JP2011217298A - Pon system, station-side device and terminal-side device thereof, and rtt correction method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain a stable link, regardless of link rates to be used together in a PON (passive optical network) system, using a plurality of link rates together.SOLUTION: The PON system includes a station-side device 1 and a terminal-side device 2 for performing optical communication via single-core optical fibers 3, 5 and uses a plurality of link rates 1G, 10G, together in the optical communication between both devices 1 and 2. The PON system includes a rate selecting section 30 for selecting one from among the plurality of link rates; an RTT (round-trip time) correction section 32 for correcting an RTT between the station and the terminal, according to the selected link rate; and a link control section 31 for controlling a link using the corrected RTT at the link rate selected.

Description

  The present invention relates to a PON (Passive Optical Network) system, station-side devices and home-side devices that are its components, and a round trip time (hereinafter referred to as “RTT”) correction method in the PON system. It is about.

A station-side device, a single-core optical fiber network that branches from an optical fiber connected to the optical fiber to a plurality of optical fibers via an optical coupler, and a home-side device connected to each end of the branched optical fiber The PON system is widely known.
Among the PON systems, 10G-EPON is a GE-PON upward compatible system that is already in operation, and performs communication by sharing a single-core optical fiber for 1G signals and 10G signals. For this reason, the station side device of 10G-EPON is configured to be able to transmit and receive both 1G signals and 10G signals.

Here, it is generally known that the power consumption of the 1G transmission / reception system is smaller than that of the 10G transmission / reception system.
For this reason, transmission / reception with 1G signal is performed when the traffic volume is small, and transmission / reception with 10G signal is performed when the traffic volume is large. Patent Documents 1 and 2 propose a method for reducing this.

Among these, Patent Document 1 relates to a method for switching an uplink rate. A station apparatus monitors the amount of uplink traffic, and if the amount is small, transfers data at a low link rate. The total power consumption can be reduced by transferring the data with.
On the other hand, Patent Document 2 relates to a downlink rate switching method, in which the station side device monitors the amount of downlink traffic and switches the link rate according to the amount, thereby reducing the overall power consumption. is there.

JP 2009-296234 A JP 2009-296231 A

However, the 1G transmission / reception system of the 10G-EPON system and the 10G transmission / reception system have different contents such as required encoding processing and decoding processing. Time also differs for each link rate.
Therefore, in a PON system using both 1G and 10G link rates, if the link rate is switched between offices, a time error is recognized due to the difference in delay time inside the device, and the RTT measured by the station side device. May become unstable and the link may be broken.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above-described conventional problems, and it is an object of the present invention to provide a PON system that can maintain a stable link regardless of the link rate used together in a PON system that uses a plurality of link rates together. .

  (1) The PON system of the present invention is a PON system that includes a station side device and a home side device that perform optical communication via a single optical fiber, and uses a plurality of link rates in optical communication between the two. A rate selection unit that selects one of the plurality of link rates, an RTT correction unit that corrects RTT between offices according to the selected link rate, and correction at the selected link rate A link control unit that controls the link using the later RTT.

  According to the PON system of the present invention, the RTT correction unit corrects the RTT between offices in accordance with the link rate selected by the rate selection unit, and the link control unit selects the link rate selected by the rate selection unit. Since the link control is performed using the corrected RTT in FIG. 5, even when the delay time inside the device varies depending on the link rate, a stable link can be maintained regardless of the link rate.

  (2) In the PON system according to the present invention, the station side device is installed in both the home and office devices in the round trip path of the control frame for the RTT measurement for at least the link rate that affects the measured value of the RTT. In the case of having a storage unit for storing data relating to the total delay time to be generated, the RTT correction unit provided in the station side device can correct the RTT independently using the data.

  (3) Further, in the PON system of the present invention, the station-side device stores a transmission delay time and a reception delay time in the device at least for the link rate that affects the measured value of the RTT. And the home-side device has a storage unit for storing the transmission delay time and reception delay time in the own device for the link rate, provided in both the station-side device and the home-side device. The RTT correction unit can share and correct the RTT.

  (4) Further, in the PON system of the present invention, both the home-side device and the home-side device in a round-trip path of a control frame for measuring the RTT for the link rate that affects at least the measured value of the RTT. In the case of having a storage unit that stores data relating to the total delay time generated therein, the RTT correction unit provided in the home-side apparatus can correct the RTT independently using the data.

  (5) The RTT correction method of the present invention is a correction method performed in the PON system according to the present invention, wherein one of a plurality of the link rates is selected, and the selected link rate And the step of correcting the RTT between the stations according to the above and the step of controlling the link using the corrected RTT at the selected link rate. Therefore, the correction method of the present invention has the same effects as the PON system of the present invention.

(6) One of the station side apparatuses of the present invention is a station side apparatus of a PON system that executes the RTT correction method of the present invention, and executes the step of correcting the RTT independently.
That is, this station side device stores data relating to the total delay time generated in both the home and office devices in the round trip path of the control frame for the RTT measurement for at least the link rate affecting the RTT measurement value. And a RTT correction unit that corrects the RTT by subtracting the total delay time from the measured value of the RTT.

The corrected RTT is the RTT of the transmission path portion that is not affected by the link rate because the total delay time generated in both the office and home equipment is subtracted in the round trip path of the control frame. Therefore, as long as the link is controlled using the corrected RTT, the link breakage at the link rate after switching does not occur.
In addition, when the station side device is configured to correct the RTT independently, there is no need to add an RTT correction function to each home side device, so there is an advantage that the operation cost of the entire PON system can be reduced. .

(7) Another station-side apparatus of the present invention is a station-side apparatus of a PON system that executes the RTT correction method of the present invention, and executes the step of correcting the RTT in a shared manner with the home-side apparatus. is there.
That is, the station side device stores a transmission delay time and a reception delay time in the own device for at least the link rate that affects the measured value of the RTT, and the reception side of the report frame. And an RTT correction unit that corrects the RTT by transmitting a gate frame having a time stamp obtained by subtracting the reception delay time and adding the transmission delay time of the own apparatus.

(8) In addition, one of the home side apparatuses of the present invention is a station side apparatus of the PON system that executes the RTT correction method of the present invention, and executes the step of correcting the RTT by sharing with the station side apparatus. To do.
In other words, this home side device has a storage unit for storing the transmission delay time and the reception delay time in its own device for at least the link rate that affects the measured value of the RTT, and automatically starts from the reception time of the gate frame. An RTT correction unit that corrects the RTT by correcting the time of the own device with a value obtained by adding the reception delay time of the device and transmitting a report frame having a time stamp obtained by adding the transmission delay time of the own device; It is characterized by having.

In the case of the station side device and the home side device that share the RTT correction, the RTT correction processing using the ranging process is performed, but even in this case, the corrected RTT is the station side device and the home side. The delay time inside the apparatus is all subtracted, and the RTT of the transmission line portion that is not affected by the link rate is obtained.
In this case, the special ranging process as described above is required, but the station side device does not have to know the delay time of the home side device. There is an advantage that correction can be performed.

(9) Another home-side device of the present invention is a home-side device of a PON system that executes the RTT correction method of the present invention, and executes the step of correcting the RTT independently.
That is, this home side device stores data relating to the total delay time generated in both the home and home devices in the round trip path of the control frame for the RTT measurement for at least the link rate that affects the RTT measurement value. And a storage unit that corrects the time of its own device by adding a delay time corresponding to the downlink in the total delay time to the time stamp value of the gate frame, and a delay corresponding to the uplink in the total delay time And an RTT correction unit that corrects the RTT by transmitting a report frame having a time stamp to which the time is added.

Since the corrected RTT in this case is also a value obtained by subtracting the total delay time generated in both the local offices in the round trip path of the control frame, the RTT of the transmission path portion that is not affected by the link rate can be obtained.
In this case, the home side device needs to know the delay time of the station side device, but the station side device does not need to know the delay time of each home side device. There is an advantage that the storage capacity of the station side device can be reduced.

  As described above, according to the present invention, a stable link can be maintained regardless of the link rate used together. For this reason, it is possible to prevent the link from being disconnected due to the switching of the link rate, and it is possible to switch the link rate stably.

1 is a schematic configuration diagram of a PON system according to an embodiment of the present invention. It is a sequence diagram which shows the multiplexing control function of the upstream signal by the station side apparatus. It is a functional block diagram of a station side device and a home side device of the PON system according to the first embodiment. It is a sequence diagram which shows the measuring method of RTT. It is a functional block diagram of a station side device and a home side device of a PON system according to a second embodiment. It is a sequence diagram which shows the measuring method of RTT. It is a functional block diagram of a station side device and a home side device of a PON system according to a third embodiment. It is a sequence diagram which shows the measuring method of RTT.

[Overall configuration of PON system]
FIG. 1 is a schematic configuration diagram of a PON system according to an embodiment of the present invention.
In FIG. 1, a station side device (OLT: Optical Line Terminal) 1 is a relay node between a higher level network and a PON system, and serves as a central station for a plurality of home side devices (OUN: Optical Network Unit) 2, 2A, 2B. It is installed at the central office of the telecommunications carrier.
Each home-side device 2, 2A, 2B is a terminal node on the home side of the PON system, and is installed in each subscriber home of the PON system.

A single-core optical fiber 3 (trunk line) that is a transmission path on the PON side of the station-side device 1 is branched into a plurality of single-fiber optical fibers (branches) 5 via an optical coupler 4 as a passive optical branching node. The home devices 2, 2A, 2B are connected to the ends of the branched optical fibers 5, respectively.
The upper interface of the station side device 1 is a multi-port that can be connected to a plurality of upper networks 6A, 6B having different transmission rates, and each of the home side devices 2, 2A, 2B is connected to the respective user networks 7A, 7B. It is connected.

In the following, when a plurality of upper networks 6A and 6B are expressed comprehensively, it is referred to as “upper network 6”, and when a plurality of user networks 7A and 7B are expressed comprehensively, “user network 7”. That's it.
Further, in FIG. 1, four home-side devices 2, 2 </ b> A, and 2 </ b> B are illustrated, but it is possible to connect 32 home-side devices 2 by branching, for example, 32 from one optical coupler 4. . Furthermore, although only one optical coupler 4 is used in FIG. 1, a larger number of home-side devices 2, 2A, 2B can be connected to the station-side device 1 by providing a plurality of optical couplers 4 in a column. Can do.

In the example of FIG. 1, the home side device 2A is an existing home side device with an uplink / downlink transmission rate of 1 Gbps, and the home side device 2B is a symmetric 10G-EPON home side with an uplink / downstream transmission rate of 10 Gbps. Device.
Further, the home-side apparatus 2 is of a link rate switching type that can be selected from two types of 1 Gbps or 10 G for both uplink and downlink transmission rates.

Therefore, the station-side apparatus 1 of the present embodiment uses two types of laser light of 1G wavelength λd1 and 10G wavelength λd2 in the downstream direction, and these are wavelength division multiplexing (WDM) systems. Is continuously transmitted.
In addition, the station side apparatus 1 of the present embodiment receives two types of laser light of the wavelength λu1 for 1G and the wavelength λu2 for 10G by the TDMA method in the upstream direction.

As described above, in the PON system of this embodiment, optical communication is performed with the upstream signal UO and the downstream optical signal DO made of laser light of two types of wavelengths both upstream and downstream, so that the media (optical fibers 3 and 5) and the station side device A WDM filter is provided between the transmitter / receiver 1 and the home-side devices 2, 2A, 2B.
In this case, only the wavelength component to be received is sent to the light receiving element, and the optical signal output from the light emitting element is wavelength-multiplexed with the received light via the WDM filter and sent to the optical fibers 3 and 5.

  In FIG. 1, the transmission rates of the upper networks 6A and 6B are 1 Gbps and 10 Gbps, respectively, and the transmission rates of the user networks 7A and 7B are 1 Gbps and 10 Gbps, respectively.

  The station side apparatus 1 contains an E / O conversion element (light emitting element) inside. This element sends a downstream optical signal UO time-division multiplexed to the home side devices 2, 2 A, 2 B to the optical fiber 3. The downstream optical signal DO is branched by the optical coupler 3 and is received by the O / E conversion elements (light receiving elements) provided in the home-side devices 2, 2A, 2B. Each home device 2, 2A, 2B receives only the data included in the downstream optical signal DO addressed to itself.

The station apparatus 1 includes an O / E conversion element (light receiving element) inside. This element receives the upstream optical signal UO sent to the optical fiber 5 from the E / O conversion elements (light emitting elements) of the home side devices 2, 2A, 2B.
When the upstream optical signal UO from each home-side device 2 is multiplexed by the optical coupler 3 and transmitted to one optical fiber 3, the station-side device 1 time-divides the transmission timing so that they do not collide with each other. Multiple control with. For this reason, as shown in FIG. 1, the upstream optical signals UO transmitted by the home devices 2, 2A, 2B are arranged on the time axis with the guard time interposed therebetween.

As described above, the station-side device 1 can perform PON communication at both 1G and 10G transmission rates. Of the subordinate home-side devices 2, 2A, 2B, the home-side device 2 Similar to the apparatus 1, PON communication at both 1G and 10G transmission rates is possible.
Therefore, the station side device 1 of the present embodiment can perform adaptive rate control with the home side device 2 to switch the link rate (1G and 10G) in each direction according to the uplink and downlink traffic volume. .

[Basic functions of the PON system]
In the PON system of this embodiment, the media access control for the home device 2 performed by the station device 1 is based on the GE-PON standard (IEEE Std 802.3ah) and the 10G-EPON standard (IEEE Std 802.3ah). It is done in accordance.
Therefore, in the following, in order to facilitate understanding of the PON system of the present embodiment, first, basic functions in the standard of the PON system will be described. In the following description, it is assumed that the home side device is the home side device 2 that supports two types of transmission rates.

<Identification function by LLID>
In the PON system, there is an RS (Reconciliation Sublayer) that acts as a mediator between a MAC (Media Access Control) layer and a physical layer, and Ethernet between the station side device 1 and the home side device 2 (Ethernet is a registered trademark. Similarly, in order to identify the frame, an identifier is embedded in a part of the preamble defined by this RS.
That is, in the PON system, since the same downstream signal reaches all the home-side devices 2 in a broadcast format, each home-side device 2 needs to determine whether or not the frame received by the home-side device 2 is addressed to itself. There is.

Therefore, in the PON system, this determination is performed using an identifier called LLID (Logical Link ID). This LLID is accommodated in the preamble of the Ethernet frame.
Note that the value of the LLID is determined by the station-side device 1 at the time of registration (discovery) of the home-side device 2, and the control unit 12 of the station-side device 1 causes LLID duplication in the home-side device 2 under its control. It is managed so that there is no.

Here, in downlink communication (communication from the OLT to the ONU direction), the control unit 12 of the station-side device 1 determines which home-side device 2 is to be transmitted for each transmission frame, and for the home-side device 2 Are embedded in the transmission frame and transmitted to the home apparatus 2.
The home side device 2 compares the LLID of the received frame with its own LLID notified from the station side device 1 in advance. If they match, the home side device 2 determines that the received frame is addressed and fetches the received frame. The received frame is discarded.

On the other hand, in uplink communication (communication from the ONU to the OLT direction), the home apparatus 2 embeds the LLID assigned to itself in a transmission frame and sends it to the station apparatus 1. The station-side device 1 determines which home-side device 2 has transmitted the frame based on the LLID value of the received frame.
As described above, when identification is performed by LLID, communication in a P2P (Point to Point) form is logically possible even in a topology form that is physically P2MP (Point to Multipoint).

<Time synchronization function>
In the PON system, in order to time-division multiplex the upstream signal of each home-side device 2, it is necessary to synchronize the time between the station-side device 1 and each home-side device 2.
Therefore, in the synchronization method proposed in the standard, the station side device 1 uses the time stamp embedded in the gate frame that is issued to the home side device 2 for permission of transmission, and synchronization between the two is performed. Maintain state.

That is, the station side device 1 transmits the current value of the master counter of its own station to the home side device 2 as time stamp information, and the home side device 2 sets the master counter value of its own station according to the received time stamp value. It is supposed to be updated.
With this method, the home device 2 can operate in an independent synchronization method. This eliminates the need for a high-accuracy PLL required for the slave synchronization device, which can contribute to cost reduction.

<MPCP function>
In the PON system, a multipoint MAC control sublayer including MPCP (Multi-point Control Protocol) which is a control protocol between the station side device 1 and the home side device 2 is adopted. The MPCP function includes the following functions 1) to 3).
In MPCP, if the handshake between the report frame and the gate frame is not performed for 1 second or more, or the measured value of the RTT changes rapidly, the link down is performed.

1) Discovery function for recognizing a plurality of home-side devices 2 by the station-side device 1 and measuring RTT necessary for communication between each home-side device 2 and the station-side device 1 and giving LLID 2) Uplink signal multiplexing control function that assigns a time slot to each home device 2 and multiplexes the upstream signal from each home device 2 on the time axis 3) Time synchronization function

<Discovery function>
When the home-side device 2 is connected to the PON, the station-side device 1 automatically finds the home-side device 2, assigns an LLID to the home-side device 2, and automatically establishes a communication link. This is the discovery function.
Specifically, the station side apparatus 1 measures RTT with the corresponding home side apparatus 2 during the period of P2MP discovery. At this time, the home side apparatus 2 performs time synchronization with the station side apparatus 1. I do.

The time of the station side device 1 and each home side device 2 is expressed by a counter incremented every 16 ns, and is synchronized in the PON system.
However, the RTT measurement and time synchronization are performed periodically (for example, every second), and are corrected as needed in the event of time deviation.

<Uplink multiplex control function>
In the PON system, since the upstream optical signal UO from each home-side apparatus 2 is joined to one optical fiber 3 by the optical coupler 4, it is necessary to control so that the upstream optical signal UO does not collide after joining.
Therefore, in the PON system, the station side device 1 serves as a control tower for the upstream signal control, and notifies each home side device 2 of transmission permission so that the upstream signal from each home side device 2 is temporally transmitted. To avoid collisions.

FIG. 2 is a sequence diagram illustrating an uplink signal multiplexing control function by the station-side device 1.
As shown in FIG. 2, upon receiving upstream data from its own user network 7, the home unit (ONU) 2 once accumulates the data in its own queue and reports the amount of data accumulated in the queue (Report). It is written in a frame and transmitted to the station side device 1.
The control unit 12 of the station side apparatus (OLT) 1 that has received the report frame uses the amount of data recorded in the report frame and the bandwidth used by the other side apparatus 2 to transmit uplink data to be assigned to the home side apparatus 2. A transmission amount (time equivalent value) and a transmission start time are calculated (dynamic bandwidth allocation), and the calculated value is recorded in a Gate frame and transmitted to the home device 2.

The home apparatus 2 that has received the gate frame transmits uplink data at a designated transmission start time in accordance with the instruction of the gate frame. When transmitting the uplink data, the home side apparatus 2 may transmit a report frame for notifying the amount of uplink data accumulated in the queue together with the next bandwidth allocation.
By repeating the above procedure, the station-side device 1 can appropriately allocate an uplink transmission band to each home-side device 2 while knowing the status of the uplink traffic in each home-side device 2.

<Dynamic bandwidth allocation function>
The station-side device 1 of the PON system uses the report frame and the gate frame to allocate a use band to each subordinate home-side device 2, but the calculation algorithm of the allocated band is out of the scope of the standard. Therefore, the description is omitted.

<OAM function>
Since the PON system is also a kind of Ethernet, it has an OAM (Operations, Administration and Maintenance) function according to the Ethernet standard. Here, OAM refers to maintenance monitoring control of devices and lines in a communication network.
For example, in the GE-PON standard (IEEE Std 802.3ah), an OAM sublayer is newly defined, and in this sublayer, a frame structure of an OAM frame and a control function using the frame are defined. .

In the PON system, the OAM frame is used between the station-side device 1 and the home-side device 2, and main functions using the OAM frame include failure notification, loopback test, link monitoring, and the like.
However, in addition to the functions defined in the standard, the system developer can also expand the OAM functions that are lacking as necessary.

[First Embodiment]
FIG. 3 is a functional block diagram of the station side device 1 and the home side device 2 of the PON system according to the first embodiment.
[Configuration of station side equipment]
As illustrated in FIG. 3, the station side device 1 includes an SNI (Service Node Interface) 11, a control unit 12, an encoding unit 13, a PON transmitting unit 14, a decoding unit 15, and a PON receiving unit in order from the left side (upper side). 16 is provided.

The control unit 12 of the station side device 1 has a data relay function, and sends a downlink frame (data) from the SNI 11 to the encoding unit 13 for each transmission rate.
The encoding unit 13 performs a predetermined encoding process on the downstream frame and sends it to the PON transmission unit 14. The PON transmission unit 14 amplifies the encoded downstream frame, converts it into an optical signal, and sends it to an optical fiber via a multiplexing / demultiplexing unit (not shown).

The encoding unit 13 is divided into a 1G encoding circuit and a 10G encoding circuit. The former performs a predetermined encoding process (for example, 8B / 10B encoding) according to the GE-PON protocol, The downstream frame is sent to the 1G transmission circuit (1G-Tx) of the PON transmission unit 14.
Further, the 10G encoding circuit of the encoding unit 13 performs a predetermined encoding process according to the 10G-EPON protocol, and sends the processed downstream frame to the 10G transmission circuit (10G-Tx) of the PON transmission unit 14. .

The encoding process of the 10G encoding circuit includes FEC (Forward Error Correction) in addition to 64B / 66B encoding. In order to prevent deterioration in reception sensitivity due to higher speed, Reed Solomon RS (225, 223) The code is indispensable by convention.
Accordingly, the processing time required for the encoding by the 10G encoding circuit is longer than that for the encoding by the 1G encoding circuit.

On the other hand, the PON receiving unit 16 photoelectrically converts the upstream optical signal and sends it to the decoding unit 15. The decoding unit 15 restores the uplink frame by performing a decoding process corresponding to the encoding process on the transmission side (home device 2), and sends the uplink frame to the control unit 12.
When the upstream frame is a control frame such as an MPCP frame or an OAM frame, the control unit 12 performs a predetermined process according to the protocol using the frame, and when the upstream frame is a data frame, Send to SNI11.

The decoding unit 15 is also divided into a 1G decoding circuit and a 10G decoding circuit, and the 1G decoding circuit performs decoding corresponding to the 8B / 10B encoding and the like according to the GE-PON regulations. Further, the 10G decoding circuit performs decoding corresponding to the 64B / 66B encoding according to the 10G-EPON protocol, FEC decoding corresponding to Reed-Solomon code, and the like.
Therefore, the processing time required for the decoding by the 10G decoding circuit varies depending on the redundancy of the FEC encoding, but is significantly longer than the decoding by the 1G decoding circuit.

  Note that the control unit 12 of the station side device 1 includes an FPGA or the like, and includes a rate selection unit 30, a link control unit 31, an RTT correction unit 32, and a reference table (storage unit) 33 as functional units programmed therein. However, these contents will be described later.

[Configuration of home-side equipment]
As shown in FIG. 3, the home-side device 1 includes, in order from the right side (user side), a UNI (User-Network Interface) 21, a control unit 22, an encoding unit 23, a PON transmission unit 24, a decoding unit 25, and a PON reception. A portion 26 is provided.

The control unit 22 of the home side apparatus 2 has a data relay function, and sends an upstream frame (data) from the UNI 21 to the encoding unit 23 for each transmission rate.
The encoding unit 23 performs a predetermined encoding process on the upstream frame and sends it to the PON transmission unit 24. The PON transmission unit 24 amplifies the encoded downstream frame, converts it into an optical signal, and sends it to an optical fiber via a multiplexing / demultiplexing unit (not shown).

The encoding unit 23 of the home device 2 is also divided into a 1G encoding circuit and a 10G encoding circuit, and the former performs a predetermined encoding process (for example, 8B / 10B encoding, etc.) according to the GE-PON protocol. Then, the processed downstream frame is sent to the 1G transmission circuit (1G-Tx) of the PON transmission unit 24.
Further, the 10G encoding circuit of the encoding unit 23 performs a predetermined encoding process according to the 10G-EPON protocol, and the downstream frame after the processing is transmitted to the 10G transmission circuit (10G-Tx) at the subsequent stage of the PON transmission unit 14. Send to.

The encoding process of the 10G encoding circuit includes FEC (Forward Error Correction) in addition to 64B / 66B encoding. In order to prevent deterioration in reception sensitivity due to higher speed, Reed Solomon RS (225, 223) The code is indispensable by convention.
Therefore, as in the case of the station side device 1, the processing time required for encoding by the 10G encoding circuit in the home side device 2 is longer than that in the case of encoding by the 1G encoding circuit.

The PON receiving unit 26 photoelectrically converts the downstream optical signal and sends it to the decoding unit 25. The decoding unit 15 performs a decoding process corresponding to the encoding process on the transmission side (station side device 1) to restore the downlink frame, and sends the downlink frame to the control unit 22.
When the downstream frame is a control frame such as an MPCP frame or an OAM frame, the control unit 22 performs predetermined processing according to the protocol using the frame, and when the downstream frame is a data frame, Send to UNI21.

The decoding unit 25 is also divided into a 1G decoding circuit and a 10G decoding circuit, and the 1G decoding circuit performs decoding corresponding to the 8B / 10B encoding and the like according to the GE-PON regulations. In addition, the 10G decoding circuit performs decoding corresponding to the 64B / 66B encoding according to the 10G-EPON protocol, and FEC decoding such as Reed-Solomon code.
Therefore, in the case of the home side apparatus 2 as well, the processing time required for decoding by the 10G decoding circuit varies depending on the redundancy of the FEC encoding, but is significantly longer than that in the case of decoding by the 1G decoding circuit.

[Control contents of station side equipment]
Among the functional units 30 to 32 of the control unit 12 of the station side device 1, the rate selection unit 30 selects a link rate operated with the home side device 2 to which the link rate is switched.
The rate selection unit 30 constantly monitors the upstream and downstream traffic volume based on, for example, the frame inflow rate from the upper side and the PON side, and when the traffic volume is relatively small, the lower transmission rate 1G When a traffic volume is relatively large, a higher transmission rate 10G is selected.

The rate selection unit 30 notifies the home apparatus 2 of the link rate selected by itself using a predetermined control frame. Upon receiving this control frame, the control unit 22 of the home-side device 2 switches the link rate of the own device to the predetermined link rate described there.
Note that, at the time of switching the link rate, the discovery is newly performed, and the RTT measurement is performed.

The link control unit 31 is a functional unit related to link connection control in the MPCP function. That is, the link control unit 31 monitors the time interval of the handshake by the corresponding report frame and gate frame with each home side device 2, and if the interval is less than 1 second, the link with the home side device 2 When the interval is 1 second or longer, the link with the home device 2 is disconnected.
Further, the link control unit 31 performs RTT measurement of each home-side device 2 every predetermined time, and maintains a link with the home-side device 2 if the measured value fluctuates within a predetermined threshold. When it changes, the link with the said home side apparatus 2 is cut | disconnected.

In the present embodiment, the link control unit 31 performs RTT measurement when the rate selection unit 30 switches to a new link rate.
At this time, the RTT correction unit 32 (to be described later) corrects the measured value of RTT, so that the rate control unit 30 executes the link control using the corrected RTTa.

  As shown in FIG. 3, the reference table 33 includes the following delay times d1 to d8 that occur inside the device when the station side device 1 and the home side device 2 transmit and receive frames. Each of the delay times d1 to d8 is specifically composed of the following times.

d1: Time required for passing through the 1G encoding circuit of the station side device 1 (transmission delay time)
d2: Time required for passing the 1G decoding circuit of the home device 2 (reception delay time)
d3: Time required for passing through the 1G encoding circuit of the home device 2 (transmission delay time)
d4: Time required for passing through the 1G decoding circuit of the station side device 1 (reception delay time)

d5: Time required for passing through the 10G encoding circuit of the station side device 1 (transmission delay time)
d6: Time required for passing the 10G decoding circuit of the home device 2 (reception delay time)
d7: Time required for passing through the 10G encoding circuit of the home device 2 (transmission delay time)
d8: Time required for passing through the 10G decoding circuit of the station side device 1 (reception delay time)

In the example of FIG. 3, the delay times d2, d3, d6, and d7 for one home-side device 2 are described, but the own device assigns an LLID to the reference table 33 of the station-side device 1. The delay times d2, d3, d6, d7 of all the home devices 2 are recorded.
Further, in the definition of the delay times d1 to d8 described above, only the processing time of the digital circuit is considered. However, when the delay of the analog circuit constituting the PON transmission unit or the reception unit is also a problem, the delay time is Should be taken into account.

Furthermore, in this embodiment, the delay times d1 to d8 are set as constant values in the reference table 33. However, in the case of a device capable of measuring the calculation time required for encoding and decoding, the device autonomously measures. The delay times d1 to d8 may be dynamically updated.
However, in this embodiment, since the station side device 1 collectively manages the delay times d2, d3, d6, d7 of the home side device 2, the home side device 2 uses these delay times d2, d3, d6, d7. When measured, it is necessary to notify the station side device 1 of the value using a predetermined control frame.

  The RTT correction unit 32 of the station side device 1 corrects the measured RTT using the delay times d1 to d8 recorded in the reference table 33. In the following, the RTT measurement method will be described first, and then the correction method will be described.

[Measurement of RTT and its correction method]
FIG. 4 is a sequence diagram showing a method of measuring RTT by the station side device 1.
In FIG. 4, the station-side device 1 is displayed as OLT, and the specific home-side device 2 that measures RTT is displayed as ONU.
In FIG. 4, it is assumed that the OLT receives a report frame (hereinafter referred to as “report R”) from the ONU when its PON counter is t4.

In this report R, the master counter value t3 of the own station when the ONU sends the report R is described as a time stamp tsr.
On the other hand, among the downstream gate frames (hereinafter referred to as “gate G”) received from the OLT before the ONU sends the report R, the latest one is the one when the OLT sends the gate G. The master counter value t1 of the local station of the OLT is written as a time stamp tsg.

Therefore, when the ONU receives the gate G, it updates the value of its master counter to the tsg value (= t1) of the gate G.
Here, the RTT of the ONU is generally calculated by the following equation.
RTT = (t4-t1)-(t3-t2)
The master counter value t2 of the ONU is updated to t1 (∴t2 = t1) by tsg (= t1) written in the gate G, so that RTT = t4−t3.

Since t3 is the same as the time stamp tsr of the report R, it can also be expressed as RTT = t4-tsr.
As described above, generally, the OLT reports RTT = t4-tsr from the reception time of the report R that is an upstream control frame (t4 in FIG. 4) and the time stamp tsr described in the report R. The RTT with the ONU that sent R can be obtained. When the synchronization of the OLT and ONU reference clocks is maintained, the RTT calculation formula described above always holds.

However, since the master counter value at the station house is the time grasped by each of the control units 12 and 22, the delay times d1 to d8 are not included.
In particular, when the link rate is 10G, FEC is included, and the delay time in the device when transmitting / receiving a 10G frame may be 4 μs or more when the encoding process and the decoding process are combined. This value can exceed the RTT of the latest home-side device 2, and in this case, the RTT is determined to be abnormal as soon as the link rate is switched from 1G to 10G, and link down may occur.

Therefore, in the present embodiment, the RTT correction unit 32 of the station side device 1 that grasps all the delay times of each home side device 2 corrects the measured value of RTT based on the table value recorded in the reference table 33. .
That is, when both the uplink and downlink link rates selected by the rate selection unit 30 are 10G, the RTT correction unit 32 obtains the corrected RTTa by the following equation.
RTTa = (t4−t1) − (t3−t2) − (d5 + d6 + d7 + d8)
= RTT- (d5 + d6 + d7 + d8)

Since the correction term (d5 + d6 + d7 + d8) on the right side in the above equation is a delay time generated inside the devices of the station side device 1 and the home side device 2 when exchanging the gate G and the report R at the up and down 10G, The corrected RTTa obtained by subtracting the internal delay time is substantially the round-trip propagation time of the transmission path portion that is not affected by the link rate.
Therefore, as long as the link is monitored with such a value of RTTa, it is possible to prevent the occurrence of link breakage at the link rate after switching.

  Thus, according to the PON system of the present embodiment, the RTT correction unit 32 corrects the RTT between the stations according to the link rate selected by the rate selection unit 30, and the link control unit 31 uses the rate selection unit. Since the link is monitored using the corrected RTT at the link rate selected in 30, even if the delay times d1 to d8 in the device differ depending on the link rate, a stable link is maintained regardless of the link rate. be able to.

In the present embodiment, since the station side device 1 is configured to correct the RTT independently, it is not necessary to add an RTT correction function to each home side device 2. For this reason, there exists an advantage that the operation cost as the whole PON system can be reduced.
When the uplink link rate is 10G and the downlink link rate is 1G, the corrected RTTa is as follows.
RTTa = RTT− (d1 + d2 + d7 + d8)

Similarly, when the uplink link rate is 1G and the downlink link rate is 10G, the corrected RTTa is as follows.
RTTa = RTT− (d5 + d6 + d3 + d4)
Further, when both the uplink and downlink link rates are 1G, the corrected RTTa is as follows.
RTTa = RTT− (d1 + d2 + d3 + d4)

  In the first embodiment, in order to calculate the total delay time generated inside the local office devices in the round trip path of the control frame (gate G and report R) for measuring the RTT, Data of each delay time d1 to d8 is individually recorded in the reference table 33. As data for obtaining the total delay time, for example, corresponding transmissions such as d1 + d2, d3 + d4, d5 + d6, and d7 + d8 are performed. Can also be recorded in the reference table in the form of the total value of the delay times of the receiver and receiver.

Further, the following four types of total delay time values generated by variations in uplink and downlink link rates may be used as data to be recorded in the reference table 33.
Total delay time for uplink 10G / downlink 10G = (d5 + d6 + d7 + d8)
Total delay time for uplink 10G / downlink 1G = (d1 + d2 + d7 + d8)
Total delay time for uplink 1G and downlink 10G = (d5 + d6 + d3 + d4)
Total delay time for uplink 1G and downlink 1G = (d1 + d2 + d3 + d4)

As described above, the data recorded in the reference table 33 may be data capable of obtaining the total delay time for each link rate by addition, or may be data of the total delay time for each link rate. .
This also applies to the case of the third embodiment in which the home side apparatus 2 performs the RTT correction process alone.

[Second Embodiment]
FIG. 5 is a functional block diagram of the station side device 1 and the home side device 2 of the PON system according to the second embodiment.
The difference between the second embodiment (FIG. 5) and the first embodiment (FIG. 3) is that the station side device 1 and the home side device 2 know only the delay time of their own devices. The devices 1 and 2 are provided with RTT correction units 32 and 34, respectively, so that both devices 1 and 2 execute the RTT correction process.

That is, in this embodiment, only the delay times d1, d5, d4, d8 of the station side device 1 are recorded in the reference table 33 of the station side device 1, and the delay times d2, d6, d3 of the home side device 2 are recorded. , D7 are not recorded.
On the contrary, only the delay times d2, d6, d3, d7 of the home device 2 are recorded in the reference table 35 of the home device 2, and the delay times d1, d5, d4, d8 of the station device 1 are recorded. It has not been.

FIG. 6 is a sequence diagram showing an RTT measurement method including a correction process that the RTT correction units 32 and 34 execute in cooperation. It is assumed here that the uplink and downlink link rates are both 10G.
Here, the gate G whose transmission time at the master counter of its own station (transmission time at the control unit 12 of the OLT) is t1 is added to the optical fiber 3 after the transmission delay time d5 of its own device has elapsed from that time t1. As shown in FIG. 6, the RTT correction unit 32 of the OLT adds a value obtained by adding the transmission delay time d5 of its own device to the transmission time t1 of the master counter of its own station (= t1 + d5) as a time stamp tsg. Transmit the gate G as a value.

On the other hand, the gate G having the time stamp tsg (= t1 + d5) controls the ONU through at least the reception delay time d6 generated in the decoding unit 25 after passing through the transmission path (optical fibers 3 and 5) independent of the link rate. The RTT correction unit 34 of the ONU that has received the gate G recognizes the reception delay time of the own device at the reception time (reception time at the ONU control unit 22) t2 of the own station. The time of the own device is corrected to the value obtained by adding d6.
Further, the ONU RTT correction unit 34 sets a timestamp (= t3 + d7) obtained by adding the transmission delay time d7 of its own device to the transmission time (transmission time of the ONU control unit 22) t3 of its own station. A report R having a tsr value is transmitted.

Then, the report R having the time stamp tsr (= t3 + d7) passes through the transmission path (optical fibers 3 and 5) independent of the link rate, and then controls the OLT through at least the reception delay time d8 generated in the decoding unit 55. The RTT correction unit 32 of the OLT that has received the report G recognizes the reception delay time d8 of the own device from the reception time t4 at the master counter of the own station. Assuming that the report R has reached the OLT, the RTT is measured by subtracting the time stamp value tsr of the report G from the time after the subtraction.
RTTa that has undergone the ranging process accompanied by the correction at both of the above-mentioned offices is expressed by the following equation. In FIG. 6 and the following equation, δ = t3−t2.

RTTa = (t4-tsr) -d8
= {T4- (t3 + d7)}-d8
= (T4−t1) −δ− (d5 + d6 + d7 + d8)
= (T4-t1)-(t3-t2)-(d5 + d6 + d7 + d8)
= RTT- (d5 + d6 + d7 + d8)

As is clear from the above equation, even in the case of RTT correction processing that is shared between the two houses using the ranging process, the corrected RTTa is the time of the master counter in the station side device 1 and the home side device 2. From the RTT based on the above, the delay time in which the influence on the RTT is a problem inside the apparatus is all subtracted, and the RTT of the transmission line portion that is not affected by the link rate is obtained.
In this embodiment, the special ranging process as described above is required. However, since the station-side device 1 does not have to grasp the delay time of the home-side device 2, the home-side devices 2 with different manufacturers are used. Both can correct RTT appropriately.

[Third Embodiment]
FIG. 7 is a functional block diagram of the station side device 1 and the home side device 2 of the PON system according to the third embodiment.
The third embodiment (FIG. 7) is different from the first embodiment (FIG. 3) in that the home device 2 knows the delay time of the station device 1 including its own device, and the home device 2 Only the RTT correction unit 34 is provided, and the home side apparatus 2 performs the RTT correction process alone.

That is, in the present embodiment, the station side device 1 does not grasp the delay times d1, d5, d4, d8 of its own device, and the delay time d2 of the home side device 2 is stored in the reference table 35 of the home side device 2. , D6, d3, d7 and delay times d1, d5, d4, d8 of the station side device 1 are recorded.
FIG. 8 is a sequence diagram illustrating an RTT measurement method including a correction process that is independently performed by the RTT correction unit 34 of the home-side apparatus 2. It is assumed here that the uplink and downlink link rates are both 10G.

As shown in FIG. 8, the OLT in this case is normally gated with the time stamp tsg value of the gate G as t1 at the transmission time (transmission time at the OLT control unit 12) t1 at the master counter of the own station. Send G.
The ONU that has received the gate G transmits a transmission delay time d5 on the OLT side that occurs in transmission / reception of the gate G, which is a downlink control frame, at the reception time (reception time at the ONU controller 22) t2 of the master counter of the local station. And the ONU side reception delay time d6 are corrected to the time of the own apparatus.

  Also, the ONU RTT correction unit 34 transmits the transmission delay time on the ONU side that occurs in transmission / reception of the report R, which is an uplink control frame, at the transmission time at the master counter of the own station (transmission time at the ONU control unit 22). A report R is transmitted with a value (= t3 + d7 + d8) obtained by adding d7 and the reception delay time d8 on the OLT side as a time stamp tsr value.

Then, the OLT that has received the report G measures the RTT by subtracting the time stamp value tsr (= t3 + d7 + d8) of the report R from the reception time t4 at the master counter of its own station.
RTTa that has undergone the ranging process accompanied by the correction in the home side apparatus 2 as described above is expressed by the following equation. In FIG. 8 and the following equation, δ = t3−t2.

RTTa = t4-tsr
= T4-t3- (d7 + d8)
= (T4-t1) -δ- (d5 + d6 + d7 + d8)
= (T4-t1)-(t3-t2)-(d5 + d6 + d7 + d8)
= RTT- (d5 + d6 + d7 + d8)

As is apparent from the above equation, even in the case of the RTT correction process using the ranging process with the correction of the home side device 2, the corrected RTTa is the time of the master counter in the station side device 1 and the home side device 2. Based on the RTT based, the delay time in which the influence on the RTT is a problem inside the apparatus is all subtracted, and the RTT of the transmission line portion not affected by the link rate is obtained.
Further, according to the present embodiment, since the station side device 1 does not need to know the delay time of each home side device 2, the storage capacity of the station side device 1 when implementing the present invention can be reduced. There are also advantages.

[Other variations]
The above embodiments are illustrative of the present invention and are not limiting. The scope of rights of the present invention is shown not by the above embodiment but by the scope of claims for patent, and includes all modifications equivalent to the scope of claims and their configurations.

For example, in the above embodiment, when the link rates to be used are 1G and 10G, the delay times d1 to d4 inside the device are taken into account even when the link rate is 1G, but these delay times d1 to d4 are RTT measurements. When the value is small enough to be ignored from the value, the RTT correction process may be performed using only the delay times d5 to d8 in the case of 10G.
Moreover, the link rate used together by the station side apparatus 1 and the home side apparatus 2 is not restricted to two types, and may be three or more types.

1 Station side device (1G, 10G combined use)
2 Home side equipment (1G, 10G combined use)
2A Home side equipment (1G)
2B Home side device (10G)
3 Optical fiber 5 Optical fiber 6A Upper network (1G)
6B Host network (10G)
7A User network (1G)
7B User network (10G)
30 rate selection unit 31 link control unit 32 RTT correction unit 33 reference table (storage unit)
34 RTT correction unit 35 Reference table (storage unit)
d1 to d8 Delay time G Gate frame R Report frame

Claims (9)

A PON system that includes a station side device and a home side device that perform optical communication via a single-core optical fiber, and that uses a plurality of link rates in optical communication between the two,
A rate selection unit for selecting one of the plurality of link rates;
An RTT correction unit that corrects round trip time (hereinafter referred to as “RTT”) between stations according to the selected link rate;
A link control unit that controls the link using the corrected RTT at the selected link rate;
PON system characterized by comprising. The station side device stores data relating to the total delay time generated in both the home and home devices in the round trip path of the control frame for the RTT measurement for at least the link rate that affects the RTT measurement value. Part
The PON system according to claim 1, wherein the RTT correction unit provided in the station-side device corrects the RTT independently using the data. The station side device has a storage unit for storing a transmission delay time and a reception delay time in the own device for at least the link rate that affects the measured value of the RTT,
The home side device has a storage unit for storing a transmission delay time and a reception delay time in the own device for the link rate,
The PON system according to claim 1, wherein the RTT correction unit provided in both the station side device and the home side device shares and corrects the RTT. The home device stores data relating to the total delay time generated in both the home devices in the round trip path of the control frame for the RTT measurement for at least the link rate that affects the measured value of the RTT Part
The PON system according to claim 1, wherein the RTT correction unit provided in the home-side apparatus independently corrects the RTT using the data. In a PON system that uses a plurality of link rates together in optical communication via a single-core optical fiber,
Selecting one of a plurality of the link rates;
Correcting RTT between offices in accordance with the selected link rate;
Performing link control using the corrected RTT at the selected link rate;
A correction method for RTT in a PON system, comprising: A station apparatus for performing the RTT correction method according to claim 5,
A storage unit for storing data relating to a total delay time generated in both local office devices in a round-trip path of a control frame for the measurement of the RTT for the link rate affecting at least the measurement value of the RTT;
A station-side apparatus of a PON system, comprising: an RTT correction unit that corrects the RTT by subtracting the total delay time from the measured value of the RTT. A station-side device of a PON system that executes the RTT correction method according to claim 5,
A storage unit for storing a transmission delay time and a reception delay time in the own device for at least the link rate affecting the measured value of the RTT;
An RTT correction unit for correcting the RTT by subtracting the reception delay time of the own device from the reception time of the report frame and transmitting a gate frame having a time stamp obtained by adding the transmission delay time of the own device;
A station apparatus of a PON system, comprising: A PON system home apparatus for executing the RTT correction method according to claim 5,
A storage unit for storing a transmission delay time and a reception delay time in the own device for at least the link rate affecting the measured value of the RTT;
The RTT is calculated by correcting the time of the own device with a value obtained by adding the reception delay time of the own device from the reception time of the gate frame and transmitting a report frame having a time stamp obtained by adding the transmission delay time of the own device. An RTT correction unit to correct;
A home-side device of a PON system characterized by comprising: A PON system home apparatus for executing the RTT correction method according to claim 5,
A storage unit for storing data relating to a total delay time generated in both local office devices in a round-trip path of a control frame for the measurement of the RTT for the link rate affecting at least the measurement value of the RTT;
A time obtained by correcting the time of its own device by adding the delay time corresponding to the downlink in the total delay time to the time stamp value of the gate frame and adding the delay time corresponding to the uplink in the total delay time An RTT correction unit for correcting the RTT by transmitting a report frame having a stamp;
A home-side device of a PON system characterized by comprising:

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