Fast handover method optimized for IEEE 802.11 networks

• US 7,676,226 B2
• Filed: 05/17/2005
• Issued: 03/09/2010
• Est. Priority Date: 05/17/2004
• Status: Active Grant

• Current Assignee: Samsung Electronics Co. Ltd.
• Original Assignee: Samsung Electronics Co. Ltd.
• Inventors: Han, Youn-hee, Lee, Choong-hun, Jang, Hee-jin
• Primary Examiner(s): Ramakrishnaiah; Melur
• Application Number:
  US11/130,250
• Publication Number:
  US 20050255847A1
• Time in Patent Office: 1,757 Days
• Field of Search: 455/457, 4554561-4566, 455436-438, 455442-444, 455524-525
• US Class Current: 455/436
• CPC Class Codes: H04W 36/0061 of neighbour cell information
• CPC Class Codes: H04W 36/00835 Determination of neighbour cell lists
• CPC Class Codes: H04W 36/302 due to low signal strength
• CPC Class Codes: H04W 84/12 WLAN [Wireless Local Area Networks]

Abstract

A fast handover method optimized for IEEE 802.11 networks. In a wireless local area system including a mobile terminal and at least two wireless access points (APs) that communicate with the mobile terminal over a unique radio channel, the fast handover method includes receiving a beacon frame signal from the serving AP and the neighbor APs of the mobile terminal; generating a first signal to determine a state of each of the neighbor APs based on the beacon frame signal received from each of the neighbor APs; comparing the first signal with predefined thresholds, classifying the neighbor APs into a detected AP, a candidate AP, and a target AP according to a result of the comparison, and storing the classification result in a neighbor AP list; and selecting an AP for the handover based on the classification result in the neighbor AP list.

Claims

1. A fast handover method for a mobile terminal in a wireless local area system including the mobile terminal and a plurality of wireless access points (APs), of which one is a serving AP currently communicating with the mobile terminal and others represent neighbor APs of the mobile terminal, that communicate with the mobile terminal over a unique radio channel, the method comprising:

receiving a beacon frame signal from the serving AP and the neighbor APs of the mobile terminal;

generating a first signal to determine a state of each of the neighbor APs based on the beacon frame signal received from each of the neighbor APs;

comparing the first signal with predefined thresholds, classifying the neighbor APs into a detected AP, a candidate AP, and a target AP according to a result of the comparison, and storing the classification result in a neighbor AP list; and

selecting an AP for the handover based on the classification result in the neighbor AP list.

2. The fast handover method of claim 1, wherein the beacon frame signal is received from the APs to the mobile terminal at intervals.

3. The fast handover method of claim 1, wherein the beacon frame signal is received from the APs to the mobile terminal when the mobile terminal requests the beacon frame signal.

4. The fast handover method of claim 1, wherein the first signal is one of signal-to-noise ratio (SNR), received signal strength indication (RSSI), bit error rate (BER), and packet error rate (PER).

5. The fast handover method of claim 4, wherein, when the first signal is the SNR, the SNR is processed to generate a Smoothed SNR, the generated Smoothed SNR is compared with predefined thresholds, and the neighbor APs are classified into a detected AP, a candidate AP, and a target AP according to the comparison.

6. The fast handover method of claim 5, wherein the Smoothed SNR is obtained from the equation:

Smoothed SNR=K×

SNR_c+(1−

K)SNR_p,

where K is a variable, SNR_c is a SNR measured at a current time, and SNR_p is a SNR measured at a previous period time.

7. The fast handover method of claim 1, wherein

the predefined thresholds include a first threshold THR_1, a second threshold THR_2, and a third threshold THR_3, and

the first threshold THR_1 is greater than the second threshold THR_2, and the second threshold THR_2 is greater than the third threshold THR_3.

8. The fast handover method of claim 1, wherein the candidate AP and the target AP establish new communications with the mobile terminal.

9. The fast handover method of claim 7, wherein the neighbor AP is determined to be in the detected AP state when the first signal is initially detected from the neighbor APs and a strength of the first signal detected from the neighbor APs is below the second threshold THR_2.

10. The fast handover method of claim 9, wherein, when the neighbor AP maintains the detected AP state when the mobile terminal does not receive the beacon frame signal from the neighbor AP more than at least one time within 100 ms intervals while the neighbor AP is in the detected AP state.

11. The fast handover method of claim 9, wherein the neighbor AP is deleted from the neighbor AP list when the mobile terminal does not receive the beacon frame signal from the neighbor AP more than at least one time within 300 ms while the detected AP is in the detected AP state.

12. The fast handover method of claim 9, wherein the neighbor AP is changed from the detected AP state to a first candidate AP state when the strength of the first signal detected from the neighbor AP exceeds the second threshold THR_2 while the neighbor AP is in the detected AP state.

13. The fast handover method of claim 12, wherein the neighbor AP is changed from the first candidate AP to the target AP state when the strength of the first signal detected from the neighbor AP exceeds a strength of the first signal detected from the other neighbor APs of the mobile terminal while the neighbor AP is in the first candidate AP state.

14. The fast handover method of claim 12, wherein the neighbor AP is changed from the first candidate AP state to the detected AP state when the strength of the first signal detected from the neighbor AP falls below the second threshold THR_2 while the neighbor AP is in the first candidate AP state.

15. The fast handover method of claim 13, wherein the neighbor AP is changed from the target AP state to a second candidate AP state when the strength of the first signal detected from the neighbor AP falls below the strength of the first signal detected from the other neighbor APs of the mobile terminal while the neighbor AP is in the target AP state.

16. The fast handover method of claim 13, wherein the neighbor AP is changed from the target AP state to the second candidate AP state when the mobile terminal does not receive the beacon frame signal from the neighbor AP more than at least one time within 100 ms intervals while the neighbor AP is in the target AP state.

17. The fast handover method of claim 15, wherein the neighbor AP is changed from the second candidate AP state to the detected AP state when the strength of the first signal detected from the neighbor AP falls below the first threshold THR_1 while the neighbor AP is in the second candidate AP state.

18. The fast handover method of claim 15, wherein the neighbor AP is changed from the second candidate AP state to the detected AP state when the mobile terminal does not receive the beacon frame signal from the neighbor AP more than at least one time within 100 ms intervals while the neighbor AP is in the second candidate AP state.

19. The fast handover method of claim 18, wherein the neighbor AP is changed from the detected AP state to the second candidate AP state when the strength of the first signal detected from the neighbor AP exceeds the first threshold THR_1 while the neighbor AP is in the detected AP state.

20. The fast handover method of claim 7, wherein a handover ready phase commences when a strength of a first signal detected from the serving AP falls below the first threshold THR_1.

21. The fast handover method of claim 20, wherein the handover ready phase occurs when the serving AP currently communicating with the mobile terminal and the neighbor AP that establishes new communications with the mobile terminal belong to different subnetworks, or when the serving AP currently communicating with the mobile terminal and the neighbor AP that establishes new communications with the mobile terminal belong to the same subnetwork.

22. The fast handover method of claim 20, wherein the handover ready phase comprises:

(a) transmitting Link_Quality_Crosses_Threshold (LQCT) Trigger information from a layer 2 to a layer 3;

(b) selecting a candidate AP and a target AP, which are each neighbor APs that are capable of establishing new communications with the mobile terminal, from among the neighbor AP list, and retrieving information relating to the selected candidate AP and target AP;

(c) transmitting a media access control (MAC) address of the mobile terminal, basic service set identifiers (BSSIDs) of the candidate APs and the target AP, and a fast binding update (FBU) message from the mobile terminal to an access router that manages the subnetwork the mobile terminal belongs to when the neighbor AP capable of establishing new communications with the mobile terminal and the serving AP currently communicating with the mobile terminal belong to different subnetworks according to the information retrieval;

(d) transmitting a handover initiation (HI) message and the MAC address of the mobile terminal from the access router that manages the subnetwork of the mobile terminal, to routers connected to the candidate APs and the target AP, respectively;

(e) transmitting a handover acknowledgement (Hack) message in response to the HI message, a router advertisement (RA) message, and Ω

, which has an ensured unique temporary address, from the routers connected to the candidate APs and the target AP, to the access router managing the subnetwork of the mobile terminal; and

(f) combining the RA message and the Ω

that are received from the routers connected to the candidate APs and the target AP, and transmitting the combined message to the mobile terminal together with a fast binding acknowledgement (FBAck) message.

23. The fast handover method of claim 22, wherein, following the (c) operation, a locking operation is performed to prevent an addition of a new candidate AP into the neighbor AP list even when the new candidate AP is discovered among the neighbor APs of the mobile terminal.

24. The fast handover method of claim 23, further comprising inserting newly discovered candidate APs into a waiting queue.

25. The fast handover method of claim 22, wherein the (d) operation comprises checking new access routers (NARs) that are connected to the target AP and the candidate APs based on the BSSIDs of the candidate APs and the target AP obtained from a CAR table.

26. The fast handover method of claim 22, wherein the BSSIDs of the candidate APs and the target AP contain MAC address information relating to the candidate APs and the target AP.

27. The fast handover method of claim 22, wherein completion of the (f) operation comprises:

signaling that the handover ready phase is completed, and

operating a timer for a predetermined time starting from the completion of the handover ready phase.

28. The fast handover method of claim 27, wherein the predetermined time is approximately 3 seconds.

29. The fast handover method of claim 27, wherein the mobile terminal enters a handover action phase when the strength of the first signal falls below the first threshold THR_2 detected from the serving AP within the predetermined time.

30. The fast handover method of claim 29, wherein, when the mobile terminal enters the handover action phase, the locking operation for the candidate APs is terminated and a target AP is re-selected from the candidate APs.

31. The fast handover method of claim 27, wherein the handover ready phase is re-preformed after the predetermined time when the strength of the first signal detected from the serving AP lies between the first threshold THR_1 and the second threshold THR_2 within the predetermined time.

32. The fast handover method of claim 31, wherein the candidate APs that are inserted into the waiting queue are updated to the neighbor AP list, and a target AP is selected from the candidate APs of the updated neighbor AP list when the handover ready phase is re-performed.

33. The fast handover method of claim 27, wherein initialization is performed to delete all of the candidate APs and the target AP from the neighbor AP list when the strength of the first signal detected from the serving AP exceeds the first threshold THR_1 within the predetermined time.

34. The fast handover method of claim 29, wherein the handover action phase comprises:

(a) transmitting Link_GoingDown (LGD) Trigger information from the layer 2 to the layer 3;

(b) transmitting a movement notification (MVN) message and an Ω

, which includes a unique temporary address, from the mobile terminal to the access router that manages the subnetwork of the mobile terminal; and

(c) tunneling by the access router that manages the subnetwork of the mobile terminal, ad transmitting a MVAck message in response to the MVN message from the access router to the mobile terminal.

35. The fast handover method of claim 34, wherein the MVN message informs the access router of information indicating the movement of the mobile terminal.

36. The fast handover method of claim 34, wherein the (c) operation intercepts, packets intended for a previous temporary address of the mobile terminal at the access routers and tunnels the intercepted packets to a new temporary address contained in the Ω

.

37. The fast handover method of claim 34, wherein the (c) operation transmits Link_Switch (LS) Trigger information from the layer 3 to the layer 2 when the MVAck message is received at the mobile terminal.

38. The fast handover method of claim 34, wherein the LS Trigger information is transmitted from the layer 3 to the layer 2 when the strength of the first signal detected from the serving AP falls below the third threshold THR_3.

39. The fast handover method of claim 34, further comprising, following the transmission of the MVN message in the (b) operation:

retransmitting the MVN message when the mobile terminal does not receive the MVAck message from the access router within approximately 10 ms; and

transmitting the LS Trigger information from the layer 3 to the layer 2 when the MVAck message is not received within 10 ms after retransmitting the MVN message.

40. The fast handover method of claim 37, further comprising, following the transmission of the LS Trigger information from the layer 3 to the layer 2:

trying a re-association to the target AP selected from the candidate APs of the neighbor AP list;

transmitting Link_Up Trigger information from the layer 2 to the layer 3 when the re-association is completed;

configuring the mobile terminal using the RA message and the new temporary address contained in the Ω

that are obtained from a previous subnetwork;

transmitting a fast neighbor advertisement (FNA) message from the mobile terminal to the access router connected to the target AP; and

transmitting the packets tunneled by routing at the access router that receives the FNA message, to the mobile terminal.

41. The fast handover method of claim 40, wherein the buffered packets are delivered to the mobile terminal when the FNA message from the mobile terminal is received while the access router connected to the target AP first receives the packets tunneled at the PAR and buffers the packets intended for the new temporary address contained in Ω

.

42. The fast handover method of claim 20, wherein the handover ready phase comprises:

(a) transmitting the LQCT Trigger information from the layer 2 to the layer 3;

(b) selecting a candidate AP and a target AP that are neighbor APs capable of establishing new communications with the mobile terminal from the neighbor AP list, and retrieving information relating to the selected candidate AP and target AP;

(c) transmitting the MAC address of the mobile terminal, the BSSIDs of the candidate APs and the target AP, and the FBU message from the mobile terminal to the access router that manages the subnetwork of the mobile terminal when the neighbor AP capable of establishing new communications with the mobile terminal belongs to the same subnetwork as the mobile terminal according to the information retrieval; and

(d) transmitting a FBAck message in response to the FBU message from the access router managing the subnetwork of the mobile terminal to the mobile terminal.

43. The fast handover method of claim 42, wherein, following the (c) operation, a locking operation is performed not allowing to add a new candidate AP into the neighbor AP list even when the new candidate AP is discovered among the neighbor APs of the mobile terminal.

44. The fast handover method of claim 43, wherein candidate APs newly discovered are inserted into the waiting queue.

45. The fast handover method of claim 42, wherein the BSSIDs of the candidate APs and the target AP contain MAC address information of the candidate APs and the target AP.

46. The fast handover method of claim 42, wherein completion of the (d) operation comprises:

signaling the completion of the handover ready phase, and

operating a timer for a predetermined time starting from the completion of the handover ready phase.

47. The fast handover method of claim 46, wherein the predetermined time is approximately 3 seconds.

48. The fast handover method of claim 46, wherein the mobile terminal enters the handover action phase when the strength of the first signal detected from the serving AP falls below the second threshold THR_2 within the predetermined time.

49. The fast handover method of claim 48, wherein, when the mobile terminal enters the handover action phase, the locking operation for the candidate APs is terminated and a target AP is re-selected from the candidate APs.

50. The fast handover method of claim 46, wherein the handover ready phase is re-preformed after the predetermined time when the strength of the first signal detected from the serving AP lies between the first threshold THR_1 and the second threshold THR_2 within the predetermined time.

51. The fast handover method of claim 50, wherein the candidate APs inserted into the waiting queue are updated to the neighbor AP list, and a target AP is selected from the candidate APs of the updated neighbor AP list when the handover ready phase is re-performed.

52. The fast handover method of claim 46, further comprising performing initialization to delete all of the candidate APs and the target AP from the neighbor AP list when the strength of the first signal detected from the serving AP exceeds the first threshold THR_1 within the predetermined time.

53. The fast handover method of claim 48, wherein the handover action phase comprises:

(a) transmitting LGD Trigger information from the layer 2 to the layer 3;

(b) transmitting a MVN message from the mobile terminal to the access router that manages the subnetwork of the mobile terminal; and

(c) buffering by the access router that manages the subnetwork of the mobile terminal, ad transmitting a MVAck message in response to the MVN message from the access router to the mobile terminal.

54. The fast handover method of claim 53, wherein the MVN message informs the access router of information indicating the movement of the mobile terminal.

55. The fast handover method of claim 53, wherein the (c) operation transmits LS Trigger information from the layer 3 to the layer 2 when the MVAck message is received at the mobile terminal.

56. The fast handover method of claim 53, wherein the LS Trigger information is transmitted from the layer 3 to the layer 2 when the strength of the first signal detected from the serving AP falls below the third threshold THR_3.

57. The fast handover method of claim 53, further comprising, following the transmission of the MVN message in the (b) operation:

retransmitting the MVN message when the mobile terminal does not receive the MVAck message from the access router within approximately 10 ms; and

transmitting the LS Trigger information from the layer 3 to the layer 2 when the MVAck message is not received within 10 ms after retransmitting the MVN message.

58. The fast handover method of claim 55, further comprising:

trying a re-association to the target AP selected from the candidate APs when the LS Trigger information is transmitted from the layer 3 to the layer 2;

transmitting Link_Up Trigger information from the layer 2 to the layer 3 when the re-association is completed;

transmitting a FNA message from the mobile terminal to the access router connected to the target AP; and

transmitting the buffered packets from the access router connected to the target AP to the mobile terminal.

59. A mobile apparatus including a memory to employ a fast handover method from an access point (AP) to another AP that is optimized for the Institute of Electrical and Electronics Engineers (IEEE) 802.11 networks, comprising:

a transceiver to receive and transmit a signal to and from a serving AP that currently communicates with the mobile apparatus and to receive beacon frame signals transmitted from neighbor APs of the mobile apparatus;

a signal processor to which the received signal and beacon frame signal are provided to process the signal of the serving AP and to process a parameter contained in each beacon frame signal;

a comparator to compare the parameter of each beacon frame signal with pre-stored thresholds in the memory;

a neighbor AP list manager to prioritize the neighbor APs in a list based on the comparison;

a timer to judge a timing of each beacon frame signal so as to signal the neighbor list manager to delete a neighbor AP from the list based on the judgments; and

a controller to determine if the serving AP signal is normal, and, if not, to commence a handover of communications to an undeleted highly prioritized neighbor AP.

60. The mobile apparatus according to claim 59, wherein the transceiver comprises a first transceiver communicates with the serving AP according to IEEE 802.11 standards.

61. The mobile apparatus according to claim 59, wherein the beacon frame signal is transmitted at 100 ms intervals.

62. The mobile apparatus according to claim 59, wherein the transceiver comprises a second receiver which itself comprises a scouter module, such that the second receiver is able to operate in an active mode to scan neighbor APs and an idle mode to suspend the scanning.

63. The mobile apparatus according to claim 59, wherein the signal processor comprises first and second signal processors to process a parameter contained in the beacon frame signal.

64. The mobile apparatus according to claim 59, wherein the parameter may be signal-to-noise ratio (SNR), received signal strength indication (RSSI), bit error rate (BER), and/or packet error rate (PER) or a combination thereof.

65. The mobile apparatus according to claim 64, wherein the first and second signal processors process the SNR contained in the beacon frame signal and obtain Smoothed SNR.

66. The mobile apparatus according to claim 59, wherein the neighbor AP list manager prioritizes the neighbor APs by classifying the neighbor APs into a detected AP, a candidate AP, and a target AP based on the comparison result received from the comparator.

67. The mobile apparatus according to claim 66, wherein, if the detected AP is switched to the candidate AP or if the candidate AP is switched to the target AP, the neighbor AP list manager updates the neighbor AP list.

68. The mobile apparatus according to claim 67, wherein the detected AP is an AP of which a signal alone is detected without an ensured quality of the radio channel, the candidate AP has a guaranteed quality of some degree, and the target AP has the largest value of the signal detected from among the neighbor APs.

69. The mobile apparatus according to claim 59, wherein the timer comprises a plurality of timers that correspond to the neighbor APs, such that the respective timers count beacon frame signal timing from the time when the beacon frame signal is received with 100 ms intervals.

70. The mobile apparatus according to claim 69, wherein when the beacon frame signal is not received from a neighbor AP with 100 ms intervals, the timer informs the neighbor AP list manager that the beacon frame signal is not received.

71. The mobile apparatus according to claim 70, wherein if the beacon frame signal is not received from a neighbor AP, the timer informs the neighbor AP list manager that the beacon frame signal is not received from a neighbor AP, and the neighbor AP list manager deletes the neighbor AP from the neighbor AP list.

72. The mobile apparatus according to claim 59, wherein if the serving AP signal is normal, the controller continues to receive data from the serving AP.

73. The mobile apparatus according to claim 72, wherein, when a signal to noise ratio (SNR) of the serving AP signal falls below a predefined first threshold THR_1 the controller controls the mobile apparatus to enter a handover ready phase.

74. The mobile apparatus according to claim 73, wherein, when the SNR of the serving AP signal falls below a predefined second threshold THR_2, the controller controls the mobile apparatus to be handed over.

75. The mobile apparatus according to claim 74, wherein, when the SNR of the serving AP signal falls below a predefined third threshold THR_3, the controller interrupts the communications between the mobile apparatus and the serving AP.

76. The mobile apparatus according to claim 75, wherein the controller reconnects the serving AP to the mobile apparatus.

77. A mobile apparatus including a memory to employ a fast handover method from an access point (AP) to another AP that is optimized for the Institute of Electrical and Electronics Engineers (IEEE) 802.11 networks, comprising:

a transceiver to receive and transmit a signal to and from a serving AP that currently communicates with the mobile apparatus and to receive beacon frame signals transmitted from neighbor APs of the mobile apparatus;

a signal processor to which the received signal and beacon frame signal are provided to process the signal of the serving AP and to process a parameter contained in each beacon frame signal;

a comparator to compare the parameter of each beacon frame signal with pre-stored thresholds in the memory;

a neighbor AP manager to prioritize the neighbor APs based on the comparison; and

a controller to determine if the serving AP signal is normal, and, if not, to commence a handover of communications to a highly prioritized neighbor AP.

78. A mobile apparatus including a memory to employ a fast handover method from an access point (AP) to another AP, comprising:

a transceiver to receive and transmit a signal to and from a serving AP that currently communicates with the mobile apparatus and to receive beacon frame signals transmitted from neighbor APs of the mobile apparatus;

a signal processor to which the received signal and beacon frame signal are provided to process the signal of the serving AP and to process a parameter contained in each beacon frame signal;

a comparator to compare the parameter of each beacon frame signal with pre-stored thresholds in the memory;

a neighbor AP list manager to prioritize the neighbor APs in a list based on the comparison;

a timer to judge a timing of each beacon frame signal so as to signal the neighbor list manager to delete a neighbor AP from the list based on the judgments; and

a controller to determine if the serving AP signal is normal, and, if not, to commence a handover of communications to an undeleted highly prioritized neighbor AP.

79. A mobile apparatus including a memory to employ a fast handover method from an access point (AP) to another AP, comprising:

a transceiver to receive and transmit a signal to and from a serving AP that currently communicates with the mobile apparatus and to receive beacon frame signals transmitted from neighbor APs of the mobile apparatus;

a signal processor to which the received signal and beacon frame signal are provided to process the signal of the serving AP and to process a parameter contained in each beacon frame signal;

a comparator to compare the parameter of each beacon frame signal with pre-stored thresholds in the memory;

a neighbor AP manager to prioritize the neighbor APs based on the comparison; and

a controller to determine if the serving AP signal is normal, and, if not, to commence a handover of communications to a highly prioritized neighbor AP.