       FIG. 3  depicts an upstream data transmitting operation performed by the subscriber device  1  according to the first embodiment, which specifically depicts an example of an operation in which upstream data is received from the lower-level device  102  and accumulated in the upstream buffer unit  12 , and the frame multiplexing unit  13  reads the upstream data accumulated in the upstream buffer unit  12 . 
     As shown in  FIG. 3 , in the subscriber device  1 , the UNI unit  11  having received the upstream data from the lower-level device  102  performs only a queue allocation and stores the upstream data in the upstream buffer unit  12 , without associating the received data with a logical link. That is, a batch of received upstream data (user frames) is stored in any of queues constituting the upstream buffer unit  12  according to the type or the like. A priority is allocated to each queue. The frame multiplexing unit  13  performs expedited read according to the priority given to the queue at the time of reading the user frame, without taking the logical link number (LLID) and the number of logical links to be used into consideration. 
     Effects of the present embodiment are explained with reference to  FIGS. 4 and 5 . A case where the logical links established between the station-side device and the subscriber device are four, that is, the LLID # 1  to a LLID # 4  is considered here. Because the subscriber device in the conventional communication system accumulates the upstream data in association with any of the logical links (LLIDs), when the upstream data to be transmitted by each logical link is accumulated in the subscriber device, the subscriber device always transmits a REPORT frame in which the LLID # 1  to the LLID # 4  and the accumulated data amount corresponding to each of the LLID # 1  to the LLID # 4  are set. As a result, because a bandwidth is allocated to each logical link individually, a burst optical signal to be transmitted from the subscriber device to the station-side device becomes as shown in  FIG. 4 , and an overhead including the laser rise time (Lon), a laser fall time (Loff), and a clock synchronization time (SyncTime), and a transmission time of the control information are required for each logical link. On the other hand, because the subscriber device  1  according to the present embodiment transmits the upstream data without taking into consideration the LLID or the number of logical links established with the station-side device  3  as described above, when the number of logical links to be used is set to one, the subscriber device  1  sets one LLID of the logical link to be used and the accumulated data amount associated with the LLID in the REPORT frame, and transmits the REPORT frame. As a result, a bandwidth is allocated to one logical link, and a burst optical signal transmitted from the subscriber device  1  to the station-side device  3  becomes as shown in  FIG. 5 . Accordingly, the overhead including the laser rise time (Lon), the laser fall time (Loff), and the clock synchronization time (SyncTime), and the transmission time of the control information can be reduced as compared to the conventional case, and the bandwidth can be efficiently used. The time required for transmitting the control information is less than a case where control described in Patent Literature 1 mentioned above is applied, thereby enabling to improve the use efficiency of the bandwidth. 
     While an example of setting an area to be used of an upstream buffer for each queue with regard to one lower-level device has been explained in the above explanations, the present embodiment can be also applied to a plurality of lower-level devices. When the number of lower-level devices is plural, the lower-level device, which is a data transmission source, is identified by, for example, a method described below, thereby enabling to execute the same control. 
     (Method a) A VLAN tag is given to a user frame transmitted from a lower-level device to identify a lower-level device. 
     (Method b) A plurality of buffer areas are provided in the upstream buffer unit  12 , and a user frame transmitted from a lower-level device is stored in a queue in a buffer area different for each lower-level device according to the priority. Data can be read from each upstream buffer area either in a unit of queue or in a unit of lower-level device. 
     (Method c) A user frame transmitted from a lower-level device is directly mapped and stored in a queue in the upstream buffer unit  12 . 
     As described above, in the communication system according to the present embodiment, the subscriber device compares the accumulated amount of the upstream data with the upstream bandwidth allocated from the station-side device. When the accumulated amount of the upstream data is more than the upstream bandwidth and the number of logical links currently used is two or more, the subscriber device decreases the number of LLIDs set in the REPORT frame to be transmitted at the time of requesting a bandwidth allocation next time than the number of LLIDs last time. Accordingly, the overhead (PON-OH) and the bandwidth (time) for transmitting the control information required for each logical link can be appropriately reduced, and the bandwidth can be efficiently used. Further, the upstream data can be efficiently transmitted in a short cycle by increasing the logical links when the upstream traffic is less, thereby enabling to reduce a delay time. For example, when a plurality of types of upstream data are accumulated in the upstream buffer unit  12  in a state with the number of logical links to be used being one, because the upstream data having a higher priority is sequentially read and transmitted, a time until the upstream data having the lowest priority is read becomes long. On the other hand, when a plurality of logical links are used, because the upstream data having a high priority and the upstream data having a low priority can be allocated to different logical links and transmitted, the delay time can be reduced in view of the entire system. 
     Furthermore, because priority control by the queues is executed without associating data with the logical link at the time of write and read of the user frame, the internal configuration of the upstream buffer unit  12  does not need to be changed at the time of changing the number of logical links to be used, and flexible operation can be performed. 
     In the present embodiment, a method of improving the bandwidth use efficiency when assuming a system in which the station-side device allocates an upstream bandwidth for each logical link and the subscriber device operates faithfully according to the determination result (the allocation result of the upstream bandwidth) by the station-side device has been explained. However, when a system in which the station-side device flexibly receives the upstream data, identical effects can be achieved by performing procedures different from those described above. Specifically, in a case where after the station-side device specifies a plurality of logical links with respect to a single subscriber device and allocates an upstream bandwidth, the station-side device operates to receive a signal normally even if the signal is transmitted over a time longer than the allocated transmission duration time by one logical link, unless the total of bandwidths allocated to the subscriber device is not exceeded (unless the subscriber device performs a transmission by using a bandwidth allocated to another subscriber device), the subscriber device can improve the bandwidth use efficiency by flexibly using the total value of the bandwidth allocated to each logical link. 
     For example, when the station-side device permits a transmission with respect to a logical link # 1  of one subscriber device at a time t 1  to a time t 2 , and permits a transmission with respect to a logical link # 2  thereof at the time t 2  to a time t 3 , if the subscriber device detects that the buffer accumulation amount is more than the allocated bandwidth by performing a process corresponding to Step S 11  described above, the subscriber device performs the transmission by the logical link # 1  over the time from t 1  to t 3 . At the time of changing the number of logical links to be used, because the subscriber device does not need to set the LLID of the logical link to be used after the change in the REPORT frame and notify it, the control procedure can be simplified, and a flexible operation can be performed. Furthermore, a time since a determination to decrease the number of logical links to be used until the transmission is started according to the determination result (a transmission with the number of logical links being decreased) can be reduced. 
     Second Embodiment 
     A second embodiment is explained next. In the first embodiment described above, a method of changing the number of logical links to be used in order to improve the bandwidth use efficiency and reduce the delay time has been explained. On the other hand, in the present embodiment, a method of controlling the number of logical links in order to reduce power consumption is explained. Configurations of the communication system and the subscriber device according to the present embodiment are similar to those of the first embodiment (see  FIG. 1 ). 
     A different point between the subscriber device  1  according to the present embodiment and the subscriber device  1  explained in the first embodiment is an operation performed by the logical-link control unit  15 , and operations of other constituent elements are similar to those of the first embodiment. In the present embodiment, only elements different from the first embodiment are explained. 
       FIG. 6  is a flowchart of a characteristic operation of the subscriber device  1  according to the second embodiment, which depicts an example of an operation of changing the number of logical links in the subscriber device  1  similarly to  FIG. 2 , which has been used for explanations of the first embodiment. 
     As shown in  FIG. 6 , in the operation of changing the number of logical links according to the present embodiment, the logical-link control unit  15  first compares a buffer usage rate of the upstream buffer unit  12  with a preset threshold (Step S 21 ). The logical-link control unit  15  calculates the buffer usage rate based on the accumulated data amount in the upstream buffer unit  12  (a buffer accumulation amount) and the buffer capacity of the upstream buffer unit  12 . A method of determining a threshold of the buffer usage rate is described later. 
     As a result of a comparison at Step S 21 , when the buffer usage rate is equal to or higher than the threshold (YES at Step S 21 ), the logical-link control unit  15  determines to maintain the number of logical links currently used and use the same number of logical links as in the current transmission at the time of the next data transmission, or determines to increase the number of logical links to be used in the next data transmission (Step S 22 ). When the buffer usage rate is high, because the buffer accumulation amount is large, if the number of logical links to be used is small, the delay time at the time of a data transmission becomes long as compared to a case where the buffer usage rate is low. Therefore, the number of logical links to be used is increased to reduce the delay time. At Step S 22 , the logical-link control unit  15  can confirm whether the buffer accumulation amount (an accumulated data amount in the upstream buffer unit  12 ) does not become larger than the allocated bandwidth due to an increase in the number of logical links to be used, and if the number of logical links does not become larger than the allocated bandwidth, the logical-link control unit  15  can increase the number of logical links. On the other hand, when the buffer usage rate is lower than the threshold (NO at Step S 21 ), and the number of logical links currently used is larger than one (YES at Step S 23 ), the logical-link control unit  15  determines to decrease the number of logical links to be used in the data transmission in the next bandwidth update cycle (Step S 24 ). When the number of logical links currently used is one (NO at Step S 23 ), the logical-link control unit  15  determines to maintain the number of logical links to be used in the data transmission in the next bandwidth update cycle (to finish the operation of changing the number of logical links without performing Step S 24 ). When the buffer usage rate is low, because the buffer accumulation amount is small, even if the number of logical links to be used is small, the delay time at the time of a data transmission does not increase so much. Therefore, the number of logical links to be used is decreased to reduce the power consumption. 
     In the transmission of the REPORT frame after the operation of changing the number of logical links is performed and the number of logical links to be used at the time of a next data transmission is determined, the number of LLIDs according to the determination result is set in the REPORT frame. This operation is similar to the operation in the communication system explained in the first embodiment. 
     The method of determining a threshold to be used in the operation of changing the number of logical links is explained below. As the method of determining a threshold, for example, (Method a) to (Method d) described below can be considered. 
     (Method a) A threshold is determined according to a power-saving-mode set value. 
     In this method, a threshold is set according to the length of a set period of the power saving state, specifically, when the period is long, the threshold is set high and when the period is short, the threshold is set low. The period of the power saving state is a period since a shift to the power saving state (a state where a transmission to the station-side device  3  is not performed) until the power saving state is released. The period of the power saving state is set, for example, in a unit of bandwidth update cycles. That is, the period of the power saving state becomes the length of an integral multiple of the bandwidth update cycle. For example, when the upstream data is not accumulated in the upstream buffer unit  12 , the station-side device  3  shifts to the power saving state, and stops the transmitting operation in the period of the length indicated by the power-saving-mode set value. The state of the upstream buffer unit  12  is confirmed at a point in time when the period is finished, and the power saving state is finished if the upstream data is accumulated, and a bandwidth for transmitting the upstream data is requested to the station-side device  3 . When the power saving mode is set, such an operation is performed. Therefore, when the period of the power saving state is long, the upstream data received from the lower-level device  102  during the period is not transmitted to the station-side device  3 , and is continuously accumulated in the upstream buffer unit  12 . As a result, the buffer accumulation amount is increased and the buffer usage rate becomes high (a mean value of the buffer usage rate increases as the period of the power saving state becomes long). Accordingly, when the period is long, the threshold is set high. In the present embodiment, while details thereof are described later, when the buffer usage rate is lower than the threshold, control for improving the power saving effect is performed by decreasing the number of logical links to be used. On the other hand, when the number of logical links to be used is decreased, the delay time tends to increase, as explained in the present embodiment. Therefore, when a priority is given to the improvement of the power saving effect, the threshold is set to a higher value, while taking the period of the length indicated by the power-saving-mode set value into consideration. When it is desired to avoid that the delay time increases (a priority is given to a reduction of the delay time), it suffices to set the threshold to a lower value, while taking the period of the length indicated by the power-saving-mode set value into consideration. 
     (Method b) A threshold is determined according to a guaranteed minimum bandwidth of an SLA (service level agreement) parameter. 
     In this method, when the guaranteed minimum bandwidth is high, a threshold is set high, and when the guaranteed minimum bandwidth is low, the threshold is set low. When the guaranteed minimum bandwidth is high, it is assumed that the data amount of the upstream data transmitted from the lower-level device  102  becomes large, and that the buffer usage rate increases (because the buffer capacity itself does not change). Accordingly, when the priority is given to the improvement of the power saving effect, it suffices that the threshold is set to a higher value, while taking the value of the guaranteed minimum bandwidth into consideration, and when the priority is given to a reduction of the delay time, it suffices that the threshold is set to a lower value, while taking the value of the guaranteed minimum bandwidth into consideration. 
     (Method c) A threshold is determined according to the delay class of the SLA parameter. 
     In this method, when the delay class is a low delay, a threshold is set low, and when the delay class is not a low delay, the threshold is set high. When the delay class is a low delay, if the buffer usage rate tends to be determined to be lower than the threshold (if the number of logical links to be used is likely to be decreased), the threshold is set to a lower value as described above, because the delay time increases. On the other hand, when the delay class permits a delay, the threshold is set to a higher value so that the number of logical links to be used can be easily decreased to improve the power saving effect. 
     (Method d) A threshold is determined according to an upstream transfer rate (1 G/10 G) of the subscriber device  1 . 
     In this method, when the upstream transfer rate is low, a threshold is set high, and when the upstream transfer rate is high, the threshold is set low. When the upstream transfer rate is low, a delay does not become a problem, and thus the threshold is set to a higher value as described above, in order that the number of logical links to be used can be easily decreased to improve the power saving effect. 
     Two or more conditions can be combined and used. For example, the threshold of the buffer usage amount can be determined based on the power-saving-mode set value and the guaranteed minimum bandwidth. 
     In the above explanations, it is determined whether to change the logical link to be used based on the comparison result between the buffer usage rate and the threshold. The buffer usage rate is calculated based on the buffer capacity and the buffer accumulation amount. However, because the buffer capacity is fixed, the buffer usage rate fluctuates depending on only the buffer accumulation amount. Accordingly, in the above operation (the operation of changing the number of logical links shown in  FIG. 6 ), the buffer accumulation amount can be used instead of the buffer usage rate. 
     Furthermore, at the time of determining the threshold, the threshold can be determined by taking it into consideration as to which is prioritized, that is, a reduction of the transmission delay time of the upstream data or power saving. In the method of determining the threshold described above, when the priority is given to the reduction of the transmission delay time, it suffices that the threshold is adjusted to be high. On the other hand, when the priority is given to a reduction of power consumption, it suffices that the threshold is adjusted to be low. 
     As described above, in the communication system according to the present embodiment, the subscriber device compares the use rate of the buffer that accumulates the upstream data with a threshold. When the use rate is lower than the threshold, and the number of the logical links currently used is two or more, the number of the LLIDs to be set in the REPORT frame transmitted at the time of requesting the bandwidth allocation next time is decreased than last time. Accordingly, an operating time of various devices involved with the data transmitting operation can be reduced, and improvement of the power saving effect can be expected (see  FIGS. 7 and 8 ). 
       FIG. 7  is a transmitting operation example in a case where a plurality of logical links (in this example, four logical links) are used.  FIG. 8  is a transmitting operation example in a case where a single logical link is used. As  FIGS. 7 and 8  are compared, it is understood that the PON-OH (the laser rise time (Lon), the laser fall time (Loff), and the clock synchronization time (SyncTime)) and the transmission time of the control information are reduced by decreasing the number of logical links to be used. That is, when data having the same size is to be transmitted, if the number of logical links to be used is decreased, the power-feeding stop period (when there are a plurality of power-feeding stop periods, the total value thereof) indicating a period that can be shifted to a low power consumption state (a state where the operation of the devices involved with the data transmitting operation is stopped) becomes long, and the power saving effect can be improved. The low power consumption state mentioned here is different from the power saving state in the “method of determining the threshold according to the power-saving-mode set value (Method a)”. 
     Third Embodiment 
     A third embodiment is explained next. In the first and second embodiments described above, the method of changing the number of logical links to be used in order to improve the bandwidth use efficiency and reduce the delay time, or to improve the power saving effect is explained. On the other hand, in the present embodiment, a method of controlling the number of logical links, which satisfies both of improvement of the bandwidth use efficiency and improvement of the power saving effect, is described. Configurations of the communication system and the subscriber device according to the present embodiment are similar to those of the first embodiment (see  FIG. 1 ). 
     A different point between the subscriber device  1  according to the present embodiment and the subscriber device  1  explained in the first and second embodiments is an operation performed by the logical-link control unit  15 , and operations of other constituent elements are similar to those of the first embodiment. In the present embodiment, only elements different from the first embodiment are explained. 
       FIG. 9  is a flowchart of a characteristic operation of the subscriber device  1  according to the third embodiment, which depicts an example of an operation of changing the number of logical links in the subscriber device  1 , as in  FIGS. 2 and 6  used for explanations of the first and second embodiments. In  FIG. 9 , processes similar to the processes explained in the first or second embodiment are denoted by like step numbers. 
     As shown in  FIG. 9 , in the operation of changing the number of logical links according to the present embodiment, the logical-link control unit  15  first compares the accumulated data amount in the upstream buffer unit  12  (a buffer accumulation amount) with the allocated bandwidth (Step S 11 ). As a result of a comparison at Step S 11 , when the buffer accumulation amount is more than the allocated bandwidth (YES at Step S 11 ), and the number of logical links currently used, that is, the number of logical links used in the data transmission at hand is larger than one (YES at Step S 12 ), the logical-link control unit  15  determines to decrease the number of logical links to be used in the data transmission in the next bandwidth update cycle (Step S 13 ). 
     On the other hand, when the buffer accumulation amount is equal to or less than the allocated bandwidth (NO at Step S 11 ), the logical-link control unit  15  compares the buffer usage rate of the upstream buffer unit  12  with a preset threshold (Step S 21 ). The threshold is determined by the method explained in the second embodiment. The logical-link control unit  15  performs Steps S 22 , S 23 , and S 24  explained in the second embodiment, according to the comparison result at Step S 21 . 
     In this manner, in the communication system according to the present embodiment, the operation combining the control procedure ( FIG. 2 ) explained in the first embodiment and the control procedure ( FIG. 6 ) explained in the second embodiment is performed to change the number of logical links to be used. Specifically, when the buffer accumulation amount is more than the allocated bandwidth, the overhead and the bandwidth for transmitting the control information required for each logical link are reduced to increase the bandwidth for transmitting the upstream data, by reducing the number of logical links to be used so that more pieces of upstream data can be transmitted. When the buffer accumulation amount is equal to or less than the allocated bandwidth and the buffer usage rate (the buffer accumulation amount) is lower than the threshold, because the buffer accumulation amount is small and the transmission delay time of the upstream data does not increase so much, the power-feeding stop period (see  FIGS. 7 and 8 ) is set long, and the overhead and the like are reduced by decreasing the number of logical links to be used, so that the power consumption can be reduced. Furthermore, when the buffer accumulation amount is equal to or less than the allocated bandwidth and the buffer usage rate is equal to or higher than the threshold, the transmission delay time of the upstream data may cause a problem. Therefore, to avoid an increase of the transmission delay time, the number of logical links to be used is maintained or increased. Accordingly, both of efficient use of the bandwidth and power saving can be achieved. 
     Fourth Embodiment 
     A fourth embodiment is explained next. In the first embodiment described above, the method in which the upstream data from the lower-level device is not distributed to the logical links, and data is read and transmitted by the queue control has been explained. On the other hand, in the present embodiment, such a method is explained that the upstream data from the lower-level device is managed for each logical link, and read control is executed in order to control the bandwidth for each logical link. Configurations of the communication system and the subscriber device according to the present embodiment are similar to those of the first embodiment (see  FIG. 1 ). 
     In the communication system according to the present embodiment, an upstream data signal transmitted from the lower-level device  102  is received by the UNI unit  11  of the subscriber device  1 , and stored in the upstream buffer unit  12  in which an area to be used is set for each logical link (that includes a plurality of areas to be used, which are associated in a one-to-one relation with each logical link). The logical-link control unit  15  monitors the accumulated data amount in the buffer area of the upstream buffer unit  12 , and determines the number of logical links according to the accumulated data amount in the upstream buffer unit  12  (the buffer accumulation amount), to notify the frame control unit  16  of the buffer accumulation amount and the determined number of logical links. The frame control unit  16  generates a control frame based on the content (the buffer accumulation amount and the number of logical links) notified from the logical-link control unit  15 . The frame multiplexing unit  13  multiplexes the control frame received from the frame control unit  16  with the upstream user frame, which is the upstream data accumulated in the upstream buffer unit  12 , and outputs the multiplexed data to the optical transmission unit  14 , while taking it into consideration that the upstream burst optical signal is transmitted from the optical transmission unit  14  in a time zone indicated by the GATE frame analysis result acquired by the frame control unit  16  (a period specified by the transmission start time and the transmission duration time). The optical transmission unit  14  converts an electric signal output from the frame multiplexing unit  13  to an optical signal, and transmits the optical signal to the station-side device  3 . 
     The operation of changing the number of logical links is performed according to the flowchart in  FIG. 2 ,  FIG. 6 , or  FIG. 9 , which have been respectively described in the first to third embodiments. 
       FIG. 10  is a figure of an upstream data transmitting operation performed by the subscriber device  1  according to the fourth embodiment, which specifically depicts an example of an operation in which upstream data is received from the lower-level device  102  and the frame multiplexing unit  13  reads the upstream data accumulated in the upstream buffer unit  12 .  FIG. 10  depicts an operation in a state where the number of logical links to be used is not reduced (an operation when the subscriber device  1  uses all the logical links to transmit data). 
     As shown in  FIG. 10 , in the subscriber device  1 , when the subscriber device  1  uses all the logical links to transmit the data, the UNI unit  11  having received the upstream data from the lower-level device  102  stores the received upstream data in a corresponding area in the upstream buffer unit  12 , specifically, in the area to be used associated with the logical link for transmitting the received upstream data. When the user frame is to be read, the frame multiplexing unit  13  reads the user frame from the area (the area to be used) associated with the logical link having the LLID set in the GATE frame indicating a bandwidth allocation result received from the station-side device  3 . 
     In  FIG. 10 , an operation example in a case where the area to be used is set for each logical link for one subscriber device  1  is shown. However, the operation example is applicable to a plurality of subscriber devices  1 . When there are a plurality of lower-level devices, the identical control can be executed by identifying a lower-level device, which is a data transmission source, by a method described below. 
     (Method a) A VLAN tag is given to a user frame transmitted from a lower-level device to identify a lower-level device. 
     (Method b) A user frame transmitted from a lower-level device is stored in an upstream buffer area different for each lower-level device. 
     (Method c) A user frame transmitted from a lower-level device is mapped on a logical link and stored in an area to be used associated with each logical link. 
       FIG. 11  is a figure of an upstream data transmitting operation performed by the subscriber device  1  according to the fourth embodiment, which specifically depicts an example of an operation in which upstream data is received from the lower-level device  102  and the frame multiplexing unit  13  reads the upstream data accumulated in the upstream buffer unit  12 , as in  FIG. 10 . However,  FIG. 11  depicts an operation in a state where the number of logical links to be used is reduced, and the frame multiplexing unit  13  uses three logical links (the logical links # 1  to # 3 ) of four logical links established between the station-side device and the subscriber device. 
     In the operation example shown in  FIG. 11 , an operation of accumulating the upstream data received from the lower-level device  102  in the upstream buffer unit  12  is similar to the operation in the state where the number of logical links to be used is not reduced (the operation shown in  FIG. 10 ). 
     On the other hand, when the user frame is read from the upstream buffer unit  12 , the frame multiplexing unit  13  handles the buffer area of the reduced logical link (a logical link not to be used) as a buffer area of an internal link, that is, as a buffer of another logical link. In the example shown in  FIG. 11 , the buffer area associated with an unused logical link # 4  is regarded as a part of the buffer area associated with the logical link # 3  to perform read. 
     In data read, for example, the priority control for each logical link is executed. When it is assumed that a plurality of the buffer areas are associated with one logical link (when the number of logical links to be used is decreased) and identification of each buffer area is required, for example, the following methods are used. 
     (Method a) A VLAN tag different for each buffer area is given to a data frame read from each buffer area associated with the same logical link to identify a buffer area. 
     (Method b) In a logical link, a transmission time of data in the respective buffer areas associated with the logical link is determined, and a control signal for a reception side to identify the transmission time of data for each buffer area is transmitted at the time of starting the transmission. As the control signal, for example, an extended OAM frame can be used. However, another frame or another control signal can be used. Furthermore, a signal can be transmitted for each buffer area or signals for all the buffer areas can be coordinated and transmitted as one signal. 
     Furthermore, when a buffer size of a part of the logical link is small with respect to the bandwidth allocated to each logical link of one subscriber device, and when there is a surplus in the bandwidth allocated to the part of the logical link, data accumulated in the buffer of another logical link can be transmitted by using the logical link having the surplus bandwidth. 
     In this manner, the subscriber device  1  according to the present embodiment accumulates the upstream data in the buffer area for each logical link established between the station-side device and the subscriber device, in the upstream buffer unit  12 . When the accumulated data is to be read, the subscriber device  1  handles the buffer area depending on the logical links currently used at that time, specifically, handles a buffer area associated with an unused logical link as a part of a buffer area associated with any of the logical links currently used, and performs data read for all the buffer areas including the buffer area associated with the unused logical link. Even when such a buffer management method is used, effects identical to those of the first to third embodiments can be achieved. 
     INDUSTRIAL APPLICABILITY 
     As described above, the communication apparatus according to the present invention is useful for a subscriber device in a point to multi-point communication system, and the communication apparatus is particularly suitable for a subscriber device that can set a plurality of logical links between a station-side device and a subscriber device. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  subscriber device 
               2  splitter 
               3  station-side device 
               11  UNI unit 
               12  upstream buffer unit 
               13  frame multiplexing unit 
               14  optical transmission unit 
               15  logical-link control unit 
               16  frame control unit 
               17  frame separation unit 
               18  optical reception unit 
               101  upper-level device 
               102  lower-level device