Clock Synchronisation Between Devices Using Message Timestamps

1 . A method, at a first device comprising a first clock, of enabling synchronisation of a second clock at a second device with the first clock, the first device further comprising a third clock, the method comprising:

transmitting a first message comprising an identifier generated in dependence on the third clock to the second device;

generating a timestamp in dependence on the time at which the first message is transmitted from the first device according to the first clock;

generating a second message comprising the identifier and the generated timestamp; and

transmitting the second message to the second device.

2 . The method as claimed in claim 1 , wherein the first clock is configured to operate to a first clock resolution and the third clock is configured to operate to a second resolution lower than the first resolution.

3 . The method as claimed in claim 1 , wherein the first device comprises a communications interface comprising the third clock, the third clock being a physical layer clock.

4 . The method as claimed in claim 1 , wherein the first message is transmitted directly to the second device.

5 . The method as claimed in claim 1 , further comprising:

determining a playback time for playing media in dependence on a time according to the first clock and an offset period of time; and

directly transmitting the determined playback time to the second device.

6 . A first device comprising a first clock, the first device for enabling synchronisation of a second clock at a second device with the first clock, the first device further comprising a third clock, the first device being configured to:

transmit a first message comprising an identifier generated in dependence on the third clock to the second device;

generate a timestamp in dependence on the time at which the first message is transmitted from the first device according to the first clock;

generate a second message comprising the identifier and the generated timestamp; and

transmit the second message to the second device.

7 . The first device as claimed in claim 6 , the first device being further configured to:

determine an event time in dependence on a time according to the first clock and an offset period of time; and,

transmit the determined event time to the second device.

8 . The first device as claimed in claim 7 , wherein the event time comprises a playback time for playing media.

9 . The first device as claimed in claim 6 , wherein the first clock is configured to operate to a first clock resolution and the third clock is configured to operate to a second resolution lower than the first resolution.

10 . The first device as claimed in claim 6 , further comprising a communications interface comprising the third clock, the third clock being a physical layer clock.

11 . The first device as claimed in claim 6 , wherein the first message is transmitted directly to the second device.

12 . The first device as claimed in claim 8 , the first device being further configured to transmit the media to the second device.

13 . The first device as claimed in claim 7 , the first device being further configured to:

transmit the first message to a third device;

determine a further event time in dependence on a time according to the first clock and an offset period of time; and

transmit the determined further event time to the third device.

14 . The first device as claimed in claim 13 , wherein the further event time comprises a further playback time for playing media.

15 . The first device as claimed in claim 6 , the first device being further configured to:

receive a third message comprising a second identifier from a fourth device;

generate a second timestamp in dependence on the time at which the third message is received at the first device according to the first clock;

generate a fourth message comprising the identifier and the generated second timestamp; and

transmit the fourth message to a fifth device.

16 . The first device as claimed in claim 15 , wherein the first and second messages are transmitted in a first communications mode and the third and fourth messages are received and transmitted in a second communications mode.

17 . The first device as claimed in claim 6 , the first device being further configured to:

receive a third message comprising a second identifier from a fourth device;

generate a second timestamp in dependence on the time at which the third message is received at the first device according to the first clock;

receive a fourth message from a fifth device comprising the second identifier and a third timestamp, the third timestamp having been generated in dependence on the time at which the fifth device received the third message from the fourth device; and

adjust the first clock in dependence on a difference between a time indicated by the second timestamp and a time indicated by the third timestamp.

18 . The first device as claimed in claim 17 , wherein the first and second messages are transmitted in a first communications mode and the third and fourth messages are received in a second communications mode.

19 . The first device as claimed claim 6 , wherein the first device is further configured to operate according to a WLAN standard and the identifier is a timing synchronisation function.

20 . A non-transitory computer readable storage medium having stored thereon computer readable instructions that, when executed at a computer system, cause the computer system to synchronise a first clock of a first device to a second clock of a second device, the first device further comprising a third clock, by:

transmitting a first message comprising an identifier generated in dependence on the third clock to the second device;

generating a timestamp in dependence on the time at which the first message is transmitted from the first device according to the first clock;

generating a second message comprising the identifier and the generated timestamp; and

transmitting the second message to the second device.