Patent Publication Number: US-2017359139-A1

Title: Synchronization with Different Clock Transport Protocols

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. application Ser. No. 15/275,316, filed Sep. 23, 2016, which claims the benefit of U.S. Provisional Application No. 62/348,782 filed Jun. 10, 2016, the contents of which are hereby incorporated by reference as if fully stated herein. 
    
    
     FIELD 
     One aspect of the disclosure herein relates to synchronizing time between devices in a network, and more particularly relates to determining the reliability of a timestamp. 
     BACKGROUND 
     Time synchronization between interconnected nodes in a network is often important to operation of the nodes. Time synchronization typically involves sharing time synchronization messages between the nodes so that nodes (often called slave nodes) may synchronize their own clocks in accordance with the clock of a node that has been designated the master node. For example, a master node sends a time synchronization message to a slave node at a first time indicated by a timestamp T 1 . The slave node receives the time synchronization message including the timestamp T 1  and notes the local time T 1 ′ when the message is received. The slave node calculates a network transit time, which is the time it takes for the time synchronization message to travel from the slave node to the master node, by sending a message to the master node at a time T 2 . The master node receives this message at a time T 2 ′ and sends a message back to the slave node including the timestamp T 2 ′. Based on T 1 , T 1 ′, T 2  and T 2 ′, the slave node calculates an offset between the clock of the master node and the clock of the slave node. The slave node may use the offset information to adjust its local clock into agreement with the clock of the master node. Fluctuations in the delay of the network, jitter or noise often affects the accuracy of a timestamp included in a time synchronization message, such that time synchronization is sometimes inaccurate between the master and slave nodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments herein are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and they mean at least one. Also, in the interest of conciseness and reducing the total number of figures, a given figure may be used to illustrate the features of more than one embodiment, and not all elements in the figure may be required for a given embodiment. 
         FIG. 1  is a representational view for explaining interconnected nodes in a first example network according to an embodiment herein. 
         FIG. 2  is a representational view for explaining interconnected nodes in a second example network according to an embodiment herein. 
         FIG. 3  is a flow diagram for explaining a time synchronization process between a slave node and a master node according to an embodiment herein. 
         FIG. 4  is a flow diagram for explaining a time synchronization process between a slave node and a master node having an intermediate node therebetween according to an embodiment herein in which chain of quality information is accumulated by the intermediate node. 
         FIG. 5  is a flow diagram for explaining a time synchronization process between a slave node and a master node having an intermediate node therebetween according to an embodiment herein in which chain of quality information is adjusted by the intermediate node. 
         FIG. 6  is a representational view for explaining an example node according to embodiments herein. 
         FIG. 7  is a representational view for explaining an example bridge element according to embodiments herein in which nodes use clock synchronization transport protocols. 
         FIG. 8  is a representation view for explaining an example node using IEEE 802.1AS as its clock synchronization transport protocol according to an embodiment herein. 
     
    
    
     DETAILED DESCRIPTION 
     Several embodiments are now explained with reference to the appended drawings. Whenever aspects are not explicitly defined, the embodiments are not limited only to the parts shown, which are meant merely for the purpose of illustration. Also, while numerous details are set forth, it is understood that some embodiments may be practiced without these details. In other instances, well-known circuits, structures, and techniques have not been shown in detail so as not to obscure the understanding of this description. 
     As used herein, the term “network” refers without limitation to any network configured to transfer data as groupings called packets. Packet networks can deliver streams of data (composed sequences of packets) to a community of devices. During transfer, packets are buffered and queued, and may experience variable delays and throughput depending on the traffic load in the network. As used herein, the term “master” or “upstream” node refers to a device or interface configured to packetize information for transfer via a packet-based network. The terms “slave” or “downstream” node refers to a device or interface configured to extract information from a packet. A “node” refers to a device which receives packets, and forwards the packets to another device. The term “timestamp” refers to any indication (sequence of characters or encoded information) of when a certain event occurred as determined by a clock of a node. These definitions are not considered to be limiting and are made only to clarify various aspects discussed herein. 
     The inventors herein have recognized that various characteristics of clocks of network nodes and links between the network nodes may impact an amount of noise included in a timestamp and the accuracy of time synchronization. For instance, nodes may be connected to a network using different types of links, such as a wired link (e.g., Ethernet) or a wireless link. In addition, each type of link may have different accuracy metrics for performance of time synchronization. Also effecting accuracy of time synchronization is the type of timestamping used by a node to generate a timestamp, such as a hardware-implemented process or a software-implemented process. Accordingly, timestamps received by a node may be reliable or unreliable depending on the type of link used by an upstream node to send the time synchronization message and timestamp and depending on the type of timestamping used by the upstream node. 
     An embodiment herein addresses the foregoing by performing time synchronization between nodes in a network with greater accuracy, even in situations where nodes are connected by different types of links and use different types of timestamping, by determining reliability of a received timestamp. Reliability of the timestamp may be determined based on several factors, including information regarding the accuracy of a clock in the upstream node (which may be provided by a manufacturer of the clock), a type of timestamping used by the upstream node (e.g., hardware or software), a type of link used by the upstream node to send the time synchronization message and the timestamp (e.g., wired or wireless) and clock drift parameters (e.g., crystal performance relative to temperature of the crystal). This quality information may be shared between the nodes in a time synchronization message or in a separate quality message. 
     In one embodiment, the quality information is received by a node, and the node generates an accumulated chain of quality information by adding its own local quality information to the received quality information and sends the accumulated quality information to a downstream node. In another embodiment, the quality information is received by a node and processed by the node to generate an adjusted timestamp and adjusted chain of quality information to send to a downstream node. 
     Based on the chain of quality information, a node determines whether a received timestamp is reliable or unreliable. For example, if the chain of quality information indicates that the type of timestamping used by a master node is hardware timestamping and that the type of link being used by the master node to send a time synchronization message including a timestamp is a wired link, a slave node determines that the master clock is reliable and that timestamps generated by the master clock are reliable (e.g., accurate within nanoseconds). In this case, the node determines parameters for a filter that is applied to a sequence of received timestamps such that the filtering is not aggressive. On the other hand, if the chain of quality information indicates that the type of timestamping used by the master node is software timestamping and that the type of link being used by the master node to send a timestamp included in a time synchronization message is a wireless link, the slave node determines that the master clock is unreliable and that timestamps generated by the master clock are unreliable (e.g. inaccurate by tens of milliseconds). In this case, the node determines parameters for a filter that is applied to a sequence of received timestamps such that the filtering is aggressive. 
     By virtue of the arrangements described herein, it is possible to determine reliability of a received timestamp by extracting chain of quality information of upstream nodes, thereby improving accuracy of time synchronization. 
       FIG. 1  is a representational view illustrating interconnected nodes in a first example network according to an embodiment herein. In the illustrated embodiment, the network  100  includes a master node  110  in communication with a number of slave nodes  120  (individually slave nodes  120   a ,  120   b ,  120   c ,  120   d ,  120   e  . . .  120   n ) through links  130  (individually links  130   a ,  130   b ,  130   c ,  130   d ,  130   e  . . .  130   n ), respectively. Nodes  110  and  120  are, for example, servers, computers (desktop, laptop, handheld, etc.), routers, firewalls, gateways, network and personal media devices, electronic devices and mobile devices, etc. 
     Each of nodes  110  and  120  is generally a time-aware system including its own local clock source, and each of slave nodes  120  is capable of synching its own local clock with the clock of a node that has been designated as a master node (such as master node  110 .) Each of nodes  110  and  120  generates a timestamp using its clock and a type of timestamping process, such as a hardware-implemented process or a software-implemented process. For example, with respect to hardware-implemented timestamping, when a message departs from or arrives at a node, special hardware generates a timestamp from the local clock. With respect to software-implemented timestamping, when a message reaches the application layer of a node, a processor executes a software program or computer-executable method stored in a memory in order to generate a timestamp based on the local clock. Generally, a timestamp generated by a hardware-implemented process is more accurate than a timestamp generated by a software-implemented process. 
     Links  130  are of a wired type (e.g., Ethernet) or a wireless type, and each type of link between master node  110  and slave nodes  120  has different accuracy metrics for performance of time synchronization. For example, a timestamp provided in a time synchronization message over a wired link type is typically more accurate than a timestamp provided in a time synchronization message over a wireless link type. 
     Using links  130 , master node  110  sends a time synchronization message to slave nodes  120  including a timestamp generated by the clock source of master node  110 . In addition, master node  110  generates and sends chain of quality information to slave nodes  120 . The chain of quality information may be included in the time synchronization message, or alternatively may be provided in a separate quality message at a same or different timing. The chain of quality information may indicate, among other things, information regarding the accuracy of the clock of master node  110 , a type of timestamping used by the master node  110  (e.g., hardware or software), a type of link used by master node  110  to send the time synchronization message including the timestamp (e.g., wired or wireless) and clock drift parameters of the clock of master node  110  (e.g., crystal performance relative to temperature of the crystal). This process is described in more detail below in connection with  FIG. 3 . 
       FIG. 2  is a representational view illustrating interconnected nodes in a second example network according to an embodiment herein. In the illustrated embodiment, the network  200  includes master node  210  in communication with intermediate node  215  through link  230 , and slave node  220  in communication with intermediate node  215  through link  235 . The network elements (nodes, links) illustrated in  FIG. 2  are similar to the network elements illustrated in  FIG. 1 ; however, the elements have a different topological arrangement in which an intermediate node ( 215 ) is provided between a master node ( 210 ) and a slave node ( 220 ). Accordingly, similar to the embodiment of  FIG. 1 , nodes  210 ,  215  and  220  are, for example, servers, computers (desktop, laptop, handheld, etc.), routers, firewalls, gateways, network and personal media devices, electronic devices and mobile devices, etc. 
     Also similar to  FIG. 1 , each of nodes  210 ,  215  and  220  is generally a time-aware system including its own local clock source, and nodes  215  and  220  are capable of synching their own local clocks with the clock of a node that has been designated as a master node (such as master node  210 .) Each of nodes  210 ,  215  and  220  generates a timestamp using a type of timestamping process, such as a hardware-implemented process or a software-implemented process. As in  FIG. 1 , with respect to hardware-implemented timestamping, when a message departs from or arrives at a node, special hardware generates a timestamp from the local clock. With respect to software-implemented timestamping, when a message reaches the application layer of a node, a processor executes a software program or computer-executable method stored in a memory in order to generate a timestamp based on the local clock. Generally, a timestamp generated by a hardware-implemented process is more accurate than a timestamp generated by a software-implemented process. 
     Links  230  and  235  are of a wired type (e.g., Ethernet) or a wireless type, and each type of link  230 ,  235  has different accuracy metrics for performance of time synchronization. As in  FIG. 1 , a timestamp provided over a wired link type is typically more accurate than a timestamp provided over a wireless link type. 
     Using link  230 , master node  210  sends a time synchronization message to intermediate node  215  including a timestamp generated by the clock source of master node  210 . In addition, master node  210  sends chain of quality information to intermediate node  215  using link  230 . The chain of quality information may indicate, among other things, information regarding the accuracy of the clock of master node  210 , a type of timestamping used by the master node  210  (e.g., hardware or software), a type of link used by master node  210  to send the time synchronization message including the timestamp (e.g., wired or wireless) and clock drift parameters of the clock of master node  210  (e.g., crystal performance relative to temperature of the crystal). 
     Using link  235 , intermediate node  215  sends a time synchronization message to slave node  220 . In addition, intermediate node  215  sends chain of quality information slave node  220 .  FIG. 4  illustrates an embodiment in which intermediate node  215  generates and sends accumulated chain of quality information, and  FIG. 5  illustrates an embodiment in which intermediate node  215  generates and sends adjusted chain of quality information. 
     The chain of quality information sent by master node  210  and intermediate node  215  may be included in the time synchronization message, or alternatively may be provided in a separate quality message at a same or different timing. 
     Although  FIGS. 1 and 2  illustrate two example network configurations, it will be understood that the disclosure herein relates to any configuration of networks, including point-to-point networks, networks connected by a bus, star networks, ring networks, mesh networks, hybrid networks, and daisy chain networks. 
     In addition, a network may have any number of nodes. For example, the network configuration of  FIG. 2  may have one or more additional intermediate nodes in addition to intermediate node  215 , provided in between master node  210  and slave node  220 . In addition, the network configuration of  FIG. 1  may have any number of intermediate nodes provided between master node  110  and slave nodes  120 . 
       FIG. 3  is a flow diagram illustrating a time synchronization process between a slave node and a master node according to an embodiment herein. In this regard, the following embodiments may be described as a process  300  and a process  350 , which are usually depicted as a flowchart, a flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed. A process may correspond to a method, a procedure, etc. Processes  300  and  350  may be performed by processing logic that includes hardware (e.g. circuitry, dedicated logic, etc.), software (e.g., embodied on a non-transitory computer readable medium), or a combination thereof. 
     In one example embodiment, process  300  is executed by master node  110  and process  350  is executed by slave nodes  120 . In this regard, although processes  300  and  350  of  FIG. 3  are described in connection with the network configuration illustrated in  FIG. 1 , it should be understood that these processes may be applied to other network configurations, including without limitation to network  200  of  FIG. 2 . 
     Referring to  FIG. 3 , at block  301  master node  110  generates a timestamp using its clock. The timestamp is included in a time synchronization message also generated by master node  110 . At block  302 , master node  110  generates a chain of quality information including a type of timestamping used by the master node  110  (e.g., hardware or software), a type of link used by master node  110  to send the time synchronization message including the timestamp (e.g., wired or wireless) and clock drift parameters of the clock of master node  110 . Clock drift parameters may include information about how resistant the master clock is to drifting, or performance of the master clock&#39;s crystal relative to temperature of the crystal, and may be provided by a manufacturer of the clock. In cases where clock drift parameters are included in the chain of quality information, information indicating a temperature of the master clock may also be included. The chain of quality information may also include any other information regarding the accuracy of the clock of master node  110 . 
     At block  303 , master node  110  sends to slave nodes  120  the time synchronization message including the timestamp and the chain of quality information. Accordingly, in the embodiment of  FIG. 3 , the chain of quality information is included in the time synchronization message. However, in other embodiments, the chain of quality information may be provided in a separate quality message at a same or different timing than the time synchronization message. In this way, the chain of quality information need not be provided with the transmission of every time synchronization message, but may be provided one time or several times during a time synchronization session or process. For example, the chain of quality information may be provided during a session at any time characteristics relating to the chain of quality information change. 
     At block  351 , slave nodes  120  receive the time synchronization message including the timestamp and the chain of quality information from master node  110 . In embodiments in which the chain of quality information is provided separately, the chain of quality information is received separately from the time synchronization message. At block  352 , slave nodes  120  calculates parameters for a filter that is applied to a sequence of timestamps received from master node  110  based on the timestamp and the chain of quality information received from master node  110 . The filter may be applied over a time period (e.g. 10 seconds) to smooth the values of the received timestamps, and the parameters may be weights indicating how aggressive the filter is (e.g., an amount of filtering, attenuation or smoothing resulting from the filter). 
     In this regard, when synchronizing its own clock with that of master node  110 , each of slave nodes  120  relates the timestamp received by master node  110  to a local timestamp generated by the local clock of the slave node indicating the time of receipt of the timestamp from master node  110 , for instance by calculating a ratio. Typically, master node  110  sends time synchronization messages including a timestamp at some time interval (e.g., 100 milliseconds). Slave node  120  receives the series of timestamps from master node  110  over some time period and stores the timestamps in a memory or buffer. Since the timestamps typically include some amount of noise, slave node  120  filters the sequence of timestamps in order to estimate the correct relationship between the local clock and the master clock. This relationship between the local clock and the master clock allows slave node  120  to adjust its own clock to generate timestamps that are closer to being in synchronization with the master clock. In other embodiments, slave node  120  processes the sequence of timestamps in order to calculate a correction value or an offset for the received timestamp. The relationship between the slave clock and the master clock and/or the correction values may be stored by slave nodes  120  in a look up table, or as a function representing the relationship. 
     When calculating parameters for the filter, slave nodes  120  determine whether a received timestamp is reliable, since the appropriate amount of filtering depends on the reliability of the received timestamp. Generally, if it is determined from the chain of quality information that the clock generating a received timestamp is reliable (e.g., a clock generated by a hardware-implementation) and that the link over which the timestamp was sent is reliable (e.g., a wired link), slave nodes  120  determine that the timestamp is reliable. In these cases, less filtering is needed to perform accurate time synchronization. Also, slave nodes  120  may decrease the number of timestamps received from master node  110  for filtering, since fewer timestamps may be needed to accurately estimate the master clock. For example, slave nodes  120  may shorten the time period over which timestamps are received by master node  110 , thereby decreasing the number of timestamps received. 
     On the other hand, if it is determined from the chain of quality information that the clock generating a received timestamp is unreliable (e.g., a clock generated by a software-implementation) and that the link over which the timestamp is provided is unreliable (e.g., a wireless link), and that the timestamp is therefore unreliable, more filtering is needed to perform accurate time synchronization. In these cases, slave nodes  120  may increase the number of timestamps received from master node  110  for filtering, since more timestamps may be needed to accurately estimate the master clock. For example, slave nodes  120  may increase the time period over which timestamps are received by master node  110 , thereby increasing the number of timestamps received. 
     Accordingly, if the chain of quality information indicates that master node  110  generates timestamps using hardware-implemented timestamping and sends time synchronization messages over an Ethernet link, less filtering is generally needed for accurate time synchronization. On the other hand, if the chain of quality information indicates that master node  110  generates timestamps using software-implemented timestamping and sends time synchronization messages over a wireless link, more filtering is needed to get a similar level of accuracy for time synchronization. 
     In cases where the chain of quality information includes clock drift parameters indicating that the master clock is drifting (e.g., due to temperature), slave nodes  120  may decrease an amount of filtering applied to the sequence of timestamps. 
     As previously mentioned, the filter may smooth the received timestamps in order to estimate the relationship between the local clock and the master clock. In some embodiments, the filter is a low pass filter that passes timestamps having a value lower than a predetermined threshold and attenuates timestamps having values higher than the predetermined threshold, where the amount of attenuation depends on the parameters calculated by slave nodes  120  based on the chain of quality information received from master node  110 . In this regard, timestamps having values higher than the predetermined threshold are reduced in amplitude by an amount determined by the parameters of the filter. 
     In other embodiments, the filter is a median filter that replaces each timestamp with the median of neighboring timestamps. A median filter is particularly useful in situations where received timestamps are unreliable such that they are outliers when determining the relationship between the slave clock and the master clock. 
     After parameters for the filter are determined and applied to a sequence of timestamps at block  352 , slave nodes  120  may adjust their own clocks to generate timestamps that are closer to being in synchronization with the master clock. In one embodiment, slave nodes  120  then apply these adjusted timestamps to extract digital information that may be included in an audio or video signal that is being streamed from one node of the network to another and is played back in real time on a slave node. As one example, slave nodes  120  apply the adjusted timestamps to extract digital information that is in a bitstream received from master node  110 . In other examples, any node of the network may be a source of the bitstream. Since slave nodes  120  use timestamps that have been adjusted according to process  300 , it is possible to render the audio or video signal to produce smooth, uninterrupted content. 
       FIG. 4  is a flow chart illustrating time synchronization between a slave node and a master node having an intermediate node therebetween according to an embodiment herein in which chain of quality information is accumulated by the intermediate node. In this regard, the following embodiments may be described as a processes  400 ,  420  and  440 , which are usually depicted as flowcharts, flow diagrams, structure diagrams, or block diagrams. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed. A process may correspond to a method, a procedure, etc. Processes  400 ,  420  and  440  may be performed by processing logic that includes hardware (e.g. circuitry, dedicated logic, etc.), software (e.g., embodied on a non-transitory computer readable medium), or a combination thereof. 
     In one example embodiment, process  400  is executed by master node  210 , process  420  is executed by intermediate node  215  and process  440  is executed by slave node  220 . In this regard, although processes  400 ,  420  and  440  of  FIG. 4  are described in connection with the network configuration illustrated in  FIG. 2 , it should be understood that these processes may be applied to other network configurations, including without limitation to network  100  of  FIG. 1 . 
     Referring to  FIG. 4 , at block  401  master node  210  generates a timestamp using its clock. The timestamp is included in a time synchronization message also generated by master node  210 . At block  402 , master node  210  generates a chain of quality information including a type of timestamping used by the master node  210  (e.g., hardware or software), a type of link used by master node  210  to send the time synchronization message including the timestamp (e.g., wired or wireless) and clock drift parameters of the clock of master node  210 . Clock drift parameters may include information about how resistant the master clock is to drifting, or performance of the master clock&#39;s crystal relative to temperature of the crystal, and may be provided by a manufacturer of the clock. In cases where clock drift parameters are included in the chain of quality information, information indicating a temperature of the master clock may also be included. The chain of quality information may also include any other information regarding the accuracy of the clock of master node  210 . 
     At block  403 , master node  210  sends to intermediate node  215  the time synchronization message including the timestamp and the chain of quality information. Accordingly, in the embodiment of  FIG. 4 , the chain of quality information is included in the time synchronization message. However, in other embodiments, the chain of quality information may be provided in a separate quality message at a same or different timing than the time synchronization message. In this way, the chain of quality information need not be provided with the transmission of every time synchronization message, but may be provided one time or several times during a time synchronization session or process. For example, the chain of quality information may be provided during a session at any time there is a change in characteristics relating to the chain of quality information. 
     At block  421 , intermediate node  215  receives the time synchronization message including the timestamp and the chain of quality information from master node  210 . In embodiments in which the chain of quality information is provided separately, the chain of quality information is received separately from the time synchronization message. At block  422 , intermediate node  215  generates a local timestamp using its clock, and at block  423 , intermediate node  215  generates local chain of quality information. This local chain of quality information includes a type of timestamping used by the intermediate node  215  (e.g., hardware or software), a type of link used by intermediate node  215  to send the time synchronization message including the timestamp (e.g., wired or wireless) and clock drift parameters of the clock of intermediate node  215 . Clock drift parameters may include information about how resistant the local clock is to drifting, or performance of the clock&#39;s crystal relative to temperature of the crystal, and may be provided by a manufacturer of the clock. In cases where clock drift parameters are included in the chain of quality information, information indicating a temperature of the clock may also be included. The chain of quality information may also include any other information regarding the accuracy of the clock of intermediate node  215 . 
     At block  424 , intermediate node  215  generates accumulated chain of quality information by adding or appending its own local chain of quality information to the chain of quality information received from master node  210 . In addition, intermediate node  215  includes its own local timestamp with the local chain of quality information. In embodiments where master node  210  provides the chain of quality information in the time synchronization message, the local chain of quality information and the local timestamp are also stored in the time synchronization message. In embodiments where the master node  210  provides the chain of quality information separate from the time synchronization message, the local chain of quality information and the local timestamp are included with the quality message or in a separate quality message. 
     Accordingly, in the embodiment of  FIG. 4 , intermediate node  215  does not process the chain of quality information received from master node  210  and instead accumulates the chain of quality information of the master node  210  with its own local chain of quality information to send to slave node  220  at block  425 . In addition, at block  425 , intermediate node  215  sends the timestamp received from master node  210  and the local timestamp generated at block  422 . 
     At block  441 , slave node  220  receives timestamps from all of the nodes in its upstream path, namely master node  210  and intermediate node  215 . At block  441 , slave node  220  also receives chain of quality information accumulated from all of the nodes in its upstream path, including local chain of quality information from intermediate node  215  and the chain of quality information received from master node  210 . 
     In embodiments where there are additional intermediate nodes in between master node  210  and slave node  220  further to intermediate node  215 , slave node  220  receives timestamps and chain of quality information accumulated from all of the nodes in its upstream path, including any of the additional intermediate nodes in the network  200 . 
     At block  442 , slave node  220  calculates parameters for a filter that is applied to the timestamps received in block  441  based on the received timestamps and the accumulated chain of quality information. The process of block  442  is similar to the process of block  352  of  FIG. 3 , and the details thereof will therefore not be discussed again here. 
     The embodiment of  FIG. 4  is especially advantageous in cases where slave node  220  does not have strict time synchronization requirements, since the resources of intermediate node  215  may be saved by not having to process received chain of quality information. As one example, master node  210  may be a desktop computer connected by an Ethernet link  230  to intermediate node  215 , and the intermediate node  215  may be a laptop computer connected by a wireless link to slave node  220  which may be an external electronic device. In this example, slave node  220  wants to synchronize its own local clock with the clock of master node  210  but also does not have strict time synchronization requirements. When master node  210  sends a time synchronization message including a timestamp to slave node  220  through intermediate node  215 , the quality of the timestamp received by the intermediate node  215  may be high, since master node  210  uses Ethernet link  230 . However, the quality of the timestamp received by the slave node  220  may be low, since intermediate node  215  uses wireless link  235 . If slave node  220  does not have strict time synchronization requirements, it would be inefficient for intermediate node  215  to process the timestamp and chain of quality information received by master node  210 , since slave node  220  does not need to receive a highly accurate timestamp. Accordingly, intermediate node  215  sends accumulated chain of quality information instead of processing the chain of quality information received from master node  210 . 
       FIG. 5  is a flow chart illustrating time synchronization between a slave node and a master node having an intermediate node therebetween according to an embodiment herein in which chain of quality information is adjusted by the intermediate node. In this regard, the following embodiments may be described as a processes  500 ,  520  and  540 , which are usually depicted as flowcharts, flow diagrams, structure diagrams, or block diagrams. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed. A process may correspond to a method, a procedure, etc. Processes  500 ,  520  and  540  may be performed by processing logic that includes hardware (e.g. circuitry, dedicated logic, etc.), software (e.g., embodied on a non-transitory computer readable medium), or a combination thereof. 
     In one example embodiment, process  500  is executed by master node  210 , process  520  is executed by intermediate node  215  and process  540  is executed by slave node  220 . In this regard, although processes  500 ,  520  and  540  of  FIG. 4  are described in connection with the network configuration illustrated in  FIG. 2 , it should be understood that these processes may be applied to other network configurations, including without limitation to network  100  of  FIG. 1 . 
     Referring to  FIG. 5 , at block  501  master node  210  generates a timestamp using its clock. The timestamp is included in a time synchronization message also generated by master node  210 . At block  502 , master node  210  generates a chain of quality information including a type of timestamping used by the master node  210  (e.g., hardware or software), a type of link used by master node  210  to send the time synchronization message including the timestamp (e.g., wired or wireless) and clock drift parameters of the clock of master node  210 . Clock drift parameters may include information about how resistant the master clock is to drifting, or performance of the master clock&#39;s crystal relative to temperature of the crystal, and may be provided by a manufacturer of the clock. In cases where clock drift parameters are included in the chain of quality information, information indicating a temperature of the master clock may also be included. The chain of quality information may also include any other information regarding the accuracy of the clock of master node  210 . 
     At block  503 , master node  210  sends to intermediate node  215  the time synchronization message including the timestamp and the chain of quality information. Accordingly, in the embodiment of  FIG. 5 , the chain of quality information is included in the time synchronization message. However, in other embodiments, the chain of quality information may be provided in a separate quality message at a same or different timing than the time synchronization message. In this way, the chain of quality information need not be provided with the transmission of every time synchronization message, but may be provided one time or several times during a time synchronization session or process. For example, the chain of quality information may be provided during a session at any time characteristics relating to the chain of quality information change. 
     At block  521 , intermediate node  215  receives the time synchronization message including the timestamp and the chain of quality information from master node  210 . In embodiments in which the chain of quality information is provided separately, the chain of quality information is received separately from the time synchronization message. At block  522 , intermediate node  215  generates an adjusted timestamp and an adjusted chain of quality information based on the timestamp and the chain of quality information received from master node  210 , as well as a local timestamp generated using the clock of intermediate node  215  and local chain of quality information. This local chain of quality information includes a type of timestamping used by the intermediate node  215  (e.g., hardware or software), a type of link used by intermediate node  215  to send the time synchronization message including the timestamp (e.g., wired or wireless) and clock drift parameters of the clock of intermediate node  215 . Clock drift parameters may include information about how resistant the local clock is to drifting, or performance of the clock&#39;s crystal relative to temperature of the crystal, and may be provided by a manufacturer of the clock. In cases where clock drift parameters are included in the chain of quality information, information indicating a temperature of the clock may also be included. The chain of quality information may also include any other information regarding the accuracy of the clock of intermediate node  215 . 
     With respect to the adjusted timestamp, intermediate node  215  performs a process similar to that at block  442  of  FIG. 4  and block  352  of  FIG. 3 , in which parameters are determined for a filter that is applied to a sequence of received timestamps. In this way, intermediate node  215  processes the timestamp and the chain of quality information received from master node  210  to generate an adjusted timestamp that is more accurate with respect to the master clock. This adjusted timestamp is provided to slave node  220  at block  523 . 
     With respect to the adjusted chain of quality information, intermediate node  215  processes the chain of quality information received from master node  210  and aggregates the information with its own local chain of quality information to re-compute an adjusted chain of quality information. For instance, master node  210  may be a desktop computer connected by a wireless link  230  to intermediate node  215 , and the intermediate node  215  may be a laptop computer connected by a wired link to slave node  220  which may be an external electronic device. In this case, the chain of quality generated by master node  210  indicates that a link  230  between master node  210  and intermediate node  215  is unreliable and the local chain of quality information generated by intermediate node  215  indicates that the link  235  between intermediate node  215  and slave node  220  is reliable. However, according to the embodiment of  FIG. 5 , intermediate node  215  aggregates its own local chain of quality information (indicating reliability) with the master node&#39;s chain of quality information (indicating unreliability) to generate an adjusted chain of quality information indicating that the link quality is unreliable and sends the adjusted chain of quality information to slave node  220  at block  523 . 
     At block  541 , slave node  220  receives the adjusted timestamp and the adjusted chain of quality information from intermediate node  215 . In embodiments where there are additional intermediate nodes in between master node  210  and slave node  220  further to intermediate node  215 , each of the intermediate nodes performs the process  520  to generate an adjusted timestamp and adjusted chain of quality information. In these embodiments, slave node  220  receives an adjusted timestamp and adjusted chain of quality information from its neighboring upstream node (e.g. the intermediate node immediately upstream from slave node  220 ). 
     At block  542 , slave node  220  calculates parameters for a filter that is applied to the timestamps received in block  541  based on the adjusted timestamp and the adjusted chain of quality information. The process of block  542  is similar to the process of block  352  of  FIG. 3 , and the details thereof will therefore not be discussed again here. 
     The embodiment of  FIG. 5  is particularly advantageous in situations where network elements are connected over unreliable links such as wireless type links but synchronicity is desired. For example, in an automobile or in home entertainment system connected via a wireless network, it may be desirable for all network devices to have highly accurate time synchronization. 
       FIG. 6  is a representational view illustrating an example node  600  according to embodiments herein. Node  600  is an example of nodes  110 ,  120 ,  210 ,  215  and  220  used for implementing the techniques disclosed herein. Node  600  includes a processor  601 , which can include one or more processing devices. Examples of processor  601  include without limitation a microprocessor, an application-specific integrated circuit (ASIC), a state machine, or other suitable processing device. Processor  601  is communicatively coupled to a computer-readable storage medium, such as memory  604 , and accesses information stored in memory  604 , such as timestamps and chain of quality information. Memory  604  also stores computer-executable instructions that when executed by processor  601  cause the processor  601  to perform the operations described herein. Memory  604  may be, for example, solid-state memories, optical and magnetic media or any other non-transitory machine-readable medium. Non-limiting examples of memory  604  include a hard drive, compact disc, flash memory, non-volatile memory, volatile memory, magnetic disk(s), etc. Node  600  also includes a network interface  603  for communicating with other nodes of the network, and clock  602  for generating timestamps. As discussed above, clock  602  may be implemented by hardware or by software. 
       FIG. 6  is merely one example of a particular implementation and is merely to illustrate the types of components that may be present in a node. While the node  600  is illustrated with various components, it is not intended to represent any particular architecture or manner of interconnecting the components; as such details are not germane to the embodiments herein. It will also be appreciated that network computers, handheld computers, mobile phones, servers, and/or other data processing systems which have fewer components or perhaps more components may also be used with the embodiments herein. Accordingly, the processes described herein are not limited to use with the hardware and software of  FIG. 6 . 
       FIG. 7  illustrates a block diagram for explaining a bridge element according to an embodiment in which nodes of the network share time synchronization messages using clock synchronization transport protocols. While the bridge element  700  is illustrated with various components, it is not intended to represent any particular architecture or manner of interconnecting the components; as such details are not germane to the embodiments herein. It will also be appreciated that network computers, handheld computers, mobile phones, servers, and/or other data processing systems which have fewer components or perhaps more components may also be used with the embodiments herein. Accordingly, the processes described herein are not limited to use with the hardware and software of  FIG. 7 . 
     In the embodiment of  FIG. 7 , network elements (such as nodes  110 ,  120 ,  210 ,  215 ,  220 ) use different clock synchronization transport protocols, such as those according to IEEE 802.1AS and IEEE 1588. IEEE 802.1AS is defined for wired Ethernet and may define various different protocols to transport time synchronization messages (on a peer to peer basis or on a link by link basis, for example). IEEE 802.1AS is a profile of IEEE 1588 which also defines various different protocols to transport time synchronization messages (e.g., on a unicast basis or multicast basis). Precise Time Protocol (PTP) is specified in IEEE 1588 and may use user datagram protocol (UDP) as its transport protocol or Ethernet as its transport protocol. 
     In situations where network elements use different clock synchronization transport protocols to share time synchronization messages, it may be advantageous to bridge between the different types of clock synchronization transport protocols in order to facilitate accurate synchronization of time across the network elements. Referring to the example node illustrated in  FIG. 6 , a node  600  includes a local clock  602  and uses a clock synchronization transport protocol, such that the node has its own time domain. In a network (such as networks  100  and  200 ), each node may run on its own time domain. 
     By way of background, conventionally, in order to negotiate between different time domains of the network elements, a best master clock algorithm is typically used to first elect a “best” clock (sometimes called a “grandmaster”) to which all other clocks of the network synch. The best master clock algorithm used by each clock synchronization transport protocol may be different. As one example, each node or network element of the network (such as networks  100  and  200 ) may share an announce message including information regarding a local clock that is used to generate a value, and the values generated by the network elements are compared to find the lowest which is selected as the grandmaster. After the grandmaster is selected, time synchronization messages are passed at an application layer after processing is performed on the time synchronization messages to negotiate the time boundary between the two time domains. 
     In the embodiment of  FIG. 7 , bridge element  700  includes interconnect structure  710 , an application layer  730 , clock  740  for generating a clock synchronization signal that may be included in time synchronization messages shared across a network, ports  720   a  and  720   b  constructed to receive and send data (including a clock synchronization signal) according to a clock synchronization transport protocol and protocol translation ports  750   a  and  750   b  constructed to translate between clock synchronization transport protocols, and to receive and send data (including a clock synchronization signal) according to various clock synchronization transport protocols. Although the embodiment of  FIG. 7  shows two ports  720   a  and  720   b  and two protocol translation ports  750   a  and  750   b , any number of such ports may be included in other embodiments, including one or more than two. By virtue of the arrangement explained by  FIG. 7 , and particularly by using ports that are able to translate between clock synchronization transport protocols (such as IEEE 802.1AS and IEEE 1588), bridge element  700  may provide a time synchronization message including a clock synchronization signal from a node using a first clock synchronization transport protocol directly to a node using a second clock synchronization transport protocol. For example, bridge element  700  may send the clock synchronization signal from a node using IEEE 802.1AS directly to a node using IEEE 1588, and vice versa. Since bridge element  700  may translate between the clock synchronization transport protocols (such as IEEE 802.1AS and IEEE 1588), it is possible to share time synchronization messages using one time domain (i.e., without negotiating a second time domain) and accuracy is thereby improved. 
     For instance, in an embodiment in which the network (such as networks  100  and  200 ) have three network elements, it may be assumed that a first node uses IEEE 802.1AS as its clock synchronization transport protocol (see, for example,  FIG. 8  below) and that a second node uses IEEE 1588 as its clock synchronization transport protocol. Referring to  FIG. 7 , in the case that bridge element receives a clock synchronization signal from the first node using IEEE802.1AS, one of ports  750   a  and  750   b  of bridge element  700  receives the clock synchronization signal from the first node. Ports  750   a  and  750   b  may be, as one example, PortSyncs defined by IEEE 802.1AS and may share time synchronization messages with other network elements also using IEEE 802.1AS as their clock synchronization transport protocol. Interconnect structure  710 , which is, as one example, a SiteSync defined by IEEE 802.1AS, receives the clock synchronization signal from one of ports  750   a  and  750   b . One of protocol translation ports  750   a  and  750   b  receives the clock synchronization signal from the interconnect structure  710 , translates the clock synchronization signal between IEEE 802.1AS and IEEE 1588, and provides the translated clock synchronization signal to the second network element using IEEE 1588. Thus, in one example embodiment, protocol translation ports  750   a  and  750   b  share time synchronization messages with network elements using IEEE 1588 as their clock synchronization transport protocol. 
     On the other hand, in the case that bridge element receives a clock synchronization signal from the second node using IEEE 1588, one of protocol translation ports  750   a  and  750   b  receives the clock synchronization signal from the second node, translates the clock synchronization signal between IEEE 1588 and IEEE 802.1AS, and provides the translated clock synchronization signal to interconnect structure  710 . Interconnect structure  710  receives the translated clock synchronization signal from the ports  750   a  and  750   b  and provides it to one of ports  720   a  and  720   b . One of ports  720   a  and  720   b  receives the translated clock synchronization signal from interconnect structure  710  and provides the translated clock synchronization signal to the first node using IEEE 802.1AS. 
     There may also be in a case in which the bridge element generates the clock synchronization signal itself. In these cases, clock  740  generates a clock synchronization signal. Interconnect structure  710  receives the clock synchronization signal from the clock  740 . One of ports  720   a  and  720   b  receives the clock synchronization signal from the interconnect structure and provides the clock synchronization signal to the first node using IEEE 802.1AS. One of protocol translation ports  750   a  and  750   b  receives the clock synchronization signal from the interconnect structure  710 , translates the clock synchronization signal between IEEE 802.1AS and IEEE 1588, and provides the translated clock synchronization signal to the second network element using IEEE 1588. 
     Furthermore, according to one example embodiment, IEEE 1588 v2 (PTP) End to End synchronization is used as a mechanism for synchronizing disjointed IEEE 802.1AS (gPTP) domains. In this embodiment, the PTP E2E is advantageously integrated with the gPTP domain. By way of background, conventional approaches typically use PTP boundary clocks and have a time domain boundary between the PTP E2E clock and the gPTP domain. In contrast, according to this example embodiment, the PTP E2E link is used as another type of full duplex point to point port within the time aware system, thereby extending the gPTP domain across the IP link and creating one gPTP domain rather than 2 gPTP domains with 2 boundary clocks and time being derived across boundary clocks. This then allows for the use of the gPTP algorithms to share a common clock amongst all of these systems. In this embodiment, the network (for example, network  100  and  200 ) is capable of sending and receiving Ethernet and/or IP packets. This is supported on both wired and wireless networks. The embodiment may be part of the software implementation of the IEEE 802.1 standard. IEEE Standard 802.1AS (“IEEE Standard for Local and Metropolitan Area Networks-Timing and Synchronization for Time-Sensitive Applications in Bridged Local Area Networks”), which is incorporated herein by reference in its entirety, provides mechanisms for synchronizing frequency of clocks, and for measuring the propagation delay across a link. It also provides a NIC-to-bridge interface in bridged network, and NIC-to-NIC in a two-machine network. It is based on IEEE 1588, and requires ingress/egress time stamping in the MAC (i.e., Sync, PDelay Request, and PDelay Response frames time-stamped on egress and ingress). 
       FIG. 8  illustrates an example node or network element  800  using IEEE 802.1AS as its clock synchronization transport protocol. Node  800  includes local clock  810  and has a first time domain, an interconnect structure such as SiteSynch  840 , and ports such as PortSync  820   a  and  820   b . In a network (such as networks  100  and  200 ), a similar and separate structure may also be running independently for a different node using IEEE 1588 as its clock synchronization transport protocol and having a second time domain. According to conventional approaches, in a network including node  800  and a node using IEEE 1588 as its clock synchronization transport protocol (not shown), the time synchronization messages are passed at the time-aware higher-layer application level  830  after processing is performed on the time synchronization messages to negotiate the time boundary between the two time domains. 
     The processes and blocks described herein are not limited to the specific examples described and are not limited to the specific orders used as examples herein. Rather, any of the processing blocks may be re-ordered, combined or removed, performed in parallel or in serial, as necessary, to achieve the results set forth above. The processing blocks associated with implementing the structures and processes disclosed herein may be performed by one or more programmable processors executing one or more computer programs stored on a non-transitory computer readable storage medium to perform the functions of the system. All or part of the network may be implemented as, special purpose logic circuitry (e.g., an FPGA (field-programmable gate array) and/or an ASIC (application-specific integrated circuit)). All or part of the network may be implemented using electronic hardware circuitry that include electronic devices such as, for example, at least one of a processor, a memory, a programmable logic device or a logic gate. Further, processes can be implemented in any combination hardware devices and software components. 
     While certain embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive, and the embodiments are not limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those of ordinary skill in the art. The description is thus to be regarded as illustrative instead of limiting.