U.S. patent application number 10/587879 was filed with the patent office on 2007-10-04 for pico cell wireless local area network (wlan).
Invention is credited to John Bongiorno, John Brancato, Tarek Ibrahim.
Application Number | 20070232307 10/587879 |
Document ID | / |
Family ID | 36068885 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070232307 |
Kind Code |
A1 |
Ibrahim; Tarek ; et
al. |
October 4, 2007 |
Pico Cell Wireless Local Area Network (Wlan)
Abstract
A pico cell wireless LAN and pre-emptive roaming algorithm is
provided to allow mobile devices in the WLAN to hand-off to another
different access point quickly and efficiently. Once associated
with an access point (AP), the mobile device receives information
about neighboring APs that may be available for hand-off during
roaming (or insufficient signal strength). Signal strength of the
associated AP is continuously monitored and if signal strength from
the associated AP falls below a threshold, the mobile device
measures signal strength of the neighboring APs in the list, ranks
them, and selects an AP for hand-off. AP load and other information
may be used to rank the neighboring APs. The mobile device
hands-off to (or associates with) one of the neighboring APs, if
appropriate. Hand-offs are attempted in order or rank. Cell sizes
in the pico cell WLAN are relatively small, on the order of 1000
square feet or less.
Inventors: |
Ibrahim; Tarek; (Hoboken,
NJ) ; Brancato; John; (Tappan, NY) ;
Bongiorno; John; (Litchfield, CT) |
Correspondence
Address: |
Docket Clerk
P O Box 800889
Dallas
TX
75380
US
|
Family ID: |
36068885 |
Appl. No.: |
10/587879 |
Filed: |
December 16, 2005 |
PCT Filed: |
December 16, 2005 |
PCT NO: |
PCT/US05/45440 |
371 Date: |
July 27, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60636741 |
Dec 16, 2004 |
|
|
|
Current U.S.
Class: |
455/436 |
Current CPC
Class: |
H04W 84/12 20130101;
H04W 36/00835 20180801; H04W 36/30 20130101 |
Class at
Publication: |
455/436 |
International
Class: |
H04Q 7/38 20060101
H04Q007/38 |
Claims
1. A method of hand-off for a mobile terminal from a first access
point to a second access point in a wireless local area network
(WLAN), the method comprising: measuring in a mobile terminal
signal to noise ratio (SNR) of first RF signals received from the
first access point; if the measured SNR of the first RF signals
exceeds a first threshold, measuring SNR of RF signals received
from a plurality of candidate access points in a roaming candidate
list stored on the mobile terminal; determining from measured SNRs
of the candidate access points whether any of the measured SNR
exceed a second threshold, and if so, identifying those candidate
access points in a new association list; selecting one of the
candidate access points in the new association list; and attempting
to associate the mobile terminal to the selected candidate access
point.
2. The method in accordance with claim 1 further comprising:
associating the mobile terminal to the first access point in the
WLAN; and receiving from the first access point the roaming
candidate list identifying the plurality of candidate access points
in the WLAN.
3. A mobile terminal operable for wireless connection to one or
more access points in a wireless local area network (WLAN), the
device comprising: means for measuring signal to noise ratio (SNR)
of first RF signals received from the first access point; if the
measured SNR of the first RF signals exceeds a first threshold,
means for measuring SNR of RF signals received from each of a
plurality of candidate access points in a roaming candidate list;
means for determining from measured SNRs of the candidate access
points whether any of the measured SNR exceed a second threshold,
and if so, identifying those candidate access points in a new
association list; means for selecting one of the candidate access
points in the new association list; and means for attempting to
associate the mobile device to the selected candidate access
point.
4. The method in accordance with claim 3 further comprising: means
for associating the mobile terminal to a first access point in the
WLAN; and means for receiving from the first access point the
roaming candidate list identifying the plurality of candidate
access points in the WLAN.
5. A state machine for use by a mobile terminal in a wireless area
network, the state machine comprising: a first state in which the
mobile terminal is associated with a first access point in the
network and signal to noise ratio (SNR) of first RF signals
received from the first access point are measured; a second state
in which SNR of RF signals received from a plurality of candidate
access points in a roaming candidate list are measured and it is
determined from measured SNRs of the candidate access points
whether any of the measured SNRs exceeds a second threshold, and if
so, identifying those candidate access points in a new association
list, the state machine transitioning from the first state to the
second if the measured SNR of the first RF signals exceeds a first
threshold; a third state in which one of the candidate access
points in the new association list is selected and an attempt is
made to associate the mobile terminal to the selected candidate
access point, the state machine transitioning from the second state
to the third state if there is at least one candidate access point
in the new association list.
6. The state machine in accordance with claim 5 further comprising:
a fourth state in which a plurality of channels associated with a
plurality of access points are scanned to determine SNRs for each
of the plurality of access points, one of the plurality of access
points is selected and the mobile terminal attempts to associate to
the selected access point, the state machine transitioning from the
fourth state to the first state when the mobile terminal associates
with the selected access point.
7. A mobile terminal for communicating with one or more access
points in a wireless local area network (WLAN), the device
comprising: a processor; a transceiver coupled to the processor; an
antenna coupled to the transceiver for receiving and transmitting
RF signals from and to the one or more access points in the WLAN;
and wherein the processor is operable for: measuring signal to
noise ratio (SNR) of first RF signals received from the first
access point, if the measured SNR of the first RF signals exceeds a
first threshold, measuring SNR of RF signals received from each of
a plurality of candidate access points in a roaming candidate list
stored in the mobile terminal, determining from measured SNRs of
the candidate access points whether any of the measured SNR exceed
a second threshold, and if so, identifying those candidate access
points in a new association list, selecting one of the candidate
access points in the new association list, and attempting to
associate the mobile device to the selected candidate access
point.
8. The mobile terminal in accordance with claim 7 wherein the
processor is further operable to: associate the mobile terminal to
a first access point in the WLAN; and receive from the first access
point the roaming candidate list identifying the plurality of
candidate access points in the WLAN.
9. A wireless local area network (WLAN), the WLAN comprising: a
plurality of sets of access points operable for communicating
wirelessly with one or more remote client devices, each set of
access points defines a cell having a predefined communication
coverage area within the WLAN; a plurality of switches
communicatively coupled to access points; and wherein the
communication coverage area of each defined cell is less than about
1000 hundred square feet and the access points in a first cell are
operable for transmitting a roaming candidate list to a mobile
device associated with one of the access points in the first cell,
the list identifying one or more neighborhood access points.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority under 35 USC 119(e) to U.S.
provisional Application Ser. No. 60/636,741, filed on Dec. 16,
2004, and which is incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates generally to communication
networks, and more particularly to a wireless local area network
having small cell coverage areas.
BACKGROUND
[0003] Current wireless systems or networks are configured with
cells having relatively large coverage areas. In an application in
which there are high number of users who may roam often within a
small coverage area, current wireless techniques may not be
applicable due to co-channel interference issues and faster than
normal requirements for a hand-off.
[0004] Accordingly, there is needed a WLAN architecture in small
area/high density/fast roaming applications having pico small cells
and method for preemptively recognizing potential hand-off
scenarios in a mobile device and applying a fast hand-off routine
or process for fast hand-offs.
SUMMARY
[0005] In accordance with one embodiment of the present invention,
there is provided a method of hand-off for a mobile terminal from a
first access point to a second access point in a wireless local
area network (WLAN). The method measures in a mobile terminal
signal to noise ratio (SNR) of first RF signals received from the
first access point. If the measured SNR of the first RF signals
exceeds a first threshold, SNR of RF signals received from a
plurality of candidate access points in a roaming candidate list
stored on the mobile terminal is measured. From the measured SNRs
of the candidate access points it is determined whether any of the
measured SNR exceed a second threshold, and if so, those candidate
access points are identified in a new association list. One of the
candidate access points in the new association list is selected and
an attempt to associate the mobile terminal to the selected
candidate access point is made.
[0006] In another embodiment, there are provided means for
performing the above method steps or functions.
[0007] In yet another embodiment, there is provided a state machine
for use by a mobile terminal in a wireless area network. The state
machine includes a first state in which the mobile terminal is
associated with a first access point in the network and signal to
noise ratio (SNR) of first RF signals received from the first
access point are measured; a second state in which SNR of RF
signals received from a plurality of candidate access points in a
roaming candidate list are measured and it is determined from
measured SNRs of the candidate access points whether any of the
measured SNRs exceeds a second threshold, and if so, identifying
those candidate access points in a new association list, the state
machine transitioning from the first state to the second if the
measured SNR of the first RF signals exceeds a first threshold; and
a third state in which one of the candidate access points in the
new association list is selected and an attempt is made to
associate the mobile terminal to the selected candidate access
point, the state machine transitioning from the second state to the
third state if there is at least one candidate access point in the
new association list.
[0008] In yet another embodiment, a mobile terminal for
communicating with one or more access points in a wireless local
area network (WLAN) is provided, the mobile terminal includes a
processor, a transceiver coupled to the processor, and an antenna
coupled to the transceiver for receiving and transmitting RF
signals from and to the one or more access points in the WLAN. The
processor is operable for: measuring signal to noise ratio (SNR) of
first RF signals received from the first access point; if the
measured SNR of the first RF signals exceeds a first threshold,
measuring SNR of RF signals received from each of a plurality of
candidate access points in a roaming candidate list stored in the
mobile terminal; determining from measured SNRs of the candidate
access points whether any of the measured SNR exceed a second
threshold, and if so, identifying those candidate access points in
a new association list; selecting one of the candidate access
points in the new association list; and attempting to associate the
mobile device to the selected candidate access point.
[0009] In still another embodiment, there is provided a wireless
local area network (WLAN). The WLAN includes a plurality of sets of
access points operable for communicating wirelessly with one or
more remote client devices, each set of access points defines a
cell having a predefined communication coverage area within the
WLAN. The WLAN also includes a plurality of switches
communicatively coupled to access points. The communication
coverage area of each defined cell is less than about 1000 hundred
square feet and the access points in a first cell are operable for
transmitting a roaming candidate list to a mobile device associated
with one of the access points in the first cell, the list
identifying one or more neighborhood access points.
[0010] Furthermore, a computer program performing one or more of
these methods is embodied on a computer readable medium and
operable for executing one or more of the methods and functions
described herein.
[0011] Other technical features may be readily apparent to one
skilled in the art from the following figures, descriptions, and
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] For a more complete understanding of the present invention,
and the advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawings,
wherein like numbers designate like objects, and in which:
[0013] FIG. 1 illustrates an example pico cell wireless local area
network or system in accordance with the present invention;
[0014] FIG. 2 depicts a block diagram of a mobile device shown in
FIG. 1;
[0015] FIG. 3 depicts a block diagram of an access point shown in
FIG. 1; and
[0016] FIG. 4 is a state diagram illustrating a fast roaming
algorithm or process in accordance with the present invention.
DETAILED DESCRIPTION
[0017] FIG. 1 illustrates an example wireless local area network
(WLAN) or system 100 in accordance with the present invention. The
WLAN or system 100 shown in FIG. 1 is for illustration purposes
only. Other embodiments of the WLAN 100 may be used without
departing from the scope of this disclosure.
[0018] In this example, the WLAN 100 includes a firewall/gateway
102 for coupling to a public network 120. The WLAN 100 further
includes one or more routers/switches (or L3/L4 switches) 103, one
or more switches 104 (e.g., L2 switch), one or more security
switches 106, a plurality of wireless access points (APs) 108 each
having an antenna (not shown). The gateway 102 functions to
communicatively couple the WLAN 100 to the public network 120, and
may further include firewall and security functionality. It may
also include router switch functionality, such as L2/L3 switching.
The WLAN 100 further includes one or more mobile devices 118
communicatively coupled wirelessly to the WLAN 100 via the APs
108.
[0019] The public network 120 may include one or more local area
networks ("LAN"), metropolitan area networks ("MAN"), wide area
networks ("WAN"), all or portions of a global network such as the
Internet, or any other communication system or systems at one or
more locations, or combination of these. Further, the network 120
may include various servers, routers, bridges, and other access and
backbone devices (not shown). In one embodiment, the network 120 is
a packet data network that utilizes any suitable protocol or
protocols, and in a specific embodiment, the network 120 (and most
components of the system 100) operates in accordance with the
Internet Protocol (IP). As will be appreciated, the concepts and
teachings of the present invention are not limited to IP, but may
be utilized in any data packet network that facilitates
communication between components coupled to the public network 120,
including Internet Protocol ("IP") packets, frame relay frames,
Asynchronous Transfer Mode ("ATM") cells, or other data packet
protocols.
[0020] The specific hardware configuration of the firewall/gateway
102, switches 103, switches 104, security switches and the access
points 108 is not essential to the present invention, and these
elements may be implemented using such components or devices as the
Nortel (Ethernet Routing Switch) ESR 8600 L2/L3 Routing Switch
(firewall/gateway 102, switches 103), Nortel (Ethernet Switch) ES
460 Switch (switches 104), WLAN 22XX/23XX Security Switches
(switches 106) and Nortel WLAN 22XX/23XX Access Points (access
points 108), available from Nortel Networks, Ltd.
[0021] In general terms, the WLAN 100 provides wireless
connectivity and communications capabilities to the mobile devices
118 when the devices are within a finite coverage area of the WLAN
100. The coverage area of the WLAN is determined by the respective
positions of the access points 108 and/or the APs antennas and the
operating parameters of the WLAN 100.
[0022] In the WLAN 100 of the present invention, the overall
coverage area is divided into a number of pico cells (this may be
done conceptually or functionally, or a combination thereof). The
term "pico" is used herein to refer to the size of the cells within
the overall coverage area. Pico cells usually are configured to
have operating coverage areas much smaller than other conventional
wireless communications systems, such as cell phone or WiFi
systems. In the present invention, the coverage areas of pico cells
are in the range of about 1000 square feet or less. In other
embodiments, the coverage areas may be 500 square feet or less, and
may range down to even about 100 square feet in area. In yet other
embodiments, coverage areas are about 200 square feet or less, or
may be in the range of about 150 to 200 square feet. The cell
coverage area sizes, as well as cell configurations, may be the
same or different for different cells.
[0023] Each access point 108 has associated therewith at least one
antenna (not shown) operable for transmitting and receiving
wireless signals, thus providing wireless connectivity to the WLAN
100 for the mobile devices 118. An access point 108 and associated
antenna may be described functionally or logically as an access
point (or as in conventional systems, a base station). In essence,
single or multiple access points or "base stations" may exist for a
cell, and multiple access points or base stations may be utilized
for redundancy purposes or for increasing the number of mobile
devices that can be serviced/connected within a given cell. Many
different cell configurations (shape and size of coverage areas,
bandwidth, number of connections, etc.) utilizing different
combinations and numbers of access points 108 are possible, as
determined or chosen to meet specifications and operating
parameters of the network to be implemented. Coverage areas of
cells may also overlap.
[0024] A given pico cell may have one or more access points
(configured using any combination of access point devices 108)
associated with the cell. For example (not shown), a particular
cell may have a specific cell coverage area using AP 108(1), AP
108(6), AP 108(11) and AP 108(13) (coupled to switches 104(1)
through 104(4), respectively) placed at one or more appropriate
predefined locations. In one embodiment, each AP 108 is assigned to
a single channel within the channel band of the wireless protocol.
Channel assignment to APs 108 is based on the desired network
configuration and coverage.
[0025] Antennas for the APs 108 may be omni-directional or
directional. In one embodiment, the antennas are directional and
the number and placement of the antennas, as well as the intended
coverage area, may be configured or chosen to meet the desired
network requirements. The antennas may be integrated with the
access point device 108, as in the case of a Nortel WLAN 22XX/23XX
Access Point device, or may be coupled via a wire link.
[0026] Again referring to FIG. 1, the WLAN 100 includes both
wireless and wireline communication links. The wireless
communication links (or wireless interface) are those links between
the mobile devices 118 and the antennas of access points 108. The
wireline communication links (or wireline interface) are those
links that connect the APs 108 to the switches 104, the switches
104 to the switches 103, the switches 103 to the security switches
104 and to the firewall/gateway 102. In other embodiments, one or
more of the wireline communication links may be wireless.
[0027] It will be understood that the wireless components or
portion of the WLAN 100 may operate utilizing any suitable wireless
protocol(s) and other communication protocol(s). In one specific
embodiment, the wireless interface and components, namely, the
access points 108 and mobile devices 118 operate in accordance with
IEEE 802.11.
[0028] As illustrated in FIG. 1, each access point 108 has a
dedicated communications link for carrying forward and reverse
traffic between the AP 108 (and the one or more associated mobile
devices 118) and an associated switch 104. Each switch 104 may have
any number of APs 108 connected thereto, and in the embodiment
shown in FIG. 1, four APs 108(1) through 108(4) are connected to
the switch 104a. The dedicated links between switches 104 and
connected APS 108 may use any suitable protocol, such as Ethernet,
and may carry one or more communications channels. For example,
each link may have 100 Mbps capability.
[0029] Optionally, each AP 108 has a second dedicated line (shown
in dotted lines) that is connected to a different switch 104. This
provides redundancy or increased operability in the network such
that if one switch 104 fails, the APs originally associated with
that switch 104 may still operate through one or more other
switches 104. The illustrated configuration provides some
resiliency and routing redundancy in the network. Other
configurations may be utilize.
[0030] Each switch 104 has two dedicated communications link for
carrying forward and reverse traffic between the switch 104 (and
the one or more associated APs 108) and two associated switches 103
(e.g., switches 103(1) and 103(3) as a pair, or switches 103(2) and
103(4) as a pair. Each switch 103 may have any number of switches
104 connected thereto, and in the embodiment shown in FIG. 1, half
of the switches 104 are connected to a pair of switches 103(1) and
103(3), while the other half are connected to a pair of switches
103(2) and 103(4). This is done for backup and loading
purposes.
[0031] In one embodiment for example, the switches 103(1) and
103(3) each handle or are assigned 50% of the load from their
connected switches 104 (primary for some and secondary for other
switches 104). In the event that one switch in the pair fails, the
other switch in the pair may take over. Switch pairs 103(2) and
103(4) are configured and operated similarly. Optionally, each
switch 103 in a switch pair may be located at a different location
or "room" to provide additional resiliency. The dedicated links
between switches 104 and connected APS 108k may use any suitable
protocol, such as Ethernet, and may carry one or more
communications channels. For example, each link may have 1 Gbps
capability over fiber.
[0032] As illustrated in FIG. 1, the switches 103 are connected to
one or more security switches 106 and the firewall/gateway 102 (and
the one or more associated mobile devices 118). Each switch 103 may
have any number of security switches 106 connected thereto, and in
the embodiment shown in FIG. 1, each switch 103(1) through 103(4)
is connected to a number of associated security switches 106a
through 106d (for example eight switches per block). Dedicated or
shared links may be used, and any suitable protocol, such as
Ethernet, may be used.
[0033] Connections between switches 103 and the gateway/firewall
102 may be similarly implemented.
[0034] The security switches 106 typically provide control and
advanced functions of the wireless control and forwarding planes of
the network 100, while the switches 103 and 104 provide control and
advanced function of the wired control and forwarding planes.
[0035] In one or more another embodiments, each of the
communications links/lines/channels between access points 108 and
switches 104, between switches 104 and switches 103, and between
switches 103 and security switches and firewall gateway 102, may be
carried on a shared data network (not shown), having high data
rates such as 10G Ethernet (and may comprise one or more physical
channels, and perhaps many logical channels, etc.).
[0036] As will be appreciated, FIG. 1 illustrates one example of an
implementation of a WLAN 100 in accordance with the present
invention. Other implementations and network configurations may be
utilized.
[0037] The mobile devices 118 represent any devices that may be
communicatively coupled wirelessly to a data network, including but
not limited to computers, modems, PDAs, routers, switches, or any
other network devices and the like. Each of the mobile devices 118
may be constructed or configured from any suitable hardware,
software, firmware, or combination thereof for transmitting or
receiving information in the WLAN 100.
[0038] Now referring to FIG. 2, there is illustrated a block
diagram of one embodiment of a mobile device 118 in accordance with
the present invention. The mobile device 118 includes a processor
or controller 200, a memory 202, an input/output interface 204, a
transceiver 206 and an antenna 208. The conventional functionality
of these components for providing wireless communication is omitted
as this is within the knowledge of a person of skill in the art. In
one embodiment, the mobile device 118 includes the components
integrated therein as part of the mobile device 118. In another
embodiment, these components and functionality are implemented as a
WLAN mobile adapter Card Bus PC card which may be installed on
various computing devices to achieve wireless connectivity. One
example of a WLAN mobile adapter are those available from Nortel
Networks, Ltd and identified as WLAN 2201/2202 Mobile Adapter.
Though the term "mobile" may be used herein to refer to the devices
118, these devices may also be stationary or fixed within the WLAN
100, and may also be referred to as "client devices" as well as
"terminals."
[0039] Now referring to FIG. 3, there is illustrated a block
diagram of one embodiment of an access point 108 in accordance with
the present invention. The access point 108 includes a processor or
controller 300, a memory 302, an input/output interface 304 and a
transceiver 306. The transceiver 306 receives/transmits signals
from/to an antenna (not shown) positioned at or near the access
point 108. The antenna may alternatively be integrated with the
access point 108. The conventional functionality of these
components for providing wireless communication is omitted as this
is within the knowledge of a person of skill in the art. Each of
the access points 108 may be constructed or configured from any
suitable hardware, software, firmware, or combination thereof for
transmitting or receiving wireless data in the WLAN 100.
[0040] Though the pico cell WLAN 100 of the present invention has
some functional and overall wireless communication similarities to
conventional wireless communications networks, different functional
and application needs of a pico cell-type WLAN require new
processes and functionality. The pico cell network configuration of
the present invention is useful when design considerations include
a high number of users and high user density within a relatively
small area, high bandwidth per user, high resiliency under single
and multiple points of failure, and the applications are sensitive
to lost data packets.
[0041] Due to unique issues raised in such environments by having
small cell sizes, the complexities of roaming (and roaming often)
and sensitivity to connection drops, the application of
conventional wireless network techniques is not practical in a pico
cell WLAN environment. It has been determined that new techniques
for handling or addressing co-channel interference and fast roaming
are beneficial in implementing a pico cell WLAN in accordance with
the present invention.
[0042] Co-channel interference is a significant problem when cell
sizes are relatively small and access points 108 (or antennas) are
close to each other. The goal is to contain the RF signal such that
its effects are reduced outside the desired cell coverage area.
Also, since cell coverage areas are designed to be relatively
small, it is beneficial to effectively limit the range of a channel
into adjacent cells. The pico cell WLAN 100 of the present
invention implements transmission power levels that are lower than
traditional wireless systems in conjunction with directional
antennas. Further, the WLAN 100 operates with and utilizes upper
and lower receive level thresholds at both the mobile devices 118
and access points 108. Levels of received RF signals are measured
(e.g., power, SNR, etc.), and when the levels are below a lower
threshold, the RF signals (or packets) are ignored (weak packets
ignored). Optionally, an upper threshold may also be used. This
reduces co-channel interference (i.e., interference on the same
channel but generated by an AP or client device using the channel
in another cell).
[0043] The present invention further provides a fast roaming
algorithm or process for use within a wireless network, such as the
WLAN 100. In one embodiment, the method is performed by the mobile
device 118 (also referred to as the client) as part of its mobile
device driver program or processing. Now referring to FIG. 4, there
is shown a diagram of a state machine implementing a fast roaming
mode or process 400 executed by the mobile device 118 within the
WLAN 100 for establishing and maintaining a connection with an
access point 108. The process 400 further implements a fast roaming
algorithm in accordance with the present invention. In one
embodiment, most or all of the process 400 is initiated and
executed when the fast roaming mode (implemented as part of a pico
cell network) is desired or appropriate. Typically, the mobile
devices 118 will be able to operate in one of two modes--standard
mode (standard roaming in a conventional wireless network) and fast
roaming mode (for roaming in a pico cell type network).
[0044] In general terms, the mobile device 118 enters the fast
roaming mode 400 when it detects an access point 108 advertising a
pico cell information element ID in its beacons. In the fast
roaming pico cell mode 400, the mobile device 118 receives a
roaming candidate list in the association response packet (or some
other transmission) when it associates or establishes a connection
to a given access point 108 and stores this list. The client 118
uses this list of roaming candidates to find another access point
when a preemptive handoff threshold is reached. During a preemptive
handoff decision phase, the client alternates between scanning
candidates on the roaming list with directed probes and moving
traffic to minimize packet latency. In the event that the
preemptive handoff fails or the client loses connectivity with the
network, an active scan of all the available channels is conducted
to find an access point.
[0045] The fast roaming process 400 may be enabled by the user via
a client utility or it will automatically be enabled when the
mobile device 118 has located an access point 108 which advertises
pico cell capability in its beacons or other responses (e.g., probe
responses, etc.). Otherwise, some conventional or standard wireless
roaming algorithm(s) will likely be implemented by the mobile
device 118 (standard mode). The fast roaming algorithm minimizes
broadcast probe request traffic and, in one embodiment, limits the
time that a client 118 is off channel to less than 100 msec out of
any 150 msec period of time while the client is associated and has
network connectivity. It also works to limit the number of clients
118 connected to a single AP 108 to meet any desirable load
balancing requirements. The two-mode operation may be ignored, and
the mobile devices 118 may always operate in accordance with the
fast roaming mode in any wireless network, if desired.
[0046] The present invention provides a method that is preemptive
in nature to allow roaming to another access point before losing
connectivity to the currently associated access point. While in the
associated state, the client 118 monitors the SNR of the associated
access point at regular intervals. In one embodiment, if the client
loses network connectivity, it will scan the list of roaming
candidates with directed probe requests before transitioning to the
state of total loss of network connectivity. When a mobile device
118 has no connection to an access point 108, it will perform an
active scan using broadcast probe requests to find an access point
108. In one embodiment, a complete active scan will take (100 msec
dwell time plus 5 msec switching overhead) per channel, and if
there are twenty channels, for example, it may take about two
seconds.
[0047] Again referring to FIG. 4, below is a more detailed
description of the process 400.
[0048] A Start state 402 is the power up or booting state of the
process 400. After start-up or initialization, the mobile device
118 enters a Connection Lost State 404 (State 7). This state is
functionally an active scan or pico cell network detection process
or state. State 404 is entered during initial power up, when
reassociation fails (from State 5), or when an association is lost
in some other manner (beacons lost/association lost, dissociation,
reset, other error) (from States 3, 6, 8 or 9), as shown in FIG.
4.
[0049] In this state 404, the mobile device 118 actively scans all
available channels looking for an access point 108 by broadcasting
request probes and receiving replies from the available channels
and may also determine that it is operating in a pico cell network
(from beacon or response probe information). From the received
information, a given channel is selected and a connection is
established (also referred to as an association) to an access point
108. The process then enters the Current AP Monitor State 406
(State 1). Various steps may be performed to establish the
connection/association, including authentication, handshaking,
encryption/security, etc. In one embodiment, the channel having the
highest signal-to-noise ratio (SNR) is selected, however other
selection criteria or information may be used, such as load
factors, network environment and operating parameters, etc.
[0050] If the client 118 does not find an access point 108 within a
specified period of time (i.e. default=10 seconds) a disconnect
timer will be used internally to notify the operating system (OS)
of the loss in media connection (link failure notification). Any
data queued from the OS network stack when in this state will only
be queued until the client TX data queues become full, at which
point any new data received may be dropped.
[0051] State 1 is the desirable stable state of being associated
with an access point 108 with a high SNR and further involves
continuous monitoring of SNRs. The goal of the fast roaming process
400 is to be in State 1 and, when not in State 1, to return to
State 1 as quickly as possible and, in one embodiment, within about
300 msec or less. This may be accomplished by using preemptive
handoffs and processing and avoiding performance of a full active
scan process (i.e., State 7) to complete a handoff.
[0052] In State 1, SNR is measured from beacons and/or overheard
packets from the associated access point 108. The measured SNR
samples are used to make state transition decisions and AP
selection decisions. There should be at a minimum at least one new
measured SNR sample every 100 msec (typical beacon interval).
[0053] In one embodiment, a filtering mechanism is employed for
detecting the changes in SNR values of the samples over time (e.g.,
reducing or eliminating state transitioning when only a single
beacon or sample is missed). These SNR samples are fed into a
recursive filter and the output of the filter used to make state
transition decisions and AP selection decisions. To make mobility
decisions at a rate of a handoff about every 5 seconds, the SNR
information needs to be sufficient to get a filter output every 100
msecs. Given a beacon interval of 100 msec there should be at least
1 SNR sample available every 100 msecs. The filter coefficients
need to be selected to allow the filter output to settle within 1
second or 10 samples. For example, given a filter time constant of
three samples, a sudden change in SNR will be detected in
approximately 300 msecs. Other methods may be utilized.
[0054] In State 1, the mobile device 118 receives from the
associated AP 108 information about other APs 108. This information
may include a list of neighborhood APs (or potential roaming
candidate APs) and other network, system or device-specific
information.
[0055] While in State 1, the process transitions to another state
when one of three events occurs, as shown. In a first event, when
there is a loss of network connectivity (association lost or
disassociation), the process transitions to a Reassociation State
408 (State 5). Optionally, if another type of error occurs, then
the process may transition directly to the Lost Connection State
404 (State 7). In a second event, if an associated client 118
misses "m" (or other desired number) beacons or receives a
disassociation request when in State 1, the client 118 transitions
to the Reassociation State 408 (State 5) where it attempts to
reassociate to the current AP. If it cannot reassociate (e.g.,
timer expires), the client 118 transitions to the active broadcast
scan state (Connection Lost State 7) and processing occurs in
accordance with that state. Alternatively (not shown), the client
118 may attempt to find another access point by using the candidate
roaming list before going to State 7.
[0056] In a third event, when the measured SNR drops below a
preemptive handoff threshold (the SNR requirement of the minimum
supported data rate plus a configurable margin, e.g., 19 dB plus 6
dB), the mobile device 118 transitions to a Candidate Search State
410 (State 3) which initiates a preemptive roaming process to find
and select a new AP 108 for association. In one embodiment, the SNR
threshold or margin may be configurable or computed by the network
100 (or its administrator) and may be generated or determined by
the associated AP 108 or switches 106 and transmitted to the client
device 118. Additionally, the threshold may be applicable network
wide or may be localized and different for different switches
and/or APs and/or protocols, etc.
[0057] State 3, generally, performs a scanning/searching and
selection process. If an access point is found with an SNR above
the measured SNR of the current AP plus a specified margin, the
client will attempt to associate with this new access point. This
margin may be the margin referred to in calculating the preemptive
handoff threshold (floor SNR for connectivity plus margin) or may
be a different one. Optionally, the margin value is configurable.
If the client cannot find a new access point or fails to associate
with the new access point, the client will maintain the current
association and keep looking for a new access point.
[0058] State 3 is the first step towards a preemptive handoff. In
this state, the client 118 sends directed probes to access points
108 in the roaming candidate list to gather the SNR and load
information from these access points in preparation for choosing a
new access point in a Fast Handoff State 412 (State 4). The roaming
candidate AP list is generated by the associated AP 108 and/or its
associated switch 106 and transmitted to the client device 118 when
entering into or transitioning to State 1 (or at the beginning or
early phase of State 1).
[0059] State 3 includes alternating scan and traffic windows.
During the scan window, directed probe requests are sent to each AP
108 in the roaming candidate list for the duration of an AP dwell
time. The length of the scan window determines the maximum amount
of time the associated AP 108 will be off channel and thus the
maximum packet delay for user traffic. During the traffic window,
the client 118 goes back to the channel of its associated access
point 108 (the current AP) to move data both up and down
(transmit/receive). In one embodiment, the default scan and traffic
windows are 50 and 25 msec, respectively. In another embodiment,
these times may be 300 msec, alternating between scan windows and
traffic windows. The maximum amount of time in this state is a
function of the environment and how fast the SNR changes.
[0060] To ensure reliable measurements or estimates of the SNR for
the current associated AP, the current AP will also be sent
directed probe requests at the end of scanning the roaming list.
The current AP is the last AP probed in order to obtain the most up
to date information of the current SNR value, though other timing
schemes may be utilized. During State 3, an average SNR of the
current AP is used as the point of reference during the selection
process rather than the SNR filter output (described previously
with respect to SNR monitoring in State 1). In the event the
current AP does not respond to any probe requests, the client 118
will assume that the current AP is out of range and will use an SNR
floor, which may be optionally configured to different values, as
the comparison value. If the current AP does not respond and no
candidate AP is valid (none responded to directed probe requests),
the client 118 will transition to State 7 and start a timer in that
state.
[0061] In one embodiment of State 3, there are two timers. The
first timer is an interrupt driven NDIS timer and the second timer
is a polled hardware timer. The probe request timeout uses the
interrupt driven NDIS timer provided by the OS (e.g., Windows CE).
This timer has a typical limited accuracy of -0/+6 msecs. Thus a 3
msec probe request timeout may take 3 to 10 msecs. The dwell time
per access point is driven by polling an NDIS hardware timer
provided by the OS. The dwell time limits the maximum number of
probe requests that will be sent by the client 118. A typical probe
request/probe response transaction takes about 0.5 to 1 msec under
no load conditions. The hardware timer is accurate to within
hundredths of microsecs. To ensure that a minimum number of probe
requests are tried per AP, a mechanism is provided to make sure a
minimum number of probe requests are attempted per AP.
[0062] Given an RF redundancy factor of three to six, there are
three to six access points on the list with a very high SNR that
will most likely cover the same area as the current associated
access point and are not likely roaming candidates. Inclusion of
more than one AP to cover a given area is provided for failure
scenarios and to satisfy peak capacity requirements. The goal is
for a roaming candidate to be on the list almost all of the time
under all conditions. Given a channel switching time of 4 msecs and
a Directed Probe Request/Probe Response transaction time of 2 msecs
under no load conditions, the SNR of an access point can be
characterized in 10 msecs or less with 3 probe requests. Given a
maximum off channel time of 100 msecs, 6 access points can be
typically characterized in 100 msecs. At the end of 100 msecs or
completion of scanning, the entire roaming list the client 118 will
enter the association state and attempt to associate with a new
access point.
[0063] To be considered for handoff, a candidate AP must have an
SNR average above the current AP's SNR average by a specified
roaming margin (e.g., 6 dB), as noted.
[0064] Thus, in State 3 (or alternatively in State 4), a new
association list is generated when one or more roaming candidate
APs meet the criteria. When the client 118 has scanned the entire
roaming candidate list `n` consecutive times with no candidate APs
having an SNR meeting the requirement (i.e., no candidate SNRs
equal to or greater than the current AP SNR plus the roaming SNR
margin), the client transitions to a Backoff State 414 (State 6).
The number `n` may be any desired number, and in one specific
embodiment, "n" equals three.
[0065] In State 6, the client 118 continues to monitor the current
AP SNR by running the sampled SNR of beacons through the SNR
filter. If the SNR filter output rises above the threshold plus the
margin value, the client will return to the Current AP Monitor
State 406 (State 1). If the client 118 misses `m` number of
beacons, the association is lost and the client 118 will transition
to the Connection Lost State 408 (State 7). The value used for "m"
may be any appropriate number, and in one embodiment, equals
three.
[0066] When State 6 is entered, a backoff timer is started. The
backoff timer is set with a value of 500 msec plus a random number
of additional milliseconds between 0-500. If the client 118 has not
lost media connection and has not transitioned back to the Current
Monitor State 1 when the backoff timer expires, the client 118
transitions to the Candidate Search State 410 (State 3) and
re-scans the candidate list in accordance therewith.
[0067] As described, in State 3 (or State 4), a new association
list is generated when one or more roaming candidate APs meet the
criteria. Once this list is generated, the process 400 transitions
to the Fast Handoff State 412 (State 4).
[0068] In State 4, the client 118 attempts to roam to a new AP
based on the SNR and load factor obtained in State 3 from the probe
responses and the switch ID obtained in State 1 from the roaming
candidate list. In one embodiment, the selection process for the
new AP operates by first ranking the association candidate APs by
SNR. The SNR ranking is from highest to lowest SNR. All roaming
candidates that have an SNR equal to or better than the current
AP's SNR plus the specified margin (e.g., current AP SNR plus 6 dB)
are identified in the new association list. The new list of
potential handoff APs is then processed and categorized as
follows:
Group 1--APs with a load factor<AP load threshold and a switch
ID equal to that of the current AP;
Group 2--APs with a load factor<AP load threshold, a switch ID
and resiliency ID equal to that of the current AP;
Group 3--APs with a load factor<AP load threshold and a switch
and resiliency ID does NOT equal to that of the current AP;
Group 4--APs with a load factor>AP load threshold and a switch
ID equal to that of the current AP;
Group 5--APs with a load factor<AP load threshold, a switch ID
and resiliency ID equal to that of the current AP; and
Group 6--APs with a load factor>AP load threshold and a switch
and resiliency ID NOT equal to that of the current AP.
[0069] Within each group, the APs are ranked by SNR. The client 118
attempts to associate with an AP starting with group 1 and moving
to group 6. Other suitable selection processes may be utilized.
[0070] The AP load threshold is a utilization factor that indicates
what percentage of the AP's capacity in terms of associated clients
is being utilized. For example, given an AP's maximum capacity
equal to 12 clients, the AP will advertise 50% utilization when
there are 6 clients associated. Other types of load information may
be used to determine the AP load threshold.
[0071] If the handoff to the new AP fails, the client 118 moves to
next best AP in the list, per the desired selection process or
criteria, and attempts to handoff. If the candidates in the
association list are exhausted with no successful handoff, the
client 118 maintains association to the current AP (or reassociates
to the current AP) and returns to State 3 to re-scan the candidate
APs in the roaming candidate list. This may be done one or more
times. Optionally, the client may transition back to State 1.
[0072] In an alternative embodiment, the handoff selection process
eliminates all APs from the association list with load utilization
greater than or equal to 110% allowing the system to accept and
report overloading situations. Next, of the top two SNR-ranked APS,
an AP is chosen whose Switch ID is equal to the current AP Switch
ID. If neither have the same Switch ID as the current Switch ID,
the highest SNR ranked AP is chosen. As will be appreciated, other
selection or ranking processes may be utilized to determine to
which candidate APs the client device 118 will attempt handoff.
[0073] As will be appreciated, the states illustrated and
identified herein are major states. These states may have one or
more sub-states associated therewith if actions are performed
transitions to other actions or states occur. Thus, some states may
not be explicitly described as a "state" but function as such.
[0074] The process 400 may optionally include one or more
additional states, such as Supplicant Exchange States 416, 418
(States 8, 9), to integrate Wireless Accelerated Roaming Protocol
(WARP) functionality into the process 400. This functionality and
its benefits and implementation are known to those skilled in the
art, and no further description thereof is provided herein.
[0075] The process 400 may optionally include a Mobility State
(State 2) (not shown). If utilized, State 2 would be entered from
State 1 when the change in SNR with respect to time exceeds a
threshold indicating that the client device is moving or the
environment around the client device is varying significantly. In
State 2, the client scans and monitors for changes in SNR with
respect to time for its roaming candidates in preparation for a
handoff.
[0076] As will be appreciated, additional processes or
functionality that may be necessary or corresponding to the process
400 are included within the WLAN 100 infrastructure (e.g., the
access points 108, security switches 106, etc.) as needed. Further
description of these processes/functionality is not necessary to
allow a person of ordinary skill in the art to implement the
components and WLAN 100 of the present invention. However, some
additional description of certain aspects of these
processes/functionality may be provided herein.
[0077] Various packets or frames (e.g., beacons, probe requests,
probe responses, association and re-association requests,
association and re-association responses, disassociation requests
or response, and perhaps others) that are transmitted/received by
the various devices in the WLAN 100 will include additional
information. The relevant information may be added to
packets/frames within a vendor-specific informational element (as
set forth in the 802.11 standard).
[0078] These frames/packets may include any or all of the following
information: pico cell network identification, WLAN capabilities,
AP details, roaming candidate AP list, mobility domain, addressing,
switch ID, list of APs, client device (station) details, etc.
[0079] In one embodiment, the association and re-association
responses include the roaming candidate list with a switch ID per
candidate. This list and associated information may be generated by
the switch 106 or AP 108 to indicate to the client 118 which other
APs will accept an association or reassociation request from the
client 118. This list may also include the list of APs information
(described further below) and AP load information (described
further below) for each listed candidate roaming AP. Further
included may be time since last reception, signal strength and
quality, and other operating parameters information for each listed
roaming candidate AP. In another embodiment, the AP load
information may be obtained from probe responses (see below).
Switch ID information may be used to minimize switch handoffs
(i.e., desirable to handoff to an AP that is configured to the same
switch 106 as the current associated AP).
[0080] In one embodiment, probe responses include load information
about the probed AP. Load information may include percent of
maximum number of allowed associate client, bandwidth utilized, or
processing load, etc. Other load information may be provided.
Beacons include information or an identifier that indicates the AP
is part of a pico cell network, which may be used by the client 118
to invoke the process 400 (or its applicable portion) described
herein.
[0081] The WLAN capabilities information are typically generated by
the switches 106 (and/or APs 108), and used to indicate features
and functions of the WLAN that a client device 118 may utilize.
This may include identifying capabilities/modes such as (1) pico
cell network (fast roaming capability or mode), (2) proxy ARP
(switch/AP capable of providing proxy ARP service for associated
client devices, (3) dynamic QOS (switch/AP capable of providing
dynamic QOS), (4) cooperative handoff (switch/AP capable of
providing cooperative handoff services), and (5) single
authentication (switch/AP provides single authentication services
as the client device roams in the same mobility group). In one
embodiment, this information is provided in beacons and probe
responses.
[0082] The AP details information is used to announce information
about an AP and its associated switch. This information may include
AP load information and addressing and mobility group information.
A client device 118 may use this information during roaming/handoff
and during other processes. Generally, all switches 106 and/or APs
in the network 100 will provide their mobility group IDs (MGID) and
controller address (i.e., IP address) in the AP details information
which is included in beacons, probe responses and association
responses. Further, association, reassociation and disassociation
requests may include AP details information which the client device
received in a previous successful association (from an association
response).
[0083] The client device (station) details information is used to
announce information about a client device. This information may
include a handoff counter (number of handoffs) and addressing and
mobility group information. This information may be used during
roaming/handoff.
[0084] The list of APs information is used by the client 118 to
identify APs to which the client can communicate and the quality of
the communication, and may include channel number and AP address,
physical types (e.g., 802.11a, 802.11b, 802.11g), signal strength
(dBm), signal quality (SNR, dB).
[0085] The following will now describe, in general terms, the
process where a client device 118 enters the network 100,
associates with an AP 108, preemptively roams, and hands-off to
another AP 108.
[0086] The client device 118 receives one or more beacons from one
or more of the APs 108. The beacon(s) includes information
identifying the network 100 as a pico cell network or network
capable of compatibility with a fast roaming and preemptive handoff
algorithm (400). In response, the client device 118 switches from a
standard roaming and handoff process to the fast roaming process
(or simply initiates the fast roaming process).
[0087] The client device scans all active channels seeking an
available access point 108 by broadcasting probe requests and
receiving probe responses from the APs 108. From the responses, the
client device 118 measures SNRs for the channels/APs and selects an
appropriate AP (and channel) for association (i.e., connection). An
association process occurs, which includes transmission of an
association request from the client device 118 to the selected AP
108 and receipt of an association response. At or during
association, the AP 108 transmits the roaming candidate list to the
client device 118. This list (and information therein) is generated
by the switches 106 from information gathered by the APs 108 in the
network (and perhaps other devices). It may optionally include AP
loading and switch ID information.
[0088] Once associated, the client device 118 continuously monitors
and measures received RF signals (beacons and/or overheard packets
or packets intended for the client device within the channel)
transmitted from the associated AP 108. If the current AP SNR falls
below a threshold having a value that is equal to the SNR
requirement to support the minimum supported data rate plus a
configurable margin, then the client device 118 will initiate a
preemptive roaming process and seek a different AP for
association.
[0089] While maintaining the association with the current AP 108,
the client device 118 transmits directed probes to all APs 108 in
the candidate roaming list and receives probe responses that
include AP load information. From the responses, the client device
118 also measures the SNR for each candidate AP. From the measured
SNR (and/or switch ID and AP load information), a new association
list is generated that identifies potential APs for handoff. The
candidate roaming APS are placed on the list when their SNR values
equal or exceed a threshold value (and/or meet other desired
criteria). This threshold value is the SNR value of the current
associated AP plus some configurable margin. The APs in the new
association list are ranked according to a predetermined process.
Other factors (AP load, switch ID) that may assist in ranking or
dropping a candidate from the list. If no APs meet the required
criteria to be on the new association list, these steps may be
repeated. If repeats occur with no success, a timed backoff state
is entered while the current AP SNR is still monitored. When the
backoff time elapses, the client device 118 returns retries these
steps again.
[0090] The client device 118 then attempts to hand-off or associate
to one of the APs 108 in the new association list in accordance
with the rankings. If unsuccessful, a new attempt is made to
another AP 108.
[0091] If a number of beacons are missed while in a current
associated state, the mobile device 118 transmits a reassociation
request in an attempt to re-associate to the current AP 108. If
unsuccessful, the mobile device 118 actively scans all APs/channel,
as described above, to associate to an AP 108/
[0092] WARP actions may optionally be included in this process at
desired points. Further, SNRs may be measured during an association
to determine the rate of change, this may be used to initiate the
preemptive roaming steps.
[0093] In some embodiments, certain functions and methods performed
by the mobile devices 118, access points 108, and/or switches 106
(and other devices are implemented or supported by a computer
program that is formed from computer readable program code and that
is embodied in a computer readable medium. The phrase "computer
readable program code" includes any type of computer code,
including source code, object code, and executable code. The phrase
"computer readable medium" includes any type of medium capable of
being accessed by a computer, such as read only memory (ROM),
random access memory (RAM), a hard disk drive, a compact disc (CD),
a digital video disc (DVD), or any other type of memory.
[0094] It may be advantageous to set forth definitions of certain
words and phrases used throughout this patent document. The terms
"include" and "comprise," as well as derivatives thereof, mean
inclusion without limitation. The term "or" is inclusive, meaning
and/or. The phrases "associated with" and "associated therewith,"
as well as derivatives thereof, may mean to include, be included
within, interconnect with, contain, be contained within, connect to
or with, couple to or with, be communicable with, cooperate with,
interleave, juxtapose, be proximate to, be bound to or with, have,
have a property of, or the like. In this document, the term
"couple," "connect" and their derivatives refer to any direct or
indirect communication between two or more elements, whether or not
those elements are in physical contact with one another.
[0095] While this disclosure has described certain embodiments and
generally associated methods, alterations and permutations of these
embodiments and methods will be apparent to those skilled in the
art. Accordingly, the above description of example embodiments does
not define or constrain this disclosure. Other changes,
substitutions, and alterations are also possible without departing
from the spirit and scope of this disclosure, as defined by the
following claims.
* * * * *