Method and apparatus for accommodating differrent clock frequencies in an ethernet passive optical network
View Patent ↗One embodiment of the present invention provides a system that accommodates different clock frequencies in an Ethernet passive optical network (EPON). The system receives a signal from an optical line terminal (OLT) at an optical network unit (ONU) and derives an OLT clock. The system also maintains a local clock. The system further receives from the OLT an assignment for an upstream transmission window, during which the ONU can transmit an upstream data burst to the OLT based on the local clock. The system adjusts the number of bits of the data burst without affecting the payload data carried in the data burst, thereby allowing the data burst to fit properly within the upstream transmission window and compensating for frequency differences between the local clock and the OLT clock. The system transmits the data burst based on the local clock in the upstream transmission window.
1. A method for accommodating different clock frequencies in an Ethernet passive optical network (EPON), the EPON including an optical line terminal (OLT) and an optical network unit (ONU), comprising:
comparing, by the ONU, an ONU clock and an OLT clock to determine whether the ONU clock is faster than the OLT clock;
determining, by the ONU, a first upstream transmission window in terms of an amount of data to be included in an upstream data burst that is to be transmitted if the OLT clock were to be used for transmission of the upstream data burst during the first upstream transmission window;
requesting, by the ONU, a second upstream transmission window, the second upstream transmission window being smaller than the first upstream transmission window when the ONU clock is faster than the OLT clock; and
transmitting by the ONU, the upstream data burst to the OLT during the second upstream transmission window, the upstream data burst including the amount of data used in determining the first upstream transmission window.
2. The method of claim 1 , further comprising:
receiving a signal from the OLT; and
deriving the OLT clock from the signal.
3. The method of claim 1 , wherein the comparing comprises:
comparing the ONU clock and the OLT clock to determine a difference between the ONU clock and the OLT clock.
4. The method of claim 1 , wherein the comparing further comprises:
comparing the ONU clock and the OLT clock to determine whether the ONU clock is slower than the OLT clock; and
requesting a third upstream transmission window, the third upstream transmission window being larger than the first upstream transmission window when the ONU clock is slower than the OLT clock.
5. The method of claim 4 , wherein the upstream data burst is characterized as including a first number of bits, and wherein the requesting the third upstream transmission window comprises:
requesting the third upstream transmission window in terms of a time needed to transmit a second number of bits when the ONU clock is slower than the OLT clock, the second number of bits being greater than the first number of bits.
6. The method of claim 1 , wherein the upstream data burst is characterized as including a first number of bits, and wherein the requesting the second upstream transmission window comprises:
requesting the second upstream transmission window in terms of a time needed to transmit a second number of bits when the ONU clock is faster than the OLT clock, the second number of bits being less than the first number of bits.
7. The method of claim 4 , further comprising:
determining a size of the third upstream transmission window in terms of an amount of data that is to be transmitted based on the OLT clock.
8. The method of claim 1 , wherein the determining the second upstream transmission window further comprises:
determining a size of the second upstream transmission window in terms of an amount of data that is to be transmitted based on the OLT clock.
9. An Ethernet passive optical network (EPON) for accommodating for different clock frequencies, comprising:
an optical network unit (ONU), coupled to an optical line terminal (OLT), configured to:
compare an ONU clock and an OLT clock to determine whether the ONU clock is faster than the OLT clock;
determine a first upstream transmission window in terms of an amount of data to be included in an upstream data burst that is to be transmitted if the OLT clock were to be used for transmission of the upstream data burst during the first upstream transmission window;
request a second upstream transmission window, the second upstream transmission window being, smaller than the first upstream transmission window when the ONU clock is faster than the OLT clock; and
transmit the upstream data burst to the OLT during the second upstream transmission window, the upstream data burst including the amount of data used in determining the first upstream transmission window.
10. The EPON of claim 9 , wherein the ONU is farther configured to compare the ONU clock and the OLT clock to determine a difference between the OLT clock and the ONU clock.
11. The EPON of claim 9 , wherein the ONU is further configured to compare the ONU clock and the OLT clock to determine whether the ONU clock is slower than the OLT clock, and to request a third upstream transmission window, the third upstream transmission window being larger than the first upstream transmission window when the ONU clock is slower than the OLT clock.
12. The EPON of claim 11 , wherein the ONU is further configured to determine a size of the third upstream transmission window in terms of an amount of data that is to be transmitted based on the OLT clock.
13. The EPON of claim 11 , wherein the ONU is further configured to transmit the upstream data burst during the third upstream transmission window.
14. The EPON of claim 9 , wherein the upstream data burst is characterized as including a first number of bits, and wherein the ONU is further configured to request the first upstream transmission window in terms of a time needed to transmit a second number of bits when the ONU clock is slower than the OLT clock, the second number of bits being greater than the first number of bits.
15. The EPON of claim 9 , wherein the upstream data burst is characterized as including a first number of bits, and wherein the ONU is further configured to request the first upstream transmission window in terms of a time needed to transmit a second number of bits when the ONU clock is faster than the OLT clock, the second number of bits being less than the first number of bits.
16. The EPON of claim 9 , wherein the ONU is further configured to determine a size of the second upstream transmission window in terms of an amount of data that is to be transmitted based on the OLT clock.
17. The EPON of claim 9 , wherein the ONU is further configured to transmit the upstream data burst during the second upstream transmission window.
18. A passive optical network (PON) for accommodating for different clock frequencies, comprising:
an optical network unit (ONU), coupled to an optical line terminal (OLT), configured to:
compare an ONU clock and an OLT clock to determine whether the ONU clock is faster than the OLT clock;
determine a first upstream transmission window in terms of an amount of data to be included in an upstream data burst that is to be transmitted if the OLT clock were to be used for transmission of the upstream data burst during the first upstream transmission window;
request a second upstream transmission window, the second upstream transmission window being smaller than the first upstream transmission window when the ONU clock is taster than the OLT clock; and
transmit the upstream data burst during the second upstream transmission window, the upstream data burst including the amount of data used in determining the first upstream transmission window.
19. The PON of claim 18 , wherein the upstream data burst is characterized as including a first number of bits, and wherein the ONU is further configured to request the second upstream transmission window in terms of a time needed to transmit a second number of bits when the ONU clock is faster than the OLT clock, the second number of bits being less than the first number of bits.
20. The PON of claim 18 , wherein the ONU is further configured to compare the ONU clock and the OLT clock to determine whether the ONU clock is slower than the OLT clock, and to request a third upstream transmission window larger than the first upstream transmission window when the ONU clock is slower than the OLT clock.