Reducing packet delay variation of time-sensitive packets

1. A method of operating a first node of a network to provide a time function in the network, the method comprising, at the first node:

generating a time function protocol data unit for supporting the time function;

generating a plurality of preamble protocol data units;

sending the plurality of preamble protocol data units to a second node via a network path before sending the time function protocol data unit; and

sending the time function protocol data unit to the second node via the same network path,

wherein the plurality of preamble protocol data units carry routing information to cause the plurality of preamble protocol data units to follow the same network path as the time function protocol data unit,

wherein the time function protocol data unit carries information causing any intermediate node on the same network path to forward the time function protocol data unit before forwarding any remaining of the plurality of preamble protocol data units, and

wherein the time function protocol data unit carries an event message of a synchronisation protocol using a two-way exchange of messages.

2. A method according to claim 1 , wherein the preamble protocol data units carry information configured to indicate to the intermediate node to forward the preamble protocol data units before other data traffic.

3. A method according to claim 1 , wherein the preamble protocol data units and the time function protocol data unit carry priority information which indicates a priority level, wherein the priority level of the preamble protocol data units is below the priority level of the time function protocol data unit.

4. A method according to claim 3 , wherein the priority level of the preamble protocol data units is immediately below the priority level of the time function protocol data unit.

5. A method according to claim 3 , wherein the priority level of the time function protocol data unit and the priority level of the preamble protocol data unit are higher than a priority level of other data traffic in the network.

6. A method according to claim 1 wherein the network permits a maximum protocol data unit size and a length of the preamble protocol data units is less than 5% of the maximum protocol data unit size.

7. A method according to claim 1 wherein the preamble protocol data unit has a length which is a minimum protocol data unit size permitted in the network.

8. A method according to claim 1 wherein the network supports a maximum protocol data unit size and a combined length of a sequence of the plurality of preamble protocol data units is equal to, or longer than, the maximum protocol data unit size.

9. A method according to claim 1 comprising:

determining when the time function protocol data unit is waiting in a queue at the first node; and

generating a sequence of the plurality of preamble protocol data units and inserting the sequence of the plurality of preamble protocol data units into a transmission queue before the time function protocol data unit is inserted into the transmission queue.

10. A method according to claim 1 , wherein the generating the plurality of preamble protocol data units comprises determining a number of preamble protocol data units based on a number of intermediate nodes to be traversed between the first node and the second node.

11. A method according to claim 1 , wherein the generating of the plurality of preamble protocol data units comprises generating a number of preamble protocol data units such that a combined length of a sequence of the plurality of preamble protocol data units is equal to, or greater than, a length of a multiple of a maximum protocol data unit size allowed in the network.

12. A method according to claim 1 comprising:

recording a timestamp of a transmission time of the time function protocol data unit; and

sending the timestamp in a subsequent time function protocol data unit.

13. A method according to claim 1 wherein the time function is one of: Precision Time Protocol and Network Time Protocol.

14. A method according to claim 13 wherein the time function protocol data unit carries a Precision Time Protocol Synch message or a Precision Time Protocol Delay Request message.

15. A method according to claim 1 comprising, at an intermediate node along the network path:

receiving the plurality of preamble protocol data units and storing the plurality of preamble protocol data units in a second queue having a second priority level;

receiving the time function protocol data unit and storing the time function protocol data unit in a first queue having a first priority level which is higher than the second priority level, wherein the plurality of preamble protocol data units are received before the time function protocol data unit, and

controlling a time order of forwarding, by the intermediate node, queued protocol data units based on the priority levels of the queues.

16. A method according to claim 15 wherein the controlling a time order of forwarding comprises forwarding the queued time function protocol data unit before forwarding any remaining queued preamble protocol data units.

17. A method according to claim 15 comprising:

receiving a data traffic protocol data unit carrying data traffic and storing the data traffic protocol data unit in a third queue having a third priority level which is lower than the second priority level; and

the controlling a time order of forwarding comprises:

forwarding, by the intermediate node, the queued preamble protocol data units before forwarding the queued data traffic protocol data unit; and

forwarding, by the intermediate node, the time function protocol data unit before forwarding any remaining queued preamble protocol data units.

18. A method according to claim 15 wherein the intermediate node allocates at least one of the preamble protocol data units to a queue according to at least one of:

priority information carried by the protocol data unit;

an indicator of a message type.

19. An apparatus for supporting a time function in a network, the apparatus configured to:

generate a time function protocol data unit for supporting the time function;

generate a plurality of preamble protocol data units;

send the plurality of preamble protocol data units to a destination node via a network path before sending the time function protocol data unit; and

send the time function protocol data unit to the destination node via the same network path,

wherein the preamble protocol data units carry routing information to cause the preamble protocol data units to follow the same network path as the time function protocol data unit,

wherein the time function protocol data unit carries information causing any intermediate node on the same network path to forward the time function protocol data unit before forwarding any remaining of the preamble protocol data units, and

wherein the time function protocol data unit carries an event message of a synchronisation protocol using a two-way exchange of messages.

20. A system comprising:

a first node and a first clock, wherein the first node is configured to:

generate a time function protocol data unit for supporting the time function,

generate a plurality of preamble protocol data units,

send the plurality of preamble protocol data units to a second node via a network path before sending the time function protocol data unit, and

send the time function protocol data unit to the second node via the same network path,

wherein the time function protocol data unit carries information causing any intermediate node on the same network path to forward the time function protocol data unit before forwarding any remaining of the preamble protocol data units, and

wherein the plurality of preamble protocol data units carry information causing the intermediate node to forward the plurality of preamble protocol data units before other data traffic;

the second node comprising a second clock;

the same network path connecting the first node to the second node, the same network path comprising at least one intermediate node,

wherein the second node is configured to:

receive the time function protocol data unit,

use the time function protocol data unit to determine a time offset between the second clock and a clock at the first node, and

synchronise the second clock with the first clock.

21. A system according to claim 20 wherein the at least one intermediate node comprises another apparatus configured to:

receive a plurality of preamble protocol data units and store the plurality of preamble protocol data units in a second queue having a second priority level;

receive a time function protocol data unit for supporting a time function and store the time function protocol data unit in a first queue having a first priority level which is higher than the second priority level, wherein the plurality of preamble protocol data units are received before the time function protocol data unit; and

control a time order of forwarding, by the intermediate node, queued protocol data units based on the priority levels of the queues.

22. A system according to claim 20 wherein the network is a wireless backhaul network and the second node is a wireless base station.

23. An apparatus for supporting a time function in a network, the apparatus comprising a processor and a memory, the memory containing instructions that when executed by the processor cause the processor to:

generate a time function protocol data unit for supporting the time function;

generate a plurality of preamble protocol data units;

control sending the plurality of preamble protocol data units to a destination node via a network path before sending the time function protocol data unit; and

control sending the time function protocol data unit to the destination node via the same network path,

wherein the plurality of preamble protocol data units carry routing information to cause the plurality of preamble protocol data units to follow the same network path as the time function protocol data unit,

wherein the time function protocol data unit carries information causing any intermediate node on the same network path to forward the time function protocol data unit before forwarding any remaining of the plurality of preamble protocol data units, and

wherein the time function protocol data unit carries an event message of a synchronisation protocol using a two-way exchange of messages.

24. An apparatus for supporting a time function in a network, the apparatus comprising:

a first generation module configured to generate a time function protocol data unit required for supporting the time function;

a second generation module configured to generate a sequence of preamble protocol data units;

an output module configured to send the sequence of preamble protocol data units to a destination node via a network path before sending the time function protocol data unit; and

the output module configured to send the time function protocol data unit to the destination node via the same network path,

wherein the sequence of preamble protocol data units carry routing information to cause the sequence preamble protocol data units to follow the same network path as the time function protocol data unit,

wherein the time function protocol data unit carries information causing any intermediate node on the same network path to forward the time function protocol data unit before forwarding any remaining of the sequence of preamble protocol data units,. and

wherein the time function protocol data unit carries an event message of a synchronisation protocol using a two-way exchange of messages.