Abstract:
To enable more precise synchronization of distributed clocks, before a high-level frame is converted to a low-level frame for transmission over a communication network, if the high-level frame includes a certain type of message, it is flagged. The flag is detected during the conversion process, and the time of detection is stored as a transmission timestamp. When a low-level frame is received from the network, it is immediately timestamped with the time of reception. The timestamped frame is then converted to a high-level frame, and if the high-level frame includes the certain type of message, the timestamp is stored as a reception timestamp.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method of synchronizing clocks in devices that transmit and receive data frames via a network, and to relevant systems and apparatus. 
         [0003]    2. Description of the Related Art 
         [0004]    Methods of synchronizing clocks in devices that exchange data frames over the Internet or another communication network are well known. Exemplary methods and apparatus are disclosed by Inomata in Japanese Patent Application Publication (JP) 2010-190635. These and many other known methods use the Precision Time Protocol (PTP) defined by the Institute of Electrical and Electronics Engineers (IEEE) in its 1588 family of standards. 
         [0005]    The IEEE 1588-2002 standard defines Sync, FollowUp, DelayReq, and DelayResp messages for use in clock synchronization. Sync and DelayReq messages are timestamped when transmitted and received, and the transmitting or receiving device stores the timestamp. Conventionally, each transmitted or received frame is analyzed in an interface layer such as a media independent interface (MII) or gigabit media independent interface (GMII) layer, and a timestamp is generated and stored if the frame includes a Sync or DelayReq message. 
         [0006]    A problem with this conventional timestamping method is that analyzing all outgoing frames on the MII or other interface to decide which frames include a Sync or DelayReq message involves a considerable processing load. The time taken to perform the analysis also lowers the precision of the timestamp. A similar problem occurs in frame reception: before storing the timestamp of a frame, the receiving apparatus must analyze the frame on the MII or other interface to determine whether the frame includes a Sync or DelayReq message, and the precision of the timestamp is reduced by the time required for the analysis. These problems make it difficult to determine the network latency precisely, which is necessary for precise clock synchronization. 
         [0007]    The problems are aggravated when security protection is employed. If the IEEE 802.1AE Media Access Control Security (MACsec) standard is used, as discussed by Ida et al. in JP 2008-42715, for example, frames on the MII or other interface are encrypted and defy analysis. The present inventor considered having the timestamp of a MACsec frame stored before the frame is encrypted at the transmitting end and after the frame is decrypted at the receiving end, but that would further lower the timestamp precision, because the time required for encryption and decryption would be added to the intrinsic network latency. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the present invention is to improve the precision of timestamps in a communication network. 
         [0009]    Another object of the invention is to reduce the interface processing load involved in generation of the timestamps. 
         [0010]    The invention provides a method of transmitting frames over a communication network. High-level frames are selectively flagged, depending on their content, at the transmitting end. Each high-level frame is then converted to a low-level (physical layer) frame and the low-level frame is transmitted over the communication network. During conversion of the high-level frame to the low-level frame, whether the high-level frame was flagged is determined. If the high-level frame has been flagged, the time of detection of the flag is stored as a transmission time. 
         [0011]    When the low-level frame is received from the communication network, a timestamp representing the time of reception is generated and added to the low-level frame. The resulting timestamped frame is then converted to a received high-level frame, and the time represented by the timestamp is selectively stored, depending on the content of the received high-level frame, as a reception time. 
         [0012]    The stored transmission and reception times exclude time spent on processes carried out in conversion between high-level and low-level frames, such as frame analysis, encryption, decryption, and data processing, so they can be used as accurate timestamps for precise clock synchronization at the receiving and transmitting ends of the communication network. 
         [0013]    The interface processing load involved in generating the timestamps is reduced at the transmitting end because frame content does not have to be analyzed during the frame conversion process or timestamp storing process. Instead, it is only necessary to check a flag. 
         [0014]    The invention also provides a frame transmitting apparatus, a frame receiving apparatus, and a frame transmission and reception system employing the method described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    In the attached drawings: 
           [0016]      FIG. 1  is a block diagram illustrating the structure of a frame receiving apparatus embodying the invention; 
           [0017]      FIG. 2  is a block diagram illustrating the structures of the MAC processor and MACsec processor in  FIG. 1 ; 
           [0018]      FIG. 3  is a flowchart illustrating a timestamp storing routine executed by the frame receiving apparatus; 
           [0019]      FIGS. 4A to 4D  illustrate frame structures used by the frame receiving apparatus; 
           [0020]      FIG. 5  is a block diagram illustrating a frame transmitting apparatus embodying the invention; 
           [0021]      FIG. 6  is a block diagram illustrating the structures of the MACsec processor and MAC processor in  FIG. 5 ; 
           [0022]      FIG. 7  is a flowchart illustrating a timestamp storing routine executed by the frame transmitting apparatus; 
           [0023]      FIGS. 8A to 8D  illustrate frame structures used by the frame transmitting apparatus; 
           [0024]      FIG. 9  is a timing diagram illustrating flag detection timing in the frame transmitting apparatus; 
           [0025]      FIG. 10  is a block diagram illustrating the general structure of a frame transmission/reception system; and 
           [0026]      FIG. 11  illustrates messages and timestamps involved in clock synchronization in the frame transmission/reception system in  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Embodiments of the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. 
       Receiving Apparatus Embodiment 
       [0028]      FIG. 1  shows a novel frame receiving apparatus  10  including a physical layer (PHY) processor  11 , a clock unit  12 , a timestamping unit  13 , a media access control (MAC) processor  14 , a MAC security (MACsec) processor  15 , a frame discriminator  16 , a timestamp storage unit  17 , and an upper layer processor  18 . 
         [0029]    The physical layer processor  11  is a receiving unit connected to a communication network  20 , such as the Internet, from which it receives and sequentially processes frames referred to below as low-level frames. The physical layer processor  11  carries out processing related to the physical layer of the Open Systems Interconnection (OSI) reference model. 
         [0030]    The clock unit  12  constantly generates current time information representing the current time. 
         [0031]    The timestamping unit  13  adds current time information generated by the clock unit  12  as a timestamp to each low-level frame that has been processed by the physical layer processor  11 . A frame with a timestamp added will be referred to as a timestamped frame. 
         [0032]    The MAC processor  14  performs processing related to the media access control (MAC) layer, which is the layer just above the physical layer, on each successive timestamped low-level frame. This processing includes, for example, address analysis and frame check sequence (FCS) confirmation. During this processing, the MAC processor  14  temporarily separates the added timestamp from the frame. 
         [0033]    The MACsec processor  15  removes security protection by decrypting and authenticating each successive timestamped frame that has been processed by the MAC processor  14 . During these MACsec processes, the MACsec processor  15  also temporarily separates the added timestamp from the frame. The MAC processor  14  and MACsec processor  15  together constitute a conversion unit. 
         [0034]    The frame discriminator  16  decides whether or not to store the timestamp of each timestamped frame that has been processed by the MACsec processor  15 . 
         [0035]    Specifically, if the timestamped frame includes a Sync or DelayReq message, the frame discriminator  16  decides to store the timestamp, removes the timestamp from the frame, passes the timestamp to the timestamp storage unit  17 , and passes the frame, without the timestamp, to the upper layer processor  18 . The frame passed to the upper layer processor  18  will be referred to below as a high-level frame. The high-level frame belongs to a layer above the physical layer, that is, to the MAC layer or a higher layer. 
         [0036]    If the timestamped frame does not include a Sync or DelayReq message, the frame discriminator  16  discards the timestamp and passes the resulting high-level frame, without the timestamp, to the upper layer processor  18 . 
         [0037]    The timestamp storage unit  17  stores the timestamps received from the frame discriminator  16 , which have been taken from frames found to include a Sync or DelayReq message. The frame discriminator  16  and timestamp storage unit  17  thus constitute a storing unit. 
         [0038]    The upper layer processor  18  performs processing related to a still higher layer, such as the network layer in the OSI reference model, on each successive high-level frame. 
         [0039]    Next, the structure of the MAC processor  14  and MACsec processor  15  will be described with reference to  FIG. 2 . 
         [0040]    The MAC processor  14  includes a timestamp separation unit  14   a , a timestamp holding unit  14   b , a data processing unit  14   c , and a timestamp restoration unit  14   d . The MACsec processor  15  includes a timestamp separation unit  15   a , a timestamp holding unit  15   b , a security protection removal processor  15   c , and a timestamp restoration unit  15   d.    
         [0041]    Timestamp separation unit  14   a  separates the timestamp that has been added to a timestamped frame supplied from the timestamping unit  13  in  FIG. 1 . 
         [0042]    Timestamp holding unit  14   b  temporarily holds the separated time stamp. 
         [0043]    Data processing unit  14   c  performs MAC processing on the data in the frame after its timestamp has been removed. 
         [0044]    Timestamp restoration unit  14   d  restores the timestamp temporarily held in timestamp holding unit  14   b  to the frame after the MAC processing by data processing unit  14   c , thereby reconfiguring the timestamped frame, and supplies the reconfigured frame to the timestamp separation unit  15   a  in the MACsec processor  15 . 
         [0045]    In timestamp separation unit  15   a , the timestamp that was added by timestamp restoration unit  14   d  to reconfigure the timestamped frame is again separated from the frame. 
         [0046]    Timestamp holding unit  15   b  temporarily holds the separated time stamp. 
         [0047]    The security protection removal processor  15   c  removes security protection from the frame after the timestamp has been removed by the timestamp separation unit  15   a.    
         [0048]    Timestamp restoration unit  15   d  restores the timestamp temporarily held in timestamp holding unit  15   b  to the frame processed by the security protection removal processor  15   c , thereby reconfiguring the timestamped frame, and supplies the reconfigured frame to the frame discriminator  16 . 
         [0049]    The timestamp storing process carried out in the frame receiving apparatus  10  will now be described with reference to  FIG. 3  and  FIGS. 4A to 4D . 
         [0050]    First, the physical layer processor  11  receives a low-level frame FR 1  from the communication network  20  (step S 11  in  FIG. 3 ). The low-level frame FR 1  includes a preamble, a start frame delimiter (SFD), a destination address, a source address, a MACsec header, encrypted data, an authentication vector, and an FCS, as shown in  FIG. 4A . 
         [0051]    Next, the clock unit  12  adds a timestamp to the low-level frame FR 1  to configure a timestamped frame FR 2  (step S 12 ). The timestamped frame FR 2  includes the destination address, source address, MACsec header, encrypted data, authentication vector, FCS, and timestamp, as shown in  FIG. 4B . 
         [0052]    Next, the MAC processor  14  performs MAC processing on the timestamped frame FR 2 , and the MACsec processor  15  performs MACsec processing to remove security protection, thereby configuring a timestamped frame FR 3  (step S 13 ). 
         [0053]    To perform MAC processing, timestamp separation unit  14   a  ( FIG. 2 ) temporarily separates the timestamp from the frame. The separated timestamp is held in timestamp holding unit  14   b , and restored to the frame by timestamp restoration unit  14   d  after the MAC processing of the data by the data processing unit  14   c . To perform MACsec processing, timestamp separation unit  15   a  ( FIG. 2 ) also temporarily separates the timestamp. The separated timestamp is held in timestamp holding unit  15   b  while security protection is being removed by the security protection removal processor  15   c , and then restored to the frame by timestamp restoration unit  15   d . These operations prevent the timestamp from being affected by the MAC and MACsec processing. The timestamped frame FR 3  includes the destination address, source address, decrypted data, and timestamp, as shown in  FIG. 4C . 
         [0054]    Next, the frame discriminator  16  analyzes the timestamped frame FR 3  and decides whether or not to store the timestamp (step S 14 ). The analysis is possible because frame FR 3  is unprotected and its data have been decrypted. If the timestamped frame FR 3  includes a Sync or DelayReq message, the frame discriminator  16  decides to store the timestamp and supplies the timestamp to the timestamp storage unit  17 , which then stores the timestamp (step S 15 ). 
         [0055]    If the frame discriminator  16  determines in step S 14  that the timestamped frame FR 3  does not include a Sync or DelayReq message, it discards the timestamp that was added to the timestamped frame FR 3  (step S 16 ). 
         [0056]    Regardless of whether or not the timestamp is stored, the frame discriminator  16  supplies the frame to the upper layer processor  18  as a high-level frame FR 4  including the destination address, source address, and decrypted data but not including the timestamp, as shown in  FIG. 4D . 
         [0057]    In the embodiment described above, the frame receiving apparatus  10  adds a timestamp to each low-level frame received from the communication network  20 , temporarily removes the timestamp to carry out MAC and MACsec processing, and restores the unaltered timestamp after these steps. The timestamped frame is then analyzed, and depending on the result of the analysis, the timestamp is stored. 
         [0058]    With this structure, the time of reception of a frame can be stored as its timestamp, so when network latency is calculated from the timestamps, the calculated latency excludes extraneous factors such as the time required for MAC processing, and for security related processing such as decryption. In addition, since the timestamp is added to a frame before the decision as to whether or not to store the timestamp is made, the calculated network latency excludes the time required to analyze the frame and make this decision. 
         [0059]    The frame receiving apparatus  10  in this embodiment can therefore provide high-precision timestamp information. 
       Transmitting Apparatus Embodiment 
       [0060]      FIG. 5  shows a novel frame transmitting apparatus  30  including an upper layer processor  31 , a flagging unit  32 , a MACsec processor  33 , a MAC processor  34 , a flag detector  35 , a clock unit  36 , a timestamp storage unit  37 , and a physical layer (PHY) processor  38 . 
         [0061]    The upper layer processor  31  generates a frame (referred to below as a high-level frame) by performing processing related to a layer such as the network layer in the OSI reference model and supplies the generated frame to the flagging unit  32 . 
         [0062]    The flagging unit  32  determines whether or not to flag each high-level frame supplied from the upper layer processor  31 . If the high-level frame includes a Sync or DelayReq message, the flagging unit  32  adds a flag to the frame and supplies the frame to the MACsec processor  33 . A high-level frame with a flag added in this way will be referred to as a flagged frame. If the high-level frame does not include a Sync or DelayReq message, the flagging unit  32  supplies the high-level frame to the MACsec processor  33  without adding the flag. 
         [0063]    The MACsec processor  33  performs encryption or other MACsec processing for security protection of each successive supplied frame. If the supplied frame is a flagged frame, the MACsec processor  33  temporarily separates the flag before performing the MACsec processing. After performing the MACsec processing, the MACsec processor  33  restores the flag to the processed frame and supplies the frame to the MAC processor  34 . 
         [0064]    The MAC processor  34  performs processing related to the MAC layer of the OSI reference model on each successive frame supplied from the MACsec processor  33 . If the supplied frame is a flagged frame, the MAC processor  34  separates the flag from the frame before performing this MAC processing. After the MAC processing, the MAC processor  34  supplies the frame, referred to below as a low-level frame, to the flag detector  35 . 
         [0065]    The flag detector  35  determines whether each frame processed by the MAC processor  34  was flagged or not. In the description below, the flag detector  35  detects the flag by means of a flag signal supplied from the MAC processor  34 . If the flag detector  35  determines that the frame was flagged, it supplies current time information generated by the clock unit  36  to the timestamp storage unit  37  as a time stamp. 
         [0066]    The clock unit  36  constantly generates current time information representing the current time. 
         [0067]    The timestamp storage unit  37  stores the timestamps of flagged frames. The timestamps are received from the clock unit  36  at the direction of the flag detector  35 . 
         [0068]    The MACsec processor  33  and MAC processor  34  constitute a conversion unit. The clock unit  36  and timestamp storage unit  37  constitute a storing unit. 
         [0069]    The physical layer processor  38  is a transmitting unit that performs processing related to the physical layer of the OSI reference model on each successive low-level frame supplied form the MAC processor  34 , and transmits the processed low-level frame to the communication network  20 . 
         [0070]      FIG. 6  shows the structure of the MACsec processor  33  and MAC processor  34 . The MACsec processor  33  includes a flag separation unit  33   a , a flag holding unit  33   b , a security processing unit  33   c , and a flag restoration unit  33   d . The MAC processor  34  includes a flag separation unit  34   a , a flag signal generator  34   b , and a data processing unit  34   c.    
         [0071]    The flag separation unit  33   a  separates the flag that has been added to a flagged frame supplied from the flagging unit  32  in  FIG. 5 . 
         [0072]    The flag holding unit  33   b  temporarily holds the separated flag. 
         [0073]    The security processing unit  33   c  performs MACsec processing on the data in the frame after its flag has been removed by the flag separation unit  33   a.    
         [0074]    The flag restoration unit  33   d  restores the flag temporarily held in the flag holding unit  33   b  to the frame after the MACsec processing by the security processing unit  33   c , thereby reconfiguring the flagged frame, and supplies the reconfigured frame to the flag separation unit  34   a  in the MAC processor  34 . 
         [0075]    In flag separation unit  34   a , the flag that was added by the flag restoration unit  33   d  to reconfigure the flagged frame is again separated from the reconfigured frame and supplied to the flag signal generator  34   b.    
         [0076]    After receiving the separated flag from the flag separation unit  34   a , the flag signal generator  34   b  generates a flag signal and supplies the flag signal to the flag detector  35  in  FIG. 5 . 
         [0077]    The data processing unit  34   c  performs data processing on the frame after its flag has been removed by the flag separation unit  34   a , thereby configuring the frame as a low-level frame, and supplies the low-level frame, without a flag, to the physical layer processor  38  in  FIG. 5 . 
         [0078]    The timestamp storing process carried out in the frame transmitting apparatus  30  will now be described with reference to  FIG. 7  and  FIGS. 8A to 8D . 
         [0079]    First, the flagging unit  32  receives a high-level frame from the upper layer processor  31  and decides whether or not to flag the frame (step S 21 ). If the high-level frame includes a Sync or DelayReq message, the flagging unit  32  adds a flag to the frame, thereby configuring it as a flagged high-level frame FS 1 , and supplies the flagged frame FS 1  to the MACsec processor  33  (step S 22 ). The flagged frame FS 1  includes the flag, a destination address, a source address, and plaintext data, as shown in  FIG. 8A . 
         [0080]    If the flagging unit  32  determines in step S 21  that the high-level frame does not include a Sync or DelayReq message, it supplies the high-level frame without a flag to the MACsec processor  33 . 
         [0081]    Next, the MACsec processor  33  performs encryption or other security protection processing on the high-level frame received from the flagging unit  32  (step S 23 ). If the high-level frame has been flagged, the MACsec processor  33  temporarily removes the flag to carry out this MACsec processing, holds the flag separately, and then restores the flag, thereby reconfiguring the flagged frame. The MACsec processor  33  supplies the reconfigured flagged frame FS 2  to the MAC processor  34 . Frame FS 2  includes the flag, destination address, and source address, a MACsec header, encrypted data, and an authentication vector, as shown in  FIG. 8B . If the high-level frame has not been flagged, the MACsec processor  33  carries out the same security processing on the frame and supplies the processed frame FS 2 , without a flag, to the MAC processor  34 . 
         [0082]    Regardless of whether the frame FS 2  received from the MACsec processor  33  is flagged or not, the MAC processor  34  carries out MAC processing on the frame FS 2  (step S 23 ). If frame FS 2  is flagged, the MAC processor  34  removes the flag, carries out the MAC processing, thereby configures a low-level frame FS 3 , and supplies the low-level frame FS 3  to the physical layer processor  38 . The low-level frame FS 3  includes the destination address, source address, MACsec header, encrypted data, authentication vector, and an FCS, as shown in  FIG. 8C . The MAC processor  34  also generates a flag signal indicating that frame FS 2  was flagged and sends the flag signal to the flag detector  35 . If frame FS 2  was not flagged, the MAC processor  34  carries out the same MAC processing, thereby configuring a low-level frame FS 3 , and supplies the low-level frame FS 3  to the physical layer processor  38 . 
         [0083]    Next, the flag detector  35  determines whether or not the frame FS 2  processed by the MAC processor  34  was flagged (step S 24 ). The flag detector  35  determines that frame FS 2  was flagged if it receives a flag signal from the MAC processor  34 . In this case, the flag detector  35  has the clock unit  36  supply current time information to the timestamp storage unit  37  as a timestamp of frame FS 2 . If the flag detector  35  determines that frame FS 2  was not flagged, the clock unit  36  does not supply a timestamp to the timestamp storage unit  37 . 
         [0084]    The timestamp storage unit  37  stores the timestamps received from the clock unit  36 . 
         [0085]    Next, the physical layer processor  38  carries out processing related to the physical layer on the low-level frame FS 3  received from the MAC processor  34 , thereby configuring a low-level frame FS 4 , and transmits frame FS 4  to the communication network  20  (step S 26 ). The low-level frame FS 4  includes a preamble, an SFD, and the destination address, source address, MACsec header, encrypted data, authentication vector, and FCS, as shown in  FIG. 8D . 
         [0086]      FIG. 9  illustrates the flag detection timing in the frame transmitting apparatus  30 . The transmit-enable (TXEN) and transmit-data (TXD) signals in  FIG. 9  are supplied from the MAC processor  34  to the physical layer processor  38 . The flag signal (FLAG) is asserted while the MAC processor  34  holds the flag of a flagged frame, and is output to the flag detector  35  at, for example, a timing TS that marks the end of the preamble data that the physical layer processor  38  adds to the TXD signal. When the flag detector  35  receives the flag signal, the timestamp storage unit  37  stores the current time information, representing time TS, as a time stamp. 
         [0087]    As described above, before carrying out MAC and MACsec processing on the high-level frames generated in the upper layer processor  31 , the novel frame transmitting apparatus  30  flags each of the high-level frames that needs to have a timestamp stored. The flag detector  35  determines whether each processed high-level frame is flagged; if the flag detector  35  detects a flag, the current time information is stored as a timestamp. 
         [0088]    With this structure, the transmission time of a frame can be stored as its timestamp, so when network latency is calculated from the timestamps, the calculated latency excludes extraneous factors such as the time required for encryption and other MACsec processing or MAC processing. In addition, since the decision as to whether or not to store the timestamp is made just by checking a flag, the calculated network latency excludes the time required to analyze the frame to determine whether or not the timestamp is needed. 
         [0089]    The frame transmitting apparatus  30  in this embodiment can therefore provide high-precision timestamp information. 
       System Embodiment 
       [0090]    Referring to  FIG. 10 , a frame transmission/reception system  40  according to the present invention includes a master apparatus  50  and slave apparatus  60  that exchange timestamp information and other data over a communication network  20 . 
         [0091]      FIG. 11  shows the transmitting and receiving sequence by which the master apparatus  50  and slave apparatus  60  maintain clock synchronization. 
         [0092]    First, the master apparatus  50 , operating as the frame transmitting apparatus  30  in the above embodiment, transmits a Sync message to the slave apparatus  60  (step S 31 ) and stores the Sync message transmission time as timestamp T 1 . The slave apparatus  60 , operating as the frame receiving apparatus  10  in the above embodiment, receives the Sync message and stores the reception time of the message as timestamp T 2 . 
         [0093]    Next, the master apparatus  50  transmits a FollowUp message including the value of timestamp T 1  as content to the slave apparatus  60  (step S 32 ). The slave apparatus  60  receives the FollowUp message and stores timestamp T 1 . 
         [0094]    Next, the slave apparatus  60 , now operating as a frame transmitting apparatus  30 , transmits a DelayReq message to the master apparatus  50  (step S 33 ) and stores the DelayReq message transmission time as timestamp T 3 . The master apparatus  50 , operating as a frame receiving apparatus  10 , receives the DelayReq message and stores the reception time of the message as timestamp T 4 . 
         [0095]    Next, the master apparatus  50  transmits a DelayResp message including timestamp T 4  as content to the slave apparatus  60  (step S 34 ). The slave apparatus  60  stores the timestamp T 4  included in the DelayResp message. 
         [0096]    These operations put the slave apparatus  60  in possession of timestamps T 1 , T 2 , T 3 , and T 4 . The slave apparatus  60  can then calculate a propagation delay time or latency D 1  from, for example, the following equation: 
         [0000]        D 1=(( T 2 −T 1)+( T 4 −T 3))/2 
         [0000]    An offset value E 1  for clock synchronization can be calculated from the following equation: 
         [0000]        E 1=( T 2 −T 1)− D 1.
 
         [0097]    As described in the preceding embodiments, the stored timestamps exclude the time required for MAC and MACsec processing, so regardless of the amount of time required for this media access control and security processing, the calculated latency D 1  includes only the propagation delay time on the communication network  20 . The slave apparatus  60  therefore need only correct its current time by the above offset value E 1  to synchronize its clock to the clock of the master apparatus  50  with high precision. The frame transmission/reception system  40  can accordingly provide high-precision clock synchronization. 
         [0098]    In the above embodiments, MAC security processing is carried out on each successive frame, but other types of security protection processing, such as Internet Protocol security (IPsec) processing, for example, may be used with the same effect. 
         [0099]    Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims.