JP2007104571A - Optical line terminal and optical network unit in optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the whole throughput of an optical communication system, by allowing the optical line terminal to discriminate a degree of error correction coding with error correction capabilities different from each other applied to signals from each of the optical network units, and applying the degree to the optical network units. <P>SOLUTION: The optical communication system 1 includes optical fibers (70, 71), the optical network units (ONU) 90, and the optical line terminal OLT 10. The optical line terminal OLT 10 applies discrimination of the degree of excellent error correction coding for signal transmission to the signals received from the ONU 90. Then the optical line terminal OLT 10 informs the ONU 90 about information denoting the degree of the error correction coding. The discrimination is carried out by using at least one of the error rate, the signal strength and the round trip time of the signal transmission. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a control station side device and a terminal station side device in an optical communication system, and more particularly to an optical communication system capable of performing P2MP (Point to MultiPoint) communication.

  An optical data communication network is used between a control station side device OLT (Optical Line Terminal; hereinafter simply referred to as a control station) and a plurality of terminal station side devices ONU (hereinafter referred to simply as a terminal station). There are two-way communication systems. Then, a single star network configuration in which the control station OLT and each terminal station ONU are connected radially with a single optical fiber has been put into practical use. In this network configuration, the system and the equipment configuration are simplified, but if one terminal station ONU occupies one optical fiber and the number of terminal station ONUs is M, the light directly connected from the control station OLT Since M fibers are required, it is difficult to reduce the cost of the system.

  Thus, a PON (Passive Optical Network) system (also referred to as PDS (Passive Double Star)) in which one optical fiber drawn from the control station OLT is shared by a plurality of terminal stations ONU has been put into practical use. This PON system is one of access schemes for optical subscribers that can be constructed at a low cost and applied to FTTx such as FTTH (Fiber To The Home) and FTTB (Fiber To The Building).

  The PON system is a control station OLT and a passive optical branching device (hereinafter simply referred to as an optical splitter) that does not require any external power supply and branches and multiplexes an input signal passively. Are connected by a single mode fiber (hereinafter simply referred to as an optical fiber). There are a plurality of terminal stations ONU, which are connected by optical fibers according to the number of terminal stations ONU. The control station OLT and the terminal station ONU of the N station are based on 1-to-N transmission connected via an optical fiber or an optical splitter. Accordingly, for example, a maximum of 32 terminal station ONUs can be accommodated in one control station OLT, and the overall equipment cost can be suppressed.

In addition, you may use the structure which inserts another optical splitter between an optical splitter and several terminal station ONU.
In addition, GE-PON (Gigabit Ethernet-Passive) is a technology that incorporates Ethernet (registered trademark) technology in the PON system, improves connection affinity with many devices, and realizes optical fiber access section communication. Optical Network) system has been put into practical use.

  In the GE-PON system, the transmission rate is constant at 1.25 Gbps for both uplink and downlink, and the minimum reception level is uniformly determined for each type of transceiver. For example, in the case of the 1000BASE-PX10 standard, the minimum reception level is defined as -24 dBm for both the control station OLT and the terminal station ONU. In the case of the 1000BASE-PX20 standard, the control station OLT is defined as -27 dBm and the terminal station ONU is defined as -24 dBm. Reed-Solomon (255, 239, 8) is designated as the error correction coding method.

In communication between the control station OLT and the terminal station ONU, a method has been proposed in which error correction coding such as Reed-Solomon is performed according to the distance of the optical fiber transmission line.
For example, in the communication system described in Patent Document 1, in the communication between the control station OLT that uses the PON system and a plurality of terminal stations ONU, the execution / error correction coding method is performed according to the distance between these stations. Communication systems that perform non-execution settings have been proposed.
JP 2004-364059 A

  By the way, in the optical communication system described in Patent Document 1, execution / non-execution of error correction processing is set according to information on only the transmission distance between the control station OLT and the terminal station ONU. However, the characteristics of the optical fiber transmission line do not depend only on the transmission distance between the control station OLT and the terminal station ONU, but the number of branches of the optical fiber transmission line, transmission luminous intensity, received signal strength, receiver performance, light receiving wavelength It depends on other environmental conditions where the optical fiber transmission line is placed. As described above, the throughput of the entire communication system cannot be improved satisfactorily only by selecting the presence / absence of error correction processing at the setting stage depending on only the transmission distance information.

  Further, the optical communication system described in Patent Document 1 mainly focuses on selecting / changing execution / non-execution of error correction processing. However, appropriate settings cannot be made individually in optical fiber transmission lines placed in various environments only by execution / non-execution of error correction processing. For this reason, in order to improve the throughput of the entire optical communication system, it is not sufficient to select execution / non-execution of error correction processing, and there is still room for improving the throughput of the entire optical communication system.

  Therefore, an object of the present invention is to determine an error rate for each optical fiber transmission line connecting a control station side device and a plurality of terminal station side devices, so that an error is detected for each terminal station side device ONU. The control station side apparatus OLT and the terminal station that can improve the throughput of the entire optical communication system including the control station side apparatus OLT and the terminal station side apparatus ONU by determining the degree of correction coding and applying it A side device ONU is provided.

  In order to achieve the above object, the control station side apparatus of the present invention includes a determination unit that determines a degree of good error correction coding for signal transmission to be transmitted to the terminal station side apparatus. ) Setting means for setting the determined degree of error correction coding for a signal transmitted from the control station side device to the terminal station side device, and / or (b) in the terminal station side device Toward this, there is a notification means for notifying the setting information of error correction coding (claim 1).

According to this configuration, the control station side device can apply good error correction coding to the optical fiber transmission line connecting the control station side device and the predetermined terminal station side device. In the error correction coding determined here, the degree can be changed by selecting the type of error correction coding or selecting the error correction coding rate.
The procedure for this application will be described as follows. The control station side device requests each terminal station side device to transmit evaluation data as a material for determining error correction capability. Upon receiving this request, the terminal station side device transmits evaluation data to the control station side device. Thereby, the control station side apparatus can measure an error rate from this data.

Next, the control station side apparatus determines the degree of good error correction coding for signal transmission to be transmitted to the terminal station side apparatus based on the measured error rate.
After that, as described in (a), the control station side apparatus sets the determined degree of error correction coding for the signal transmitted to the terminal station side apparatus, and transmits data. Moreover, you may notify the setting information of error correction encoding toward the said terminal station side apparatus like said (b).

Thereby, the terminal station side apparatus can know the degree of error correction coding. Therefore, the terminal station side apparatus can transmit data to the control station side apparatus at the level of the good error correction coding after the next time.
The control station side device can perform good error correction processing with any terminal station side device by performing such determination processing for each terminal station side device. Throughput of the entire communication system can be improved.

  On the other hand, the trigger for determining the error correction capability of the above-described error correction coding was performed by a request from a control station side device to a predetermined terminal station side device, but can be performed without a request from the control station side device. . At this time, the control station side device measures the error rate of the arbitrary data received from the arbitrary terminal station side device, so that the control station side device transmits the optical fiber between the control station side device and the terminal station side device. The approximate optical loss of the path is known, and the error correction capability based on the degree of error correction coding can be determined for the terminal station side apparatus.

  The determination means includes an error rate of a signal transmitted from the terminal station side device to the control station side device, and a signal strength of a signal transmitted from the terminal station side device to the control station side device. The degree of error correction coding may be determined using at least one of round trip time as time spent for communication between the control station side device and the terminal station side device (claim 2). .

  In addition, the terminal station side apparatus of the present invention includes a determination unit that determines a degree of good error correction coding for signal transmission to be transmitted to the control station side apparatus, and (c) the terminal station side A setting means for setting the determined degree of error correction coding for a signal transmitted from a device to the control station side device, and / or (d) an error toward the control station side device. Notification means for notifying setting information of correction encoding is provided (claim 3).

According to this configuration, each terminal station side device can apply good error correction coding to the optical fiber transmission line connecting the control station side device and the terminal station side device.
The procedure for this application will be described as follows. Each terminal station side apparatus requests the control station side apparatus to transmit evaluation data as a material for determining error correction capability. Upon receiving this request, the control station side device transmits evaluation data to the terminal station side device. Thereby, the terminal station side apparatus can measure the error rate from this data.

Next, the terminal station side apparatus determines the degree of good error correction coding for signal transmission to be transmitted to the control station side apparatus based on the measured error rate.
Thereafter, the terminal pole side apparatus sets the determined degree of error correction coding to the signal transmitted to the control station side apparatus as in (c), and transmits data. Further, as in (d) above, the error correction coding setting information may be notified to the control station side device.

Thereby, the control station side apparatus can know the degree of error correction coding. Therefore, the control station side apparatus can transmit data to the terminal station side apparatus at the degree of the good error correction coding from the next time onward.
By performing such determination processing by each terminal station side device, it becomes possible to perform good error correction processing for the signal transmission between any terminal station side device and the control station side device. Therefore, the throughput of the entire optical communication system can be improved.

  Although the above-described error correction coding error correction capability is determined by a request from each terminal station side device to the control station side device, it can be performed without a request from the terminal station side device. At this time, by measuring the error rate of arbitrary data received by the terminal station side device from the control station side device, the terminal station side device transmits optical fibers between the control station side device and the terminal station side device. The approximate optical loss of the path is known and the degree of error correction coding can be determined.

  The determination means includes an error rate of a signal transmitted from the control station side device to the terminal station side device, and a signal strength of a signal transmitted from the control station side device to the terminal station side device. The degree of the error correction coding may be determined using at least one of the following.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a schematic diagram showing a configuration example of an optical communication system in which a control station OLT 10 and a plurality of terminal stations ONUs 90 according to an embodiment of the present invention are connected by optical fibers via an optical splitter. .
A PON system 1 as an optical communication system includes a control station side device (hereinafter simply referred to as “control station”) OLT 10 provided in a control station side station and a terminal station side device (hereinafter simply referred to as “ ONU 90 (90A to 90E). In the PON system 1, the control station OLT 10 and the terminal station ONU 90 are connected via a trunk optical fiber 70, an optical splitter 80, and a branch optical fiber 71.

Each terminal station ONU 90 is installed in a subscriber's house and is provided with a network interface (not shown) in order to connect to a terminal device such as a personal computer that can enjoy an optical network service.
The optical splitter 80 includes a star coupler that does not require any external power supply and can passively branch and multiplex signals input from the optical fibers (70 and 71).

  The optical communication system including the control station OLT 10 and the terminal station ONU 90 of the present invention incorporates the Gigabit Ethernet (TM) technology into the PON (Passive Optical Network) technology, and has a base of 1.25 Gbps. A GE-PON (Gigabit Ethernet-Passive Optical Network) system that realizes optical fiber access section communication at a band speed is adopted.

According to the GE-PON system, the control station OLT 10 and the terminal station ONU 90 communicate with each other in units of variable length frames. This frame includes a GE-PON header including a logical link identifier and data of 64 bytes or more. The maximum data size is generally about 1530 bytes.
First, a downlink frame signal sent from a higher-level network such as the Internet network to the control station OLT 10 is subjected to a predetermined bridge process in the control station OLT 10 to specify a logical link to be relayed. At this time, the control station OLT 10 adds a GE-PON header including a logical link identifier to the frame signal. Then, it is converted into an optical signal by the control station OLT 10 and sent to the trunk optical fiber 70. The optical signal sent to the trunk optical fiber 70 is branched by a plurality of optical splitters 80 and sent to the terminal station ONU 90 via the branch optical fiber 71. At this time, only the terminal station ONU 90 including the logical link can capture a predetermined optical signal. Then, the terminal station ONU 90 that has fetched the frame signal relays the home network interface and sends data to a terminal device such as a personal computer (not shown).

  On the other hand, the upstream optical signal includes an upstream frame signal from each terminal station ONU 90. The upstream optical signal needs to be transmitted so that the optical signals from the individual terminal stations ONU 90 do not compete with each other in time. Therefore, the control station OLT 10 assigns a window (hereinafter simply referred to as a window) during which an upstream optical signal may be transmitted to each terminal station ONU 90 and notifies it as a control frame. The terminal station ONU 90 assigned the window transmits an upstream optical signal to the assigned window. Therefore, the upstream optical signal between the terminal stations ONU 90 can avoid contention between optical signals.

  Further, each terminal station ONU 90, when given a certain window, can continuously send frame signals as long as it fits in that window. At this time, it is necessary for the control station OLT 10 and the terminal station ONU 90 to share a clock that records the same time. This clock transmits time information in the control frame when communicating the control frame. The time can be adjusted by including it in the.

Further, in each terminal station 90 in the PON system 1, for example, individual optical fiber transmission lines such as the characteristics of the light receiving element of the terminal station ONU 90, the light receiving wavelength, the transmission distance of the optical fibers 70 and 71, and the light receiving characteristics of the optical splitter. Depending on the characteristics, the received optical signal waveform and level differ.
For example, in the network of the PON system 1 shown in the figure, the optical fiber transmission path between the control station OLT 10 and the terminal stations ONU 90B, 90D, 90E is an optical fiber transmission between the control station OLT 10 and the terminal stations ONU 90A, 90C _{1 to} 90C _4. This is because the optical loss can be made relatively small because the distance is relatively shorter than the road and the number of branching floors by the optical splitter 80 is small. Therefore, by adding short redundant data (for example, 16 symbols) in the terminal stations ONU 90B, 90D, and 90E, the error rate can satisfy the specification range. The “redundant data” is data calculated by encoding calculation in the original data divided into a predetermined length (for example, 239 bytes), and is added to the divided original data, and the code word It is generated as data having a length (for example, 255 bytes). Further, the number of bytes that can be corrected for errors is determined by the length of the redundant data portion.

On the other hand, in the terminal stations ONU 90A, 90C _{1 to} 90C ₄ , the number of branching floors by the optical splitter 80 is large, and the error rate satisfies the specification range, so that it is longer than the redundant data added to the terminal stations ONU 90B, 90D, 90E. Redundant data (for example, 32 symbols) is added. This is because, for example, the optical loss increases due to a long distance from the control station OLT 10 (terminal station ONU 90A) or through a plurality of optical splitters 80 (terminal stations ONU 90C _{1 to} C ₄ ).

  As described above, in the data transmission / reception between the control station OLT 10 and each terminal station ONU 90, not only the distance from the control station OLT 10 to the terminal station ONU 90 but also the optical loss for each optical fiber transmission line is considered. The degree of correction encoding can be determined. The degree of this error correction coding can be changed by selecting the type of error correction coding or selecting the error correction coding rate. And error correction capability can be selected according to each optical fiber transmission line. Thereby, error correction processing can be performed satisfactorily for each terminal station ONU 90, and as a result, throughput as a transmission rate of the entire PON system 1 can be improved.

FIG. 2 is a schematic diagram showing the basic configuration of the control station OLT 10 and the terminal station ONU 90 in the PON system 1.
The control station OLT 10 converts a frame signal as an electrical signal from the transmission control unit 11 that performs communication with a host network and control of a frame signal generated for each terminal station ONU 90 into an optical signal. The optical signal generator 12, the wavelength multiplexing filter 13 for separating the optical signal sent from the terminal station ONU 90 via the optical fiber, and the optical signal from the terminal station ONU 90 sent via the wavelength multiplexing filter 13 And a light receiver 14 for converting the signal into an electric signal.

  On the other hand, the terminal station ONU 90 separates an optical signal multiplexed and transmitted from the control station OLT 10 and a light receiving unit that converts the optical signal separated by the wavelength multiplexing filter 91 into an electric signal. The transmission control unit 93 that performs communication control such as compensating the electrical signal of the optical device 92 and the light receiver 92 and sends the packet signal to the personal computer 99, and the packet signal from the personal computer 99 through the transmission control unit 93 And an optical signal generator 94 for converting into an optical signal.

  Then, the optical signal generator 12 of the control station OLT 10 and the optical signal generator 94 of the terminal station ONU 90 emit light strongly for a period corresponding to 1 of a baseband signal that is an electric signal representing information desired to be transmitted to each other, and correspond to 0. Lights up weakly for a period of time. Thereby, a rectangular signal of NRZ (Non Return to Zero) can be transmitted at both ends across the control station OLT 10 and the terminal station ONU 90.

Hereinafter, a series of signals exchanged between the control station OLT 10 and the terminal station ONU 90 will be described.
The wavelength multiplexing filter 13 of the control station OLT 10 separates the optical signal emitted from the terminal station ONU 90 and reaching through the optical fiber 70 and guides it to the light receiver 14. The light receiver 14 converts an optical signal from the terminal station ONU 90 into an electric signal by a light receiving element member such as a photodiode for converting the optical signal into an electric signal, and the electric signal is transmitted to the transmission control unit 11 as a frame signal. send. Then, the transmission control unit 11 can send a signal from the terminal station ONU 90 to the Internet network.

The signal as a code stream converted by the transmission control unit 11 is a 1000BASE-X 10-bit signal that is one of the Gigabit Ethernet standards, and the serialized 1.25 Gbps signal is 0. 1 is a baseband signal that alternates frequently.
On the other hand, the transmission control unit 11 adds a GE-PON header to be sent to the terminal station ONU 90 to the frame signal received from the host network. Then, the frame signal is sent to the optical signal generator 12.

The optical signal generator 12 controls the light emission of the laser diode (for example, one emitting light having a wavelength of 1490 nm) by using the frame signal as the received electric signal as the driving current of the laser diode, and the optical signal is generated from the electric signal. Convert to.
Next, the wavelength multiplexing filter 13 receives the optical signal generated by the optical signal generator 12 and guides the optical signal to the trunk optical fiber 70. Then, the optical signal is sent to the terminal station ONU 90 via the trunk optical fiber 70, the optical splitter 80, and the branch optical fiber 71.

  The optical signal sent to the terminal station ONU 90 is sent to the light receiver 92 via the wavelength multiplexing filter 91. The light receiver 92 converts the received optical signal into an electrical signal. The frame signal as the electrical signal is sent to the transmission control unit 93, and only the signal corresponding to the terminal station ONU 90 is decoded. The decoded frame signal is sent to the personal computer 99 as a packet signal via an interface of the terminal station ONU 90 (not shown).

On the other hand, the packet signal from the personal computer 99 is sent to the transmission control unit 93 of the terminal station ONU 90, and the transmission control unit 93 adds data such as a predetermined header.
The signal to which a predetermined header or the like is added is sent to the optical signal generator 94 and converted from an electric signal to an optical signal. The optical signal generator 94 emits light having a predetermined wavelength (specifically, light having a wavelength range of about 1310 nm) by a laser diode having a wavelength characteristic different from the wavelength emitted by the optical signal generator 12. The conversion to the optical signal by the optical signal generator 94 is performed, for example, by using the received electric signal as a driving current of the laser diode and controlling the light emission of the laser diode.

Next, the wavelength multiplexing filter 91 receives the optical signal sent from the optical signal generator 94 and guides this optical signal to the branch optical fiber 71. Then, this optical signal is sent to the control station OLT 10 via the branch line optical fiber 71, the optical splitter 80, and the trunk line optical fiber 70.
Here, since the wavelength range of light used for downlink communication in the control station OLT 10 and the wavelength range of light used for uplink communication in the terminal station ONU 90 are different, the control station OLT 10 and the terminal station ONU 90 are connected by a single optical fiber. can do.

  By the way, in the IEEE 802.3ah (IEEE: Institute of Electrical and Electronics Engineers) standard, which is a specification of 1 Gbps using an optical fiber, the specification of the spectrum width used in the GE-PON system is defined. ing. IEEE 802.3ah defines the 1000BASE-PX10 standard for transmission up to 10 km and the 1000BASE-PX20 standard for transmission up to 20 km. In consideration of waveform distortion caused by chromatic dispersion of the optical fiber in long-distance optical transmission, the spectrum width of 1000BASE-PX20 for long transmission lines is defined narrower than that of 1000BASE-PX10. In IEEE 802.3ah, the bit error rate BER (Bit Error Rate: hereinafter simply referred to as a transmission error rate) of a transmission signal is 10 ^ (-12) or less (^ represents a power multiplier. The same applies hereinafter. ) Quality is required.

However, the state of the optical fiber transmission path is different such that the transmission distance of the optical fiber and the number of branching stages are different for each terminal station ONU 90 accommodated by the control station OLT 10 and the reception sensitivity of the light receiver 92 is different. Since they are different, the light reception state during optical communication from the control station OLT 10 is different for each terminal station ONU 90.
In the present invention, redundant data in the encoding process is set in accordance with the characteristics of the optical fiber transmission line in which each terminal station ONU 90 is placed. Thereby, for example, when the characteristics of the optical fiber transmission line are good, it is possible to avoid a decrease in transmission speed by not redundantly lengthening redundant data. On the other hand, when the characteristics of the optical fiber transmission line are poor, the redundant data can be lengthened to maintain the communication quality. Thereby, error correction processing can be performed satisfactorily for each terminal station ONU 90, and as a result, the throughput of the entire PON system 1 can be improved.

  In the determination of the error rate used in the PON system 1, for example, when the control station OLT 10 and the terminal station ONU 90 can perform error correction encoding corresponding to each, the error correction capability of the control station OLT 10 and the terminal station ONU 90 is used. Switch according to the value of. This error rate is measured using the probability of processing in the 8B10B decoding process of received data, the probability of a frame being discarded in the check of CRC8 (Cyclic Redundancy Check) and FCS (Frame Check Sequence) fields at the MAC frame processing stage, and the like. .

Further, due to the characteristics of the PON system 1, it is possible to increase the accuracy of the error rate value itself by checking the error rate of the data transmitted to the other terminal station ONU 90 on the terminal station ONU 90 side.
For example, distribution is performed according to four ranks (ranks 1 to 4), rank 1 is an optical fiber transmission line that does not receive much of the characteristics of the optical fiber transmission line, and optical loss is relatively not recognized, and rank 4 is optical. It is assumed that the optical fiber transmission line has a good optical characteristic and a relatively large optical loss.

Hereinafter, a method for determining the degree of error correction coding of uplink data transmitted from a predetermined terminal station ONU 90 to the control station OLT 10 will be described.
First, consider a situation in which the control station OLT 10 and a predetermined terminal station ONU 90 (here, the terminal station ONU 90C ₂ ) transmit and receive data to and from each other.
The control station OLT 10 requests the terminal station ONU 90C ₂ to transmit evaluation data as a material for determining the degree of error correction coding. Upon receiving this request, the terminal station ONU 90C ₂ transmits evaluation data to the control station OLT 10. Thereby, the control station OLT 10 can measure the error rate from this data.

Next, the control station OLT 10 determines the degree of error correction coding of the uplink transmission path based on the measured error rate. Here, it is assumed that rank 2 is selected.
Thereafter, the control station OLT 10 transmits the data to the terminal station ONU 90C ₂ by performing error correction coding with redundant data having a length defined as rank 2 added to the transmission data.

Or, the control station OLT10 to the terminal station ONU90C _2, may simply transmit the data containing information that the rank 2.
On the other hand, the above-described determination of the degree of error correction coding is performed by a request from the control station OLT 10 to the predetermined terminal station ONU 90, but can be performed without a request from the control station OLT 10. At this time, the control station OLT 10 measures the error rate of arbitrary data received from the terminal station ONU 90C ₂ , so that the control station OLT 10 can obtain an approximate optical loss in the optical fiber transmission line between the control station OLT 10 and the terminal station ONU 90C _2. Thus, it is possible to determine the degree of error correction coding in the upstream transmission path. Here, it is assumed that rank 3 is selected.

Thereafter, the control station OLT 10 transmits, to the terminal station ONU 90C ₂ , error correction encoded data in which redundant data having a length defined as rank 3 is added to the transmission data. This transmission is performed to notify the terminal station ONU 90C ₂ of the rank. Alternatively, data including information indicating that the rank is 3 is simply transmitted.
The terminal station ONU90C ₂ receives the error correction encoded data to which redundant data having a length defined as rank 3 is added, or receives data including information indicating that it is rank 3 You can know the rank value.

Therefore, in the subsequent transmission, error correction coding with the redundant data of rank 3 added is performed on the transmission signal to the control station OLT 10 and transmitted to the control station OLT 10.
In this way, the control station OLT 10 can satisfactorily receive data that has been appropriately error correction encoded from the predetermined terminal station ONU 90.
In the determination work here, round trip time and error rate values are used as determination elements. Further, the degree of error correction coding specifically changes the length of redundant data of Reed-Solomon coding.

Below, the control performed when performing an error correction process in the PON system 1 is demonstrated.
FIG. 3 is a diagram for explaining the round trip time.
The round trip time represents the time spent for round-trip communication between the control station OLT 10 and the terminal station ONU 90.

First, when transmitting data to be transmitted to the terminal station ONU 90, the control station OLT 10 includes information on the transmission time T1 representing the time on the control station OLT 10 in the data (a). When receiving the transmission data, the terminal station ONU 90 adjusts the clock of the terminal station ONU 90 itself to the transmission time T1 included in the data (b).
Next, when transmitting data to be transmitted to the control station OLT 10, the terminal station ONU 90 includes information on the time T2 as the transmission time measured from the transmission time T1 in the data (c). The control station OLT 10 stores the time T3 when this data is received (d).

Here, the control station OLT 10 compares the time T2 included in the received data with the time T3 when the data is received. Then, it is determined that the round trip time is a difference T3−T2 between time T3 and time T2.
Since the round trip time indicates the time spent for the round-trip communication between the control station OLT 10 and the predetermined terminal station ONU 90, the approximate distance between the control station OLT 10 and the terminal station ONU 90 can be measured. Thereby, since the relationship between the distance of the optical fiber transmission line and the optical loss is known, the control station OLT 10 can determine the degree of error correction coding suitable for the predetermined terminal station ONU 90.

By the way, depending on the communication quality, inversion may occur in bits in transmission data. The process of calculating the rate of inversion occurring in transmission data, that is, the error rate, by the following three methods will be described.
FIG. 4 is a diagram for explaining an error rate calculation method using a frame error rate.

The frame error rate is a method of measuring the inversion of bits in a certain MAC frame in transmission data and calculating the error rate based on the measured value. The frame error rate can be calculated by performing two types of CRC encoding calculations and counting the frames as defective frames.
First, the CRC8 field in the preamble is read on the data stream. In the CRC8 field, a calculated value of 8-bit CRC encoding on the transmission side is stored. As a result, the calculated value when CRC encoding is performed from 8 bits on the receiving side is compared with the value stored in the CRC8 field at the time of transmission, and the count is performed when there is a mismatch, so that the control station OLT10 And the error rate between the predetermined terminal station ONU 90 can be measured.

On the other hand, the FCS field on the MAC frame stores a calculation result when 32-bit CRC encoding is performed at the time of data transmission. The error rate between the control station OLT 10 and the predetermined terminal station ONU 90 can be measured by comparing the calculated values of the FCS field and the 32-bit CRC encoding at the time of reception and counting when they do not match.
FIG. 5 is a diagram for explaining a method of calculating an error rate using the 8B10B encoding / restoration rate.

  There is a process of converting binary data into 10-bit data when data is signaled during transmission. On the other hand, there is a process of converting 10-bit data into binary data when receiving the transmission signal and reading the signal as data. If there is at least one inversion in 10-bit data, the corresponding binary data cannot be restored. In this way, the unrecoverable rate is obtained by counting the binary data that can no longer be used and calculating the rate with respect to the MAC frame.

In communication from the transmission side to the reception side, data is transmitted from the data stream of the MAC frame as follows.
First, on the transmission side, when data is signaled during transmission, binary data included in the data stream as a MAC frame on the transmission side is converted into 10-bit data (a). Here, conversion from 8-bit data to 10-bit data is performed according to a predetermined conversion rule. Then, the 10-bit data is converted into an NRZ signal (b).

The optical signal as the NRZ signal is sent to the receiving station via the optical fiber transmission line (c).
The data stream on the receiving side is converted from the NRZ signal to 10 bits (d), and this 10 bit data is converted to binary data (e). Here, the conversion from 10 bits to 8 bits is performed according to a predetermined conversion rule.

As described above, the binary data on the data stream is converted from 8 bits to 10 bits and then reconverted from 10 bits to 8 bits.
However, if there is at least one inversion in the bits included in the 10-bit data, the corresponding binary data cannot be restored in (e). As a result, the error rate is calculated by counting the number of binary data that could not be subjected to the 8B10B encoding / restoration with respect to the number of binary data included in the data stream.

FIG. 6 is a diagram for explaining a method of calculating an error rate using a bit error rate.
When the communication quality is not extremely bad, it is very rare that one or more inversions exist in the received 10-bit data as in the 8B10B encoding restoration as described above. The bit error rate is to calculate the bit error rate from the above-mentioned 8B10B encoding / restoration rate.

At this time, it can be said that one bit inversion exists in the binary data that could not be encoded and restored from 10 bits to 8 bits. Accordingly, the error rate is calculated by counting the number of inverted bits (that is, the number of binary data that cannot be restored by 8B10B encoding) with respect to the total data amount of the data stream.
4 to 6, the error rate for the optical fiber transmission line between the control station OLT 10 and the terminal station ONU 90 can be calculated.

FIG. 7 is a diagram for explaining error correction capability in error correction coding. FIG. 8 is a diagram showing the relationship between the redundant data of the code word and the amount of information with respect to the effective data D.
The MAC frame is divided together with the redundant data P so as to have a predetermined codeword length at the time of transmission. The error correction coding here uses Reed-Solomon coding, 8 bits (1 byte) as one symbol, and 255 bytes as the code word length. The code word includes data D having a length of 255 bytes or less and redundant data P calculated in the process of performing encoding calculation.

For example, the total number of bytes of the MAC frame is 1100 bytes, and the number of bytes of redundant data P is 16 bytes. At this time, since the effective data D and the redundant data P are combined to 255 bytes, the MAC frame is divided into _five blocks B _{1 to} B ₅ .
Block B ₁ represents, contains 255 bytes of effective and 239 bytes of data D _1, and 16 bytes of the calculated redundant data P ₁ from the valid data D _1.

The block B ₂ .about.B _4, like the block B _1, contains 255 bytes and 239 bytes of valid data D, a 16-byte redundant data P calculated from the effective data D.
Block B ₅ is included and 144 bytes of valid data D _5, and 16 bytes of the redundant data P ₅ calculated from the effective data D ₅ is. Then, in order to make the block B ₅ 255 bytes, an area of 0 pad filled with “0” bits is formed in the portion not including the valid data D ₅ and the redundant data P ₅ , and as a result, The block B ₅ has an information amount of 255 bytes.

Although the length of the redundant data P is 16 bytes here, the length of the redundant data P can be changed as shown in FIG.
For example, by shortening the length of the redundant data P, the amount of information for the effective data D is reduced and the error correction capability is reduced. At this time, for example, when the optical fiber transmission path is relatively short and the optical loss is relatively small, the throughput can be improved by shortening the redundant data P.

On the other hand, by increasing the length of the redundant data P, the amount of information for the data D increases and the error correction capability increases. At this time, for example, when the optical fiber transmission line is relatively long and a large amount of optical loss is observed, the error correction capability can be improved by lengthening the redundant data P.
In the above description, the control performed by the transmission control unit 11 on the control station OLT 10 side is targeted. On the contrary, the error rate of the downlink signal received by the terminal station ONU 90 from the control station OLT 10 by a similar method. Based on the measurement result, it is possible to perform error correction coding by adding redundant data having a predetermined length in the control station OLT 10 to the downlink transmission data.

In this case, as shown in FIG. 2, a situation is considered in which the control station OLT 10 and a predetermined terminal station ONU 90 (here, the terminal station ONU 90C ₂ ) transmit and receive data to and from each other.
Next, the terminal station ONU 90C ₂ requests the control station OLT 10 to transmit evaluation data as a material for determining error correction capability. Upon receiving this request, the control station OLT 10 transmits evaluation data to the terminal station ONU 90C ₂ . Thereby, the terminal station ONU 90C ₂ can measure the error rate from this data.

Next, the terminal station ONU 90C ₂ determines the degree of error correction coding for the control station OLT 10 based on the measured error rate. Here, it is assumed that rank 2 is selected.
Thereafter, the terminal station ONU 90C ₂ transmits the uplink transmission data after performing error correction coding to which redundant data having a length defined as rank 2 is added, for example. This transmission is performed to notify the control station OLT 10 of the rank. Alternatively, information including redundant data of a predetermined length may be simply added to the uplink transmission data for the control station OLT 10 and transmitted.

Note that the determination of the degree of error correction coding described above is performed by a request from the predetermined terminal station ONU 90 to the control station OLT 10, but can be performed without a request from the predetermined terminal station ONU 90. At this time, the terminal station ONU 90C ₂ measures the error rate of arbitrary data received from the control station OLT 10, so that the terminal station ONU 90C ₂ can measure the approximate optical fiber transmission path between the control station OLT 10 and the terminal station ONU 90C _2. The loss is known and the degree of error correction coding for the control station OLT 10 can be determined. Here, it is assumed that rank 3 is selected.

After that, the terminal station ONU 90C ₂ transmits the uplink data to the control station OLT 10 by performing error correction coding with redundant data having a length defined as rank 3 added to the transmission data. Alternatively, information indicating that the rank is 3 may be simply added to the uplink transmission data for the control station OLT 10 and transmitted.
The control station OLT 10 can know the rank by receiving redundant data with a predetermined rank or information on the rank itself. Therefore, the degree of error correction coding corresponding to the rank can be set, and the downlink signal to be transmitted to the terminal station ONU 90 can be error-correction coded and transmitted from the next time onward.

In this way, each terminal station ONU 90 can satisfactorily receive the signal subjected to error correction coding from the control station OLT 10.
In addition to the measurement of the round trip time and various error rates described above, the degree of error correction coding may be determined using the values of received signal strength, transmission luminous intensity, and jitter.
Further, even when the characteristics of the optical fiber transmission line change, the applied error correction capability can be changed as appropriate according to the determination work.

  In addition, various design changes can be made within the scope of matters described in the claims.

It is the schematic which shows the structural example of the optical communication system connected with the optical fiber via the optical splitter between the control station OLT concerning one Embodiment of this invention, and several terminal station ONU. It is the schematic which shows the basic composition of the control station OLT and the terminal station ONU in a PON system. It is a figure for demonstrating round trip time. It is a figure for demonstrating the calculation method of the error rate using a frame error rate. It is a figure for demonstrating the calculation method of the error rate using the 8B10B encoding unrestorable ratio. It is a figure for demonstrating the calculation method of the error rate using a bit error rate. It is a figure for demonstrating the error correction capability in error correction encoding. It is a figure showing the relationship between the redundant data of a codeword, and the information amount with respect to the number of effective data.

Explanation of symbols

1 PON system (optical communication system)
10 Control station side device OLT
11 Transmission Control Unit 12 Optical Signal Generator 90 Terminal Station Side Device ONU
93 Transmission Control Unit 94 Optical Signal Generator D _{1 to} D ₅ Valid Data P _{1 to} P ₅ Redundant Data

Claims (4)

A control station side device that is included in an optical communication system and can communicate with a terminal station side device through an optical fiber,
The control station side device
A determination means for determining a degree of good error correction coding for signal transmission to be transmitted to the terminal station side device;
The control station side apparatus which has either or both of following (a) and (b).
(A) a setting means for setting the determined degree of error correction coding for a signal transmitted from the control station side device to the terminal station side device;
(B) Notification means for notifying the terminal station side device of error correction coding setting information. The determination means includes
An error rate of a signal transmitted from the terminal station side device to the control station side device, a signal strength of a signal transmitted from the terminal station side device to the control station side device, the control station side device, and The control station side apparatus according to claim 1, wherein the degree of the error correction coding is determined using at least one of a round trip time as a time spent for communication with the terminal station side apparatus. A terminal station side device that is included in an optical communication system and can communicate with a control station side device through an optical fiber,
The terminal station side device
Determining means for determining the degree of good error correction coding for signal transmission to be transmitted to the control station side device;
The terminal station side apparatus which has either or both of following (c) and (d).
(C) Setting means for setting the determined degree of error correction coding for a signal transmitted from the terminal station side device to the control station side device;
(D) Notification means for notifying the control station side device of error correction coding setting information. The determination means includes
Using at least one of an error rate of a signal transmitted from the control station side device to the terminal station side device and a signal strength of a signal transmitted from the control station side device to the terminal station side device The terminal station apparatus according to claim 3, wherein the degree of error correction coding is determined.

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