U.S. patent application number 10/214271 was filed with the patent office on 2003-11-06 for collaboration between wireless lan access points.
This patent application is currently assigned to Extricom Ltd.. Invention is credited to Shpak, Eran.
Application Number | 20030206532 10/214271 |
Document ID | / |
Family ID | 31714241 |
Filed Date | 2003-11-06 |
United States Patent
Application |
20030206532 |
Kind Code |
A1 |
Shpak, Eran |
November 6, 2003 |
Collaboration between wireless lan access points
Abstract
A method for mobile communication includes arranging a plurality
of access points in a wireless local area network (WLAN) to
communicate on a common frequency channel with a mobile station.
Upon receiving at one or more of the access points an uplink signal
transmitted over the WLAN by the mobile station on the common
frequency channel, the access points receiving the uplink signal
arbitrate among themselves so as to select one of the access points
to respond to the uplink signal. A response is then transmitted
from the selected one of the access points to the mobile
station.
Inventors: |
Shpak, Eran; (Tel Aviv,
IL) |
Correspondence
Address: |
WELSH & KATZ, LTD
120 S RIVERSIDE PLAZA
22ND FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
Extricom Ltd.
Herzeliya
IL
|
Family ID: |
31714241 |
Appl. No.: |
10/214271 |
Filed: |
August 7, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60377645 |
May 6, 2002 |
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60377646 |
May 6, 2002 |
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60377647 |
May 6, 2002 |
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60377650 |
May 6, 2002 |
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Current U.S.
Class: |
370/322 ;
370/315; 370/330 |
Current CPC
Class: |
H04W 92/20 20130101;
H04W 84/12 20130101; H04W 88/08 20130101; H04J 13/0048 20130101;
H04W 52/08 20130101; H04W 52/40 20130101; H04W 48/20 20130101 |
Class at
Publication: |
370/322 ;
370/315; 370/330 |
International
Class: |
H04J 001/10; H04Q
007/00; H04B 007/212 |
Claims
1. A method for mobile communication, comprising: arranging a
plurality of access points in a wireless local area network (WLAN)
to communicate on a common frequency channel with a mobile station;
receiving at one or more of the access points an uplink signal
transmitted over the WLAN by the mobile station on the common
frequency channel; arbitrating among the access points receiving
the uplink signal so as to select one of the access points to
respond to the uplink signal; and transmitting a response from the
selected one of the access points to the mobile station.
2. A method according to claim 1, wherein the access points have
respective service areas, and wherein arranging the plurality of
the access points comprises arranging the access points so that the
service areas substantially overlap.
3. A method according to claim 1, wherein arranging the plurality
of the access points comprises arranging the access points to
communicate with the mobile station substantially in accordance
with IEEE Standard 802.11.
4. A method according to claim 3, wherein arbitrating among the
access points comprises selecting the one of the access points to
respond to the uplink signal within a time limit imposed by the
IEEE Standard 802.11 for acknowledging the uplink signal.
5. A method according to claim 1, wherein arbitrating among the
access points comprises sending messages over a shared
communication medium linking the access points.
6. A method according to claim 5, wherein sending the messages
comprises sending broadcast messages from the access points
receiving the uplink signal to the plurality of the access
points.
7. A method according to claim 6, wherein sending the broadcast
messages comprises sending the messages over the shared
communication medium from two or more of the access points
simultaneously using a multiple access protocol.
8. A method according to claim 7, wherein the multiple access
protocol comprises a code division multiple access (CDMA)
protocol.
9. A method according to claim 5, wherein arbitrating among the
access points comprises receiving and processing the messages at
each of the plurality of the access points, so that each of the
access points determines which one of the access points is to be
selected to respond to the uplink signal.
10. A method according to claim 9, wherein processing the messages
comprises selecting, responsive to the messages, the one of the
access points that was first to receive the uplink signal.
11. A method according to claim 5, wherein sending the messages
comprises indicating in the messages a strength of the uplink
signal received by each of the access points.
12. A method according to claim 5, wherein arranging the plurality
of the access points comprises linking the access points in a local
area network (LAN) for conveying data to and from the mobile
station, the LAN comprising a cable having multiple conductors, and
wherein sending the messages comprises passing the messages over
one or more of the conductors on the cable that are not used for
conveying the data to and from the mobile station.
13. A system for mobile communication, comprising: a communication
medium; and a plurality of access points interconnected by the
medium and arranged in a wireless local area network (WLAN) to
communicate on a common frequency channel with a mobile station,
the access points being adapted, upon receiving at one or more of
the access points an uplink signal transmitted over the WLAN by the
mobile station on the common frequency channel, to arbitrate among
the access points receiving the uplink signal by sending messages
over the medium so as to select one of the access points to respond
to the uplink signal, and to transmit a response from the selected
one of the access points to the mobile station.
14. A system according to claim 13, wherein the access points have
respective service areas and are arranged so that at least some of
the service areas substantially overlap.
15. A system according to claim 13, wherein the access points are
adapted to communicate with the mobile station substantially in
accordance with IEEE Standard 802.11.
16. A system according to claim 15, wherein the access points are
adapted to select the one of the access points to respond to the
uplink signal within a time limit imposed by the IEEE Standard
802.11 for acknowledging the uplink signal.
17. A system according to claim 1, wherein the medium is configured
as a shared communication medium.
18. A system according to claim 17, wherein the access points
receiving the uplink signal are adapted to broadcast the messages
over the medium to the plurality of the access points.
19. A system according to claim 18, wherein the access points are
adapted so that two or more of the access points may broadcast the
messages substantially simultaneously using a multiple access
protocol.
20. A system according to claim 19, wherein the multiple access
protocol comprises a code division multiple access (CDMA)
protocol.
21. A system according to claim 17, wherein each of the plurality
of the access points is adapted to receive and process the messages
so as to determine which one of the access points is to be selected
to respond to the uplink signal.
22. A system according to claim 21, wherein the access points are
adapted to select, responsive to the messages, the one of the
access points that was first to receive the uplink signal to
respond to the uplink signal.
23. A system according to claim 17, wherein the access points are
adapted to indicate in the messages a strength of the uplink signal
received by each of the access points.
24. A system according to claim 13, and comprising a local area
network (LAN), which comprises a cable having multiple conductors
linking the access points, wherein the access points are adapted to
convey data to and from the mobile station over the LAN, and
wherein the communication medium comprises one or more of the
conductors on the cable that are not used for conveying the data to
and from the mobile station.
25. Access point apparatus for deployment in a wireless local area
network (WLAN) as one of a plurality of access points for mobile
communication, the apparatus comprising: a radio transceiver, which
is configured to communicate on a predetermined frequency channel
with a mobile station; and a message processor, which is adapted,
when the transceiver receives an uplink signal transmitted over the
WLAN by the mobile station on the predetermined frequency channel,
to carry out an arbitration protocol together with others of the
access points receiving the uplink signal so as to select one of
the access points to respond to the uplink signal, and to control
the transceiver so that the transceiver returns a response to the
mobile station subject to the arbitration protocol.
26. Apparatus according to claim 25, wherein the transceiver is
adapted to communicate with the mobile station substantially in
accordance with IEEE Standard 802.11.
27. Apparatus according to claim 26, wherein the message processor
is adapted to carry out the arbitration protocol so as to select
the one of the access points to respond to the uplink signal within
a time limit imposed by the IEEE Standard 802.11 for acknowledging
the uplink signal.
28. Apparatus according to claim 25, wherein the message processor
is adapted to carry out the arbitration protocol by sending and
receiving messages to and from the others of the access points over
a shared communication medium linking the access points.
29. Apparatus according to claim 28, wherein the messages comprise
broadcast messages, and wherein the message processor is adapted to
send one of the broadcast messages when the transceiver receives
the uplink signal.
30. Apparatus according to claim 29, wherein the message processor
is adapted to send the messages over the shared communication
medium using a multiple access protocol.
31. Apparatus according to claim 30, wherein the multiple access
protocol comprises a code division multiple access (CDMA)
protocol.
32. Apparatus according to claim 28, wherein the message processor
is adapted to process the messages that it receives from the others
of the access points so as to determine the one of the access
points that was first to receive the uplink signal, and to select
the one of the access points that was first to receive the uplink
signal to respond to the uplink signal.
33. Apparatus according to claim 28, wherein the message processor
is adapted to include in the messages that it send a strength of
the uplink signal received by the transceiver.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Applications 60/377,645, 60/377,646, 60/377,647 and
60/377,650, all of which were filed May 6, 2002, and are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to wireless
communications, and specifically to methods and devices for
improving the performance of wireless local area networks.
BACKGROUND OF THE INVENTION
[0003] Wireless local area networks (WLANs) are gaining in
popularity, and new wireless applications are being developed. The
original WLAN standards, such as "Bluetooth" and IEEE 802.11, were
designed to enable communications at 1-2 Mbps in a band around 2.4
GHz. More recently, IEEE working groups have defined the 802.11a,
802.11b and 802.11g extensions to the original standard, in order
to enable higher data rates. The 802.11a standard, for example,
envisions data rates up to 54 Mbps over short distances in a 5 GHz
band, while 802.11b defines data rates up to 22 Mbps in the 2.4 GHz
band. In the context of the present patent application and in the
claims, the term "802.11" is used to refer collectively to the
original IEEE 802.11 standard and all its variants and extensions,
unless specifically noted otherwise.
[0004] The theoretical capability of new WLAN technologies to offer
enormous communication bandwidth to mobile users is severely
hampered by the practical limitations of wireless communications.
Indoor propagation of radio frequencies is not isotropic, because
radio waves are influenced by building layout and furnishings.
Therefore, even when wireless access points are carefully
positioned throughout a building, some "black holes" generally
remain--areas with little or no radio reception. Furthermore,
802.11 wireless links can operate at full speed only under
conditions of high signal/noise ratio. Signal strength scales
inversely with the distance of the mobile station from its access
point, and therefore so does communication speed. A single mobile
station with poor reception due to distance or radio propagation
problems can slow down WLAN access for all other users in its basic
service set (BSS--the group of mobile stations communicating with
the same access point).
[0005] The natural response to these practical difficulties would
be to distribute a greater number of access points within the area
to be served. If a receiver receives signals simultaneously from
two sources of similar strength on the same frequency channel,
however, it is generally unable to decipher either signal. The
802.11 standard provides a mechanism for collision avoidance known
as clear channel assessment (CCA), which requires a station to
refrain from transmitting when it senses other transmissions on its
frequency channel. In practice, this mechanism is of limited
utility and can place a heavy burden on different BSSs operating on
the same frequency channel.
[0006] Therefore, in 802.11 WLANs known in the art, access points
in mutual proximity must use different frequency channels.
Theoretically, the 802.11b and 802.11g standards define 14
frequency channels in the 2.4 GHz band, but because of bandwidth
and regulatory limitations, WLANs operating according to these
standards in the United States actually have only three different
frequency channels from which to choose. (In other countries, such
as Spain, France and Japan, only one channel is available.) As a
result, in complex, indoor environments, it becomes practically
impossible to distribute wireless access points closely enough to
give strong signals throughout the environment without substantial
overlap in the coverage areas of different access points operating
on the same frequency channel.
SUMMARY OF THE INVENTION
[0007] It is an object of some aspects of the present invention to
provide methods and devices for enhancing the coverage and speed of
WLAN systems.
[0008] In preferred embodiments of the present invention, a WLAN
system comprises multiple wireless access points distributed within
a service region. In order to provide complete coverage of the
service region, with strong communication signals throughout the
region, the access points are preferably closely spaced, and their
areas of coverage may substantially overlap one another. In order
to deal with this overlap, the access points communicate among
themselves using a novel protocol over a high-speed, low-latency
communication medium. Preferably, the medium comprises a dedicated
wire or fiberoptic network, but wireless media may also be used for
this purpose.
[0009] When a mobile station sends an uplink message attempting to
initiate communications in a given frequency channel, a number of
access points operating in this frequency channel may typically
receive the message. These access points arbitrate among themselves
by communicating over the dedicated medium, in order to decide
which of the access points will communicate with this mobile
station. This arbitration process is preferably repeated each time
a mobile station in the WLAN service region sends a new uplink
message. Problems of overlapping coverage areas and collisions are
thus resolved, typically in favor of the access point that is
closest to the mobile station in question. The access points may
therefore be deployed within the service region as closely as
desired, so that mobile stations everywhere in the service region
experience good radio coverage, without "black holes," and can
operate at optimal speed.
[0010] There is therefore provided, in accordance with a preferred
embodiment of the present invention, a method for mobile
communication, including:
[0011] arranging a plurality of access points in a wireless local
area network (WLAN) to communicate on a common frequency channel
with a mobile station;
[0012] receiving at one or more of the access points an uplink
signal transmitted over the WLAN by the mobile station on the
common frequency channel;
[0013] arbitrating among the access points receiving the uplink
signal so as to select one of the access points to respond to the
uplink signal; and
[0014] transmitting a response from the selected one of the access
points to the mobile station.
[0015] Typically, the access points have respective service areas,
and arranging the plurality of the access points includes arranging
the access points so that the service areas substantially
overlap.
[0016] In a preferred embodiment, arranging the plurality of the
access points includes arranging the access points to communicate
with the mobile station substantially in accordance with IEEE
Standard 802.11. Preferably, arbitrating among the access points
includes selecting the one of the access points to respond to the
uplink signal within a time limit imposed by the IEEE Standard
802.11 for acknowledging the uplink signal.
[0017] Preferably, arbitrating among the access points includes
sending messages over a shared communication medium linking the
access points, wherein sending the messages includes sending
broadcast messages from the access points receiving the uplink
signal to the plurality of the access points. Further preferably,
sending the broadcast messages includes sending the messages over
the shared communication medium from two or more of the access
points simultaneously using a multiple access protocol. Most
preferably, the multiple access protocol includes a code division
multiple access (CDMA) protocol.
[0018] Alternatively or additionally, arbitrating among the access
points includes receiving and processing the messages at each of
the plurality of the access points, so that each of the access
points determines which one of the access points is to be selected
to respond to the uplink signal. Typically, processing the messages
includes selecting, responsive to the messages, the one of the
access points that was first to receive the uplink signal.
Alternatively or additionally, sending the messages includes
indicating in the messages a strength of the uplink signal received
by each of the access points.
[0019] In a preferred embodiment, arranging the plurality of the
access points includes linking the access points in a local area
network (LAN) for conveying data to and from the mobile station,
the LAN including a cable having multiple conductors, and sending
the messages includes passing the messages over one or more of the
conductors on the cable that are not used for conveying the data to
and from the mobile station.
[0020] There is also provided, in accordance with a preferred
embodiment of the present invention, a system for mobile
communication, including:
[0021] a communication medium, and
[0022] a plurality of access points interconnected by the medium
and arranged in a wireless local area network (WLAN) to communicate
on a common frequency channel with a mobile station, the access
points being adapted, upon receiving at one or more of the access
points an uplink signal transmitted over the WLAN by the mobile
station on the common frequency channel, to arbitrate among the
access points receiving the uplink signal by sending messages over
the medium so as to select one of the access points to respond to
the uplink signal, and to transmit a response from the selected one
of the access points to the mobile station.
[0023] There is additionally provided, in accordance with a
preferred embodiment of the present invention, access point
apparatus for deployment in a wireless local area network (WLAN) as
one of a plurality of access points for mobile communication, the
apparatus including:
[0024] a radio transceiver, which is configured to communicate on a
predetermined frequency channel with a mobile station; and
[0025] a message processor, which is adapted, when the transceiver
receives an uplink signal transmitted over the WLAN by the mobile
station on the predetermined frequency channel, to carry out an
arbitration protocol together with others of the access points
receiving the uplink signal so as to select one of the access
points to respond to the uplink signal, and to control the
transceiver so that the transceiver returns a response to the
mobile station subject to the arbitration protocol.
[0026] The present invention will be more fully understood from the
following detailed description of the preferred embodiments
thereof, taken together with the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a block diagram that schematically illustrates a
WLAN system, in accordance with a preferred embodiment of the
present invention;
[0028] FIG. 2 is a schematic illustration of a mobile station
communicating with multiple wireless access points, in accordance
with a preferred embodiment of the present invention;
[0029] FIG. 3 is a flow chart that schematically illustrates a
method for establishing a communication link between a mobile
station and a wireless access point, in accordance with a preferred
embodiment of the present invention;
[0030] FIG. 4 is a block diagram that schematically illustrates
communication links among multiple access points in a WLAN system,
in accordance with a preferred embodiment of the present
invention;
[0031] FIG. 5 is a block diagram that schematically illustrates
communication and power links between an access point and a hub in
a WLAN system, in accordance with a preferred embodiment of the
present invention; and
[0032] FIG. 6 is a block diagram that schematically illustrates a
message packet exchanged between access points in a WLAN system, in
accordance with a preferred embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0033] FIG. 1 is a block diagram that schematically illustrates a
wireless LAN (WLAN) system 20, in accordance with a preferred
embodiment of the present invention. System 20 comprises multiple
access points 22, which are configured for data communication with
mobile stations 24. The mobile stations typically comprise
computing devices, such as desktop, portable or handheld devices,
as shown in the figure. In the exemplary embodiments described
hereinbelow, it is assumed that the access points and mobile
stations communicate with one another in accordance with one of the
standards in the IEEE 802.11 family and observe the 802.11 medium
access control (MAC) layer conventions. Details of the 802.11 MAC
layer are described in ANSI/IEEE Standard 801.11 (1999 Edition),
and specifically in Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications, which is
incorporated herein by reference. The principles of the present
invention, however, are not limited to the 802.11 standards, and
may likewise be applied to substantially any type of WLAN,
including HiperLAN, Bluetooth and hiswan-based systems.
[0034] Access points 22 are typically connected to an Ethernet hub
26 by a wired LAN 28. The LAN serves as a distribution system (DS)
for exchanging data between the access points and the hub. This
arrangement enables mobile stations 24 to send and receive data
through access points 22 to and from an external network 30, such
as the Internet, via an access line 32 connected to hub 26. LAN 28
is typically capable of carrying data at high speeds--greater than
the aggregate speed of wireless communications between the access
points and mobile stations. Message latency on the LAN is high,
however, generally on the order of milliseconds, due mainly to
collision avoidance mechanisms that are inherent in the operation
of Ethernet and other conventional LANs.
[0035] In addition to the conventional DS provide by LAN 28, access
points 22 are also connected by a novel shared communication medium
34 to a MAC collaboration hub 36. Medium 34 may comprise
substantially any suitable high-speed communication means,
including wire, fiberoptics, or even free-space optical or radio
communications (in an allowed frequency band that does not
interfere with WLAN operation). For the sake of economy, medium 34
preferably comprises wires that run parallel to LAN 28. For
example, medium 34 may comprise a twisted pair of wires that
already exists in cabling of LAN 28, but which is not required for
carrying LAN data. The function of MAC collaboration hub 36 is
simply to connect medium 34 in such a way as to allow all access
points 22 to broadcast and receive messages to and from all other
access points. Therefore, unlike Ethernet hub 26, MAC collaboration
hub 36 typically need not include a switch. Exemplary
implementations of medium 34 and hub 36 are described hereinbelow
with reference to FIGS. 4 and 5. Although the hub-and-spokes
topology shown in FIGS. 1, 4 and 5 is generally the most convenient
way to configure medium 34, alternative configurations will be
apparent to those skilled in the art and are considered to be
within the scope of the present invention.
[0036] FIG. 2 is a schematic illustration of simultaneous radio
communications between mobile station 24 and multiple access points
22 in system 20, in accordance with a preferred embodiment of the
present invention. It is assumed that the access points labeled
AP1, AP2 and AP3 are all operating on the same band, over which
mobile station 24 seeks to communicate. (Access points AP4 and AP5
are assumed to be operating in a different band, and thus do not
participate directly in this communication process.) Radio waves
40, 42 and 44 reach mobile unit 24 from AP1, AP2 and AP3,
respectively, with similar amplitudes. By the same token, radio
messages transmitted by mobile unit 24 are received at about the
same time by AP1, AP2 and AP3. In WLAN systems known in the art,
under these circumstances, mobile station 24 would receive downlink
messages from two or more access points 22, which would probably
result in inability of the mobile station to communicate with any
of the access points. In preferred embodiments of the present
invention, access points AP1, AP2 and AP3 communicate with one
another over medium 34 in order to resolve this conflict, as
described hereinbelow.
[0037] FIG. 3 is a flow chart that schematically illustrates a
method for establishing communications between mobile station 24
and one of access points 22 in system 20, in accordance with a
preferred embodiment of the present invention. Access points 22
(say AP1, AP2 and AP3) transmit beacon signals on their common
frequency channel, at a beacon transmission step 50. In accordance
with the 802.11 standard, the beacon signals transmitted by any
given access point provide the time base with which the mobile
station should synchronize its communications and indicate the BSS
identification (BSSID) of the access point. The BSSID can be
regarded as the MAC address of the access point. In 802.11 WLAN
systems known in the art, each access point has its own unique
BSSID. In system 20, however, access points AP1, AP2 and AP3 share
the same BSSID, so that they appear logically to the mobile station
to be a single, extended, distributed access point, which has
multiple antennas at different locations. The time bases of AP1,
AP2 and AP3 are mutually synchronized using medium 34, and the
beacon signals transmitted by the access points are interlaced to
avoid collision between them.
[0038] When mobile station 24 receives a beacon signal of
sufficient strength, it extracts the BSSID and time base from the
signal, at a beacon processing step 52. This step, as well as
subsequent steps taken by the mobile station, is completely in
accordance with the 802.11 standard. In other words, the present
invention can be implemented in a manner that is transparent to and
requires no modification of legacy mobile stations. Using the time
base and BSSTD it has acquired, mobile station 24 sends an uplink
signal, in the form of an association request message that is
addressed to the BSSID and indicates the MAC address of the mobile
station, at an association request step 52.
[0039] Ordinarily, in a conventional WLAN, the access point to
which the association request is addressed will answer immediately
with an acknowledgment (ACK). If the mobile station does not
receive the ACK within a given timeout period, typically 10 .mu.s,
it submits an automatic repeat request (ARQ) . Ultimately, the
mobile station will treat the association request as having failed
if it does not receive the required ACK. Therefore, to maintain
802.11 compatibility in system 20, one--and only one--of access
points AP1, AP2 and AP3 must return an ACK to mobile station 24
within the 10 .mu.s limit.
[0040] To determine which of the access points will respond to the
association request message, access points AP1, AP2 and AP3 carry
out an arbitration procedure using medium 34. For this purpose, all
access points that received the association request message from
mobile station 24 broadcast messages over medium 34, at a broadcast
step 56, giving notice to the other access points that they have
received an uplink message. Each broadcast message indicates the
identity of the access point sending the message (i.e., a unique,
internal identity, not the BSSID) and the MAC address of the mobile
station in question. Preferably, to reduce the length of the
broadcast message, the MAC address of the mobile station is
hashed.
[0041] The access points send their messages over medium 34 in
accordance with a predetermined protocol that makes it possible to
distinguish messages sent simultaneously (or almost simultaneously)
by different access points. For example, a time division multiple
access (TDMA) protocol may be used, in which each access point has
its own, assigned time slot. Alternatively, a code division
multiple access (CDMA) protocol is used, as described below with
reference to FIG. 6. Further alternatively, a frequency division
multiplexing scheme may be used (or if medium 34 is implemented as
a fiberoptic network, wavelength division multiplexing, as is known
in the art).
[0042] The access points receive and process the broadcast messages
sent over medium 34, at a processing step 58. Each access point is
able to determine whether it was first to send its message, or
whether another access point preceding it, by comparing the time of
receipt of these broadcast messages to the time at which the access
point sent its own broadcast message. (Access points operating on
other frequency channels, as well as access points on the same
frequency channel that did not receive an uplink signal from the
mobile station identified in the broadcast message, may ignore the
message.) Typically, the access point that was able to send its
broadcast message first in response to an uplink message from a
given mobile station is in the best position to continue
communications with the mobile station, since this access point is
generally the closest one to the mobile station. Therefore, all the
access points independently choose this first access point to
respond to mobile station 24. Alternatively, other criteria, such
as received signal power, may be applied in choosing the "winning"
access point, as long as the criteria are applied uniformly by all
the access points. Preferably, if a deadlock occurs (such as when
two access points send their broadcast messages at the same
instant), a predetermined formula is applied by all the access
points to resolve the deadlock uniformly.
[0043] The winning access point sends the required ACK message to
mobile station 24, at an acknowledgment step 60. As noted above,
the ACK must be sent within a short time, typically 10 .mu.s, and
steps 56, 58 and 60 must all be completed within this time. Access
points 22 are able to meet this time constraint by using medium 34
as a dedicated, shared medium for this purpose, and by implementing
a fast arbitration protocol, based on short broadcast messages, as
described above. After sending the ACK, the winning access point
typically sends an association response message to mobile station
24, and then continues its downlink transmission to the mobile
station as appropriate, at a downlink step 62.
[0044] The winning access point continues serving the mobile
station until the mobile station sends another uplink message, at a
new uplink step 64. The arbitration protocol described above is
then repeated, starting from step 56. A different access point may
be chosen to serve the mobile station in the next round,
particularly if the mobile station has moved in the interim. Even
if the mobile station has moved, there is no need to repeat the
association protocol. As noted above, all the access points belong
to the same BSS, as though they were a single extended access
point. Therefore, the same association of the mobile station is
therefore maintained even if the arbitration process among the
access points chooses a different "winner" to respond to the next
uplink packet from the mobile station.
[0045] FIG. 4 is a block diagram that schematically shows details
of communications by access points 22 over medium 34, in accordance
with a preferred embodiment of the present invention. Medium 34 in
this embodiment comprise pairs of wires 68 connecting each of
access points 22 to hub 36. Hub 36 comprises a splitter 70, which
joins wires 68 in such a way that medium 34 functions as a shared
medium, i.e., so that signals transmitted onto wires 68 by any of
access points 22 are received by all the other access points on
medium 34. In the simplified embodiment shown in FIG. 4, splitter
70 comprises a passive, inductive coupler, which couples together
all the pairs of wires. Alternatively or additionally, splitter 70
may comprise one or more amplifiers or other active elements, as
are known in the art.
[0046] Each access point 22 comprises a message processor 72 for
communicating with the other access points over medium 34 and
carrying out the MAC-level collaboration protocol described above.
Message processor 72 typically comprises a transmit circuit 74 and
a receive circuit 76, for transmitting and receiving broadcast
messages over medium 34. Preferably, to meet the timing
requirements of an 802.11 WLAN, as noted above, circuits 74 and 76
and medium 34 operate with a bandwidth of at least 30 MHz. Message
processor 72 interacts with and controls a WLAN transceiver 78, in
compliance with the collaboration protocol. Transceivers 78
communicate over the air with mobile stations 24 in accordance with
the applicable WLAN standards.
[0047] FIG. 5 is a block diagram that shows further details of
communications between one of access points 22 and hubs 26 and 36,
in accordance with a preferred embodiment of the present invention.
For the sake of simplicity, only a single access point is shown in
this figure. Typically, multiple access points are connected in
like manner, as shown in FIG. 4.
[0048] In the present embodiment, a multi-conductor cable 86 is
used to connect access points 22 in LAN 28. Typically, cable 86
comprises CAT-5 cabling, as is common in Ethernet LANs. Two twisted
pairs of wires 82 and 84 (the 1-2 and 3-6 pairs in a CAT-5 cable)
are used for transmitting and receiving data packets over LAN 28,
between Ethernet hub 26 and an Ethernet interface 80 in access
point 22. These data packets may comprise data sent between mobile
stations 24 and network 30, via the access points. A remaining
twisted pair 88 (the 4-5 pair) is not generally used for LAN data
communications. Therefore, pair 88 serves as medium 34, carrying
MAC collaboration messages between message processor 72 and hub 36.
This novel use of pair 88 eliminates the need for separate wiring
of medium 34.
[0049] In some LANs (and particularly LANs that are used to connect
wireless access points), pair 88 is also used to convey DC power to
access points 22. A power distribution hub 90, associated with
Ethernet hub 26, is connected by pair 88 to a power supply circuit
92 in access point 22. In accordance with the IEEE 802.3af draft
standard, hub 90 supplies 48 VDC over pair 88. This voltage is
stepped down and regulated by power supply circuit 92 in order to
provide operating power to the communication circuits of access
point 22. The DC level on the wires of pair 88, however, does not
prevent pair 88 from serving as medium 34. Rather, message
processor 72 comprises a high-frequency coupler 94, typically an
inductive coupler, which separates the high-speed communication
traffic on medium 34 from the DC power.
[0050] FIG. 6 is a block diagram that schematically illustrates a
broadcast packet 100 sent over medium 34 by one of access points
22, in accordance with a preferred embodiment of the present
invention. Packet 100 is used by the access points to convey
broadcast notice messages when they receive uplink communications
from one of mobile stations 24, as described above with reference
to FIG. 3 (step 56). The present embodiment assumes that the access
points communicate over medium 34 using a CDMA protocol. CDMA has
the advantage, by comparison with TDMA, that it allows all the
access points to broadcast simultaneously and does not require a
master clock, delay compensation or an intelligent central
unit.
[0051] Packet 100 comprises a preamble 102, which is typically made
up of a synchronization word 104 and an access point identifier
106. As noted above, identifier 106 is a proprietary, internal
identification code, which uniquely identifies the access point
sending the packet. Message processor 72 in each of access points
22 preferably has a set of data masks, which correspond
respectively to preambles 102 of all the other access points that
are configured to transmit and receive on the same WLAN frequency
channel. As the message processor receives data over medium 34, it
compares the data against each of its masks in order to detect the
beginning of a new packet and the identity of the access point that
sent the packet. Walsh codes may be used advantageously for this
purpose, as is known in the CDMA art.
[0052] Preamble 102 is followed by a broadcast message 108, which
identifies the mobile station that sent the uplink message reported
by packet 100. After message processor 72 has succeeded in decoding
preamble with one of its data masks, it uses the same data mask to
decode message 108. The message processor thus identifies both the
mobile station that sent the uplink message and the access point
that received it first, and in this way is able to decide which
access point should respond to the uplink message, as described
above. optionally, message 108 may include other parameters, such
as the power level of the received uplink message and/or an
identification of the antenna on which the access point received
the message. (For diversity purposes, access points generally have
multiple antennas.) These additional parameters may be used, in
addition to or instead of the time of receipt of packet 100, in
arbitrating among the access points.
[0053] As noted above, although preferred embodiments are described
herein with reference to particular types of wireless and wired
LANs and particular communication standards, the principles of the
present invention are similarly applicable to other types of LANs
and WLANs, which may operate in accordance with other standards. In
addition, these principles may be applied in wireless personal area
networks (PANs), as defined by IEEE Standard 802.15, including
ultra-wide band (UWB) PANs. It will thus be appreciated that the
preferred embodiments described above are cited by way of example,
and that the present invention is not limited to what has been
particularly shown and described hereinabove. Rather, the scope of
the present invention includes both combinations and
subcombinations of the various features described hereinabove, as
well as variations and modifications thereof which would occur to
persons skilled in the art upon reading the foregoing description
and which are not disclosed in the prior art.
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