System and method for performing clock synchronization of nodes connected via a wireless local area network

• US 20030172179A1
• Filed: 11/07/2002
• Published: 09/11/2003
• Est. Priority Date: 03/07/2002
• Status: Active Grant

• Current Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Sato, Takashi, Choi, Sunghyun, del Prado Pavon, Javier, Nandagopalan, Sai Shankar, Bennet, Jeff
• Granted Patent: US 7,120,092 B2
• Time in Patent Office: Days
• US Class Current: 709/236
• CPC Class Codes: H04J 3/0655 using timestamps

Abstract

A clock synchronization method and apparatus is disclosed for use in a communication system including a plurality of wireless nodes communicatively coupled via a wireless network, each of the plurality of wireless nodes having a local time base, and one of the plurality of wireless nodes being designated as a master node having a master time base which serves as a master clock against which the local time bases are synchronized. The clock synchronization method includes the steps of periodically transmitting synchronization frames to the plurality of non-master nodes so as to adjust the slave clocks associated with the respective non-master nodes. The synchronization frames are distributed from the master node at near-periodic intervals and includes a cycle time value that corresponds to the end of the previously transmitted synchronization frame. The slave clocks (i.e., non-master nodes) receiving the synchronization frame determine the cycle time value at the point of reception of the synchronization frame and adjusts their clocks by calculating a difference value between the received cycle time and a previously saved local cycle time value.

Claims

1. In a communication system including a plurality of non-master nodes being communicatively coupled via a network, each of said plurality of non-master nodes having a local time base, one of said plurality of non-master nodes being designated as a master node having a master time base which serves as a master clock against which said non-master node'"'"'s local time bases are synchronized, a clock synchronization method for adjusting the local time bases, the method comprising the steps of:

in an ith iteration cycle, at said master node;

(1) transmitting a synchronization frame, F_i, including an i−

1th iteration cycle master node time value, a(n<sub>i−

1</sub>), and an i−

1th iteration cycle sequence number, n<sub>i−

1</sub>;

(2) detecting, during transmission from said master node, a predefined symbol position associated with said transmitted synchronization frame, F_i;

(3) saving an ith iteration cycle master node time value, a(n_i), corresponding to a time of detection of said predefined symbol position; and

(4) saving an ith iteration cycle sequence number, n_i, corresponding to said synchronization frame, F_i;

at each of said non master nodes;

(a) receiving said synchronization frame, F_i;

(b) detecting, during reception of said synchronization frame, F_i, said predefined symbol position;

(c) saving an ith iteration cycle non-master node cycle time value, b(n_i), corresponding to a time of detection of said predefined symbol position;

(d) saving said ith iteration cycle sequence number, n_i corresponding to said synchronization frame, F_i;

(e) computing a cycle time difference value as, a(n<sub>i−

1</sub>)−

b(n<sub>i−

1</sub>), wherein b(n<sub>i−

1</sub>) is an i−

1th iteration cycle non-master node time value; and

(f) adjusting said non-master node'"'"'s local time base in accordance with said computed cycle time difference value.

2. The method of claim 1, wherein in an i+1th iteration cycle, a(n_i−

1) and b(n<sub>i−

1</sub>), stored in said ith iteration cycle, are used to compute said cycle time difference value.

3. The method according to claim 2, wherein a(n_i) and n_i, are transmitted in a next synchronization frame, F_i+1.

4. The method of claim 1, wherein said network is one of a wired and a wireless network.

5. The method of claim 2, wherein said wireless network operates in accordance with the IEEE 802.11 wireless standard.

6. The method of claim 1, wherein said synchronization frame F_i and said next synchronization frame, F_i+1 are transmitted at near-periodic intervals.

7. The method of claim 1, further comprising the step of correcting the saved cycle time value, a(n_i), at the master node, by subtracting an offset value prior to said step (3), said corrected cycle time value computed as:

a(n_i)_corrected=cycle_time_{(at observed PHY-TXEND.confirm)}−

offset

where;

offset=a delay associated with a time difference between the observation of the said symbol position of said synchronization frame F_i and said next synchronization frame, F_i+1 at a higher layer of the hierarchy and the time of detection at the physical layer; and

;

PHY-TXEND.confirm=a time that the physical layer confirms the detection of said symbol position.

8. The method of claim 1, further comprising the step of correcting the saved cycle time value, b(n_i), at each of said non-master nodes, by subtracting an offset value prior to step (iv), computed as:

b(n_i)_corrected=cycle_time_{(at observed PHY-RXEND.indication)}−

offset

where;

offset=a delay associated with a time difference between the observation of said symbol position of said synchronization frame F_i and said next synchronization frame, F_i+1 at a higher layer of the non-master node hierarchy and the time of detection at the physical layer; and

PHY-RXEND.indication=a time that the physical layer confirms the detection of said symbol position.

9. The method of claim 1, wherein the sequence number, n_i of the synchronization frame F_i and the sequence number n_i+1 of the next synchronization frame, F_i+1, corresponds to a 2-octet sequence control field of the synchronization frame.

10. The method of claim 1, wherein the synchronization frame F_i and said next synchronization frame, F_i+1 are medium access control (MAC) frames.

11. The method of claim 1, wherein the synchronization frame F_i and said next synchronization frame, F_i+1 are identified at each of said non master nodes by a source address and a destination address.

12. The method of claim 11, wherein the destination address is a reserved multicast medium access control (MAC) address, reserved for transmitting clock synchronization information.

13. In a communication system including a plurality of non-master nodes said plurality of non-master nodes being communicatively coupled via a network, each of said plurality of non-master nodes having a local time base, one of said plurality of non-master nodes designated as a master node having a master time base which serves as a master clock against which said non-master local time bases are synchronized, a clock synchronization method for adjusting the local time bases, the method comprising the steps of:

(a) transmitting an ith synchronization frame F_i from said master node over said network to said plurality of non-master nodes;

(b) saving a cycle time value, a(n_i), corresponding to a time of detection of a symbol position of the ith synchronization frame F_i and a sequence number n_i associated with said ith synchronization frame F_i;

at said plurality of non-master nodes;

(c) receiving said ith synchronization frame F_i;

(d) detecting the symbol position of said ith synchronization frame F_i;

(d) saving a cycle time value, b(n_i), associated with a time of detection of the symbol position and said sequence number n, associated with said ith synchronization frame F_i;

(e) computing a cycle time difference value as;

a(n<sub>i−

1</sub>)−

b(n<sub>i−

1</sub>); and

(f) adjusting said local time bases in accordance with said computed cycle time difference value.

14. The communication system of claim 13, further comprising the step of transmitting an i+1th synchronization frame, F_i+1, from said master node in a next iteration of said clock synchronization method, said next synchronization frame, F_i+1, including said cycle time value, a(n_i), and said sequence number n, saved at the ith iteration cycle at said step (b).

15. In a communication system including a plurality of non-master nodes said plurality of non-master nodes being communicatively coupled via a network, each of said plurality of non-master nodes having a local time base, one of said plurality of non-master nodes designated as a master node having a master time base which serves as a master clock against which said non-master local time bases are synchronized, a clock synchronization method for adjusting the local time bases, the system comprising:

means for transmitting a current synchronization frame, F_i;

means for detecting a symbol position during on air transmission of the current synchronization frame, F_i over said wireless network;

means for saving a cycle time value associated with a time of detection of the detected symbol position during said transmission, and a sequence number of said current synchronization frame;

means for receiving said transmitted current synchronization frame;

means for detecting the symbol position during on air reception of the received current synchronization frame over said network;

means for saving a cycle time value associated with a time of detection of the symbol position of the reception of said current synchronization frame and a sequence number n of said current synchronization frame;

means for transmitting a next synchronization frame including the saved symbol position and sequence number corresponding to said current synchronization frame;

means for computing a cycle time value difference; and

means for adjusting said local time base in accordance with said computed time difference.