Methods and apparatus for optical networks

1. A method comprising:

automatically measuring a transit time of a signal between ends of an optical cable by forming a measurement marker word with a loop back code, and transmitting that word in one direction between the ends of the cable, and then responsive to that code, forming a return measurement marker word and transmitting it in the opposite direction between ends of the cable and generating a representation of elapsed time thereof;

automatically determining a degree of chromatic dispersion between the ends using the representation of elapsed time; and

compensating for the chromatic dispersion.

2. A method as in claim 1 which includes transmitting traffic on the cable while measuring the transit time.

3. A method as in claim 1 where generating the representation of elapsed time includes measuring the transit time for a signal to travel in the one direction between the ends and opposite the one direction.

4. A method as in claim 1 which includes providing an optical supervisory channel in the cable and transmitting the measurement marker word in the supervisory channel.

5. A method as in claim 4 which includes providing a timer for generating a representation of elapsed time.

6. A method as in claim 5 which includes initiating operation of the timer when the marker word is transmitted.

7. A method as in claim 6 which includes terminating operation of the timer when the marker word returns to an initial transmitting end.

8. A method comprising:

transmitting traffic optically between first and second sites;

generating a marker word at one site and transmitting the marker word between sites simultaneously with transmitting traffic, and generating a revised marker word which includes generating a loop back performed indicator and incorporating it into the revised marker word and transmitting that word back to the one site while determining a transit time for each direction; and

terminating transmission of the marker word when the word is received at the one site, and making a determination of chromatic dispersion.

9. A method as in claim 8 which includes transmitting the word on an optical supervisory channel.

10. A method as in claim 8 where on receipt of the loop back performed indicator at the one site, transmission is terminated.

11. A method as in claim 10 where transit time is determined by initiating operation of a timer when the marker word is transmitted between sites.

12. A method as in claim 11 where operation of the timer is terminated when the loop back performed indicator is received at the one site.

13. A control system for use with a traffic carrying optical fiber, the system comprising:

at least one measurement module to be coupled to the optical fiber, the module includes circuitry to determine a transit time between first and second ends of the optical fiber, while carrying traffic, the module includes circuitry to loop back a received measurement marker word and incorporate a loop back performed indicator, and, where the module includes circuitry to accumulate a representation of the transit time; and

circuitry responsive to the representation of transit time, to determine a degree of chromatic dispersion.

14. A system as in claim 13 which includes a multi-state control element that generates the measurement marker word.

15. A system as in claim 13 which includes a timer to accumulate the representation.

16. An apparatus as in claim 13 where the chromatic dispersion determining circuitry includes a plurality of pre-stored values of effective group index of refraction.

17. An apparatus as in claim 13 where the chromatic dispersion determining circuitry includes at least one pre-stored chromatic dispersion constant.

18. A system as in claim 13 which includes a dispersion compensation element coupled to the circuitry.

19. An apparatus for compensating chromatic dispersion of an optical fiber which includes an optical supervisory channel, where the apparatus comprises:

first and second spaced apart network elements, the elements are each couplable to the optical supervisory channel;

first and second substantially identical measurement modules with the first module coupled to the first element and the second module coupled to the second element, at least one of the modules includes a timer to accumulate a representation of a round trip transit time of a measurement marker word between the elements, the modules each include control circuitry that receives an input signal to initiate a round trip measurement transmission between the elements while traffic is being carried between elements where the control circuitry inserts a loop back code into the measurement marker word; and

additional circuitry, responsive to contents of the timer, to determine chromatic dispersion of the fiber between the elements.

20. An apparatus for compensating chromatic dispersion of an optical fiber which includes an optical supervisory channel, where the apparatus comprises:

first and second spaced apart network elements, the elements are each couplable to the optical supervisory channel;

first and second substantially identical measurement modules with the first module coupled to the first element and the second module coupled to the second element, at least one of the modules includes a timer to accumulate a representation of a round trip transit time of a measurement marker word between the elements, the modules each include control circuitry that receives an input signal to initiate a round trip measurement transmission between the elements while traffic is being carried between elements wherein the control circuitry also initiates operation of the timer, and responsive to a received measurement transmission, a receiving module enters a loop back performed indicator thereinto and returns the transmission to a transmitting module which in response thereto terminates operation of timer; and at least one device that compensates chromatic dispersion, coupled to one of the elements.

21. An apparatus as in claim 20 where each module includes circuitry to enter a loop back indicium into the measurement transmission to the other module and where the other module responds thereto and enters the loop back performed indicator into the received transmission.

22. An apparatus as in claim 21 where in the absence of the loop back indicium, the other module does not return the transmission to the transmitting module.

23. A method comprising:

selecting one of a measurement mode, or a non-measurement mode, and responsive to selecting the measurement mode, automatically measuring a transit time of a signal between ends of an optical cable by forming a measurement marker word with a loop back code, and transmitting that word in one direction between the ends of the cable, and then responsive to that code, forming a return measurement marker word and transmitting it in the opposite direction between ends of the cable and generating a representation of elapsed time thereof;

automatically determining a degree of chromatic dispersion between the ends using the representation of elapsed time; and

compensating for the chromatic dispersion.

24. A method as in claim 23 which includes, incorporating a loop back performed indictor into the return measurement marker word.

25. An apparatus for use with an optical transmission channel comprising:

first and second substantially identical network elements, the elements are each couplable to the optical channel and wherein the elements each include control circuitry that receives an input signal to initiate a round trip measurement transmission between the elements, via the channel, while traffic is being carried therebetween and where the control circuitry at a receiving element responds to a received measurement transmission from a transmitting element by inserting a loop back performed code into a representation of that transmission before returning it to the transmitting element.

26. An apparatus as in claim 25 where each element includes chromatic dispersion determination circuitry.