METHOD OF CONTROLLING HANDOVER IN MOBILE COMMUNICATION NETWORKS AND APPARATUS AND SYSTEM IMPLEMENTING THE METHOD

Abstract

A method of controlling selection of an access point includes controlling one or more further access points having service areas adjacent or overlapping with a service area of a first access point, to which a first mobile terminal is associated, to send probe request messages to the first mobile terminal, and determining, for each of the one or more further access points and from a response received from the first mobile terminal in response to the corresponding probe request message, a respective first value representing a quality of a connection between the respective access point and the first mobile terminal. Selected ones of the one or more further access points are controlled to accept a future association request from the first mobile terminal, while the not selected access points are controlled to reject a future association request from the mobile terminal.

Claims

1. A method of controlling association of a mobile terminal to an access point in a wireless communication system comprising at least two access points, each access point being adapted to provide data communication with the mobile terminal within a service area of the access point through a wireless interface over a selected one of a number of channels, the method comprising: one or more candidate access points having a service area adjacent or overlapping with a service area of a first access point to which a first mobile terminal is associated, receiving a command to send probe request messages to the first mobile terminal; each of the one or more candidate access points determining, from a response received from the first mobile terminal in response to the corresponding probe request message, a respective first value representing a quality of a connection between the respective candidate access point and the first mobile terminal; each of the one or more candidate access points receiving a command controlling the respective candidate access point to selectively accept or reject a future association request issued by the first mobile terminal.

2. The method of claim1, wherein the wireless interface of each one of the candidate access points is initially set to a channel that is different from the channel over which the first mobile terminal is associated with the first access point, wherein, for each of the candidate access points and prior to sending the probe request message to the first mobile terminal, the method further includes: switching the wireless interface to the channel over which the first mobile terminal is associated with the first access point, and, upon expiration of a predetermined time after sending the probe request message or upon receiving a response to the probe request message, switching to the channel that was initially set.

3. The method of claim 1, further including sending probe request messages only in a time interval during which a respective candidate access point performs no active wireless communication with a further mobile terminal.

4. The method of claim 1, wherein the command to send probe request messages is received from the first access point or from a control unit that is in communication with the at least two access points of the wireless communication system.

5. The method of claim 1, further including monitoring one or more attributes of a connection between the first access point and the first mobile terminal, determining, from the monitored attributes, a second value representing the quality of the connection, and triggering sending the command to the candidate access points to send probe request messages only in case the second value is at or below a predetermined value.

6. The method of claim 5, wherein one or more of monitoring, determining and triggering the command for controlling are performed in the first access point or in a control unit that is in communication with the at least two access points of the wireless communication system.

7. The method of claim 1, wherein the command controlling the candidate access points to selectively accept or reject an association request from the first mobile terminal is received from the first access point or from a control unit that is in communication with the at least two access points of the wireless communication system, in response to messages received from the candidate access points transmitting the respective first values.

8. The method of claim 1, wherein controlling each of the candidate access points to accept or reject a future association request from the first mobile terminal includes one or more of accepting a future association request only in case the respective first value is at or exceeds a predetermined value, accepting a future association request only in case fewer than a predetermined number of further mobile stations are attached to the respective candidate access point, or accepting a future association request only in case an aggregate amount of data traffic of the respective candidate access point is below a predetermined value over a predetermined time interval.

9. The method of claim 1, wherein the probe request message is of a type that must be responded to irrespective of an established association with an access point, including an ADDBA-request or a QoS control frame.

10. The method of claim 1, wherein each access point transmits status messages including a value indicating a quality of a connection with a mobile terminal to a control unit.

11. The method of claim 1, wherein the quality of a connection with a mobile terminal is determined based on one or more of signal strength, bit error rate, block error rate, data rate, modulation scheme, and signal-to-noise ratio.

12. The method of claim 1, wherein an access point that does not receive a response from the mobile terminal in response to the probe request message returns a message in reply to the command to send probe request messages indicating lack of a response from the mobile terminal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In the following the invention will be described with reference to the drawings, in which

[0052] FIG. 1 is a schematic and simplified overview of a network architecture in which the inventive method is executed;

[0053] FIG. 2 represents an exemplary schematic flow diagram of an embodiment of the method;

[0054] FIG. 3 shows an exemplary message flow diagram in accordance with a first aspect of the invention;

[0055] FIG. 4 shows an exemplary message flow diagram in accordance with a second aspect of the invention;

[0056] FIG. 5 is an exemplary schematic block diagram of a first AP in accordance with the invention; and

[0057] FIG. 6 is an exemplary schematic block diagram of a second AP in accordance with the invention.

[0058] In the figures like elements are designated with the same reference numerals.

[0059] FIG. 1 shows a schematic and simplified overview of a network architecture in which the inventive method is executed. A mobile terminal (MT) 102 is connected wirelessly to a first access point (AP) 104. The connection is established over one of a plurality of channels. In the figure the connection is on channel 11. MT 102 is also within the service areas of a second and a third AP 106, 108. First, second and third APs 104, 106, 108 are also linked over a LAN 110, which is further connected to the internet 116 over a switch 112 and a router 114.

[0060] If, for some reason, first AP 104 decides that the link quality with MT 102 is not sufficient anymore, first AP 102 could simply disassociate MT 102 in order to force it to re-establish a better connection with one of the second or third APs 106, 108. Prior art approaches would leave it to MT 102 to figure out to which AP to connect. In accordance with the present invention the second and third APs 106, 108 will predetermine if MT 102 is allowed to associate with one of them. Since second AP 106 operates on channel 1, which is different from the channel that MT 102 and first AP 104 currently are linked on, second AP 106 cannot simply perform an evaluation of the link quality. The same is true for third AP 108. Hence, they would need to switch to the channel that MT 102 is currently using for assessing the link quality. Since neither of the second or third AP 106, 108 knows if or when the quality of the link between MT 102 and first AP 104 is insufficient they will only perform a link assessment upon a trigger signal. Such trigger signal is typically sent from the AP that is currently associated with the MT, in the figure first AP 104 would send such trigger signal, e.g. through LAN management protocol. Also, since the APs operate on different channels, and since MT 102 is only associated with first AP 104, and is not associated with second or third AP 106, 108, only a basic link assessment is possible, e.g. using RSSI. As discussed further above, when an AP is not busy transmitting or receiving, it can perform a scan on other than its own operating channels, for checking a link quality with a MT that might potentially associate in the future.

[0061] Since merely listening to a MT that potentially associates in the future, so-called sniffing, might not result in accurate RSSI readings, so-called action frames, e.g. ADDBA request, or QoS control frames, e.g. QoS NULL, subtype 1100, for determining RSSI can be used. Any device receiving such transmissions must reply with either an ACK message or a rejected because OBSS request-message. The resulting RSSI reading is then used for determining whether to accept or reject future association requests from the MT.

[0062] In FIG. 1 the RSSI readings from second and third AP 106, 108 are transmitted to first AP 104, which can decide to maintain the existing association, e.g. because the other links have an even less quality than the existing link, or can instruct one of the second and third APs 106,108 to accept an impending association request from MT 102, and then disassociate with MT 102.

[0063] FIG. 2 shows an exemplary flow diagram of an embodiment of a method in accordance with the invention. The flow diagram depicts the method steps as performed in an AP in accordance with the invention, once it has received a trigger to perform the off channel link check. Step 202 determines the available free time for performing the off channel link check. If no sufficient time is available, no-branch of step 204, checking the available time is repeated. If sufficient time is available for performing the off channel link check, in full or partially, the AP switches to a target channel that is different from its initial or own channel it normally operates on, step 206. Then, the AP sends a link check frame, step 208, receives a response to the link check frame, step 210, and records the RSSI reading, step 212. Sending and receiving the link check frames may be repeated for increased accuracy. Finally, the AP switches back to the initial channel it operated on before, step 214.

[0064] FIG. 3 shows an exemplary message flow diagram in accordance with a first aspect of the invention. When AP 1, which has an active association with MT, determines that the link quality is below a predetermined threshold, step 301, it sends a message to AP2, AP3, . . . , APn, triggering those APs to issue probe request messages to MT, step 302. Each of the addressed APs subsequently sends the probe request messages to MT, step 303, and receives corresponding responses, step 304. Once the addressed APs have received responses they determine the link quality and report the determined link quality to AP1 that initiated the probing, step 305. AP1, after receiving reports on the link quality from all addressed APs, selects one or more of the APs as appropriate for future association with MT, and instructs the APs accordingly to accept or reject future association requests from MT, step 306.

[0065] FIG. 4 shows an exemplary message flow diagram in accordance with a second aspect of the invention. The initial situation corresponds to the one described in FIG. 3: AP1 has an active association with MT and determines that the link quality is below a predetermined threshold, but, unlike as shown in FIG. 3, in FIG. 4 a control unit CU is present, which coordinates and controls the association of MT to any of the APs. AP1 sends a message to CU, informing CU about a low link quality, step 401. CU sends a message to AP2, AP3, . . . , APn, triggering those APs to issue probe request messages to MT, step 402. Each of the addressed APs subsequently sends the probe request messages to MT, step 403, and receives corresponding responses, step 404. Once the addressed APs have received responses they determine the link quality and report the determined link quality to CU that initiated the probing, step 405. CU, after receiving reports on the link quality from all addressed APs, selects one or more of the APs as appropriate for future association with MT, and instructs the APs accordingly to accept or reject future association requests from MT, step 406.

[0066] FIG. 5 shows an exemplary schematic block diagram of a first AP 500 suitable to perform a method in accordance with the invention. Microprocessor 502 is linked to random access memory (RAM) 504, wide area network (WAN) and/or local area network (LAN) interface 506, non-volatile memory 508 and wireless interface 516 via data bus 518. Data bus 518 is not necessarily a single data bus but may also be implemented as a system of separate buses each connecting all or only some components of the AP, as appropriate and required for control and data flow. Non-volatile memory 508 holds program instructions which, when executed by microprocessor 502, perform one or more of the various aspects and embodiments of the method described in this patent specification. The microprocessor is of a commonly known and used type and may be constructed in accordance with any suitable one of the known architectures, e.g. x86, x86-64, ARM, MIPS, etc. Random access memory 504 may temporarily store data and/or program instructions when performing the method. The non-volatile memory may be implemented through hard disk drives (HDD), Flash-Memory, or a combination thereof. Other non-volatile memory types such as MRAM, optical storage, etc. can also be used in the AP without leaving the scope of the present invention.

[0067] FIG. 6 is an exemplary schematic block diagram of a second AP 600 in accordance with the invention. AP 600 comprises all elements shown and discussed with reference to FIG. 5, and the corresponding elements are referenced with similar reference numerals in the 600-range. In addition to the AP shown in FIG. 5 AP 600 of FIG. 6 has a second wireless interface, allowing for concurrent data traffic with one or more associated MTs and probing link quality with non-associated MTs, as described further above in one or more aspects of the invention.

[0068] It will be appreciated by those skilled in the art that the block diagrams presented herein represent conceptual views of illustrative system components and/or circuitry embodying the principles of the invention. Similarly, it will be appreciated that any flow charts and the like represent various processes which may be substantially represented in computer readable storage media and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.

[0069] Although the invention was described above with reference to the examples shown in the appended drawings, it is obvious that the invention is not limited to these, but may be modified by those skilled in the art without departing from the scope and spirit of the invention. For example, the invention is not restricted to WLAN networks according to the IEEE 802.11 standard only, but can be used in connection with other wireless systems, such as systems based on the BRAN (Broadband Radio Access Networks) standard, or any other network type in which a MT is connected to a single AP at a time.