Method and apparatus for frequency and timing distribution through a packet-based network

• US 20030137997A1
• Filed: 04/24/2002
• Published: 07/24/2003
• Est. Priority Date: 01/24/2002
• Status: Active Grant

• Current Assignee: Google Technology Holdings LLC (Alphabet Inc.)
• Original Assignee: RadioFrame Networks, Inc. (Motorola Solutions, Inc.)
• Inventors: Keating, Pierce V.
• Granted Patent: US 7,099,354 B2
• Time in Patent Office: Days
• US Class Current: 370/519
• CPC Class Codes: H04J 3/0667 Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays arrangements for monitoring round trip delays in packet switching networks H04L43/0864

Abstract

A reference frequency is distributed through a packet-based network to remote elements in a system. Timing packets are periodically sent from a master timing element, to be received by at least one peripheral timing element. Echo messages are sent to the master timing element by each peripheral timing element after a unique delay, in response to the reception of a timing packet. Loopback delay measurements are included in each timing packet for each peripheral timing element. Each peripheral timing element locks a loop using only timing packets which incur a minimum loopback delay.

Claims

1. A method of frequency and timing distribution between a master timing element and at least one peripheral timing element interconnected through a packet-based network, the method comprising:

periodically transmitting a timing packet from the master timing element according to a timing reference, each peripheral timing element coupled to receive the timing packets;

transmitting an echo packet to the master timing element from each peripheral timing element after a timing packet is received by the same peripheral timing element;

measuring a loopback delay between the start of the transmission of the timing packet and the reception of a corresponding echo packet for each peripheral timing element;

reading a plurality of loopback delay values corresponding to a peripheral timing element over time and designating the lowest loopback delay value for the peripheral timing element as the nonblocked loopback delay for that peripheral timing element; and

locking a loop in each peripheral timing element using only timing packets which incur a nonblocked loopback delay for the corresponding peripheral timing element as a reference.

2. The method of claim 1, further comprising:

transmitting each echo packet after a unique delay with respect to each peripheral timing element in order to reduce the likelihood of interblocking delays between echo packets.

3. The method of claim 2, further comprising:

storing a loop phase from a locked loop of each peripheral timing element when a timing packet is received in each corresponding peripheral timing element; and

designating the loop phase minus said unique delay and minus one half the nonblocked loopback delay as a phase reference for a peripheral timing element if the loop phase corresponds to a timing packet which incurred a nonblocked loopback delay value for the same peripheral timing element.

4. A synchronous, frequency-locked distributed system interconnected by a packet-based network comprising:

a timing reference;

a master timing element coupled to periodically transmit timing packets on the network according to the timing reference;

at least one peripheral timing element coupled to receive the timing packets, each peripheral timing element coupled to transmit an echo message on the network to the master timing element after a timing packet is received;

a locked loop included in each peripheral timing element;

wherein a loopback delay is determined according to when each echo message is received by the master timing element, the loopback delay corresponding to each echo message included in a payload field of the following timing packet to be transmitted;

wherein a minimum loopback delay is determined corresponding to each peripheral timing element, one half of said minimum loopback delay being representative of a nonblocked network path delay from the master timing element to a peripheral timing element; and

wherein the locked loop in each peripheral timing element is coupled to lock using only timing packets which incur a minimum loopback delay.

5. The synchronous, frequency-locked distributed system of claim 4, wherein each peripheral timing element transmits an echo message after a unique delay.

6. The synchronous, frequency-locked distributed system of claim 5, further comprising:

means to designate a phase reference in each locked loop according to the reception of timing packets which incur a minimum loopback delay to a corresponding peripheral timing element minus one half of the minimum loopback delay.

7. The synchronous, frequency-locked distributed system of claim 6, wherein the system is a distributed radio system, which includes at least one radio interface unit for wireless communication.

8. A method of synchronization between a master timing element and at least one peripheral timing element interconnected through a packet-based network, the method comprising:

periodically transmitting a timing packet from the master timing element according to a timing reference, the at least one peripheral timing element coupled to receive the timing packets;

transmitting an echo packet to the master timing element from the at least one peripheral timing element after a timing packet is received by the at least one peripheral timing element;

measuring a loopback delay related to the timing between the transmission of the timing packet and the reception of the corresponding echo packet from the at least one peripheral timing element;

utilizing a plurality of loopback delay values that are measured over time related to the at least one peripheral timing element to determine a designated loopback delay for the at least one peripheral timing element; and

utilizing timing packets which incur the designated loopback delay as a reference for synchronizing the at least one peripheral timing element and the master timing element.

9. The method of claim 8, wherein the designated loopback delay for the at least one peripheral timing element is the minimum loopback delay of the plurality of loopback delays.

10. The method of claim 8, wherein the loopback delays are measured at the master timing element between the transmission of the timing packet and the reception of a corresponding echo packet.

11. The method of claim 8, wherein the at least one peripheral timing element and the master timing element are synchronized by locking a loop in the at least one peripheral timing element using only timing packets which incur the designated loopback delay as a reference.

12. The method of claim 8, wherein the at least one peripheral timing element comprises a plurality of peripheral timing elements, each of the peripheral timing elements transmitting echo packets in response to timing packets from the master timing element, and each of the peripheral timing elements having an associated designated loopback delay.

13. The method of claim 12, further comprising:

transmitting each echo packet after a unique delay with respect to each peripheral timing element in order to reduce the likelihood of interblocking delays between echo packets.

14. The method of claim 13, wherein each peripheral timing element is synchronized with the master timing element by locking a loop in each peripheral timing element using only timing packets which incur the designated loopback delay for the corresponding peripheral timing element as a reference.

15. The method of claim 14, further comprising:

storing a loop phase from a locked loop of each peripheral timing element when a timing packet is received in each corresponding peripheral timing element; and

designating the loop phase minus the unique delay and minus one half the designated loopback delay as a phase reference for a peripheral timing element if the loop phase corresponds to a timing packet which incurred a designated loopback delay value for the same peripheral timing element.

16. The method of claim 8, wherein the packet-based network that interconnects the master timing element and the at least one peripheral timing element comprises a wireless LAN.

17. The method of claim 8, wherein the packet-based network that interconnects the master timing element and the at least one peripheral timing element comprises an Ethernet network.

18. A synchronous distributed system interconnected by a packet-based network, the system comprising:

a timing reference;

a master timing element coupled to periodically transmit timing packets on the network according to the timing reference;

at least one peripheral timing element coupled to receive the timing packets, the at least one peripheral timing element coupled to transmit an echo message on the network to the master timing element after a timing packet is received, the at least one peripheral timing element comprising a locked loop; and

wherein the master timing element measures the timing between the transmission of the timing packets and the receipt of the echo messages, and determines a designated loopback delay for the at least one peripheral timing element, the locked loop of the at least one peripheral timing element being coupled to lock using only timing packets which incur the designated loopback delay.

19. The system of claim 18, wherein the designated loopback delay is determined according to the minimum loopback delay that is measured by the master timing element.

20. The system of claim 18, wherein the at least one peripheral timing element comprises a plurality of peripheral timing elements, each peripheral timing element transmitting echo messages in response to timing packets received from the master timing element, the timing of the transmission of the echo messages from the peripheral timing elements being coordinated by unique delays for each of the peripheral timing elements such that in response to timing packets that are simultaneously sent to the peripheral timing elements at least some of the echo messages will be transmitted to the master timing element at different times.