Passive optical network system and ranging system thereof

1. A passive optical network system comprising:

a station-side communication device;

a plurality of subscriber-side communication devices;

an optical splitter and a plurality of optical fibers connecting the plurality of subscriber-side communication devices to the station-side communication device,

wherein the station-side communication device determines timing of transmitting a signal to the station-side communication device from the subscriber-side communication devices for communications between the station-side communication device and the subscriber-side communication devices,

the station-side communication device comprising:

a threshold control part detecting a voltage level of a received signal converted into an electrical signal from an optical signal transmitted from one of the plurality of subscriber-side communication devices;

a signal detection part detecting breaks in the received signal;

a transmission granting part determining the timing at which transmission of optical signal from each of the subscriber-side communication devices is granted; and

a reset timing generation part generating a reset signal for resetting the voltage level detected by the threshold control part when it is notified from the signal detection part that there has been detected a break in the received signal, wherein:

a distance measurement request signal for measuring distances between said respective plural subscriber-side communication devices and the station-side communication device is transmitted to the plurality of subscriber-side communication devices, and response signals from the plurality of subscriber-side communication devices in response to the distance measurement request signal are received to calculate transmission delay times of optical signals from the subscriber-side communication devices; and

while the transmission granting part is notifying the subscriber-side communication devices that distance measurement is carried out, when the signal detection part detects a break in a received response signal from one of the subscriber-side communication devices, the reset timing generation part sends a reset signal to the threshold control part, and then the threshold control part executes reception of a response signal from another subscriber-side communication device.

2. The passive optical network system according to claim 1 , wherein in each of the plurality of subscriber-side communication devices, when the distance measurement request signal is received from the station-side communication device, the response signal is delayed at random and the delayed response signal is transmitted to the station-side communication device.

3. A passive optical network system comprising:

a station-side communication device;

a plurality of subscriber-side communication devices;

an optical splitter and a plurality of optical fibers connecting the plurality of subscriber-side communication devices to the station-side communication device, wherein:

the station-side communication device determines timing of transmitting a signal to the station-side communication device from the subscriber-side communication devices for communications between the station-side communication device and the subscriber-side communication devices,

the station-side communication device comprises:

a threshold control part detecting a voltage level of a received signal converted into an electrical signal from an optical signal transmitted from one of the plurality of subscriber-side communication devices, holding a threshold value corresponding to the detected voltage level, and identifying the received signal by using the threshold value to output the identified received signal;

a signal detection part detecting breaks in the received signal output from the threshold control part;

a transmission granting part determining the timing at which transmission of optical signal from each of the subscriber-side communication devices is granted; and

a reset timing generation part generating a reset signal for instructing the threshold control part to reset the threshold value when it is notified from the signal detection part that there has been detected a break in the received signal;

a distance measurement request signal for measuring distances between said respective plural subscriber-side communication devices and the station-side communication device is transmitted to the plurality of subscriber-side communication devices, and response signals from the plurality of subscriber-side communication devices in response to the distance measurement request signal are received to calculate transmission delay times of optical signals from the subscriber-side communication devices; and

while the transmission granting part is notifying the subscriber-side communication devices that distance measurement is carried out, when the signal detection part detects a break in a received response signal from one of the subscriber-side communication devices, the reset timing generation part sends a reset signal to the threshold control part, and then the threshold control part executes reception of a response signal from another subscriber-side communication device.

4. The passive optical network system according to claim 3 , wherein in each of the plurality of subscriber-side communication devices, when the distance measurement request signal is received from the station-side communication device, the response signal is delayed at random and the delayed response signal is transmitted to the station-side communication device.

5. A ranging method of measuring distances between a station-side communication device and subscriber-side communication devices in an optical network system comprising the station-side communication device and a plurality of subscriber-side communication devices connected via an optical splitter and a plurality of optical fibers, the method comprising the steps of:

notifying each of the subscriber-side communication devices that a distance between the station-side communication device and any one of the subscriber-side communication devices is being measured;

transmitting a distance measurement request signal for measuring distances to each of the plurality of subscriber-side communication devices;

receiving response signals transmitted from the subscriber-side communication devices in response to the distance measurement request signal; and

calculating a signal transmission delay time between the station-side communication device and each of the subscriber-side communication devices, wherein:

in the step of receiving response signals, when a break in a response signal transmitted from a first subscriber-side communication device is detected, a receiving circuit in the station-side communication device receiving a response signal from any one of the subscriber-side communication devices is reset to execute a reception of a response signal from a second subscriber-side communication device in the receiving circuit; and

in the step of calculating the signal transmission delay time, the delay time is calculated in a time period when the distance between the station-side communication device and each of the subscriber-side communication devices is measured.

6. The ranging method according to claim 5 , wherein in the step of receiving response signals, when the break in the response signal transmitted from the first subscriber-side communication device is detected, a voltage level of the receiving circuit is reset, and the receiving circuit starts to receive the response signal from the second subscriber-side communication device, the receiving circuit receiving a signal based on the voltage level detected from an electrical signal converted from an optical signal transmitted from one of the plurality of subscriber-side communication devices.

7. The ranging method according to claim 5 , wherein in each of the plurality of subscriber-side communication devices, when the distance measurement request signal is received from the station-side communication device, the response signal is delayed at random and the delayed response signal is transmitted to the station-side communication device.