Timing source
A timing source is provided for sending timing information via a packet network. The source comprises a first clock for generating the timing information and a packet forming section for forming a sequence of packets. An output section in the form of a packet launch control section transmits the packets via the network as packet bursts, each of which comprises a plurality of packets with the intervals between consecutive packets of each burst being less than the intervals between consecutive packets. A time-stamping section inserts into each packet a transmission time derived from the timing information generated by the clock.
1. A timing source for sending timing information via a packet network, said source comprising:
a first clock for generating said timing information;
a packet forming section for forming a sequence of packets;
an output section for transmitting said packets of said sequence via said network as packet bursts, each of which comprises a plurality of said packets with a time difference between consecutive packets of a same burst being less than a time difference between consecutive bursts, and said time difference between packets in the same burst being greater than a length of said packet, so that individual delays in transit times of at least some packets of the sequence are arranged to be minimized and to be consistent with each other across the network; and
a time-stamping section for inserting into each said packet of said sequence a transmission time derived from said timing information generated by said first clock.
2. A source as claimed in claim 1 , in which said output section is arranged to vary said time difference between transmitting said consecutive packets of each said burst.
3. A source as claimed in claim 1 , in which said output section is arranged to vary a number of said packets in each said burst.
4. A source as claimed in claim 1 , in which said output section is arranged to vary the sizes of the packets.
5. A source as claimed in claim 1 , in which said output section is arranged to vary said time difference between said consecutive bursts.
6. A source as claimed in claim 2 , in which said output section is arranged to vary transmission times of said packets so as to reduce transit times to a destination.
7. A source as claimed in claim 6 , in which said output section is arranged to vary said transmission times in accordance with measured said transit times.
8. A source as claimed in claim 7 , in which said transit times are measured at said destination.
9. A source as claimed in claim 7 , in which said transit times are for packets received from said destination via a return path and are measured at said source.
10. A source as claimed in claim 5 , in which said output section is arranged to vary transmission times of said bursts so as to reduce transit times to a destination.
11. A source as claimed in claim 10 , in which said output section is arranged to vary said transmission times in accordance with measured said transit times.
12. A source as claimed in claim 11 , in which said transit times are measured at said destination.
13. A source as claimed in claim 11 , in which said transit times are for packets received from said destination via a return path and are measured at the said source.
14. A source as claimed in claim 1 , in which said output section is arranged to transmit one of said bursts and said packets of each said burst at an interval equal to a product of a positive integer and a frame rate of a time division multiplex path in said network.
15. A source as claimed in claim 1 , in which said output section is arranged to transmit one of said bursts and said packets of each said burst at an interval equal to a product of a positive integer and 125 microseconds.
16. A source as claimed in claim 1 , in which said packets contain padding data and are for conveying said timing information only.
17. A source as claimed in claims 1 , in which said packets contain payload data associated with said timing information.
18. A source as claimed in claim 17 in which said packet-forming section is arranged to form said packets from incoming data of a constant data rate on a communication path.
19. A source as claimed in claim 18 , in which said incoming data represent real-time events.
20. A source as claimed in claim 19 , in which said incoming data comprise at least one of audio and video.
21. A source as claimed in claim 18 , in which said communication path comprises a time division multiplex path.
22. A combination of:
a timing source for sending timing information via a packet network, said source comprising a first clock for generating said timing information, a packet forming section for forming a sequence of packets, an output section for transmitting said packets of said sequence via said network as packet bursts, each of which comprises a plurality of said packets with a time difference between consecutive packets of a same burst being less than a time difference between consecutive bursts, and said time difference between packets in the same burst being greater than a length of said packet, so that individual delays in transit times of at least some packets of the sequence are arranged to be minimized and to be consistent with each other across the network, and a time-stamping section for inserting into each said packet of said sequence a transmission time derived from said timing information generated by said first clock; and
a destination apparatus comprising a second adjustable clock and an arrangement for adjusting said second clock into synchronism with said first clock.
23. A combination as claimed in claim 22 , in which said adjusting arrangement comprises a destination time-stamping arrangement for determining a local time of arrival of each said packet of said sequence and a control circuit for adjusting said second clock as a function of differences in packet transit times.
24. A combination as claimed in claim 23 , in which said packet transit times are minimum packet transit times in consecutive time periods.
25. A method of sending timing information via a packet network, said method comprising the steps of:
generating said timing information; forming a sequence of packets;
transmitting said packets of said sequence via said network as packet bursts, each of which comprises a plurality of said packets with a time difference between consecutive packets of a same burst being less than a time difference between consecutive bursts and said time difference between packets in the same burst being greater than a length of said packet, so that individual delays in transit times of at least some packets of the sequence are arranged to be minimized and to be consistent with each other across the network; and
inserting into each said packet of said sequence a transmission time derived from said timing information.