CN115473602B

CN115473602B - Time synchronization method for integrated inter-satellite and intra-satellite communications based on TSN

Info

Publication number: CN115473602B
Application number: CN202210903620.0A
Authority: CN
Inventors: 崔钊婧; 何熊文; 詹盼盼; 王丹; 王铎; 张东璞; 阎冬; 王帅; 蒋帅; 顾明; 闫春香; 张天为
Original assignee: Beijing Institute of Spacecraft System Engineering
Current assignee: Beijing Institute of Spacecraft System Engineering
Priority date: 2022-07-28
Filing date: 2022-07-28
Publication date: 2025-03-21
Anticipated expiration: 2042-07-28
Also published as: CN115473602A

Abstract

The present invention provides a time synchronization method for integrated inter-satellite and intra-satellite communications based on TSN, which can implement a time-sensitive network protocol within the inter-satellite satellite to achieve time synchronization of inter-satellite and intra-satellite data communications. The present invention studies the corresponding scheduling mechanism algorithms respectively according to the business requirements of intra-satellite and inter-satellite communications, implements a time-sensitive network protocol within the inter-satellite satellite, and performs data communication, which involves the TSN standard. The present invention has low time synchronization requirements while meeting time-sensitive business requirements, and the implementation cost and complexity are reduced compared to the TTE network. The TTE clock synchronization is based on the IEEE 1588 protocol, and its time synchronization accuracy reaches 100ns; the TSN clock synchronization is based on the IEEE802.1AS protocol, and the end-to-end synchronization time is maintained on the devices running and controlling the applications.

Description

Time synchronization method for inter-satellite and intra-satellite integrated communication based on TSN

Technical Field

The invention relates to the technical field of spatial information networks, in particular to a time synchronization method for inter-satellite and intra-satellite integrated communication based on TSN.

Background

With the development of spatial information networks, new services, especially time-sensitive services, are continuously emerging, and new demands are put on information transmission and processing. Space-time sensitive services, i.e. space monitoring information, space detection information, voice services, video services, etc., place higher demands on the real-time and deterministic nature of data transmission. The real-time performance of the space time-sensitive service needs to reach the sub-millisecond level in different occasions, and the certainty needs to be predictability of each transmission of the time-sensitive service adapting to different applications. In addition, as space tasks become more complex, the requirements on communication performance of the inter-satellite and inter-satellite space information systems are increasingly strict due to the cooperation enhancement among a plurality of test tasks. On one hand, high bandwidth, high reliability and high real-time performance of the spacecrafts in-satellite information system are required, and on the other hand, the spacecrafts are networked through inter-satellite links, and the inter-satellite links also have the characteristics of low time delay, high reliability and high resource utilization rate. When a plurality of tasks with larger data transmission quantity occupy network resources at the same time, network delay and network congestion are caused, data forwarding efficiency is reduced, and how to guarantee real-time performance and certainty of an intra-satellite Ethernet and an inter-satellite link network is a key for improving network communication performance.

Time sensitive networks are a general term for a series of standard technologies, and a novel network with time characteristics is constructed on the basis of a traditional Ethernet, so that data transmission capacity with deterministic time delay is provided. The TSN is properly modified by integrating key technologies such as time synchronization, flow control, gate control scheduling, preemption mechanism and the like, so that time sensitive service data transmission can be realized based on the traditional Ethernet, and meanwhile, the compatibility of common service data transmission is ensured, thereby ensuring the deterministic time delay of data transmission in a network at a link layer.

However, there are also some serious challenges in directly applying time-sensitive network technology in the aerospace field. The specific problems of the wired and wireless integrated time-sensitive system are as follows:

(1) The problem of accuracy and reliability of time synchronization in the space is that for the outer space environment of a spacecraft, data transmission in the spacecraft is easily affected by outer space radiation due to the fact that the protection of a geomagnetic field is not available, so that the problem that the accuracy or stability of a time synchronization result obtained by a time synchronization method used in a time sensitive network used on the ground is insufficient is caused. Therefore, how to improve the accuracy and reliability of clock synchronization in satellite systems is an important issue to be resolved.

(2) The inter-satellite time synchronization accuracy problem is that in a space network environment, a spacecraft or a satellite is in a continuous motion state, the distance between propagation paths between two nodes can be changed, the space transmission paths are also highly asymmetric, and the path delay between the nodes can not be obtained through a measurement mode in an IEEE 802.AS protocol. Therefore, how to reduce the influence of the motion of the spacecraft or the satellite on the inter-satellite time delay measurement, so as to improve the accuracy of inter-satellite time synchronization is a problem to be solved.

(3) The problem of inter-satellite data transmission delay certainty is that because an inter-satellite transmission link has time-varying characteristics, compared with a ground fixed network, the problem of end-to-end delay uncertainty can be caused by adopting fixed communication speed between satellites. Therefore, how to solve the problem of overall delay uncertainty caused by inter-satellite propagation delay variation due to satellite relative motion.

(4) The time sensitive network can ensure low delay, low jitter and zero congestion loss of key data stream transmission, so that key services are not influenced by non-key services. In the TSN IEEE 802.1Qbv protocol, although a scheduling mechanism of gate operation is proposed, data is placed in different queues according to priority, and then is scheduled according to a gating list, so as to ensure low latency requirements of key services, a specific method for obtaining the gating list is not provided in the protocol.

Disclosure of Invention

In view of the above, the present invention provides a time synchronization method for integrated inter-satellite and intra-satellite communication based on TSN, which can implement a time sensitive network protocol in inter-satellite and realize time synchronization of inter-satellite and intra-satellite data communication.

In order to achieve the above purpose, the technical scheme of the invention is as follows:

The time synchronization method for inter-satellite and intra-satellite integrated communication based on TSN comprises the following specific steps of A, establishing a master-slave topological relation by selecting a master clock, establishing a synchronous system in the whole PTP network, B, setting a synchronous local clock scheme, calculating time deviation between local clocks of all slave nodes and a master clock by using a local clock synchronization algorithm through exchange of PTP data messages between master nodes and slave nodes of the network, adjusting the local clocks to synchronize with the master clock, C, establishing a wired wireless integrated time-sensitive network overall architecture, acquiring global topology and resource state information of a satellite network by a TSN controller satellite, setting a time delay integrated communication protocol, and setting a time delay integrated communication network, F, D, reserving a time delay integrated network communication network, and F.

In the step B, the local clock is synchronized based on an IEEE 1588v1 protocol, the clock in the whole PTP network is divided into a common clock and a boundary clock according to the number of PTP communication ports on the clock, the local clock is synchronized based on the IEEE 1588v2 protocol, and the local clock is synchronized based on an IEEE 802.1AS protocol.

When synchronizing local clocks based on IEEE 1588v1 protocol, synchronizing the local clock on each port with a level master port, wherein the synchronization system establishment flow is as follows:

1) In the initial state, each node port can monitor Sync data frames in the network in a designated time;

If the Sync data frame is not received, changing the state of the node into Pre_Master, and assuming the node as a Master clock node;

2) The port state is kept in a pre_Master within a certain time, if a Sync data frame is received within a port designated time, the port state is determined by an optimal Master clock algorithm, if the port is judged to be the Master clock, the Sync frame is periodically sent, if the port is judged to be the slave clock, the Sync frame is received, the deviation is calculated, the local clock is corrected, if the Sync data frame is not received within the time period, the state is changed to the Master clock, and the timing sending of the Sync data frame is started;

3) The states of the master clock and the slave clock change with changes in clock performance and operating state.

In the step B, the synchronous local clock based on the IEEE 1588v2 protocol is improved and expanded based on the IEEE 1588v1 protocol, and the method comprises the steps of newly adding an independent message mode for measuring network path delay between point-to-point, newly adding a transparent clock model, wherein the transparent clock comprises an end-to-end transparent clock and a point-to-point transparent clock, and adding a single-step clock model.

In the step B, a BCMA master clock algorithm is adopted for synchronizing the local clock based on the IEEE 1588v1 protocol, synchronizing the local clock based on the IEEE 1588v2 protocol, and synchronizing the local clock based on the IEEE 802.1AS protocol.

In the step C, the following architecture design is divided:

1) A single domain wired and wireless integrated time sensitive network architecture;

2) A multi-domain wired and wireless integrated time sensitive network architecture;

3) Dynamic distributed inter-satellite network time reference establishment and synchronization.

In the step D, the TSN protocol modifies the data link layer based on the ethernet, and combines clock synchronization and time-sensitive service scheduling to implement traffic shaping, scheduling and frame preemption, thereby guaranteeing time delay bounded transmission of space time-sensitive service in the star.

Wherein in said step E, additional requirements in the form of IEEE 802.3 full duplex ethernet links and IEEE 802.11 wireless networks and ethernet passive optical networks are ensured by the IEEE 802.1AS protocol.

In the step F, path delay between two nodes is obtained based on multiple measurements of IEEE 802.1AS, and the influence of satellite motion characteristics, relativistic effects and propagation path asymmetry on time synchronization accuracy is comprehensively considered, so AS to realize inter-satellite time synchronization.

The beneficial effects are that:

1. the invention respectively researches corresponding scheduling mechanism algorithms aiming at service requirements of inter-satellite and inter-satellite communication, realizes a time sensitive network protocol in the inter-satellite and performs data communication, and relates to TSN standards. According to the invention, the time synchronization requirement is low under the condition of meeting the time-sensitive service requirement, the realization cost and the complexity are reduced compared with a TTE network, the TTE clock synchronization is based on an IEEE 1588 protocol, the time synchronization precision reaches 100ns, the TSN clock synchronization is based on an IEEE802.1AS protocol, and the end-to-end synchronization time is maintained on equipment running and controlling the application.

2. The invention has flexible network topology structure, supports the dynamic addition of new service and reduces the overhead of system re-planning. When new time sensitive service is added into the network, TTE needs to re-plan the task and update the time schedule, while TSN only schedules according to the priority of the service, without re-distributing the resource.

3. The invention has simple data exchange structure, effectively reduces the data exchange cost, and the TTE exchanges the time trigger service and the event trigger service respectively, which is equivalent to the simultaneous operation of two switches, and the TSN does not change the structure of the existing Ethernet switch and only needs to process data at the outlet port of the switch.

4. The present invention ensures the additional requirements required for IEEE 802.3 full duplex ethernet links and IEEE 802.11 wireless networks and ethernet passive optical network formats through the IEEE 802.1AS protocol.

5. The invention obtains the path delay between two nodes based on the repeated measurement of IEEE 802.1AS, comprehensively considers the influence of satellite motion characteristics, relativistic effects and the asymmetry of propagation paths on time synchronization precision, and realizes inter-satellite time synchronization.

Drawings

FIG. 1 is a schematic view of the overall structure of the present invention;

FIG. 2 is an IEEE 1588 clock synchronization state transition diagram of the present invention;

FIG. 3 is a PTP synchronization principle of the present invention;

FIG. 4 is a schematic diagram of the point-to-point delay measurement of the present invention;

FIG. 5 is a diagram of a single domain wired and wireless integrated time sensitive network architecture of the present invention;

FIG. 6 is a diagram of a multi-domain wired and wireless integrated time sensitive network architecture of the present invention;

FIG. 7 is a schematic diagram of a delay measurement mechanism of the present invention;

Fig. 8 is a clock synchronization measurement flow chart of the present invention.

Detailed Description

The invention will now be described in detail by way of example with reference to the accompanying drawings.

The invention provides a time synchronization method of inter-satellite integrated communication based on TSN, which is based on a time sensitive network, wherein the time sensitive network comprises a TSN standard, and the TSN standard comprises IEEE 802.1ASbt, IEEE 802.1Qbu, IEEE 802.1Qbv, IEEE 802.1Qca, IEEE 802.1Qcc, IEEE 802.1CB, IEEE 802.1Qci and IEEE 802.1Qch protocols. The time synchronization method comprises the following specific steps:

Step A, selecting a synchronization system, wherein the IEEE 802.1ASbt protocol is simplified and modified based on the IEEE 1588 protocol, namely an accurate clock protocol PTP, the protocol uses an optimal master clock algorithm, and a master-slave topological relation is established by selecting a master clock, so that the synchronization system is established in the whole PTP network;

Setting a synchronous local clock scheme, wherein a protocol uses a local clock synchronization algorithm, calculates time deviation between local clocks of all slave nodes and a master clock through exchange of PTP data messages between master nodes and slave nodes of a network, and adjusts the local clocks to be synchronous with the master clock, wherein the method comprises the following steps of:

Based on IEEE 1588v1 protocol, the clock in the whole PTP network can be divided into a common clock and a boundary clock according to the number of PTP communication ports on the clock;

step C, establishing a wired and wireless integrated time sensitive network overall architecture, acquiring global topology and resource state information of a satellite network through a TSN controller satellite, responding to service demands, completing network configuration, routing table issuing, resource reservation and the like, and realizing time delay deterministic demands of integrated communication between the satellites;

Step D, establishing a wired and wireless integrated time-sensitive network communication protocol, wherein the TSN protocol realizes flow shaping, scheduling, frame preemption and the like by modifying a data link layer based on the Ethernet and combining clock synchronization and time-sensitive service scheduling, thereby guaranteeing the time delay bounded transmission of the space time-sensitive service in the star;

step E, setting intra-satellite time synchronization, and ensuring additional requirements required by the forms of an IEEE 802.3 full duplex Ethernet link, an IEEE 802.11 wireless network and an Ethernet passive optical network through an IEEE 802.1AS protocol;

And F, setting inter-satellite time synchronization, obtaining path delay between two nodes based on multiple times of measurement of IEEE 802.1AS, comprehensively considering influence of satellite motion characteristics, relativistic effects, asymmetric propagation paths and the like on time synchronization precision, and realizing inter-satellite time synchronization.

In the step B, based on the IEEE 1588v1 protocol, the local clock is synchronized, and the clocks in the whole PTP network can be divided into a common clock and a boundary clock according to the number of PTP communication ports on the clock, wherein only one common clock exists, and a plurality of boundary clocks exist. A boundary clock, such as a switch or router, is typically used at a network node where certainty is low, as shown in fig. 1, and PTP communications are performed independently on each port. Specifically, only one slave port is allowed to exist on the boundary clock, and is communicated with the master port of the upper node, so that the local clock of the slave port is synchronized with the master port of the stage. The other ports are master ports and communicate with slave ports of the downstream nodes. The boundary clock can be connected with different network protocols, and the synchronous system establishment flow is as follows:

if the Sync data frame is not received, the node state is changed to Pre_Master, and the node is assumed to be the Master clock node. The node port state now behaves as a master clock, but the Sync frame is not sent.

2) The port state remains pre_master for a certain time, if a Sync data frame is received within a port specified time, the port state is determined by the optimal Master clock algorithm. If the port is judged to be the master clock, the Sync frame is periodically transmitted, and if the port is judged to be the slave clock, the Sync frame is received, the deviation is calculated, and the local clock is corrected.

If the port does not receive the Sync data frame in the period, the state is changed to the master clock, and the timing of the Sync data frame transmission is started.

3) The states of the master and slave clocks change as the clock performance and operating state change, and fig. 2 illustrates state transitions in BMCA.

In the step B, the synchronous local clock based on the IEEE 1588v2 protocol is improved and expanded based on the IEEE 1588v1 protocol, and the method comprises the steps of newly adding an independent message mode for measuring network path delay between the points, newly adding a transparent clock model, wherein the transparent clock comprises an end-to-end transparent clock and a point-to-point transparent clock, and adding a single-step clock model.

The PTP synchronization principle of the present invention is shown in FIG. 3, and the time deviation adjustment implementation mechanism specifically includes that a slave node receives a Sync data frame and records a receiving time T2 and a Follow_Up frame with Sync frame sending time information T1. In addition, the slave node sends a delay_req data frame to the master node and records a sending time T3, and then receives a feedback frame delay_resp of the master node, and the feedback frame delay_resp has four time stamps T1-T4 obtained from a clock at a time T4 when the master node receives the delay_req frame. Assuming that the time offset between the master and slave nodes and the propagation Delay on the path are Offsett and Delay, respectively, the following equation can be obtained:

T2=Offest+Delay+T1 (1)

T4=T3-Offset+Delay (2)

according to the above formulas (1) and (2), the values of Offset and Delay are calculated:

Delay=[(T2-T1)+(T4-T3)]/2 (3)

Offset=[(T2-T1)-(T4-T3)]/2 (4)

The slave node device adjusts the local time accordingly, and clock synchronization between the master device and the slave device is achieved.

The time synchronization algorithm of the PTP v1 protocol can achieve sub-microsecond accuracy but relies on the symmetry of the links between the network nodes. For the asymmetric link delay condition in the network, the round trip transmission delay difference needs to be calculated through an additional asymmetric algorithm, and the link delay is compensated. The point-to-point delay measurement schematic diagram of the present invention is shown in fig. 4.

In the step C, the following architecture design is divided:

1) In a single satellite subnet, in order to meet the time delay bounded communication requirement in the subnet, a satellite is selected to serve as a TSN controller for acquiring global state information in the subnet and issuing decisions. The single-domain wired and wireless integrated time-sensitive network architecture is shown in fig. 5, and mainly adopts the idea that a control plane is separated from a data plane, wherein a TSN controller satellite is mainly responsible for satellite network topology discovery, traffic monitoring, configuration of a forwarding information table and the like. The control plane specifically comprises the following steps that when a data stream or a service request is newly added, a TSN controller satellite calculates according to the acquired topological state information and the service delay requirement, the acquired network configuration parameters are respectively issued to satellites in a subnet (red dotted line), the satellites in the subnet can use corresponding strategies when forwarding the TSN stream according to the received configuration information, and finally, the data plane forwards the TSN data stream according to the decision of the control plane (green dotted line).

2) The multi-domain wired and wireless integrated time-sensitive network architecture is characterized in that a single-domain wired and wireless integrated time-sensitive network architecture can only meet the communication requirement in one subnet, when the network scale is enlarged, the coverage and management and control capability of one TSN controller satellite is limited, and the acquisition of the topology state information of the whole network can not be realized, so that the multi-domain wired and wireless integrated time-sensitive network architecture is provided for a large-scale TSN (e.g. satellites at different orbit heights) satellite network. As shown in fig. 6, a TSN controller satellite is selected in each subnet, and the TSN controller satellites can acquire the topology of the whole network and the resource status information through information interaction, so as to provide time delay deterministic service for time-sensitive service in cross-domain transmission.

3) The time reference of the dynamic distributed inter-satellite network is established and synchronized, namely, in a satellite network architecture integrating wire and wireless, relevant decisions such as scheduling, configuration and the like are issued through a TSN controller, however, how to select satellite nodes representing the TSN controller in the dynamic distributed satellite network. Taking inter-satellite time synchronization as an example, in order to establish a time reference for time synchronization in a dynamic distributed satellite network, a time reference center selection method with the shortest total network synchronization time is proposed, and all satellites are synchronized to the reference.

For a time synchronization network, the time required for synchronization of the satellites furthest from the reference center is longer than that of the other satellites when all the satellite synchronization targets of the network are reached, and by distance is meant the length of the propagation delay of two satellite information, for example, the distance between two satellites with direct communication links is the propagation delay between two satellites, and the distance between two satellites without direct links is the sum of the propagation delays of several links. The smaller the distance of the satellite furthest from the reference center in the overall network, the less time it takes for the overall network to synchronize in time, and the better the performance.

Based on the analysis, for a network topology, i is taken as a starting point, the shortest delay path of the network topology reaching any satellite in the network is found, and the longest delay path is selected, namely the shortest time for completing time synchronization of the whole network by taking i as a synchronization center. The following objective function can be obtained:

min(T_i) (5);

Where T _i denotes the minimum time for the entire network to complete time synchronization with i as the synchronization center, and as such, the objective function can be translated into:

Where H _i,j represents the set of all possible synchronization delays from i to j. By determining the minimum maximum path delay from satellite i, the delay required by the whole network to complete time synchronization when i is taken as a synchronization reference center can be determined, and the delay measurement mechanism of the invention is shown in fig. 7. By comparison, the best synchronization reference center in the network can be found. A flow chart of the clock synchronization measurement of the present invention is shown in fig. 8.

The invention is simulated and verified by using an EXata 5.1 simulation satellite network, the IEEE 802.1AS time synchronization protocol in a TSN protocol cluster developed in the EXata is tested and the result is analyzed, the wired network in the built test scene is developed based on the IEEE802.3 full-duplex Ethernet, and the verification result shows that the method effectively realizes time synchronization.

In summary, the above embodiments are only preferred embodiments of the present invention, and are not intended to limit the scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A time synchronization method for integrated inter-satellite and intra-satellite communications based on TSN, characterized in that the method is based on a time-sensitive network, the time-sensitive network includes the TSN standard, and the TSN standard includes IEEE 802.1ASbt, IEEE802.1Qbu, IEEE 802.1Qbv, IEEE 802.1Qca, IEEE 802.1Qcc, IEEE 802.1CB, IEEE 802.1Qci and IEEE 802.1Qch protocol; the specific steps of time synchronization are as follows: Step A. By selecting a master clock, establishing a master-slave topology relationship, and establishing a synchronization system in the entire PTP network; Step B. Setting a synchronous local clock scheme, the protocol uses a local clock synchronization algorithm, and through the exchange of PTP data messages between the master and slave nodes of the network, calculates the time deviation between the local clock of each slave node and the master clock, and adjusts the local clock to synchronize it with the master clock; Step C. Establishing the overall architecture of a wired and wireless integrated time-sensitive network, obtaining the global topology and resource status information of the satellite network through the TSN controller satellite, and responding to business needs, completing network configuration, routing table issuance and resource reservation, and realizing the time delay determinism requirements of integrated intra-satellite and inter-satellite communication; Step D. Establishing a wired and wireless integrated time-sensitive network communication protocol; Step E. Setting intra-satellite time synchronization; Step F. Setting inter-satellite time synchronization; In the step B, the local clock is synchronized based on the IEEE 1588v1 protocol, and the clocks in the entire PTP network are divided into ordinary clocks and boundary clocks according to the number of PTP communication ports on them; Based on IEEE 1588v2 protocol to synchronize local clocks; based on IEEE 802.1AS protocol to synchronize local clocks; when synchronizing local clocks based on IEEE 1588v1 protocol, synchronize the local clock on each port with the master port; the synchronization system establishment process is as follows:

1) In the initial state, each node port will listen to the Sync data frame in the network within the specified time; if a Sync data frame is received, the node port will determine the port state according to the best master clock algorithm;

If no Sync data frame is received, the node status changes to Pre_Master and assumes itself as the master clock node;

2) The port state remains Pre_Master for a certain period of time: If a Sync data frame is received within the specified time of the port, the port state is determined by the best master clock algorithm; if the port is determined to be a master clock, Sync frames will be sent periodically; if it is determined to be a slave clock, Sync frames will be accepted, deviations will be calculated, and the local clock will be corrected; if the port does not receive a Sync data frame within this period of time, the state will be changed to a master clock, and Sync data frames will be sent regularly;

3) The status of the master clock and the slave clock changes with the change of clock performance and operating status;

In the step B, synchronizing the local clock based on the IEEE 1588v2 protocol is an improvement and extension based on the IEEE 1588v1, and the specific methods are as follows: adding an independent message mode for measuring the network path delay between points; adding a transparent clock model; the transparent clock includes an end-to-end transparent clock and a point-to-point transparent clock; and adding a single-step clock model.

2. The method as claimed in claim 1 is characterized in that in step B, synchronizing the local clock based on the IEEE 1588v1 protocol, synchronizing the local clock based on the IEEE 1588v2 protocol, and synchronizing the local clock based on the IEEE 802.1AS protocol all adopt the BCMA master clock algorithm.

3. The method according to claim 2, characterized in that, in step C, the architecture design is divided into the following:

1) Single-domain wired and wireless integrated time-sensitive network architecture;

2) Multi-domain wired and wireless integrated time-sensitive network architecture;

3) Establishment and synchronization of dynamic distributed intersatellite network time base.

4. The method as claimed in claim 3 is characterized in that in step D, the TSN protocol modifies the Ethernet-based data link layer, combines clock synchronization and time-sensitive service scheduling to implement traffic shaping, scheduling and frame preemption, and ensures the bounded transmission of space time-sensitive services within the satellite.

5. The method according to claim 2 or 4, characterized in that in said step E, the additional requirements required for the IEEE 802.3 full-duplex Ethernet link and the IEEE 802.11 wireless network and Ethernet passive optical network are ensured by the IEEE 802.1AS protocol.

6. The method as claimed in claim 5 is characterized in that, in step F, the path delay between two nodes is obtained based on multiple measurements of IEEE 802.1AS, and the influence of satellite motion characteristics, relativistic effects and propagation path asymmetry on time synchronization accuracy are comprehensively considered to achieve inter-satellite time synchronization.