JP4172475B2 - Station side optical communication equipment - Google Patents

Description

  The present invention relates to a station-side optical communication apparatus in which a plurality of subscriber-side optical communication apparatuses are connected by branching one optical fiber and perform packet communication with the subscriber.

There is an optical fiber network for bidirectional communication between a station side optical communication device and a plurality of subscriber side optical communication devices. As an example, one optical fiber is shared by a plurality of subscriber side optical communication devices. PON (Passive
There is a form of Optical Network.
The station side optical communication apparatus is often referred to as OLT (Optical Line Terminal), and the subscriber side optical communication apparatus is often referred to as ONU (Optical Network Unit).

A general form of the PON is as shown in FIG.
The OLT 2 and the optical splitter 3 are connected by a single trunk optical fiber 4, and the optical splitter 3 and the ONUs 1a to 1c are connected by a plurality of branch optical fibers 5a to 5c.
Such a conventional PON type optical fiber network is disclosed in Japanese Patent Laid-Open No. 2005-20417 (Patent Document 1).

In such a PON, for data transmission from the OLT 2 to the ONUs 1a to 1c (hereinafter referred to as “downlink transmission”), the OLT 2 regularly transmits data for each of the ONUs 1a to 1c sequentially in packets, and the ONUs 1a to 1c. Receives packets addressed to itself by reading destination information included in each packet.
On the other hand, regarding data transmission from each ONU 1a to 1c to the OLT 2 (hereinafter referred to as "upstream transmission"), packets may collide if the transmission timing of each ONU 1a to 1c is different, so that each ONU 1a to 1c Is transmitted according to the communication timing specified by the OLT 2. Further, this communication timing is determined by a control signal transmitted down to each of the ONUs 1a to 1c.

Further, since the distances of the branch optical fibers 5a to 5c connecting the optical splitter 3 and the plurality of ONUs 1a to 1c vary depending on the locations where the ONUs 1a to 1c are installed, the attenuation rate of the signals passing through the branch optical fibers 5a to 5c Will be different.
For this reason, the OLT 2 must make a logical (0, 1) decision on signals of different magnitudes transmitted from the ONUs 1a to 1c, and various circuits are considered for performing this logical decision, but a simple circuit. A continuous system is sometimes used as a system that has a good S / N ratio and is resistant to noise. As an example of this continuous logic determination circuit, conventionally, for example, a receiving circuit 90 as shown in FIG.

The outline of the receiving circuit 90 in the OLT 2 includes a photodiode 91, an amplifier 92, an AC coupling circuit 93, a comparator 94, and a clock data recovery circuit 95 as shown in FIG.
Uplink transmission data from each of the ONUs 1 a to 1 c is received as an optical signal by the photodiode 91 of the receiving circuit 90 of the OLT 2 through the trunk optical fiber 4, converted into an electric signal, and input to the amplifier 92.
The signal amplified by the amplifier 92 is logically determined by the comparator 94 via the AC coupling circuit 93 that cuts the DC component, and input to the clock data recovery circuit 95.
The clock data recovery circuit 95 outputs data while reproducing the clock signal in accordance with the data logically determined by the comparator 94.

Incidentally, the signal input to the comparator 94 is affected by the time constant as shown in FIG. 10 due to the capacitance of the AC coupling circuit 93.
FIG. 10 shows a case where, for example, packet-like signals Z1 and Z2 are sequentially input to the comparator 94 as uplink transmission data, and the magnitude of the signal Z1 is larger than that of the signal Z2.

In this case, the influence of the time constant of the AC coupling circuit 93 appears in the periods S1 and S2, which are the head portions of the signals Z1 and Z2, and the signal is not stabilized, causing the following problems.
The data structure of the signals Z1 and Z2 can be schematically illustrated as shown in FIG. 11, and conventionally, a synchronization bit of a fixed synchronization time D for synchronization is provided at the head of each signal. . That is, a fixed-length synchronization bit is provided. In general, in a PON type optical fiber network, the head part of a signal transmitted from each ONU is an idle pattern signal of an encoding method 8B10B (IBM) in the case of GEPON (Gigabit Ethernet PON), for example. Is repeated to form a synchronization bit, which is called a preamble.

However, as described above, the head portion of the signal is unstable due to the influence of the time constant described above, and the synchronization bit in the period S2 becomes 0 level L0 or less as shown in FIG. is there. For this reason, the fixed synchronization time D of the synchronization bit needs to be longer than at least the period S2.
In particular, in the combination of the preceding and following signals such as the signals Z1 and Z2, when the previous signal Z1 is larger than the subsequent signal Z2 and the intensity difference is large, the head of the signal Z2 starts from the rear end of the signal Z1. The influence of the time constant until the part (the beginning of the period S2) becomes significant.

For this reason, the circuit cannot follow the change in the intensity of the signal, and the unstable period S2 of the leading portion of the subsequent signal Z2 tends to become longer.
Therefore, in the past, in order to cope with such a difference in signal strength, the fixed synchronization time D was set to be longer for a certain time. As a result, the bandwidth of data to be reduced is narrowed, resulting in a problem that the communication speed becomes slow.

Japanese Patent Laid-Open No. 2005-20417

  Therefore, the present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a station side optical communication apparatus capable of shortening the synchronization time and increasing the communication speed as compared with the prior art.

In order to solve the above-mentioned problem, the invention according to claim 1 is directed to each subscriber-side optical communication device with a communication timing in a state where a plurality of subscriber-side optical communication devices are connected by branching one optical fiber. By assigning the burst signal from each subscriber side optical communication device by time division, in the station side optical communication device,
Station-side optical communication apparatus, the preamble length to be added to the beginning portion of each burst signal to be received by the front and rear of the combination of the received burst signal, and the synchronization time calculating unit that calculates for each subscriber side optical communication apparatus,
A communication unit that transmits a control signal that can be changed to the calculated preamble length to the synchronization time change receiving unit on the subscriber side optical communication device side ; and
An optical communication apparatus on the station side is provided.

According to this configuration, the head portion of the signal is changed according to the signal status, as compared with the conventional case where a synchronization bit having a uniform fixed synchronization time (fixed length) is provided at the head portion of the signal. in subjecting the preamble length so (hereinafter, synchronization time referred to as the length of) can be variably set, to calculate the minimum synchronization time to take the communication synchronization can be set to as Become.
Since the synchronization time can be shortened in this way, the synchronization bit band can be narrowed and the data band to be originally communicated can be expanded. As a result, the communication speed can be improved.

The station-side optical communication apparatus, have a storage unit for storing the intensity of the burst signal every source communication device for each burst signal,
The synchronization time calculating unit, according to the intensity difference between the front and rear of the combination of the burst signals stored in the storage unit, it is preferable that calculates the preamble length.
In the calculation of the preamble length, when there is no difference in intensity and when the intensity of the subsequent signal is greater than the intensity of the previous signal, the preamble length of the subsequent burst signal is changed to be shorter .

As a result, when an element that affects the time constant such as a capacitor is provided in the receiving circuit of the station side optical communication device, the previous signal strength is greater than the subsequent signal strength in the combination before and after the signal. If the intensity difference is large, the influence of the time constant in the head portion of the subsequent signal becomes significant, so that stable synchronization can be established by extending the synchronization time.
On the other hand, if there is no difference in intensity between the previous signal and the subsequent signal in the combination before and after the signal, or if the subsequent signal strength is equal to or higher than the previous signal strength, the subsequent signal Since the influence of the time constant at the top of the signal becomes small, the synchronization time can be shortened.

The station-side optical communication device is a combination of a synchronization detection circuit that measures the time required for synchronization from the actually received burst signal and a subscriber-side optical communication device that has transmitted the burst signal immediately before the measured time. Having a storage unit for storing each
The synchronization time calculating unit may calculates a length of the synchronization time based on the measured time of each of the combinations is stored in the storage unit.
As a result, the optical communication apparatus on the station side measures and stores the time actually required for synchronization, so regardless of the method of the logic determination circuit (even if the logic determination is performed by looking at the intensity at the head of each burst signal) ), The length of the synchronization time can be calculated quickly and accurately based on the stored contents.

Further, by this, since the station-side optical communication apparatus is stored by measuring the time required for actually synchronized with each combination of the subscriber-side optical communication apparatus has transmitted the signals before and after, the station-side optical communication apparatus At the time when the communication timing of each subscriber-side optical communication device is determined, it is possible to obtain an accurate synchronization time immediately by referring to the stored contents.

As described above, the length of the synchronization time (preamble) depends on the status of the received signal, as compared with the case where the synchronization bit of the fixed synchronization time (fixed length) is uniformly provided at the head portion of the signal as in the past. (Length) can be made variable, so it is possible to set the minimum synchronization time required to synchronize communication, and the communication speed is improved by increasing the proportion of time for data exchange. Can be made.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
The station-side optical communication apparatus 20 (hereinafter referred to as “OLT 20”) of the present invention shown in FIG. 1 is a PON-type trunk optical fiber, similar to the conventional station-side optical communication apparatus (OLT 2 in FIG. 8) already described. 4 is connected.
The trunk optical fiber 4 is branched by the optical splitter 3 and connected to the subscriber side optical communication devices 10a to 10c (hereinafter referred to as “ONUs 10a to 10c”) via branch optical fibers 5a to 5c.
Since each of the ONUs 10a to 10c has the same function, the relationship between the OLT 20 and the ONU 10a will be described in the following description.

  The outline of the OLT 20 includes, for example, a control unit 21 that includes an arithmetic processing unit and performs processing such as calculation and determination, a storage unit 23, and a communication unit 25 that transmits and receives signals using light. Moreover, the control part 21 is provided with the synchronous time calculation part 22 and the time division designation | designated part 24, for example.

  The synchronization time calculation unit 22 calculates the minimum required length of the synchronization time in the signal transmitted by the ONUs 10a to 10c for each ONU 10a to 10c. That is, the length of the synchronization bit provided at the head portion of the signal is determined.

The storage unit 23 stores an operation program of the OLT 20, data indicating the strength of signals received from the ONUs 10a to 10c, and the like.
The time division designation unit 24 designates timing for transmitting data transmitted by each of the ONUs 10a to 10c as a packet burst signal.

  In this embodiment, the synchronization time calculation unit 22 and the time division designation unit 24 are programs that can be executed by the control unit 21 including an arithmetic processing unit, and are actually stored in the storage unit 23. However, since the function is performed by being executed by the control unit 21, the synchronization time calculation unit 22 and the time division designation unit 24 are shown in the control unit 21 in FIG. 1. Yes. However, it goes without saying that the synchronization time calculation unit 22 and the time division designation unit 24 may be formed by hardware.

  The communication unit 25 includes a receiving circuit that receives a burst signal transmitted from each of the ONUs 10a to 10c, and a transmission circuit that the OLT 20 transmits various data and control signals to each of the ONUs 10a to 10c. A control signal is transmitted to each of the ONUs 10a to 10c via the communication unit 25, so that a burst signal attached with a synchronization signal having a specified length at a communication timing specified by each of the ONUs 10a to 10c can be transmitted. It is configured. (Details will be described later)

Here, the receiving circuit provided in the communication unit 25 is, for example, as shown in FIG.
The outline of the receiving circuit 250 includes a photodiode 251, an amplifier 252, an AC coupling circuit 253, a comparator 254, and a clock data recovery circuit 255.

Data from each of the ONUs 10 a to 10 c described above is received as optical signals by the photodiode 251 of the receiving circuit 250 of the OLT 20 via the trunk optical fiber 4, converted into electric signals, and input to the amplifier 252.
The signal amplified by the amplifier 252 is logically determined by the comparator 254 via the AC coupling circuit 253 that cuts the DC component, and input to the clock data recovery circuit 255.

  The clock data recovery circuit 255 outputs the data logically determined from the data output terminal 257 while reproducing and outputting the clock signal from the clock signal terminal 256 according to the data logically determined by the comparator 254. .

Further, the comparator 254 detects the intensity of the burst signal transmitted from each of the ONUs 10 a to 10 c according to the timing designated by the time division designation unit 24 of the OLT 20 and outputs it from the intensity output terminal 258.
These clock signal terminal 256, data output terminal 257, and intensity output terminal 258 are connected to the OLT side control unit 21.

  The outline of the ONUs 10a to 10c includes, for example, a control unit 11 that includes an arithmetic processing device and performs processing such as calculation and determination, a storage unit 13, and a communication unit 15 that transmits and receives optical signals. Moreover, the control part 11 is provided with the synchronous time change acceptance part 12 and the transmission data generation part 14, for example.

  The synchronization time change receiving unit 12 is calculated by the synchronization time calculation unit 22 of the OLT 20 and receives the control signal transmitted via the communication unit 25 via the communication unit 15. Adjust the synchronization bit to the optimal length found. Thus, the ONU 10a is set to actually transmit a signal having a synchronization time with a length adjusted to be optimal.

The storage unit 13 is an operation program of the ONU 10a (in this embodiment, a program for causing the control unit 11 to function as the synchronous time change receiving unit 12 or the transmission data generation unit 14), data received from the OLT 20, and setting values. Etc. are memorized.
The transmission data generation unit 14 generates data transmitted by the ONU 10a as a packet burst signal.

The ONU side communication unit 15 is for transmitting and receiving data to and from the OLT 20, and transmits the signal generated by the transmission data generation unit 14 at the timing specified by the time division specifying unit 24 of the OLT 20. is there.
As described above, the OLT 20 and the ONUs 10a to 10c are connected as a PON type optical fiber network.

In such a configuration, for example, a case where signals (burst signals) P1, P2, and P3 as shown in FIG.
In the following description, it is assumed that the signal P1 is a signal transmitted from the ONU 10a, the signal P2 is a signal transmitted from the ONU 10b, and the signal P3 is a signal transmitted from the ONU 10c.

Further, the signal P1 and the signal P2 have substantially the same intensity, and the signal P2 has a higher intensity than the signal P3, and D is the conventional fixed synchronization time described with reference to FIG.
First, when the signal P <b> 2 is input to the comparator 254, the comparator 254 outputs the strengths of the signals P <b> 1 and P <b> 2 from the strength output terminal 258 and inputs them to the control unit 21.

  FIG. 4 shows the intensities of the signals P1 to P3 measured for each ONU (the subscriber side optical communication apparatus that has transmitted the signals P1 to P1 and hereinafter also simply referred to as a transmission source) and stored in the storage unit 23. (For convenience, the symbol attached to the signal also represents its strength). The length of the synchronization bit of the signals P1 to P3 received first from each of the ONUs 10a to 10c is a conventional fixed synchronization time indicated by a symbol D in FIG.

The synchronization time calculation unit 22 calculates the synchronization time of the signal P2 according to the intensity difference (P1-P2) input to the OLT side control unit 21 by receiving the signals P1 to P3. In the case of the example shown in FIG. 3, the intensity difference (P1−P2) is almost zero (the intensity of the signal P1 and the signal P2 is substantially the same). Is not affected by the time constant of the AC coupling circuit 253.
Therefore, the synchronization time calculation unit 22 calculates a synchronization time t2 that is shorter than the conventional fixed synchronization time D as the synchronization time of the signal P2 that the ONU 10b will emit in the future.

As a calculation method in this case, for example, a function for calculating an appropriate synchronization time from the intensity difference between the preceding and following signals is determined in advance, and the synchronization time calculating unit 22 substitutes the intensity difference between the preceding and following signals into the function. There is a method for calculating the synchronization time.
This function is preferably, for example, such that the larger the signal after the previous signal, the shorter the synchronization time, while the smaller the signal after the previous signal, the longer the synchronization time.
By defining the function in this way, when there is almost no difference in the intensity of the preceding and following signals as described above, the subsequent signal is not significantly affected by the time constant, so that the synchronization time can be shortened.

When the signal P3 is input to the comparator 254, the comparator 254 outputs the intensity difference P3 of the signal P3 from the intensity output terminal 258 and inputs it to the OLT control unit 21.
And the synchronous time calculation part 22 calculates the synchronous time of the signal P3 according to an intensity | strength difference (P2-P3) similarly to the above-mentioned.

In this case, since the intensity difference (P2−P3) is large, the influence of the time constant by the AC coupling circuit 253 is large on the leading portion of the signal P3 which is the subsequent signal in the combination of the front and rear of the signal.
Therefore, the synchronization time calculation unit 22 calculates the synchronization time t3 close to the conventional fixed synchronization time D as the synchronization time of the signal P3 emitted from the ONU 10c in the future by substituting the intensity difference (P2-P3) into the above function. .

  From the above, if there is no difference in intensity between the previous signal and the subsequent signal in the combination before and after the signal, or if the intensity of the subsequent signal is greater than the intensity of the previous signal, the subsequent signal is AC. Since it is not easily influenced by the time constant of the coupling circuit 253, the synchronization time calculation unit 22 performs a process of calculating a later signal synchronization time to be shorter. Note that the length t1 of the synchronization time of the burst signal transmitted from the ONU 10a assigned first in the series of communication timings is set according to the strength of the signal P1 received from one ONU 10a, and when the strength P1 is high. If it is short and the strength P1 is low, it is lengthened.

In other words, the synchronization time calculation unit 22 “the transmission timing of the signals P1 to P3 determined by the time division designation unit 24 (combination of signals before and after)” and “the intensity of the signals P1 to P3 stored in the storage unit 23” Thus, it is possible to easily calculate the intensity difference between the front and rear signals.
The lengths t1 to t3 between the synchronizations for the respective subscriber-side optical communication devices calculated by the synchronization time calculation unit 22 are determined using the control signals transmitted from the communication unit 25 of the OLT 20 to the respective ONUs 10a to 10c. To 10 c of synchronization time change acceptance unit 12 (synchronization time change acceptance unit 12 causes storage unit 13 to store indication values of synchronization times t1 to t3 included in the control signal)

  By performing such processing, if the subsequent signal is not easily affected by the AC coupling circuit 253 in the combination before and after the signal, the synchronization time of the subsequent signal is calculated to be short. Compared to the case where the synchronization time is set to be longer than normal, the time required for synchronization is shortened and the bandwidth of data to be originally communicated can be expanded. Faster and better communication performance.

  On the other hand, when the subsequent signal strength is smaller than the previous signal strength and easily affected by the AC coupling circuit 253, the synchronization time of the subsequent signal is lengthened, so that the interval between the OLT 20 and the ONUs 10a to 10c is long. It is possible to calculate the minimum necessary synchronization time in accordance with the signal strength.

  Further, the intensity of the signal mainly depends on the distance from the optical splitter 3 to each of the ONUs 10a to 10c, and the intensity of the signals P1 to P3 emitted from each of the ONUs 10a to 10c does not basically change, so the ONUs 10a to 10c shown in FIG. It is not necessary to re-detect the intensity of each signal many times.

  Further, since the communication timing does not change unless the connection state of the ONUs 10a to 10c to the optical fibers 4 and 5a to 5c changes, the synchronization time calculation unit 22 always performs the synchronization time based on the intensity output from the intensity output terminal 258. Need not be calculated, and signal processing among the comparator 254, the OLT side control unit 21, and the synchronization time calculation unit 22 can be reduced. In addition, it is also possible to simply calculate the synchronization time of the signal based on only the intensity of the head portion of the signal input to the comparator 254 without calculating the intensity of the preceding and following signals. It is.

As described above, in the PON type optical fiber network, if the installation location or user of the ONU is not changed, the distance from the optical splitter 3 to each ONU is also not changed, so that the ONUs 10a to 10c received on the OLT 20 side are not changed. The intensity of the signal does not change.
Therefore, in a PON type optical fiber network in which a user is fixed, if the combination of signals before and after being input to the comparator 254 is the same, the synchronization time calculated by the synchronization time calculation unit 22 is also the same value. It becomes.

Therefore, the synchronization time for various combinations of the preceding and following signals is calculated in advance, and as shown in FIG. 5, “the source of the previous signal”, “the source of the subsequent signal”, and “synchronization time” are associated. The synchronization time correspondence table is stored in the storage unit 23 in advance.
In this case, when the combination of the preceding and succeeding signals is determined by the time division designation unit 24, the synchronization time calculation unit 22 can extract the synchronization time corresponding to this combination from the synchronization time correspondence table, and calculate the synchronization time. Processing can be reduced.

Further, as described above, instead of obtaining the synchronization signal according to the difference in intensity between the preceding and succeeding signals, the time actually required for synchronization can be determined as the synchronization time.
In this case, the receiving circuit of the OLT side communication unit 25 of the OLT 20 is configured as a receiving circuit 250a shown in FIG.
The receiving circuit 250a is obtained by removing the strength output terminal 258 from the receiving circuit 250 described in FIG. 2 and providing a synchronization detection circuit 261 at the subsequent stage of the clock data recovery circuit 255.

The synchronization detection circuit 261 determines that synchronization has been established by detecting that the clock data recovery circuit 255 has successfully reproduced the idle bit signal of the synchronization bit, and outputs that synchronization has been established as a synchronization detection bit. The data is transmitted from the terminal 262 to the OLT side control unit 21. (In the case of GEPON as an example of the synchronization bit, the 8B10B idle pattern signal is repeated in the preamble. The synchronization detection circuit 261 determines that synchronization has been established when the preamble idle pattern signal is detected.)

  Then, the synchronization time calculation unit 22 detects synchronization from the rise of the burst signal P1 transmitted from each of the ONUs 10a to 10c (at the time when the transmission timing determined by the time division designating unit 24 starts) in the combination before and after the signal. The synchronization time included in the burst signal transmitted from each of the ONUs 10a to 10c is calculated using the time actually required until the bit can be detected by the synchronization detection circuit 261.

As shown in FIG. 7, the time required for the synchronization is preferably stored in the storage unit 23 for each combination with the previous signal source (ONUs 10 a to 10 c).
For example, when the transmission source of the previous signal is the ONU 10a and the transmission source of the subsequent signal is the ONU 10b, the time required for the synchronization of the subsequent signal is stored in the storage unit 23 as T12.
As a result, when the time division designation unit 24 designates the combination of the preceding and following signals as “the previous signal is the ONU 10a and the subsequent signal is the ONU 10b”, the synchronization time calculation unit 22 performs the processing before and after being stored in the storage unit 23. The time T12 required for synchronization for each combination of signals is referred to, and the time obtained by adding a slight margin to this time T12 is obtained as the “synchronization time of the subsequent signal”.

In this way, if the combination of signal sources before and after and the time T12... Actually required for synchronization are stored in association with each other, the synchronization time calculation unit 22 can easily perform only the combination of signals before and after. An optimum synchronization time can be obtained.
Further, in this case, only the three items of “Prior signal source”, “Subsequent signal source”, and “Time required for synchronization” are stored in the storage unit 23. Therefore, the necessary storage capacity can be suppressed.

The schematic block diagram of the optical fiber network using the station side optical communication apparatus of this invention. The schematic block diagram of the receiving circuit 250 provided in the communication part 25 of OLT20. The figure which showed the front-back relationship of the signal received by the receiving circuit 250. FIG. The conceptual diagram of the strength of the signal for each ONU memorize | stored in the memory | storage part 23. FIG. The conceptual diagram which showed the correspondence of the intensity | strength difference of the signal before and behind memorize | stored in the memory | storage part 23, and synchronous time. The schematic block diagram of the receiving circuit 250a when the time actually required for the synchronization is set as the synchronization time. The conceptual diagram which showed the correspondence of the combination of the front and back signals, and the time actually required for the synchronization. 1 is a schematic configuration diagram of a general optical fiber network. The schematic block diagram of the receiving circuit by the side of the conventional OLT. The figure which showed the front-back relationship of the signal received with the conventional receiving circuit. The schematic diagram which showed the data structure of the signal received with the conventional receiving circuit.

Explanation of symbols

10a ONU
11 ONU side control part 12 Synchronous time change acceptance part 20 OLT
22 period calculation unit 24 time division designation unit 250 receiving circuit 251 photodiode 252 amplifier 253 AC coupling circuit 254 comparator 255 clock data recovery circuit

Claims (4)

A burst signal from each subscriber-side optical communication device is obtained by assigning communication timing to each subscriber-side optical communication device in a state where a plurality of subscriber-side optical communication devices are connected by branching one optical fiber. In the station side optical communication device that can receive the time-division,
The station side optical communication apparatus calculates the preamble length attached to the head part of each burst signal to be received, and calculates the synchronization time calculation unit for each subscriber side optical communication apparatus according to the combination of the received burst signal before and after ,
A control signal to be changed to the calculated preamble length, and a communication unit to be transmitted to the synchronization time change receiving part of the subscriber side optical communication apparatus,
A station side optical communication apparatus characterized by comprising: The station side optical communication device has a storage unit that stores the intensity of the burst signal for each burst signal source communication device,
The station-side optical communication apparatus according to claim 1, wherein the synchronization time calculation unit calculates the preamble length according to an intensity difference in a combination of before and after burst signals stored in the storage unit. In the calculation of the preamble length, when the intensity of the case and the signal after the intensity difference is no greater than the strength of the signal before, according to claim 2 which is changed shorter preamble length of a burst signal after Station side optical communication device. The station-side optical communication device is a combination of a synchronization detection circuit that measures the time required for synchronization from the actually received burst signal and a subscriber-side optical communication device that has transmitted the burst signal immediately before the measured time. Having a storage unit for storing each
The synchronization time calculating unit, a station-side optical communication apparatus according to claim 1 and calculates the preamble length based on the measurement time of each of the combinations is stored in the storage unit.

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