JP2007295151A - Pon system and station side apparatus and terminal used for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology capable of arranging UP signals from terminals, without causing collision among the UP signals or the need for precondition of clock synchronization, among a station side apparatus and the terminals. <P>SOLUTION: In a PON system disclosed herein that includes a station side apparatus 1; an optical fiber network 10 with a configuration, wherein an optical fiber 5 connected to the station-side apparatus 1 is branched into a plurality of optical fibers 7, 8, 9 via a photocoupler 6, and the plural terminals 2, 3, 4 respectively connected to the terminations of the branched optical fibers 7, 8, 9, wherein the station-side apparatus 1 obtains a round trip time (RTT), on the basis of the difference between a time stamp TS included in a report frame (REPORT), corresponding to a particular gate frame (GATE) and a count of its own PON counter and distributes a subsequent gate frame (GATE) to the terminals 2, 3, 4 at a transmission start time resulting from subtracting the round trip time (RTT), so that the PON system allows the station-side apparatus 1 to distribute the subsequent gate frame (GATE) by allowing a time margin, capable of absorbing variations in the time stamp TS. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a PON system in which communication between a station-side device and a plurality of terminal devices is time-division multiplexed, and a station-side device and a terminal device used therefor.

A PON system (Passive Optical Network System) is a P2MP (P2MP) in which one station side device (OLT: Optical Line Terminal) and a plurality of user terminal devices (ONU: Optical Network Unit) are connected via a passive element such as an optical coupler. Point To Multipoint) type optical fiber network system. Among these PON systems, GE-PON (Gigabit Ethernet-PON) is an economical implementation of a gigabit class transmission system based on Ethernet (registered trademark) technology. It was established as IEEE 802.3ah ^TM in 2004. This is one of the high-speed optical access methods standardized in June.
In the PON system described above, a TDM (Time Division Multiplexing) method for aligning and transmitting signals for each terminal device is used for the downlink signal transmitted from the station side device to each terminal device. For uplink signals transmitted to the apparatus, a TDMA (Time Division Multiple Access) system is employed in which optical signals are transmitted at correct timing so that the signals do not collide with each other. In order to accurately perform the signal transmission timing in the TDMA system, it is necessary that the clock of the terminal device and the clock of the station side device are accurately synchronized.

For this reason, for example, in the standard based on IEEE802.3ah ^TM , the station side device counts the local clock to generate a clock of the station side device (hereinafter referred to as a PON counter), and the PON downstream signal transmission clock is generated locally. It is specified that the PON control frame transmitted to the terminal device is synchronized with the clock and the value of the PON counter at the time of transmission is described as a time stamp (hereinafter sometimes abbreviated as TS).
Also, as a terminal device, the local clock is synchronized with the clock included in the PON reception signal, the local clock is counted to generate the PON counter of the terminal device, and when the PON control frame is received, Update the PON counter with the indicated time stamp value, transmission permission is indicated by the value of the PON counter, and the terminal device transmits to the PON when its own PON counter is within the specified range, at this time Is synchronized with a local clock (see Non-Patent Document 1).

  Further, in the above standard, regarding the ranging process for obtaining the round-trip propagation time, when the station side device receives the PON control frame from the terminal device, the time stamp attached to the PON control frame, its own PON counter, Describes a method for obtaining the round-trip propagation time between the terminal device and the terminal device. The station-side device that has obtained the round-trip propagation time considers the round-trip propagation time and distributes the subsequent PON control frame to each terminal device.

On the other hand, in the PON system, it is expected that the transmission speed will be further increased in the future, but in order to respond to such a request for high speed, a multi-rate burst circuit that generates a signal of a different rate for each time slot is added. A service capacity increasing method in a PON system that increases the service capacity for each terminal device is known (see Patent Document 1).
Thus, even when the transmission speed for a specific terminal device is increased, it is necessary to introduce a new high-speed service while maintaining the existing service, and it is necessary to coexist with a plurality of transmission rates. The communication between the station side device and each terminal device is performed at a plurality of different transmission rates (hereinafter referred to as multi-rate PON).

IEEE Std 802.3ah (registered trademark) -2004 (64.Multipoint MAC Control) JP-A-8-8954 (Claim 1)

  The standard is based on the premise that the station side device continuously transmits a signal having a fixed transmission rate in the downlink direction, and the terminal device reproduces a clock from the fixed rate downlink signal, Increment its own PON counter based on the clock. As described above, in the PON system assumed by the standard, since the clock is synchronized between the station side device and the terminal device, the station side device and the station side device can be connected regardless of the length of the interval at which the time stamp arrives. The synchronization of the PON counter of the terminal device is supposed to be accurate.

However, for example, in the above-described multi-rate PON, since the transmission rate in the downlink direction is not fixed, the PON counter of the terminal device incremented by the clock regenerated from the downlink signal becomes faster or slower. An accurate PON counter cannot represent the exact time.
In multi-rate PON, it may be assumed that a signal of a certain transmission rate (for example, an ultra-high speed 10G transmission rate) cannot be correctly recognized depending on the capability of the terminal device. Operation based on continuous clock synchronization by signals is not desirable. Therefore, in general, in the multi-rate PON, the clocks of the station side device and each terminal device are independent. However, although the ideal frequencies of these clocks match, in reality, a minute mismatch within the specified range is unavoidable.

As described above, in a multi-rate PON that does not require strict clock synchronization, when a terminal device transmits a PON control frame, the TS attached to that frame is the value of the PON counter of the station side device at that time. Does not match.
That is, the round trip time measured by the station side device (hereinafter sometimes abbreviated as RTT (Round Trip Time)) is the PON counter when the station side device receives a PON control frame such as a report frame from the terminal device. Is calculated by subtracting the time stamp TS of the PON control frame from the value of Tr (noted as Tr). If the clock synchronization between the station side device and the terminal device is not strictly achieved, the time stamp itself is Since the difference between the clocks of both devices is included, an accurate round-trip propagation time cannot be calculated.

Assuming that the frequency difference between the clocks of the station side device and the terminal device is constant, the time lag based on this frequency difference updates its PON counter with the TS of the PON control frame received by the terminal device. To the time until the PON control frame is further transmitted to the station side device.
Therefore, when the round trip propagation delay is calculated with RTT = Tr-TS according to the standard, and the transmission permission time of the subsequent terminal device is determined, there is a possibility that the uplink signals collide between the plurality of terminal devices, Further, when the difference from the immediately preceding RTT exceeds a certain value, there is a risk that the link will be disconnected.

In the PON system in which the station side device distributes the PON control frame in consideration of the round trip propagation time to each terminal device in view of such a situation, even if the clock synchronization between both devices is not premised, each terminal It is a first object of the present invention to make it possible to arrange upstream signals from the apparatus without colliding.
In the PON system in which the station side device distributes the PON control frame considering the round trip propagation time to each terminal device, the round trip propagation time can be accurately obtained without assuming clock synchronization between the two devices. The second object is to make it possible to efficiently arrange uplink signals from each terminal device without colliding.
Further, in the PON system in which the station side device distributes the PON control frame considering the round-trip propagation time to each terminal device, the clock frequency difference between both devices is measured, and the upstream signal from each terminal device is obtained. A third object is to enable more efficient arrangement without causing a collision.

  According to a first aspect of the present invention, there is provided a station side device, an optical fiber network configured to branch from an optical fiber connected to the station side device to a plurality of optical fibers via an optical coupler, and each of the branched optical fibers. A plurality of terminal devices each connected to the terminal, and the station side device transmits the PON control frame from the difference between the time stamp included in the received PON control frame and its PON counter. In the PON system that obtains the round-trip propagation time between them and distributes the gate frame to the terminal device at the transmission start time obtained by subtracting the round-trip propagation time from the time at which the uplink signal from the terminal device arrives, The gate frame is distributed to each of the terminal devices with an allowance for absorbing the fluctuation of the time stamp at the time when the upstream signal arrives. To.

In the first aspect of the present invention, “assuming a margin for absorbing fluctuations at the time when the uplink signal arrives” means that the uplink signals of the terminal apparatuses do not collide even if such fluctuations occur. It means that transmission permission is given by setting a sufficient guard time between bursts.
According to the first aspect of the present invention, it is assumed that a time for arriving the upstream signal with a margin capable of absorbing time stamp fluctuations generated due to a clock frequency difference between the station side device and each terminal device. Since it is distributed, the upstream signal from each terminal device can be arranged without colliding without synchronizing the clock between the station side device and each terminal device. Achieved.

The temporal fluctuation caused by the clock frequency difference between the station side device and each terminal device is not only the time stamp included in the report frame but also the transmission start time of each terminal device and the transmission duration time of each terminal device. Can also affect.
Therefore, in the first aspect of the present invention, fluctuations in the transmission start time and transmission duration of each terminal device generated due to the frequency difference are also evaluated, and a margin that can absorb these fluctuations is expected at the time when the upstream signal arrives. It is preferable to distribute the gate frame. When evaluating the variation of the transmission start time of the terminal device, it is possible to avoid the collision of the uplink signal due to the clock frequency difference even when the transmission permission to the terminal device is reserved much earlier. Further, when evaluating the variation in the transmission duration time of the terminal device, it is possible to avoid the collision of the uplink signals due to the clock frequency difference even when the time is considerably increased.

  According to a second aspect of the present invention, there is provided a station-side device, an optical fiber network configured to branch from an optical fiber connected to the station-side device to a plurality of optical fibers via an optical coupler, and each of the branched optical fibers. A plurality of terminal devices respectively connected to the terminal, and the station side device transmits the report frame from a difference between a time stamp included in a report frame corresponding to a specific gate frame and its own PON counter. In the PON system that obtains a round-trip propagation time between the terminal device and distributes the gate frame to the terminal device at a transmission start time obtained by subtracting the round-trip propagation time from the time at which the uplink signal from the terminal device arrives. A time stamp included in the report frame in which a transmission start time is set to be closest to an arrival time of the specific gate frame; And measuring the round-trip propagation time by.

According to the second aspect of the present invention, since the round-trip propagation time is measured by the time stamp included in the report frame whose transmission start time is set closest to the arrival time of the specific gate frame, The report frame is returned to the station side device with almost no transmission waiting time. For this reason, even if there is a clock frequency difference between the station side device and the terminal device, the difference hardly affects the time stamp from the terminal device, and the round-trip propagation time can be obtained accurately.
Therefore, according to the second aspect of the present invention, the round-trip propagation time can be accurately obtained without assuming clock synchronization between both apparatuses, and the second object is achieved.

Further, in the second aspect of the present invention, in the exchange of the original gate frame and report frame for the purpose of bandwidth allocation of the uplink signal, it is not necessary to update the round-trip propagation time every time a report frame is received. If the transmission waiting time (the time from the reception of the gate frame to the transmission of the report frame) is a predetermined short time, when the original report frame is received, the ranging process is performed to update the round-trip propagation time. If the update condition is not sufficient, the round-trip propagation time ranging process may be executed again.
For this reason, the number of times of the ranging process is reduced and the calculation load on the station side apparatus is reduced as compared with the conventional case (in the case of the standard) in which the round-trip propagation time is calculated and updated every time a report frame is received. be able to.

Also, in the exchange of the original gate frame and report frame for the purpose of bandwidth allocation of the uplink signal, the station side device does not immediately transmit the original gate frame when the bandwidth allocation amount is calculated, but at that time It is preferable to transmit the original gate frame with a delay near the transmission start time.
In this case, by delaying the transmission time of the gate frame close to the transmission start time, the transmission waiting time at the terminal device can be shortened as much as possible, and the time stamp of the terminal device based on the frequency difference of the clock between both devices can be reduced. There is an advantage that blurring is reduced and the above update condition is easily satisfied.

In the second aspect of the present invention, the station side device may transmit a gate frame corresponding to two or more report frames having different time intervals. In this case, it is possible to calculate a plurality of round trip propagation times using the time stamps of the two or more report frames, and from the difference between the round trip propagation times, the frequency difference of the clock between the station side device and each terminal device. The clock difference measurement process can be performed.
Therefore, the variation in the transmission start time and transmission duration of each terminal device that can occur due to the frequency difference thus determined is obtained, and a margin for absorbing this variation is estimated at the time when the upstream signal arrives, and thereafter By distributing the gate frame, the upstream signal from each terminal apparatus can be arranged without colliding without synchronizing the clock between the station side apparatus and each terminal apparatus. Is achieved.

  Further, when performing the above-described clock difference measurement process, the terminal device may be caused to notify each terminal device of the frequency difference obtained by the clock difference measurement process. In this case, if each terminal device that has received the notification adjusts its own clock so that the frequency difference is reduced, the time lag between the two devices is reduced, and the upstream signal is more efficiently transmitted. Can be arranged.

Also, each terminal device that has received the notification and the station side device performs the notification and the adjustment until the frequency difference falls within a predetermined range, and then performs main signal communication including band allocation communication. For example, when an upstream signal is arranged, it is not necessary to compensate for the time lag caused by the clock frequency difference in the station side device, and the processing becomes simple.
Further, when the station side device observes that the frequency difference of the clock has deviated from the predetermined range, the deviated frequency difference is notified to each terminal device, and each terminal device that has received the notification notifies the deviated frequency. You may adjust own clock so that a difference may become small.

As described above, according to the present invention, in a PON system in which a station-side device distributes a PON control frame considering round-trip propagation time to each terminal device, each terminal device can be used without assuming clock synchronization between both devices. Can be arranged without colliding the upstream signal from.
Further, according to the present invention, in the PON system, it is possible to accurately obtain the round-trip propagation time without assuming clock synchronization between both devices. Thereby, it is possible to efficiently arrange the uplink signals from the respective terminal devices without colliding. Further, according to the present invention, in the PON system, it is possible to measure the difference in clock frequency between the two devices. As a result, the upstream signals from the terminal devices can be arranged more efficiently without colliding.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Overall configuration of PON system]
FIG. 1 is a schematic configuration diagram of a multi-rate PON system assumed by the present invention.
In FIG. 1, a station side device (OLT) 1 is installed in a telephone office or the like as a central station for a plurality of terminal devices (ONUs) 2, 3, and 4, and each terminal device 2, 3, and 4 is a PON. Installed in the subscriber home of the system. One optical fiber (main line) 5 is connected to the station side apparatus 1. The optical fiber 5 is branched into a plurality of optical fibers (branch lines) 7, 8, 9 via an optical coupler 6, thereby forming an optical fiber network 10. The terminal devices 2, 3, and 4 are connected to the ends of the branched optical fibers 7, 8, and 9, respectively. The station side device 1 is connected to the host network 11, and the terminal devices 2, 3, 4 are connected to the respective user networks 12, 13, 14.

In FIG. 1, for the sake of simplicity, an example in which three terminal apparatuses 2, 3, and 4 are connected is illustrated. However, in practice, 32 terminal apparatuses are divided into 32 branches from one optical coupler 6. It is possible to connect. In addition, FIG. 1 illustrates a topology with only one optical coupler 6, but by arranging a plurality of stages of optical couplers 6 having a small number of branches in a column, it is possible to make terminal devices distributed over a wide area short. It is also possible to connect to the station side device 1 by a fiber.
In this PON system, downstream optical wavelength and upstream optical wavelength are divided and wavelength division multiplexing (WDM) is performed. That is, a laser beam having a single wavelength λ1 is used for uplink communication between the station side device 1 and the terminal devices 2, 3, and 4, and a single wavelength λ2 different from the wavelength λ1 is used for downlink communication. Laser light is used.

Accordingly, a WDM filter is provided between the transmitter / receiver of the PON medium (optical fibers 5, 7, 8, 9) and the station side device 1 and each of the terminal devices 2, 3, 4, and only the wavelength component to be received is provided. The optical signal sent to the receiver and output from the transmitter is multiplexed with the received light via the WDM filter and sent to the optical fibers 5, 7, 8 and 9. The wavelengths λ1 and λ2 can be selected in the range of 1260 nm ≦ λ1 ≦ 1360 and 1480 nm ≦ λ2 ≦ 1500 in accordance with IEEE Std 802.3ah ^™ -2004 Clause 60.
This PON system is a multi-rate PON system in which burst transmission is performed at a multi-rate in the downstream direction, and the transmission rates in the upstream direction of the terminal apparatuses 2, 3, and 4 are different from each other. In the example of FIG. 1, the information communication rates of the terminal apparatuses 2, 3, and 4 are 1G, 2G, and 10 Gbps, respectively, and a 2 Gbps signal and a 10 Gbps signal are time-division multiplexed based on a 1 Gbps GE-PON. Is transmitted to the station-side device 1.

[Configuration of station side equipment]
FIG. 2 shows an example of the PON interface unit of the station side apparatus 1 according to the present invention.
As shown in FIG. 2, a frame to be transmitted in the downlink direction is once stored in the downlink buffer 16 and then transmitted to the encoding circuit 18 based on an instruction from the transmission control circuit 17. The encoding circuit 18 encodes the transmission frame based on a predetermined method and a predetermined rate, and sends it to the transmitter 19 as a serial bit signal. The transmitter 19 converts the serial bit signal into an optical signal, bidirectionally multiplexes it with the upstream optical signal by a WDM filter (not shown), and then sends it to the optical fiber 5 of the PON system.

  As for the optical signal received from the optical fiber 5 of the PON system, only the upstream optical signal is input to the receiver 20 by a WDM filter (not shown), converted into an electrical signal by the receiver 20, and then input to the data recovery circuit 21. The The data recovery circuit 21 restores the bit data at a rate modulated by the transmission source terminal devices 2, 3, and 4, and sends the bit data to the code synchronization circuit 22. The code synchronization circuit 22 detects a code boundary from the restored bit data based on the coding method performed by the transmission source terminal devices 2, 3, and 4, and performs decoding processing to restore the frame. The restored frame is input to the frame type classification circuit 23, and the PON control frame and other frames are classified. The PON control frame is sent to the control frame processing circuit 25 through the time stamp extraction circuit 24.

  The time stamp extraction circuit 24 reads the time stamp TS and sends it to the PON time management circuit 25. The control frame processing circuit 26 determines the type of the PON control frame, sends information relating to the registration processing of the terminal devices 2, 3, and 4 to the registration processing circuit 27, and sends information relating to bandwidth allocation to the terminal devices 2, 3, 4. The data is sent to the allocation circuit 28. The registration processing circuit 27 instructs the band allocation circuit 28 to start the registration processing, and exchanges a PON control frame based on a sequence determined between the terminal devices 2, 3 and 4 that have requested registration, The terminal devices 2, 3, and 4 are registered or the registration is rejected.

When registering the terminal devices 2, 3, and 4, the registration processing circuit 27 instructs the bandwidth allocation circuit 28 to continuously allocate the bandwidth to the terminal devices 2, 3, and 4.
The band allocation circuit 28 allocates a transmission permission for polling the registration request based on an instruction to start the registration process from the registration processing circuit, and issues a gate frame (PON control frame for band allocation). 26. When the bandwidth allocation circuit 28 receives a report frame (a PON control frame for bandwidth allocation request) from the control frame processing circuit 26, the bandwidth allocation circuit 28 executes bandwidth allocation processing based on a predetermined policy, and each terminal device 2, 3, 4 is assigned, and the control frame processing circuit 26 is instructed to issue a gate frame.

When receiving a gate frame issuance instruction from the registration processing circuit 27 or the band allocation circuit 28, the control frame processing circuit 26 configures a corresponding gate frame and sends it to the transmission control circuit 17. Details of the sequence related to registration and bandwidth allocation of the terminal devices 2, 3, and 4 will be described later.
The clock generation circuit 29 oscillates autonomously within a predetermined frequency range around the ideal frequency f, and generates a reference clock. The PON time management circuit 25 counts the reference clock to generate a PON counter, and distributes this PON counter to each main part of the PON interface unit. When receiving the time stamp of the PON control frame, the PON time management circuit 25 calculates the round-trip propagation time RTT between the terminal devices 2, 3, and 4 that are the transmission source of the frame, and uses this RTT information as the PON interface. Distribute to each major part of the department.

At this time, the PON time management circuit 25 examines a change from the immediately preceding RTT, and generates an alarm if the fluctuation exceeds a predetermined range. When receiving this alarm, the registration processing circuit 27 performs necessary processing such as refusing registration of the terminal devices 2, 3 and 4.
When the transmission control circuit 17 receives the PON control frame from the control frame processing circuit 26, the transmission control circuit 17 temporarily stores the PON control frame in the downlink buffer 16, and transmits it to the PON in preference to the transmission of the general downlink frame. The transmission process is as described above. At this time, referring to the latest value of the PON counter, the time stamp TS is added to the PON control frame, and the frame check sequence is set to a value reflecting the value of TS.

[Configuration of terminal device]
FIG. 3 shows an example of the PON interface unit of the terminal devices 2, 3, and 4 according to the present invention.
The optical signals received from the PON optical fibers 7, 8, 9 are input only to the downstream optical signal by the WDM filter (not shown) to the receiver 31, converted into an electrical signal by the receiver 31, and then the data recovery circuit 32. Is input. The data recovery circuit 32 restores the bit data at a predetermined rate (1G, 2G, or 10G in the present embodiment) assumed by the terminal devices 2, 3, and 4, and sends the bit data to the code synchronization circuit 33. The code synchronization circuit 33 detects a code boundary from the restored bit data based on an encoding method performed by the transmission source station-side apparatus 1 and performs a decoding process to restore a frame.

The restored frame is input to the frame type classification circuit 34, and the PON control frame and other frames are classified. The PON control frame is sent to the control frame processing circuit 36 via the time stamp extraction circuit 35.
The time stamp extraction circuit 35 reads the time stamp TS, and the PON time management circuit 37
Send to. The control frame processing circuit 36 determines the type of the PON control frame and sends information related to the registration processing to the registration processing circuit 38. If it is related to transmission permission (gate frame), the transmission control circuit 39 is notified of transmission permission information described in the gate frame.

  The transmission permission information is composed of a data type (registration request or normal request: presence / absence of bandwidth request for normal request), transmission start time, and transmission duration. The registration processing circuit 38 exchanges PON control frames based on a sequence determined with the station side device 1 to register the terminal devices 2, 3 and 4. When receiving an instruction to issue a PON control frame from the registration processing circuit 38, the control frame processing circuit 36 configures a corresponding PON control frame and sends it to the transmission control circuit 39. Details of the sequence related to registration and transmission permission of the terminal devices 2, 3, and 4 will be described later.

  The clock generation circuit 40 oscillates autonomously within a predetermined frequency range around the ideal frequency f and generates a reference clock. The PON time management circuit 37 counts the reference clock and generates a PON counter. The value of the PON counter can be referred from the transmission control circuit 39. When the time stamp TS of the PON control frame is received, the PON counter is updated with the value. At this time, the change with the immediately preceding PON counter is examined, and if the fluctuation exceeds a predetermined range, an alarm is generated. In response to this alarm, the registration processing circuit 38 performs necessary processing such as withdrawal of registration.

A frame to be transmitted in the upstream direction is temporarily stored in the upstream buffer 41. The state of the upstream buffer 41 is managed by the transmission control circuit 39. When receiving the PON control frame to be transmitted from the control frame processing circuit 36, the transmission control circuit 39 temporarily stores it in the upstream buffer 41. In addition, when a request for bandwidth request is instructed by transmission permission, a report frame (a kind of PON control frame) reflecting the state of the upstream buffer 41 is generated.
When the transmission control circuit 39 receives a transmission permission notification from the control frame processing circuit 36, if the transmission permission data type is a normal request, the transmission control circuit 39 transmits when the PON counter reaches the transmission start time. The device 43 is instructed to emit light, and the light emission instruction is continued until the transmission duration elapses.

At this time, prior to the light emission instruction, the upstream buffer 41 is instructed to take out an amount of transmission frames corresponding to the transmission duration. However, since it is necessary to transmit a report frame when a band request forcing is instructed, the report frame is inserted into the transmission frame sequence while subtracting the amount of data extracted from the uplink buffer 41 accordingly. At this time, as an extreme example, only a report frame may be transmitted.
On the other hand, when the terminal device 2, 3, 4 is in an unregistered state and receives a registration request PON control frame from the control frame processing circuit 36, the transmission permission data type is a registration request. The control circuit 39 instructs the transmitter to emit light after waiting for a random time within the range of the transmission duration from the time when the PON counter reaches the transmission start time, and instructs to emit light only for the time to send the registration request PON control frame. To last. At this time, prior to the light emission instruction, the upstream buffer 41 is instructed to take out the frame.

The report frame and the frame extracted from the upstream buffer 41 are encoded by the encoding circuit 42 based on a predetermined method and a predetermined rate, and are sent to the transmitter as a serial bit signal.
At this time, if the frame to be transmitted is a PON control frame, the latest value of the PON counter is referred to and a time stamp TS is attached to the frame. The transmitter 43 converts a serial bit signal into an optical signal during a period of light emission instruction, and sends it to the PON. On the other hand, the transmitter 43 does not emit light during a period when no light emission instruction is given.

[Calculation method of round-trip propagation time (RTT)]
FIG. 4 is a sequence diagram illustrating a method of measuring the round trip propagation time (RTT) between the terminal devices 2, 3, and 4 by the station side device 1. In FIG. 4, the station-side device 1 is displayed as OLT, and a specific terminal device that measures RTT is displayed as ONU (A).
In FIG. 4, it is assumed that the OLT receives an upstream PON control frame US1 from the ONU (A) when its PON counter is T4. In this US1, the value T3 of the PON counter of the ONU (A) when the ONU (A) sends out the control frame US1 is described as a time stamp TS.

On the other hand, if the most recent downlink PON control frame received from the OLT before the ONU (A) sends US1 is DS1, this DS1 contains the data when the OLT sends the PON control frame. A value T1 of the PON counter of the OLT is written as a time stamp TS. When ONU (A) receives DS1, it updates the value of its PON counter with the TS value (= T1) of DS1.
Here, the round-trip propagation time of the ONU (A) is generally calculated by the following equation.
RTT (A) = (T4-T1)-(T3-T2)
Further, T2 of ONU (A) is updated to T1 by TS (= T1) written in DS1 (∴T2 = T1), and eventually RTT (A) = T4-T3. Since T3 is the same as the time stamp TS of the control frame US1, it can also be expressed as RTT (A) = T4-TS.

As described above, generally, the OLT transmits the control frame as ONT (RTT = Tr−TS) from the reception time Tr of the downlink control frame (T4 in FIG. 4) and the TS described in the control frame. When the RTT between the ALT and the reference clock of the OLT and the ONU (A) is maintained, the above-described formula for calculating the round-trip propagation time is always established accurately.
When the OLT allocates an upstream band to the ONU (A), the OLT gives the ONU (A) a gate frame in which transmission permission information is described in advance. The transmission permission information includes a transmission start time and a transmission duration. The transmission start time is obtained by subtracting RTT from the time at which the upstream signal from ONU (A) arrives. Here, when the upstream signal from each ONU is densely arranged, the time at which the upstream signal arrives is the time when the upstream signal from the ONU that is supposed to perform upstream transmission immediately before the ONU (A) ends. A predetermined guard time is added in order to absorb the shake due to accuracy.
However, when the synchronization of the reference clock of the OLT and the ONU (A) is not maintained, the T3 as the time stamp TS of the upstream PON control frame US1 is the true T3 when compared with the time of the OLT In this case, (Tr-TS) does not represent a true RTT. If the frequency difference between the reference clocks of OLT and ONU (A) is constant (≠ 0), the time difference between (Tr−TS) and RTT increases as the time interval between T2 and T3 in FIG. 4 increases.

Therefore, in this embodiment, the OLT evaluates the fluctuation of the time stamp TS generated by the clock frequency difference between the OLT and the ONU (A), and distributes the subsequent gate frame in anticipation of a margin that can absorb this fluctuation. By adopting this transmission control method, the upstream signals can be arranged without colliding even without assuming clock synchronization between the OLT and the ONU (A).
Further, in the present embodiment, the OLT measures the RTT by the time stamp TS included in the report frame US1 in which the transmission start time T3 is set to the closest time of the arrival time T2 of the specific gate frame DS1, whereby the OLT and the ONU Even if the clock synchronization with (A) is not assumed, the RTT can be obtained accurately.

[Transmission control method considering clock difference]
FIG. 5 is a sequence diagram illustrating an example of the transmission control method and the ranging process.
In the example of FIG. 5, the above transmission control method is employed for ONU registration. In FIG. 5, the station-side device 1 is displayed as OLT, and a specific terminal device that performs ranging processing is displayed as ONU (B).
As shown in FIG. 5, first, the OLT broadcasts to the ONU (B) a transmission permission frame DGATE (a registration gate frame which is a kind of PON control frame) for polling a registration request. The OLT stores the value Tdg (DGATE transmission time) of the DGATE time stamp TS for later processing.

When the ONU (B) to be newly registered receives DGATE, it updates its PON counter with the TS written in this DGATE. Then, a registration request frame REGREQ (a kind of PON control frame) is sent to the OLT within the transmission permission time designated by DGATE.
Therefore, when the OLT accepts an ONU (B) registration request, the OLT transmits a registration frame REGISTER (a kind of PON control frame) to the ONU (B). Further, the OLT sends a normal transmission permission frame GATE (a normal gate frame which is a kind of PON control frame) to the ONU (B) in order to send a registration confirmation frame REGACK (a kind of PON control frame) from the ONU (B). Send to.

As a response to REGISTER, ONU (B) sends REGACK during the transmission permission period described in GATE. The OLT receives REGACK, thereby completing the ONU (B) registration procedure.
In the present embodiment, the ONU shown in FIG. 3 has a configuration in which only data is extracted from the downstream signal and no clock is extracted, as is apparent from the clock included in the continuous signal from the OLT. Do not synchronize the clock. Accordingly, the frequency of the OLT and ONU reference clocks do not exactly match, but the frequency difference is configured to be within a certain range with the ideal frequency f as a reference. If the frequency difference of the clock is df (expressed as a ratio), the upper limit dfmax can usually be set to about ± 100 ppm.

When the OLT receives the REGREQ from the ONU (B), the OLT obtains the first-stage RTT1 (B) by the equation (Tr-TS), that is, (REGREQ reception time-TS described in the REGREQ).
Further, the OLT is a time-dependent fluctuation in the time stamp TS of the REGREQ from the formula (TS-Tdg), that is, (the transmission time of TS-DGATE described in the REGREQ) and the dfmax. The upper limit ΔRmax of is evaluated. Specifically, ΔRmax is calculated by (TS−Tdg) × dfmax.

When the OLT gives a GATE for REGACK to the ONU (B) (transmission duration is TL (B)), there is no collision with the transmission of other ONUs in relation to the transmission permission given to the other ONUs. In addition, the arrival time TT (B) to the OLT is determined so that the upstream band can be used without waste, and the transmission start time is determined in consideration of RTT1 (B) (specifically, RTT1 (B) is subtracted). Decide. Since the time TE (B) at which all the upstream signals of the ONU (B) have arrived is TE (B) = TT (B) + TL (B), it is expressed as ONU (ONU (N) to be transmitted immediately after the ONU (B). The time TT (N) at which the upstream signal) arrives at the OLT is assumed to be obtained by adding a predetermined burst gap to TE (B).
Further, the OLT calculates a variation (ΔSmax) of the transmission start time interpreted by the ONU (B) from the difference between the transmission start time and the time when the GATE will be transmitted and the dfmax, and the dfmax is calculated as ONU (B The fluctuation (ΔLmax) caused in the transmission duration of) is also calculated.

Then, the OLT takes into account these fluctuations and the above ΔRmax, and describes the further adjusted transmission start time in the GATE so as not to collide with the transmission of the previous ONU.
More specifically, when the ONU (B) reference clock is faster than the OLT reference clock, the RTT1 (B) is smaller than the actual time, but the ONU (B) starts transmitting an upstream signal. Is earlier than the OLT time. That is, the upstream signal from ONU (B) arrives earlier than TT (B) by ΔSmax−ΔRmax.
Conversely, when the ONU (B) reference clock is faster than the OLT reference clock, the upstream signal from the ONU (B) arrives earlier by ΔRmax−ΔSmax. Since the OLT does not know whether the reference clock of the ONU (B) is fast or slow, the OLT changes the TT (B) to TT so that the upstream signal from the ONU (B) does not collide with the transmission of the previous ONU. (B) Correction to + | ΔRmax−ΔSmax |
When the reference clock of ONU (B) is late, the time when all the upstream signals have arrived is delayed based on the OLT time, so there is a possibility of colliding with the upstream signal of ONU (N). In order to prevent this, TT (N) is corrected to TT (N) + ΔSmax + ΔLmax. Here, it is assumed that the original TT (N) is obtained based on the corrected TT (B). When the OLT gives GATE to the ONU (N), the OLT further performs the same adjustment as the ONU (B) (the same applies hereinafter). Since these adjustments are pursuing limits in terms of efficiency, variations at the expense of some efficiency are within the scope of the present invention.

  As described above, in the PON system of this embodiment, the variation in the time stamp TS included in the registration request frame (REGREQ in FIG. 5) generated by the frequency difference between the clocks of the OLT and the ONU (B), and the ONU (B) Since the fluctuation of the transmission start time and the transmission continuation time is calculated and the subsequent gate frame (GATE in FIG. 5) is distributed in consideration of a margin that can absorb this fluctuation, between the OLT and the ONU (B) Thus, the upstream signals from other ONUs can be arranged without colliding without clock synchronization.

[Ranging considering the clock difference]
As shown in FIG. 5, the OLT according to the present embodiment performs the second registration process by the following method after performing the registration process.
The OLT sends out a GATE forcing a report frame (a kind of PON control frame, which describes a bandwidth request). The purpose of the GATE and REPORT is to increase the accuracy of the second ranging process, and the transmission start time is as close as possible to the time when the ONU (B) will receive the GATE.

That is, the REPORT transmission start time from the ONU (B) is the arrival time of the GATE to the ONU (B) (the PON counter of the ONU (B) is updated with the GATE time stamp TS. It is set to the nearest distance that is only a slight time difference ΔT. Specifically, in IEEE 802.3ah ^TM , the time difference ΔT is 16 μs or more, and is set to a short time (ΔTmin) of about 16 μs here. This short time is about 1/100 of the upper limit (1 ms) of the time difference recognized in IEEE802.3ah ^™ .
Then, when the OLT receives the REPORT corresponding to the GATE for recalculating the RTT, the OLT obtains an accurate RTT2 (from REPORT reception time-REPORT time stamp TS) from the formula (Tr-TS). B).

  As described above, in the PON system according to the present embodiment, RTT2 (B) is measured by the time stamp TS included in the REPORT whose transmission start time is set closest to the arrival time of the GATE. Therefore, the ONU (B) REPORT is returned to the OLT with almost no transmission waiting time. For this reason, even if there is a clock frequency difference between OLT and ONU (B), the difference hardly affects the time stamp TS of ONU (B), and RTT2 (B) can be obtained accurately. Therefore, the round-trip propagation time can be accurately obtained without assuming clock synchronization between the OLT and the ONU (B), and since the round-trip propagation time is accurate, the upstream signal from each ONU is as dense as possible. It can also be arranged.

By the way, as the OLT of the present embodiment, an OLT that is selectively updated in accordance with a certain update condition is employed instead of calculating and updating the RTT with the formula (Tr-TS) every time a PON control frame is received. Can do.
For example, in GATE-REPORT for the purpose of original bandwidth allocation, the elapsed time in the ONU (the time from the reception of GATE until the start of transmission by the ONU) is within a predetermined range (for example, within 20 μs) ( Hereinafter, the RTT may be updated when the OLT receives the REPORT.

In this case, whether the GATE-REPORT with the ONU did not satisfy the RTT update condition within a predetermined time limit (for example, within 1 s) after updating the RTT of a specific ONU, Alternatively, when the difference between (Tr−TS) when the PON control frame is received from the ONU and the RTT at that time exceeds a certain value, the OLT executes the second-stage ranging process again. You can do it.
For this reason, the number of times of the ranging process is reduced and the calculation load on the OLT can be reduced as compared with the conventional case where the RTT is calculated and updated every time REPORT is received (in the case of the standard).

Furthermore, in this embodiment, the OLT does not immediately transmit GATE when the bandwidth allocation amount is calculated based on REPORT, but intentionally delays the elapsed time in the ONU as short as possible. Also good. Specifically, with respect to the time at which the report is desired to arrive, the time obtained by subtracting RTT2 (B) and ΔRmin is the limit, and the time is delayed to near this limit. In this case, the transmission waiting time in the ONU (B) can be shortened as much as possible by intentionally delaying the transmission time of the GATE, based on the frequency difference of the clock between the OLT and the ONU (B). The fluctuation of the time stamp TS of the ONU (B) is reduced, and the RTT update condition is easily satisfied.
Accordingly, in this case, since REPORT almost certainly satisfies the RTT update condition, the RTT may be updated by the equation (Tr-TS) each time REPORT is received.
Moreover, by intentionally delaying the GATE transmission for the main signal, the fluctuation of the transmission start time is reduced, and the upstream signals can be arranged more densely.

[Measurement of RTT and clock difference]
FIG. 6 is a sequence diagram illustrating a ranging process according to another embodiment of the present invention.
As shown in FIG. 6, in this alternative embodiment, the OLT sends out one GATE forcing the transmission of two REPORTs in the second stage ranging process in which an accurate RTT is calculated. This GATE includes one time stamp TS and two transmission permissions.
At this time, the transmission permission for the first first REPORT is the GATE reception time and the latest transmission time, similar to the REPORT transmission permission in the ranging process. On the other hand, it is desirable that the transmission permission for the second second REPORT should be as long as possible in the ONU (B).

Therefore, the equation (Tr-TS) when the first REPORT is received reflects the exact RTT of the OLT and ONU (B), and the RTT between these two REPORTs is transmitted. Since the change is relatively small, it can be said that the difference between (Tr-TS) at the time of receiving the first REPORT and that at the time of receiving the second REPORT is due to the difference in clock frequency between the OLT and the ONU.
Therefore, in this alternative embodiment, the OLT calculates the clock frequency difference df together with the RTT from the above measurement. Therefore, in this case, the transmission start time can be adjusted by GATE in consideration of the actually measured deviation due to the df, not the temporal fluctuation due to the dfmax that is a preset fixed value.
In this case, since OLT knows whether the reference clock of ONU (B) is fast or slow, it is considered that ΔSmax and ΔLmax take positive and negative values (the slower one is positive), and the correction of TT (B) is as follows: TT (B) −ΔSmax is set, and TT (N) is corrected by TT (N) + ΔSmax + ΔLmax (the same applies hereinafter). Since these adjustments are pursuing limits in terms of efficiency, variations at the expense of some efficiency are within the scope of the present invention.

  In the example of FIG. 6, the case where the transmission time of the first REPORT is set close to the reception time of GATE is illustrated, but the method for measuring df is not limited to this. That is, although the accuracy is inferior, RTT and df may be calculated from two independent GATE-REPORTs having different elapsed times in the ONU. In this case, since the clock frequency of the OLT or ONU changes with time due to environmental conditions or the like, the calculation of df is continuously performed in the same manner as the RTT. If the measurement conditions are not satisfied in the original GATE-REPORT process, the second-stage ranging is started.

[Modification of terminal device]
FIG. 7 shows the PON interface unit of the terminal devices 2, 3, and 4 according to another embodiment of the present invention.
The interface units of the terminal devices 2, 3, and 4 shown in FIG. 7 have substantially the same configuration as that shown in FIG. 3, and therefore, the same components are denoted by the same reference numerals and description thereof is omitted. Only the differences will be described here.

In this other embodiment, the reference clock generation circuit is configured by a VCXO 45 whose frequency can be controlled from the outside, and the control frame processing circuit 36 performs frequency control of this VCXO 45, which is different from the case of FIG.
In the initial state, the control frame processing circuit 36 sets the frequency of the VCXO 45 to the median value. At this time, the actual frequency of the VCXO 45 is located in a frequency range determined around the ideal frequency f. The OLT corresponding to the ONU in FIG. 7 can perform ONU registration processing and ranging processing as in the OLT in FIG. 2, and obtain the clock frequency difference df as a result of the second stage ranging processing. And

.
Therefore, the OLT notifies the ONU of a difference in the reference clock frequency from the ONU as a kind of PON control frame based on the frequency difference df.
On the other hand, in the ONU of this another embodiment, when receiving the PON control frame, the control frame processing circuit 36 controls the VCXO 45 so that the frequency deviation df becomes small. The notification of the frequency deviation may be information on the frequency difference df itself or simple information on whether to make it faster or slower. Further, instead of the PON control frame, another type of frame such as an OAM frame may be used for this notification.

If such a ranging process and the adjustment of the reference clock in the ONU are performed until the frequency difference df falls within a sufficiently small predetermined range, then main signal communication including the original band allocation communication is performed in the OLT. Since it is not necessary to compensate for the time lag based on the clock frequency difference, the OLT process that takes into account the df lag may be skipped.
Alternatively, the processing that takes into account the deviation due to df may not be implemented in the OLT by concentrating after the ONU registration and adjusting the ONU reference clock. In this case, the change of the frequency difference df is traced by continuous ranging, and readjustment may be performed when the deviation becomes large.

  In FIG. 1 of the present embodiment, the case where the present invention is applied to the multi-rate PON system is illustrated, but the present invention can also be adopted in a normal PON system in which the transmission rate of each terminal device is fixed.

1 is a schematic configuration diagram of a multi-rate PON system assumed by the present invention. FIG. It is a circuit diagram which shows an example of the PON interface part of the station side apparatus of this invention. It is a circuit diagram which shows an example of the PON interface part of the terminal device of this invention. It is a sequence diagram which shows the measuring method of a round-trip propagation time (RTT). It is a sequence diagram which shows an example of the transmission control method and ranging process of this invention. It is a sequence diagram which shows the ranging process of another embodiment of this invention. It is a circuit diagram which shows the PON interface part of the terminal device which concerns on another embodiment of this invention.

Explanation of symbols

1 Station side device 2 Terminal device (for 1G)
3 Terminal device (for 2G)
4 Terminal device (for 10G)
5 Optical fiber 6 Optical coupler 7 Optical fiber 8 Optical fiber 9 Optical fiber 10 Optical fiber network GATE Gate frame REPORT Report frame TS Time stamp RTT Round-trip propagation time

Claims (14)

A station-side device, an optical fiber network configured to branch from an optical fiber connected to the station-side device to a plurality of optical fibers via an optical coupler, and a plurality of devices connected to the ends of the branched optical fibers, respectively And the terminal side device obtains the round-trip propagation time between the terminal device that sent the PON control frame from the difference between the time stamp included in the received PON control frame and its own PON counter. In the PON system that distributes the gate frame to the terminal device at the transmission start time obtained by subtracting the round-trip propagation time from the time at which the uplink signal from the terminal device arrives,
A PON system that distributes a gate frame to each of the terminal devices in consideration of a time when the station side device can absorb the fluctuation of the time stamp at a time when the upstream signal arrives.   2. The PON system according to claim 1, wherein the station side device distributes the gate frame to each of the terminal devices in consideration of a time when the uplink signal arrives at a time when the station device can absorb a change in transmission start time of each of the terminal devices.   3. The PON system according to claim 1, wherein the station-side device distributes the gate frame to each of the terminal devices in anticipation of a time when the uplink signal arrives at a time when the station-side device can absorb a variation in transmission duration of each of the terminal devices. . A station-side device, an optical fiber network configured to branch from an optical fiber connected to the station-side device to a plurality of optical fibers via an optical coupler, and a plurality of devices connected to the ends of the branched optical fibers, respectively Round-trip propagation between the terminal device that sent the report frame from the difference between the time stamp included in the report frame corresponding to the specific gate frame and its own PON counter. In the PON system for obtaining the time and distributing the gate frame to the terminal device at the transmission start time obtained by subtracting the round-trip propagation time from the time at which the uplink signal from the terminal device arrives.
The PON system, wherein the station side device measures the round-trip propagation time based on a time stamp included in the report frame whose transmission start time is set closest to the arrival time of the specific gate frame.   In exchange of a gate frame and a report frame for the purpose of bandwidth allocation of an uplink signal, the station side device has an update condition as to whether or not a transmission waiting time at the terminal device is a predetermined short time. The PON system according to claim 4, wherein the round-trip propagation time is updated by performing the ranging process only when a report frame that satisfies a condition is received.   In the exchange of the gate frame and the report frame for the purpose of bandwidth allocation of the uplink signal, the station side device does not transmit the gate frame immediately when the bandwidth allocation amount is calculated, but the transmission start time from that time. The PON system according to claim 4 or 5, wherein the gate frame is transmitted with a delay in the vicinity. The station side device, based on the difference between the round-trip propagation times measured using the time stamps of two or more report frames with different time intervals returned in response to the specific gate frame, Perform clock difference measurement processing to measure the clock frequency difference with each terminal device,
7. The gate frame is distributed to each of the terminal devices in consideration of a margin that can absorb a change in transmission start time of each of the terminal devices that may occur due to this frequency difference at the time of arrival of the uplink signal. The PON system according to item 1.   8. The PON system according to claim 7, wherein a gate frame is distributed to each of the terminal devices in consideration of a margin for absorbing a variation in transmission duration of each of the terminal devices that may be generated due to the frequency difference at a time when the uplink signal arrives. .   The station side device notifies the terminal device of the frequency difference obtained by the clock difference measurement process, and the terminal device that has received the notification adjusts its clock so that the frequency difference becomes small. Item 9. The PON system according to Item 7 or 8.   The terminal device that has received the notification with the station side device performs the notification and the adjustment until the frequency difference falls within a predetermined range, and then performs main signal communication including band allocation communication. PON system.   When the station side device observes that the frequency difference has deviated from a predetermined range, the terminal device is notified of the deviating frequency difference, and the terminal device that has received the notification The PON system according to claim 9 or 10, wherein the PON system adjusts its own clock to be small. Reciprocal propagation between a plurality of terminal devices connected in a P2MP form via optical fibers and the terminal device that sent out the PON control frame based on the difference between the time stamp included in the received PON control frame and its own PON counter In the station side device of the PON system that obtains the time and distributes the gate frame to the terminal device at the transmission start time obtained by subtracting the round-trip propagation time from the time at which the uplink signal from the terminal device arrives.
A station-side device of a PON system, wherein a gate frame is distributed to each of the terminal devices in consideration of a time for absorbing the fluctuation of the time stamp at a time when the upstream signal arrives. Between a terminal device that is connected to a plurality of terminal devices via an optical fiber in the P2MP form and that has transmitted the report frame based on a difference between a time stamp included in a report frame corresponding to a specific gate frame and its own PON counter In the station-side device of the PON system that distributes the gate frame to the terminal device at the transmission start time obtained by subtracting the round-trip propagation time from the time at which the uplink signal from the terminal device arrives.
A station-side apparatus of a PON system, wherein the round-trip propagation time is measured by a time stamp included in the report frame whose transmission start time is set closest to the arrival time of the specific gate frame. When the local time reaches the transmission start time set in the gate frame distributed from the station side device and connected to the station side device via the optical fiber in the P2MP form, the transmission of the uplink signal is started, and the report When transmitting a frame, in the terminal device of the PON system in which the local time when transmitting the report frame is described in the report frame,
By receiving a control frame including information on the frequency difference of the clock with the station side device from the station side device and adjusting its own clock so that this frequency difference becomes small, it becomes a downlink signal from the station side device. A terminal device of a PON system that generates uplink signal transmission timing with an independent clock without performing network synchronization based on the terminal device.

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