U.S. patent application number 11/003210 was filed with the patent office on 2005-04-21 for method for performing handoff in wireless network.
This patent application is currently assigned to SAMSUNG ELECTRONICS CO., LTD.. Invention is credited to Arbaugh, William Albert, Jang, Kyung-Hun, Lee, In-Sun, Mishra, Arunesh, Shin, Min-Ho.
Application Number | 20050083887 11/003210 |
Document ID | / |
Family ID | 32108174 |
Filed Date | 2005-04-21 |
United States Patent
Application |
20050083887 |
Kind Code |
A1 |
Lee, In-Sun ; et
al. |
April 21, 2005 |
Method for performing handoff in wireless network
Abstract
A method for minimizing handoff latencies when a handoff is
performed in a wireless network. An access point (AP) or base
station associated to a current wireless station (STA) allows
information required for a reassociation to the STA to be
propagated to handoff-capable neighboring APs or base stations.
When the STA moves, a neighboring AP or base station performs the
reassociation to the STA on the basis of context. When a handoff
procedure is performed, the time taken to receive context of a
corresponding STA is reduced, such that a fast handoff can be
implemented.
Inventors: |
Lee, In-Sun; (Seoul, KR)
; Jang, Kyung-Hun; (Suwon-si, KR) ; Shin,
Min-Ho; (Laurel, MD) ; Arbaugh, William Albert;
(Ellicott City, MD) ; Mishra, Arunesh; (Greenbelt,
MD) |
Correspondence
Address: |
DILWORTH & BARRESE, LLP
333 EARLE OVINGTON BLVD.
UNIONDALE
NY
11553
US
|
Assignee: |
SAMSUNG ELECTRONICS CO.,
LTD.
GYEONGGI-DO
MD
THE UNIVERSITY OF MARYLAND COLLEGE PARK
COLLEGE PARK
|
Family ID: |
32108174 |
Appl. No.: |
11/003210 |
Filed: |
December 3, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11003210 |
Dec 3, 2004 |
|
|
|
10703516 |
Nov 10, 2003 |
|
|
|
60425109 |
Nov 8, 2002 |
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Current U.S.
Class: |
370/331 ;
370/338 |
Current CPC
Class: |
H04W 48/20 20130101;
H04W 84/12 20130101; H04W 12/06 20130101; H04W 88/08 20130101; H04W
36/0016 20130101; H04W 36/08 20130101; H04W 84/045 20130101; H04W
92/20 20130101; H04W 36/0038 20130101; H04W 48/16 20130101; H04W
36/0033 20130101; H04W 40/00 20130101; H04W 84/18 20130101; H04W
36/0055 20130101 |
Class at
Publication: |
370/331 ;
370/338 |
International
Class: |
H04B 007/212 |
Goverment Interests
[0002] 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.
Claims
What is claimed is:
1. 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 comprising: 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.
2. 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 comprising: 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.
3. The method according to claim 2, wherein re-association of the
station and a handoff-capable neighboring access point is performed
on the basis of the propagated context, in a handoff of the station
to the handoff-capable neighboring access point.
4. The method according to claim 3, wherein the handoff-capable
neighboring access point propagates information including the
context corresponding to the station to one or more handoff-capable
access point of the wireless network neighboring the
handoff-capable neighboring access point, in response to
re-association with the station.
5. The method according to claim 3, wherein in response to the
context corresponding to the station not being present in the
handoff-capable neighboring access point, the handoff-capable
neighboring access point forms a channel with a prior access point
of the station and receives the context from the prior access
point.
6. A computer-readable medium comprising computer-executable
instructions for performing the operations recited in claim 2.
7. 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 comprising: 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.
8. A computer-readable medium comprising computer-executable
instructions for performing the operations recited in claim 7.
Description
PRIORITY
[0001] 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.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] 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.
[0005] 2. Description of the Related Art
[0006] 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.
[0007] 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.
[0008] FIG. 1 is a view illustrating the architecture of a
conventional wireless local area network (WLAN) for supporting the
infrastructure mode.
[0009] Referring to FIG. 1, a plurality of access points (APs) 120a
and 120b are connected through one distribution system (DS) 110.
The DS 110 is implemented with a wired network. A communication
path is formed between the plurality of APs 120a and 120b. The
plurality of APs 120a and 120b form constant service areas, and
serve as bridges between STAs 130a, 130b, 130c and 130d and the DS
110. One AP and the STAs associated with the AP form a basic
service set (BSS). In other words, a unique BSS is formed on an
AP-by-AP basis, and service is provided on a BSS-by-BSS basis. A
plurality of BSSs formed by the APs 120a and 120b can be extended
to extended service sets (ESSs). The STAs 130a, 130b, 130c and 130d
must undergo an authentication procedure to access the WLAN through
the APs 120a and 120b to which the STAs 130a, 130b, 130c and 130d
belong. In other words, the STAs 130a, 130b, 130c and 130d are
permitted to access the network through the authentication
procedure. There is provided state information required so that the
STAs 130a, 130b, 130c and 130d can access the network according to
the authentication procedure. The state information contains
encryption information (based on an encryption code) used to
transfer data to the DS 110.
[0010] In the WLAN based on the architecture shown in FIG. 1, a
wireless station (STA) has mobility and hence can move from one BSS
to another BSS. In this case, a handoff is required so that service
being received from the one BSS can be continuously provided to the
STA by another BSS. An AP to which the STA had physical layer
connectivity prior to the handoff is referred to as a "prior-AP",
while a new AP to which the STA acquires physical layer
connectivity after the handoff is referred to as a "post-AP".
[0011] 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.
[0012] Logical steps based on the handoff procedure are classified
into a discovery phase and a reauthentication phase.
[0013] 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.
[0014] 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.
[0015] FIG. 2 is a view illustrating a handoff procedure in the
conventional WLAN. It is assumed in FIG. 2 that the discovery phase
is performed in the active mode. The handoff procedure shown in
FIG. 2 is divided into a probe phase 210 and a reassociation phase
220.
[0016] Referring to FIG. 2, a wireless station (STA) sensing the
need for the handoff transmits a probe request message to a
plurality of unspecified APs at step 212. The probe request message
is defined as information for asking each AP whether or not the
handoff can be successfully performed. Upon receiving the probe
request message, the APs transmit probe response messages to the
STA at step 214. Here, the fact that certain APs have received the
probe request message means that the APs are adjacent to the STA.
Thus, the APs capable of receiving the probe request message are
determined to be potential APs. The STA repeatedly performs the
above-described operation on a channel-by-channel basis.
[0017] 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.
[0018] 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.
[0019] 1. Probe Delay: Messages transmitted for an active scan at
the probe phase 210 shown in FIG. 2 are probe messages. The latency
for this process is referred to as a probe delay. The STA transmits
a probe request message and waits for responses from APs on each
channel. The time during which the STA waits on a particular
channel after sending the probe request message corresponds to
probe-wait latency. This is determined to be a time difference
between subsequent probe request messages. Here, the time is
subsequent between PROBE REQUEST MESSAGES on differing channels.
According to the above procedure, it has been found that the
traffic on the channel and the timing of probe response messages
affect the probe-wait time.
[0020] 2. Authentication Delay: This is the latency (not shown in
FIG. 2) incurred during which authentication frames are exchanged.
Authentication consists of two or four consecutive frames depending
on the authentication method used by the AP. Some wireless network
interface cards (NICs) try to initiate a reassociation prior to the
authentication, which causes an additional delay in the handoff
process.
[0021] 3. Reassociation Delay: This is the latency incurred during
which reassociation frames are exchanged in the reassociation phase
220 shown in FIG. 2. If an authentication process is successful,
the STA sends a reassociation request frame to the AP, receives a
reassociation response frame, and completes the handoff. Where the
IAPP procedure between a new AP and other APs is additionally
required, the reassociation delay will further increase.
[0022] 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.
SUMMARY OF THE INVENTION
[0023] Accordingly, it is an aspect of the present invention to
provide a method for minimizing handoff latencies.
[0024] 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.
[0025] 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.
[0026] 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).
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] 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:
[0032] FIG. 1 is a view illustrating the architecture of a
conventional wireless local area network (WLAN);
[0033] FIG. 2 is a view illustrating a handoff procedure in the
conventional WLAN;
[0034] FIGS. 3A and 3B are views illustrating an operation for
generating a neighborhood graph in accordance with an embodiment of
the present invention;
[0035] FIG. 4 is a conceptual view illustrating the handoff
procedure in accordance with the embodiment of the present
invention;
[0036] FIG. 5 is a view illustrating the handoff procedure in a
wireless local area network (WLAN) in accordance with the
embodiment of the present invention; and
[0037] FIG. 6 is a flow chart illustrating operations of access
points (APs) in accordance with the embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] 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.
[0039] 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.
[0040] According to another aspect, as illustrated in FIG. 3B, a
data structure may be provided for containing information on one or
more potential APs for each of the APs. In this case, such a
structure may be provided in a medium accessible by each of the
APs. Methods for generating the neighborhood graph and a handoff
procedure based on the proactive caching technique using the
neighborhood graph will be described in detail herein below.
[0041] 1. Generation of Neighborhood Graph
[0042] 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.
[0043] 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.
[0044] 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.
[0045] FIG. 3A is a view illustrating an exemplary arrangement of
APs configuring the WLAN to which an embodiment of the present
invention is applied; and FIG. 3B is a view illustrating an
exemplary neighborhood graph capable of being generated by the
arrangement of APs shown in FIG. 3A.
[0046] As shown in FIG. 3A, an AP_C is installed in a closed space
with one gateway. Thus, a path in which the STA located at AP_C can
move is defined by an AP_B. This means that only a handoff process
between AP_C and AP_B can be performed with respect to the STA
located at AP_C. The STA located at AP_B can move not only to AP_A,
AP_D and AP_E but also to AP_C, which are installed at passages (as
physical connections). In other words, the STA located at AP_B
allows a handoff process to be performed between AP_B and all other
APs shown in FIG. 3A. APs to which the STA located at AP_A can
directly associate without going through any other AP are defined
by AP_B and AP_E. Thus, the STA located at AP_A allows a handoff
process to be performed between AP_A and AP_B or AP_E. The STA
located at AP_E can directly associate to all APs other than AP_C
among APs shown in FIG. 3A. This means that the STA located at AP_E
allows a handoff process to be performed between AP_E and any AP
except for AP_C. APs to which the STA located at AP_D can directly
associate without going through any other AP are defined by AP_B
and AP_E. Thus, the STA located at AP_D allows a handoff process to
be performed between AP_D and AP_B or AP_E. A reason why a handoff
between AP_D and AP_A is not permitted is because a distance
between AP_D and AP_A is outside a predetermined threshold distance
range.
[0047] FIG. 3B shows a neighborhood graph generated by the
connection relationships between the above-described APs. The
neighborhood graph shown in FIG. 3B shows the connection
relationships between all APs configuring the WLAN. In accordance
with the present invention, each AP only needs to recognize
potential APs capable of being associated therewith. For example,
AP_A only needs to recognize AP_B and AP_E as its potential APs,
while AP_B only needs to recognize AP_A, AP_C, AP_D and AP_E as its
potential APs. As described above, while not illustrated, a
neighborhood graph on each AP may be generated by the manager or
can be automatically generated according to an existing handoff
procedure.
[0048] 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.
[0049] 2. Proactive Caching Technique
[0050] 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.
[0051] 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 FIG. 4. FIG. 4 is
a conceptual view illustrating a handoff procedure based on the
proactive caching technique in accordance with the embodiment of
the present invention. Here, it is assumed that a wireless station
(STA) moves from an AP_A to an AP_B.
[0052] Referring to FIG. 4, the STA sends an
association/reassociation request to AP_A at step 1. AP_A performs
different operations according to whether the association or
reassociation request is received from the STA.
[0053] 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.
[0054] 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.
[0055] 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 FIG. 4. However, where a plurality of APs are
present as potential APs, the context is propagated to the
plurality of APs. AP_B stores the context transferred from AP_A in
a cache. After moving to AP_B through a predetermined path, the STA
sends a reassociation request to AP_B at step 3. In response to the
reassociation request, AP_B performs communication with the STA
according to the context previously transferred from AP_A. In other
words, the reassociation between AP_B and the STA is performed
according to the context. Thus, the present invention reduces a
time delay incurred during the IAPP procedure and hence improves a
communication rate.
[0056] 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.
[0057] 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
FIG. 5. FIG. 5 is a view illustrating a handoff procedure using the
proactive caching technique in the WLAN in accordance with the
embodiment of the present invention.
[0058] Referring to FIG. 5, context of a corresponding STA is
transferred from a prior-AP to a post-AP before a reassociation
process for the handoff is performed. In FIG. 5, it is assumed that
AP_A is the prior-AP and AP_B is the post-AP. Furthermore, it is
assumed that the context of the corresponding STA is already
temporarily stored.
[0059] Referring to FIG. 5, the STA sends a reassociation request
message to AP_A at step 501. At this time, AP_A may already have
stored the context of the STA using the proactive caching
technique. Otherwise, if AP_A has not stored the context of the
STA, AP_A can receive the context of the STA from the WLAN through
the typical authentication procedure or receives the context of the
STA from an AP at which the STA was previously located through the
IAPP procedure. AP_A transmits a reassociation response message to
the STA on the basis of the temporarily stored context
corresponding to the STA at step 503. Then, AP_A propagates the
temporarily stored context to a potential AP, i.e., AP_B at step
505. At this time, information of the potential AP can be obtained
from the above-described neighborhood graph. It is assumed that the
number of potential APs is one as shown in FIG. 5, but a plurality
of potential APs can be present. If the multiple potential APs are
present, AP_A propagates the context of the STA to the plurality of
potential APs. AP_B temporarily stores the context corresponding to
the STA propagated from AP_A.
[0060] 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.
[0061] 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.
[0062] 3. Description of Operation in Accordance With the Present
Invention
[0063] 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 FIG. 6. A procedure of
receiving and storing context received from neighboring APs, a
procedure of performing an operation in response to an association
request, and a procedure of performing an operation in response to
a reassociation request will now be described with reference to
FIG. 6.
[0064] Referring to FIG. 6, the AP determines whether context
corresponding to a specific wireless station (STA) is received from
handoff-capable neighboring APs that are managed by a neighborhood
graph at step 610. Upon receiving the context corresponding to the
specific STA, the AP proceeds to step 612, and stores the received
context in its own cache. Although, FIG. 6 illustrates the steps
for receiving and storing the security context, these steps are not
necessarily required to perform the following steps 614-626.
[0065] 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.
[0066] 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.
[0067] 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.
[0068] 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.
[0069] 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.
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