Method for time synchronization, first node, second node, network

Abstract

An embodiment of the present disclosure provides a time synchronization method, which is used for a first node. At least one output port of the first node is connected to an input port of a second node through an out-of-band synchronization link, and at least one other output port is connected to a third node through an in-band synchronization link. The method includes: sending out-of-band time synchronization information to the second node through the out-of-band synchronization link, where the out-of-band time synchronization information includes the grandparent clock identifier of the first node. An embodiment of the present disclosure also provides a time synchronization method for a second node, a first node, a second node, and a network.

Description

Technical Field

[0001] Embodiments of the present disclosure relate to the field of communication technologies, and particularly to a method for time synchronization, a first node, a second node, and a network. Background Art

[0002] With the development of communication technologies, especially the development of 5G technologies, the requirements for time synchronization between network devices in a network are getting higher and higher.

[0003] Time synchronization is divided into two methods: in-band synchronization and out-of-band synchronization. As the network becomes more and more complex, there are network devices in the network that can use both in-band and out-of-band methods for time synchronization, resulting in the easy existence of a timing loop in the network. Summary of the Invention

[0004] Embodiments of the present disclosure provide a method for time synchronization, a first node, a second node, and a network.

[0005] In a first aspect, embodiments of the present disclosure provide a method for time synchronization for a first node. At least one output port of the first node is connected to an input port of a second node through an out-of-band synchronization link, and at least one other output port is connected to a third node through an in-band synchronization link. The method includes:

[0006] Sending out-of-band time synchronization information to the second node through the out-of-band synchronization link, where the out-of-band time synchronization information includes the grandparent clock identifier of the first node.

[0007] In some embodiments, the out-of-band synchronization link is a 1PPS+TOD synchronization link.

[0008] In some embodiments, the out-of-band time synchronization information further includes the first priority, the second priority, and the hop count of the first node.

[0009] In a second aspect, embodiments of the present disclosure provide a method for time synchronization for a second node. At least one input port of the second node is connected to an output port of a first node through an out-of-band synchronization link, and at least one other input port is connected to a third node through an in-band synchronization link. The method includes:

[0010] Receiving the out-of-band time synchronization information sent by the first node and the in-band synchronization information sent by at least one of the third nodes, where the out-of-band time synchronization information includes the grandparent clock identifier of the first node;

[0011] When the grandparent clock identifier of the first node is the identifier of the second node, determine a preferred node from all third nodes according to the in-band time synchronization information, and synchronize the time of the second node to the time of the preferred node;

[0012] When the grandparent clock identifier of the first node is not the identifier of the second node, determine a preferred node from the first node and all third nodes according to the out-of-band time synchronization information and the in-band time synchronization information, and synchronize the time of the second node to the time of the preferred node.

[0013] In some embodiments, the in-band synchronization link is a Precision Time Protocol (PTP) link.

[0014] In some embodiments, the out-of-band synchronization link is a 1PPS+TOD synchronization link.

[0015] In some embodiments, between receiving the out-of-band time synchronization information sent by the first node, the in-band synchronization information sent by at least one of the third nodes, and determining a preferred node from the first node and all third nodes according to the out-of-band time synchronization information and the in-band time synchronization information, further include: mapping the input port of the 1PPS+TOD synchronization link of the second node to the input port of the Precision Time Protocol link.

[0016] In some embodiments, mapping the input port of the 1PPS+TOD synchronization link of the second node to the input port of the Precision Time Protocol link includes: establishing a 1PPS+TOD reference source data set according to the out-of-band time synchronization information, and the grandparent clock identifier of the 1PPS+TOD reference source data set is the grandparent clock identifier of the first node.

[0017] In some embodiments, the out-of-band time synchronization information further includes the priority 1, priority 2, and hop count of the first node; the 1PPS+TOD reference source data set further includes the priority 1, priority 2, and hop count of the first node.

[0018] In a third aspect, an embodiment of the present disclosure provides a first node. At least one output port of the first node is connected to an input port of a second node through an out-of-band synchronization link, and at least one other output port is connected to a third node through an in-band synchronization link. The first node includes:

[0019] A sending module, configured to send out-of-band time synchronization information to the second node through the out-of-band synchronization link, where the out-of-band time synchronization information includes the grandparent clock identifier of the first node.

[0020] Fourthly, embodiments of the present disclosure provide a second node. At least one input port of the second node is connected to an output port of a first node through an out-of-band synchronization link, and at least one other input port of the second node is connected to a third node through an in-band synchronization link. The second node includes:

[0021] A receiving module, configured to receive out-of-band time synchronization information sent by the first node and in-band synchronization information sent by at least one of the third nodes. The out-of-band time synchronization information includes a grandparent clock identifier of the first node.

[0022] A first processing module, configured to, when the grandparent clock identifier of the first node is the identifier of the second node, determine a preferred node from all the third nodes according to the in-band time synchronization information, and synchronize the time of the second node to the time of the preferred node.

[0023] A second processing module, configured to, when the grandparent clock identifier of the first node is not the identifier of the second node, determine a preferred node from the first node and all the third nodes according to the out-of-band time synchronization information and the in-band time synchronization information, and synchronize the time of the second node to the time of the preferred node.

[0024] Fifthly, embodiments of the present disclosure provide a network, which includes:

[0025] At least one first node; at least one second node; at least one third node;

[0026] At least one output port of the first node is connected to an input port of the second node through an out-of-band synchronization link;

[0027] At least one other output port of the first node is connected to the third node through an in-band synchronization link;

[0028] At least one other input port of the second node is connected to the third node through an in-band synchronization link.

[0029] In the time synchronization method, first node, second node, and network according to embodiments of the present disclosure, the grandparent clock identifier of the first node is added to out-of-band time synchronization information and sent to the second node. After receiving the out-of-band time synchronization information of the first node, if the second node finds that the grandparent clock identifier of the first node is its own identifier, it performs time synchronization only according to the in-band time synchronization information received from the third node. When the grandparent clock identifier of the first node is the second node, it indicates that the first node, the second node, and other nodes in the network have formed a time loop. At this time, the second node performs time synchronization only according to the in-band time synchronization information sent by the third node. That is to say, the time of the second node will no longer be synchronized with the first node, achieving the effect of breaking the loop. At the same time, since the grandparent clock identifier of the first node is an existing parameter and no new parameter needs to be added, it is convenient to be compatible with the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In the drawings of the embodiments of the present disclosure:

[0031] Figure 1 is a flowchart of a time synchronization method for a first node provided by an embodiment of the present disclosure;

[0032] Figure 2 is a flowchart of a time synchronization method for a second node provided by an embodiment of the present disclosure;

[0033] Figure 3 is a schematic diagram of the relationship between network devices in the network;

[0034] Figure 4 is a schematic diagram of the structure of the TOD frame of the 1PPS+TOD message;

[0035] Figure 5 is a block diagram of a first node provided by an embodiment of the present disclosure;

[0036] Figure 6 is a block diagram of a second node provided by an embodiment of the present disclosure;

[0037] Figure 7 is a block diagram of a network provided by an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] To enable those skilled in the art to better understand the technical solutions of the embodiments of the present disclosure, the time synchronization method, first node, second node, and network provided by the embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings.

[0039] Embodiments of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings. However, the illustrated embodiments may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0040] The accompanying drawings of the embodiments of the present disclosure are used to provide a further understanding of the embodiments of the present disclosure, and constitute a part of the specification. Together with the embodiments of the present disclosure, they are used to explain the present disclosure and do not constitute a limitation to the present disclosure. By describing the detailed exemplary embodiments with reference to the accompanying drawings, the above and other features and advantages will become more apparent to those skilled in the art.

[0041] Embodiments of the present disclosure may be described with reference to plan views and / or cross-sectional views by means of ideal schematic diagrams of the present disclosure. Therefore, the example illustrations may be modified according to manufacturing techniques and / or tolerances.

[0042] In the case of no conflict, the embodiments of the present disclosure and the features in the embodiments may be combined with each other.

[0043] The terms used in the present disclosure are only for describing specific embodiments and are not intended to limit the present disclosure. As used in the present disclosure, the term "and / or" includes any and all combinations of one or more of the related listed items. As used in the present disclosure, the singular forms "a" and "the" are also intended to include the plural forms unless the context clearly indicates otherwise. As used in the present disclosure, the terms "comprising", "made of", specify the presence of the stated features, wholes, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, wholes, steps, operations, elements, components and / or their groups.

[0044] Unless otherwise defined, all terms (including technical and scientific terms) used in the present disclosure have the same meaning as commonly understood by those of ordinary skill in the art. It will also be understood that terms such as those defined in common dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted as having an idealized or overly formal meaning unless the present disclosure clearly defines otherwise.

[0045] Embodiments of the present disclosure are not limited to the embodiments shown in the accompanying drawings, but include modifications to the configurations formed based on manufacturing processes. Therefore, the regions illustrated in the accompanying drawings have schematic attributes, and the shapes of the regions shown in the figures illustrate the specific shapes of the regions of the elements, but are not intended to be restrictive.

[0046] In some prior arts, with reference to Figure 3 , there are multiple network devices in the network at the same time, that is, network device (or referred to as node) 1 ( Figure 3 NE1 inFigure 3 in NE2) and network device 3( Figure 3 in NE3), where there is an out-of-band synchronization link between NE1 and NE2, NE1 is the input end of the out-of-band synchronization link, and NE2 is the output end of the out-of-band synchronization link; there are in-band synchronization links between other adjacent network devices (i.e., between NE1 and NE3 and between NE2 and NE3).

[0047] Among them, the in-band synchronization link refers to a link established by network devices in the network for time synchronization in an in-band manner. The ports at both ends of this link are generally Ethernet ports and run the IEEE1588 protocol (the precision clock synchronization protocol standard for network measurement and control systems), such as a PTP (Precision Time Protocol) link.

[0048] The out-of-band synchronization link refers to a link established by network devices in the network for time synchronization in an out-of-band manner. The ports at both ends of this link are generally GPS (Global Positioning System) interfaces and run protocols such as NEMA (National Marine Electronics Association), UBX (a protocol message developed by U-blox), and CMTOD (China Mobile Time of Day), such as a 1PPS (Pulse Per Second)+TOD (Time of Day) synchronization link.

[0049] Before time synchronization, it is necessary to configure clock parameters for each network device in the network. The clock parameters mainly include: clock identifier (ID), priority 1 (priority1), priority 2 (priority2), clock class, etc.

[0050] During the time synchronization process, two network devices connected by an in-band synchronization link can send in-band time synchronization information to each other. For example, NE1 and NE3 can send in-band time synchronization information to each other. The in-band time synchronization information includes the information required by the BMC (Best Master Clock) algorithm in the IEEE 1588 protocol (Precision Clock Synchronization Protocol Standard for Network Measurement and Control Systems), specifically: Grandmaster Identity, priority1, priority2, Clock Class, offsetScaledLogVariance, stepRemoved, clockAccuracy, timeSource, etc.

[0051] Among them, the in-band time synchronization information is carried in the announce message. At the very beginning of the time synchronization process, that is, when the time of the network devices in the network has not been synchronized with other network devices and is still their own time, the Grandmaster Identity in the in-band time synchronization information is the clock identity configured for this network device, and priority1 and priority2 are also the priority1 and priority2 configured for this network device.

[0052] After receiving the in-band time synchronization information, the network device establishes a reference source data set according to the specific information in the in-band time synchronization information. The reference source data set includes information such as Grandmaster Identity, priority1, and priority2 in the in-band time synchronization information.

[0053] Two network devices connected by an out-of-band synchronization link can only send out-of-band time synchronization information from the input end to the output end of the out-of-band synchronization link. For example, NE1 can send out-of-band time synchronization information to NE2, but NE2 cannot send out-of-band time synchronization information to NE1. The out-of-band time synchronization information carries information such as the time offset (leaps) between GPS and UTC (Coordinated Universal Time) and the status of the second pulse, but it cannot carry information such as Grandmaster Identity, priority1, priority2, Clock Class, offsetScaledLogVariance, stepRemoved, clockAccuracy, and timeSource required by the BMC algorithm in the IEEE 1588 protocol.

[0054] After receiving the out-of-band time synchronization information, the network device establishes a reference source data set according to the out-of-band time synchronization information. The reference source data set should also include information such as Grandmaster Identity, priority1, and priority2. However, since these information are not included in the out-of-band synchronization information, the reference source data set uses the clock identity configured for NE2 as the Grandmaster Identity, and the configured priority1 and priority2 as priority1 and priority2.

[0055] Refer to Figure 3 Assume that during the process of configuring clock parameters for network devices in a network, the clock identifiers of NE1, NE2, and NE3 are their respective MAC (Media Access Control Address) addresses; the priority1 and priority2 of NE1 and NE3 are configured as 128 and 30 respectively (the priorities are represented in the form of A / B in the figure, where A is priority1 and B is priority2), while the priority1 and priority2 of NE2 are configured as 128 and 20 respectively.

[0056] After receiving the out-of-band time synchronization information from NE1 through the out-of-band synchronization link, NE2 needs to map the port connected to the out-of-band synchronization link or NE1 to the port connected to the in-band synchronization link, that is, establish a reference source data set according to the out-of-band time synchronization information from NE1. Since the out-of-band time synchronization information does not carry information such as priority1 and priority2, when NE2 establishes a reference source data set for the out-of-band time synchronization information from NE1, the grandparent clock of this reference source data set is represented by the MAC address of NE2, and priority1 and priority2 use the priority1 and priority2 of NE2, that is, 128 and 20 respectively.

[0057] NE2 receives the in-band time synchronization information from NE3 through the in-band time synchronization link. This in-band time synchronization information carries information such as the grandparent clock identifier, priority1, priority2, and clock level. Since it is the initial time synchronization process and NE3 has not synchronized the time of other network devices, the grandparent clock identifier in the in-band time synchronization information from NE3 is the MAC address of NE3, and priority1 and priority2 are the priority1 and priority2 of NE3, that is, 128 and 30 respectively.

[0058] Since the value of priority2 of the reference source data set constructed according to the out-of-band time synchronization information is lower than the priority2 of the reference source data set constructed according to the in-band time synchronization information, NE2 selects the network device connected by the out-of-band synchronization as the time source according to the BMC algorithm, that is, NE2 synchronizes the time of NE1, sets the port connected to NE1 to the Slave state (slave port state), and sets the port connected to NE3 to the Master state (master port state).

[0059] NE2 sends in-band time synchronization information to NE3 through the in-band time synchronization link. At this time, since NE2 has selected the network device connected by the out-of-band synchronization as the time source, the grandparent clock identifier of the in-band time synchronization information sent by NE2 to NE3 is the MAC address of NE2, priority1 is 128, and priority2 is 20.

[0060] NE3 not only receives in-band time synchronization information from NE2, but also receives in-band time synchronization information from NE1. NE1 does not synchronize the time of other network devices. Therefore, the grandparent clock identifier in the in-band time synchronization information from NE1 is the MAC address of NE1, and the priority levels 1 and 2 are the priority levels 1 and 2 of NE3, which are 128 and 30 respectively. Obviously, since the value of the priority level 2 of the reference source data set constructed according to the in-band time synchronization information from NE2 is lower than that of the reference source data set constructed according to the in-band time synchronization information from NE1, NE3 selects NE2 as the time source according to the BMC algorithm, that is, NE3 synchronizes the time of NE2.

[0061] NE3 sends in-band time synchronization information to NE1 through the in-band time synchronization link. At this time, since N3 has selected the network device connected by the out-of-band synchronization as the time source, the grandparent clock identifier of the in-band time synchronization information sent by NE3 to NE1 is the grandparent clock identifier in the in-band time synchronization information sent by NE2 to NE3, that is, the MAC address of NE2. Similarly, the priority level 1 is 128 and the priority level 2 is 20.

[0062] After receiving the in-band time synchronization information from NE3, since the current clock parameters of NE3 are better than those of NE1 (the value of the priority level 2 of NE3 is lower than that of the priority level 2 of NE1), NE1 selects the time source of NE3 as its own time source according to the BMC algorithm, that is, NE1 synchronizes the time of NE3. And NE3 synchronizes the time of NE2, so NE1 also synchronizes the time of NE2. As described above, essentially the time of NE2 is synchronized with that of NE1. However, since the out-of-band time link cannot carry the grandparent clock identifier information, NE2 "thinks" that it is its own time source, thus forming a time loop of NEl-NE2-NE3-NE1.

[0063] In some related technologies, by hierarchically classifying network devices and setting different priorities for network devices at different levels, referring to Figure 3 , NE1 and its upstream node (such as NE3) are classified into the first level, and the downstream node of NE1 (such as NE2) is classified into the second level. Among them, the first-level priority of the first level is higher than the second-level priority of the second level. The time source selection is completed through different-level priorities to break the loop. However, since new parameters are introduced, it is not easy to be compatible with the existing technology.

[0064] In the first aspect, referring to Figure 1, an embodiment of the present disclosure provides a time synchronization method for a first node. The first node has at least one output port connected to an input port of a second node through an out-of-band synchronization link, and at least one other output port connected to a third node through an in-band synchronization link.

[0065] The first node refers to a network device in a network. It is connected to at least one other network device (i.e., the third node) in the network through an in-band synchronization link. At the same time, as the input end of the out-of-band synchronization link, it is connected to a network device (i.e., the second node) in the network that serves as the output end of the out-of-band synchronization link through the out-of-band synchronization link.

[0066] There may be multiple network devices in a network that meet the conditions of being connected to at least one other network device in the network through an in-band synchronization link and serving as the input end of the out-of-band synchronization link, and being connected to a network device in the network that serves as the output end of the out-of-band synchronization link through the out-of-band synchronization link. In different time synchronization processes, the first node can be different network devices that meet this condition.

[0067] The time synchronization method of the embodiment of the present disclosure specifically includes:

[0068] S101. Send out-of-band time synchronization information to the second node through the out-of-band synchronization link. The out-of-band time synchronization information includes the grandparent clock identifier of the first node.

[0069] The first node sends out-of-band time synchronization information to the second node in the network through the out-of-band synchronization link. Different from the above out-of-band time synchronization information, the out-of-band time synchronization information of the embodiment of the present disclosure carries the grandparent clock identifier of the first node.

[0070] Among them, when the first node has not synchronized the time of other network devices in the network, the grandparent clock identifier of the first node is the clock identifier of the first node, which can specifically be the MAC address of the first node; when the first node has synchronized the time of other network devices in the network, the grandparent clock identifier of the first node is the clock identifier of the time source that the first node synchronizes to. For example, referring to Figure 3 , if the time source of NE1 is NE2, then the grandparent clock identifier of NE1 is the MAC address of NE2.

[0071] The time synchronization method according to the embodiments of the present disclosure sends the grandparent clock identifier of the first node to the second node by adding it to the out-of-band time synchronization information. After the second node receives the out-of-band time synchronization information of the first node, if it is found that the grandparent clock identifier of the first node is the identifier of the second node, the second node only performs time synchronization according to the in-band time synchronization information received from the third node. When the grandparent clock identifier of the first node is the second node, it means that the first node, the second node, and other nodes in the network have formed a time loop. At this time, the second node only performs time synchronization according to the in-band time synchronization information sent by the third node. That is to say, the time of the second node will no longer be synchronized with the first node. As shown in reference to Figure 3 , that is, the time of NE2 will no longer be synchronized with NE1, and the time loop of NEl-NE2-NE3-NE1 will not be formed, achieving the effect of breaking the loop. At the same time, since the grandparent clock identifier of the first node is an existing parameter and no new parameter needs to be added, it is convenient to be compatible with the prior art.

[0072] In some embodiments, the out-of-band synchronization link is a 1PPS+TOD synchronization link.

[0073] The out-of-band synchronization link may specifically be a 1PPS+TOD synchronization link, and the in-band synchronization link may specifically be a PTP link.

[0074] In some related technologies, the second pulse state of the TOD frame carrying the time information message in the 1PPS+TOD packet sent from the input end to the output end of the 1PPS+TOD synchronization link in the network can be corresponded to the clock level of PTP. The specific correspondence is shown in the following table:

[0075]

[0076] By corresponding the second pulse state of the TOD frame carrying the time message in the 1PPS+TOD packet to the clock level of PTP, during the process of constructing the reference source data set according to the out-of-band time synchronization information, the clock level can be obtained according to the 1PPS+TOD packet and used as the clock level of the constructed reference source data set, solving the problem that the clock level cannot be carried in the out-of-band time synchronization information.

[0077] In some embodiments, the out-of-band time synchronization information further includes the priority 1, priority 2, and hop count of the first node.

[0078] In addition to the grandparent clock identifier of the first node, the out-of-band time synchronization information sent by the first node may further include the priority 1, priority 2, and hop count of the first node. Of course, other information such as time stability and clock accuracy may also be included.

[0079] Among them, similar to the grandfather clock identifier of the first node, when the first node has not synchronized the time of other network devices in the network, the priority 1 and priority 2 of the first node are the priority 1 and priority 2 of the first node. As shown in Figure 3 , they are 128 and 30; when the first node has synchronized the time of other network devices in the network, the priority 1 and priority 2 of the first node are the priority 1 and priority 2 carried in the in-band time synchronization information corresponding to the time synchronized by the first node, that is, the priority 1 and priority 2 of the time source of the first node. As shown in Figure 3 , if the time source of NE1 is NE2, then the priority 1 and priority 2 of NE1 are the priority 1 and priority 2 of NE2, that is, 128 and 20.

[0080] The hop count refers to the number of other network devices that the current network device needs to pass through to connect to its time source through the in-band synchronization link. As shown in Figure 3 , the time sources of NE3 and NE1 are both NE2. NE3 is directly connected to NE2 through the in-band synchronization link, while NE1 is connected to NE2 through NE3 via the in-band synchronization link. Naturally, the hop count of NE3 is lower than that of NE1.

[0081] By sending the priority 1, priority 2, and hop count to the second node, the accuracy of the second node in constructing the reference source dataset based on the priority 1, priority 2, and hop count is improved, which is convenient for the second node to select the correct time source according to the constructed reference source dataset.

[0082] As shown in Figure 3 , for the out-of-band time synchronization information received by NE2 from NE1 through the out-of-band synchronization link, if the out-of-band time synchronization information carries information such as the grandfather clock identifier, priority 1, priority 2, and hop count, when NE2 maps the port connected to the out-of-band synchronization link or NE1 to the port connected to the in-band synchronization link, that is, when establishing a reference source dataset based on the out-of-band time synchronization information from NE1, the grandfather clock, priority 1, and priority 2 of the reference source dataset can directly use the grandfather clock identifier, priority 1, and priority 2 in the out-of-band time synchronization information, that is, the priority 1 and priority 2 of NE1, instead of using the default configured priority 1 and priority 2 of NE2. At this time, since the values of the priority 1 and priority 2 of the reference source dataset constructed based on the out-of-band time synchronization information are the same as those of the reference source dataset constructed based on the in-band time synchronization information, NE2 may select NE3 as its data source at this time, which can also avoid loop formation to a certain extent.

[0083] Specifically, the grandfather clock identifier, priority 1, priority 2, and hop count of the first node in the out-of-band time synchronization information are carried in the 1PPS+TOD packet and sent to the second node.

[0084] The structure of the TOD frame of the specific 1PPS+TOD message refers to Figure 4 , which includes a frame header composed of two bytes SYNCCHAR1 and SYNCCHAR2, a message header composed of two bytes CLASS (message class) and ID (message ID), a two-byte LENGTH BIG Endian (message length field), a Payload (payload field), and an FCS (frame check sequence field).

[0085] Among them, SYNCCHAR1 occupies one byte, which is a fixed value of 0X43, representing the character "C" in ASCII code; SYNCCHAR2 occupies one byte, which is a fixed value of 0X4D, representing the character "M" in ASCII code. The message class occupies one byte, which specifies the basic classification of the TOD message, and the message ID occupies one byte, which defines the number of the specific TOD message. The effective range of the calculation of the message length field only includes the net payload of the message (i.e., the payload field), excluding the frame header, the message header, the message length field itself, and the frame check sequence field. The payload field is the specific message content, and currently this field has 16 bytes. The frame check sequence field is used for frame check, and the generating polynomial of the frame check sequence is: G(x) = x 8 +x 5 +x 4 +1.

[0086] There are two specific types of 1PPS+TOD messages. One carries time information messages, and the other carries time status messages. The following table shows the specific composition of the payload field of the TOD frame of the 1PPS+TOD message carrying time information messages. The payload field of the TOD frame of the 1PPS+TOD message carrying time information messages has 7 reserved bytes.

[0087]

[0088]

[0089] The following table shows the specific composition of the payload field of the TOD frame of the 1PPS+TOD message carrying time status messages. The payload field of the TOD frame of the 1PPS+TOD message carrying time status messages has 11 reserved bytes.

[0090]

[0091] Since the grandparent clock identifier requires 8 bytes, and priority 1, priority 2, and hop count each require 1 byte, the grandparent clock identifier, priority 1, priority 2, and hop count together occupy 11 bytes. The grandparent clock identifier, priority 1, priority 2, and hop count can be sent to the second node in the reserved field of the payload domain of the TOD frame of the 1PPS+TOD message carrying the time status message. The following table shows the specific composition of the payload domain of the TOD frame of the 1PPS+TOD message carrying the extended time status message, which carries the grandparent clock identifier, priority 1, priority 2, and hop count in the bytes with the last byte offsets of 5, 6, 7, and 8.

[0092]

[0093] Using the existing message to send information such as the grandparent clock identifier, priority 1, priority 2, and hop count is easy to be compatible with the existing technology, and since there is no need to send a new message, it also saves transmission resources. Of course, other methods can also be used to send the grandparent clock identifier, priority 1, priority 2, and hop count, such as carrying them together in the reserved fields of the payload domains of the TOD frames of the 1PPS+TOD messages carrying the time status message and the 1PPS+TOD messages carrying the time information message.

[0094] In a second aspect, referring to Figure 2 , the embodiments of the present disclosure provide a time synchronization method for a second node. At least one input port is connected to the output port of the first node through an out-of-band synchronization link, and at least one other input port is connected to the third node through an in-band synchronization link.

[0095] Among them, the second node refers to a network device in the network, which is connected to at least one other network device (i.e., the third node) in the network through an in-band synchronization link. At the same time, as the output end of the out-of-band synchronization link, it is connected to the network device (i.e., the first node) in the network that serves as the input end of the out-of-band synchronization link through the out-of-band synchronization link.

[0096] There may be multiple network devices in a network that meet the conditions of being connected to at least one other network device in the network through an in-band synchronization link and serving as the output end of the out-of-band synchronization link to be connected to the network device in the network that serves as the input end of the out-of-band synchronization link through the out-of-band synchronization link. In different time synchronization processes, the second node can be different network devices that meet this condition.

[0097] The time synchronization method of this embodiment specifically includes:

[0098] S201. Receive out-of-band time synchronization information sent by the first node and in-band synchronization information sent by at least one third node. The out-of-band time synchronization information includes the grandparent clock identifier of the first node.

[0099] The second node receives the out-of-band time synchronization information sent by the first node connected to it via an out-of-band synchronization link. Meanwhile, it receives the in-band time synchronization information sent by at least one third node connected to it via an in-band synchronization link.

[0100] Among them, the out-of-band time synchronization information includes not only information such as the time offset between GPS and UTC and the status of the second pulse, but also the grandparent clock identifier of the first node; the in-band time synchronization information includes the grandparent clock identifier of the third node, priority 1, priority 2, hop count, etc.

[0101] S202. When the grandparent clock identifier of the first node is the identifier of the second node, determine a preferred node from all the third nodes according to the in-band time synchronization information, and synchronize the time of the second node to the time of the preferred node.

[0102] After receiving the out-of-band time synchronization information of the first node, if the second node finds that the grandparent clock identifier of the first node in the out-of-band time synchronization information is the identifier of the second node, such as the MAC address of the second node, the second node constructs a reference source data set for each third node according to the in-band time synchronization information sent by the third node connected to the second node. The reference source data set corresponding to each third node should include information such as the grandparent clock identifier, priority 1, priority 2, hop count, etc. in the in-band time synchronization information sent by the third node.

[0103] The second node uses the BMC algorithm to determine a third node as the preferred node from the third nodes corresponding to the received in-band time synchronization information according to the reference source data set corresponding to each third node, and synchronizes the time of the second node with the time of the preferred node.

[0104] S203. When the grandparent clock identifier of the first node is not the identifier of the second node, determine a preferred node from the first node and all the third nodes according to the out-of-band time synchronization information and the in-band time synchronization information, and synchronize the time of the second node to the time of the preferred node.

[0105] After receiving the out-of-band time synchronization information of the first node, if the second node finds that the grandparent clock identifier of the first node in the out-of-band time synchronization information is not the identifier of the second node, the second node constructs a reference source dataset for each third node according to the in-band time synchronization information sent by the third node connected to the second node. The reference source dataset corresponding to each third node should include information such as the grandparent clock identifier, priority 1, priority 2, and hop count in the in-band time synchronization information sent by the third node. At the same time, map the input port connected to the first node to the port of the in-band synchronization link, that is, construct a reference source dataset for the first node according to the out-of-band time synchronization information sent by the first node. The reference source dataset corresponding to the first node should include the grandparent clock identifier of the first node. If there is no information such as priority 1, priority 2, and hop count in the out-of-band synchronization information, the priority 1 and priority 2 of the second node are used as priority 1 and priority 2 in the reference source dataset corresponding to the first node.

[0106] The second node uses the BMC algorithm to determine a third node as the preferred node from the third nodes and the first node corresponding to the reference source datasets of the first node and each third node according to the in-band time synchronization information received, and synchronizes the time of the second node with the time of the preferred node.

[0107] The time synchronization method of the embodiments of the present disclosure sends the grandparent clock identifier of the first node to the second node by adding it to the out-of-band time synchronization information. After the second node receives the out-of-band time synchronization information of the first node, if it is found that the grandparent clock identifier of the first node is the identifier of the second node, time synchronization is performed only according to the in-band time synchronization information of the received third node. When the grandparent clock identifier of the first node is the second node, it means that the first node, the second node, and other nodes in the network have formed a time loop. At this time, the second node performs time synchronization only according to the in-band time synchronization information sent by the third node. That is to say, the time of the second node will no longer be synchronized with the first node. As shown in Figure 3 That is, the time of NE2 will no longer be synchronized with NE1, and a time loop of NEl-NE2-NE3-NE1 will not be formed, achieving the effect of breaking the loop. At the same time, since the grandparent clock identifier of the first node is an existing parameter, there is no need to add new parameters, which is convenient for compatibility with the prior art.

[0108] In some embodiments, the in-band synchronization link is a Precision Time Protocol link.

[0109] In some embodiments, the out-of-band synchronization link is a 1PPS+TOD synchronization link.

[0110] Specifically, the out-of-band synchronization link may be a 1PPS+TOD synchronization link, and the in-band synchronization link may be a PTP link.

[0111] In some related technologies, the second pulse state of the TOD frame carrying the time information message in the 1PPS+TOD message sent from the input end to the output end of the 1PPS+TOD synchronization link in the network can be corresponded to the clock level of PTP.

[0112] By corresponding the second pulse state of the TOD frame carrying the time message in the 1PPS+TOD message to the clock level of PTP, during the process of constructing the reference source data set according to the out-of-band time synchronization information, the clock level can be obtained according to the 1PPS+TOD message and used as the clock level of the constructed reference source data set, thus solving the problem that the clock level cannot be carried in the out-of-band time synchronization information.

[0113] In some embodiments, between receiving the out-of-band time synchronization information sent by the first node and the in-band synchronization information sent by at least one third node and determining a preferred node from the first node and all third nodes according to the out-of-band time synchronization information and the in-band time synchronization information, it further includes: mapping the input port of the 1PPS+TOD synchronization link of the second node to the input port of the Precision Time Protocol link.

[0114] In some embodiments, mapping the input port of the 1PPS+TOD synchronization link of the second node to the input port of the Precision Time Protocol link includes: establishing a 1PPS+TOD reference source data set according to the out-of-band time synchronization information, and the grandparent clock identifier of the 1PPS+TOD reference source data set is the grandparent clock identifier of the first node.

[0115] After the second node receives the out-of-band time synchronization information of the first node, if it is found that the grandparent clock identifier of the first node in the out-of-band time synchronization information is not the identifier of the second node, the second node constructs the reference source data sets of each third node according to the in-band time synchronization information sent by the third node connected to the second node. The reference source data set corresponding to each third node should include information such as the grandparent clock identifier, priority 1, priority 2, hop count, etc. in the in-band time synchronization information sent by the third node.

[0116] At the same time, map the input port connected to the first node (i.e., the port of the 1PPS+TOD synchronization link) to the input port of the Precision Time Protocol link. The specific mapping process can be: constructing the reference source data set of the first node according to the out-of-band time synchronization information sent by the first node. The reference source data set corresponding to the first node should include the grandparent clock identifier of the first node. If there is no information such as priority 1, priority 2, hop count, etc. in the out-of-band synchronization information, the priority 1 and priority 2 of the second node are used as priority 1 and priority 2 for the reference source data set corresponding to the first node.

[0117] In some embodiments, the out-of-band time synchronization information further includes the priority 1, priority 2, and hop count of the first node; the 1PPS+TOD reference source dataset further includes the priority 1, priority 2, and hop count of the first node.

[0118] The out-of-band time synchronization information sent by the first node to the second node further includes the priority 1, priority 2, and hop count of the first node, and the priority 1, priority 2, and hop count of the first node are the priority 1, priority 2, and hop count of the time source it synchronizes with.

[0119] Then, after the second node receives the out-of-band time synchronization information of the first node, if it is found that the grandparent clock identifier of the first node in the out-of-band time synchronization information is not the identifier of the second node, when constructing the reference source dataset of the first node according to the out-of-band time synchronization information sent by the first node, the reference source dataset corresponding to the first node should include information such as the grandparent clock identifier, priority 1, priority 2, hop count, etc. of the out-of-band time synchronization information sent by the first node.

[0120] The second node uses the BMC algorithm to determine a third node as the preferred node from the third node corresponding to the in-band time synchronization information received and the first node according to the reference source datasets corresponding to the first node and each third node, and synchronizes the time of the second node with the time of the preferred node.

[0121] By receiving the priority 1, priority 2, and hop count of the first node, the second node improves the accuracy of the reference source dataset of the first node constructed according to the priority 1, priority 2, and hop count (because the priority 1, priority 2, and hop count of the first node are used instead of the priority 1, priority 2 of the second node), which is convenient for the second node to select the correct time source according to the constructed reference source dataset.

[0122] Refer to Figure 3 , the specific process of the time synchronization method according to the embodiments of the present disclosure may be: If the time source of NE1 is the best, after NE2 receives the out-of-band time synchronization information sent by NE1 and the in-band time synchronization information sent by NE3, it recovers the reference source dataset of NE3 from the in-band time synchronization information received from NE3. The reference source dataset includes information such as the grandparent clock identifier, priority 1, priority 2, hop count, etc. in the in-band time synchronization information sent by NE3. Perform port mapping on the out-of-band time synchronization information received from NE1 and construct the reference source dataset corresponding to NE1. Among them, information such as the grandparent clock identifier, priority 1, priority 2, hop count, etc. is extracted from the out-of-band time synchronization information, and the clock level can be mapped from the received second pulse state, and the clock accuracy and time stability are filled in by default.

[0123] At this time, the grandparent clock identifier in the reference source dataset corresponding to NE1 is the MAC of NE1, which is inconsistent with the MAC of NE2 itself. Mixed source selection is performed based on the reference source dataset corresponding to NE1 and the reference source dataset corresponding to NE3: According to the BMC algorithm, the hop count of the reference source dataset corresponding to NE1 is less than that of the reference source dataset from NE3, the time source of NE1 is superior to that of NE3, and the time of NE2 is synchronized with NE1.

[0124] Similarly, if the time source of NE3 is the best, after receiving the out-of-band time synchronization information sent by NE1 and the in-band time synchronization information sent by NE3, NE2 restores the reference source dataset of NE3 from the in-band time synchronization information received from NE3. This reference source dataset includes information such as the grandparent clock identifier, priority 1, priority 2, hop count, etc. in the in-band time synchronization information sent by NE3. Port mapping is performed on the GPS synchronization message received from NE1 and the GPS time source dataset is constructed (i.e., the reference source dataset corresponding to NE1).

[0125] At this time, the grandparent clock identifier in the reference source dataset corresponding to NE1 is the MAC of NE3, which is inconsistent with the MAC of NE2 itself. Mixed source selection is performed based on the reference source dataset corresponding to NE1 and the reference source dataset corresponding to NE3: According to the BMC algorithm, the hop count of the reference source dataset corresponding to NE1 is greater than that of the reference source dataset from NE3, the time source of NE3 is superior to that of NE1, and the time of NE2 is synchronized with NE3.

[0126] If the time source of NE2 is the best, after receiving the out-of-band time synchronization information sent by NE1 and the in-band time synchronization information sent by NE3, NE2 restores the reference source dataset of NE3 from the in-band time synchronization information received from NE3. This reference source dataset includes information such as the grandparent clock identifier, priority 1, priority 2, hop count, etc. in the in-band time synchronization information sent by NE3. The GPS time source dataset is restored from the GPS synchronization message received from NE1.

[0127] At this time, the grandparent clock identifier in the reference source dataset corresponding to NE1 is the MAC of NE2, which is consistent with the MAC of NE2 itself. It indicates that there may be a time loop. Therefore, only the reference source dataset corresponding to NE3 can participate in source selection: According to the BMC algorithm, NE2 is used as the time source, the port decision of the connection between NE2 and NE3 is set as the master port, and the port of NE3 corresponding to the connection with NE2 is set as the slave port.

[0128] Thirdly, referring to Figure 5, an embodiment of the present disclosure provides a first node. At least one output port of the first node is connected to an input port of a second node through an out-of-band synchronization link, and at least one other output port is connected to a third node through an in-band synchronization link. The first node includes:

[0129] A sending module, configured to send out-of-band time synchronization information to the second node through the out-of-band synchronization link. The out-of-band time synchronization information includes the grandparent clock identifier of the first node.

[0130] In the first node of the embodiment of the present disclosure, by adding the grandparent clock identifier of the first node to the out-of-band time synchronization information and sending it to the second node, after the second node receives the out-of-band time synchronization information of the first node, if it is found that the grandparent clock identifier of the first node is the identifier of the second node, the second node only performs time synchronization according to the in-band time synchronization information received from the third node. When the grandparent clock identifier of the first node is the second node, it means that the first node, the second node, and other nodes in the network have formed a time loop. At this time, the second node only performs time synchronization according to the in-band time synchronization information sent by the third node. That is to say, the time of the second node will no longer be synchronized with the first node. For example, referring to Figure 3 , that is, the time of NE2 will no longer be synchronized with NE1, and the time loop of NEl-NE2-NE3-NE1 will not be formed, achieving the effect of breaking the loop. At the same time, since the grandparent clock identifier of the first node is an existing parameter and no new parameter needs to be added, it is convenient to be compatible with the prior art.

[0131] In a fourth aspect, referring to Figure 6 , an embodiment of the present disclosure provides a second node. At least one input port of the second node is connected to an output port of the first node through an out-of-band synchronization link, and at least one other input port is connected to a third node through an in-band synchronization link. The second node includes:

[0132] A receiving module, configured to receive the out-of-band time synchronization information sent by the first node and the in-band synchronization information sent by at least one third node. The out-of-band time synchronization information includes the grandparent clock identifier of the first node;

[0133] A first processing module, configured to, when the grandparent clock identifier of the first node is the identifier of the second node, determine a preferred node from all third nodes according to the in-band time synchronization information, and synchronize the time of the second node to the time of the preferred node;

[0134] A second processing module, configured to, when the grandparent clock identifier of the first node is not the identifier of the second node, determine a preferred node from the first node and all third nodes according to the out-of-band time synchronization information and the in-band synchronization information, and synchronize the time of the second node to the time of the preferred node.

[0135] In an embodiment of the present disclosure, the second node sends the grandfather clock identifier of the first node to the second node by adding it to the out-of-band time synchronization information. After the second node receives the out-of-band time synchronization information of the first node, if it is found that the grandfather clock identifier of the first node is the identifier of the second node, the second node performs time synchronization only according to the in-band time synchronization information received from the third node. When the grandfather clock identifier of the first node is the second node, it means that the first node, the second node, and other nodes in the network have formed a time loop. At this time, the second node performs time synchronization only according to the in-band time synchronization information sent by the third node. That is to say, the time of the second node will no longer be synchronized with the first node. As shown in Figure 3 , that is, the time of NE2 will no longer be synchronized with NE1, and the time loop of NEl-NE2-NE3-NE1 will not be formed, achieving the effect of breaking the loop. At the same time, since the grandfather clock identifier of the first node is an existing parameter, there is no need to add new parameters, which is convenient for compatibility with the prior art.

[0136] Fifthly, as shown in Figure 7 , an embodiment of the present disclosure provides a network, which includes:

[0137] At least one output port of the first node is connected to an input port of the second node through an out-of-band synchronization link;

[0138] At least one other output port of the first node is connected to the third node through an in-band synchronization link;

[0139] At least one other input port of the second node is connected to the third node through an in-band synchronization link.

[0140] In the network according to the embodiment of the present disclosure, the second node sends the grandfather clock identifier of the first node to the second node by adding it to the out-of-band time synchronization information. After the second node receives the out-of-band time synchronization information of the first node, if it is found that the grandfather clock identifier of the first node is the identifier of the second node, the second node performs time synchronization only according to the in-band time synchronization information received from the third node. When the grandfather clock identifier of the first node is the second node, it means that the first node, the second node, and other nodes in the network have formed a time loop. At this time, the second node performs time synchronization only according to the in-band time synchronization information sent by the third node. That is to say, the time of the second node will no longer be synchronized with the first node. As shown in Figure 3 , that is, the time of NE2 will no longer be synchronized with NE1, and the time loop of NEl-NE2-NE3-NE1 will not be formed, achieving the effect of breaking the loop. At the same time, since the grandfather clock identifier of the first node is an existing parameter, there is no need to add new parameters, which is convenient for compatibility with the prior art.

[0141] Those of ordinary skill in the art will understand that all or some of the steps, systems, and functional modules / units in the devices disclosed above can be implemented as software, firmware, hardware, and appropriate combinations thereof.

[0142] In a hardware implementation, the division between the functional modules / units mentioned in the above description does not necessarily correspond to the division of physical components; for example, one physical component can have multiple functions, or one function or step can be executed by several physical components working together.

[0143] Some or all physical components can be implemented as software executed by a processor, such as a central processing unit (CPU), a digital signal processor, or a microprocessor, or can be implemented as hardware, or can be implemented as an integrated circuit, such as an application-specific integrated circuit. Such software can be distributed on a computer-readable medium, which can include a computer storage medium (or non-transitory medium) and a communication medium (or transitory medium). As is well known to those of ordinary skill in the art, the term computer storage medium includes volatile and non-volatile, removable and non-removable media implemented in any method or technology for storing information, such as computer-readable instructions, data structures, program modules, or other data. Computer storage media include, but are not limited to, random access memory (RAM, more specifically SDRAM, DDR, etc.), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory (FLASH), or other magnetic disk memories; compact disc read-only memory (CD-ROM), digital versatile disc (DVD), or other optical disc memories; magnetic cassettes, tapes, magnetic disk storage, or other magnetic memories; and any other medium that can be used to store the desired information and can be accessed by a computer. In addition, as is well known to those of ordinary skill in the art, a communication medium typically contains computer-readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transmission mechanism, and can include any information delivery medium.

[0144] The present disclosure has disclosed example embodiments, and although specific terms have been used, they are used only and should be construed only as having a general illustrative meaning and not for the purpose of limitation. In some instances, it will be apparent to those skilled in the art that, unless otherwise expressly stated, the features, characteristics, and / or elements described in connection with a particular embodiment can be used alone or in combination with the features, characteristics, and / or elements described in connection with other embodiments. Thus, those skilled in the art will understand that various forms and details can be changed without departing from the scope of the present disclosure as set forth by the appended claims.

Claims

1. A time synchronization method for a first node. The first node has at least one output port connected to an input port of a second node through an out-of-band synchronization link, and at least one other output port connected to a third node through an in-band synchronization link. The method includes: Sending out-of-band time synchronization information to the second node through the out-of-band synchronization link. The out-of-band time synchronization information includes the grandparent clock identifier of the first node; When the grandparent clock identifier of the first node is the identifier of the second node, the second node determines a preferred node from all the third nodes according to the in-band time synchronization information sent by each of the third nodes, and synchronizes the time of the second node to the time of the preferred node; When the grandparent clock identifier of the first node is not the identifier of the second node, map the input port connected to the first node as the port of the in-band synchronization link, and determine a preferred node from the first node and all the third nodes according to the out-of-band time synchronization information and the in-band time synchronization information sent by each of the third nodes, and synchronize the time of the second node to the time of the preferred node.

2. The method according to claim 1, wherein The out-of-band synchronization link is a 1PPS+TOD synchronization link.

3. The method according to claim 1, wherein, The out-of-band time synchronization information further includes the priority 1, priority 2 and hop count of the first node.

4. A time synchronization method for a second node. The second node has at least one input port connected to an output port of a first node through an out-of-band synchronization link, and at least one other input port connected to a third node through an in-band synchronization link. The method includes: Receiving the out-of-band time synchronization information sent by the first node and the in-band time synchronization information sent by at least one of the third nodes. The out-of-band time synchronization information includes the grandparent clock identifier of the first node; When the grandparent clock identifier of the first node is the identifier of the second node, determine a preferred node from all the third nodes according to the in-band time synchronization information, and synchronize the time of the second node to the time of the preferred node; When the grandparent clock identifier of the first node is not the identifier of the second node, map the input port connected to the first node as the port of the in-band synchronization link, and determine a preferred node from the first node and all the third nodes according to the out-of-band time synchronization information and the in-band time synchronization information, and synchronize the time of the second node to the time of the preferred node.

5. The method according to claim 4, wherein The in-band synchronization link is a Precision Time Protocol link.

6. The method according to claim 5, wherein, The out-of-band synchronization link is a 1PPS+TOD synchronization link.

7. The method according to claim 6, wherein, The mapping of the input port connected to the first node as the port of the in-band synchronization link includes: Mapping the input port of the 1PPS+TOD synchronization link of the second node as the input port of the Precision Time Protocol link.

8. The method according to claim 7, wherein, The mapping of the input port of the 1PPS+TOD synchronization link of the second node as the input port of the Precision Time Protocol link includes: Establish a 1PPS+TOD reference source data set according to the out-of-band time synchronization information, and the grandparent clock identifier of the 1PPS+TOD reference source data set is the grandparent clock identifier of the first node.

9. The method according to claim 8, wherein, The out-of-band time synchronization information further includes the priority 1, priority 2, and hop count of the first node; the 1PPS+TOD reference source data set further includes the priority 1, priority 2, and hop count of the first node.

10. A first node, at least one output port of the first node is connected to an input port of a second node through an out-of-band synchronization link, and at least one other output port is connected to a third node through an in-band synchronization link. The first node includes: A sending module, configured to send out-of-band time synchronization information to the second node through the out-of-band synchronization link, where the out-of-band time synchronization information includes the grandparent clock identifier of the first node. The second node is configured to, when the grandparent clock identifier of the first node is the identifier of the second node, determine a preferred node from all third nodes according to the in-band time synchronization information sent by each third node, and synchronize the time of the second node to the time of the preferred node. When the grandparent clock identifier of the first node is not the identifier of the second node, map the input port connected to the first node to the port of the in-band synchronization link, determine a preferred node from the first node and all third nodes according to the out-of-band time synchronization information and the in-band time synchronization information of each third node, and synchronize the time of the second node to the time of the preferred node.

11. A second node, at least one input port of the second node is connected to an output port of a first node through an out-of-band synchronization link, and at least one other input port is connected to a third node through an in-band synchronization link. The second node includes: A receiving module, configured to receive the out-of-band time synchronization information sent by the first node and the in-band time synchronization information sent by at least one third node, where the out-of-band time synchronization information includes the grandparent clock identifier of the first node. A first processing module, configured to, when the grandparent clock identifier of the first node is the identifier of the second node, determine a preferred node from all third nodes according to the in-band time synchronization information, and synchronize the time of the second node to the time of the preferred node. A second processing module, configured to, when the grandparent clock identifier of the first node is not the identifier of the second node, map the input port connected to the first node to the port of the in-band synchronization link, determine a preferred node from the first node and all third nodes according to the out-of-band time synchronization information and the in-band time synchronization information, and synchronize the time of the second node to the time of the preferred node.

12. A network, the network includes: At least one first node; At least one second node; At least one third node; The first node has at least one output port connected to the input port of the second node through an out-of-band synchronization link, and the first node includes the first node as described in claim 10; The first node also has at least one other output port connected to the third node through an in-band synchronization link; The second node also has at least one other input port connected to the third node through an in-band synchronization link, and the second node includes the second node as described in claim 11.

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