JP4052288B2 - Receiver in optical communication system - Google Patents

Description

  The present invention relates to an optical communication system in which a master station and a plurality of slave stations are connected by an optical fiber via a passive optical splitter.

  In a system for bidirectional communication between a master station and a plurality of slave stations using an optical data communication network, there is a network configuration in which the master station and each slave station are radially connected by a single optical fiber. It has been put into practical use (Single Star). This network configuration simplifies the system and equipment configuration, but since one slave station occupies one optical fiber (if the number of slave stations is N, N optical fibers are required), the system is low. It is difficult to make a price.

  Therefore, a PON (Passive Optical Network) system (also referred to as PDS (Passive Double Star)) in which one optical fiber is shared by a plurality of slave stations has been proposed. In this PON system, a master station and a passive optical branching unit are connected by an optical fiber, and an optical branching unit and a plurality of slave stations are connected by a plurality of optical fibers. Further, a configuration in which another optical branching device is inserted between the optical branching device and the plurality of slave stations is also employed.

In the PON system, the downstream optical signal transmitted from the master station to a specific slave station is distributed to all the slave stations, but only the slave station of the destination address takes in the optical signal. In addition, multicast or broadcast is also used to transmit a downstream optical signal to an unspecified slave station without specifying a destination address.
In the PON system, an optical signal from a slave station installed in a location close to the master station or a slave station with few branches, and an optical signal from a slave station installed in a location far from the master station or a slave station that has passed multiple branches. The optical reception intensity at the master station differs greatly from the signal. This is because light is attenuated when propagating through the optical fiber and is further attenuated every time it passes through the optical branching unit.
Japanese Patent Laid-Open No. 11-355218

In the optical receiver of the master station, if the light receiving sensitivity is always maximized, a weak optical signal can be received with high sensitivity, but if a strong optical signal is input, it is easily saturated and accurate optical signal reception is possible. Can not be.
Therefore, this light receiving sensitivity is varied to be large for weak input optical signals and small for strong input optical signals.

  In the method of changing the light receiving sensitivity, it is conceivable to measure the intensity of the optical signal from each slave station and feedback control the bias voltage of the light receiving element (Patent Document 1). However, the optical signal from each slave station is considered. In addition to the very short rise time (about 1 nanosecond), the feedback loop has a time constant, and therefore there is a problem that it cannot sufficiently follow the rise of the received optical signal.

  Therefore, in the optical communication system in which the master station and a plurality of slave stations are connected with an optical fiber via an optical branching unit, the present invention can be used in advance when the optical reception intensity from the slave station can be predicted. It is an object of the present invention to provide a receiving apparatus in an optical communication system in which the light receiving sensitivity can be set and the light receiving intensity from the slave station cannot be predicted.

The receiving apparatus in the optical communication system of the present invention includes a light receiving element that receives an upstream optical signal from a slave station, a sensitivity control unit that sets a light receiving sensitivity of the light receiving element, an identification number of the slave station, and light from the slave station When communicating with a specific slave station and a storage unit that stores the received light intensity of the signal, at the time before receiving the upstream optical signal from the specific slave station, from the slave station written in the storage unit when to set the receiving sensitivity of the light receiving element in accordance with the data of the received light intensity of the optical signal and receives the upstream optical signal from the child station, it communicates with unspecified slave station, the uplink from each slave station Before receiving the optical signal, the light receiving sensitivity of the light receiving element is set to a low level to receive an upstream optical signal from each slave station, and then the light receiving sensitivity of the light receiving element is changed to a high level. Te, a reception control unit waits for retransmission of the optical signal from the slave station It is as it has.

  According to the above configuration, when communicating with a specific slave station, the optical signal from the slave station written in the storage unit at the time before receiving the upstream optical signal from the specific slave station Since the light receiving sensitivity of the light receiving element is set according to the intensity data, it is possible to always receive the upstream optical signal with the optimum sensitivity. When communicating with unspecified slave stations, the light receiving sensitivity of the light receiving element is set to a low level before receiving the upstream optical signal from each slave station. An upstream optical signal from (for example, a nearby slave station) can be normally received. If the light receiving sensitivity of the light receiving element is set to a high level from the beginning, it is possible to prevent the light receiving element from being damaged when a strong optical signal is input. And if there is an upstream optical signal that could not be received normally due to insufficient sensitivity, it can be detected by error checking, optical level detection, etc., so the light receiving sensitivity of the light receiving element is changed to a low level, Wait for the next optical transmission. Thereby, even when the optical reception intensity from the slave station cannot be predicted, the light receiving sensitivity of the master station can always be set to an optimum value, and the upstream optical signals from all the slave stations can be received.

The receiving apparatus in an optical communication system of the present invention, a light receiving element for receiving the upstream optical signal from the slave station, and a sensitivity control unit that sets the receiving sensitivity of the light receiving element, the light receiving upstream optical signals from the slave station An optical level detector for detecting the intensity, a storage unit for storing the identification number of the slave station and the received light intensity of the optical signal from the slave station, and an upstream optical signal from the slave station when communicating with an unknown slave station in the time before receiving configured to receive the uplink signals from the slave station sets the receiving sensitivity of the light receiving element to a low level, stored in the storage unit for each slave station a received light intensity of the optical signal, then A reception control unit that changes the light receiving sensitivity of the light receiving element to a high level and waits for retransmission of the optical signal from the slave station.

  When receiving the upstream optical signal from the slave station, the receiver sets the light receiving sensitivity of the light receiving element to a low level and communicates the upstream signal from the slave station when communicating with an unknown slave station. The received light intensity of the optical signal is stored in the storage unit for each slave station. When there is an upstream optical signal that could not be received normally due to insufficient sensitivity, it can be detected by error checking, light level detection, etc., so the light receiving sensitivity of the light receiving element is changed to a low level and the next signal from the slave station is changed. Wait for transmission. Thereby, the upstream optical signal from all the slave stations can be received, and the received light intensity of the optical signal from each slave station can be stored in the storage unit.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a block diagram showing a PON system. The PON system component in the station building is referred to as a master station or station side device, and the PON system component in the subscriber premises is referred to as a slave station or subscriber side device. The PON system includes a master station 1, a plurality of slave stations 5, and optical branching devices 3a and 3b. The master station 1 and the optical branching device 3a are connected by an optical fiber 2, and the optical branching device 3a and the optical branching device are connected. The optical fiber 4 connects the optical branching device 3b, the optical branching device 3a and the slave station 5, and the optical branching device 3b and the slave station 5, respectively. A single mode fiber is used as the optical fiber. Each of the optical branching devices 3a and 3b is configured by a star coupler.

The downstream optical signal from the master station 1 to the slave station 5 and the upstream optical signal from the slave station 5 to the master station 1 are each composed of packets.
The master station 1 receives a packet sent from a host network (such as the Internet), sends it to the slave station 5 through an optical fiber, receives a packet sent from the slave station 5, and sends it to the host network have.

The master station 1 includes an optical transmission line termination device OLT (Optical Line Terminals), a layer 2 switch, and the like, which are connection ends with the optical fiber 2.
The slave station 5 includes a personal computer installed in the house, an optical subscriber line terminating device ONU (Optical Network Unit) that transmits and receives broadband optical signals of the personal computer to the optical network, and the like.

  Briefly describing the operation of the PON system, a downstream packet that enters the master station 1 from a higher-level network is subjected to predetermined processing by the layer 2 switch in the master station 1. Then, it is transmitted to the optical fiber through the optical transmission line termination device OLT. The optical signal transmitted to the optical fiber is branched by the optical branching devices 3a and 3b and transmitted to the slave station 5 connected to the optical branching devices 3a and 3b. And decrypt the packet.

On the other hand, the upstream packet transmitted from the slave station 5 is transmitted to the master station 1 via the optical branching units 3a and 3b. In the master station 1, after a predetermined process is performed by the layer 2 switch, it is transmitted to the upper network.
At the time of starting up the PON system or the like, since it is unclear which slave station is connected to the PON system, the master station 1 performs a discovery operation.

The master station 1 broadcasts from the master station 1 to all the slave stations 5. Packets are returned from each slave station 5 at random timing. In this case, if there is a packet received normally, the received light level of the slave station that transmitted the packet and the identification number (ID) of the slave station are stored. In this way, the slave station connected to the PON system is specified.
After specifying the slave station, communication is performed on a one-to-one basis with each slave station. This is called “unicast” for broadcasting.

  At the time of unicast, it is necessary that uplink packets transmitted from the slave stations 5 do not compete with each other in time. Therefore, when transmitting a packet from the master station 1 to the slave station 5, an upstream optical signal time slot (hereinafter simply referred to as "slot") is assigned to each slave station 5. The slave station 5 to which the slot is assigned transmits an uplink packet to the assigned slot. Therefore, contention for upstream packets between the slave stations 5 is avoided. Note that the clock must be shared between the master station 1 and the slave station 5, but the time adjustment of this clock is performed by including time information in the packet when performing packet communication. It can be carried out.

Whether it is broadcast or unicast, the master station 1 receives an upstream packet from each slave station 5, but the optical signal strength differs depending on the slave station 5. For example, the length of the optical fiber path is different for each slave station 5, and it differs depending on whether or not it passes through the optical branching unit 3 b.
FIG. 2 is a received waveform diagram for explaining the difference in the intensity of the upstream packet from each slave station 5, with the horizontal axis representing time and the vertical axis representing received intensity. Packets 1 to 4 received from different slave stations 5 have different reception strengths.

  FIG. 3 is a block diagram showing a configuration of the optical transmission line terminating device OLT in the master station 1. The optical transmission line terminator OLT is provided with a WDM (Wavelength Division Multiplexing) filter that separates and extracts an optical signal received from each slave station through an optical fiber. The upstream optical signal received through the WDM filter is detected by an avalanche photodiode (APD) having an optical multiplication function, and the light reception signal of the APD is a transimpedance amplifier for converting a current signal into a voltage signal. 11 to the post-amplifier 16. Further, a light level detector 17 for monitoring the intensity of the light reception signal of the APD is provided.

  Further, a variable voltage bias power supply 12 for providing a bias voltage to the APD is provided. In principle, the voltage variable bias power supply 12 can employ a circuit in which the feedback voltage of the voltage variable DC-AC converter is variable. Specifically, a voltage variable type DC-DC converter power supply circuit (for example, Linear Technology LT1930 / LT1930A) developed for APD can be employed.

Further, an optical reception control circuit 13 for controlling the bias voltage of the APD is provided. The optical reception control circuit 13 includes a reception intensity storage unit (referred to as “LLID table 14”) 14 that stores reception intensity from each slave station 5, and reception control that determines an optimum bias voltage based on the stored reception intensity. Part 15.
The functions described below of the reception control unit 15 are executed by a computer provided in the optical transmission line terminating device OLT using a predetermined program.

  The reception control unit 15 measures the intensity of light received from each slave station 5 and stores the intensity information in the reception intensity storage unit 14. The measurement time point may be measured every time an uplink signal is received from the slave station, or may be measured every time a predetermined time has elapsed since the previous measurement. The measurement may be performed once or several times a day. As a measurement method, the current flowing through the APD may be measured, and the received light intensity level may be determined by the post amplifier.

Further, the reception control unit 15 manages the time schedule of the slots allocated to the slave station 5. Based on this time schedule, it is known in advance which slave station 5 receives the uplink packet at which time.
FIG. 4 is a graph showing the relationship between the APD multiplication factor M and the reverse bias voltage. The reception control unit 15 sets a large multiplication factor M for a small reception intensity, and sets a small multiplication factor M for a strong input optical signal. For this purpose, the optical reception control circuit 13 has the LLID table 14 that defines the relationship between the reception intensity and the optimum bias voltage of the APD based on this graph. The optical reception control circuit 13 uses this LLID table 14 to adjust the APD bias voltage.

Hereinafter, the operation of the reception control unit 15 during the discovery operation of the master station will be described with reference to a flowchart (FIG. 5).
First, the optical reception control circuit 13 sets the control signal CNT to the bias power supply 12 to 0 (step S1). CNT = 0 is a control state for setting the APD reverse bias voltage to a low value and setting the APD sensitivity low.

The reception control unit 15 performs broadcast transmission in a state where CNT = 0 (step S2).
Each slave station receives a broadcast optical signal from the master station, transmits an upstream optical signal, and returns a reply. In order to prevent collision, it is preferable that the time for transmitting the upstream optical signal is different for each slave station. Therefore, it is preferable to assign a different time for each slave station. For example, it may be a time that each slave station randomly generates with a random number generator.

  When receiving the upstream optical signal from the slave station (step S3), the master station checks whether a normal signal has been received (step S4). If the signal is received normally, the received light level and identification number (ID) from the slave station are stored and written in the LLID table 14 (step S5). Then, a reply to the effect is sent to the slave station (step S6), and CNT = 0 is set (step S7).

  FIG. 6 shows an example of the structure of the LLID table 14. The upper column of the LLID table 14 is a column for entering the identification number of the slave station. In the middle column, enter the level of light received from the slave station. The contents of the control signal CNT are entered in the lower column. CNT = 0 is a control state for setting the APD reverse bias voltage to a low value to set the sensitivity of the APD low, and CNT = 1 sets the APD reverse bias voltage to a high value to set the APD reverse bias voltage to a low value. This is a control state for setting the sensitivity high.

  After performing the processes of steps S5 and S6, it is estimated that there are slave stations that have not been received yet. As described above, the length of the optical fiber path differs from one slave station 5 to another, and whether it passes through the optical splitter 3b or not, the upstream optical signal intensity from the slave station may be weak. Because there is. There is also a collision between signals. Therefore, it is considered that an error has occurred for these unknown slave stations (NO in step S4), the control signal CNT is set to 1 (step S8), and the next reception is waited for a predetermined time.

  On the other hand, for the slave station, when a normal signal is not received at the master station, a reply (step S6) is not sent to the slave station. Know what you did not get. In this case, the slave station retransmits the upstream optical signal after waiting for a certain time. This “time” may be a predetermined time that is different for each slave station, or may be a time randomly generated by a random number generator. However, do not exceed the upper limit time. The “upper limit time” is a predetermined time during which the master station waits for reception from the slave station in step S8.

In this way, the reception control unit 15 sets the control signal CNT to 1 for the upstream optical signal from the slave station that has not been able to receive a normal signal in a state in which the sensitivity of the APD is set low after the broadcast. Can be received again with the sensitivity set to high.
Next, the operation of the optical reception control circuit 13 during normal communication after the end of the discovery operation will be described using a flowchart (FIG. 7).

First, the optical reception control circuit 13 sets the control signal CNT to the bias power supply 12 to 0, sets the reverse bias voltage of the APD to a low value, and sets the sensitivity of the APD to low (step T1).
Then, the transmission mode of the transmission frame acquired from the upper layer circuit is confirmed (step T2), and transmission is started toward a predetermined slave station (step T3).

FIG. 8 shows an example of a transmission frame. The header of the transmission frame describes the transmission mode (broadcast or unicast), the slave station identification number ID that is the transmission destination, and the address to the data to be transmitted.
When the transmission mode is unicast (YES in step T4), the uplink received light level registered in the slave station is confirmed with reference to the LLID table 14 based on the identification number of the slave station of the transmission destination (step S4). T5).

When the upstream received light level of the slave station registered in the LLID table 14 is “strong” (step T6), the upstream optical signal from the slave station is received and received with the APD sensitivity remaining low. Thereafter, the received light level is confirmed and overwritten on the LLID table 14 (step T7).
If the uplink received light level of the slave station registered in the LLID table 14 is “weak”, in order to increase the sensitivity of APD, CNT = 1 is set (step T8), and the uplink from the slave station is set. Receive an optical signal. After reception, the received light level is confirmed and overwritten on the LLID table 14 (step T7).

  If the transmission mode is broadcast in step T4, broadcast transmission is performed for all the slave stations as in FIG. Each slave station receives the broadcast optical signal from the master station, transmits an upstream optical signal, and returns a reply, and the master station receives the upstream optical signal from the slave station (step T9). After receiving the upstream optical signal from the slave station, the master station transmits a confirmation signal to the slave station for reception confirmation.

In step T9, there may be a slave station that cannot be received because the upstream optical signal from the slave station is weak. Therefore, it is determined that these slave stations are in error (YES in step T10), and the control signal CNT is set to 1, that is, the APD reverse bias voltage is set to a high value to set the APD sensitivity high (step S10). T11).
Even if the slave station receives the broadcast optical signal from the master station, transmits the upstream optical signal, and sends a reply, the slave station cannot receive the confirmation signal from the master station, and therefore transmits the upstream optical signal again. At this time, since the APD sensitivity is set high in the master station (step T11), the upstream optical signal from the slave station can be normally received. The master station receives the upstream optical signal from the slave station, confirms the received optical level, and overwrites the LLID table 14 (step T7).

As described above, the optical reception control circuit 13 refers to the reception light level written in the LLID table 14 during unicast, and sets the APD sensitivity to be high if the reception light level is low. The upstream optical signal from can be normally received.
At the time of broadcasting, in the state where the APD sensitivity is set low after the broadcast, the upstream optical signal from the slave station that cannot receive a normal signal can be received again in the state where the APD sensitivity is set high.

Thus, in any case, it is possible to communicate with the slave station connected to the optical communication system without omission.
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 a block diagram which shows the whole PON system. It is a received waveform diagram for explaining the difference in the intensity of the upstream optical signal from each slave station 5. 2 is a block diagram showing a configuration of an optical transmission line terminating device OLT in a master station 1. FIG. It is a graph which shows the relationship between the multiplication factor M of APD, and a reverse bias voltage. 7 is a flowchart illustrating an operation of the optical reception control circuit 13 during a master station discovery operation. 4 is a structural diagram of an LLID table 14. FIG. 6 is a flowchart illustrating an operation of the optical reception control circuit 13 during normal reception after the end of the discovery operation. It is a structure diagram of a transmission frame.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Master station 2 Optical fiber 3a, 3b Optical branching device 4 Optical fiber 5 Slave station 11 Transimpedance amplifier 12 Bias power supply 13 Optical reception control circuit 14 LLID table 15 Reception control part 16 Post amplifier 17 Optical level detection part APD Avalanche photodiode OLT Optical transmission line terminator ONU of master station Optical subscriber line terminator LD of slave station LD Laser diode LDD Laser diode drive circuit WDM Wavelength separation filter

Claims (4)

A receiver provided in the master station in an optical communication system in which a master station and a plurality of slave stations are connected with an optical fiber via an optical splitter,
A light receiving element for receiving an upstream optical signal from the slave station;
A sensitivity control unit for setting the light receiving sensitivity of the light receiving element;
A storage unit for storing the identification number of the slave station and the received light intensity of the optical signal from the slave station;
When communicating with a particular slave station, at the time prior to receiving the upstream optical signal from that particular slave station, depending on the data of the received light intensity of the optical signal from the child station written in the storage unit When receiving the upstream optical signal from the slave station by setting the light receiving sensitivity of the light receiving element, when communicating with an unspecified slave station, at the time before receiving the upstream optical signal from each slave station, Set the light receiving sensitivity of the light receiving element to a low level to receive the upstream optical signal from each slave station, then change the light receiving sensitivity of the light receiving element to a high level and retransmit the optical signal from the slave station And a reception control unit that waits for a reception apparatus in an optical communication system. A receiver provided in the master station in an optical communication system in which a master station and a plurality of slave stations are connected with an optical fiber via an optical splitter,
A light receiving element for receiving an upstream optical signal from the slave station;
A sensitivity control unit for setting the light receiving sensitivity of the light receiving element;
An optical level detector that detects the received light intensity of the upstream optical signal from the slave station;
A storage unit for storing the identification number of the slave station and the received light intensity of the optical signal from the slave station;
When communicating with an unknown slave station, at the time before receiving the upstream optical signal from the slave station, the light receiving sensitivity of the light receiving element is set to a low level to receive the upstream signal from the slave station, A reception control unit that stores the received light intensity of the optical signal in each storage unit in the storage unit, and then waits for retransmission of the optical signal from the slave station by changing the light receiving sensitivity of the light receiving element to a high level. A receiving apparatus in an optical communication system. The receiving device according to claim 1, wherein the light receiving element is a light receiving element having a photomultiplier action. The receiving apparatus according to claim 1, wherein the time during which the reception control unit waits for retransmission of an optical signal from a slave station is a predetermined time.

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