Clock generating method and apparatus for an asynchronous transmission
Clock generating method and apparatus for an asynchronous transmission, wherein a plurality of actual receiving signal arrival times are detected and averaged, to thereby avoid influences of randomly distributed delay variations upon signal transmission. Clock synchronization between a transmitting clock and a receiving clock is recovered by correcting the receiving clock on the basis of the obtained average of the actual arrival times and an expected arrival time. Thereby, overflow and underflow of a receiver buffer can be prevented.
1. A clock generating method for an asynchronous transmission, comprising:
determining a plurality of actual signal arrival times for a number of samples;
averaging the plurality of actual signal arrival times over the number of samples;
correcting a timing of a receiving clock on a basis of an average of the plurality of actual signal arrival times and an expected signal arrival time;
deriving an expected signal arrival time from the receiving clock; and
determining a frequency difference between a frequency corresponding to an average of the plurality of actual signal arrival times and a frequency of said receiving clock, and changing the frequency of the receiving clock according to said frequency difference;
wherein the number of samples is set such that a time-dependent cell delay variation of actual signals being asynchronously transmitted has a mean value of zero.
2. The method according to claim 1 , wherein determining further comprises counting a time period between arrival of a first signal and arrival of a subsequent second signal.
3. The method according to claim 2 , wherein averaging further comprises storing counted time periods and calculating an average of stored time periods.
4. The method according to claim 1 , wherein the asynchronous transmission is an ATM transmission and the signal is an ATM cell.
5. A clock generating apparatus for asynchronous transmission comprising:
means for determining an average of actual signal arrival times over a number of samples and for generating a control signal on a basis of a determined average of the actual signal arrival times and an expected signal arrival time; and
means for correcting a timing of a receiving clock on a basis of the control signal;
wherein the number of samples is set such that a time-dependent cell delay variation of actual signals being asynchronously transmitted has a mean value of zero.
6. The apparatus according to claim 5 , wherein the means for correcting comprises a voltage controlled oscillator.
7. The apparatus according to claim 5 , wherein the means for determining comprises:
means for detecting an actual arrival time of a signal;
means for averaging a plurality of detected actual signal arrival times over the number of samples in order to obtain an average of the actual signal arrival times; and
means for comparing and correction control the average of the actual signal arrival times with the expected signal arrival time and for generating the control signal in accordance with a comparison result, wherein the expected signal arrival time is derived from the receiving clock.
8. The apparatus according to claim 7 , wherein the means for determining comprises means for storing a plurality of detected actual signal arrival times.
9. The apparatus according to claim 7 , wherein the means for detecting comprises a timer.
10. The apparatus according to claim 7 , wherein means for comparing and correction control comprises a phase detector, and wherein a polarity of the control signal is changed in accordance with a result of comparison.
11. The apparatus according to claim 5 , wherein the asynchronous transmission is an ATM transmission and the signal is an ATM cell.