United States Patent (19]
`Boer et al.
`
`11111~111111111111111
`US005706428A
`5,706,428
`[111 Patent Number:
`Jan. 6, 1998
`[45] Date of Patent:
`
`[54] MULTIRATE WIRELESS DATA
`COMMUNICATION SYSTEM
`
`[75]
`
`Inventors: Jan Boer. Odijk; Wllhelmus Josephus
`Diepstraten. Diessen; Adriaan
`Kamennan. Nieuwegein; Hendrik van
`Bokhorst, Nijkerk; Hans van Driest.
`Bilthoven, all of Netherlands
`
`[73] Assignee: Lucent Technologies Inc., Murray Hill.
`N.J.
`
`[21] Appl. No.: 615,408
`
`Mar. 14, 1996
`
`[22] Filed:
`Int. Cl.6
`.............................. H04Q 1130; H04L 12128
`[51]
`[52] U.S. Cl ........................... 395/200; 370/349; 370/342;
`370/465; 3751202; 3751206; 375/347
`[58] Field of Search ..................................... 3701349. 342.
`370/338, 465; 3751202. 206, 347, 349;
`395/200.13. 200.l
`
`[56]
`
`References Cited
`
`U.S. PJXI'ENT DOCUMENTS
`
`5,206,881
`5,379,290
`5,592,468
`
`. ....................... 375/1
`4/1993 Messenger et al.
`1/1995 Kleijne ................................... 370/85.2
`1/1997 Sato ........................................ 370/252
`
`OfHER PUBLICATIONS
`
`Wilkinson Tom; "High Data Rate Radio LANs". IEEE. pp.
`3/1-3/8. No Date.
`Hayes. Victor; "Standardization Efforts for Wireless LANs".
`IEEE Network Magazine, pp. 19-20, Nov. 1991.
`
`"Welcome to IEEE P802.ll"; Working Group for Wireless
`Local Area Networks; Set-up on Dec. 17, 1996. update of
`May 20, 1997.
`
`"Bell Labs Unveils 10-Megabit Wireless-Network Technol(cid:173)
`ogy, Offering Five Times Today's Highest Data-Transmis(cid:173)
`sion Capacity"; ICA New Product Announcment. Apr. 22,
`1997.
`
`Primary Examiner-Jam.es P. Trammell
`Assistant Examiner-Shah Kaminis
`Attorney, Agent, or Firm-Christopher N. Malvone
`
`[57]
`
`ABSTRACT
`
`A wireless LAN includes first stations adapted to operate at
`a 1 or a 2 Mbps data rate and second stations adapted to
`operate at a 1.2.5 or 8 Mbps data rate. The 1 and 2 Mbps
`rates use DBPSK and DQPSK modulation, respectively. The
`5 and 8 Mbps rates use PPM/DQPSK modulation. All four
`data rates use direct sequence spread spectrum (DSSS)
`coding. All transmitted messages start with a preamble and
`header at the 1 Mbps rate. The header includes fields
`identifying the data rate for the data portion of the message,
`and a length field. For a 2 Mbps transmission the length field
`identifies the number of bytes in the data field. For a 5 or 8
`Mbps the length field identifies the number of bytes in the
`data field which. if transmitted at 2 Mbps, would take the
`same transmission time of the data field, and is thus a
`fraction ¥s or ¥e of the actual number of the bytes. With this
`arrangements. all the stations are interoperable in a
`co-existent manner in the LAN.
`
`6 Claims, 6 Drawing Sheets
`
`DECREMENT
`DATA RATE
`SCCOUNT=O
`
`524
`
`I
`
`500
`
`Exhibit 3001
`
`

`
`U.S. Patent
`
`Jan. 6, 1998
`
`Sheet 1of6
`
`5,706,428
`
`14
`
`16
`
`ACCESS
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`
`17
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`
`FIG.1
`
`Exhibit 3001
`
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`Exhibit 3001
`
`

`
`U.S. Patent
`
`Jan. 6, 1998
`
`Sheet 5 of 6
`
`5,706,428
`
`START
`
`502
`
`506
`
`CONFIGURED
`FOR ACK RECEIPT
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`
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`
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`
`END
`
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`
`FIG.7
`
`Exhibit 3001
`
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`

`
`1
`MULTIRATE WIRELESS DATA
`COMMUNICATION SYSTEM
`
`2
`FIG. 8 is a block diagram of a modified embodiment of a
`LAN station.
`
`5,706,428
`
`FIELD OF THE INVENTION
`This invention relates to wireless data communication 5
`systems.
`
`DEfAILED DESCRIPITON OF THE
`INVENTION
`Referring first to FIG. l, there is shown a preferred
`embodiment of a wireless LAN (local area network) 10 in
`which the present invention is implemented. The LAN 10
`includes an access point 12, which serves as base station,
`10 and is connected to a cable 14 which may be part of a
`backbone LAN (not shown), connected to other devices
`and/or networks with which stations in the LAN 10 may
`communicate. The access point 12 has antennas 16 and 17
`for transmitting and receiving messages over a wireless
`l5 communication channel.
`The network 10 includes mobile stations 18, referred to
`individually as mobile stations 18-1, 18-2. and having
`antennas 20 and 21. referred to individually as antennas
`20-1, 20-2 and 21-1. 21-2. The mobile stations 18 are
`20 capable of transmitting and receiving messages selectively
`at a data rate of 1 Mbps (Megabit per second) or 2 Mbps,
`using DSSS (direct sequence spread spectrum) coding.
`When operating at the 1 Mbps data rate. DBPSK
`(differential binary phase shift keying) modulation of the RF
`25 carrier is utilized, and when operating at the 2 Mbps data rate
`DQPSK (differential quadrature phase shift keying) modu(cid:173)
`lation of the RF carrier is utilized. Thus, it will be appreci(cid:173)
`ated that both data rates are equivalent to a symbol rate of
`1 MBaud (Megabaud), i.e. 1 symbol per second. Preferably
`30 the DSSS code utilized is an 11-chip Barker code having the
`values
`+1. -1. +1, +l. -1. +l. +1, +1. -1. -1. -1. the leftmost
`chip being the first in time.
`Also included in the LAN 10 are further mobile stations
`22, referred to individually as stations 22-1 and 22-2. and
`having antennas 24 and 25, referred to individually as
`antennas 24-1, 24-2 and 25-1. 25-2. The stations 22 can
`operate at a 1 Mbps or a 2 Mbps data rate, using the same
`modulation and DSSS coding as the stations 18. and in
`addition can also operate at two higher data rates. namely 5
`Mbps and 8 Mbps. These 5 and 8 Mbps data rates utilize
`PPM/DQPSK (pulse position modulation-differential
`quadrature phase shift keying) in combination with the
`11-chip Barker code mentioned hereinabove. At the 5 Mbps
`data rate there are used 1 out of 8 possible PPM positions,
`whereby there are 5 encoded bits per symbol interval (3
`position bits plus 2 bits for quadrature phase information).
`At the 8 Mbps data rate the I- and Q-components are used
`separately. Thus there are 3 position bits for the
`so I-component, 3 position bits for the Q-component. and 2 bits
`for quadrature phase infonnation. It will be appreciated that
`the 5 and 8 Mbps data rates correspond to a 1 Mbaud symbol
`rate, just as do the 1 and 2 Mbps data rates.
`From the above description, it will be appreciated that the
`55 LAN 10 contains mobile stations 18 of a first type (operating
`at 1or2 Mbps data rates) and mobile stations 22 of a second
`type (operating at 1.2.5 or 8 Mbps data rates). However. as
`will be explained hereinbelow, there is fully interoperable
`operation at the 1 and 2 Mbps data rates, and further, the
`60 stations 22 can operate at their higher data rates of 5 and 8
`Mbps, in a manner c~existent with the operation of the
`staions 18.
`Referring now to FIG. 2. there is shown a functional block
`diagram illustrating, for a station 18, the interconnection of
`65 the functional blocks which relate to the implementation of
`the present invention. The block 30 represents a MAC
`(medium access control) control unit which includes four
`
`BACKGROUND OF THE INVENTION
`With a view to obviating the need for wired cabling
`connections between stations in local area networks (LAN s ),
`wireless local area networks have been developed, and are
`now commercially available. These wireless local area net(cid:173)
`works employ stations, which may be data processing
`devices (such as PCs) having a wireless communication
`capability.
`In view of this development, there is being produced
`IEEE standard 802.11, currently available in draft form,
`which specifies appropriate standards for use in wireless
`LAN s. This standard specifies two possible data rates for
`data transmission, namely 1 Mbps (Megabit per second) and
`2 Mbps. Accordingly, manufacturers have produced com(cid:173)
`mercially available systems operating at these data rates.
`However, it may be advantageous to provide systems oper(cid:173)
`ating at higher data rates, which are not in accordance with
`the standard.
`It is an object of the present invention to provide a method
`of operating a wireless local area network station which
`enables communication between stations operating at dif(cid:173)
`ferent data rates.
`
`SUMMARY OF THE INVENTION
`Therefore, according to the present invention, there is
`provided a method of operating a wireless local area net(cid:173)
`work station adapted to transmit and receive messages at a 35
`plurality of data rates, wherein said messages include an
`initial portion and a data portion, including the steps of:
`transmitting the initial portion of a message to be transmitted
`by a station at a first predetermined one of a first plurality of
`data rates; including in said initial portion a data rate 40
`identification segment identifying a selected one of a second
`plurality of data rates, at which said data portion is to be
`transmitted, and a length segment representing the length of
`time which would be required for a transmission of said data
`portion at one of said first plurality of data rates; and 45
`transmitting said data portion at said selected one of said
`second plurality of data rates.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`One embodiment of the invention will now be described
`by way of example, with reference to the accompanying
`drawings, in which:
`FIG. 1 is a block diagram of a wireless LAN embodying
`the present invention;
`FIG. 2 is a block diagram of a wireless LAN station
`capable of operating at two data rates;
`FIG. 3 is a block diagram of a wireless LAN station
`capable of operating at four data rates;
`FIG. 4 is a diagram illustrating the fonnat of a data
`message circulating in the LAN;
`FIG. 5 is a diagram illustrating the format of a first type
`of acknowledgement message;
`FIG. 6 is a diagram illustrating the format of a second type
`of acknowledgement message;
`FIG. 7 is a flowchart illustrating the operation of an
`automatic data rate selection procedure; and
`
`Exhibit 3001
`
`

`
`5,706.428
`
`3
`state machines. namely a MAC control state machine
`C-MST 32. a MAC management state machine M-MST 34,
`a transmitter state machine T-MST 36 and a receiver state
`machine R-MST 38. The MAC control unit 30 is shown as
`connected over a line 40 to a 1-out-of-2 rate selector 42 and
`a scrambler 44. The rate selector 42 and scrambler 44 are
`connected to a 1-out-of-2 encoder 46 which encodes the data
`bits from the scrambler 44 in accordance with the selected
`1 or 2 Mbps data rate. The output of the encoder 46 is
`connected to a spreader 48 which effects the above(cid:173)
`discussed spread spectrum coding and applies the signal to
`an RF front-end transmitter 50 for application to the antenna
`20.
`The receive antenna 21 is connected to an RF front-end
`receiver 52 which is connected to a correlator 54 which
`effects a correlation to "despread" the received signal. A first
`output of the correlator 54 is connected to carrier detector
`56. A second output of the correlator 54 is connected to a
`1-out-of-2 detector/decoder 58 which has an output con(cid:173)
`nected to an input of a descrambler 60. The output of the
`descrambler 60 is connected over a line 62 to the MAC
`control unit 30 and to a 1-out-of-2 rate selector 64 which has
`an output connected to the detector/decoder 58 to control the
`detector/decoder 58 appropriately in accordance with con(cid:173)
`trol information contained in received messages.
`Referring now to HG. 3, there is shown a functional block 25
`diagram illustrating the interconnection of the functional
`blocks included in a station 22, which relate to the imple(cid:173)
`mentation of the present invention. The arrangement of
`functional blocks for the stations 22 is similar to that of the
`functional blocks shown in HG. 2 for the station 18.
`Consequently, similar functional blocks in FIG. 3 are pre(cid:173)
`fixed by an initial 1. It will be appreciated that an important
`difference is that the rate selectors 142 and 164 are 1-out(cid:173)
`of-4 rate selectors, rather than 1-out -of-2 rate selectors. as
`are the selectors 42 and 64 in FIG. 2. Similarly, the encoder
`146 is a 1-out-of-4 encoder and the detector/decoder 158 is
`a detector/decoder for a selected one of four possible data
`rates. It will be appreciated that these differences arise since
`the station 22 is capable of operation at one of four possible
`data rates, whereas the station 18 is capable of operating
`only at one of two possible rates.
`Referring now to FIG. 4. there is shown the format of a
`typical message 200 used in the LAN 10. The message 200
`includes a 128-bit SYNC (synchronisation) field 202, a
`16-bit SFD (start of frame delimiter) field 204, an 8-bit
`SIGNAL field 206 (to be explained), an 8-bit SERVICE field
`208 (to be explained). a 16-bit LENGTH field 210 (to be
`explained). a 16-bit CRC check field 212, which provides a
`CRC check for the portions 206. 208 and 210, and finally a
`D..UA field 214 which comprises a variable number of data
`"octets", that is 8-bit data segments, sometimes referred to
`as "bytes". The fields 202 and 204 are together conveniently
`referred to as a preamble 216 and the fields 206. 208, 210
`and 212 are together conveniently referred to as a header
`218.
`With regard to the message 200, HG. 4, it should be
`understood that the preamble 216 and header 218 are always
`transmitted at the 1 Mbps rate using DBPSK modulation.
`The subsequent D..UA field 214, however, may be transmit(cid:173)
`ted at a selected one of the four possible rates 1, 2. 5 or 8
`Mbps, using the modulation and coding discussed herein(cid:173)
`above. Of course. the stations 18 are capable of transmitting
`at the 1 and 2 Mbps rates only, whereas the stations 22 can
`transmit the D..UA field 214 at a selected one of the four data
`rates.
`In more detail concerning the fonnat of the message 200.
`the SYNC field 202 consists of 128 bits of scrambled "l"
`
`4
`bits, enabling a receiving device to perform the necessary
`operations for synchronisation. The SFD field 204 consists
`of a predetermined 16-bit field identifying the impending
`start of the header 218. The SIGNAL field 206 has a first
`5 predetermined value if the DATA field 214 is transmitted at
`the 1 Mbps rate and a second predetermined value if the
`D..UA field 214 is transmitted at the 2, 5 or 8 Mbps rates.
`The SERVICE field 208 has a first predetermined value
`(typically all zero bits) for the 1 and 2 Mbps rates. a second
`10 predetermined value for the 5 Mbps rate and a third prede(cid:173)
`termined value for the 8 Mbps rate. It should be understood
`at this point that the stations 18. adapted to operate at the 1
`and 2 Mbps rates only, ignore the SERVICE field 208. This
`aspect will be discussed more fully hereinafter. The
`15 LENGTH field 210 contains, if the bit rate is designated as
`1 or 2 Mbps, a value corresponding to the actual number of
`octets in the DATA field 214. However for the 5 and 8 Mbps
`rates, the LENGTH field 210 contains a value which is a
`fraction, ¥s and %, times the actual number of octets in the
`20 D..UA field 214, respectively. These values correspond to
`the length in octets of a transmission at 2 Mbps which would
`give the same transmission time of the DATA field 214.
`which is actually transmitted at 5 Mbps. or 8 Mbps respec-
`tively.
`Referring briefly to FIG. 1. it should be understood that
`the LAN 10 operates on a CSMA/CA (carrier sense multiple
`access with collision avoidance) protocol. According to this
`protocol. if a station wishes to transmit a message, it first
`senses the transmission channel. If the channel is sensed as
`30 free and has been free for a predetermined, interframe
`spacing time, then the message is transmitted immediately.
`If the channel is sensed as busy, then access is deferred until
`the channel becomes free and remains free for the short
`interframe spacing time. However, transmission of the mes-
`35 sage does not then take place immediately, but is further
`deferred for a random backoff time. This procedure allevi(cid:173)
`ates contention problems where multiple stations are waiting
`to transmit. Of course, collisions are not completely avoided
`by this CSMA/CA protocol. but the chance of a collision is
`40 rendered very small.
`In connection with the above, it should be noted that a
`station 18 will sense that the channel is busy only if the
`signal level is above a predetermined threshold level,
`referred to as the defer threshold level. and a simple DSSS
`45 type of signal is sensed. Thus a station 18 will not defer if,
`when it wishes to transmit. it senses a transmission involving
`a PPM type coding as well as DSSS coding, such as is used
`for the 5 and 8 Mbps transmissions of a station 22. In these
`circumstances the station 18 and 22 may mutilate each
`50 other's transmissions. It is in order to alleviate this problem.
`that the HEADER 218 of the messages transmitted by the
`stations 22 contains a representation of ¥s and ¥e times the
`actual number of octets in the DATA field 214 since this
`representation causes any station receiving it to defer for the
`55 length of time corresponding to the specified number of
`symbol intervals. regardless of the type of DSSS coding
`used.
`The data rate capability of each station 18, 22 is supplied
`to the access point 12 in an initial access point association
`60 procedure when the station is initially operated in the LAN
`10. Briefly. this procedure involves a transmission by the
`station of an association request frame and the consequent
`transmission by the access point 12 of an association
`response frame. The data rate capability of the station is then
`65 stored in a table (not shown) at the access point 12. which
`associates the ID of the station with a representation of the
`data rate capabilities of the station. Also, the association
`
`Exhibit 3001
`
`

`
`5,706,428
`
`10
`
`5
`response message informs the newly associated station of
`the data rate capabilities of the other stations in the network
`10.
`A further feature of the present embodiment is that an
`acknowledgement procedure is utilised, that is, for each 5
`directed message transmitted by a station an ACK
`(acknowledgement) message is expected to be received in
`response. With this in mind, and referring to FIGS. 2 and 3,
`the operation of the MAC control units 30 and 130 will be
`briefly described.
`The MAC management state machine M-MST 134
`(FIG.3) includes a table (not shown) containing information
`relating to other stations, identified by their station ID code.
`Such table contains, for each station ID. counter values for
`the number of frames correctly received from that station,
`the number of frames transmitted to that station, for which
`an ACK frame has been correctly received, and the number
`of frames transmitted to that station for which an ACK frame
`has not been correctly received, together with the applied
`data rate, for each direction of frame transmission. The
`MAC control state machine C-MST 132 handles the control
`of the transmitter and receiver state machines T-MST 136
`and R-MST 138. The transmit state machine T-MST 136
`handles the timed control and the forwarding of the frames
`200 (FIG.4) over the line 140 for transmission. The receive
`state machine R-mst 138 handles the timed control and
`foiwarding of the frames 200 from the line 162 to the MAC
`control unit 130.
`When a station 22 is to transmit a frame to a destination
`station, it accesses the table stored in the M-MST 134 to
`ascertain the data rate to be applied to the transmission to
`that station. The C-MST 132 inserts the preamble 216 and
`header 218 in the frame 200 (FIG. 4), and ascertains the data
`rate information from the table in the M-MST 134. Also, the
`C-MST 132 adds the LENGfH field 210, which, for 5 and
`8 Mbps bits rates is, as discussed hereinabove, a fraction, 7's
`or %, of the actual length in octets of the DATA field 214.
`As discussed hereinabove, the SIGNAL field 206 used at 5
`and 8 Mbps data rate is the same as the SIGNAL field 206
`for the 2 Mbps data rate.
`With regard to transmission of a message by a station such
`as 18, if the channel is clear, a transmission can be initated.
`At the beginning of a transmission, the scrambler 44 will
`receive as an input the 128 bit SYNC field 202, followed by
`the SFD field 204, the header 218 and the DATA field 214. 45
`The rate selector 42 utilizes the SIGNAL field 206 to control
`the encoder 46 such that. after the last bit of the header
`portion 218, the data rate is maintained at 1 Mbps DBPSK
`mode or switched to the 2 Mbps DQPSK mode. The encoder
`46 thus provides appropriately modulated signals at the 1
`MBaud rate for application to the spreader 48 where the
`DSSS coding is effected. The RF transmitter 50 then effects
`conventional filtering, up-mixing and power amplification to
`provide a signal for application to the antenna 20.
`With regard to receiving a signal in a station 18, when the
`channel is active, the carrier detector 56 provides a signal
`indicating the presence of a signal received by the antenna
`21. The received signal is fed to the RF receiver 52, which
`effects conventional filtering, automatic gain control and
`down-mixing. The output signal from the RF receiver 52 is
`applied to the correlator 54, which produces a spike(cid:173)
`waveform output signal. The detector/decoder 58 initially
`operates at the 1 Mbps data rate, and provides an output
`signal which is applied to the descrambler 60. After the SFD
`field 204 has left the descrambler 60 the rate selector 64 uses
`the SIGNAL field 206 to determine whether the detector/
`decoder 58 should remain in the 1 Mbps mode or switch to
`
`6
`the 2 Mbps mode. If such switching takes place, then the
`DATA field 214 will be descrambled in the descrambler 60
`and applied to the MAC control unit 30, using a 2 MHz
`clock.
`In a station 22 which is to transmit a message, the C-MST
`132 inserts the preamble 216 and header 218. As mentioned
`hereinabove, the SIGNAL field 206 is the same for the 5 and
`8 Mbps data rates as for the 2 Mbps data rate. but the
`SERVICE field 208 differs. The rate selector 142 uses the
`SIGNAL and SERVICE fields 206, 208 to decide whether or
`not the encoder 146 should switch to the 2. 5 or 8 Mbps
`modes. If rate switching is to take place. then after the last
`bit of the header 218 has passed through, the rate selector
`142 provides a control signal to the encoder, to switch from
`operation in the 1 Mbps DBPSK mode to the 2 Mbps
`15 DQPSK mode, 5 Mbps PPM/QPSK mode or the 8 Mbps
`PPM/QPSK mode, whereby the DATA field 214 is encoded
`in the selected manner.
`In a station 22 which is receiving a message. the rate
`selector 164 uses the SIGNAL and SERVICE fields 206, 208
`20 to determine whether to remain in the 1 Mbps mode or
`switch to the 2,5 or 8 Mbps mode. If the SIGNAL field 206
`indicates the 2 Mbps mode, then the rate selector 164
`provides, after the last bit of the header 218 has passed, a
`control signal to the detector/decoder 158 to switch to the 2,
`25 5 or 8 Mbps mode, dependent on the value of the SERVICE
`field 208. Thus the DATA field 214 is descrambled in the
`descrambler 160. and clocked into the MAC control unit 130
`at the appropriate 2, 5 or 8 Mbps clock rate. The C-MST 132
`determines if an incoming message is addressed to its own
`30 station, using a destination address included in the data field
`214 of the message 200. If the address matches, and the
`C-MST has checked a CRC field (not shown) that is part of
`the data field 214, then assuming there is no error, the
`C-MST forwards the data field 214 for further processing in
`35 the station, and forwards the data rate information to the
`M-MST 134, for storage in the aforementioned table under
`the relevant station ID. Note also that. following the receipts
`of the header 218, and assuming a correct CRC check for the
`CRC field 212, the rate selector 164 is controlled to operate
`40 the detector/decoder 158 at the correct signalling rate of 1,
`2, 5 or 8 Mbps, as indicated by the contents of the SIGNAL
`and SERVICE fields 206 and 208. An octet counter (not
`shown) is updated until the last detected symbol of the data
`field 214 has been processed.
`As mentioned above. the table in the M-MST 134 stores
`the data rates that will be used for transmissions to the
`stations identified by their ID. Referring now to FIG.S, there
`is shown the format of an ACK (acknowledgement) message
`300 used in the LAN 10. The format of the ACK message
`50 300 is generally similar to the format of the message 200
`(FIG. 4), and includes a SYNC field 302 and a SFD (start of
`frame delimiter) field 304, a SIGNAL field 306. a SERVICE
`field 308, a LENGTH field 310, and a CRC field 312. The
`fields 302 and 304 form a preamble 316 and the fields 306,
`55 308, 310 and 312 form a header 318. Also included in the
`ACK message 300 is a DATA field 314 which contains a
`16-bit FRAME CONTROL (FC) field 320, a 16-bit DURA(cid:173)
`TION field 322, a 48-bit RECElVER ADDRESS (RA) field
`324 and a 32-bit CRC check field 326. Thus the DATA field
`60 314 contains a total of 14 octets. The ACK message DATA
`field 314 may be transmitted at the 1 Mbps rate or the 2
`Mbps rate, as identified in the SIGNAL field 306. The ACK
`frame 300 is used by the stations 18 and is also used by the
`stations 22 when operating at the 1 or 2 Mbps rate. However,
`65 when operating at the 5 or 8 Mbps rate, the stations 22
`preferably use a shorter ACK message, having the format
`shown in FIG. 6.
`
`Exhibit 3001
`
`

`
`5,706,428
`
`10
`
`7
`8
`automatic data rate selection procedure has been described.
`Referring to F1G. 6, there is shown the format of a short
`At a lower data rate the transmission of data is more robust
`ACK message 400, preferably used by the stations 22 when
`because the detection margin is larger at lower data rates. At
`operating at the 5 or 8 Mbps rate. The short ACK message
`a higher data rate the requirements with regard to channel
`400 includes a 76-bit SYNC field and an SFD (start-of-
`frame delimiter) field 404, together forming a preamble 406. 5 conditions such as SNR. SlR (co-channel interference) and
`delay spread, are more stringent. If a station 22 doesn't
`The preamble 406 is followed by a data field 408 which
`include an 8-bit station ID field 410 and a 2-bit field 412
`receive the expected ACK message in return correctly and in
`identifying a preferred data rate. The preferred data rate is
`due time. it will retransmit the original message packet at a
`lower data rate. If a station 22 does receive the expected
`derived in a receiving station, dependent on receive quality
`condition and a SNR (signal-to-noise) value with respect to
`ACK messages correctly and in due time from a particular
`a message received from a transmitting station.
`station for a predetermined number of successive times, then
`Referring now to F1G. 7, there is shown a fl.owchart 500
`it will transmit the next message to that station at a higher
`illustrating an automatic data rate update procedure for the
`data rate. In this way the stations 22 adapt the operating data
`data rate to be used in the transmit mode, which is imple-
`rate dependent on channel conditions (degradation by
`mented in the preferred embodiment described herein for a
`noise--SNR, time dispersion in the channel-delay spread)
`station 22. The fl.owchart 500 begins at start block 502. 15 and co-channel interference (SlR).
`As mentioned above. the stations 22 preferably use a short
`Accordingly, from the start block 502, the flowchart 500
`proceeds to block 504. where a determination is made as to
`ACK message (FIG. 6) when operating at the 5 or 8 Mbps
`whether the data rate is 5 or 8 Mbps. If so, the flowchart
`data rates. This ACK message has a duration of only 90
`proceeds to block 506 (to be described). If not, the flowchart
`microseconds, in contrast to the ACK message 300 of F1G.
`proceeds to block 508 where a determination is made as to 20 5, which lasts for about 300 microseconds at the 1 Mbps
`rate, or about 250 microseconds at the 2 Mbps rate. It will
`whether the ACK has been received and within a predeter-
`mined time-out time. If yes. the fl.owchart proceeds to block
`be appreciated that stations 18 which detect the transmis-
`510. where a successive correct (SC) count value is incre-
`sions of a short ACK message 400 (F1G.6) will defer until
`mented. Next, as seen in block 512, a check is made as to
`the ACK message has ended. since the ACK message 400
`whether the SC count value is greater than a predetermined 25 uses DBPSK modulation at the 1 MBaud symbol rate and its
`value, selected as value 9, by way of example. In other
`transmission time is much less than the transmission time of
`words, a check is made as to whether more than nine
`alongACKmessage300(FIG.5).Anadvantageofusingthe
`successive ACK messages have been correctly and timely
`short ACK message 400 (F1G.6) is a significant reduction of
`received. If yes. the fl.owchart proceeds to block 514 where
`the overhead-in-time per transmission.
`a check is made as to whether the local SNR (signal-to-noise 30
`After a station 22 has transmitted a message using the 5
`ratio) value is greater than a predetermined value, suitable
`or 8 Mbps rate, it expects a short ACK message 400 within
`a time-out period of 30 microseconds. If a carrier signal is
`for data rate incrementation. (The SNR is the ratio of
`received signal strength during the reception of the ACK
`detected before 20 microseconds have expired after the end
`message to the average silence level during periods at which
`of the abovementioned 30 microsecond period, and if the
`no carrier signal is being received). If the SNR value is 35 4-bit SFD pattern in the SFD field 404 is recognized. then
`suitable, then the flowchart proceeds to block 516, where a
`the receipt of a short ACK message is confirmed and the
`data rate incrementation i