`
`US 20020071448A1
`
`(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0071448 A1
`(43) Pub. Date: Jun. 13, 2002
`
`Cervello ct al.
`
`(54) COLLISION AVOIDANCE IN IEEE 802.11
`CONTENTION FREE PERIOD (CFP) WITH
`()VERLAPPlNG BASIC SERVICE SETS
`(BSSS)
`
`(76)
`
`Inventors: Gerard Cervello, Barcelona (ES);
`Sunghyun Choi, Montrale, NJ (US)
`
`Correspondence Address:
`Philips Electronics North America Corporation
`580 White Plains Road
`Tarrytown, NY 10591 (US)
`
`(21) App]. No.:
`
`09/896,716
`
`(22)
`
`Filed:
`
`Jun. 30, 2001
`
`Related US. Application Data
`
`(63) Non-provisional of provisional
`60/217,146, filed on Jul. 7, 2000.
`
`application No,
`
`Publication Classification
`
`Int. Cl.7
`(51)
`(52) US. Cl.
`
`H0411 [2/42, H04L 12/43
`370/445; 370/449
`
`(57)
`
`ABSTRACT
`
`A medium access control (MAC) protocol is provided for
`avoiding collisions from stations (STAs) comprising two or
`more IEEE 802.11 basic service sets (BSSs) collocated and
`operating in the same channel during contention [re-e periods
`(CFPs). The MAC protocol includes hardware/software for
`utilizing ready-to-send(RTS)/clear-to-send(CTS) exchange
`during CFPs to avoid potential collision from STAs in
`overlapping BSSs and hardware/software for providing
`overlapping network allocation vectors (ONAV) in addition
`to a network allocation vector (NAV), the ONAV included to
`facilitate the effectiveness of the RTS/CTS during CFPs.
`
`
`
`Delay {due to a busy medium]
`
`Foreshorhened CFP
`CFP Repetition Interval
`
`
`Contention Period
` Contention Period
`Contention Free Period
`
`DCF
`DCF
`
`
`
`
` Variable Length
`{per SuperFrame)
`
`Busy
`Medium
`
`B = Beacon Frame
`
`
`
`APPLE 1011
`
`1
`
`APPLE 1011
`
`

`

`Patent Application Publication
`
`Jun. 13, 2002 Sheet 1 0f 6
`
`US 2002/0071448 A1
`
`I
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`

`

`Patent Application Publication
`
`Jun. 13, 2002 Sheet 2 0f 6
`
`US 2002/0071448 A1
`
`Delay (due to a busy medium)
`
`Fareshortened CFP
`CFP Repetition interval
`
`
`CF Period
`Contention Period
`Contention Period
`
`
`
`DEF
`
`DCF
`
`
`Busy
`;
`H
`
`
`PCP
`
`
`
`
`
`Variable Length
`(per SuperFrame)
`
`
`
`
`Fig. 2
`
`B - Beacon Frame
`
`3
`
`

`

`Patent Application Publication
`
`Jun. 13, 2002 Sheet 3 0f 6
`
`US 2002/0071448 A1
`
`
`Contention Free Pariod Repetition Interval
`Contention Free Period
`SlFS
`SIFS PIFS
`
`SIFS
`
`1:. .
`,
`,
`Contention Period
`_
`In mm l-_.-..
`m -U2~
`-UG+
`D =F
`I m“
`'
`I A“
`CF‘End
`s:ntb;al:1ol:t
`
`I
`
`SIFS
`
`'
`--|
`
`I
`
`I
`
`'
`
`NAV
`
`‘
`
`,
`
`.
`
`SlFs
`SIFS
`”0
`su=s
`RESPW“
`
`t" CF‘P‘M
`‘
`‘
`
`Reset NAV
`
`,.
`
`7 i“ i
`
`'1’
`
`
`
`Coordinator
`Ux = Frames
`sent bypailed
`statlons
`
`4
`
`

`

`Patent Application Publication
`
`Jun. 13, 2002 Sheet 4 0f 6
`
`US 2002/0071448 A1
`
`
`
`5
`
`

`

`Patent Application Publication
`
`Jun. 13, 2002 Sheet 5 0f 6
`
`US 2002/0071448 A1
`
`(Data +) CF-Poll
`
`Duration/ID (D1111)
`
`
`
`
`
`
`
`
`STALI
`
`V
`
`‘
`
`f Data + CF-Ack
`
`
`
`Duration/TD (DurZ)
`
`
`
`Fig. 5
`
`6
`
`

`

`Patent Application Publication
`
`Jun. 13, 2002 Sheet 6 0f 6
`
`US 2002/0071448 A1
`
`i Desired to transmit
`
`(data+)CF-poll to STAi
`
`.
`
`
`
`
`
`STA i received frame(s)
`fi'om overlappin -
`
` Yes
`
`
`
`
`
`
`Size of (data+)CF-poll
`over RtsThreshol ,
`
`STA i had more errors
`
`than Th_error?
`
`No
`
`Transmit (data+)CF—poil
`without RTS/CTS
`
`Transmit RTS first
`
`Fig. 6
`
`7
`
`

`

`US 2002/0071448 A1
`
`Jun. 13, 2002
`
`COLLISION AVOIDANCE IN IEEE 802.11
`CONTENTION FREE PERIOD (CFP) WITH
`OVERLAPPING BASIC SERVICE SETS (BSSS)
`
`BACKGROUND OF THE INVENTION
`
`[0001]
`
`1. Field of the Invention
`
`[0002] The present invention relates to a wireless medium
`access control
`(MAC) protocol, and more particularly
`relates to a hybrid wireless MAC protocol which uses a
`Ready To Send(RTS)/Clear To Send(CTS) exchange during
`a contention free period (CFP) in order to avoid contention
`from Stations (STAs)
`in overlapping basic service sets
`(BSSs).
`
`[0003]
`
`2. Description of Related Art
`
`[0004] The wireless communication market has lately
`enjoyed tremendous gIowth and is now capable of reaching
`every place on earth. Hundreds of millions of people
`exchange information every day using pagers, cellular tele-
`phones and other wireless communication products. Wire-
`less communication has broken the harnesses of Wireline
`networks, allowing users to access and share information on
`a global scale nearly everywhere they venture.
`
`[0005] Standard LAN protocols (Wireline), such as ETH-
`ERNE'1"1'M, operate on Wireline networks using various
`MAC protocols, e.g., carrier sense multiple access with
`collision detection (CSMA/CD), at fairly high speeds with
`inexpensive connection hardware which provides an ability
`to bring digital networking to practically any computer.
`Until recently, however, LANs were limited to physical,
`hard-wired (Wireline) infrastructure. Even with phone dial-
`ups, network nodes were limited to access through Wireline
`connections. Wireline communications, however, have set
`the stage for wireless communications.
`
`[0006] Since the recent development of wireless LANs,
`many network users, such as mobile users in business, the
`medical professions, industry, universities, etc., have ben-
`efited from the enhanced communication ability of wireless
`I.ANs, i.e., increased mobility. Uses for wireless network
`access are practically unlimited. In addition to increased
`mobility, wireless LANs offer increased flexibility. Com-
`pared to Wireline counterparts, however, wireless networks
`are known to have much less bandwidth, and hence it is
`highly desirable to utilize the wireless link bandwidth elli-
`ciently.
`
`[0007] To that end, commonly owned pending application
`Ser. No. 09/732,585, filed Dec. 8, 2000, and entitled: A
`Wireless MAC Protocol Based On A Hybrid Combination
`Of Slot Allocation, Token Passing and Polling For Isochro-
`nous Traflic, discloses a mechanism for increasing the
`efficiency of bandwidth use. The ’585 application, incorpo-
`rated herein by reference, utilizes a hybrid MAC protocol
`with a combination of bandwidth allocation, a variation on
`conventional token passing and polling to regulate isochro-
`nous traffic efliciently within a wireless network with “hid-
`den” terminals.
`
`[0008] The IEEE standard for wireless LAN protocol is
`identified as “Standard for Information '1‘eehnology-'l‘ele-
`communications and information exchange between sys-
`tems-Local
`and metropolitan
`area
`networks-Specific
`Requirements—Part 11: Wireless LAN Medium Access
`
`Control (MAC) and Physical Layer (PHY),” 1999, which
`will be referred to hereinafter as IEEE 802.11. IEEE 802.11
`
`specifies parameters of both the physical (PHY) and medium
`access control (MAC) layers of the network. The PHY
`network may handle transmission of data between nodes by
`either direct sequence spread spectrum (DSSS)/complemen-
`tary code keying (CCIQ supporting 1-11 Mbps, frequency-
`hopping spread spectrum (PHSS) supporting 1 or 2 Mbps,
`infrared (IR) pulse position modulation supporting 1 or 2
`Mbps, or orthogonal
`frequency division multiplexing
`(OFDM) modulation supporting 6-54 Mbps.
`
`[0009] The MAC layer is a set of protocols which is
`responsible for maintaining order in the use of a shared
`medium. IEEE 802.11 specifies a carrier sense multiple
`access with collision avoidance (CSMA/CA) protocol for
`use as a random access protocol technique. A CSMA pro-
`tocol operates as follows. A station (STA) senses the
`medium, which,
`if busy, defers transmission of its data
`packet to a later time. Aproblem will arise in the case where
`two (2) STAs sense the medium as being free, for example,
`contemporaneously, and each transmit a data packet at the
`same time resulting in a collision. Note that in wireless
`environment, transmitting and receiving at the same time is
`almost impossible even with a full duplex radio due to the
`high signal attenuation. That is, if one senses the medium
`while it transmits a packet, it will only sense its own packet
`even if the packet is colliding with another packet in the
`medium.
`
`[0010] Moreover, in such a Wireless LAN system, not all
`STAs can “hear” each other. The 802.11 standard includes
`
`collision avoidance (CA) mechanism in order to minimize
`collisions, which could arise from two STAs, transmitting at
`the same time.
`
`[0011] The conventional mechanism attempts to overcome
`the problem by implementing the following rules. 1. If a
`station wishing to transmit a data packet senses that the
`medium is busy, it defers its transmission. If the station
`“listens” for a random length of time and finds the medium
`free, the STAwill then transmit. As the reader can guess, this
`is certainly not a complete solution to the above-stated
`problem. 2. Alternatively, the receiving station implements
`a cycle redundancy check (CRC) of the received packet and
`sends an acknowledgment packet (ACK) to the transmitting
`station, indicating to the transmitting STA that no collision
`has occurred. If the transmitting station does not receive the
`ACK, it retransmits its data packet until it actually receives
`the ACK, or discards the data. As with rule 1., this is not a
`complete solution.
`
`radio transmissions based on IEEE
`[0012] Moreover,
`802.11 may also be ineffective because transmitting nodes
`within the wireless LAN cannot hear any other node in the
`system (network) which may be transmitting. That is, the
`transmitting node’s own signal is presumably stronger than
`any other signal arriving at the node. The problem can be
`analogized to the problem of hearing impairment, that is,
`some nodes are hearing impaired for any of various reasons.
`
`[0013] Hidden nodes or stations (STAs) prevent efficient
`use of bandwidth as a result of their hearing impairment to
`certain transmissions. For example, FIG. 1 shows a con-
`ventional wireless local area network (WLAN) composed of
`an access point (AP) and a number of stations (STAs).
`WLAN operation therein is based on the premise that the AP
`
`8
`
`

`

`US 2002/0071448 A]
`
`Jun. 13, 2002
`
`can communicate with all STAs directly over the wireless
`link while STAs can communicate each other depending on
`the relative locations due to their
`limited transmission
`ranges,
`
`In order to reduce the probability of two STAs
`[0014]
`transmitting data which will collide because the STAs are
`not aware of the other’s presence (can not “hear” each other)
`defines a virtu a1 carrier sense (VCS) mechanism The STA
`to transmit sends a short control packet referred to as a
`request to send(RTS) packet. The RTS includes identifica-
`tion of the STAsource, its destination, and the duration of its
`data packet transmission time and time for receipt of the
`ACK packet. The destination STA responds with a clear to
`send(CTS) if the medium is free, also including time dura-
`tion information. All STAs receiving either the RTS or CTS
`packets set their virtual carrier sense indicators, referred to
`as network allocation vectors (NAV), for the given time
`period, and utilize same with their physical carrier sensing
`mechanism when sensing the medium (see FIG. 2). This
`reduces the probability of collision.
`
`In prior art FIG. 1, STA 1 is seen as clearly able to
`[0015]
`communicate by its access point AP] with STA 2 by its
`access point AP2, either directly or in one hop, but not with
`STA3 and its access point AP3. In FIG. 1, a circle around
`each STA (and access point A) represents the corresponding
`transmission range, where STAs 1 and 3 are called hidden
`terminals to each other since they cannot know even the
`existence of each other without the help of the access point
`A hi between. Note that the communication between STAs
`
`1 and 3 should be performed via the access point A.
`
`[0016] The IEEE 802.11 MAC sub-layer defines two
`functions for accessing the Wireless medium: distributed
`coordination function (DCF) and point coordination func-
`tion (PCP), as seen in FIG. 2. The DCF is used to transmit
`asynchronous data based on Carrier Sense Medium Access
`with Collision Avoidance (CSMA/CA) mechanism, while
`the PCP uses a polling mechanism for a “nearly isochro-
`nous” service.
`
`[0017] The PCF is implemented on top of the DCF, and
`controlled by a Point Coordinator (PC) which resides inside
`the access point (AP). An example of the PCF access is
`shown in FIG. 3FIG. 3. The transmission time is divided
`into super-frames, where each super-frame is composed of a
`Contention Free Period (CFP) and a Contention Period (CP).
`During the CFP, the PCP is used for accessing the medium,
`while the DCF is used during the CP. The duration of a
`super-frame is referred to as Contention Free Period Rep-
`etition Interval (CFPRI). A CFP starts by a beacon frame
`sent by the AP or PC. A CFP starts with a beacon frame and
`finishes with a CF-End frame, both transmitted by the AP
`(See FIG. 3). The beacon includes the information about the
`real duration of the CFP to update the network allocation
`vector (NAV) of the STAs as well as the network synchro-
`nization information. A Target Beacon Transmission Time
`(TBTT) indicates the time when the AP attempts to transmit
`a beacon, so TBTTs re peat every beacon period. A CFPRI is
`composed of a number of beacon periods. In some situa-
`tions, the transmission of the beacon frame can be delayed
`if a DCI’ frame from the previous repetition interval carries
`over into the current interval. This situation is known as
`stretching, and can be seen in FIG. 2 as ‘Delay (due to a
`busy medium)’. During the CFP, there is no competition for
`
`the medium. The AP polls each STA asking for pending
`frames to be transmitted. In case the STA has any, it will
`transmit a frame. If the AP receives no response from a
`polled STA after waiting for a point inter-frame space (PIPS)
`interval (FIG. 3), it will poll the next STA.
`
`[0018] FIG. 4 highlights a situation of overlapping among
`two BSSs in order to use it for the presentation of our
`invention. For example, a circle around each STA (and AP)
`which represents the transmission range of the STA. STAs;l
`belongs to the B85 of APX, which is called BSSX. The APs
`can always reach all the STAs belonging to its B53, and
`therefore, all the STAs can always reach its own AP. Unless
`stated otherwise, the effects of the overlapping STAs will be
`considered with respect to STA“ belonging to BSSI. In the
`overlapping RSS situation of FIG. 4, API can hear STAL1
`and STA];2 (B381); (2) STA],l can hear APl, STALZ, and
`STAZ‘J; and (>3) STA;l can hear APl, STA”, and STAB.
`Then, in BSSZ, (1) AP2 can bear STA”; andl(2) STA;1 can
`hearAPz, STAlpl, and STA”. This can happen, for example,
`in a block of/ offices, where the B555 located in two
`neighboring offices, apartments, etc., interfere to each other.
`
`[0019] Here, the main concern is the performance of the
`CFP under PCF in B88; in the existence of the overlapping
`BSSz. For example, the transmission from STAL1 to AP1
`during a CFP can collide with the transmission from STAQ1
`to AP2. This kind of collision during the CFP can result in
`severe degradation of the effectiveness of the PCF in terms
`of the throughput, and it makes really difficult to support
`QoS using this polling-based PCF.
`
`OBJECTS AND SUMMARY OF THE
`INVENTION
`
`It is therefore an object of the present invention to
`[0020]
`provide a wireless MAC protocol, and a wireless LAN
`system rising the MAC protocol which overcomes the short-
`comings of the prior art.
`
`It is also an object of the invention to provide a
`[0021]
`hybrid Wireless MAC protocol for isochronous trallic sup-
`port which uses a novel Ready To Send(RTS)/Clear To
`Send(CTS) exchange during a contention free period (CFP)
`in order to avoid contention from Stations (STAs) in over-
`lapping BSSs.
`
`It is yet another object of the present invention to
`[0022]
`define a new counter called Overlapping Network Allocation
`Vector (ONAV) to render the RTS/CTS during CFP truly
`elfective even in the existence of the STAs in CFP in the case
`of overlapping BSSs.
`
`It is still yet another aspect of the invention to
`[0023]
`provide a hybrid wireless MAC protocol for isochronous
`traffic support which uses a novel Ready To Send(RTS)/
`Clear T o Send(CTS) exchange during a contention free
`period (CFP) in order to avoid contention from Stations
`(STAs) in overlapping BSSs, combined with a new counter
`called Overlapping Network Allocation Vector (ONAV) to
`render the RTS/CTS during CFP truly effective even in the
`existence of the STAs in CFP in the case of overlapping
`BSSs. To that, end, the present invention sets forth a medium
`access control (MAC) protocol for avoiding collisions from
`stations (STAs) comprising two or more IEEE 802.11 basic
`service sets (BSSs) collocated and operating in the same
`channel during contention free periods (CFPs). The MAC
`
`9
`
`

`

`US 2002/0071448 A]
`
`Jun. 13, 2002
`
`U.)
`
`protocol includes hardware or software for utilizing request-
`to-send (RTS)/clear-to-send(CTS) exchange during CFPs to
`avoid potential collision from STAs in overlapping B885
`and hardware or software for providing overlapping network
`allocation vectors (ONAV) in addition to a network alloca—
`tion vector (NAV),
`the ONAV included to facilitate the
`effectiveness of the hardware or software for utilizing.
`
`[0024] Ase-cond embodiment of the invention embodies a
`wireless local area network (WLAN) which utilizes the
`inventive MAC protocol described above.
`
`[0025] A third embodiment of the present invention pro-
`vides a method for avoiding collisions from stations (STAs)
`comprising two or more IEEE 802.11 basic service sets
`(BSSs) collocated and operating in the same channel during
`contention free periods (CFPs). The MAC protocol includes
`hardware or software for utilizing request-to-send (RTS);
`cle ar-to-send(CTS) exchange during CFPs to avoid potential
`collision from STAs in overlapping BSSs. The method may
`further include steps for providing an overlapping network
`allocation vectors (ONAV) in addition to a network alloca-
`tion vector (NAV),
`the ONAV included to facilitate the
`effectiveness of the collision avoidance.
`
`[0026] A fourth embodiment of this invention includes a
`computer programmable device readable by machine, tan-
`gibly embodying a program] of instructions executable by
`machine to perform the method steps of the invention set
`forth herein.
`
`BRIEF DESCRIPTION OI; THE DRAWING
`FIGURES
`
`[0027] FIG. 1 is a schematic diagram depicting a conven-
`tional wireless network comprising stations which may be
`“hidden” from each other;
`
`[0028] FIG. 2 is a schematic diagram depicting the coex-
`istence of CFP and CP in a super-frame, and an example of
`stretching and foreshortened CFP;
`
`[0029] FIG. 3 is a schematic diagram of the PCF working
`in the CFP in accordance with the present invention;
`
`[0030] FIG. 4 is a schematic diagram depicting a situation
`of overlapping BSSs;
`
`[0031] FIG. 5 is a timing diagram of a R'I‘S/C'IS exchange
`during CFP; and
`
`[0032] FIG. 6 is a schematic flow diagram depicting an
`embodiment of a number of steps for implementing a DMR
`algorithm of this invention.
`
`DETAILED DESCRIPTION OF TIIE
`INVENTION
`
`[0033] As mentioned herein. one embodiment of the
`invention comprises means for utilizing an innovative
`Medium Access Control (MAC) protocol for isochronous
`traffic support which uses a novel Request To Send(RTS)/'
`Clear To Send(CTS) exchange during a contention free
`period (CFP) in order to avoid contention from Stations
`(S'I‘As) in overlapping BSSs, and method for implementing
`same.
`
`[0034] Said first embodiment may further comprise a new
`counter called Overlapping Network Allocation Vector
`
`(ONAV) to render the R'I‘S/C'I‘S during CFP truly efieetive
`even in the existence of the STAs in CFP in the case of
`
`overlapping B583.
`
`[0035] A third embodiment of the invention provides a
`hybrid wireless MAC protocol for isochronous trafiic sup-
`port which uses a novel Ready To Send(RTS)/'Clear To
`Send(CTS) exchange during a contention free period (CFP)
`in order to avoid contention from Stations (.S'I‘As) in over-
`lapping BSSs, combined with a new counter called Over—
`lapping Network Allocation Vector (ONAV) to render the
`RTS/CTS during CFP truly effective even in the existence of
`the S'I‘As in CFP in the ease of overlapping BSSS.
`
`[0036] More particularly, the inventions disclosed herein
`utilize a work frame which could included and used in a
`conventional WLAN such as that shown in FIG. 1. Within
`the invention,
`the network frame starts with a beacon
`transmission from the access point (AP) to all the stations
`(STA) in the receiving range of the AP (as shown in FIGS.
`2 and 3). The beacon includes the information about the
`time slot allocation for each STA, synchronization informa-
`tion and information about the AP itself, which is necessary
`for new STAs to associate with the AP.
`
`[0037] FIG. 5 shows the exchange. of the RTS/CTS frames
`during a CFP. Those skilled in the art will realize that we
`have ignored the short
`inter-frame space (SH-S)
`time
`between two exchanging frames, e.g., between RTS and
`CTS, for the simplicity of the explanation. The frame
`formats of RTS/CTS frames transmitted during the CFP are
`the same as the original frame formats defined in IEEE
`802.11 specification. The only difference is the way to
`specify/update the Duration/ID field (time) as is set forth in
`the frame header. For the RTS/CTS during the CFP,
`the
`Du ratioanD fields are calculated as follows. First, the Dura-
`tion/ID of the RTS is:
`
`in FIG. 5):
`DuratiouflD iu RTS frame (i.e., Durl
`[duration of CTS]+[duration of
`(Drum) CFiPoll
`frame]+[duratiou of CF-ACK]
`
`[0038] The skilled artisan will understand that the AP1
`cannot know how long STA1 1 will transmit per being polled,
`i.e., the duration of Data+Cli'-ACK frame, shown in FIG. 5
`is not known to the AP in advance. Now, per receiving the
`RTS, the STA responds with CTS with the Duration/ID field
`calculated as follows:
`
`DuratioanD in CTS frame (i.e., Dur‘Z in FIG. 5')=
`[DuratiouxID specified in the received RTS frame]—
`[duration of CTS]+[duralion of Dala+CF-ACK]
`
`[0039] Also shown in FIG. 5, STAly1 sets rip the NAV with
`the Durationle value in the received CTS frame, and will
`not interfere with the transmission between STAG:1 to AP1
`following the RTS/CTS exchange.
`
`If the RTS/CTS exchange is not successful, for
`[0040]
`example, assuming STAL1 did not receive the RTS frame
`correctly, which could occur due to the bad channel condi-
`tion or an erroneous transmission of S'I'AZJ, then AP1 will
`defer the polling to STA“ to a future time, and will poll
`another STA, for example, STAL2 in the situation under
`consideration. More particularly, if the underlying wireless
`channel involves bursty errors often, exchanging the RTS/
`CTS before the polling can minimize the potential failure of
`the actual data transmission, which can result in the severe
`throughput degradation. The present inventions address and
`correct for such situations as a by-product.
`
`10
`
`10
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`

`

`US 2002/0071448 A1
`
`Jun. 13, 2002
`
`the proposed
`[0041] The skilled artisan will note that
`RTS/CTS may be quite effective when is STAM in the CP
`under DCF. That is, by having a non-zero NAV due to the
`reception of the CTS from STAN, in the above example,
`STA” will never initiate a frame transmission. However, if
`STAZ')1 in the CFP under PCF, this RTS/CTS exchange will
`not be effective. This is a result of the fact that STA;1 will
`set up its NAV at the beginning of the CFP of 1388;, and the
`reception of the CTS will not update the NAV at all.
`Moreover, according to the 802.11 PCF mechanism, STAZ1
`will transmit a frame if it has any data for transmission upon
`being polled by AP2 irrespective of the value of its NAV.
`Such transmission by STA;1 may result in a collision with
`another transmission during the CFP of STAzsl’s neighbor-
`ing BSS. In order to obviate this undesirable situation, the
`present inventions define and utilize a useful new counter
`called overlapping network allocation vector
`(ONAV).
`Termed differently,
`the present
`inventions will maintain
`ONAV as well as the original NAV for each STA in a system.
`
`[0042] There are two rules regarding the ONAV which are
`implemented by any of the inventions set forth herein. The
`first ONAV rule requires that the ONAV is updated with the
`same rule of the original/existing conventional methods for
`updating a NAV as set forth in IEEE 802.11 MAC with one
`exception. The exception is that the ONAV is updated by the
`frames from neighboring BSSs only, not the frames from its
`own BSS. The second ONAV rule requires that if a STA has
`a non-zero ONAV, the STA will not respond to the CF-poll
`from its AP. Accordingly, if a STA being polled does not
`respond, the AP will assume that (1) the CF-poll was not
`received correctly, or (2) the STA has non-zero ONAV. The
`AP will then automatically defer the polling to the future.
`
`[0043] The present inventions which include the ONAV as
`well as the NAV, and the means for implementing ONAV
`rules 1 and 2,
`is that
`the undesirable potential collision
`discussed above can be eliminated. Moreover, the RTS/CTS
`exchanges consume the precious wireless bandwidth redun—
`dantly if they are used where (1) there is no contention from
`overlapping BSSs, and (2) the bursty errors are very rare. On
`the other hand, if the RTS/CTS exchange is not used where
`it could be useful, the transmitted frames result in a waste of
`bandwidth due to the unsuccessful transmissions. Therefore,
`the present invention also defines several decision-making
`rules (DMR), as is the case with the RTS/CTS in the DCF.
`The DMR can be implemented by a system control software
`of the MAC implementation.
`
`[0044] The first DMR rule regarding the decision on
`whether to use RTS/CTS during CFP before a particular
`(data+)Cli'-poll frame (that is, whether to use ONAV rules 1
`and 2) requires that each STA report to its AP if there are
`STAs, which belong to other BSSs, Within its coverage area.
`By receiving a frame with a BSSID different from its own,
`a STA can detect the existence of such STAs easily. For each
`data transmission to/from such a STA, the AP may initiate
`the RTS/CTS exchange during CFP.
`
`[0045] The second DMR rule regarding the decision on
`whether to use RTS/CTS during CFP before a particular
`(data+)CF-poll frame requires that the transmission of the
`(data+)CF-poll frame which is larger than a predetermined
`threshold size results in the AP requesting the R'l‘S/C'I‘S
`exchange during CFP. The second DMR rule is applied to
`the RTS/CTS decision in the DCF via the parameter
`RtsThreshold.
`
`[0046] The third DMR rule regarding the decision on
`whether to use RTS/CTS during CFP before a particular
`(data+)CF-poll frame is based on observing a number of
`transmission failures to/from a STA which is greater than a
`predetermined threshold value. In consequence, the present
`invention requires that the AP may request the RTS/CTS
`exchange during CFP for the data transmissions involving
`this STA, effective for time-varying channel with bursty
`errors.
`
`[0047] FIG. 6 shows the diagram for the above-described
`DMR algorithm.
`
`What is claimed is:
`
`1. Amedium access control (MAC) protocol for avoiding
`collisions from stations (STAs) comprising two or more
`IEEE 802.11 basic service sets (RSSs) collocated and oper-
`ating in the same channel during contention free periods
`(Cli'Ps), comprising:
`
`ready—to—send—(RTS)/elear—to—
`utilizing
`for
`means
`send(CTS) exchange during CFPs to avoid potential
`collision from STAs in overlapping B885; and
`
`means for providing overlapping network allocation vec-
`tors (ONAV) in addition to a network allocation vector
`(NAV), the ONAV included to facilitate the eifective-
`ness of the means for utilizing.
`2. The MAC protocol defined by claim 1, further com-
`prising means for updating the duratioanD of both the RTS
`and CTS frames during the CFP and maintaining that the
`overlapping STA(s) will not
`interfere with transmission
`between and STA and its corresponding AP following the
`RTS/CTS exchanges.
`3. The MAC protocol defined by claim 2, wherein the
`means for updating the duration/ID of the RTS frame by
`adding the time length of the CTS to the (duration of the CF
`data minus the poll frame) and the duration of the CF—ACK,
`and for updating the duration/ID of the CTS frame by adding
`the duration/ll) specified in the received RTS frame with the
`duration of the data and the time length if the CF-ACK and
`subtracting the duration time of the CTS from same.
`4. The MAC protocol defined by claim 2, including means
`for determining if the RTS/CTS exchange is successful, and,
`if unsuccessful, deferring polling of the STA by its corre-
`sponding AP to a future time, where the AP then polls
`another STA.
`
`5. The MAC protocol defined by claim 1, further com-
`prising means for implementing ONAV rules.
`6. The MAC protocol defined by claim 1, further com-
`prising means for determining when to implement the RTS/
`CTS during CFP.
`7. A method for implementing a medium access control
`(MAC) protocol
`for avoiding collisions from stations
`(STAs) comprising two or more IEEE 802.11 basic service
`sets (BSSs) collocated and operating in the same channel
`during contention free periods (CFPs), the method compris-
`ing the steps:
`
`ready-to-send-(RTS)(«"clear-to-send(CTS)
`utilizing
`exchange during CFPs to avoid potential collision from
`STAs in overlapping BSSs; and
`
`vectors
`allocation
`network
`overlapping
`providing
`(ONAV) in addition to a network allocation vector
`(NAV), the ONAV included to facilitate the effective-
`ness of the Step of utilizing.
`
`11
`
`11
`
`

`

`US 2002/0071448 A]
`
`Jun. 13, 2002
`
`L11
`
`8. A wireless local area network (WLAN) which imple-
`ments the MAC protocol defined in claim 1.
`9. A program storage device readable by a machine,
`tangibly embodying a program of instructions executable by
`the machine to perform method steps for which define a
`medium access control (MAC) protocol for implementing a
`medium access control (MAC) protocol [or avoiding colli-
`sions from stations (S'l'As) comprising two or more IEEE
`802.11 basic service sets (BSSs) collocated and operating in
`the same channel during contention free periods (CFPs), the
`method comprising the steps:
`
`ready-to-send-(RTS),"elear-to -sen d(CTS)
`utilizing
`exchange during CFPs to avoid potential collision from
`S'l‘As in overlapping B885; and
`
`vectors
`allocation
`network
`overlapping
`providing
`(ONAV) in addition to a network allocation vector
`(NAV), the ONAV included to facilitate the effective-
`ness of the Step of utilizing.
`
`10. A medium access control (MAC) protocol for avoid-
`ing collisions from stations (STAs) comprising two or more
`IEEE 802.11 basic service sets (BSSs) collocated and oper-
`ating in the same channel during contention free periods
`(CFPs), comprising:
`ready-to-send-(RTS):'clear-to-
`means
`for
`utilizing
`send(CTS) exchange during CFPs to avoid potential
`collision from STAs in overlapping B555.
`11. A medium access control (MAC) protocol for avoid-
`ing collisions from stations (VS'l'As) comprising two or more
`IEEE 802.11 basic service sets (BSSs) collocated and oper-
`ating in the same channel during contention free periods
`(CFPs), comprising:
`means for providing overlapping network allocation vec-
`tors (ONAV) in addition to a network allocation vector
`(NAV), the ONAV included to facilitate the effective-
`ness said collision avoidance.
`
`12
`
`12
`
`