Optical TDM Transmission Device, Synchronization Method, and Synchronization Program

Abstract

According to the present invention, in an optical TDM transmission system in which a plurality of nodes (40) transmit data signals in a state where time slots are synchronized between the nodes, control is performed such that: in a first time period which is before data signals are superposed, each of the nodes (40) receives, for a plurality of times, timing signals cyclically transmitted to the node (40), and stores reception cycles and reception time points of the received timing signals as reception timings of the timing signals; and, in a second time period in which data signals are superposed, each of the nodes (40) causes a data signal transmitted from the node itself to be superposed on a time slot identified by using the stored reception timings as a reference.

Description

Technical Field

[0001] The present invention relates to an optical TDM transmission device, a synchronization method, and a synchronization program.

Background Art

[0002] In order to perform full-mesh optical connection between a plurality of communication stations in a transmission network, an increase in the number of optical paths is required. As one technique for realizing an increase in the number of optical paths, optical time-division multiplexing (TDM) is effective.

[0003] FIG. 15 is an explanatory diagram showing signals used in optical TDM transmission and signals used in a normal transmission network. Signal 801 is a "continuous signal" in which a single signal occupies a transmission path of the same line and the same wavelength. The continuous signal is an optical signal used in an existing link system. Signal 802 is a "burst signal" in which a plurality of signals divide a transmission path of the same line and the same wavelength with a fixed time width (time slot: TS) separated on the time axis. The burst signal is an intermittent optical signal on the time axis, and in TDM transmission, data is superimposed on this burst signal for transmission. There is a signal-free interval (guard time: GT) with almost zero optical power between burst signals.

[0004] In the data transfer method of optical TDM transmission, a plurality of nodes (transmission devices) connected to the same line transmit burst signals on the same wavelength with a time difference from each other to avoid signal collisions. A state in which burst signals can reach the receiving end of the destination without colliding with each other is called a synchronized state.

[0005] In optical TDM transmission, by synchronizing the timing between the transmission nodes of burst signals, a synchronous state is achieved where signals do not collide. To do this, it is necessary for each node to transmit and switch the path of the burst signal after subtracting the signal propagation delay time between the nodes. In addition, the optical fiber that propagates the signal has an optical distance that fluctuates over time due to temperature changes. Therefore, in order to achieve high-precision synchronization for optical TDM transmission, each node needs to synchronize the timing with high precision while also considering the deviation of the signal propagation delay time caused by the temporal variation of the optical distance.

[0006] As one of the most common existing technologies for synchronization considering signal propagation delay and propagation delay variation, there is the optical TDM transmission method of PON (Passive Optical Network) that is widespread in current optical access networks. In PON, it is a form of a P2MP (Point to Multi-point) type optical access system with a tree structure, in which a master node called an OLT (Optical Line Terminal) placed in a building of a communication carrier and nodes called ONUs (Optical Network Unit) placed in each user's home are connected via a transmission path composed of an optical splitter and an optical fiber, which are passive components. The OLT measures the RTT (Round Trip Time) with each ONU in advance, calculates the timing at which signals from each ONU do not collide, notifies each ONU, and thereby synchronizes between the ONUs. On top of that, the OLT instructs each ONU of the signal transmission timing.

[0007] FIG. 11 is a configuration diagram of a TDM-PON system described in Non-Patent Document 1. In the optical transmission system, the OLT is connected to each ONU by an optical fiber. The optical fiber from the OLT is branched to each ONU via an optical splitter SP. Hereinafter, the signal flow from the OLT to the ONU is referred to as a "downlink signal", and the signal flow from the ONU to the OLT is referred to as an "uplink signal".

[0008] FIG. 12 is a sequence diagram showing the signal flow in the TDM-PON system of FIG. 11. The OLT transmits a downstream continuous signal for RTT measurement to the ONU at time t0. The destination ONU sets the time to t0 at the timing when it receives the downstream continuous signal from the OLT (S11). When the time reaches t1 specified by the OLT, the ONU transmits an upstream burst signal for RTT measurement. The OLT acquires the time t2 when it receives the upstream burst signal (S12). Thereby, the OLT calculates the RTT between itself and the ONU based on the timestamps from time t0 to t2. Based on the calculated RTT, the OLT transmits the transmission timing of the data signal using burst light to the ONU (S13). The ONU transmits the data signal using burst light based on the notified timing (S14).

[0009] Figure 13 is a configuration diagram of a TDM-PON system in a 1-to-N optical connection (P2MP optical connection) network described in Non-Patent Documents 1 and 2. In this configuration diagram, one OLT and N ONUs (here N = 3) are connected. A downstream continuous signal P11 from the OLT to each ONU and a signal P13 in which an upstream burst signal P12 from each ONU to the OLT is multiplexed propagate between nodes. The downstream signal P11 is, for example, the Sync (synchronization) signal (S11) and the timing notification signal (S13) used in the RTT measurement of FIG. 12. The upstream signal P12 is, for example, the Sync signal (S12) in FIG. 12 and the data signal for other ONUs (S14). The OLT achieves a synchronization accuracy of about several tens to several hundreds [ns] by measuring the RTT every time each ONU transmits and receives a data signal (S14).

Prior Art Documents

Non-Patent Documents

[0010]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0011] Consider expanding the above-mentioned 1-to-N optical connection (P2MP optical connection) network in FIG. 13 to an N-to-M optical connection (MP2MP optical connection) network. FIG. 14 is a configuration diagram of a TDM-PON system in an N-to-M optical connection network. In this configuration diagram, a set of N ONUs (here N = 3) arranged on the left side of the drawing is regarded as the transmission side of the Metro network, and a set of M ONUs (here M = 3) arranged on the right side of the drawing is regarded as the reception side of the Metro network. Then, the left and right Metro networks are connected by the Core network to form a direct optical connection. Note that the ONU on the transmission side indicates the equivalent of the ONU that is the source of the data signal P12. The ONU on the reception side indicates the equivalent of the ONU that receives the data signal P13.

[0012] There is also a data signal P14 transmitted (returned) from the ONU on the reception side to the ONU on the transmission side. This data signal P14 from the ONU on the reception side to the ONU on the transmission side has the same direction as the downstream signal P11 in FIG. 13 towards the ONU on the transmission side. In the existing PON system, a Sync signal is added to the data signal by electrical processing in the OLT, and the downstream signal is transmitted to each ONU as a continuous signal. However, in an N-to-M optical connection network without electrical termination between nodes, a Sync signal cannot be added to the data signal P14 at the equivalent of the OLT and transmitted as a continuous signal. Therefore, the conventional synchronization method used in the existing TDM-PON system cannot be directly applied to an N-to-M optical connection network.

[0013] FIG. 16 is an explanatory diagram of the transmission cycle of TDM transmission. Time slots are segmented every period T and assigned to each ONU. This period is called the TDM frame length. For example, the first ONU can transmit one data signal in the first period T and another data signal in the next period T. The GT provided between time slots serves to absorb signal deviation (synchronization error).

[0014] FIG. 17 is an explanatory diagram of signal collision of burst signals flowing from left to right. In reference numeral 811, the first data signal 811A, the second data signal 811B, and the third data signal 811C are sequentially superimposed on the optical fiber. Here, due to a large synchronization error between ONUs, a signal collision occurs because the time slot of the first data signal 811A and the time slot of the second data signal 811B partially overlap. In reference numeral 812, the first data signal 812A, the second data signal 812B, and the third data signal 812C are sequentially superimposed on the optical fiber. Here, since the synchronization error between ONUs is small, the time slots of each data signal can be correctly superimposed without overlapping.

[0015] That is, as shown in reference numeral 812, it is necessary to suppress the synchronization error between ONUs to an order that can be compensated by GT. On the other hand, in the existing synchronization method based on RTT measurement, since a synchronization error due to signal processing delay fluctuation associated with packet processing occurs to some extent, it is difficult to achieve a dramatic improvement in synchronization accuracy. Also, as long as electrical processing is not performed in the network, a separate wavelength is required for the downstream signal from the OLT, and as the number of ONUs increases, the load concentrates on the OLT that synchronizes with each of the multiple ONUs, and RTT measurement for each wavelength is required for high-precision synchronization. In order to achieve high-precision synchronization for optical TDM transmission in an N-to-M optical connection network, it is necessary to establish a timing adjustment method that realizes high-precision synchronization not based on RTT measurement.

[0016] Therefore, the main problem of the present invention is to avoid collisions between data signals during multiplexing in an N-to-M optical connection optical TDM transmission network without measuring the RTT for each node.

Means for Solving the Problem

[0017] In order to solve the above problems, the optical TDM transmission device of the present invention has the following characteristics. The present invention is in an optical TDM transmission system in which a plurality of optical TDM transmission devices transmit data signals with the time slots between nodes synchronized. Each of the optical TDM transmission devices In a first period before superimposing the data signal, receives a plurality of timing signals periodically transmitted to each of the optical TDM transmission devices, and stores the reception time and reception period of the received timing signal as the reception timing of the timing signal. In a second period in which the data signal is superimposed, controls to superimpose the data signal transmitted from itself on a time slot grasped based on the stored reception timing.

Advantages of the Invention

[0018] According to the present invention, in an N-to-M optical TDM transmission network, collisions between data signals during multiplexing can be avoided without measuring the RTT for each node.

Brief Description of the Drawings

[0019]

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Mode for Carrying Out the Invention

[0020] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0021] FIG. 1 is a configuration diagram of a TDM transmission system in an N-to-M optical connection network. The optical TDM transmission system in Fig. 1 is provided with a T (Timing) transmitter 30 instead of an OLT that measures the RTT (2-way synchronization process) with each ONU on the transmission side in Fig. 14. The T transmitter 30 transmits a timing signal P15 to each node on the transmission side using one time slot on the same wavelength as the data signal. Each node on the transmission side receives the timing signal P15, and stores the period and reception time of the received timing signal P15 as its reception timing. By each node grasping the reception timing of the timing signal, it is possible to accurately grasp the time slot for superimposing the data signal (1-way synchronization process).

[0022] The timing signal P15 is transmitted in the following two-stage period. The first period is a preparation period for each node to grasp the transmission timing of the data signal for each wavelength. In this period, the transmission of the data signal is prohibited. In the first period, the T transmitter 30 transmits only the periodic timing signal P15, and each node stores the received timing signal P15 as the reception timing. By a plurality of nodes receiving the timing signal P15 for each wavelength at which the data signal is transmitted, an inter-node time slot synchronization state in which the propagation delay time for each wavelength is indirectly corrected is achieved. That is, in the first period before superimposing the data signal, each node receives the timing signal periodically transmitted to each node a plurality of times, and stores the reception time and reception period of the received timing signal as the reception timing of the timing signal.

[0023] The second period is an operation period in which signal transmission is permitted according to the time slot for superimposing the data signal grasped by each node in the first period. In the second period, each node superimposes the data signal P12 on the time slot grasped based on the reception timing of the timing signal P15. Due to the second period, a signal P13 in which the timing signal P15 and the data signal P12 are multiplexed at the same wavelength flows on the optical path. Each node on the reception side receives the desired data signal P13 from the signal P13.

[0024] Furthermore, also in the second period as in the first period, the T transmitter 30 may continue to transmit the periodic timing signal P15. In that case, each node adjusts (fine time correction) the reception timing stored previously from the timing signal P15 received in the second period, and updates it as the current reception timing. Thereby, each node can grasp the time slots for superimposing the data signal continuously and with high precision by autonomous extraction of the timing signal in the physical layer not only in the first period but also in the second period. This means that it is possible to continuously ensure the time slot synchronization state with high precision even in an environment where the physical properties of the optical fiber change due to temperature changes or the like.

[0025] FIG. 2 is a configuration diagram when the N-to-M optical connection network of FIG. 1 is applied to a network having a ring topology. In FIG. 1, three nodes are arranged on the left side of the drawing as the transmission side, and three nodes are arranged on the right side of the drawing as the reception side. In FIG. 2, four nodes are arranged on the left side of the drawing as the transmission side, and five nodes are arranged on the right side of the drawing as the reception side. The T transmitter 30 transmits the timing signal T to the four nodes on the transmission side. That is, the starting point of the timing signal T is the T transmitter 30 in both FIG. 1 and FIG. 2. The four nodes on the transmission side superimpose the data signal D on the timing signal T. The five nodes on the reception side receive the timing signal T and the data signal D.

[0026] FIG. 3 is an explanatory diagram of the T transmitter 30 in the second period. The T transmitter 30 transmits the timing signal P15 to the node A. The node A receives the timing signal P15 and transmits the data signal transmitted from the node A using the same wavelength as the timing signal P15. Node B extracts only the timing signal P15 from among the timing signal P15 and the data signal transmitted from Node A, and transmits the data signal transmitted from Node B using the same wavelength as the timing signal P15. Node C extracts only the timing signal P15 from among the timing signal P15 and the data signals transmitted from Nodes A and B, and transmits the data signal transmitted from Node C using the same wavelength as the timing signal P15. As a result, the timing signal P15 and the data signals P12 transmitted from each of Nodes A, B, and C are multiplexed as signal P13.

[0027] FIG. 4 is an explanatory diagram of time slots constituting the burst signal P13 in FIG. 3. Reference numeral 101 denotes a time slot of wavelength λ1 at the time when the T transmitter 30 first transmits the timing signal T. Each node receives the timing signal T. The timing signal T is periodically inserted according to a pattern for each signal shown by reference numeral 103, and the other time slots are empty slots. Reference numeral 102 denotes a time slot of wavelength λ1 at the time when a certain node transmits (superimposes) a data signal as a burst signal (burst light) with respect to the empty slot of reference numeral 101.

[0028] FIG. 5 is a time-series time slot diagram showing a method for extracting a timing signal. FIG. 6 is an enlarged time slot diagram of a part of FIG. 5. The coloring pattern of each signal is the same as reference numeral 103 in FIG. 4. The T transmitter 30 transmits the timing signal T at a timing T0 every predetermined period = TDM frame length × N (for example, N = 100). Each node regards the signal received at a signal extraction timing within a timing T0 ± GT every predetermined period as a new timing signal T. Each node corrects the predetermined period and the timing T0 based on the new timing signal T, and sets the next signal extraction timing. That is, each node sets and corrects the reception timing which is the reference for superimposing the data signal D when receiving the timing signal. In this way, by memorizing the reception timing once, it becomes possible to continuously grasp and extract while correcting the reception timing of the subsequent timing signal T.

[0029] FIG. 7 is a configuration diagram of the T transmitter 30. The T transmitter 30 includes a clock unit 31, a counter management unit 32, a control unit 33, and a timing signal transmission unit 34. The clock unit 31 supplies a clock to the counter management unit 32. The counter management unit 32 counts, as a counter value, the time from when it transmits a timing signal to when it transmits the next timing signal based on the clock from the clock unit 31. The control unit 33 controls the timing signal transmission unit 34 to continuously transmit the timing signal periodically based on the counter value notified from the counter management unit 32. The timing signal transmission unit 34 transmits the timing signal according to the control of the control unit 33. As a result, each node on the transmission side can continuously receive the timing signal periodically.

[0030] FIG. 8 is a configuration diagram of the node 40. This node 40 is, for example, three transmission-side nodes arranged on the left side of FIG. 1 and four transmission-side nodes arranged on the left side of FIG. 2. The node 40 includes a burst optical reception unit 41, a high-speed AD conversion unit 42, a waveform memory unit 43, a timing control unit 44, a TS (Time Slot) control unit 45, a counter management unit 46, a clock unit 47, and a burst optical transmission unit 48. The burst optical reception unit 41 receives burst light (burst signal) including the timing signal. The high-speed AD conversion unit 42 performs high-speed AD conversion on the received burst light and notifies the waveform memory unit 43 of the timing signal of the conversion result. The waveform memory unit 43 notifies the timing control unit 44 of the waveform memory (stored timing signal) of the periodic timing signal. Therefore, the timing control unit 44 controls the waveform memory unit 43 to autonomously extract the periodic timing signal.

[0031] The timing control unit 44 notifies the reception timing obtained from the previous timing signal (in the waveform memory) to the TS control unit 45 and the counter management unit 46. Furthermore, the timing control unit 44 grasps the current reception timing and corrects the reception timing of the timing signal stored up to the previous time. Based on the reception timing notified by the timing control unit 44, the TS control unit 45 controls the time slot for superimposing the signal transmitted by its own device so as not to collide with other signals. The counter management unit 46 notifies the counter value to the timing control unit 44 and the TS control unit 45. As described with reference to FIG. 7, the counter value indicates the time from when the timing signal is transmitted until the next timing signal is transmitted. The clock unit 47 supplies a clock to the counter management unit 46. The burst optical transmission unit 48 superimposes an arbitrary signal on the TS designated by the TS control unit 45 and transmits a data signal.

[0032] FIG. 9 is a sequence diagram showing the details of the operation of the node 40. In the first period, the burst optical reception unit 41 receives burst light and converts it into an electrical signal (O / E conversion: Optical / Electrical conversion) (S111). The high-speed AD conversion unit 42 removes the noise of the electrical signal (A / D conversion: Analog / Digital conversion) (S121). The waveform memory unit 43 notifies the timing control unit 44 of the waveform memory of the electrical signal (S131). The timing control unit 44 notifies the reception time of the timing signal to the counter management unit 46 (S141).

[0033] The clock unit 47 supplies a clock to the counter management unit 46 (S161). The counter management unit 46 starts a counter based on the clock (S151). The counter management unit 46 notifies the timing control unit 44 of the counter value (S152). The timing control unit 44 grasps the period of the timing signal (S142). The counter management unit 46 notifies the waveform memory unit 43 of the counter value (S153). The timing control unit 44 notifies the waveform memory unit 43 of the next reception time (S143).

[0034] In the second period, the burst optical reception unit 41 receives burst light and converts it into an electrical signal (S112). The high-speed AD conversion unit 42 removes the noise of the electrical signal (A / D conversion) (S122). The waveform memory unit 43 notifies the timing control unit 44 of the waveform memory within an arbitrary range (S132). The timing control unit 44 grasps the reception timing of the timing signal and notifies the counter management unit 46 (S144). The clock unit 47 supplies a clock to the counter management unit 46 (S162). The counter management unit 46 corrects the counter value based on the clock (S154).

[0035] The timing control unit 44 notifies the TS control unit 45 of the reference (reception timing) of the time slot (S145). The counter management unit 46 notifies the TS control unit 45 of the counter value (S156). The TS control unit 45 determines a TS (time slot) for superimposing the data signal D (S171). The burst optical transmission unit 48 superimposes an arbitrary signal on the TS specified by the TS control unit 45 and transmits a data signal (S181).

[0036] FIG. 10 is a hardware configuration diagram of the optical TDM transmission device (node 40 in FIG. 8). The optical TDM transmission device is configured as a computer 900 having a CPU 901, a RAM 902, a ROM 903, an HDD 904, a communication I / F 905, an input / output I / F 906, and a media I / F 907. Similarly, the T transmitter 30 in FIG. 7 is also configured as a computer 900. The communication I / F 905 is connected to an external communication device 915. The input / output I / F 906 is connected to an input / output device 916. The media I / F 907 reads and writes data from / to a recording medium 917. Further, the CPU 901 controls each unit by executing a program (also called an application or an app for short) read into the RAM 902. This program can also be distributed via a communication line or recorded on a recording medium 917 such as a CD-ROM and distributed.

[0037] [Effect] In the optical TDM transmission system in which a plurality of nodes 40 transmit data signals in a state where time slots between the nodes are synchronized, each node 40 receives a plurality of timing signals periodically transmitted to each node 40 in a first period before superimposing the data signal, stores the reception time and reception period of the received timing signal as the reception timing of the timing signal, and is characterized in that, in a second period in which the data signal is superimposed, it controls to superimpose the data signal transmitted from itself on the time slot grasped based on the stored reception timing.

[0038] Thereby, each node 40 on the transmission side stores the period and reception time of the received timing signal as the reception timing in itself, thereby synchronizing with other nodes 40 (1-way synchronization process). Thereby, in an N-to-M optical connection network, signal collisions during multiplexing can be avoided without measuring the RTT for individual nodes 40.

[0039] The present invention is characterized in that each node 40 receives a timing signal transmitted to each node 40 also in the second period, and corrects the stored reception timing based on the reception time and reception period of the received timing signal.

[0040] Thereby, each node 40 can perform continuous timing adjustment not only in the first period but also in the second period. Therefore, it is possible to maintain a highly accurate time slot synchronization state between nodes even in an environment where the physical properties of the optical fiber change due to temperature changes or the like.

Explanation of Reference Numerals

[0041] 30 Transmitter 31 Clock Unit 32 Counter Management Unit 33 Control Unit 34 Timing Signal Transmission Unit 40 Node (Optical TDM Transmission Device) 41 Burst Optical Receiver 42 High-Speed AD Converter 43 Waveform Memory Unit 44 Timing Control Unit 45 TS Control Unit 46 Counter Management Unit 47 Clock Unit 48 Burst Optical Transmitter

Claims

1. In an optical TDM transmission system in which a plurality of optical TDM transmission devices transmit data signals in a state where time slots between nodes are synchronized, each of the optical TDM transmission devices, in a first period before superimposing the data signal, receives a timing signal periodically transmitted to each of the optical TDM transmission devices a plurality of times, and stores the reception time and reception period of the received timing signal as the reception timing of the timing signal, and in a second period of superimposing the data signal, controls to superimpose the data signal transmitted from itself on a time slot grasped based on the stored reception timing. Optical TDM transmission device.

2. Each of the optical TDM transmission devices also receives the timing signal transmitted to each of the optical TDM transmission devices in the second period, and corrects the stored reception timing based on the reception time and reception period of the received timing signal. The optical TDM transmission device according to Claim 1.

3. In a synchronization method executed by an optical TDM transmission system in which a plurality of optical TDM transmission devices transmit data signals in a state where time slots between nodes are synchronized, each of the optical TDM transmission devices, in a first period before superimposing the data signal, receives a timing signal periodically transmitted to each of the optical TDM transmission devices a plurality of times, and stores the reception time and reception period of the received timing signal as the reception timing of the timing signal, and in a second period of superimposing the data signal, controls to superimpose the data signal transmitted from itself on a time slot grasped based on the stored reception timing. Synchronization method.

4. A synchronization program for causing a computer to function as the optical TDM transmission device according to Claim 1 or Claim 2.

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