1. Field of the Invention
The present invention relates to a method for performing a handoff in a fast and secure wireless network, and more particularly to a method for minimizing handoff latencies.
2. Description of the Related Art
Conventionally, a local area network (LAN) is a collection of personal terminals, main frames and workstations coupled to a communication link within a distance of 300 meters or less. The LAN is a high-speed communication network for allowing employees in a company to be aware of information, i.e., a distance in which an electric current or radio wave signal can be correctly transferred between the personal terminals, to commonly and most effectively use equipment installed in the company's building. As LANs, wired networks for directly transferring an electrical signal through the communication link have been initially used. The trend has been to replace the wired networks with wireless networks for transferring a signal using a radio wave in accordance with the development of wireless protocols. LANs based on these wireless networks are referred to as wireless local area networks (WLANs). WLANs are based on Institute of Electrical and Electronics Engineers (IEEE) 802.11. IEEE 802.11-based WLANs have seen immense growth in the last few years. It is predicted that the IEEE 802.11-based WLANs will be rapidly developed in the future because of an advantageous effect of convenient network connectivity.
IEEE 802.11 allows for two operating modes, i.e., an ad hoc mode and an infrastructure mode, in relation to a media access control (MAC) layer. In the ad hoc mode, two or more wireless stations (STAs) recognize each other and establish a peer-to-peer communication without any existing infrastructure. Meanwhile, in the infrastructure mode, there is a fixed entity called an access point (AP) that bridges all data between the STAs associated with the AP. The AP and the STAs associated with the AP form a basic service set (BSS) communicating on the unlicensed radio frequency (RF) spectrum.
Referring to
In the WLAN based on the architecture shown in
The conventional handoff procedure refers to the mechanism or sequence of messages exchanged between the APs and the STA. In the conventional handoff procedure, physical layer connectivity and state information must be transferred from one AP to another AP with respect to the STA in consideration. The handoff is a physical layer function carried out by at least three participating entities, i.e., an STA, a prior-AP and a post-AP. The state information that is transferred typically consists of the client credentials (which allow the STA to gain network access) and some accounting information. An operation for transferring the state information can be performed by an inter access point protocol (IAPP). For an IEEE 802.11 network that has no access control mechanism, there would be a nominal difference between a completion association and a handoff/reassociation. Looking at it another way, handoff latency would be strictly greater than association latency as there is an additional inter-access point communication delay involved.
Logical steps based on the handoff procedure are classified into a discovery phase and a reauthentication phase.
1. Discovery Phase: Attributing to mobility, the signal strength and the signal-to-noise ratio of a signal from the STA's current AP (or prior-AP) might degrade and cause it to initiate a handoff. At this point, the STA might not be able to communicate with its current AP (or prior-AP). Thus, the STA needs to find potential APs in range to associate to. This is accomplished by a MAC layer function (or scan function). During a scan, the STA listens for beacon messages sent out periodically by APs at a rate of 10 ms, on assigned channels. Thus, the STA can create a priority list, i.e., a list of APs prioritized by the received signal strength. Two kinds of scanning methods defined in the standard are based on an active mode and a passive mode. As the names suggest, in the active mode, apart from listening to beacon messages (which is passive), the STA sends additional probe broadcast packets on each channel and receives responses from APs. Thus, the STA actively searches or probes for potential APs.
2. Reauthentication Phase: The STA sends a reauthentication request to potential APs according to the priority list in the above-described discovery phase. The reauthentication phase typically involves an authentication and a reassociation to the post-AP. The reauthentication phase involves the transfer of credentials and other state information from the prior-AP. As mentioned earlier, this can be achieved through a protocol such as the IAPP. The reauthentication phase includes an authentication phase and a reassociation phase.
Referring to
On the other hand, the STA performs the reassociation phase 220 according to priorities of the potential APs registered in a priority list created in the discovery phase. The STA transmits a reassociation request message to a new AP at step 222. In response to the reassociation request message, the new AP performs an inter access point protocol (IAPP) procedure with other APs, i.e., a prior AP of the STA, at step 230. Through the IAPP procedure, the new AP receives credentials and other state information assigned to the STA. Then, the new AP transmits, to the STA, a reassociation response message to the reassociation request message at step 224.
As described above, the conventional handoff procedure starts when the STA transmits a probe request message and ends when the STA receives a reassociation response message. During the handoff procedure, three types of delay are incurred as in the following. The three types of delay include a probe delay incurred in the discovery phase, an authentication delay incurred in the authentication phase and a reassociation delay incurred in the reassociation phase.
1. Probe Delay: Messages transmitted for an active scan at the probe phase 210 shown in
2. Authentication Delay: This is the latency (not shown in
3. Reassociation Delay: This is the latency incurred during which reassociation frames are exchanged in the reassociation phase 220 shown in
According to the above, messages during the probe delay form the discovery phase, while the authentication and reassociation delays form the reauthentication phase. Apart from the latencies discussed above, there will potentially be a bridging delay caused by the time taken for the MAC address updates to Ethernet switches which form the distribution system (i.e., the backbone Ethernet). It can be seen that many latencies are incurred while a handoff between an STA and APs is performed in the conventional WLAN. There are problems in that the latencies not only affect the quality of service (QoS) but also disable high-speed roaming.
Accordingly, it is an aspect of the present invention to provide a method for minimizing handoff latencies.
It is another aspect of the present invention to provide a method for transferring state information of a corresponding wireless station (STA) to access points (APs) before a handoff is performed.
It is yet another aspect of the present invention to provide a handoff method capable of eliminating a tunneling procedure between a prior-access point (AP) and a post-AP and a procedure of transferring state information of a corresponding wireless station (STA) through the tunneling procedure.
It is still another aspect of the present invention to provide a method for generating a neighborhood graph needed to send state information of a wireless station (STA) to potential access points (APs).
It is still yet another aspect of the present invention to provide a method for propagating state information of a wireless station (STA) to neighboring access points (APs) on the basis of a neighborhood graph.
To achieve the above and other aspects of the present invention, there is provided a method for enabling access points to support a handoff for at least one station in a wireless network, the wireless network including the access points for covering constant service areas and the station associated with the one of the access points for receiving communication service. The method includes: generating a neighborhood graph configured by handoff-capable neighboring access points; receiving information including context of a station propagated from a prior access point of the station, the prior access point being one of the neighboring access points; and propagating information including the context corresponding to the station to the neighboring access points of the neighborhood graph in response to association or re-association with the station using the received context.
Additionally, there is provided a method for enabling access points to support a handoff for at least one station in a wireless network, the wireless network including the access points for covering constant service areas and the station associated with the one of the access points for receiving communication service. The method includes: obtaining information including context corresponding to a station in response to an association request and associating with the station; and propagating information including the context to handoff-capable neighboring access points of the wireless network upon associating with the station.
Additionally, there is provided a method for performing a handoff between access points and a station in a wireless network, the wireless network including the access points for covering constant service areas and the station associated to one of the access points for receiving communication service. The method includes: outputting an association or re-association response message according to context in response to an association or re-association request message of the station; propagating the context to handoff-capable neighboring access points of the wireless network, where the context corresponding to the station is stored in an access point receiving the association or re-association request message; outputting the association or re-association response message after obtaining the context from a prior access point of the station; and propagating the context to the handoff-capable neighboring access points, where the context is not present in the access point receiving the association or re-association request message.
The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
A preferred embodiment of the present invention will now be described in detail with reference to the annexed drawings. In the following description, the present invention proposes the preferred embodiment to achieve the above and other objects. However, other embodiments of the present invention can be drawn from the following description of the present invention.
According to an aspect of the present invention, a proactive caching technique is adopted to reduce a reassociation delay. In order for the proactive caching technique to be adopted, a procedure of propagating state information of a corresponding wireless station (STA), i.e., context, from a prior-access point (AP) to potential APs is performed irrespective of a handoff process. The potential APs are a set of APs with which the STA can associate from the prior AP of the STA. In order for the context of the STA to be sent to the potential APs as described above, the potential APs may be managed on each AP. For this, the APs may generate and manage a neighborhood graph. The neighborhood graph defines connection relationships between the potential APs and the prior AP in the handoff process.
According to another aspect, as illustrated in
1. Generation of Neighborhood Graph
In accordance with the present invention, a neighborhood graph is formed by the arrangement of APs configuring a wireless local area network (WLAN). As potential APs corresponding to each of the APs configuring the WLAN are different, the generation of the neighborhood graph is achieved on an AP-by-AP basis. Three types of neighborhood graph generation methods are disclosed below. The first generation method allows a manager to manually generate the neighborhood graph. The first generation method allows the manager to configure and register neighborhood graphs on the AP-by-AP basis according to the arrangement of the APs and also allows the manager to update a neighborhood graph when the arrangement of APs is changed therein. The second generation method allows the manager to register the first neighborhood graph and also allows the neighborhood graph to be automatically changed when the arrangement of the APs is changed therein. The third generation method allows neighborhood graphs to be automatically generated on an AP-by-AP basis. In the third generation method, a handoff is performed on the basis of an existing handoff procedure to generate or update a neighborhood graph. In other words, a procedure of confirming connection relationships on an AP-by-AP basis is performed in the third generation method. For example, where the STA located at an AP_A tries to first perform a handoff process to an AP_B by which no handoff for the STA was previously performed, the AP_B performs an inter-access point protocol (IAPP) procedure to receive context corresponding to the STA from the AP_A. Then, AP_A and AP_B confirm the existence of the interconnection relationship therebetween for the handoff, such that a corresponding neighborhood graph can be generated or updated. After the neighborhood graph is updated, the handoff can be performed with respect to the STA that desires to move from AP_A to AP_B or from AP_B to AP_A without the IAPP procedure.
A physical path connected between APs and a distance between the APs should be considered so that any one of the three types of generation methods can generate a neighborhood graph. In other words, the APs configuring the WLAN must be able to be physically connected to each other without going through any other AP so that connection relationships can be formed on the basis of the neighborhood graph. Furthermore, two APs physically connected to each other should be within a threshold distance range. Where the two APs are far away from each other, a handoff may be performed according to an initial procedure for allowing a new AP to support communication.
An example of generating a neighborhood graph to be applied in accordance with an embodiment of the present invention will now be described in detail.
As shown in
An operation _for allowing each AP to automatically generate the neighborhood graph will now be described. Upon receiving a reassociation request message from a wireless station (STA), an arbitrary AP determines whether temporarily stored context corresponding to the STA is present. At this point, the arbitrary AP becomes a post-AP for the STA. The fact that the context is present means that a neighborhood graph with a prior-AP from which the STA moves is formed. On the other hand, if the context is not present, it can be determined that the neighborhood graph with the prior-AP from which the STA moves is not formed. In this case, the post-AP receives the context corresponding to the STA from the prior-AP through the existing IAPP, updates the neighborhood graph and forms a connection with the prior-AP. In accordance with a handoff procedure of the present invention, the handoff can be performed with respect to the STA that moves from the prior-AP after the connection is formed.
2. Proactive Caching Technique
In a proactive caching technique according to an embodiment of the present invention, each AP recognizes its potential APs. Context of the STA belonging to the AP is sent to the potential APs. Even though the STA belonging to an arbitrary AP moves to any AP connected to the arbitrary AP, the time required for a reassociation phase in the handoff procedure is minimized. That is, the proactive caching technique is based on some locality principle of mobility. In this environment, a reassociation pattern of the STA will be the sequence of APs that the STA gets associated with in a given interval of time.
Proactive caching techniques for reducing a reassociation delay in accordance with the embodiment of the present invention will now be described in detail with reference to
Referring to
When the association request is received, AP_A performs an authentication process for the STA on the basis of a typical initial authentication procedure. If the authentication process is completed, AP_A sends, to the STA, a response message to the association request.
When the reassociation request is received, AP_A performs different operations according to whether or not context corresponding to the STA has been temporarily stored. If the context corresponding to the STA has been temporarily stored, AP_A sends a response message to the STA in response to the reassociation request. On the other hand, if the context corresponding to the STA has been not temporarily stored, AP_A receives the context from an AP at which the STA was previously located through the typical IAPP procedure. Then, the response message to the reassociation request is sent to the STA. The STA performs communication with AP_A by receiving the response message from AP_A.
On the other hand, AP_A transfers the context, such as security context, corresponding to the STA to AP_B indicating a potential AP in a handoff at step 2. Only one AP is shown as the potential AP in
An embodiment of the present invention employs proactive caching technique in which context of a corresponding STA can be provided to at least one predicted AP to which the STA moves. In other words, in order for the proactive caching technique to be applied, an operation for transferring context of a corresponding STA from a prior-AP to a post-AP is performed. Furthermore, each AP is able to predict information about potential post-APs so that the proactive caching technique may be applied. This has been described above in relation to the neighborhood graph.
In accordance with an embodiment of the present invention, a method of reducing a reassociation delay using the proactive caching technique will now be described in detail with reference to
Referring to
Referring to
At the time of the need for a handoff to AP_B, the STA sends a reassociation request message to AP_B at step 507. Upon receiving the reassociation request message, AP_B determines whether the temporarily stored context corresponding to the STA is present. If the temporarily stored context corresponding to the STA is present in AP_B, AP_B transmits a reassociation response message to the STA on the basis of the context at step 509. As authentication is completed between the STA and AP_B, communication between the STA and AP_B is enabled. Since the AP_B includes the context of the STA, further/another authentication may also be readily performed.
Where the proactive caching technique is applied as described above, a state in which each AP cannot store context propagated from neighboring APs may be incurred due to insufficient storage, for example, cache capacity. In this case, the AP sequentially deletes the oldest contexts so that newly propagated context can be stored.
3. Description of Operation in Accordance With the Present Invention
An operation of the AP when a handoff procedure is performed in accordance with an embodiment of the present invention will now be described in detail with respect to
Referring to
Accordingly, the AP determines, at step 614, whether an association request has been received from the STA, and determines, at step 616, whether a reassociation request has been received from the STA. If the association request has been received from an arbitrary STA, the AP proceeds to step 618 and performs a typical authentication procedure with an authentication server provided in a wireless network. Then, the AP configures context corresponding to the STA and stores the configured context in its own cache. In step 616, if the reassociation request has been received, the AP determines that the STA has moved from another AP. Then, the AP proceeds to step 620 and determines whether context corresponding to the STA stored in the internal cache is present. If the context corresponding to the STA is not present in the internal cache, the AP proceeds to step 622. At the above step 622, the AP performs a typical IAPP procedure, and obtains the context corresponding to the STA from another AP at which the STA was previously located. If the AP recognizes another AP at which the STA was previously located, the IAPP procedure is performed only for the already recognized AP.
When the AP proceeds from the above step 618, 620 or 622 to step 624, the AP sends a response message to the STA. The response message corresponds to the association/reassociation request. Then, the AP proceeds to step 626 after sending the response message, the AP refers to a neighborhood graph managed thereby and propagates the context of a corresponding STA to neighboring APs. This is to implement a fast handoff when a corresponding STA moves to any neighboring AP.
As apparent from the above description, the present invention can provide a method for simplifying a handoff procedure in a wireless local area network (WLAN), reducing a reassociation delay, and enabling a wireless station (STA) to quickly communicate with an access point (AP) to which the STA moves. Furthermore, the method in accordance with the present invention can provide not only secure quality of service but also high-speed roaming service.
Furthermore, the present invention is applicable to all wireless communication systems and technologies, and as such may be utilized with CDMA, TDMA, FDMA, IMT, GSM, etc. systems and equipment, as well as IEEE 802.11 technology and equipment. APs as described above are analogous to base stations in telecommunication systems, while STAs are analogous to mobile terminals or stations.
Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope of the invention. Therefore, the present invention is not limited to the above-described embodiments and drawings.
This application is a Continuation of U.S. patent application Ser. No. 10/703,516, which claims priority to a provisional application entitled “A METHOD FOR FAST AND SECURE WIRELESS LOCAL AREA NETWORK HANDOFFS”, filed in the United States Patent and Trademark Office on Nov. 8, 2002 and assigned Ser. No. 60/425,109, the contents of which are hereby incorporated by reference.
This invention was made with Government support under Contract No. 60NANB1D0113 awarded by the National Institute of Standards and Technology. The U.S. Government has certain rights in the invention.
Number | Date | Country | |
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60425109 | Nov 2002 | US |
Number | Date | Country | |
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Parent | 10703516 | Nov 2003 | US |
Child | 11003210 | Dec 2004 | US |