Patent Publication Number: US-7916758-B2

Title: Method and system for precise-clock synchronization, and device for precise-clock frequency/time synchronization

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/582,002, filed on Oct. 20, 2009, which claims priority to Chinese Patent Application No. 200810224615.7, filed on Oct. 21, 2008, both of which are hereby incorporated by reference in their entireties. 
    
    
     FIELD OF THE TECHNOLOGY 
     The present invention relates to the field of communication technology, and more particularly, to a method and system for precise-clock synchronization and a device for precise-clock frequency/time synchronization. 
     BACKGROUND 
     Internet Protocol (IP)-based communication networks have not reduced demands for clock synchronization. On the contrary, for demands of cost, safety, and services, the IP clock synchronization becomes more and more important. Among the various existing IP clock technologies, such as circuit emulation services over packet switched (CESoP), timing over packet (ToP), network time protocol (NTP), the ToP technology is a relatively new technology. For the IP clock technology, the Institute for Electrical and Electronics Engineers (IEEE) also introduced an IEEE 1588 precision time protocol (PTP) standard. The IEEE 1588 PTP not only inherits the characteristics of performing frequency synchronization on master and slave clocks in the technology above, but also implements time synchronization, that is, phase synchronization. 
     However, in the IEEE 1588 PTP, it is necessary to collect all time stamps transmitted between the master clock and the slave clock. The time stamps are stamped by the time stamp engines located at the master clock side and the slave clock side. The time stamps mainly include transmitting time and receiving time of information such as a synchronous packet (Sync packet), follow up information (Follow_Up information), delay request (Delay_Req) packet, and a delay response (Delay_Resp) packet. The frequency synchronization adjustment and time synchronization adjustment of the master clock and the slave clock can be performed only after collecting sufficient information of time stamps between the master clock and the slave clock. 
     In the process of implementing the present invention, the inventor finds that there are at least the following disadvantages in the conventional art. Too much link bandwidth resources are occupied in the aforesaid process. Especially, when the network transmission conditions are deteriorated and the frequency of transmitting packets needs to be increased for frequency synchronization, such a disadvantage that the clock synchronization occupies too much link bandwidth resources becomes more apparent. In addition, when only one time stamp engine exists at the slave clock side, the time of stamping a time stamp at a local position of the slave clock has to be different from the time of the local frequency synchronization; otherwise, due to the local frequency adjustment, the value of the local time stamp is influenced. Therefore, correct calculation is not able to be performed according to the obtained time stamps, and the frequency synchronization/time synchronization is thus influenced. 
     SUMMARY 
     Embodiments of the present invention provide a precise-clock synchronization method and system and a precise-clock frequency/time synchronization device, so that the frequency synchronization and time synchronization of the clocks do not interfere with each other, and the occupied link bandwidth resources are greatly reduced. 
     In an embodiment, the present invention provides a precise-clock synchronization method, which includes the following steps. 
     A time stamp is obtained. The time stamp includes at least an original time stamp about the time when a master clock side transmits each synchronous packet (Sync packet) and a relative arrival time stamp about the time that a slave clock side receives each Sync packet. 
     It is determined whether a clock frequency at the slave clock side and a clock frequency at the master clock side are synchronous or not according to each original time stamp and the corresponding relative arrival time stamp. 
     If the clock frequency at the slave clock side and the clock frequency at the master clock side are asynchronous, the clock frequency at the slave clock side is adjusted to become synchronous with the clock frequency at the master clock side. 
     In another embodiment, the present invention provides a precise-clock frequency synchronization device. The precise-clock frequency synchronization device includes a relative time stamp engine, a first obtaining module, a determining module, and a frequency adjusting module. 
     The relative time stamp engine is adapted to provide a relative arrival time stamp about the time when a slave clock side receives a Sync packet. 
     The first obtaining module is adapted to obtain a time stamp. The time stamp includes at least an original time stamp about the time when the master clock side transmits each Sync packet and a relative arrival time stamp about the time when the slave clock side receives each Sync packet. 
     The determining module is adapted to determine whether a clock frequency at the slave clock side and a clock frequency at the master clock side are synchronous or not according to each original time stamp and its corresponding relative arrival time stamp. 
     The frequency adjusting module is adapted to adjust the clock frequency at the slave clock side to be synchronous with the clock frequency at the master clock side if the clock frequency at the slave clock side and the clock frequency at the master clock side are asynchronous. 
     In another embodiment, the present invention provides a precise-clock time synchronization device. The precise-clock time synchronization device includes an absolute time stamp engine, a second obtaining module, a second calculating module, and a time adjusting module. 
     The absolute time stamp engine is adapted to provide an absolute arrival time stamp about the time when a slave clock side receives a Sync packet and a delay request packet (Delay_Req) time stamp about the time when the slave clock side transmits a Delay_Req. 
     The second obtaining module is adapted to obtain a time stamp including an original time stamp and an absolute arrival time stamp of the Sync packet transmitted by the master clock side, a Delay_Resp time stamp transmitted by the master clock side, and a Delay_Req time stamp transmitted by the slave clock side. 
     The second calculating module is adapted to calculate a time offset according to the obtained original time stamp, absolute arrival time stamp, Delay_Req time stamp, and Delay_Resp time stamp. 
     The time adjusting module is adapted to adjust the clock time at the slave clock side to be synchronous with the clock time at the master clock side by using the time offset. 
     In another embodiment, the present invention provides a precise-clock synchronization system. The precise-clock synchronization system includes a precise-clock frequency synchronization device and a precise-clock time synchronization device. 
     The precise-clock frequency synchronization device is adapted to adjust a clock frequency at a slave clock side to be synchronous with a clock frequency at the master clock side according to an original time stamp about the time when the master clock side transmits a Sync packet and a relative arrival time stamp about the time when the slave clock side receives the Sync packet. 
     The precise-clock time synchronization device is adapted to calculate a time offset according to the original time stamp of the Sync packet and an absolute arrival time stamp about the time when the slave clock side receives the Sync packet, as well as a Delay_Req time stamp and Delay_Resp time stamp, and to adjust the clock time at the slave clock side to be synchronous with the clock time at the master clock side by using the time offset. 
     As can be seen from the technical solution above, in the embodiments of the present invention, the precise-clock synchronization method and system and the precise-clock frequency/time synchronization device may accomplish frequency synchronization and time synchronization of the master clock and the slave clock independently by using time stamps provided by two different time stamp engines, so that the frequency synchronization and time synchronization of the clocks do not interfere with each other, thus greatly reducing the occupied link bandwidth resources. 
     The present invention is further illustrated in detail below through specific embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To illustrate the technical solutions according to the embodiments of the present disclosure more clearly, the accompanying figures for describing the embodiments are introduced briefly in the following. Apparently, the accompanying drawings in the following description are only some embodiments of the present disclosure; persons of ordinary skill in the art can derive other drawings according to the accompanying drawings without paying any creative efforts. 
         FIG. 1  is a schematic structural view of a synchronization message exchange scheme of the IEEE 1588 PTP; 
         FIG. 2  is a schematic flow chart of a common precise-clock synchronization method; 
         FIG. 3  is a schematic flow chart of a precise-clock synchronization method according to a first embodiment of the present invention; 
         FIG. 4  is a schematic flow chart of a precise-clock synchronization method according to a second embodiment of the present invention; 
         FIG. 5  is another schematic flow chart of the precise-clock synchronization method according to the second embodiment of the present invention; 
         FIG. 6  is a schematic flow chart of a precise-clock synchronization method according to a third embodiment of the present invention; 
         FIG. 7  is a schematic flow chart of a precise-clock synchronization method according to a fourth embodiment of the present invention; 
         FIG. 8  is a schematic structural view of a precise-clock frequency synchronization device according to an embodiment of the present invention; 
         FIG. 9  is a schematic structural view of a precise-clock time synchronization device according to an embodiment of the present invention; and 
         FIG. 10  is a schematic structural view of a precise-clock synchronization system according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The technical solutions in the embodiments of the present invention are described clearly below with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are only a part of embodiments of the present invention, rather than all the embodiments. On the basis of the embodiments of the present invention, all the other embodiments obtained by those of ordinary skill in the art without carrying out creative activities fall within the protection scope of the present invention. 
       FIG. 1  is a schematic structural view of a synchronization message exchange scheme of the IEEE 1588 PTP. Referring to  FIG. 1 , the simple descriptions are provided as follows. 
     First, a master clock transmits a certain synchronous packet (Sync packet) periodically, usually once every two seconds. The Sync packet includes a time stamp precisely describing an estimated time of transmitting a data packet. A slave clock measures an accurate receiving time t 2  for receiving the Sync packet. As the Sync packet includes the estimated time of transmitting the data packet instead of a practical time, the master clock further transmits a piece of Follow_Up information after transmitting the Sync packet. The Follow_Up information is added with a time stamp accurately recording a practical time t 1  of transmitting the Sync packet. In such a manner, the slave clock obtains the time stamps t 1  and t 2  after receiving the Follow_Up information. 
     Subsequently, the slave clock transmits a delay request packet (Delay_Req) at a time t 3  after the Sync packet is received. After the master clock receives the Delay_Req, the master clock carries a time stamp including an accurate receiving time t 4  in a delay response packet (Delay_Resp) and transmits the Delay_Resp to the slave clock. In this way, the slave clock obtains the time stamps t 1 , t 2 , and t 3  when transmitting the Delay_Req, and obtains time stamps t 1 , t 2 , t 3 , and t 4  when receiving the Delay_Resp. 
     The slave clock calculates a time offset between the master clock and the slave clock, and a delay of the transmission link by using the collected t 1 , t 2 , t 3 , and t 4 . The time offset between the master clock and the slave clock is a value of “Δ” in  FIG. 1 , and is used to modify a time difference between the master clock and the slave clock. The delay of the transmission link is a value of “Delay” in  FIG. 1 , and is used to measure a delay time caused by the network transmission. The slave clock utilizes a clock restoration algorithm through the two values, so that the master clock and the slave clock become synchronous in time and frequency. 
     It should be noted that, the Delay_Req is transmitted randomly, and has no time limits. Also, in the whole process of measurement, it is assumed that the transmission medium is uniform and symmetric. 
       FIG. 2  is a schematic flow chart of a common precise-clock synchronization method. Referring to  FIG. 2 , when the exchange of synchronization messages in  FIG. 1  is finished, the following steps are performed locally, that is, at the slave clock side. 
     In step  201 , a time stamp is obtained. That is, four time stamps t 1 , t 2 , t 3 , and t 4  are obtained. 
     The t 2  and t 3  are obtained from a local absolute time stamp engine. The time represented in the absolute time stamp engine is an absolute time, which has to be synchronous with the master clock. 
     In step  202 , a time offset is calculated periodically. 
     In step  203 , time adjustment is performed on the slave clock by directly using the time offset calculated each time, so as to make the time of the slave clock synchronous with the time of the master clock, and update the time of the absolute time stamp engine synchronously. 
     In step  204 , in a certain period, a statistical calculation for phase-discrimination results is performed on the multiple time offsets obtained by periodical calculation, so as to obtain an optimal time offset. 
     Step  204  and step  203  may be performed at the same time. 
     In step  205 , high frequency components in the time offset obtained in step  204  are filtered, such as a jitter value of the network, or transient value jump occurred due to occasional errors when the time stamp engine processes data, and the changing trend of the time offset is monitored. 
     Specifically, if the time offset obtained at a later time is always greater than the time offset obtained at a former time, that is, the time offset is in a forward change, it shows that a local drive frequency, that is, the drive frequency of slave-clock timing is higher than that of master-clock timing; otherwise, the drive frequency of the slave-clock timing is lower than that of the master-clock timing. 
     In step  206 , conversion of reverse control is performed according to the changing trend of the time offset, the time offset is converted into a signal that is able to be identified by a local voltage controlled oscillator (VCO) or direct digital synthesizer (DDS), so as to control the adjustment of the local oscillator such as the VCO or DDS. As a negative feedback loop is used, reverse adjustment is performed, so as to realize the frequency synchronization of the master clock and the slave clock, and the frequency synchronization result is used to update the frequency of the absolute time stamp engine synchronously. 
     After step  203  and steps  204  to  206  are performed, both the time and frequency of the absolute time stamp engine are updated. Next time a local time stamp is obtained from the absolute time stamp engine after synchronous update, and in this embodiment, it is the step of obtaining t 2  and t 3 . Steps  201  to  206  are continuously repeated, until the frequency and time locks a prime reference source, that is, the master clock. 
     The embodiment of precise-clock synchronization method is characterized in that, only one absolute time stamp engine exists, so the time of the absolute time stamp engine is changed after either time synchronization or frequency synchronization; and all synchronization messages exchanged in  FIG. 1  need to be processed, so as to obtain the time offset required in both frequency restoration (frequency synchronization) and time setting (time synchronization). 
     The solution in the embodiment of the present invention utilizes two time stamp engines. One is a relative time stamp engine, which does not need time setting. The other one is an absolute time stamp engine representing an absolute time. The two time stamp engines are used to perform frequency and time synchronization for the master clock and the slave clock respectively, so that the frequency synchronization and the time synchronization of the master clock and the slave clock may be performed independently. Also, when frequency synchronization is performed for the relative time stamp engine, the synchronization message does not need to be processed. 
       FIG. 3  is a schematic flow chart of a precise-clock synchronization method according to a first embodiment of the present invention. Referring to  FIG. 3 , the precise-clock synchronization method includes the following steps. 
     In step  301 , a time stamp is obtained. The time stamp includes at least an original time stamp about the time when a master clock side transmits each Sync packet and a relative arrival time stamp about the time when each Sync packet is received. 
     The original time stamp is a time of transmitting the Sync packet stamped at the master clock side, that is, the time of the master clock. The relative time stamp is a local time when the slave clock side receives the Sync packet, that is, the time of the slave clock. 
     In step  302 , it is determined whether a clock frequency at the slave clock side is synchronous with a clock frequency at the master clock side or not according to each original time stamp and its corresponding relative arrival time stamp. 
     In step  302 , several manners are available for determining whether the clock frequency at the slave clock side and the clock frequency at the master clock side are synchronous or not according to each original time stamp and its corresponding relative arrival time stamp. For example, the determination may be formed through a changing trend of a relative delay, that is, a changing trend of a time difference between the original time stamp and the relative arrival time stamp. Alternatively, the determination may also be formed through comparison between a time interval of adjacent original time stamps and a time interval of similar adjacent relative arrival time stamps, and the like. 
     In step  303 , if the clock frequency at the slave clock side and the clock frequency at the master clock side are asynchronous, the clock frequency at the slave clock side is adjusted to become synchronous with the clock frequency at the master clock side. 
     The precise-clock synchronization method provided in this embodiment is able to be applied in a situation not requiring the time synchronization of the master clock and the slave clock or having a low requirement on the time synchronization or having good time synchronization. As the frequency synchronization of the master clock and the slave clock may be realized by using the Sync packet only, the bandwidth is greatly saved. Especially when the network is deteriorated, a packet transmission frequency needs to be increased to obtain more sampling points to ensure clock synchronization quality, the advantage becomes more apparent. 
       FIG. 4  is a schematic flow chart of a precise-clock synchronization method according to a second embodiment of the present invention. Referring to  FIG. 4 , the precise-clock synchronization method includes the following steps. 
     In step  401 , a Sync packet transmitted by a master clock side is received. 
     In step  402 , a time stamp is obtained from the Sync packet. The time stamp includes an original time stamp about the time when the master clock side transmits each Sync packet and a relative arrival time stamp about the time when each Sync packet is received. 
     As shown in  FIG. 1 , the original time stamp of the Sync packet is a time t 1  when the master clock side transmits the Sync packet. The relative arrival time stamp of the Sync packet is transmitted locally, that is, transmitted by the relative time stamp engine at the slave clock side. The relative time stamp engine does not need time setting and is only used for frequency synchronization. Therefore, the relative arrival time stamp obtained from the relative time stamp engine is indicated by t 2 ′, denoting a relative arrival time, rather than an absolute arrival time. 
     In step  403 , a relative delay between the relative arrival time stamp t 2 ′ and the original time stamp t 1  is calculated. 
     The relative delay=the relative arrival time stamp t 2 ′−the original time stamp t 1 . Alternatively, the relative delay=the original time stamp t 1 −the relative arrival time stamp t 2 ′. In the embodiment of the present invention, the equation that the relative delay=the relative arrival time stamp t 2 ′−the original time stamp t 1  is used. 
     In step  404 , a first statistics period is set. Within the first statistics period, a minimum relative delay is selected from a plurality of calculated relative delays. 
     Within a certain statistics period, one minimum relative delay is selected from the plurality of relative delays calculated by using step  403  many times. The minimum relative delay represents the best network transmission performance, which is able to show the transmission delay characteristic of the physical layer more practically. It is equivalent to that the jittering of the network is filtered once. 
     In step  405 , a second statistics period is set, and a changing trend of the minimum relative delay within the second statistics period is monitored. 
     The second statistics period in step  405  is a time period including a plurality of continuous first statistics periods. Within this time period of the second statistics period, the changing trend of the obtained minimum relative delay is monitored. The changing trend may include that the value of the minimum relative delay gradually increases or decreases. 
     In addition, before step  405 , the method further includes filtering the selected minimum relative delay, so as to filter high frequency components. The first statistics period and second statistics period may be set by a timer respectively. 
     In step  406 , it is determined whether the clock frequency at the slave clock side and the clock frequency at the master clock side are synchronous or not according to the changing trend of the minimum relative delay monitored in step  405 . 
     Specifically, if the values of two adjacent minimum relative delays gradually increase or decrease, it indicates that the clock frequency at the slave clock side and the clock frequency at the master clock side are not synchronous, so step  407  is performed. If the value of the minimum relative delay is constant, it indicates that the clock frequency at the slave clock side and the clock frequency at the master clock side are synchronous. 
     In step  407 , the clock frequency at the slave clock side is adjusted to become synchronous with the clock frequency at the master clock side. 
     In step  406 , if it is found that the values of two adjacent minimum relative delays gradually increase by statistics, it indicates that the local relative time stamp engine, that is, the relative time stamp engine at the slave clock side runs is faster than the time engine at the master clock side, denoting that the frequency of the slave clock is higher than the frequency of the master clock. On the contrary, it denotes that the frequency of the slave clock is lower than the frequency of the master clock. Therefore, if the frequency of the slave clock is higher than the frequency of the master clock, in step  407 , a variation of the minimum relative delay is converted into a signal value that can be identified by the oscillator. The output frequency of the local oscillator is then turned slower. The local oscillator may be a VCO or a DDS. 
       FIG. 5  is another schematic flow chart of the precise-clock synchronization method according to a second embodiment of the present invention. Referring to  FIG. 5 , the precise-clock synchronization method includes the following steps. 
     In step  501 , a Sync packet transmitted by a master clock side is received. 
     In step  502 , a time interval of adjacent original time stamps and a time interval of similar corresponding adjacent relative arrival time stamps are compared. 
     In step  503 , it is determined whether a clock frequency at a slave clock side and a clock frequency at the master clock side are synchronous or not according to a comparison result. 
     It is assumed that a periodic interval that the master clock side transmits the Sync packet is ΔTT. A time interval that the slave clock side receives the corresponding Sync packet is ΔTR. If the ΔTT is not equal to the ΔTR, it indicates that the clock frequency at the slave clock side and the clock frequency at the master clock side are asynchronous, and step  503  is performed. On the contrary, if the intervals are the same, it indicates that the clock frequency at the slave clock side and the clock frequency at the master clock side are synchronous. 
     In step  504 , the clock frequency at the slave clock side is adjusted to become synchronous with the clock frequency at the master clock side. 
     For the precise-clock synchronization method provided in this embodiment, the frequency synchronization of the slave clock and the master clock may be performed as long as an original time stamp and a relative arrival time stamp of the Sync packet are obtained locally, that is, at the slave clock side. The method may be applied in a situation having low requirement on the time synchronization of clock or having good time synchronization, so the bandwidth may be greatly saved. Especially, when the network is deteriorated, the packet transmission frequency needs to be increased to obtain more sampling points to ensure clock synchronization quality, the advantages becomes more apparent. 
       FIG. 6  is a schematic flow chart of a precise-clock synchronization method according to a third embodiment of the present invention. Referring to  FIG. 6 , the differences between the flow in  FIG. 6  and that in the second embodiment shown in  FIGS. 4 and 5  are described as follows. The obtained time stamp further includes an absolute arrival time stamp, a Delay_Req time stamp, and a Delay_Resp time stamp of the Sync packet. After the frequency synchronization is accomplished, the time of the master clock and the slave clock may be synchronized according to the calculated time offset. Taking the precise-clock synchronization method in steps  401  to  407  as an example, the time synchronization has the following two situations. 
     In the first situation, the method further includes the following steps after receiving the Sync packet transmitted by the master clock side in step  401 . 
     In step  601 , a Delay_Req packet is transmitted, and a Delay_Resp packet returned by the master clock side after it receives the Delay_Req packet is received. 
     In step  602 , a time stamp including an absolute arrival time stamp, a Delay_Req packet time stamp, and a Delay_Resp packet time stamp of the Sync packet is obtained. 
     The absolute arrival time stamp, the Delay_Req time stamp, and the Delay_Resp time stamp of the Sync packet are an accurate receiving time t 2  when the slave clock side receives the Sync packet, a time t 3  when the slave clock transmits the Delay_Req, and an accurate receiving time t 4  of the master clock introduced in the synchronization message exchange scheme in the IEEE 1588 PTP, respectively. 
     In step  603 , a time offset is calculated according to the obtained time stamp. 
     At this time, the time stamp used to calculate the time offset obtained in step  603  includes the original time stamp t 1  obtained in step  402 , the absolute arrival time stamp t 2 , the Delay_Req time stamp t 3 , and the Delay_Resp time stamp t 4  obtained in step  602 . The process of calculating the time offset according to t 1 , t 2 , t 3 , and t 4  are as shown in  FIGS. 1 and 2 , and the description of which is not repeated here. 
     After the synchronization of the clock frequency at the slave clock side and the clock frequency at the master clock side in step  407  is performed, the method further includes step  604 , in which a time synchronization enable signal is received, and the clock time of the slave clock side is adjusted to be synchronous with the clock time at the master clock side by using the time offset in step  603 . 
     In  FIG. 6 , steps  401  and  601  among the steps are not shown. 
     In the second situation which is not shown in  FIG. 6 , after step  407  is performed so that the clock frequency at the slave clock side and the clock frequency at the master clock side are synchronous, the method further includes the following steps. 
     In step  601 ′, a time synchronization enable signal is received. 
     In step  602 ′, a Delay_Req is transmitted at the time when the time synchronization enable signal is received, and a Delay_Resp returned by the master clock side after it receives the Delay_Req is received. 
     In step  603 ′, a time stamp including an original time stamp and an absolute arrival time stamp, a Delay_Req time stamp, and a Delay_Resp time stamp of the Sync packet is obtained. 
     In step  604 ′, a time offset is calculated according to the obtained time stamp. The time stamp includes the original time stamp t 1 , the absolute arrival time stamp t 2 , the Delay_Req time stamp t 3 , and the Delay_Resp time stamp t 4 . 
     In step  605 ′, the clock time at the slave clock side is adjusted to become synchronous with the clock time at the master clock side by using the time offset. 
     As can be seen from the precise-clock synchronization method provided in this embodiment, when the time and frequency of the master clock and the slave clock need to be synchronous, the relative delay time may be calculated according to the relative arrival time stamp of the Sync packet provided by the relative time stamp engine and the time offset may be calculated according to the absolute arrival time stamp of the Sync packet provided by the absolute time stamp engine at the same time. Next, after the frequencies of the master clock and the slave clock are synchronized by using the relative delay time, the time of the master clock and the slave clock are then synchronized by using the time offset according to the enable indication after the frequency synchronization is accomplished. Alternatively, the relative delay time may also be calculated first, and the time offset is then calculated to perform time synchronization on the master clock and the slave clock a relatively long time after the frequencies of the master clock and the slave clock are synchronized. In this way, the frequency synchronization and the time synchronization may be separately performed and do not interfere with each other, thus saving the bandwidth. 
       FIG. 7  is a schematic flow chart of a precise-clock synchronization method according to a fourth embodiment of the present invention. Referring to  FIG. 7 , the precise-clock synchronization method includes the following steps. 
     In step  701 , a time stamp including an original time stamp, an absolute arrival time stamp, and a Delay_Resp time stamp of a Sync packet transmitted by a master clock side, as well as a Delay_Req time stamp transmitted by the slave clock side are obtained. 
     The master clock side transmits the Sync packet. The slave clock side transmits a Delay_Req after receiving the transmitted Sync packet for a set period of time. The master clock side transmits a Delay_Resp after receiving the Delay_Req. The slave clock side receives the Delay_Resp. Time stamps, i.e. the time stamps obtained in step  701  are stamped in the packets. 
     In step  702 , a time offset is calculated according to the obtained original time stamp, absolute arrival time stamp, Delay_Req time stamp, and Delay_Resp time stamp. 
     In step  703 , the clock time at the slave clock side is adjusted to become synchronous with the clock time at the master clock side by using the time offset. 
     In this embodiment, the time of the master clock and the slave clock may be synchronized independently by calculating the obtained time stamps. 
     Before the clock time at the slave clock side is adjusted by using the time offset in step  703 , or before the slave clock side transmits the Delay_Req, the method may further include receiving a time synchronization enable signal to enable the time synchronization process. The time that the time synchronization enable signal is received before the slave clock side transmits the Delay_Req is a time of the arrival set time. The time synchronization enable signal may be a completion indication for the synchronization between the clock frequency at the slave clock side and the clock frequency at the master clock side. From the descriptions of the precise-clock synchronization method in the first, second, and third embodiments, the method of realizing frequency synchronization of the master clock and the slave clock may be used in this embodiment to be combined with the time synchronization. 
       FIG. 8  is a schematic structural view of a precise-clock frequency synchronization device according to an embodiment of the present invention. Referring to  FIG. 8 , the precise-clock frequency synchronization device includes a relative time stamp engine  81 , a first obtaining module  82 , a determining module  83 , and a frequency adjusting module  84 . The relative time stamp engine  81  provides a relative arrival time stamp about the time when the slave clock side receives a Sync packet. The first obtaining module  82  obtains a time stamp, and the time stamp includes at least an original time stamp about the time when the master clock side transmits each Sync packet and a relative arrival time stamp about the time when the slave clock side receives each Sync packet. The determining module  83  determines whether the clock frequency at the slave clock side and the clock frequency at the master clock side are synchronous or not according to each original time stamp and its corresponding relative arrival time stamp. The frequency adjusting module  84  adjusts the clock frequency at the slave clock side to be synchronous with the clock frequency at the master clock side if the clock frequency at the slave clock side and the clock frequency at the master clock side are asynchronous. A plurality of manners exists for the determining module  83  to determine whether the clock frequency at the slave clock side and the clock frequency at the master clock side are synchronous or not according to each original time stamp and its corresponding relative arrival time stamp. For example, the determination may be formed through a changing trend of the relative delay, that is, a changing trend of a time difference between the original time stamp and the relative arrival time stamp. Alternatively, the determination may be formed through the comparison between a time interval of the adjacent original time stamps and a time interval of similar corresponding adjacent relative arrival time stamps, and the like. 
     In this embodiment, the determining module  83  in the precise-clock frequency synchronization device specifically includes a first calculating module  831 , a comparing/selecting module  832 , a monitoring module  833 , and a change determining module  834 . The first calculating module  831  calculates a relative delay between the relative arrival time stamp and the original time stamp. The comparing/selecting module  832  selects a minimum relative delay from a plurality of calculated relative delays within a first statistics period. The monitoring module  833  monitors changes of the plurality of minimum relative delays within a second statistics period. The change determining module  834  determines whether the clock frequency at the slave clock side and the clock frequency at the master clock side are synchronous or not according to a changing trend of the plurality of minimum relative delay. If values of two adjacent minimum relative delays gradually increase or decrease, it indicates that the clock frequency at the slave clock side and the clock frequency at the master clock side are asynchronous. If a value of the minimum relative delay is constant, it indicates that the clock frequency at the slave clock side and the clock frequency at the master clock side are synchronous. The monitoring module  833  further has a function of filtering the selected minimum relative delays to filter high frequency components. 
     In this embodiment, the precise-clock frequency synchronization device further includes a timing module  85  adapted to set the first statistics period and the second statistics period. 
     The specific process that the precise-clock frequency synchronization device provided in this embodiment performs frequency synchronization of the master clock and the slave clock is as described in the embodiments of the methods, which may be separately applied in the situation not requiring the time synchronization of the master clock and the slave clock or having low requirement on the time synchronization or having good time synchronization. As the use of the Sync packet may realize the frequency synchronization of the master clock and the slave clock, the bandwidth is greatly saved. Especially when the network is deteriorated and packet transmission frequency needs to be increased to ensure the clock quality, the advantage becomes more apparent. 
       FIG. 9  is a schematic structural view of a precise-clock time synchronization device according to an embodiment of the present invention. Referring to  FIG. 9 , the precise-clock time synchronization device includes an absolute time stamp engine  91 , a second obtaining module  92 , a second calculating module  93 , and a time adjusting module  94 . The absolute time stamp engine  91  provides an absolute arrival time stamp about the time when a slave clock side receives a Sync packet and a Delay_Req time stamp about the time when the slave clock side transmits a Delay_Req. The second obtaining module  92  obtains a time stamp including an original time stamp and an absolute arrival time stamp of the Sync packet transmitted by the master clock side, a Delay_Resp time stamp transmitted by the master clock side, and a Delay_Req time stamp transmitted by the slave clock side. The second calculating module  93  calculates a time offset according to the obtained original time stamp, absolute arrival time stamp, Delay_Req time stamp, and Delay_Resp time stamp. The time adjusting module  94  adjusts the clock time at the slave clock side to be synchronous with the clock time at the master clock side by using the time offset. 
     In this embodiment, the precise-clock time synchronization device further includes a receiving module  95  adapted to receive a time synchronization enable signal. 
     The specific process that the precise-clock time synchronization device provided in this embodiment performs time synchronization of the master clock and the slave clock is as described in the methods in the embodiments. The time of the master clock and the slave clock may be synchronized separately by calculating the obtained time stamps. 
       FIG. 10  is a schematic structural view of a precise-clock synchronization system according to an embodiment of the present invention. Referring to  FIG. 10 , the precise-clock synchronization system includes a precise-clock frequency synchronization device  8  and a precise-clock time synchronization device  9 . The precise-clock frequency synchronization device  8  adjusts a clock frequency at a slave clock side to be synchronous with a clock frequency at a master clock side according to an original time stamp about the time when the master clock side transmits a Sync packet and a relative arrival time stamp about the time when the slave clock side receives the Sync packet. The precise-clock time synchronization device  9  calculates a time offset according to the original time stamp of the Sync packet and an absolute arrival time stamp about the time when the slave clock side receives the Sync packet, as well as a Delay_Req time stamp and a Delay_Resp time stamp, and adjusts the clock time at the slave clock side to be synchronous with the clock time at the master clock side by using the time offset. Specific structures of the precise-clock frequency synchronization device  8  and the precise-clock time synchronization device  9  are described in the embodiment of the precise-clock frequency synchronization device shown in  FIG. 8  and the embodiment of the precise-clock time synchronization device shown in  FIG. 9 , and the descriptions of which are not repeated here. 
     The specific process of the precise-clock synchronization system provided in this embodiment performing frequency synchronization, time synchronization and their combination of the master clock and the slave clock is as described in the embodiments of the methods. When the time and frequency of the master clock and the slave clock need to be synchronized, the relative delay time may be calculated according to the relative arrival time stamp of the Sync packet provided by the relative time stamp engine and the time offset may be calculated according to the absolute arrival time stamp of the Sync packet provided by the absolute time stamp engine at the same time. Next, after the frequencies of the master clock and the slave clock are synchronized by using the relative delay time, the time of the master clock and the slave clock are then synchronized by using the time offset according to an enable indication after the frequency synchronization is accomplished. Alternatively, the relative delay time may be calculated first, and the time offset is then calculated to perform time synchronization of the master clock and the slave clock, after accomplishing the frequency synchronization of the master clock and the slave clock for a long time. In this way, the frequency synchronization and the time synchronization may be separately processed and do not interfere with each other, thus saving the bandwidth. 
     Those of ordinary skill in the art should understand that all or a part of processes in the method according to the embodiments may be implemented by a computer program instructing relevant hardware. The program may be stored in a computer readable storage medium. When the program is executed, the processes of the methods according to the embodiments of the present invention are performed. The storage medium may be a magnetic disk, an optical disk, a read-only memory (ROM), a random access memory (RAM), or the like. 
     Finally, it should be noted that the above embodiments are merely provided for describing the technical solutions of the present invention, but not intended to limit the present invention. It should be understood by those of ordinary skill in the art that although the present invention has been described in detail with reference to the foregoing embodiments, modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent replacements can be made to some technical features in the technical solutions, and such modifications or replacements do not cause the essence of corresponding technical solutions to depart from the spirit and scope of the present invention.