Communication Apparatus and Delay Detecting Method

1. A communication apparatus that performs periodical communication with other communication apparatus connected via a transmission line, the communication apparatus comprising: a clock configured to measure time; a communicating unit configured to transmit and receive a communication frame; a time-stamp generating unit configured to generate a time stamp using the clock during transmission or during reception of the communication frame transmitted and received by the communication apparatus; a transmission-data storing unit configured to store periodical transmission data to be transmitted in periodically-transmitted communication frame; a reception-data storing unit configured to store periodical reception data received in periodically-received communication frame; a frame processing unit configured to generate a refresh instruction frame, of the communication frame, including a data refresh instruction to the other communication apparatus, the periodical transmission data from the transmission-data storing unit, and a frame transmission time, which is the time stamp of transmission timing acquired from the time-stamp generating unit, and to store, upon receiving a refresh instruction frame from the other communication apparatus, the periodical reception data included in the refresh instruction frame into the reception-data storing unit; and a one-way-delay detecting unit configured to determine, upon receiving the refresh instruction frame, whether a next refresh instruction frame is received within a first delay allowable time after the refresh instruction frame is received last time, and to determine, when the next refresh instruction frame is received within the first delay allowable time, according to whether a transmission time of the refresh instruction frame from the other communication apparatus to the communication apparatus is within a second delay allowable time, whether a delay has occurred in a communication frame transmitted from the other communication apparatus, wherein the one-way-delay detecting unit uses, as the transmission time, a difference between a frame reception time acquired from the time-stamp generating unit at the reception time of the refresh instruction frame and the frame transmission time stored in the refresh instruction frame.

2. The communication apparatus according to claim 1 , further comprising a round-trip-delay detecting unit configured to transmit a request frame to the other communication apparatus at time other than a periodical communication time and determine that a delay occurs when a response frame corresponding to the request frame is not received within a round-trip delay allowable time after the request frame is transmitted.

3. The communication apparatus according to claim 1 , further comprising a loss detecting unit configured to acquire, upon receiving the refresh instruction frame, a present frame reception time from the time-stamp generating unit, compare a difference between the present frame reception time and a last frame reception time acquired from the time-stamp generating unit when the refresh instruction frame was received last time with a loss evaluation time indicating a loss of a communication frame, and determine a loss of the communication frame.

4. The communication apparatus according to claim 3 , wherein, when a region for storing the frame transmission time in the communication frame is “a” bits and width of the clock is “b” bits (>a), the frame processing unit has a function of storing high-order (b-a) bits of the clock as high-order bit information when connection to the other communication apparatus is established and transmitting a communication frame including the high-order bit information when connection establishment is requested and a function of generating, during the periodical communication, a refresh instruction frame in which a low-order “a” bits of a time stamp obtained from the time-stamp generating unit is stored in a region where the frame transmission time is stored, and the loss detecting unit sets a value of the frame transmission time in the refresh instruction frame as a value of the “b” bits using the high-order bit information and performs loss detection for the refresh instruction frame.

5. The communication apparatus according to claim 1 , wherein the frame processing unit further has a function of transmitting, at a predetermined interval from a start of the periodical communication, the refresh instruction frame including a measurement instruction for a clock offset, and transmitting, upon receiving a response frame to the refresh instruction frame including the measurement instruction, the refresh instruction frame including a calculation instruction for a clock offset and a frame reception time indicating reception timing of the response frame.

6. The communication apparatus according to claim 1 , wherein, when a region for storing the frame transmission time in the communication frame is “a” bits and width of the clock is “b” bits (>a), the frame processing unit has a function of storing high-order (b-a) bits of the clock as high-order bit information when connection to the other communication apparatus is established and transmitting a communication frame including the high-order bit information when connection establishment is requested and a function of generating, during the periodical communication, a refresh instruction frame in which a low-order “a” bits of a time stamp obtained from the time-stamp generating unit is stored in a region where the frame transmission time is stored, and the one-way-delay detecting unit sets a value of the frame transmission time in the refresh instruction frame as a value of the “b” bits using the high-order bit information and performs one-way delay detection.

7. The communication apparatus according to claim 6 , wherein the frame processing unit has a function of generating, when the refresh instruction frame is transmitted, a check code from the high-order (b-a) bits of the frame transmission time, a header section of the refresh instruction frame, and a data section in which the transmission data is stored and includes the check code in the refresh instruction frame and a function of correcting, when the refresh instruction frame is received, the high-order bit information based on a size relation between low-order “a” bits of a reception time in the own apparatus of the refresh instruction frame and the frame transmission time of “a” bits in the refresh instruction frame, generating a check code from the corrected high-order bit information and the header section and the data section of the received communication frame, and determining whether the check code coincides with a check code in the received refresh instruction frame.

8. The communication apparatus according to claim 1 , further comprising a clock-offset storing unit configured to store a clock offset, which is an offset time of the clock of the own apparatus with respect to a clock included in the other communication apparatus, wherein the time-stamp generating unit generates, when the refresh instruction frame transmitted by the own communication apparatus is transmitted or received, a time stamp obtained by correcting time obtained from the clock with the clock offset.

9. The communication apparatus according to claim 8 , further comprising a loss detecting unit configured to acquire, when the refresh instruction frame is received, a present frame reception time from the time-stamp generating unit, compare a difference between the present frame reception time and a last frame reception time acquired from the time-stamp generating unit when the refresh instruction frame was received last time with a loss evaluation time indicating a loss of a communication frame, and determine a loss of the communication frame.

10. The communication apparatus according to claim 9 , wherein, when a region for storing the frame transmission time in the communication frame is “a” bits and width of the clock is “b” bits (>a), the frame processing unit has a function of storing, when connection to the other communication apparatus is established, as high-order bit information, high-order (b-a) bits of a clock of the other communication apparatus stored in a communication frame transmitted from the other communication apparatus and a function of generating, during the periodical communication, a refresh instruction frame in which a low-order “a” bits of a time stamp obtained from the time-stamp generating unit is stored in a region where the frame transmission time is stored, and the loss detecting unit sets a value of the frame transmission time in the refresh instruction frame as a value of the “b” bits using the high-order bit information and performs loss detection for the communication frame.

11. The communication apparatus according to claim 8 , further comprising a clock-offset calculating unit configured to store, when the refresh instruction frame including a measurement instruction for a clock offset is received from the other communication apparatus, the frame transmission time included in the refresh instruction frame including the measurement instruction as a master transmission time, store reception timing of the refresh instruction frame including the measurement instruction as a slave reception time, and store transmission timing of a response frame to the refresh instruction frame including the measurement instruction as a slave transmission time and store, when the refresh instruction frame including a calculation instruction for a clock offset is received from the other communication apparatus, a frame reception time indicating reception timing of the response frame stored in the refresh instruction frame including the calculation instruction as a master reception time and calculate the clock offset using the master transmission time, the slave reception time, the slave transmission time, and the master reception time.

13. The communication apparatus according to claim 11 , wherein, when a region for storing the frame transmission time in the communication frame is “a” bits and width of the clock is “b” bits (>a), the frame processing unit has a function of storing, when connection to the other communication apparatus is established, as high-order bit information, high-order (b-a) bits of a clock of the other communication apparatus stored in a communication frame transmitted from the other communication apparatus and a function of generating, during the periodical communication, a refresh instruction frame in which a low-order “a” bits of a time stamp obtained from the time-stamp generating unit is stored in a region where the frame transmission time is stored, and the offset calculating unit sets the master transmission time, the slave reception time, the slave transmission time, and the master reception time as a value of the “b” bits using the high-order bit information and performs calculation of the clock offset.

14. The communication apparatus according to claim 8 , wherein, when a region for storing the frame transmission time in the communication frame is “a” bits and width of the clock is “b” bits (>a), the frame processing unit has a function of storing, when connection to the other communication apparatus is established, as high-order bit information, high-order (b-a) bits of a clock of the other communication apparatus stored in a communication frame transmitted from the other communication apparatus and a function of generating, during the periodical communication, a refresh instruction frame in which a low-order “a” bits of a time stamp obtained from the time-stamp generating unit is stored in a region where the frame transmission time is stored, and the one-way-delay detecting unit sets a value of the frame transmission time in the refresh instruction frame as a value of the “b” bits using the high-order bit information and performs one-way delay detection.

15. A delay detecting method by any one of two communication apparatuses connected via a transmission line in a communication system in which periodical communication is performed between the two communication apparatuses, the delay detecting method comprising: a first timer starting step of starting a timer when the periodical communication is started; a first one-way-delay determining step of determining whether a refresh instruction frame including a refresh instruction from other communication apparatus is received within a predetermined time from the start of the timer; a frame-reception-time acquiring step of acquiring, when the refresh instruction frame is received at the first one-way-delay determining step, a frame reception time of reception timing of the refresh instruction frame; a frame-transmission-time acquiring step of acquiring a frame transmission time, which is a transmission time of the refresh instruction frame by the other communication apparatus stored in the refresh instruction frame; a second one-way-delay determining step of determining presence or absence of delay occurrence based on a difference between the frame reception time at the reception time of the refresh instruction frame and the frame transmission time stored in the refresh instruction frame; and a timer restarting step of restarting the timer after the second one-way-delay determining step, wherein the refresh instruction frame further includes data of the periodical communication.

16. The delay detecting method according to claim 15 , further comprising: a second timer starting step of transmitting a request frame to the other communication apparatus and starting the timer before the periodical communication is performed; and a round-trip-delay determining step of determining whether a response frame to the request frame is received from the communication apparatus within a predetermined time.

17. The delay detecting method according to claim 15 further comprising: a last-frame-transmission-time acquiring step of acquiring, upon receiving a communication frame indicating refresh preparation completion from the other communication apparatus, as a last frame transmission time, a frame transmission time of the communication frame stored in the communication frame; a present-frame-transmission-time acquiring step of acquiring, upon receiving the refresh instruction frame from the other communication apparatus next, as a present frame transmission time, a frame transmission time of the refresh instruction frame stored in the refresh instruction frame; and determining whether a difference between the present frame transmission time and the last frame transmission time is within a loss evaluation time in which a loss of the communication frame is not determined to have occurred.

18. The delay detecting method according to claim 17 , further comprising a last-frame-transmission-time resetting step of setting, when it is determined in the frame-loss determining step that a loss of the communication frame has not occurred, a value of the present frame transmission time as the last frame transmission time, wherein processing from the present-frame-transmission-time acquiring step is repeatedly executed.

19. The delay detecting method according to claim 15 , further comprising: a clock-offset-measurement instructing step in which a master station, which is a communication apparatus including a reference clock of the two communication apparatus, transmits, when timing for calculation of a clock offset that is a shift of time of a clock of a slave station, which is the other communication apparatus, with respect to the clock of the master station comes, to the slave station, a first refresh instruction request frame obtained by including a measurement instruction for the clock offset in a periodically-transmitted refresh instruction frame including a data refresh instruction to the slave station, periodical transmission data to be periodically transmitted, and a frame transmission time of the frame; a request-frame-reception processing step in which, upon receiving the first refresh instruction request frame, the slave station stores, as a master transmission time, the frame transmission time included in the refresh instruction request frame and stores, as a slave reception time, reception timing of the first refresh instruction request frame; a response-frame transmitting step in which the slave station transmits, to the master station, a refresh instruction response frame obtained by imparting a function of a response to the first refresh instruction request frame to a periodically-transmitted refresh instruction frame including a data refresh instruction to the master station and a periodical transmission data and stores a transmission time of the refresh instruction response frame as a slave transmission time; a clock-offset-calculation instructing step in which, upon receiving the refresh instruction response frame, the master station acquires a reception time of the refresh instruction response frame as a master reception time and transmits, to the slave station, a second refresh instruction request frame obtained by including a calculation instruction for the clock offset in a periodically-transmitted refresh instruction frame including a data refresh instruction to the slave station, a periodically-transmitted periodical transmission data, and the master reception time; and a clock-offset calculating step in which, upon receiving the second refresh instruction request frame, the slave station calculates, after acquiring the slave reception time, the clock offset of the slave station using the master transmission time, the slave reception time, the slave transmission time, and the master reception time.