Method and apparatus for time synchronization 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 wireless network, comprising:
an access point comprising a first physical layer counter, wherein the access point is arranged to encode and transmit a timing synchronization frame over the wireless network, the timing synchronization frame comprising a timestamp derived from the first physical layer counter; and
a wireless device comprising a second physical layer counter, receiver circuitry and an interface,
wherein the wireless device is arranged to receive the timing synchronization frame and decode the timestamp, control the second physical layer counter in dependence on the timestamp, and, responsive to receiving the timing synchronisation frame, to output a signal from the interface for use by a synchronisation function used to regulate playback of synchronised media from the wireless device.
2. The wireless network according claim 1 , wherein the wireless device further comprises a further counter controlled by the synchronisation function.
3. The wireless network according to claim 2 , wherein the wireless device is further configured to use the signal from the interface to trigger the reading of the further counter and form a pair of values from the timestamp and the further counter reading for use in the synchronisation function.
4. The wireless network according to claim 2 , wherein the further counter is used to regulate playback of synchronised media from the wireless device.
5. A wireless device comprising:
a physical layer comprising: a counter; and receiver and decoder circuitry for repeatedly receiving and decoding a timing synchronisation frame and a timestamp included therein, wherein the counter is controlled in dependence on the timestamp; and
an interface connected to the physical layer and arranged, responsive to the physical layer receiving the timing synchronisation frame, to output a signal for use by a synchronisation function used to regulate playback of synchronised media from the wireless device.
6. The wireless device according to claim 5 , wherein the wireless device further comprises a second counter.
7. The wireless device according to claim 6 , wherein the signal is used to trigger the reading of the second counter.
8. The wireless device according to claim 7 , wherein the second counter reading and the timestamp form a pair of values used in the synchronisation function.
9. The wireless device according to claim 8 , wherein the pair of values are processed to perform clock rate compensation at the wireless device.
10. The wireless device according to claim 8 , wherein the wireless device is further arranged to transmit the pair of values to a further device in a response frame.
11. The wireless device according to claim 6 , wherein the second counter is a software counter.
12. The wireless device according to claim 6 , wherein the second counter is a monotonic higher layer counter.
13. The wireless device according to claim 6 , wherein the signal is used to trigger an update to a second counter.
14. The wireless device according to claim 6 , wherein the second counter is used to regulate playback of synchronised media from the wireless device.
15. The wireless device according to claim 5 , wherein the wireless station is arranged to control the physical layer counter by updating the counter value with a value derived from the timestamp.
16. The wireless device according to claim 5 , wherein the timing synchronization frame is a broadcast beacon frame.
17. The wireless device according to claim 16 , wherein the broadcast beacon frame is a Wi-Fi TSF frame.
18. A method for performing synchronisation at a wireless device comprising a physical layer having receiver circuitry and a counter and an interface connected to the physical layer, the method comprising:
at the receiver circuitry, repeatedly receiving and decoding a timing synchronisation frame and a timestamp included therein;
controlling the counter in dependence on the timestamp; and
responsive to the physical layer receiving the timing synchronisation frame, outputting a signal from an interface for use by a synchronisation function used to regulate playback of synchronised media from the wireless device.
19. The method according to claim 18 , further comprising the signal triggering the reading of a second counter at the wireless device.
20. The method according to claim 19 , further comprising forming a pair of values from the second counter reading and the timestamp for use in the synchronisation function.