Method and apparatus for time synchronisation in wireless networks
A wireless media distribution system is provided comprising an access point ( 6 ) for broadcasting media and a plurality of stations ( 2 ) for reception and playback of media. Each station is configured for receiving and decoding a timestamp in a beacon frame transmitted repeatedly from the access point. This is used to control the output signal of a station physical layer clock ( 12 ) which is then used as a clock source for an application layer time synchronisation protocol. This application layer time synchronisation protocol can then be used in the station to control an operating system clock ( 8 ) for regulating playback of media.
1. A physical layer clock, comprising:
a counter configured to increment in dependence on a clocking signal, wherein the physical layer clock is configured to:
receive a timestamp value,
determine an error between a current counter value and the received timestamp value,
adjust the clocking signal in dependence on the determined error, and
provide the adjusted clocking signal to the counter,
wherein the counter is configured to be overwritten with the received timestamp value.
2. The physical layer clock according to claim 1 , wherein the physical layer clock comprises an oscillator configured to generate the clocking signal.
3. The physical layer clock according to claim 2 , wherein the oscillator is a free running crystal oscillator.
4. The physical layer clock according to claim 1 , wherein the physical layer clock receives the timestamp value from receive and decode circuitry, the receive and decode circuitry being configured to repeatedly receive and decode a broadcast beacon frame and a timestamp included therein.
5. The physical layer clock according to claim 4 , wherein the receive and decode circuitry is configured to overwrite the counter with the received timestamp value.
6. The physical layer clock according to claim 4 , wherein the beacon frames are beacon frames of a Wi-Fi protocol.
7. The physical layer clock according to claim 1 , wherein the received timestamp value is a value of a counter at an access point.
8. The physical layer clock according to claim 1 , wherein the physical layer clock comprises an error calculation unit which is configured to determine the error between the current counter value and the received timestamp value before the counter is overwritten with the received value in the timestamp.
9. The physical layer clock according to claim 8 , wherein the error calculation unit is configured to determine the error in dependence on a number of ticks of the counter between receiving the beacon frame and determining the current counter value.
10. The physical layer clock according to claim 1 , wherein the physical layer clock comprises a rate adjustment module which is configured to adjust the clocking signal in dependence on the determined error and is coupled between the oscillator and the counter.
11. The physical layer clock according to claim 10 , wherein the rate adjustment module is a phase locked loop.
12. The physical layer clock according to claim 10 , wherein the rate adjustment module is configured to determine the adjusted rate in dependence on time elapsed since a previous correction and the determined error.
13. The physical layer clock according to claim 1 , wherein the timestamp is a timing synchronisation function of a Wi-Fi protocol.
14. A wireless station comprising:
the physical layer clock as set forth in claim 1 ; and
receive and decode circuitry configured to:
repeatedly receive and decode a broadcast beacon frame and a timestamp included therein, and
overwrite the counter with the received timestamp value.
15. The wireless station according to claim 14 , wherein the wireless station is configured to play media in dependence on the output clock signal.
16. The wireless station according to claim 14 , further comprising a software clock configured to use the physical layer clock as a time source.
17. The wireless station according to claim 16 wherein the software clock is used to control the playback of media.
18. A wireless media distribution system comprising:
a wireless station as set forth in claim 14 ; and
an access point (AP) for broadcasting media, wherein the AP is configured to transmit repeatedly a beacon frame which includes a timestamp derived from a physical layer clock in the AP.
19. A method for synchronising a physical layer clock of a wireless station to a physical layer clock of an access point, the method comprising:
receiving at the physical layer clock of the wireless station a timestamp value;
incrementing a counter in dependence on a clocking signal;
determining an error between a current counter value and the received timestamp value;
overwriting the counter with the received timestamp value, and
adjusting the clocking signal in dependence on the determined error; and
providing the adjusted clocking signal to the counter.