JP5067610B2 - Station side terminal equipment in PON system - Google Patents

Description

  The present invention relates to a PON system in which a plurality of slave stations are connected by optical fibers to perform optical communication.

  An optical data communication network is used between the station side terminal equipment OLT (Optical Line Terminal: optical subscriber line station side terminal equipment) and a plurality of slave stations ONU (Optical Network Unit: optical subscriber side terminal). There is an optical communication system that performs two-way communication. In this system, a single star network configuration in which the station side terminal device OLT and each slave station ONU are radially connected by a single optical fiber is put into practical use. In this network configuration, the system and equipment configuration are simplified, but if one slave station ONU occupies one optical fiber and the number of slave station ONUs is N, it is directly connected from the station side terminal device OLT. N optical fibers are required.

Therefore, a PON (Passive Optical Network) system (also referred to as a PDS (Passive Double Star)) in which one optical fiber drawn from the station-side terminal device OLT is shared by a plurality of slave stations ONU has been put into practical use. .
In the PON system, a station-side terminal device OLT and a passive optical branching device (hereinafter simply referred to as an optical coupler) that branches and multiplexes signals to a plurality of slave station ONUs passively (hereinafter simply referred to as an optical coupler) Are connected by a single mode fiber (hereinafter simply referred to as an optical fiber).

Thereby, many slave station ONUs can be assigned to one station side terminal device OLT, and the overall equipment cost can be suppressed.
Note that another optical coupler may be inserted between the optical coupler and the plurality of slave station ONUs.
In the PON system, the downstream optical signal transmitted from the station side terminal device OLT to the specific slave station ONU is simultaneously distributed to all the slave stations ONU, but the logical link identifier is added, Only the slave station ONU constituting the link can capture the optical signal. The upstream optical signal transmitted from the slave station ONU is assigned a transmission time for each slave station ONU so that the slave station ONUs do not collide with each other.

FIG. 7 is a block diagram showing an internal configuration of the station-side terminal device OLT 110 in the conventional GE-PON system (PON system defined by IEEE 802.3ah) 100.
The host network interface 150 is an interface for connecting to a host network such as the Internet, and is a Gigabit Ethernet such as 1000BASE-T (Ethernet is a registered trademark). A GE-PON MAC_LSI (hereinafter referred to as MAC_LSI) 140 performs GE-PON control conforming to IEEE802.3ah, and performs upstream transmission of a plurality of slave station ONUs (190a to 190i) connected on the PON transmission line 160. Control timing. The optical transmission / reception unit 120 performs E / O conversion for converting the electrical signal sent from the MAC_LSI 140 into an optical signal, and O / E conversion for converting the optical signal sent from the slave station ONU 190 into an electrical signal.

FIG. 8 shows a schematic graph (a) showing an upstream burst signal transmitted by each slave station ONU 90 and a schematic graph (b) when using the same time axis coordinates.
The upstream optical signal includes an upstream packet (referred to as a burst signal) from each slave station ONU 190. The upstream optical signal needs to be transmitted so that the optical signals from the respective slave station ONUs 190 do not compete with each other in time. For this purpose, the station-side terminal device OLT 110 assigns a time window (hereinafter simply referred to as a time window) during which an upstream optical signal may be transmitted to each slave station ONU 190, and each slave station ONU 190 uses a control frame. Notify

The slave station ONU 190 assigned the time window transmits the upstream optical signal in the assigned time window. Therefore, the competition of the upstream optical signal between each slave station ONU 190 is avoided.
On the other hand, there is a desire to increase the number of slave station ONUs 190 per one station side terminal device OLT110. For this purpose, an optical coupler 180 must be added.

However, in this case, there is a problem that the optical transmission line loss due to passing through the plurality of optical couplers 180 increases.
Therefore, the station-side terminal device is provided with a plurality of N optical transmission / reception units, and the slave stations are divided into N groups with respect to the slave stations of N or more stations, and the slave stations of each group are grouped by an optical coupler. A station-side terminal device having a circuit configuration for connecting each optical coupler to each optical transceiver and taking the logical sum of the electrical signals from the N optical transceivers and sending it to the upper circuit is known. (See Patent Document 1 below).

According to this station-side terminal device, by providing a plurality of N optical transceivers, the number of slave stations that can be accommodated in one station-side terminal device is increased without increasing the optical transmission line loss. it can.
JP 2006-352350 A Japanese Patent Laying-Open No. 2005-039309 "1.25Gbit / s burst receiver using ADP" 2003 IEICE Communication Society B-10-40

As can be seen from FIG. 8B, the intensity of the optical signal from each slave station ONU 190 varies depending on the slave station ONU190. This is because the number of optical couplers passing through each slave station ONU 190 and the transmission distance of the optical fiber are different.
The optical receiver circuit (not shown) provided in the optical transceiver 120 must adjust the optical signal reception sensitivity for each burst signal, and is a short time ("guard time") when the burst signal is switched. 8 (b) is used to switch the gain of the light receiving element to switch the gain of the light receiving element, and a station side terminal device that can follow a rapid change in optical reception intensity and a large drop is known (see above). Patent Document 2). The guard time Tg is set to 512 nanoseconds, for example.

For example, when an avalanche photodiode (hereinafter referred to as APD) is used as the light receiving element in the optical transceiver 120, a schematic circuit diagram of the receiver of the optical transceiver 120 is shown in FIG.
This APD is given a bias voltage with its cathode grounded via a capacitor C of about 100 pF for stable operation. Further, in order to change the gain of the APD, it is necessary to change the bias voltage of the APD within a large range of several volts to several tens of volts. For this reason, a time constant is generated, and in order to change the gain of the APD with a short guard time Tg, it is necessary to charge and discharge the capacitor C at high speed. This complicates the circuit configuration of the voltage control circuit 210 and increases the circuit scale.

  Therefore, an object of the present invention is to provide a station-side terminal device that can easily adjust the gain of the optical receiving unit in a station-side terminal device including a plurality of optical receiving units.

In order to achieve the above object, a station-side terminal apparatus according to the present invention provides a control for distributing and managing a time window in order to time-division multiplex an upstream optical signal transmitted from a slave station to a plurality of slave stations. A plurality of optical receivers that divide a circuit and each slave station into groups, and that are connected to each group and convert optical signals transmitted from the slave stations in the group into electrical signals; and the plurality of optical receivers A logical sum part for taking the logical sum of the electrical signals from the control circuit, the control circuit so that the different optical receivers sequentially receive the upstream optical signals from each of the time-division multiplexed slave stations, that is, the same as the light receiving unit does not receive continuously, Ru der to perform the distribution of the time window.
Furthermore, in order to check which optical receiver each slave station is connected to, the station-side terminal device of the present invention has a look-up table (LUT) that stores information on which optical receiver each slave station is connected to. )have. Based on the contents of the station-side terminal device, scheduling can be performed so that different optical receivers sequentially receive uplink burst signals from time-division multiplexed slave stations.

  Here, the number of slave stations accommodated by one station-side terminal device is m, and the number of optical receivers is n. Further, the number m is a number equal to or less than the maximum number (for example, 256) of logical links that can be managed by the station-side terminal device. Although the name “optical receiver” was given by paying attention to the optical receiver function of the optical receiver, the optical receiver functioned as described later in “Best Mode for Carrying Out the Invention” May be used in combination.

Information on a time window for time-division multiplexing of an optical signal transmitted from the slave station is added from the station-side terminal device corresponding to each slave station, and distributed to a plurality of optical receivers. Each optical receiver has an E / O conversion circuit that converts an electrical signal into an optical signal. The converted optical signal is sent to each slave station via an optical fiber or the like.
On the other hand, for an upstream signal, a plurality of n optical receivers in the station-side terminal device have an O / E conversion circuit that converts an optical signal transmitted from each slave station into an electrical signal. Then, the electrical signals converted by the respective optical receivers are logically summed with respect to the electrical signals by an OR circuit and sent to the control circuit.

  By the way, since the control circuit distributes the time window corresponding to each slave station, each slave station can send a signal to the station-side terminal device at a time corresponding to the time window. For this reason, the signals sent from the slave stations are sent to the control circuit via the optical receiver without colliding with each other. Therefore, since the control circuit can cope with signals from the slave stations of the plurality of m stations whose control frames are managed, the station-side terminal device can communicate with the slave stations of the plurality of m stations.

Therefore, according to this configuration, the PON system can increase the number of slave stations that can be accommodated in one station-side terminal device.
In a feature of the present invention, the control circuit distributes the time window so that different optical receivers sequentially receive upstream optical signals from the slave station ONUs that are time-division multiplexed. For example,
Optical receiver 0 ... ONU00, ONU01, ONU02
Optical receiver 1... ONU10, ONU11, ONU12
Optical receiver 2... ONU20, ONU21, ONU22
As shown, when there are three optical receivers, and each of the optical receivers is connected with three slave stations ONU (the first subscript represents the number of the connected optical receiver),
ONU00, ONU10, ONU20, ONU01, ONU11, ...
The time window for receiving from each slave station ONU is received so that different optical receivers sequentially receive upstream optical signals from the ONUs that are time-division multiplexed (ie, there is no order of ONU00 and ONU01). Schedule.

Thereby, each optical receiver does not receive the subsequent burst signal with a short guard time Tg. That is, when viewed from the optical receiver, it seems that the guard time, which is the interval between burst signals, is longer than usual. Thereby, each optical receiver can easily and reliably adjust the gain of the optical receiver .

The control circuit of the station-side terminal device monitors any one optical signal is received by the light receiving unit, Ru can configure the LUT based on the results of this monitoring. Thereby, it is possible to dynamically cope with a change in the configuration of the PON system.
If the control circuit requests each slave station to transmit an optical signal in turn and performs the monitoring based on detecting the reception detection signal of the optical receiver, the currently connected child because the station is able to know that through either of the light receiving portion, Ru can be easily configured the LUT. Then, the monitoring is stopped during a period in which reception detection signals of a plurality of optical receiving units are in competition, and when there is no competition, the reception signals of the optical receiving units are detected. As a result, it is possible to accurately know which optical receiver the connected slave station passes through.

Further, if the control circuit is capable of instructing reception disable to each optical receiving unit, the station-side terminal device can establish a specific optical receiving unit when establishing a connection with the slave station. only the output enable, not good when receiving the upstream signal. Since reception is possible only when the ONU is connected to an enabled optical receiver, it is possible to determine which optical receiver the ONU is connected to.

As described above, according to the present invention, different optical receivers can sequentially receive the upstream optical signals from each of the time-division-multiplexed slave stations, so that the time constant when the optical receiver performs gain control can be set. It is possible to have a margin.
In particular, when a weak signal is received after receiving a burst signal with a high optical level, the reception sensitivity deteriorates (for example, non-patent document 1 points out deterioration due to fluctuations in the APD bias voltage) due to the configuration of the receiving circuit. However, since the guard time interval is substantially widened, it is possible to reduce the amount of deterioration in reception sensitivity.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 shows a PON (Passive Optical Network) which is an optical communication system in which a single-mode optical fiber (70 and 71) is connected between a station-side terminal device OLT10 and a plurality of slave station ONUs 90 via an optical coupler 80. 1 is a schematic diagram illustrating a configuration example of a system 1. FIG.
The PON system 1 incorporates Gigabit Ethernet (Ethernet is a registered trademark) technology into the PON technology, and realizes optical fiber access section communication at a baseband speed of 1.25 Gbps. (Gigabit Ethernet-Passive Optical Network) system is adopted.

In this PON system 1, a main line optical fiber 70 and an optical branching device in which a station side terminal device OLT 10 provided in a station building and a slave station ONU 90 provided in a plurality of subscriber houses are directly connected to the station side terminal device OLT 10. And the branch optical fiber 71 directly connected to the slave station ONU 90.
The optical coupler 80 is a coupler that passively branches and multiplexes a signal from an input signal without requiring an external power supply.

The slave station ONU 90 includes a network interface for connecting a terminal that enjoys an optical network service, such as a personal computer installed in the subscriber's home.
The station-side terminal device OLT 10 includes optical transmission / reception units (also referred to as optical modules) 20A to 20N to be described later, and a plurality of trunk optical fibers 70A to 70N are connected to the optical transmission / reception units 20A to 20N. .

  Here, the trunk optical fiber 70A, the optical coupler 80A connected to the trunk optical fiber 70A, the branch optical fiber 71A, and the slave station ONU 90A are collectively referred to as an A group (the suffix of the symbol has A). Hereinafter, similarly, they are referred to as a B group to an N group. In the following description, the N group will be described. The same applies to the other groups unless otherwise specified.

  The total number of slave station ONUs 90 accommodated in the PON system 1 is m, and the total number of trunk optical fibers 70 connected to the station-side terminal device OLT10 is n, in other words, the station-side terminal device OLT10. The total number of the PON transmission lines 60 included in n is n sets. Further, the total number of optical couplers 80N in the N group is s, and the total number of branch optical fibers in the PON transmission line 60N is i, in other words, the total number of slave stations ONU 90 in the N group is i station.

The trunk optical fiber 70N is connected to i branch optical fibers 71N1 to 71Ni directly connected to the slave stations ONUs 90N1 to 90Ni of i stations via s optical couplers 80N. In other words, in the N group of one station-side terminal device OLT10, the slave stations ONUs 90N1 to 90Ni of i stations are accommodated.
Specifically, when the total n of the PON transmission lines 60 is 8 (groups) and the total i of the slave station ONUs 90 is 32 (stations), the station-side terminal device OLT 10 of one station is the maximum of logical links. The number of 256 child station ONUs 90 equal to the maximum allocation number of 256 (pieces) can be accommodated.

FIG. 2 is a block diagram showing an internal configuration of the station-side terminal device OLT 10.
The station-side terminal device OLT10 is connected to an upper network interface 50 connected to an upper network such as the Internet network, a GE-PON MAC_LSI (hereinafter simply referred to as MAC_LSI) 40 as a control circuit corresponding to Gigabit Ethernet, and the MAC_LSI 40. It has n optical transmission / reception parts 20A-20N. Further, the station-side terminal device OLT 10 includes a logical sum circuit 30 that is a means for performing a logical sum on electric signals from the plurality of optical transmission / reception units 20A to 20N to the MAC_LSI 40.

Note that when the bias condition for realizing the logical sum by the connection is satisfied, the logical sum circuit 30 is not necessary, and the connection may be simply made. This connection substantially performs a logical sum function.
Each of the optical transmission / reception units 20A to 20N is connected to a corresponding PON transmission path 60A to 60N, respectively. Each PON transmission line 60A to 60N is connected to each child station ONU 90 as described above.

According to the GE-PON system, communication between the station-side terminal equipment OLT 10 and each slave station ONU 90 is performed in units of variable-length frames. The frame structure is composed of a GE-PON header including a logical link identifier and data of 64 bytes or more. The maximum size of data is generally about 1530 bytes.
First, a downstream packet that enters the station-side terminal device OLT 10 from an upper network is sent to the MAC_LSI 40 via the upper network interface 50. The MAC_LSI 40 performs a predetermined bridge process, and specifies a logical link to be relayed. The MAC_LSI 40 adds, for example, a GE-PON header including a logical link identifier corresponding to the slave station ONU 90Ni to the frame, to the signal corresponding to each slave station ONU 90. Then, the MAC_LSI 40 transmits the same frame information to each of the optical transceivers 20A to 20N.

Receiving the same frame information, each of the optical transmission / reception units 20A to 20N performs the same processing and converts an electrical signal into an optical signal (E / O conversion). Therefore, the predetermined packet information received from the Internet network is converted into the same optical signal via the respective optical transceivers 20A to 20N, and transmitted through the PON transmission lines 60A to 60N of the groups A to N.
A series of frame information is branched into i branch optical fibers 71N1 to 71Ni by an optical coupler 80N through a trunk optical fiber 70N of each PON transmission line 60N. Then, it is sent to each slave station ONU 90N1 to 90Ni through i branch optical fibers 71N1 to 71Ni. Each slave station ONU 90N1 to 90Ni captures only frame information to which a logical link identifier corresponding to the slave station ONU 90N1 to 90Ni is added, and relays the packet to the home network interface.

Next, communication in the upstream direction, that is, the direction from the slave station ONU 90 to the station-side terminal device OLT 10 will be described.
The upstream optical signal includes an upstream packet from each slave station ONU 90. The upstream optical signal needs to be transmitted so that the optical signals from the respective slave station ONUs 90 do not compete with each other in time. For this purpose, the station-side terminal device OLT 10 includes a storage unit 41 that stores a lookup table (LUT) that is referred to when assigning a time window. The storage unit 41 includes a rewritable memory such as a hard disk or a flash memory. The MAC_LSI 40 refers to the LUT, assigns a time window (hereinafter simply referred to as a time window) during which an upstream optical signal may be transmitted to each slave station ONU 90, and time-division multiplexes the transmitted optical signal. Each slave station ONU 90 is notified as a control frame.

The slave station ONU 90 assigned with the time window transmits the upstream optical signal within the assigned time window. Therefore, the competition of the upstream optical signal between each slave station ONU 90 is avoided.
Note that the clock must be shared between the station-side terminal device OLT 10 and each slave station ONU 90. The time adjustment of this clock is performed when the MAC_LSI 40 communicates the control frame with the time information. It can be done by including it in

In this way, the station-side terminal device OLT 10 can receive a burst optical signal including a series of packet signals from each slave station ONU 90.
As illustrated in FIG. 8, the burst optical signal is a finite-time optical signal sequence in which the light emission state is changed with a baseband signal of 1.25 Gbps. At the beginning of the frame sequence, there is a period for stabilizing the light emission state and light reception state, and for recovering the baseband signal on the receiving side (these are collectively referred to as the synchronization adjustment period (Sync Time)). Transmits a unique signal sequence. Further, the tail part of the frame sequence has an adjustment period for quenching.

FIG. 3 shows an example of a look-up table (LUT) that is referred to when the MAC_LSI 40 of the station-side terminal device OLT 10 assigns time windows so that optical signals from the respective slave station ONUs 90 do not compete with each other in time. FIG.
The MAC_LSI 40 assigns a time window to one of the slave station ONUs 90A connected to the optical transmitter / receiver 20A, for example, the slave station ONU 90A1, and then assigns the next time window to any of the slave stations ONU 90B1 connected to the optical transmitter / receiver 20B. Further, after assigning a time window to the slave station ONU 90B1, the next time window is assigned to any slave station ONU 90C1 connected to a different optical transceiver unit, for example, the optical transceiver unit 20C. In this way, time windows are assigned so that the same optical transceiver does not receive continuously.

  By the control for assigning the time window, the optical transmission / reception unit 20A that first receives the optical signal of the slave station ONU 90A1 is several times to several tens of times longer than the guard time Tg until the optical signal of the slave station ONU 90A2 is received next. Double time is secured. In this long time, reception gain control may be performed in preparation for optical reception from the slave station ONU 90A2 that is expected to be received next. That is, when the voltage control circuit 210 (see FIG. 9) of the optical transmission / reception unit charges and discharges the capacitor C, the charge and discharge can be performed with a sufficient time. Therefore, a high capability as the instantaneous light bulb supply capability of the voltage control circuit 210 is not required, the internal impedance of the output terminal of the voltage control circuit 210 may be high to some extent, and the configuration of the voltage control circuit 210 can be simplified. In addition, since a capacitor having a large capacity can be used as the capacitor C, the cut-off frequency of the low-pass filter composed of the capacitor C and the resistor R can be reduced, and the advantage of being strong against noise from the voltage control circuit side. There is also.

Next, how to make an LUT will be described.
When the configuration of the PON system 1 including the slave station ONU 90 is fixed, if one LUT is created first, the LUT can be used all the time.
However, since the configuration of the PON system 1 usually fluctuates (for example, a slave station is generated when the power switch is turned on and disappears when the power switch is turned off), the LUT must be recreated frequently.

The discovery procedure is executed when the LUT is reconfigured.
The discovery procedure refers to a procedure for periodically inspecting the presence / absence of connection of the slave station ONU 90, the intensity of the upstream optical signal from each slave station ONU 90, and the round trip time (round trip time) to each slave station ONU 90.
As shown in FIG. 4, the MAC_LSI 40 can acquire a signal detection (SD) signal from each optical transmission / reception unit 20 to notify that an optical signal has been received.

In this discovery procedure, the MAC_LSI 40 requests each of the slave station ONUs 90 for an upstream optical signal containing the slave station identification code in order.
The optical transmission / reception unit 20 that has received the optical signal from the slave station performs O / E conversion into an electrical signal, and sends the converted electrical signal to the OR circuit 30. The OR circuit 30 transfers the frame information that has become an electric signal to the MAC_LSI 40. At this time, the MAC_LSI 40 can detect which optical transmission / reception unit 20 has received the upstream frame information by the SD signal.

  Since the SD signal is not necessarily intended for detection of a burst optical signal, the response time for frame detection may be slow. In this case, it is assumed that SD signals of a plurality of optical receiving units are generated at the same time, and it is impossible to correctly determine which optical receiving unit is receiving. 5A to 5D illustrate how the SD signals of the optical receivers A and B are simultaneously generated. Therefore, in this case, the discovery procedure is started or restarted after all the SD signals become Low. As shown in FIGS. 5 (e) and 5 (f), after the competition of the SD signal is eliminated, the upstream optical signal including the slave station identification code is requested again from the slave station ONU, and the upstream optical signal and the SD Signal.

Thereby, since there is no contention for the SD signal, it is possible to correctly determine which optical transmission / reception unit 20 has received the upstream frame information.
Therefore, it is possible to identify which slave station ONU 90 is suspended by which optical transmission / reception unit 20. Based on this hanging information, it is possible to create an LUT for time window allocation so that the same optical transceiver 20 does not continuously receive upstream signals.

  FIG. 6 is a block diagram showing another configuration example of the station-side terminal device OLT 10 for creating the LUT. In this configuration, the MAC_LSI 40 can supply a disable signal for stopping the reception function of each optical transceiver 20 to each optical transceiver 20 instead of detecting an SD signal. The MAC_LSI 40 scans the disable signal with respect to the all-optical transmission / reception unit 20 and receives the upstream signal from the slave station ONU 90, thereby determining which optical transmission / reception unit 20 the slave station ONU 90 sends the signal to. Can be determined. Therefore, as in the case of FIG. 4, it is possible to create an LUT for time window allocation so that the same optical transmission / reception unit 20 does not continuously receive upstream signals. As an example of the disable signal, an output on / off command signal of the optical transceiver 20 is opened.

  Although the embodiments of the present invention have been described above, the embodiments of the present invention are not limited to the above-described embodiments, and various modifications can be made within the scope of the present invention.

It is the schematic which shows the structural example of the PON system which connected between the station side terminal apparatus OLT and several subunit | mobile_unit ONU with the optical fiber via the optical coupler. It is a block diagram which shows the internal structure of the station side terminal apparatus OLT. It is a figure which shows an example of the look-up table (LUT) referred when MAC_LSI40 of the station side terminal apparatus OLT10 allocates a time window. It is a block diagram which shows the example of 1 structure of the station side terminal apparatus OLT10 for making LUT. It is a graph which shows the waveform of the upstream optical signal and SD signal when the upstream optical signal containing a slave station identification code is requested | required of the slave station ONU. It is a block diagram which shows the other structural example of the station side terminal apparatus OLT10 for making LUT. It is a block diagram which shows the internal structure of the station side terminal apparatus OLT in the conventional PON system. It is a schematic graph (a) which shows the burst signal of the up direction which each sub_station | mobile_unit ONU controlled by the control frame transmits, and a schematic graph (b) when using the same time-axis coordinate. It is a figure which shows the schematic circuit structural example of an optical transmission / reception part.

Explanation of symbols

1 PON system 10 station side terminal equipment OLT
20 Optical transceiver 30 OR circuit 40 GE-PON MAC_LSI (control circuit)
41 Look-up table (LUT) storage unit 50 Host network interface 60 PON transmission line 70 Trunk optical fiber 71 Branch optical fiber 80 Optical coupler (optical splitter)
90 Slave station ONU
210 Voltage control circuit

Claims (5)

A station-side terminal device used in a PON (Passive Optical Network) system that connects multiple slave stations with optical fibers to perform optical communication,
A control circuit that distributes and manages a time window for time-division multiplexing of upstream optical signals transmitted from the slave stations to a plurality of slave stations;
A plurality of optical receivers that divide each slave station into groups, are connected to each group, and convert optical signals transmitted from the slave stations in the group into electrical signals;
A logical sum unit for taking a logical sum of the electrical signals from the plurality of optical receiving units,
The control circuit has a look-up table (LUT) storing information indicating which optical receiver each slave station is connected to, and using this LUT, the uplink from each slave station time-division multiplexed The station-side terminal device characterized in that the time window is distributed and managed so that different optical receivers sequentially receive optical signals.   The station side terminal according to claim 1, wherein the control circuit distributes and manages the time window so that the same optical receiver does not continuously receive the upstream optical signal from each of the time-division multiplexed slave stations. Station equipment. Wherein the control circuit, one of the monitors whether the optical signal is received by the light receiving unit, the claim 1 or claim 2 station-side terminal device according to create the LUT based on the result of the monitoring. The control circuit requests each slave station to transmit an optical signal in turn, performs the monitoring based on detecting the reception detection signal of the optical receiver, and the reception detection signals of a plurality of optical receivers The station-side terminal device according to claim 3, wherein the monitoring is stopped during a period of contention. The station side terminal apparatus according to claim 3 , wherein the control circuit is capable of commanding reception disable to each optical receiving unit.

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