United States Patent [19]
`Boer et a].
`
`[54] MULTIRATE WIRELESS DATA
`COMMUNICATION SYSTEM
`
`[75] Inventors: Jan Boer. Odijk; Wilhelmus Josephus
`Diepstraten. Diessen; Adriaan
`Kamerman. Nieuwegein; Hendrik van
`Bokhorst. Nijkerk; Hans van Driest.
`Bilthoven. all of Netherlands
`
`[73] Assignee: Lucent Technologies Inc.. Murray Hill.
`NJ.
`
`[21] Appl. No.: 615,408
`'[22] Filed:
`Mar. 14,1996
`[51] Int. Cl.6 ......................... .. H04Q 1/30; H04L 12/28
`[52] US. Cl. ........................ .. 395/200; 370/349; 370/342;
`370/465; 375/202; 375/206; 375/347
`[58] Field of Search ................................... .. 370/349. 342.
`370/338. 465; 375/202. 206. 347, 349;
`395/200.13. 200.1
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`5,206,881
`5,379,290
`5,592,468
`
`4/1993 Messenger et a1. ...................... .. 375/1
`1/1995 Kleijne ..... ..
`370/852
`1/1997 Sato ................................... .. 370/252
`
`OTHER PUBLICATIONS
`
`Wilkinson Tom; “High Data Rate Radio LANs”. IEEE. pp.
`3/ 1-3/8. No Date.
`Hayes. Victor; “Standardization E?orts for Wireless LANs”.
`IEEE Network Magazine. pp. 19-20, Nov. 1991.
`
`‘
`
`US005706428A
`
`‘
`
`[11] Patent Number:
`[45] Date of Patent:
`
`5,706,428
`Jan. 6, 1998
`
`“Welcome to IEEE P802.11”; Working Group for Wireless
`Local Area Networks; Set-up on Dec. 17. 1996. update of
`May 20. 1997.
`“Bell Labs Unveils IO-Megabit Wireless-Network Technol
`ogy. Offering Five Times Today's Highest Data-Transmis
`sion Capacity"; ICA New Product Announcment. Apr. 22.
`1997.
`
`Primary Examiner-James P. Trammell
`Assistant Examiner—Shah Kaminis
`Anomey Agent, or F irm-Christopher N. Malvone
`
`[57]
`
`ABSTRACT
`
`A wireless LAN includes ?rst 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 ?elds
`identifying the data rate for the data portion of the message.
`and a length ?eld. For a 2 Mbps n'ansmission the length ?eld
`identi?es the number of bytes in the data ?eld. For a 5 or 8
`Mbps the length ?eld identi?es the number of bytes in the
`data ?eld which. if transmitted at 2 Mbps. would take the
`same transmission time of the data ?eld. and is thus a
`fraction 7/5 or 2/; 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
`
`START
`
`502
`
`DATA
`RATE:5OR \ YES
`8 MBPS
`v
`
`506
`
`CONFIGURED
`FOR ACK RECEIPT
`CONTROL
`
`YES
`
`DECREMENT
`DAYA RATE
`5 C COUNT = D
`
`AC K
`DEFINED RATE
`> DATA RATE
`
`524
`
`INCREME NT DATA RATE
`5 C COUNT = 0
`
`Exhibit 1204 01/12
`
`

`

`US. Patent
`
`Jan. 6, 1998
`
`Sheet 1 of 6
`
`5,706,428
`
`16
`
`F ACCESS EV 17
`12A POINT
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`
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`
`N
`
`182 5
`
`P2} 2
`
`-
`
`Exhibit 1204 02/12
`
`

`

`US. Patent
`
`Jan. 6, 1998
`
`Sheet 2 of 6
`
`5,706,428
`
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`EXNbH120403H2
`
`Exhibit 1204 03/12
`
`
`
`
`
`
`
`
`
`

`

`US. Patent
`
`Jan. 6, 1998
`
`Sheet 3 of 6
`
`5,706,428
`
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`Exhibit 1204 04/12
`
`Exhibit 1204 04/12
`
`
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`
`
`
`
`
`

`

`US. Patent
`
`Jan. 6, 1998
`
`Sheet 4 of 6
`
`5,706,428
`
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`Exhibit 1204 05/12
`
`Exhibit 1204 05/12
`
`
`
`

`

`US. Patent
`
`Jan. 6, 1998
`
`Sheet 5 of 6
`
`5,706,428
`
`506
`
`CONFIGURED
`FOR ACK RECEIPT
`CONTROL
`7
`
`YES
`
`NO
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`504
`
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`
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`DECREMENT
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`
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`524
`
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`
`51 6
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`
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`500
`
`STORE DATA RATE {518
`STORE s c COUNT
`
`END
`
`FIG.7
`
`Exhibit 1204 06/12
`
`

`

`US. Patent
`
`Jan. 6, 1998
`
`Sheet 6 of 6
`
`5,706,428
`
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`Exhibit 1204 07/12
`
`
`
`
`
`
`
`

`

`5 ,706,428
`
`1
`MULTIRATE WIRELESS DATA
`COMMUNICATION SYSTEM
`
`FIELD OF THE INVENTION
`
`10
`
`This invention relates to wireless data communication 5
`systems.
`BACKGROUND OF THE INVENTION
`With a view to obviating the need for wired cabling
`connections between stations in local area networks (LAN 5).
`wireless local area networks have been developed. and are
`now commercially available. These wireless local area net
`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. cln'rently available in draft form.
`which specifies appropriate standards for use in wireless
`LANs. This standard speci?es two possible data rates for
`data transmission. namely 1 Mbps (Megabit per second) and
`2 Mbps. Accordingly. manufacturers have produced com
`mercially available systems operating at these data rates.
`However. it may be advantageous to provide systems oper
`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
`ferent data rates.
`
`25
`
`30
`
`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 format of a data
`message circulating in the LAN;
`FIG. 5 is a diagram illustrating the format of a ?rst type
`of acknowledgement message;
`FIG. 6 is a diagram illustrating the format of a second type
`of acknowledgement message;
`FIG. 7 is a ?owchart illustrating the operation of an
`automatic data rate selection procedure; and
`
`55
`
`65
`
`2
`FIG. 8 is a block diagram of a modi?ed embodiment of a
`LAN station.
`DETAILED DESCRIPTION OF THE
`INVENTION
`Referring ?rst to FIG. 1. 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.
`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
`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
`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
`carrier is utilized. and when operating at the 2 Mbps data rate
`DQPSK (diiferential quadrature phase shift keying) modu
`lation of the RF carrier is utilized. Thus. it will be appreci
`ated that both data rates are equivalent to a symbol rate of
`1 MBaud (Megabaud). i.e. l symbol per second. Preferably
`the DSSS code utilized is an ll-chip Barker code having the
`values
`+1. —1. +1. +1. —1. +1. +1. +1. -1. —l. -l. the leftmost
`chip being the ?rst 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—di?’erential
`quadrature phase shift keying) in combination with the
`ll-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
`I-component. 3 position bits for the Q-oomponent. and 2 bits
`for quadrature phase information. It will be appreciated that
`the 5 and 8 Mbps data rates correspond to a 1 Mbaud symbol
`rate. just as do the l and 2 Mbps data rates.
`From the above description. it will be appreciated that the
`LAN 10 contains mobile stations 18 of a ?rst type (opm'ating
`at l or 2 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 l and 2 Mbps data rates. and further. the
`stations 22 can operate at their higher data rates of 5 and 8
`Mbps. in a manner co-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
`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
`
`SUMMARY OF THE INVENTION
`Therefore, according to the present invention. there is
`provided a method of operating a wireless local area net
`work station adapted to transmit and receive messages at a
`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 ?rst predetermined one of a ?rst plurality of
`data rates; including in said initial portion a data rate
`identi?cation 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 ?rst plurality of data rates; and
`transmitting said data portion at said selected one of said
`second plurality of data rates.
`
`35
`
`45
`
`Exhibit 1204 08/12
`
`

`

`5,706,428
`
`25
`
`30
`
`35
`
`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 l-out-of-2 rate selector 42 and
`a scrambler 44. The rate selector 42 and scrambler 44 are
`connected to a l-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 eifects the above
`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 ?rst
`output of the correlator 54 is connected to carrier detector
`56. A second output of the correlator 54 is connected to a
`l-out-of-2 detector/decoder 58 which has an output con
`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
`trol information contained in received messages.
`Referring now to FIG. 3. there is shown a functional block
`diagram illustrating the interconnection of the functional
`blocks included in a station 22. which relate to the irnple
`mentation of the present invention. The arrangement of
`functional blocks for the stations 22 is similar to that of the
`functional blocks shown in FIG. 2 for the station 18.
`Consequently. similar functional blocks in FIG. 3 are pre
`?xed by an
`1. It will be appreciated that an important
`difference is that the rate selectors 142 and 164 are l-out
`of-4 rate selectors. rather than l-out -of-2 rate selectors. as
`are the selectors 42 and 64 in FIG. 2. Similarly. the encoder
`146 is a l-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 di?erences 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) ?eld 202. a
`16-bit SFD (start of frame delimiter) ?eld 204. an 8-bit
`SIGNAL ?eld 206 (to be explained), an 8-bit SERVICE ?eld
`208 (to be explained). a 16-bit LENGTH ?eld 210 (to be
`explained). a 16-bit CRC check ?eld 212. which provides a
`CRC check for the portions 206. 208 and 210. and ?nally a
`DATA ?eld 214 which comprises a variable number of data
`“octets". that is 8-bit data segments. sometimes referred to
`as "bytes". The ?elds 202 and 204 are together conveniently
`referred to as a preamble 216 and the ?elds 206. 208. 210
`and 212 are together conveniently referred to as a header
`218.
`With regard to the message 200. FIG. 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 DATA ?eld 214. however. may be transmit
`ted at a selected one of the four possible rates 1. 2. 5 or 8
`Mbps. using the modulation and coding discussed herein
`above. Of course. the stations 18 are capable of transmitting
`at the l and 2 Mbps rates only. whereas the stations 22 can
`transmit the DATA ?eld 214 at a selected one of the four data
`rates.
`In more detail concerning the format of the message 200.
`the SYNC ?eld 202 consists of 128 bits of scrambled “1"
`
`4
`bits, enabling a receiving device to perform the necessary
`operations for synchronisation. The SFD ?eld 204 consists
`of a predetermined 16-bit ?eld identifying the impending
`start of the header 218. The SIGNAL ?eld 206 has a ?rst
`predetermined value if the DATA ?eld 214 is transmitted at
`the 1 Mbps rate and a second predetermined value if the
`DATA ?eld 214 is transmitted at the 2. 5 or 8 Mbps rates.
`The SERVICE ?eld 208 has a ?rst predetermined value
`(typically all zero bits) for the 1 and 2 Mbps rates. a second
`predetermined value for the 5 Mbps rate and a third prede
`termined value for the 8 Mbps rate. It should be understood
`at this point that the stations 18. adapted to operate at the l
`and 2 Mbps rates only. ignore the SERVICE ?eld 208. This
`aspect will be discussed more fully hereinafter. The
`LENGTH ?eld 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 ?eld 214. However for the 5 and 8 Mbps
`rates. the LENGTH ?eld 210 contains a value which is a
`fraction. 3/5 and 2At. times the actual number of octets in the
`DATA ?eld 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 ?eld 214.
`which is actually transmitted at 5 Mbps. or 8 Mbps respec
`tively.
`Referring brie?y 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 ?rst
`senses the transmission channel. If the channel is sensed as
`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
`sage does not then take place immediately. but is further
`deferred for a random backoif time. This procedure allevi
`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
`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
`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
`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 2/s and 34; times the
`actual number of octets in the DATA ?eld 214 since this
`representation causes any station receiving it to defer for the
`length of time corresponding to the speci?ed 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
`access point association
`procedure when the station is initially operated in the LAN
`10. Brie?y. 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
`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
`
`45
`
`50
`
`55
`
`65
`
`Exhibit 1204 09/12
`
`

`

`5.706.428
`
`10
`
`25
`
`30
`
`35
`
`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
`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
`brie?y described.
`The MAC management state machine M-MST 134
`(FIG.3) includes a table (not shown) containing information
`relating to other stations. identi?ed by their station E) 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 (FIGA) over the line 140 for transmission. The receive
`state machine R-mst 138 handles the timed control and
`forwarding 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-MSI‘ 134 to
`ascertain the data rate to be applied to the transmission to
`that station. The C-MSI‘ 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-MS'I‘ 134. Also. the
`C-MST 132 adds the LENGTH ?eld 210. which. for 5 and
`8 Mbps bits rates is. as discussed hereinabove. a fraction. 2{5
`or 7/3. of the actual length in octets of the DATA ?eld 214.
`As discussed hereinabove. the SIGNAL ?eld 206 used at 5
`and 8 Mbps data rate is the same as the SIGNAL ?eld 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 ?eld 202. followed by
`the SFD ?eld 204. the header 218 and the DATA ?eld 214.
`The rate selector 42 utilizes the SIGNAL ?eld 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 l
`MBaud rate for application to the spreader 48 where the
`DSSS coding is effected. The RF transmitter 50 then effects
`conventional ?ltering. tip-mixing and power ampli?cation 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 ?ltering. automatic gain control and
`down-mixing. The output signal from the RF receiver 52 is
`applied to the correlator 54. which produces a spike
`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 dcscrambler 60. After the SFD
`?eld 204 has left the descrambler 60 the rate selector 64 uses
`the SIGNAL ?eld 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 ?eld 214 will be descrambled in the dcscrambler 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-MSl"
`132 inserts the preamble 216 and header 218. As mentioned
`hereinabove. the SIGNAL ?eld 206 is the same for the 5 and
`8 Mbps data rates as for the 2 Mbps data rate. but the
`SERVICE ?eld 208 differs. The rate selector 142 uses the
`SIGNAL and SERVICE ?elds 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
`DQPSK mode. 5 Mbps PPM/QPSK mode or the 8 Mbps
`PPM/QPSK mode. whereby the DATA ?eld 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 ?elds 206. 208
`to determine whether to remain in the 1 Mbps mode or
`switch to the 2.5 or 8 Mbps mode. If the SIGNAL ?eld 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.
`5 or 8 Mbps mode. dependent on the value of the SERVICE
`?eld 208. Thus the DATA ?eld 214 is descrambled in the
`dcscrambler 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
`station. using a destination address included in the data ?eld
`214 of the message 200. If the address matches. and the
`C-MST has checked a CRC ?eld (not shown) that is part of
`the data ?eld 214. then assuming there is no error. the
`C-MST forwards the data ?eld 214 for further processing in
`the station. and forwards the data rate information to the
`M-MST 134, for storage in the aforementioned table under
`the relevant station 11). Note also that. following the receipts
`of the header 218. and assuming a correct CRC check for the
`CRC ?eld 212. the rate selector 164 is controlled to operate
`the detector/decoder 158 at the correct signalling rate of l.
`2. 5 or 8 Mbps. as indicated by the contents of the SIGNAL
`and SERVICE ?elds 206 and 208. An octet counter (not
`shown) is updated until the last detected symbol of the data
`?eld 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 identi?ed by their ID. Referring now to FIGS. there
`is shown the format of anACK (acknowledgement) message
`300 used in the LAN 10. The format of the ACK message
`300 is generally similar to the format of the message 200
`(FIG. 4). and includes a SYNC ?eld 302 and a SFD (start of
`frame delimiter) ?eld 304. a SIGNAL ?eld 306. a SERVICE
`?eld 308. a LENGTH ?eld 310. and a CRC ?eld 312. The
`?elds 302 and 304 form a preamble 316 and the ?elds 306.
`308. 310 and 312 form a header 318. Also included in the
`ACK message 300 is a DATA ?eld 314 which contains a
`l6-bit FRAME CONTROL (FC) ?eld 320. a l6-bit DURA
`TION ?eld 322. a 48-bit RECEIVER ADDRESS (RA) ?eld
`324 and a 32-bit CRC check ?eld 326. Thus the DATA ?eld
`314 contains a total of 14 octets. The ACK message DATA
`?eld 314 may be transmitted at the 1 Mbps rate or the 2
`Mbps rate. as identi?ed in the SIGNAL ?eld 306. The ACK
`frame 300 is used by the stations 18 and is also used by the
`stations 22 when operating at the l or 2 Mbps rate. However.
`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.
`
`45
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`50
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`55
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`65
`
`Exhibit 1204 10/12
`
`

`

`10
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`20
`
`25
`
`7
`Referring to FIG. 6. there is shown the format of a short
`ACK message 400. preferably used by the stations 22 when
`operating at the 5 or 8 Mbps rate. The short ACK message
`400 includes a 76-bit SYNC ?eld and an SFD (start-of
`?'ame delimiter) ?eld 404. together forming a preamble 406.
`The preamble 406 is followed by a data ?eld 408 which
`include an 8-bit station ID ?eld 410 and a 2-bit ?eld 412
`identifying a preferred data rate. The preferred data rate is
`derived in a receiving station. dependent on receive quality
`condition and a SNR (signal-to-noise) value with respect to
`a message received from a transmitting station.
`Referring now to FIG. 7. there is shown a ?owchart 500
`illustrating an automatic data rate update procedure for the
`data rate to be used in the transmit mode. which is irnple
`mented in the preferred embodiment described herein for a
`station 22. The ?owchart 500 begins at start block 502.
`Accordingly. from the start block 502. the ?owchart 500
`proceeds to block 504. where a determination is made as to
`whether the data rate is 5 or 8 Mbps. If so. the ?owchart
`proceeds to block 506 (to be described). If not. the ?owchart
`proceeds to block 508 where a determination is made as to
`whether the ACK has been received and within a predeter
`mined time-out time. If yes. the ?owchart proceeds to block
`510. where a successive correct (SC) count value is incre
`mented. Next. as seen in block 512. a check is made as to
`whether the SC count value is greater than a predetermined
`value. selected as value 9. by way of example. In other
`words. a check is made as to whether more than nine
`successive ACK messages have been correctly and timely
`received. If yes. the ?owchart proceeds to block 514 where
`a check is made as to whether the local SNR (signal-to-noise
`ratio) value is greater than a predetermined value. suitable
`for data rate incrementation. (The SNR is the ratio of
`received signal strength during the reception of the ACK
`message to the average silence level during periods at which
`no carrier signal is being received). If the SNR value is
`suitable. then the ?owchart proceeds to block 516. where a
`data rate incrementation is implemented (if the maximum
`data rate is not already being used). and the SC (successive
`correct) count value is reset to zero. Thereafter. the data rate
`value and SC count value are stored (block 518). and the
`?owchart ends at block 520.
`Returning to block 508, if an ACK message is not
`received correctly and within the predetermined time
`interval. then the ?owchart proceeds to block 522 where the
`SC count value is reset to zero and the data rate is decre
`mented (if the minimum data rate is not already being used).
`and the ?owchart proceeds over line 524 to block 518 where
`the new data rate and SC count value are stored. It should be
`noted also that if either block 512 or block 514 results in a
`negative determination. the ?owchart also proceeds over
`line 524 to block 518.
`Returning now to block 504. if it is determined that the
`data rate is 5 or 8 Mbps. then the ?owchart proceeds to block
`506. where a determination is made as to whether the system
`is con?gured for overruling the preferred data rate by a data
`rate de?ned by monitoring the receipt of ACK messages. If
`no. the ?owchart proceeds to block 508. previously dis
`cussed If yes. the ?owchart proceeds to block 526. where a
`determination is made as to whether the preferred data rate
`de?ned in the short ACK message 400 (FIG. 6) is greater
`than the actual data rate of the original message being
`acknowledged. If so. the ?owchart proceeds to block 516
`where the data rate is incremented and SC count value is
`reset to zero. If not. the ?owchart proceeds directly to block
`518 where the data rate and SC count value are stored.
`To summarise the procedure described above with refer
`ence to the ?owchart 500. it will be appreciated that an
`
`35
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`40
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`45
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`50
`
`5 ,706.428
`
`8
`automatic data rate selection procedure has been described
`At a lower data rate the transmission of data is more robust
`because the detection margin is larger at lower data rates. At
`a higher data rate the requirements with regard to channel
`conditions such as SNR. SIR (co-channel interference) and
`delay spread. are more stringent. If a station 22 doesn’t
`receive the expected ACK message in return correctly and in
`due time. it will retransmit the original message packet at a
`lower data rate. If a station 22 does receive the expected
`ACK messages correctly and in due time from a particular
`station for a predetermined number of successive times. then
`it will transmit the next message to that station at a higher
`data rate. In this way the stations 22 adapt the operating data
`rate dependent on channel conditions (degradation by
`noise-SNR. time dispersion in the channel-delay spread)
`and co-channel interference (SIR).
`As mentioned above. the stations 22 preferably use a short
`ACK message (FIG. 6) when operating at the 5 or 8 Mbps
`data rates. This ACK message has a duration of only 90
`microseconds. in contrast to the ACK message 300 of FIG.
`5. which lasts for about 300 microseconds at the 1 Mbps
`rate. or about 250 microseconds at the 2 Mbps rate. It will
`be appreciated that stations 18 which detect the transmis
`sions of a short ACK message 400 (FIG.6) will defer until
`the ACK message has ended. since the ACK message 400
`uses DBPSK