JP2004064494A - Atm-pon system and its distance measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a distance measuring ATM-PON system by which the effective utilization of bands is allowed by reducing the ineffective use of bands in the incoming direction which occurs when a distance measurement window is opened. <P>SOLUTION: This ATM-PON system has a station-side device and a plurality of subscriber-side devices, and performs distance measurement for measuring a distances between the station-side device and subscriber-side devices. The station-side device is provided with a radio clock and sends out a distance measuring cell used for measuring the distance to the subscriber-side device measured by acquiring the sending-out time of the cell from the radio clock and inserting a time into the cell. The subscriber-side device is also provided with a radio clock, and upon receiving the distance measuring cell from the station-side device, compares the sending-out time inserted into the cell with the receiving time of the cell acquired from its radio clock, calculates the sending-out timing of an incoming cell sent out from the device from a difference between the sending-out time and receiving time, and transmits the incoming cell to the station-side device at calculating sending-out timing. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a PON (Passive Optical Network) system in which an OLT (Optical Line Termination) and a plurality of ONUs (Optical Network Units) are connected via a star coupler. Mode) The present invention relates to an ATM-PON system that performs communication using cells.
[0002]
[Prior art]
A public network for providing a uniform communication service to an unspecified number of subscribers is divided into an access system and a relay system with a subscriber exchange as a boundary. The access system is a network that connects the subscriber's equipment used by the subscriber, such as telephones and information processing devices, to the exchange. The relay system is a local network that provides communication services in the city area, and a long-distance communication service. Is a hierarchically constructed network that includes a toll network that provides Internet services and an international network that provides communication services with foreign countries. Generally, the access system is called an access network system.
[0003]
ATM-PON is an access network that connects the optical line terminal OLT and the optical network unit ONU installed in the exchange using an optical fiber, and is used to economically construct a high-speed and large-capacity network. It is being widely adopted as a method.
[0004]
FIG. 5 is a block diagram showing the configuration of the ATM-PON system.
[0005]
As shown in FIG. 5, the ATM-PON system has an OLT 1 and a plurality of ONUs 2 ₁ to 2 _n (n is a positive integer) connected to each other via a star coupler 3. Configuration.
[0006]
Star coupler 3 is installed in the subscriber's home, apartment and office buildings or the like away from the exchange, it accommodates a plurality of subscriber line from the subscriber side device ONU 2 ₁ to 2 _n, respectively.
[0007]
Downlink cells transmitted from the OLT ₁ to the ONUs 2 ₁ to 2 _n are multiplexed in the OLT 1 and transmitted to the star coupler 3 through a transmission line using an optical fiber. Star coupler 3 distributes the multiplexed signal received from the station-side device OLT1 to all subscriber units ONU 2 ₁ to 2 _n. Subscriber unit ONU 2 ₁ to 2 _n, only the extraction cell of the subject apparatus from the received signal, discarding the cells of other subscriber units ONU addressed.
[0008]
On the other hand, the uplink cells transmitted from the subscriber units ONU2 ₁ to _2n are time-division multiplexed by the star coupler 3 and then transmitted to the optical line terminal OLT1. At this time, in order to time-division multiplex the uplink cells of the subscriber units ONU2 ₁ to _2n installed at various positions, the transmission timing of the cells by the respective subscriber units ONU2 ₁ to _2n is shifted. It is necessary to prevent cell collision. Therefore, in the ATM-PON system, the distance measuring process of measuring the optical line terminal OLT1 and the subscriber-side device ONU 2 ₁ to 2 _n of the distance to each is performed. Incidentally, the transmission timing of the cell are unique to the subscriber unit ONU 2 ₁ each to 2 _n, referred to as "expected phase" in the following.
[0009]
In the conventional distance measurement processing, a period called a distance measurement window in which the uplink cells are not transmitted from the subscriber-side devices ONU2 ₁ to _2n is provided, and the station-side device OLT1 transmits the subscriber-side devices ONU2 ₁ to sending the distance measuring grant is a cell of the distance measurement start for the 2 _n, subscriber unit by measuring the reception time of the PLOAM cell sent back from the subscriber-side device ONU 2 ₁ to 2 _n for that transmission time ONU2 had calculated the distance between the _{1 ~2 n.}
[0010]
FIG. 6 is a flowchart showing a procedure of a distance measuring process of the conventional ATM-PON system. FIG. 7 is a timing chart showing the relationship between the distance measurement window and the transmission timing of the PLOAM cell.
[0011]
As shown in FIG. 6, in the conventional distance measurement processing, when the optical line terminal OLT1 starts the distance measurement processing (step A1), first, a distance measurement window is opened and a distance measurement target, for example, a subscriber side apparatus ONU2 sends a grant for distance measurement with respect to ₁ (step A2). Here, other subscriber units ONU 2 ₂ to 2 _n while the distance measuring windows are open can not sends uplink cell.
[0012]
Distance subscriber unit ONU 2 ₁ of the measurement grant received returns a PLOAM cell to the line terminal OLT1 as a response to the grant for distance measurement (step A3).
[0013]
The OLT ₁ determines whether or not a PLOAM cell from the ONU 21 has been received within the distance measurement window (step A4). If a PLOAM cell has been received within the distance measurement window, the distance measurement is not performed. determining successful (step A5), the distance measures the time from transmission of the measuring grant to the reception of the PLOAM cell, calculates the distance between the subscriber unit ONU 2 _1, which returned the PLOAM cell. Then, return the result of the calculation to the subscriber side device ONU 2 _1, and terminates the distance measurement (step A6).
[0014]
On the other hand, when no PLOAM cell is received within the distance measurement window, it is determined that the distance measurement has failed (step A7), and the process returns to step A1 and repeats the processes up to step A4. Examples of the failure of the distance measurement include, for example, a case where the subscriber unit ONU cannot receive the distance measurement grant due to a power failure or a failure in the subscriber line.
[0015]
Station apparatus OLT1 executes in the order the same processing for all the subscriber units ONU 2 ₁ to 2 _n, and calculates the distance to each subscriber unit ONU 2 ₁ to 2 _n, respectively.
[0016]
In an ATM-PON system in which the distance between the OLT ₁ and the ONUs 21 to _2n is 20 km at the maximum, the OLT 1 corresponds to the maximum delay time of the distance as shown in FIG. A distance measurement window is secured for a time corresponding to 69 cells.
[0017]
[Problems to be solved by the invention]
As described above, in the conventional ATM-PON system, the distance measurement process is started in the optical line terminal OLT regardless of the state of the subscriber-side device ONU, and if the distance measurement fails, the distance is measured until the distance measurement succeeds. A measurement window is opened and a grant for distance measurement is transmitted to the subscriber unit ONU.
[0018]
Therefore, for example, when the subscriber unit ONU is in a power-off state for repairing a failure, the distance measurement does not succeed until the failure is recovered and the operation state is established, so that the optical line terminal OLT is connected to the subscriber unit. The distance measurement process is repeated until the ONU enters the operation state.
[0019]
As described above, when the distance measurement window is open, the upstream cell cannot be transmitted from the subscriber unit ONU. Therefore, the more the subscriber unit ONU in the non-operation state due to the power cutoff or the failure of the transmission line, the more the distance becomes. The number of measurement processes increases, and the use of useless transmission bandwidth due to opening the distance measurement window increases.
[0020]
For example, in the case of an ATM-PON system in which the distance measurement processing is performed at a period of 100 ms, if the subscriber unit ONU is in a power-off state for one hour for repairing a failure, the distance measurement processing is performed 36,000 times per hour. Become. Here, in a system in which the maximum distance between the optical line terminal OLT and the subscriber unit ONU is 20 km, a distance measurement window for 69 cells is opened in one distance measurement process, so that the transmission band for 2484000 cells in one hour. Will be wasted.
[0021]
This corresponds to about 110 Mbytes in terms of data amount, which means that about 110 Mbytes of data could not be transmitted in one hour. Furthermore, if a failure in distance measurement occurs due to power interruption or transmission line failure in more subscriber-side devices ONUs, the transmission bandwidth is wasted by that number.
[0022]
The present invention has been made in order to solve the problems of the conventional technology as described above, and reduces unnecessary use of bandwidth in the uplink direction caused by opening of a distance measurement window, and makes effective use of bandwidth. An object of the present invention is to provide a distance measuring method to be realized.
[0023]
[Means for Solving the Problems]
To achieve the above object, an ATM-PON system of the present invention comprises:
A plurality of subscriber devices,
An ATM-PON system for performing a distance measurement process for measuring a distance between the optical line terminal and a subscriber device, respectively.
The station apparatus,
Equipped with a radio clock,
The transmission time of the distance measurement cell used for the distance measurement processing is acquired from the radio clock, and the transmission time is inserted into the distance measurement cell and transmitted to the subscriber side device that is the distance measurement target,
The subscriber device,
Equipped with a radio clock,
When receiving the distance measurement cell from the optical line terminal, the transmission time inserted in the distance measurement cell is compared with the reception time of the distance measurement cell obtained from the radio clock, and the difference is automatically determined. The transmission timing of the uplink cell transmitted from the device is calculated, and the uplink cell is transmitted to the optical line terminal at the calculated transmission timing.
[0024]
At this time, the station apparatus
If the uplink cell is returned at the expected phase specific to the subscriber unit, the distance measurement is determined to be successful and the distance measurement process ends,
If the uplink cell is not returned in the expected phase, it may be determined that the distance measurement has failed, and the transmission of the distance measurement cell to the subscriber unit may be repeated.
[0025]
On the other hand, the distance measuring method for an ATM-PON system according to the present invention measures a distance between the station-side device and the subscriber device, which is executed by an ATM-PON system having a station-side device and a plurality of subscriber-side devices. An ATM-PON system distance measuring method for performing
In advance, the station apparatus and the subscriber unit are provided with radio clocks, respectively, and the transmission time of the distance measurement cell used for the distance measurement processing is obtained from the radio clock provided in the station apparatus, The transmission time is inserted into the distance measurement cell and transmitted from the station-side device to the subscriber-side device whose distance is to be measured,
The subscriber device receiving the distance measurement cell from the station device, the transmission time inserted in the distance measurement cell and the distance measurement cell obtained from the radio clock provided in the subscriber device. The reception time is compared, the transmission timing of the uplink cell transmitted from the subscriber side device is calculated from the difference, and the uplink cell is transmitted from the subscriber side device to the station side device at the calculated transmission timing. How to send.
[0026]
At this time, when the uplink cell is returned to the optical line terminal at the expected phase specific to the subscriber side device, it is determined that the distance measurement has been successful, and the distance measurement process ends, and the When the uplink cell is not returned to the optical line terminal, it may be determined that the distance measurement has failed, and the transmission of the distance measuring cell from the optical line terminal to the subscriber side device may be repeated.
[0027]
In the ATM-PON system and the distance measuring method as described above, the station-side device acquires the transmission time of the distance measuring cell from the radio clock, inserts the time into the distance measuring cell, and sets the subscriber side as the distance measurement target. Transmitted to the device, the subscriber device that has received the distance measurement cell, compares the transmission time of the cell inserted in the distance measurement cell with the reception time of the distance measurement cell obtained from the radio clock, By calculating the transmission timing of the uplink cell from the difference and transmitting the uplink cell to the optical line terminal at the calculated transmission timing, only transmitting the distance measurement cell from the optical line terminal to the subscriber side device, The subscriber unit can calculate the transmission timing of the uplink cell corresponding to the distance between the own unit and the office unit.
[0028]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, the present invention will be described with reference to the drawings.
[0029]
In the ATM-PON system of the present invention, the station-side device OLT and the subscriber-side device ONU are provided with radio clocks, respectively, to recognize a standard time, which is a common time, and to transmit a downstream cell from the station-side device OLT. The distance between the two devices is calculated from the difference between the time and the time when the subscriber unit ONU receives the downlink cell.
[0030]
FIG. 1 is a block diagram showing the configuration of the OLT OLT of the ATM-PON system of the present invention, and FIG. 2 is a block diagram showing the configuration of the ONU of the ATM-PON system of the present invention. . Note that the configuration of the ATM-PON system is the same as the conventional configuration shown in FIG. 6, and a description thereof will be omitted.
[0031]
As shown in FIG. 1, an OLT included in an ATM-PON system of the present invention is an IF processing which is an interface for transmitting and receiving uplink cells and downlink cells to and from a subscriber apparatus ONU via a star coupler. Unit 11, a downlink signal transmitting unit 12 for transmitting a downlink cell, an uplink signal receiving unit 13 for receiving an uplink cell, and a downlink cell transmission time inserting unit for inserting a standard time obtained by a radio clock into the downlink cell. 14 and a control unit 15 for controlling the operations of the downlink signal transmission unit 12, the uplink signal reception unit 13, and the downlink cell transmission time insertion unit 14 during the distance measurement processing.
[0032]
The IF processing unit 11 has a configuration including an optical wavelength demultiplexer 111 for separating a signal for each subscriber unit ONU, and an optical fiber 112 as a transmission line. And a transmission time insertion unit 142 for inserting a standard time into a cell. The downlink cell transmission time insertion unit 14 and the control unit 15 are configured by, for example, a CPU or a DSP that executes predetermined processing according to a program.
[0033]
When executing the distance measurement process, the downlink cell transmission time insertion unit 14 acquires the time at the time of transmission of the distance measurement cell from the radio clock of the standard time recognition / holding unit 141, and the transmission time insertion unit 142 converts the time information into the distance. It is inserted into the measurement cell and transmitted to the downlink signal transmitting unit 12. The downlink signal transmission unit 12 transmits the distance measurement cell including the time information received from the downlink cell transmission time insertion unit 14 to the subscriber unit ONU to be distance measured via the IF processing unit 11.
[0034]
The control unit 15 determines that the distance measurement has been successful when the uplink cell is returned at the expected phase from the subscriber unit ONU, and ends the distance measurement process. If the uplink cell is not returned at the expected phase, it is determined that the distance measurement has failed, and the distance measurement process is repeated.
[0035]
As shown in FIG. 2, the subscriber unit ONU included in the ATM-PON system of the present invention is an IF processing which is an interface for transmitting and receiving an upstream cell and a downstream cell to / from the station OLT via a star coupler. Unit 21, a downlink signal receiving unit 22 for receiving downlink cells, an uplink signal transmitting unit 23 for transmitting uplink cells, and a standard time obtained by a radio clock and time information inserted in the distance measuring cell. The configuration includes a downlink cell reception time comparison unit 24 for comparison, and a control unit 25 for controlling the operations of the downlink signal reception unit 22, the uplink signal transmission unit 23, and the downlink cell reception time comparison unit 24 during the distance measurement processing. is there.
[0036]
The IF processing unit 21 includes an optical wavelength demultiplexer 211 that separates a signal addressed to the own device, and an optical fiber 212 that is a transmission line. The downlink cell reception time comparison unit 24 includes a radio clock. The configuration includes a standard time recognition / holding unit 241 and a reception time comparison unit 242 that compares the standard time with the time information inserted into the distance measurement cell. The downlink cell reception time comparison unit 24 and the control unit 15 are configured by, for example, a CPU or a DSP that executes predetermined processing according to a program.
[0037]
At the time of performing the distance measurement processing, the subscriber unit ONU receives the distance measurement cell by the downlink signal receiving unit 22 and inserts it into the distance measurement cell received by the reception time comparison unit 242 of the downlink cell reception time comparison unit 24. The extracted time information is extracted. Further, the reception time comparison unit 242 acquires the time at the time of receiving the distance measurement cell from the radio clock of the standard time recognition / holding unit 241, and extracts the time information extracted from the distance measurement cell and the time at the time of reception of the distance measurement cell. Is calculated, and a Td value (a value that matches the expected phase), which is a time during which the own apparatus waits when transmitting an uplink cell, is calculated from the difference.
[0038]
The control unit 25 controls the transmission time of the uplink cell from the uplink signal transmission unit 23 based on the Td value calculated by the downlink cell reception time comparison unit 24.
[0039]
Next, the procedure of the distance measurement processing of the ATM-PON system of the present invention will be described with reference to FIGS.
[0040]
FIG. 3 is a flow chart showing the procedure of the distance measurement process of the ATM-PON system of the present invention, and FIG. 4 is a schematic diagram showing an example of the format of the distance measurement cell transmitted from the optical line terminal to the subscriber side device. .
[0041]
As shown in FIG. 3, when the power of the station-side device OLT and the subscriber-side device ONU is turned on, they are still in a non-operation state, and a common standard time is recognized by a built-in radio clock (step B1).
[0042]
Next, when the distance measurement processing is started in the station side apparatus OLT (step B2), the station side apparatus OLT generates a distance measurement cell by the control unit 15, and sends the distance measurement target subscriber side apparatus ONU to the distance measurement target. The cell for use is transmitted (step B3). At this time, the distance measurement cell is generated, for example, in the format shown in FIG.
[0043]
In FIG. 4, MESSAGE PON ID is an identifier for identifying the subscriber unit ONU, and MESSAGE ID is an identifier for identifying the type of transmission cell (here, the cell for distance measurement). The year display, month display, day display, 24-hour display, minute display, second display, and sub-second display are each time information, and Don't care is an unused area. CRC Byte is information used to check whether data has been transmitted correctly.
[0044]
The optical line terminal OLT obtains the time at which the distance measurement cell is transmitted from the radio clock by the downlink cell transmission time insertion unit 14, and displays the time information as shown in FIG. 4, such as the year display, month display, day display, 24-hour display, It is inserted into the area of minute display, second display and less than 1 second display, and transmitted to the subscriber unit ONU via the downstream signal transmission unit 12.
[0045]
Upon receiving the distance measurement cell, the subscriber unit ONU extracts the time information inserted in the received distance measurement cell by using the downlink cell reception time comparison unit 24, and also measures the distance from its own radio clock. The time at the time of receiving the distance measurement cell is obtained, and the time information (transmission time of the distance measurement cell) extracted from the distance measurement cell is compared with the time at which the distance measurement cell is received (the reception time of the distance measurement cell). Then, the difference is calculated (step B4).
[0046]
Subsequently, the Td value of the own device is calculated from the difference between the transmission time and the reception time of the distance measuring cell (step B5). Thereafter, the subscriber unit ONU transmits an uplink cell based on the Td value.
[0047]
The optical line terminal OLT determines whether or not the upstream cell transmitted from the subscriber side device ONU has been received at an expected phase corresponding to the distance to the device (step B6). Determines that the distance measurement has been successful (step B7), and ends the distance measurement process (step B8).
[0048]
On the other hand, if the uplink cell transmitted from the subscriber-side device ONU is not received at the expected phase of the device, it is determined that the distance measurement process has failed (step B9), and the process returns to step B2 and returns to step B2. The processing up to B6 is repeated.
[0049]
Therefore, in the ATM-PON system of the present invention, only the distance measurement cell is transmitted from the OLT OLT to the ONU, and the ONU corresponds to the distance between the OLT and the OLT. It is possible to calculate the transmission timing of the uplink cell to be performed, which eliminates the need to provide a distance measurement window at the time of the distance measurement processing as in the related art. Therefore, there is no period during which the uplink cell cannot be transmitted, and even when the station-side device repeats the distance measurement process for a long time due to disconnection of the transmission line with the subscriber-side device, useless use of the bandwidth in the uplink is reduced, and Effective utilization becomes possible.
[0050]
【The invention's effect】
Since the present invention is configured as described above, the following effects can be obtained.
[0051]
The station side device obtains the transmission time of the distance measurement cell from the radio clock, inserts it into the distance measurement cell, sends it to the subscriber device to be measured, and receives the distance measurement cell. The apparatus compares the transmission time of the cell inserted in the distance measurement cell with the reception time of the distance measurement cell obtained from the radio clock, calculates the transmission timing of the uplink cell from the difference, and calculates the calculated transmission timing. By transmitting the uplink cell to the optical line terminal at the transmission timing, only the distance measurement cell is transmitted from the optical line terminal to the optical line terminal, and the optical line terminal can reduce the distance between the local device and the optical line terminal. The transmission timing of the corresponding uplink cell can be calculated.
[0052]
Therefore, unlike the related art, there is no need to provide a distance measurement window at the time of the distance measurement processing, so that there is no period during which the uplink cell cannot be transmitted, and the station apparatus performs the distance measurement processing due to a transmission line disconnection with the subscriber apparatus. Even in the case of repeating for a long time, useless use of the band in the uplink direction is reduced, and the band can be effectively used.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an optical line terminal OLT included in an ATM-PON system of the present invention.
FIG. 2 is a block diagram showing a configuration of a subscriber unit ONU included in the ATM-PON system of the present invention.
FIG. 3 is a flowchart showing a procedure of a distance measuring process of the ATM-PON system of the present invention.
FIG. 4 is a schematic diagram showing an example of a format of a distance measurement cell transmitted from a station side device to a subscriber side device.
FIG. 5 is a block diagram illustrating a configuration of an ATM-PON system.
FIG. 6 is a flowchart showing a procedure of a distance measurement process of the conventional ATM-PON system.
FIG. 7 is a timing chart showing a relationship between a distance measurement window and PLOAM cell transmission timing.
[Explanation of symbols]
11, 21 IF processing unit 12 Downlink signal transmission unit 13 Uplink signal reception unit 14 Downlink cell transmission time insertion unit 15, 25 Control unit 22 Downlink signal reception unit 23 Uplink signal transmission unit 24 Downlink cell reception time comparison unit 111, 211 Optical wavelength Demultiplexers 112, 212 Optical fibers 141, 241 Standard time recognition / holding unit 142 Transmission time insertion unit 242 Reception time comparison unit

Claims (6)

An optical line terminal,
A plurality of subscriber devices,
An ATM-PON system for performing a distance measurement process for measuring a distance between the optical line terminal and a subscriber device, respectively.
The station apparatus,
Equipped with a radio clock,
The transmission time of the distance measurement cell used for the distance measurement processing is acquired from the radio clock, and the transmission time is inserted into the distance measurement cell and transmitted to the subscriber side device that is the distance measurement target,
The subscriber device,
Equipped with a radio clock,
When receiving the distance measurement cell from the optical line terminal, the transmission time inserted in the distance measurement cell is compared with the reception time of the distance measurement cell obtained from the radio clock, and the difference is automatically determined. An ATM-PON system for calculating a transmission timing of an uplink cell transmitted from a device and transmitting the uplink cell to the optical line terminal at the calculated transmission timing. The station apparatus,
If the uplink cell is returned at the expected phase specific to the subscriber unit, the distance measurement is determined to be successful and the distance measurement process ends,
The ATM-PON system according to claim 1, wherein when the uplink cell is not returned in the expected phase, it is determined that the distance measurement has failed, and the transmission of the distance measurement cell to the subscriber unit is repeated. A station-side device and an ATM-PON system having a plurality of subscriber-side devices, wherein the ATM-PON system has a distance measuring method of an ATM-PON system for measuring a distance between the station-side device and a subscriber device.
In advance, the station device and the subscriber device are each provided with a radio clock,
The transmission time of the distance measurement cell used for the distance measurement process is obtained from the radio clock provided in the station side device, and the transmission time is inserted into the distance measurement cell and the distance measurement target is transmitted from the station side device. Sent to a certain subscriber device,
The subscriber device receiving the distance measurement cell from the station device, the transmission time inserted in the distance measurement cell and the distance measurement cell obtained from the radio clock provided in the subscriber device. The reception time is compared, the transmission timing of the uplink cell transmitted from the subscriber side device is calculated from the difference, and the uplink cell is transmitted from the subscriber side device to the station side device at the calculated transmission timing. ATM-PON system distance measurement method for transmission. If the uplink cell is returned to the optical line terminal at the expected phase specific to the subscriber unit, the distance measurement is determined to be successful, and the distance measurement process ends,
4. When the uplink cell is not returned to the optical line terminal at the expected phase, it is determined that distance measurement has failed, and the transmission of the distance measuring cell from the optical line terminal to the subscriber side device is repeated. The distance measuring method of the ATM-PON system described. An ATM-PON system that performs a distance measurement process for measuring a distance between devices, a station-side device that transmits and receives uplink cells and downlink cells to and from a subscriber device,
A downlink signal transmitting unit that transmits the downlink cell to the subscriber device,
An uplink signal receiving unit that receives the uplink cell from the subscriber device,
With a radio clock, a transmission time of a distance measurement cell used for the distance measurement process is acquired from the radio clock, and a downlink cell transmission time insertion unit that inserts the transmission time into the distance measurement cell,
If the uplink cell is returned at the expected phase specific to the subscriber unit, the distance measurement is determined to be successful, the distance measurement process is terminated, and the uplink cell is not returned at the expected phase. A control unit that determines that the distance measurement has failed and repeats the distance measurement process,
An optical line terminal having: An ATM-PON system that performs a distance measurement process for measuring a distance between devices, a subscriber-side device that transmits and receives uplink cells and downlink cells to and from a station-side device,
An uplink signal transmitting unit that transmits the uplink cell to the optical line terminal,
A downlink signal receiving unit that receives the downlink cell from the optical line terminal,
A radio timepiece, when receiving a distance measurement cell used for the distance measurement process, the transmission time of the distance measurement cell inserted in the distance measurement cell and the distance measurement cell obtained from the radio timepiece; And a downlink reception time comparison unit that calculates the transmission timing of an uplink cell from the own apparatus from the difference,
A control unit that transmits an uplink cell from the uplink signal transmission unit based on the transmission timing calculated by the downlink cell reception time comparison unit,
A subscriber device having:

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