Abstract:
A handoff method is provided in a communication network including a plurality of fixed nodes attached to a router connected to at least one external network, and at least one mobile node associated with a current fixed node. The method detects movement of a mobile node; prepares a handoff of the mobile node from a current fixed node to potential fixed nodes upon detecting movement of the mobile node; decides whether to handoff to one of the potential fixed nodes as a target fixed node; and performs the handoff from the current fixed node to the target fixed node. The handoff method enables the mobile station to quickly move its association from one access point to another by minimizing a probe delay which is a primary contributor to handoff delay.

Description:
PRIORITY  
       [0001]     This application claims priority to an application entitled “Seamless Handoff Method In Wireless Local Area Network” filed with the Korean Intellectual Property Office on May 29, 2004 and assigned Serial No. 2004-38663, the contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to wireless local area networks (WLANs), and in particular, to a seamless handoff method in the WLAN.  
         [0004]     2. Description of the Related Art  
         [0005]     A wireless local area network (WLAN) is a type of local area network (LAN) that uses high frequency radio waves rather than wires to communicate between terminals (stations). The WLAN is practical to use in an environment where wire distribution is not allowed or the deployment of the network changes frequently. The WLAN uses a random access technique for media access to allow the stations to competitively transmit data. Since, in the random access scheme, data loss occurs due to the collisions of packets on the radio media and it is difficult to detect the data collisions at the transmitter by nature of the wireless media, a collision avoidance technique is required. Generally, the WLAN adopts carrier sense multiple access/collision avoidance (CSMA/CA), as the collision avoidance scheme, in which collisions or errors can be indirectly monitored during a basic procedure of the data transmission from the transmitter to the receiver. If the transmitter does not receive an acknowledgement of the transmitted data, it retransmits the data after a predetermined time lapses.  
         [0006]     Typically, the WLAN has a plurality of Basic Service Sets (BSSs), with each BSS having an Access Point (AP) connected to a wired network, and a plurality of wireless stations (STAs).  
         [0007]     Communication within the BSS is provided through the AP such that the STAs are required to associate with an AP in order to access to the network. Thus, when deciding to establish an association with the network, the STA first searches for an AP to be associated with before transmitting data. During the course of data transmission after the association, the STA may roam out of the service area of the current AP. In this case, since the APs use different channels to prevent from interference with each other, the STA must release the association with the current AP and establish a new association with a new AP, which is called a “handoff” in wireless communication terminology. Typically, the BSSs have coverage areas overlapping each other such that the STAs can roam from one area to another within the WLAN and maintain communication anywhere in the WLAN.  
         [0008]     Handoff can occur when the STA roams within the BSS or moves into range of other BSSs. Handoff delay is a significant issue for real time multimedia services such as voice-over-IP, video teleconference, and the like.  
         [0009]      FIG. 1  shows a typical WLAN structure. In  FIG. 1 , three access points  11 ,  12 , and  13  are connected to the Internet  17  through a gateway  15  and their coverage areas overlap with each other. The access points  11 ,  12 , and  13  are assigned different radio channels, i.e., respective channel  1 , channel  6  and channel  11 . In this example, AP 2   12  communicates with STA  10  through the channel  6 .  
         [0010]     When STA  10  moves from service area (B) of the AP 2   12  to service area (A or C) of the AP 1   11  or AP 3   13  while communicating with the AP 2   12 , handoff occurs. Typically, the handoff procedure comprises scanning, authentication, and association processes. Scanning is a process that attempts to discover the existence of APs and to gather information on the APs. There are two ways for a mobile node to scan for existing APs, active scanning and passive scanning.  
         [0011]     For passive scanning, the STA listens for beacon frames, which are periodically transmitted by APs. For active scanning, the STA sends out a probe frame on specific channel to solicit response frames from the APs.  
         [0012]     In the passive scanning technique, it typically takes more than 300 msec for receiving the beacon frames from the APs and obtaining information on the APs from the beacon frames. Also, in the active scanning technique, it typically takes more than 300 msec since the STA sends the probe frame when it does not receive the beacon frame within a predetermined time and to obtain the information on the APs from the response frame. After obtaining the information on the APs the STA determines whether or not to join the Basic Service Set (BSS). If the STA determines to join the BSS, the STA enters the authentication process.  
         [0013]     Authentication is a process of determining whether the STA is what it is declared to be for security by communicating information between the STA and AP.  
         [0014]     After passing the authentication process, the STA enters the association process. In the association process, the STA and the AP exchange information on the capability of the BSS, and a distribution system service (DSS) is triggered in order for the AP to communicate with another APs to exchange frames for the STA in their respective BSSs, forward frames to follow mobile stations as they move from one BSS to another, and exchange frames with a wired network. After the STA associates with an AP through the association process, the STA can transmit data.  
         [0015]      FIG. 2   a  and  FIG. 2   b  are message flowcharts for illustrating a conventional passive scanning-based handoff technique and a conventional active scanning-based handoff technique, respectively. As shown in  FIG. 2   a  and  FIG. 2   b , the probe delays are main contributors to the entire handoff latency in both cases.  
         [0016]      FIG. 3   a  and  FIG. 3   b  are pie charts for illustrating percentages of delays occurring during the handoff procedure in the conventional passive scanning-based handoff technique and the active scanning based handoff technique, respectively. As shown in  FIG. 3   a  and  FIG. 3   b , the probe delay is dominant in both the passive scanning-based handoff and active scanning-based handoff.  
         [0017]     Thus, there is a need to reduce the probe delay for implementing seamless handoff in the WLAN.  
       SUMMARY OF THE INVENTION  
       [0018]     The present invention has been made in an effort to solve the above problems and an object of the present invention is to provide a handoff method in WLAN, which is capable of minimizing the entire handoff latency.  
         [0019]     It is another object of the present invention to provide a handoff method enabling the mobile station to quickly move its association from one access point to another by minimizing a probe delay which is a primary contributor to the handoff delay.  
         [0020]     It is another object of the present invention to provide a handoff method which can be adopted between different systems by exchanging upper layer information between the access points before the handoff occurs.  
         [0021]     To achieve the above objects, a handoff method is provided for a communication network including a plurality of fixed nodes attached to a router connected to one or more external networks and at least one mobile node associated with a current fixed node. The handoff method detects movement of a mobile node; prepares a handoff of the mobile node from a current fixed node to potential fixed nodes upon detecting the movement of the mobile node; decides to implement the handoff to one of the potential fixed nodes as a target fixed node; and performs the handoff from the current fixed node to the target fixed node.  
         [0022]     Preferably, detecting movement of the mobile node is accomplished by monitoring, at each fixed node, channel states of the neighbor channels; and determining the movement of the mobile node on the basis of changes of the channel states.  
         [0023]     Preferably, preparing the handoff includes transmitting the channel states from each fixed node to the router; creating, at the router, a list of the potential fixed nodes in a prioritized manner; transmitting the list from the router to the mobile node; and preparing, at the potential fixed nodes, for the handoff.  
         [0024]     Preferably, deciding the handoff includes selecting one among the potential fixed nodes as the target fixed node according to priorities of the potential fixed nodes at the list.  
         [0025]     In another aspect of the present invention, a handoff method is provided for a wireless local area network (WLAN) including a plurality of access points (APs) attached to a router which stores a neighbor graph (NG) and is connected to one or more external networks and at least one mobile node (MN) associated with a current access point. The handoff method detects movement of the MN by eavesdropping on neighbor channels, and decides a handoff of the MN from a current AP to a new AP upon detecting the movement of the MN; exchanges handoff initiation information between the new AP and the MN upon deciding the handoff; and associates the MN with the new AP on the basis of the handoff information.  
         [0026]     Preferably, detecting the movement of the MN is accomplished by monitoring, at each AP, traffic on the neighbor channels; extracting header information from the traffic; checking changes of the traffic based on the header information; and determining the movement of the MN on the basis of the changes.  
         [0027]     Preferably, handoff is decided by reporting, at each AP, the movement of the MN to the router; transmitting the NG stored in the router to the MN; and selecting, at the MN, the new AP on the basis of the NG. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     The above and other objects, features and adventures of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0029]      FIG. 1  is a schematic diagram illustrating a conventional WLAN structure;  
         [0030]      FIG. 2   a  is a flow diagram illustrating a conventional handoff method using a passive scanning procedure;  
         [0031]      FIG. 2   b  is a flow diagram illustrating a conventional handoff method using an active scanning procedure;  
         [0032]      FIG. 3   a  is a chart illustrating delay distributions in the entire handoff latency according to a conventional passive scanning-based handoff;  
         [0033]      FIG. 3   b  is a chart illustrating delay distribution in the entire handoff latency according to a conventional active scanning-based handoff;  
         [0034]      FIG. 4  is an exemplary schematic diagram illustrating a WLAN structure according to a first embodiment of the present invention;  
         [0035]      FIG. 5  is a flowchart for illustrating the seamless handoff method according to the first embodiment of the present invention; and  
         [0036]      FIG. 6  is a flow diagram for illustrating message flows between a NG server, APs, and STAs according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0037]     Preferred embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. In the drawings, the same or similar reference numbers will be used throughout the drawings to refer to the same or like parts.  
         [0038]     In the present invention, an RF module dedicatedly responsible for reception is added to the AP.  
         [0039]      FIG. 4  is an exemplary schematic diagram illustrating a WLAN structure according to a first embodiment of the present invention.  
         [0040]     As shown in  FIG. 4 , the WLAN includes three access points APs  41 ,  42 , and  43  associated with each other so as to overlap their service areas, and the APs  41 ,  42 , and  43  are connected to the Internet  47  through a gateway  45 . In this embodiment, it is assumed that a STA  40  currently associated with the AP  42  in the coverage area (cell)  42   b  moves into a coverage area  43   b  of the AP  43 . In this case since the APs  41 ,  42  and  43  use different channels, channel  1 , channel  6 , and channel  11 , respectively, a handoff occurs. The APs  41 ,  42 , and  43  are provided with dedicated reception modules  41   a ,  42   a , and  43   a , respectively.  
         [0041]     Each reception module monitors the states of the radio channels. Thus, if a STA  40  comes close to an AP  43 , AP  43  provides STA  40  with AP information of itself, such as channel number, Medium Access Control (MAC) address, Basic Service Set Identification (BSSID), and the like for supporting a seamless handoff.  
         [0042]     Upon receiving the AP information from AP  43 , STA  40  decides handoff based on the AP information gathered from AP  43  and the currently associated AP  42 .  
         [0043]     In order to support the seamless handoff, handoff prediction information, called trigger information, is transferred to an upper layer. Upon receiving the trigger information, the upper layer performs various operations so as to prepare the handoff.  
         [0044]     Now, the handoff operation in the above structured WLAN will be described in detail.  FIG. 5  is a flowchart for illustrating the seamless handoff method according to the first embodiment of the present invention.  
         [0045]     Once each AP is turned on, the AP receives the radio frames transmitted from the STAs located in its service area by means of the dedicated reception module and extracts MAC headers from the radio frames. By analyzing the information contained in these MAC headers, the AP acquires information of the respective STAs, such as the types of STAs.  
         [0046]     Sequentially, the dedicated reception module monitors the neighbor channels so as to detect movements of STAs at step S 501 . The AP determines whether or not a movement of the STA associated thereto is detected at step S 502 . If a movement of the STA is detected, the STA tries to share the information of the STA and the AP itself at step S 503 .  
         [0047]     Upon acquiring the STA information, the AP sends the STA movement information together with AP information of itself such as received signal strength indicator (RSSI), channel number, service type, and the like to the neighbor APs. The information on the STAs and APs is shared in this manner.  
         [0048]     Upon reception of the STA movement information and the AP information, the neighbor AP sends the AP information of itself to the STA at step S 504 . The AP information includes the RSSI, channel number, and service classes.  
         [0049]     Upon receiving the AP information of the neighbor AP, the STA determines whether or not to handoff from the current AP to the neighbor AP on the basis of the strength of RSSI, channel number, and service classes included in the AP information at step S 505 . If the AP information satisfies the handoff conditions, the STA requests a handoff initiation to the neighbor AP through a handoff initiation request message at step S 506 , and otherwise, the STA sends an update request to the AP at step S 510 .  
         [0050]     Upon reception of the handoff initiation request message, the neighbor AP performs an authentication process and other handoff related operations so as to provide the STA with the service without interruption at step S 507 . At this time, in order to prepare the expected handoff, trigger information is transferred to an upper layer. Also, the neighbor AP transmits an acknowledgement, in response to the handoff initiation request, by means of a handoff initiation response message.  
         [0051]     Here, the STA determines whether or not the handoff initiation response message has been received at step  508 . If the STA receives the handoff initiation response message, the STA executes the handoff at step S 509  and ends the handoff procedure, and otherwise, the STA resends the handoff initiation request message at predetermined times, and the process returns to step S 507 .  
         [0052]     The handoff is carried out using the channel information of the neighbor AP such as BSS ID, MAC address, and RSSI, and the network is updated after the handoff is completed in order for other APs to share the status information of the AP and STA.  
         [0053]      FIG. 6  is a flow diagram illustrating message flows between a NG server, APs, and STAs according to a second embodiment of the present invention.  
         [0054]     The structure of the WLAN of the second embodiment is identical with that of the first embodiment except that the gateway is replaced with a neighbor graph server. In the second embodiment of the present invention, the neighbor graph server is denoted by the reference numeral  45  that had identified the gateway in the first embodiment.  
         [0055]     The neighbor graph server  45  stores a neighbor graph for the purpose of keeping track of the neighbor APs and provides STA  40  with the information of the neighbor APs once the STA&#39;s movement is detected. NG  45  contains the MAC addresses and IP addresses of pairs of AP and neighbor AP pairs, and channels through which each pair of APs can communicate.  
         [0056]     Upon receiving the APs information from NG server  45 , STA  40  selects one AP from among the APs listed in the neighbor AP information as a handoff target AP.  
         [0057]     In  FIG. 6 , firstly AP 1   41 , AP 2   42 , AP 3   43 , and STA  40  register their own information to the neighbor graph (NG) server  45  through registration messages such that NG server  45  configures a neighbor graph on the basis of the information contained in the registration messages. The registration message received from each AP or STA includes a MAC and IP addresses of itself, channel numbers, channel utilization, and etc.  
         [0058]     Upon receiving the registration messages from APs  41 ,  42 , and  43 , and STA  40 , NG server  45  sends acknowledgements to the respective APs  41 ,  42 , and  43  and STA  40  in response to the respective registration messages.  
         [0059]     Each AP monitors the neighbor channels by means of its RF module so as to detect the movement of STA  40 . In this embodiment, STA  40  moves from AP 2 &#39;s coverage area  42   b  to AP 3 &#39;s coverage area  43   b  (See  FIG. 4 ).  
         [0060]     Once an STA&#39;s movement is detected, AP 2   42  sends a movement detection message to NG server  45 . The movement detection message includes RSSI and link quality of the current associated AP.  
         [0061]     Upon reception of the movement detection message, NG server  45  sends an acknowledgement to AP 2   42  in response to the movement detection message and sends a handoff candidate AP message to STA  40  via AP 2   42 . The handoff candidate AP message includes MAC and IP addresses and channel information of the candidate APs. Accordingly, STA  40  selects one of the neighbor APs as a handoff target AP on the basis of the information on the APs in the handoff candidate AP message. In the second embodiment of the present invention, AP 3   43  is the handoff target AP.  
         [0062]     Upon reception of the handoff candidate information message, STA  40  sends an acknowledgement to NG server  45  via AP 2   42  in response to the handoff candidate AP message and then sends a handoff initiation request message to AP 3   43  on the basis of the information contained in the handoff candidate AP message. In response to the handoff initiation request message from STA  40 , AP 3   43  sends a handoff initiation response message to STA  40 .  
         [0063]     Sequentially, upon reception of the handoff initiation response message, STA  40  sends a handoff execution request message to AP 3   43 . Upon reception of the handoff execution request message, AP 3   43  sends a handoff execution response message to STA  40  and sends an update message to NG server  45 . The update message contains information on the neighbor APs and STAs associated the current AP.  
         [0064]     Upon reception of the update message from AP 3   43 , NG server  45  transmits an acknowledgement to AP 3   43  and updates the neighbor graph.  
         [0065]     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.  
         [0066]     As described above, in the handoff method of the present invention, each AP equipped with a reception-dedicated RF module monitors the neighbor channels and reports movement of the STA to a router whenever movement is detected, and the router provides the STA with information on the neighbor APs upon acquiring information the movement of the STA, such that the STA can quickly move its association from one AP to another without interruption or requirement of a probe phase.  
         [0067]     Also, in the handoff method of the present invention, since an STA quickly knows the neighbor AP&#39;s information by means of neighbor graph server, it is possible to minimize the probe delay which is the primary contributor to the overall handoff latency.  
         [0068]     Also, since the STA can obtain the AP&#39;s information and the handoff candidate AP can prepare Layer  3  handoff prior to the handoff execution, the seamless handoff method of the present invention can be adopted for inter-system handoff.  
         [0069]     Furthermore, in the handoff method according to the present invention, since the handoff occurs in a seamless manner it can support real time multimedia services such as VoIP, video teleconference, etc.