WO2012065334A1

WO2012065334A1 - Method, device and system for realizing time synchronization in time division multiplexing network

Info

Publication number: WO2012065334A1
Application number: PCT/CN2010/080049
Authority: WO
Inventors: 詹喻平
Original assignee: 中兴通讯股份有限公司
Priority date: 2010-11-19
Filing date: 2010-12-21
Publication date: 2012-05-24
Also published as: CN102468898A; CN102468898B

Abstract

A method, device and system for realizing time synchronization in Time Division Multiplexing (TDM) network are provided in the present invention. Wherein the method includes: a clock master site obtains time information from outside as a reference time, wherein the clock master site is an edge site of the TDM network; the clock master site gradually implements the time synchronization with other sites in the TDM network in a manner of time slot transmission according to the obtained reference time. According to the invention, problems are solved that the Global Positioning System (GPS) synchronization technology has a higher cost and a higher security risk and a synchronization manner including a timestamp protocol packet has not higher precision, thus providing a safeguard for time transfer application in the TDM network.

Description

TECHNICAL FIELD The present invention relates to the field of communications, and in particular to a method, device and system for implementing time synchronization in a time division multiplexed network. BACKGROUND OF THE INVENTION With the development of networks and services, there is an increasing demand for time transmission. At this stage, there is no way to effectively transfer time in a transmission network. Time synchronization includes both the synchronization of the clock frequency and the synchronization of the clock phase, and the phase of the clock is represented by a numerical value, that is, a time. At present, most of the time synchronization technologies widely used in the industry are solved by GPS (Global Positioning System), and each site in the technology is equipped with a GPS module for synchronizing the station to GPS time. In addition, there is a technique for time synchronization through a protocol packet, which mainly implements time synchronization between sites by using a time-stamped protocol packet, which is usually used in an IP network to solve the IP network time. Synchronization issue. However, when the time is passed through the protocol packet, the accuracy of the time synchronization can only reach the second-second level standard due to the delay jitter problem in the system.

TDM (Time Division Multiplex and Multiplexer) networks (such as SDH optical network, microwave network) as a bearer network, there is also a need for time synchronization. If the above GPS synchronization technology is used, the cost will be relatively high, and also There is a certain security risk. If the time-stamped protocol packet synchronization method is used, there is a problem that the synchronization accuracy is not high. SUMMARY OF THE INVENTION A primary object of the present invention is to provide a method, device and system for implementing time synchronization in a time division multiplexing network to solve the above-mentioned GPS synchronization technology with high cost, high security risk, and time-stamped protocol packet. The problem of low synchronization accuracy is not high. According to an aspect of the present invention, a method for implementing time synchronization in a time division multiplexed TDM network is provided, including: the clock primary station acquires time information from the outside world as a reference time, and the clock primary station is a boundary station of the TDM network; The clock master station performs time synchronization with the other stations in the TDM network in a time slot transmission manner according to the obtained reference time. According to another aspect of the present invention, an apparatus for implementing time synchronization in a time division multiplexed TDM network is provided, including: a reference time acquisition module, configured to acquire time information from the outside of the TDM network as a reference time; and a time synchronization module, It is used to perform time synchronization in a time slot transmission manner with other stations in the TDM network according to the reference time. According to still another aspect of the present invention, an apparatus for implementing time synchronization in a time division multiplexed TDM network is provided, including: a reference time determining module, configured to perform time synchronization with a superior station in a time slot transmission manner, and to synchronize the The time information is used as its own reference time; the time synchronization module is configured to perform time synchronization with the lower-level station in a time slot transmission manner according to its own reference time. According to still another aspect of the present invention, a system for implementing time synchronization in a time division multiplexing TDM network is provided, including: a clock primary site and a first level site, and the clock primary site includes: a first reference time determining module, For obtaining time information from the outside of the TDM network as a reference time; the first time synchronization module is configured to perform time synchronization with the first-level station in a time slot transmission manner according to the reference time; the first-level site includes: the second reference The time determining module is configured to use the time information synchronized with the clock main station as its own reference time. The second time synchronization module is configured to perform time synchronization with the lower station in a time slot transmission manner according to its own reference time. Through the invention, the time synchronization is performed step by step for each station in the TDM network in a time slot transmission manner, because the time slot transmission mode is a time stamp sent in a specified time slot, and there is no delay jitter and asymmetry of the back and forth path. It ensures the time synchronization accuracy between the sites, solves the problem that the GPS synchronization technology has higher cost, higher security risk, and the synchronization mode of the time-stamped protocol packet is not high, which provides guarantee for the time transmission application of the TDM network. . BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are set to illustrate,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 1 is a flowchart of a method for implementing time synchronization in a TDM network according to Embodiment 1 of the present invention; FIG. 2 is a schematic diagram of networking of a microwave transmission network according to Embodiment 2 of the present invention; A block diagram of a service processing principle in a microwave transmission network according to Embodiment 2 of the present invention; 4 is a block diagram showing the principle of time transfer of a hollow port of a microwave transmission network according to Embodiment 2 of the present invention; FIG. 5 is a schematic diagram of message interaction of a time interval of a hollow port of a microwave transmission network according to Embodiment 2 of the present invention; FIG. 7 is a structural block diagram of an apparatus for implementing time synchronization in a TDM network according to Embodiment 3 of the present invention; FIG. 8 is a block diagram of an apparatus for implementing time synchronization in a TDM network according to Embodiment 3 of the present invention; A block diagram of a device for implementing time synchronization in a TDM network; FIG. 9 is a block diagram showing a system structure for implementing time synchronization in a TDM network according to Embodiment 5 of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. It should be noted that the embodiments in the present application and the features in the embodiments may be combined with each other without conflict. The embodiment of the present invention considers that the TDM network is different from the pure IP network, and uses the characteristics of the fixed time slot transmission time in the TDM network to transmit timestamp information, thereby achieving time synchronization between sites within the TDM network. Based on this, an embodiment of the present invention provides a method, device, and system for implementing time synchronization in a TDM network. Embodiment 1 FIG. 1 is a flowchart of a method for implementing time synchronization in a TDM network according to an embodiment of the present invention. The method includes the following steps: Step S102: The clock master station acquires time information from the outside world as a reference time. The clock primary site is a boundary site of the TDM network; wherein the clock primary site is a site selected in advance from the boundary site of the TDM network, and the time information source of the site may be obtained from the GPS or through other networks. The device in (for example, IP network) is obtained after time synchronization (such as time-stamped protocol packet method;). Step S104, the clock master station performs time synchronization with the other stations in the TDM network in a time slot transmission manner according to the reference time. The time slot in the time slot transmission mode refers to a TDM time slot. In specific implementation, the TDM network may include multiple sites inside. For the sake of clarity, the site adjacent to the clock primary site is used as the first-level site, and the next site adjacent to the first-level site is used as the second-level site. , and so on. Step S104 may specifically include:

1) The clock primary station synchronizes with the first-level station in a TDM time slot transmission manner according to the reference time, and the first-level station is a station adjacent to the clock main station in the TDM network;

2) The first-level site takes the synchronized time information as its own reference time; 3) For all the stations in the TDM network except the clock main station, the upper-level station uses the time slot transmission mode and the lower level according to its own reference time. The site performs time synchronization, and the upper site and the lower site are two adjacent nodes in the TDM network. The foregoing step 1) can be implemented as follows: The clock master station sends a synchronization message to the first-level station on the designated time slot of the TDM frame, where the synchronization message carries the current transmission timestamp T 1 ; After receiving the above synchronization message, the station extracts T1 and records the time of receiving the synchronization message.

The first level station returns a delay request message on the specified time slot of the TDM frame, where the delay request message carries the current timestamp T3; after the clock main station receives the delay request message, the record receives the Time delay request message

T4, and returning T4 to the first-level station by using a delay response message; the first-level station extracts T4 after receiving the delay response message, and calculates a time deviation from the clock main station according to T1, T2, T3, and T4, Time synchronization compensation is performed according to the time deviation. For example: The time deviation of the first level station from the clock main station = { ( T1-T2 ) + ( T4-T3 ) } ÷ 2. In this embodiment, time synchronization is performed in a time slot transmission manner for each station in the TDM network. Because of the timestamp sent in the specified time slot, there is no delay jitter and round-trip path asymmetry, which ensures the between the sites. Time synchronization accuracy, solving the high cost and safe wind of GPS synchronization technology The problem of high risk and low synchronization of time-stamped protocol packets provides a guarantee for time synchronization applications in TDM networks. Embodiment 2 This embodiment provides a method for implementing time synchronization in a TDM network. The method is described by taking the networking diagram of the microwave transmission network shown in FIG. 2 as an example. The microwave transmission system belongs to the TDM transmission mode. At the two ends of the Qibo air interface, one is "Master Port", denoted by M; the other is "Slave", denoted by S, as shown in Figure 2. In this embodiment, a primary site is set as a "clock master site" in the microwave transmission network, and the clock primary site is located at the network boundary, and is synchronized with the upper-level high-precision time by the 1588 V2 protocol (or clock interface), and the other sites are slave stations. Each interface of the primary site only has the primary synchronization mode (Master); the slave clock accepting port is set to the slave synchronization mode (Slave). When the slave station needs to provide the clock source to other slave sites, the clock sending interface needs to be set to the primary synchronization mode. (Master). In the microwave transmission network, the slave station tracks the master clock in master-slave mode, and the master clock is used as the reference, and is transmitted down to the end station. The time synchronization method of this example includes the following steps. 4: The first step: 4 The wave network extracts the frequency and time information from the outside. As shown in Figure 2, a clock master station is set up in the microwave network, and the clock master station extracts the time from the outside. The time information source can be GPS or Obtained from the external device through the synchronization method with timestamp protocol packet. Step 2: Synchronize the frequency of each station in the microwave network. As shown in Figure 2, the "Slave" endpoint can extract the clock frequency of the "Master" endpoint from the air interface (microwave radio frequency set) The core supports this feature ;), and synchronizes the local system clock to the air interface clock.

"Master-Slave," the clock synchronization mode is a prior art mode. In this manner, each slave station clock in the entire oscillating network can be synchronized to the clock master station, thereby achieving frequency synchronization of the entire oscillating network site. The third step: 4 time synchronization network time synchronization. As shown in Figure 3, the microwave transmission equipment is generally composed of IDU equipment (Indoor Unit, also known as indoor unit;), ODU (Outdoor Unit, also known as outdoor unit) equipment and antenna. Mainly complete power amplification and RF transceiver function. IDU equipment is mainly composed of signal processing module, MODEM (modulation / Demodulation module, AFE (Analog Front End) module; The functions of each module are as follows: Signal processing module: mainly multiplex/demultiplex processing and signal encoding/decoding functions of various services; MODEM ( MOCDEM): Mainly complete the modulation/demodulation function of the signal;

AFE module: Complete conversion between digital signal and analog signal; ODU device: Mainly complete power amplification and RF signal transmission and reception.

The "MODEM" module, the "AFE" module, the "ODU" device and the antenna together form the physical transmission channel of the service. The function of time transfer is implemented in the signal processing module. As shown in FIG. 4, in order to realize time transfer, two function modules of "time stamp processing module" and "time counter" are provided in the signal processing module. The "Timestamp Processing Module" completes the function of inserting/extracting timestamp information. The "Time Counter" module completes local time generation and provides local time information for the "Insert Time Stamp Module". Taking the networking structure of FIG. 2 as an example, in the microwave network, the steps and principles of time transmission include: time synchronization from the master end to the slave end, and the master end has a standard time locally (ie, the reference time of the station where the end is located) The Slave side calculates the time difference between the two ends of "Master" and "Slave" through "Sync,", "Delay_Req," and "Delay_Resp," the timestamp information on the three interactive messages, and tracks the synchronization. 5, including the following process:

1) The "Master" side sends a "Sync" message (ie, a synchronization message) in a time slot in the TDM frame, and a local timestamp (T1) is placed in the message.

2) The "Slave" side receives the "Sync" message and records the local time (T2) at the time of reception.

3) The "Slave" end returns a "Delay_Req," message (ie, a delay request message) in a time slot in the TDM frame, and the timestamp (T3) is stamped in the message.

4) The "Master" side receives the "Delay_Req" message and records the local time (T4) at the time of reception. 5) The "Master" side passes the T4 timestamp back to the "Slave" side via the "Delay_Resp" message (ie the delayed response message). 6) The "Slave" end calculates the time deviation from the "Master" end according to T 1 , T2 , Τ 3 and Τ 4 , and compensates to achieve the synchronization requirement. As shown in FIG. 5, T1 is the time when the master sends the kth Sync message packet, T2 is the time when the slave receives the kth Sync message packet, T3 is the transmission time of the kth Delay_Req message packet of the slave, and the T4 bit receives the kth. The time of the Delay_Req message packet. Support:

^Toffset : time deviation between "Master" and "Slave";

Asynck, Adelay.k: Two synchronized inter-day P鬲; d ^rk , ^djk : Master to Slave and Slave to Master delay; Then, the message delivery delay between the master port and the slave port are:

If the delay from Master to Slave to Slave to Master is equal (ie, the back and forth path delay is symmetric;), the time deviation between Master and "Slave" is:

According to the time deviation calculated above, the Slave can adjust the local time counter to achieve time synchronization with the "Master". Step 4: The microwave network provides time information externally. The microwave network stations achieve time synchronization through the above steps, so that each The site has standard time information. When the microwave network is connected to other networks as a bearer network, it provides standard time information to the external device and realizes the function of transmitting time. The networking structure of the wired and microwave hybrid transmission network shown in Figure 6 For example, the boundary node in the microwave network performs time synchronization with the station from the wired network through the related technology, and uses the synchronized time information as the reference time, and performs time synchronization of each station in the microwave network according to the above method. Embodiment 3 FIG. 7 is a structural block diagram of an apparatus for implementing time synchronization in a TDM network according to an embodiment of the present invention. The time synchronization apparatus may be disposed on a clock master station in the above embodiment, and includes: a reference The time obtaining module 72 is configured to obtain time information from the outside of the TDM network as a reference time. The time synchronization module 74 is connected to the reference time acquiring module 72, and is configured to transmit time slots according to the reference time and other stations in the TDM network. Time synchronization is performed step by step. The specific manner of time synchronization in the time slot transmission mode can be implemented in the manner of the embodiment 1 or the embodiment 2, and details are not described herein again. In this embodiment, time synchronization is performed in a time slot transmission manner for each station in the TDM network. Because the timestamp sent in the specified time slot in the TDM network does not have a delay jitter problem, the time between the sites is ensured. The synchronization accuracy solves the problem that the cost of the GPS synchronization technology is high, the security risk is high, and the synchronization mode of the time-stamped protocol packet is not high, which provides a guarantee for the time transmission application of the TDM network. Embodiment 4 FIG. 8 is a structural block diagram of another apparatus for implementing time synchronization in a TDM network according to an embodiment of the present invention, which may be disposed on a site inside a TDM network in the above embodiment. The reference time determining module 82 is configured to perform time synchronization with the upper station in a time slot transmission manner, and use the synchronized time information as its own reference time. The time synchronization module 84 is connected to the reference time determining module 82. Time synchronization with the subordinate station in time slot transmission according to its own reference time. The specific manner of time synchronization in the time slot transmission mode can be implemented in the manner of the embodiment 1 or the embodiment 2, and details are not described herein again. Preferably, the time synchronization module 84 includes: a synchronization receiving unit, configured to receive a synchronization message from a higher-level site, where the synchronization message carries a first transmission timestamp T1 of the upper-level station; extracts T1 from the synchronization message, and records a time T2 at which the synchronization message is received; Returning the delay request message to the upper station on the designated time slot of the TDM frame, wherein the delay request message carries the current timestamp T3; the delay response receiving unit is configured to receive the delay response message from the upper station, The delay response message carries a second transmission timestamp T4 of the upper station; T4 is extracted from the delay response message; and a synchronization compensation unit is configured to calculate a time deviation from the clock main station according to T1, Τ2, Τ3, and Τ4, Time synchronization compensation is performed according to the time deviation. For example: The time deviation of the first level station from the clock main station = { ( T1-T2 ) + ( Τ 4-Τ3 ) } ÷ 2. In this embodiment, time synchronization is performed in a time slot transmission manner for each station in the TDM network. The time slot transmission mode is a time stamp sent in a specified time slot, and there is no delay jitter problem, which ensures the inter-site between the sites. The time synchronization accuracy solves the problem that the cost of the GPS synchronization technology is high, the security risk is high, and the synchronization mode of the time-stamped protocol packet is not high, which provides a guarantee for the time transmission application of the TDM network. Embodiment 5 FIG. 9 is a structural diagram of a system for implementing time synchronization in a TDM network according to an embodiment of the present invention, including: a clock master station 90 and a first level station 92, wherein the clock master station 90 is located in the TDM. The first-level site 92 is an internal site of the TDM network and is connected to the clock master site 90. The clock master site 90 includes: a first reference time determining module 902, configured to acquire time information from the outside of the TDM network as a reference time. The first time synchronization module 904 is connected to the first reference time determining module 902, configured to perform time synchronization with the first-level station 92 in a time slot transmission manner according to the reference time; the first-level station 92 includes: a second reference time determination The module 922 is configured to use the time information synchronized with the clock main site as its own reference time. The second time synchronization module 924 is connected to the second reference time determining module 922, and is configured to use the reference time and the lower-level site according to itself. Time synchronization is performed in time slot transmission mode. The first time synchronization module 904 includes: a first sending unit, configured to send a synchronization message to the first-level station 92 on a designated time slot of the TDM frame, where the synchronization message carries a current transmission timestamp T1; After receiving the delay request message, recording the time T4 of receiving the delay request message, and returning T4 to the first level station 92 through the delay response message; the second time synchronization module 924 includes: a synchronous receiving unit, After receiving the synchronization message, extract T1, and record the time T2 of receiving the synchronization message; the response unit is configured to return a delay request message on the specified time slot of the TDM frame, where the delay request message carries the current timestamp T3; a delay response receiving unit, configured to receive a delay response message and extract T4; a synchronization compensation unit, configured to calculate a time deviation from the clock main station 90 according to T1, T2, Τ3, and Τ4, and perform time synchronization according to the time deviation make up. Preferably, the system further includes: n stations, where η is an integer greater than or equal to 1; the level station includes: a time receiving module, configured to perform time synchronization with the N-1th station in a time slot transmission manner, The time information after synchronization is used as its own reference time; the time sending module is configured to perform time synchronization with the N+1th station in a time slot transmission manner according to its own reference time; where Ν=η+1. In this embodiment, time synchronization is performed in a time slot transmission manner for each station in the TDM network. The time slot transmission mode is a time stamp sent in a specified time slot, and there is no delay jitter problem, which ensures the inter-site between the sites. The time synchronization accuracy solves the problem that the cost of the GPS synchronization technology is high, the security risk is high, and the synchronization mode of the time-stamped protocol packet is not high, which provides a guarantee for the time transmission application of the TDM network. From the above description, it can be seen that the present invention achieves the following technical effects: The above embodiment can accurately transmit the characteristics of time information by using the TDM working mode inside the TDM network, and use the time slot transmission mode to perform time synchronization step by step. It greatly improves the transmission accuracy of time in the TDM network, and at the same time effectively improves the time pass of the entire network and saves costs. Obviously, those skilled in the art should understand that the above modules or steps of the present invention can be implemented by a general-purpose computing device, which can be concentrated on a single computing device or distributed over a network composed of multiple computing devices. Alternatively, they may be implemented by program code executable by the computing device, such that they may be stored in the storage device by the computing device and, in some cases, may be different from the order herein. Perform the steps shown or described, or separate them into individual integrated circuit modules, or make multiple modules or steps in them Implemented as a single integrated circuit module. Thus, the invention is not limited to any specific combination of hardware and software. The above is only the preferred embodiment of the present invention, and is not intended to limit the present invention, and various modifications and changes can be made to the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the scope of the present invention are intended to be included within the scope of the present invention.

Claims

Claim

A method for implementing time synchronization in a time division multiplexing TDM network, characterized in that it comprises:

The clock main station obtains time information from the outside as a reference time, and the clock main station is a boundary station of the TDM network;

The clock master station performs time synchronization with the other stations in the TDM network in a time slot transmission manner according to the obtained reference time.

The method according to claim 1, wherein the clock master station performs time synchronization with the other stations in the TDM network in a time slot transmission manner according to the obtained reference time: The clock master station performs time synchronization with the first level station according to the reference time in a time slot transmission manner, where the first level station is a station adjacent to the clock main station in the TDM network;

The first-stage station uses the synchronized time information as its own reference time; for each level of the TDM network except the clock main station, the upper-level station transmits the time slot according to its own reference time. The mode is time synchronized with the lower-level site, where the upper-level site and the lower-level site are two adjacent nodes in the TDM network.

The method according to claim 2, wherein the clock master station performs time synchronization with the first-level station in a time slot transmission manner according to the reference time, including:

The clock master station sends a synchronization message to the first-level station on the specified time slot of the TDM frame, where the synchronization message carries the current transmission timestamp T1;

Receiving T1 after receiving the synchronization message, and recording a time T2 at which the synchronization message is received;

The first stage station returns a delay request message on the specified time slot of the TDM frame, where the delay request message carries the current timestamp T3;

After receiving the delay request message, the clock master station records the time T4 of receiving the delay request message, and returns T4 to the first level station by using a delay response message. After receiving the delay response message, the first-stage station extracts T4, calculates time offset from the clock main station according to T1, T2, Τ3, and Τ4, and performs time compensation according to the time deviation.

The method according to claim 3, wherein the calculating, by the first-stage station, the time deviation from the clock main station according to T1, Τ2, Τ3, and Τ4 includes:

The time deviation of the first level station from the clock main station is = { ( T1 - T2 ) + ( Τ 4 Τ 3 ) } ÷ 2 .

5. A device for implementing time synchronization in a time division multiplexing TDM network, characterized in that it comprises:

a reference time acquisition module, configured to acquire time information from the outside of the TDM network as a reference time;

The time synchronization module is configured to perform time synchronization in a time slot transmission manner with other stations in the TDM network according to the reference time.

6. A device for implementing time synchronization in a time division multiplexing TDM network, characterized in that it comprises:

a reference time determining module, configured to perform time synchronization with the upper station in a time slot transmission manner, and use the synchronized time information as its own reference time;

The time synchronization module is configured to perform time synchronization with the subordinate station in a time slot transmission manner according to the reference time of the self.

The device according to claim 6, wherein the time synchronization module comprises: a synchronization receiving unit, configured to receive a synchronization message from a higher-level site, where the synchronization message carries a Sending a timestamp T1; extracting T1 from the synchronization message, and recording the time Τ2 of receiving the synchronization message;

a response unit, configured to return a delay request message to the upper-level station on a specified time slot of the TDM frame, where the delay request message carries a current timestamp Τ3; a delay response message of the upper-level station, wherein the delay response message carries a second transmission time stamp Τ4 of the upper-level station; extracting Τ4 from the delayed response message; The synchronization compensation unit is configured to calculate a time deviation from the clock main station according to T1, Τ2, Τ3, and Τ4, and perform time synchronization compensation according to the time deviation.

8. A system for implementing time synchronization in a time division multiplexing TDM network, characterized by comprising: a clock primary site and a first level site,

The clock main site includes: a first reference time determining module, configured to acquire time information from the outside of the TDM network as a reference time; and a first time synchronization module, configured to time with the first level station according to the reference time Gap transmission mode for time synchronization;

The first level station includes: a second reference time determining module, configured to use time information synchronized with the clock main station as its own reference time; and a second time synchronization module, configured to use the reference time of the self Time synchronization with the subordinate station in time slot transmission mode.

9. The system of claim 8 wherein:

The first time synchronization module includes: a first sending unit, configured to send a synchronization message to the first-level station on a designated time slot of the TDM frame, where the synchronization message carries a current transmission timestamp T1; a sending unit, configured to: after receiving the delay request message, record a time Τ4 of receiving the delay request message, and return Τ4 to the first-level station by using a delay response message;

The second time synchronization module includes: a synchronization receiving unit, configured to extract T1 after receiving the synchronization message, and record a time Τ2 of receiving the synchronization message; and a response unit, configured to return on the specified time slot of the TDM frame a delay request message, wherein the delay request message carries a current time stamp Τ3; a delay response receiving unit, configured to receive the delay response message and extract Τ4; a synchronization compensation unit, configured to use, according to Tl, Τ2, Τ3, and Τ4 calculate the time offset from the clock master station, and perform time synchronization compensation based on the time offset.

The system according to claim 9, wherein the system further comprises: n stations, wherein η is an integer greater than or equal to 1;

The first level site includes:

a time receiving module, configured to perform time synchronization with the N-1th station in a time slot transmission manner, and use the synchronized time information as its own reference time; a time sending module, configured to perform time synchronization with the N+1th station in a time slot transmission manner according to the reference time of the self;

Where N=n+ 1.