United States Patent
`
`
`
`
`
`Natarajan et al.
`
`[19]
`
`
`.
`
`
`
`USOO524l 542A
`[11] Patent Number:
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`
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`
`
`
`[45] Date of Patent:
`
`
`
`5,241,542
`
`
`Aug. 31, 1993
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`[75]
`
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`[54] BATTERY EFFICIENT OPERATION OF
`
`
`
`
`
`
`
`SCHEDULED ACCESS PROTOCOL
`Inventors: Kadathur S. Natarajan, Millwood;
`
`
`
`
`
`
`
`Chin-Chi Huang, Yorktown Heights,
`both of N'Y'
`
`
`
`
`
`International Business Machines
`Corporation, Armonk, N.Y.
`
`
`
`
`
`
`[73] Assignee:
`
`
`
`
`
`
`
`
`[2]] App]. No; 749,234
`
`1221
`um
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`
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`
`
`[51]
`Int. c1.s ...................... .. H04B 7/212; HO4B 7/26
`[52] U.S. Cl. .................................. 370/953; 370/85.2;
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`455/333; 455/541; 455/343
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`[58] Field of Search ................. .. 370/95.1, 95.3, 104.1,
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`370/29’ 351’ g5_7, 94_1; 455/511, 54.1’ 343,
`
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`3&3; 340/82544, 31 L1; 379/53
`
`
`I55}
`
`
`
`References Cited
`us. PATENT DOCUMENTS
`
`
`.,
`
`""""""""""
`Z‘;-Islgfirgt
`
`
`
`
`2/1991 Davis _____H
`340/82544
`4,995:O99
`
`
`
`
`3/1992 Hewitt ............................... .. 455/343
`5,o9s.3os
`
`
`
`
`
`5,l44.296 9/1992 DeLuca et al.
`................... .. 455/343
`
`
`
`
`
`
`9/1992 Wieczorek et al.
`.............. .. 370/95.1
`5,150,361
`
`
`
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`
`
`
`
`
`
`MEIBILE STATIUN
`
`
`
`l0,12,14,ElR16
`
`
`
`SDFTVARE
`
`
`fig
`
`
`44
`
`
`
`
`
`
`
`
`
`
`FOREIGN PATENT DOCUMENTS
`2232326 12/1990 Umted Kingdom .................. 370/29
`
`
`
`
`
`
`
`OTHER PUBLICATIONS
`William Stallings, “Data and Computer Communica-
`
`
`
`
`
`
`
`
`
`
`
`
`tions", Macmillan Publishing Company,
`1985, PP-
`312-315.
`
`
`
`
`Olms
`Primary Examiner—Douglas
`
`
`
`:i::::::;;i§;:;i"::;£*;s_i:::E:2?1mo1d
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`7
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`method and apparatus for conserving battery power
`
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`In 8 Wireless link adapter of 8 battery Operated C°m°
`
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`puter such as a portable laptop computer, as controlled
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`by a scheduled multiaccess protocol. The portable com-
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`network. The scheduled access multiaccess protocol is
`implemented to effectively conserve battery power by
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`suitable control of the state of the controller, the trans-
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`mitter and receiver units at the wireless link adapterby
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`scheduling when the adapter 1s
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`

`
`U.S. Patent
`
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`
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`Aug. 31, 1993
`
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`
`Sheet 1 of 9
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`5,241,542
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`2
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`

`
`U.S. Patent
`
`Aug. 31, 1993
`
`Sheet 2 of 9
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`5,241,542
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`U.S. Patent
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`Aug. 31, 1993
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`Sheet 3 of 9
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`
`U.S. Patent
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`Aug. 31, 1993
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`Sheet 4 of 9
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`5,241,542
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`

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`U.S. Patent
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`Aug. 31, 1993
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`Sheet 5 of 9
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`5,241,542
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`80
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`RECEIVE HEADER AH
`
`
`FDR PERIUD A
`
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`
`I
`
`32.
`
`
`FIG. 8A
`
`
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`
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`INCLUDED IN THE
`AM I
`
`
`
`LIST UF RECEIVING
`
`
`MUBILE UNITS
`
`
`RLIST=(
`U1,...,UN)?
`
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`NU
`
`
`
`
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`
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`i-——MY PUSITIDN IN RIIISTJ
`,_
`
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`SLEEP_DURATIElN-— Z S]
`
`J'=0
`
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`86
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`
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`FIG. 8
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`
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`SLUTS HAVE ELAPSED;
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`RECEIVE PACKETS FUR
`
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`THE NEXT S; SLDTS
`
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`6
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`

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`U.S. Patent
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`Aug. 31, 1993
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`Sheet 6 of 9
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`5,241,542
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`
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`FIG. 8B
`
`
`
`
`96
`
`
`
`
`
`
`
`TURN RECEIVER EJFFJ
`
`
`
`ENTER SLEEP MUDEJ
`
`
`WAKE UP AFTER
`
`SLEEP_DURATIUN
`
`
`SLEITS HAVE ELAPSED
`
`
`
`
`
`
`
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`
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`RECEIVE HEADER BH FUR PERIUD B
`
`
`
`
`
`TURN RECEIVER UFF FDR DURATIEIN TB
`
`
`
`
`
`98
`
`
`
`
`
`IN
`AM I
`
`
`
`
`THE LIST UF MUBILE
`
`
`
`
`UNITS ALLDVED TU TRANSMIT IN THIS
`
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`
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`
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`
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`
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`ENTER SLEEP MUDE
`
`
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`
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`
`
`SLEEP_DURATIE|N
`
`
`SLUTS HAVE ELAPSED
`
`
`
`we
`
`
` 110
`
`
`
`TURN TRANSMITTER UN}
`
`
`
`TRANSMIT FUR ‘ti SLDT_S1
`
`
`
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`
`SLEEP_DURATIDN --—TB -_ 2+}
`1 =4
`
`
`
`
`
`
`7
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`

`
`US. Patent
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`Aug. 31, 1993
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`Sheet 7 of 9
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`5,241,542
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`111
`
`
`
`
`
`
`ENTER SLEEP MODE FOR TRANSMITTER
`
`
`FUR SLEEP_DURATIElN
`
`
`WAKE UP RECEIVER AFTER
`
`
`
`PERIUD B HAS ELAPSED
`
`
`
`
`
`
`FIG. 8C
`
`
`
`
`113
`
`
`
`
`
`
`
`RECEIVE HEADER CH FUR PERIUD C
`114
`
`
`
`
`AND TURN RECEIVER EIFF
`
`116
`
`
`
`
`
`no I HAVE ANY
`
`
`
`
`PACKET TD TRANSMIT IN RANDUM
`
`
`
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`
`ACCESS MUDE (PERIDD c > 7
`
`NU
`
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`132
`G__IG.8m
`
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`YES
`
`
`
`
`118
`(FIG.8D)
`
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`FDLLDV SLDTTED PRDTEJCDL AND SCHEDULE
`
`
`
`
`
`TRANSMISSIDN IN SLUT T INTO THE FUTURE
`
`
`
`SLEEP_DURATIUN <-— T-1
`
`180
`
`
`
`
`
`
`TURN TRANSMITTER DFFJ
`
`
`
`ENTER SLEEP MDDEI
`
`
`
`128
`
`
`
`
`
`
`
`WAKE UP TRANSMITTER SLEEP_DURATIDN
`HAS ELAPSED AND TRANSMIT AT SLUT T
`
`
`
`
`
`
`
`
`
`
`
`
`8. THEN T30 T13 SLEEP (TRANSMITTER)
`
`134
`
`
`
`
`
`
`
`136
`
`
`
`
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`
`
`
`
`
`
`
`
`WAKE UP RECEIVER AND RECEIVE
`ACK/NAK MESSAGE AT SLnT
`
`
`
`
`
`
`
`
` T +A(A= DELAY FUR GENERATING
`
`
`
`THE ACK/NAK MESSAGE) &
`
`
`
`
`
`GD TD SLEEP RECEIVER
`
`
`
`8
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`

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`U.S. Patent
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`
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`Aug. 31, 1993
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`Sheet 8 of 9
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`5,241,542
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`128
`FIG. so
`
`
`
`
`
`
`ANY MORE PACKET
`YES
`
`
`
`
`
`
`TD TRANSMIT IN RANDUM
`
`
`ACCESS MEIDE 7
`
`
`ND
`
`
`
`13.?
`(FIG.8C)
`
`
`118
`(FIG.8C)
`
`
`
`130
`
`
`
`
`SLEEP_DURATIl'JN <‘-' REMAINING
`
`
`
`TIME IN PERIEID C
`
`134
`
`
`
`
`
`
`
`
`
`TURN TRANSMITTER AND RECEIVER DFFJ
`
`
`
`ENTER SLEEP MUDE
`
`
`
`136
`
`
`
`138
`
`
`
`
`
`
`WAKE UP AFTER
`SLEEP_DURATIUN
`
`
`HAS ELAPSED
`
`
`
`
`
`
`CHANGE CARRIER FREQUENCY
`
`
`
`(IN A FREQUENCY HEIPPING SYSTEM)
`
`
`
`140
`
`
`
`
`
`
`
`80
`
`(FIG.8A)
`
`9
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`

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`U.S. Patent
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`Aug. 31, 1993
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`Sheet 9 of 9
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`158
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`158
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`PRUXIM_ RADIU
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`WAKEUP
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`166
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`154
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`}
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`MEMURY
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`AND LEJCIC
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`156
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`RADIEI
`CONTROLLER
`150
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`10
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`‘W315
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`TRANSMIT
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`4 so
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`SLEEP
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`T§§gR
`AND
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`SLEEP
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`TIMEUUT
`VAKEUP
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`1
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`5,241,542
`
`BATTERY EFFICIENT OPERATION OF
`
`
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`SCHEDULED ACCESS PROTOCOL
`
`
`
`
`5
`
`FIELD OF THE INVENTION
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`
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`The invention is in the field of wireless communica-
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`tions, and in particular is directed to power conserva-
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`tion due to wireless communication. Specifically, the
`
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`
`invention is directed to battery efficient operation of 10
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`wireless link adapters of mobile computers as controlled
`by multiaccess protocols used in wireless communica-
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`
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`tion.
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`15
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`20
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`25
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`30
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`35
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`BACKGROUND OF THE INVENTION
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`In order to obtain true portability in micro-computers
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`and workstations, battery powered operation is essen-
`tial. Moreover, given the capacity versus size limita-
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`tions of known batteries, it is essential to minimize total
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`power consumption in order to extend the operating life
`of the batteries.
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`It is relatively easy to reduce battery consumption by
`an initial 60 to 70 percent in the computer portion of a
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`mobile station. This initial savings can be accomplished
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`by simply turning selected pieces of hardware in the
`computer portion off when they are not being used. The
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`last 30 to 40 percent savings in the computer portion
`becomes increasingly more difficult to achieve, while
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`simultaneously becoming increasingly more valuable in
`terms of extending battery life. This is due to the inverse
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`relationship between battery life and battery load. Ac-
`cordingly, savings that would seem trivial in off-line
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`applications, might be momentous in a battery powered
`environment.
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`While the above is directed to the computer portion,
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`to date no work has been done relative to the wireless
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`link adapter portion of the mobile station.
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`There has been recent work directed to the design of
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`multiaccess protocols for portable mobile computer 40
`users, as well as movable boundary protocols for sup-
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`porting integrated voice/data users in mobile indoor
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`radio networks. The schemes proposed to date do not
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`take into explicit account the effective conservation of
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`battery power used by the multiaccess scheme relative 45
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`to the wireless link adapter.
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`Since portable laptop computers run on battery
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`power, the implementation of a multiaccess protocol
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`described in this invention attempts to minimize the
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`consumption of battery power in a wireless link adapter
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`to the minimum amount required, as a function of the
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`protocol.
`According to this invention, several techniques are
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`disclosed for minimizing the battery power wasted at
`the wireless link adapters of mobile units, as controlled
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`by a multiaccess protocol for wireless communication.
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`DISCLOSURE OF THE INVENTION
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`50
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`55
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`Method and apparatus is described for the battery
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`efficient operation of wireless link adapters of mobile
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`computers as controlled by scheduled multiaccess pro-
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`tocols for wireless communication.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`65
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`FIG. 1 is a pictorial diagram showing an indoor digi-
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`tal data communication system of the type in which the
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`invention is implemented;
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`11
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`2
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`FIG. 2 is a block diagram of the system shown in
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`FIG. 1 illustrating the basic components of a mobile
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`station and a base station;
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`FIG. 3 is a block diagram of the radio system used in
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`the implementation of a preferred embodiment of the
`invention;
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`FIG. 4 is a diagram of the frame structure of a multi-
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`access protocol for describing the battery efficient oper-
`ation of a wireless link adapter according to the inven-
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`tion;
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`FIG. 5 is a diagram of a Receiving Users Index mes-
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`sage as a bit-vector 64 bits long;
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`FIG. 6 is a diagram of receiver allocations for the
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`multiaccess protocol;
`FIG. 7 is a diagram of transmitter allocations for the
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`multiaccess protocol;
`,
`
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`FIG. 8A—8D, when taken together as shown in FIG.
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`8, are block diagram representations of how the battery
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`efficient operation of the wireless link adapter is con-
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`trolled by the multiaccess protocol, as implemented
`according to the invention;
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`FIG. 9 is a block diagram of how a controller in the
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`wireless link adapter sets a time to accomplish the im-
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`plementation set forth in FIGS. 8A-8D.
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`DETAILED DESCRIPTION OF THE
`
`
`INVENTION
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`The invention is described relative to operation in a
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`wireless radio communications link. It is to be appreci-
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`ated that the invention is also applicable to other wire-
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`less communication links such as infrared links as well
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`as microwave links. FIG. 1 depicts mobile stations 10,
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`12, 14, and 16 that communicate via wireless links.
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`Gateways, referred to as base stations, are augmented
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`according to the invention to provide certain radio
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`system management functions which coordinate the
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`mobile stations’ access to the common radio channel.
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`Communications between mobile stations is supported
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`via relay through the base stations 26 and 28.
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`As shown in more detail in FIG. 2, a base station 26
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`or 28, which may be conventional microcomputer, has
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`a LAN adapter 30 inserted in a bus slot and connected
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`to LAN cabling 32. A server 18, typically also a con-
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`ventional microcomputer and including one or more
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`direct access storage devices (DASDs) such as hard
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`disks (not shown), also has a LAN adapter 34 inserted in
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`a bus slot and connected to LAN cabling 32. The LAN
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`adapters 30 and 34 and the LAN cabling 32 together
`with LAN software constitute the LAN 24. The LAN
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`24 is of conventional design and does not form part of
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`the invention. The base station 26 or 28 also has an RF
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`transceiver adapter 36 implemented as a printed circuit
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`card which is inserted in a bus slot of the base station.
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`The transceiver adapter 36 includes a spread spectrum
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`transceiver of conventional design. The transceiver
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`adapter 36 has an antenna 38 by which a radio link 40 is
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`established with one or more remote or mobile stations,
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`10, 12, 14, or 16. The mobile station may itself be a hand
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`held or laptop computer such as an IBM PS/2 Model
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`L40 SX laptop computer as described in Technical
`
`
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`
`
`Reference Manual, Order Number: S/5F-2270, Part
`
`
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`l5F2270, which may be ordered from an IBM
`No.
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`
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`Authorized Dealer.
`
`
`IBM and PS/2 are registered Trademarks of Interna-
`
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`tional Business Machines Corporation. The laptop com-
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`like the base station,
`is provided with an
`puter
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`antenna 42
`and a
`transceiver
`adapter 44,
`also
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`implemented as
`a printed circuit card which is
`
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`bus
`slot of
`computer. The
`inserted
`in
`a
`the
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`11
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`

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`5,241,542
`
`I0
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`15
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`20
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`25
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`30
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`35
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`45
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`50
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`3
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`transceiver adapter 44, like transceiver adapter 36, in-
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`cludes a spread spectrum transceiver of similar design.
`The base station and the mobile stations are further
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`provided with software, generally indicated by refer-
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`ence numerals 46 and 48, respectively, which support
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`their respective transceiver adapters.
`FIG. 3 shows the radio system common to both the
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`mobile stations and the base stations of FIG. 1. The
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`radio system includes a transceiver adapter 36 or 44
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`connected to a computer 50 via a computer's bus inter-
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`face 52. The transceiver adapter 36 section is itself di-
`vided into an RF transceiver 54, which may be com-
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`mercially available spread spectrum transceiver, and a
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`dedicated microprocessor system 56 which controls the
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`transceiver via an interface 58. The microprocessor
`system 56 further includes a system interface 60 which
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`interfaces the transceiver adapter 36 section to the com-
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`puter section 50. The microprocessor system includes a
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`dedicated microprocessor 62 containing high-resolution
`time interval determination hardware or “timers” typi-
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`cal of real-time microprocessor systems.
`Microprocessor 62 is connected by a memory bus 64
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`to program storage 66 and data storage 68 as well as to
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`interfaces 60 and 58 providing attachment to bus inter-
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`face 52 and RF transceiver 54, respectively. Program
`storage 66 is typically read only memory (ROM), while
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`data storage 68 is static or dynamic random access
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`memory (SRAM or DRAM). Packets received or to be
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`sent are held in data storage 68 and communicated to or
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`from the RF transceiver 54 via interface 58 under con-
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`trol of serial channels and a direct memory access
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`(DMA) controller (not shown) which is part of the
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`microprocessor 62. The function of these serial channels
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`is to encapsulate data and control
`information in an
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`HDLC (high-level data link control) packet structure
`and provide the packet in serial form to the RF trans-
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`ceiver 54. For more information on the HDLC packet
`
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`structure, see, for example, Mischa Schwartz, Telecom-
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`munication Networks: Protocols, Modeling and Analy-
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`sis, Addison-Wesley (1988).
`When a packet is received through the RF trans-
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`ceiver 54, the serial channels check the packet destina-
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`tion address, check for errors, and deserialize the packet
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`to data storage 68. The serial channels must have the
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`capability to recognize a specific adaptor address as
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`well as a broadcast address. Specific microprocessors
`
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`with appropriate serial channel and timer facilities in-
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`
`
`clude the Motorola 68302 and the National Semicon-
`
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`
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`
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`ductor HPC4640OE microprocessors.
`
`
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`The computer 50 runs an operating system 70 which
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`supports one or more user application programs 72. The
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`operating system 70 may include a communications
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`manager 74, or the communications manager 74 may
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`itself be an application program installed on the com-
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`55
`puter. In either case, the communications manager 74
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`controls a device driver 76 via the operating system 70.
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`The device driver 76, in turn, communicates with the
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`transceiver adapter 36 or 44 via bus interface 52.
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`In general,
`the main idea for minimizing battery
`power consumed by wireless link adapters at the mobile
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`units is as follows. The transmitter and receiver units at
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`the communication adapter(s) of a portable terminal
`
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`expend some power that depend on their states (OFF
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`
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`
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`state/ON state/ACTIVE ON). The ratio of transmit-
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`to-receive power depends also on the type of wireless
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`link adapter used (radio or infrared). Scheduled access
`
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`
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`multiaccess protocols can be implemented to effectively
`
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`conserve battery power by suitable control of the state
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`4
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`of transmitter and receiver units at the portable units
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`(i.e., by scheduling when they should be turned ON or
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`OFF). A desirable solution is one in which the transmit-
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`ter (or receiver) consumes power only when it is ac-
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`tively transmitting a message (or actively receiving a
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`message). Traditional multiaccess protocols do not have
`
`
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`
`
`the above desirable characteristics because:
`A receiver may consume power while waiting to
`
`
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`
`
`receive a packet. Some examples are as follows:
`
`
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`
`
`A mobile station may be waiting to hear a polling
`
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`message from the base station station before it can
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`transmit a packet.
`
`
`Outbound messages are broadcast from the base sta-
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`tion. Mobiles keep their receiver ON in order to
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`receive packets that may be addressed to them.
`
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`Battery power is wasted in receiving packets that
`are addressed to others.
`
`
`
`A transmitter may consume power while waiting to
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`
`
`transmit a packet.
`
`
`In the scheme described here, a scheduled multiac-
`
`
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`
`
`cess protocol is used in which time is divided into fixed-
`
`
`
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`
`
`
`
`length frames, and frames are divided into slots as
`
`
`
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`
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`
`
`
`shown in FIG. 4. It is to be appreciated that different
`
`
`
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`
`
`
`frame divisions and header lengths and content may be
`
`
`
`
`
`
`
`
`utilized in the practice of the invention, and the scheme
`
`
`
`
`
`
`
`
`
`
`
`
`
`set forth here is merely exemplary.
`The beginning of the frame is a header G of fixed
`
`
`
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`
`
`
`
`
`
`
`
`
`
`length FH. The frame is divided into multiple subframes
`as described below.
`
`
`
`Period A for broadcast of packets from base station to
`
`
`
`
`
`
`mobile units (outbound traffic), with a header AH
`
`
`
`
`
`
`
`
`for period A.
`Period B for the contention-free transfer of all traffic
`
`
`
`
`
`from mobile units to base station (inbound traffic),
`
`
`
`
`
`
`
`with a header B}! for period B.
`
`
`
`Period C for the transfer of all bursty data traffic in a
`
`
`
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`contention mode from mobile units to base station
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`(inbound traffic), with a header CH.
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`Referring to FIG. 1, in conjunction with FIG. 4, in
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`interval A the base station 26 or 28 controls the trans-
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`missions outbound to the mobile units 14. The corre-
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`sponding header (AH) control information for this in-
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`terval is broadcast reliably by the base station, and is
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`assumed to be received correctly by relevant mobile
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`stations. The header includes:
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`A list of mobile users (U 1, U2, .
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`receiving data from the base station in the current
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`frame and the order in which they will receive
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`packets.
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`Bandwidth allocated to users in this frame (S1, S2,
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`, S,,), where S,-, is the number of packets that will
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`be directed to User U; from the base station in the
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`current frame.
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`In the description that follows, a transmitter or re-
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`ceiver is considered ON or awake when it is in a normal
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`running mode. A transmitter or receiver is considered
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`OFF or asleep when it is in an idle or standby mode.
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`The following is a general description of the method
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`of battery efficient control of a wireless link adapter as
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`controlled by a scheduled multiaccess protocol for
`wireless communication as shown in FIG. 4, a more
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`detailed description is set
`forth relative to FIGS.
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`8A-8D. On correct reception of the above broadcast
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`infonnation as shown in FIG. 4, a mobile unit that is not
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`included in Header AH can turn its receiver OFF for a
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`time duration TA (total number of slots allocated to
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`interval A). The adapter of each receiving mobile unit
`can compute exactly when it should be ready to receive
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`65
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`12
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`12
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`5
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`packets from the base station (add up the slots allocated
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`to all receiving units that precede it). Each receiving
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`mobile unit goes to sleep after scheduling to wake itself
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`up at its designated time for receiving data. After re-
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`ceiving its packets, the mobile unit goes to sleep for the
`remainder of period A. At the end of the A interval, all
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`mobile units turn their receiver ON and receive Header
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`BH corresponding to the B interval.
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`Header BH contains an ordered list of users that will
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`be allowed to transmit to the base station in the current
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`frame.
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`, V,,) that are
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`A list of mobile stations (V 1, V2,
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`allowed to transmit packets to the base station in
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`the current frame and the order in which they
`should transmit.
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`Bandwidth allocated to mobile stations in this frame
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`(t1, t2,
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`that the mobile station V,- can transmit in the cur-
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`rent frame.
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`Using the information regarding the number of pack-
`ets that each user can transmit, each mobile unit can
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`compute exactly when it should begin its transmission.
`Once each mobile station computes its exact time for
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`transmission, it can schedule to wake up its transmitter
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`at its designated time and then go to sleep (i.e., shut both
`it s transmitter and receiver OFF). At its designated
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`time, the transmitter at a mobile station can turn ON
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`and transmit for a fixed period of time whose duration
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`depends on the number of slots allocated to it.
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`It is pointed out that explicit polling messages are not
`used to address users individually. The advantage of
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`avoiding explicit polling on a peruser basis is the follow-
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`ing. Each polling message spacing incurs a fixed over-
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`head time per station that is polled. The overhead is
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`independent of whether the station has anything to
`transmit or not. The polling overhead is the sum of the
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`following components:
`Time to transmit a polling message.
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`Effective propagation time for the polling message
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`(this includes the delay due electromagnetic radia-
`tion plus the radio turnaround time from receive-
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`to-transmit mode at the mobile unit that responds
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`to a poll message and radio turnaround time at the
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`base station itself) could significantly impair perfor-
`mance in radio-based networks.
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`The execution of scheduled access in the manner
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`described here has the following advantages over ex-
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`plicit polling schemes. They are summarized below:
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`Effective elimination of the polling overhead for all
`inbound traffic from mobile users.
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`More importantly, the mobile stations economize the
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`use of battery power by utilizing the control infor-
`mation contained in the AH (Outbound) and BH
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`(Inbound) headers and scheduling their transmit-
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`55
`ters/receivers to be turned ON just in time and
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`turned OFF at the earliest opportunity.
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`In interval C (Random Access or Contention Mode),
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`only those mobile units that do not wish to transmit go
`to sleep (by turning both their transmitter and receiver
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`OFF) till the end of the current frame. The inbound
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`traffic from remaining mobile units may include: regis-
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`tration requests (that enable mobile stations to request
`the services of the base station), bandwidth reservation
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`requests for use in interval B, single packet messages
`etc. A mobile unit executing a random access protocol
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`wakes up its transmitter at its designated time and trans-
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`mits and then goes to sleep. The receiver at the mobile
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`unit can wake itself up to receive acknowledgement
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`5
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`10
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`15
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`20
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`25
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`30
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`35
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`45
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`50
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`60
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`65
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`13
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`5,241,542
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`6
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`messages at the correct time and then go to sleep. It is
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`likely that battery power savings resulting from effi-
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`cient operation of interval C may be significantly less
`compared to those realized in the contention-free inter-
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`vals A and B.
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`Next a brief outline of a scheme for implementing
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`battery efficient execution of a simple version of the
`Scheduled Access Protocol for wireless communication
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`is described. The implementation is based on the con-
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`cept of broadcasting short user activity (in transmit and
`receive modes) indexing messages in the header sections
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`of a frame. For purpose of description, assume that in
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`each frame a mobile unit is allocated at least one slot for
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`receiving traffic and at least one slot for transmitting
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`'-
`traffic.
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`In Period A, before the base station broadcasts mes-
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`sages to receivers, it includes a Receiving Users Index
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`in the Header AH section of the frame. The Receiving
`Users Index is a coded description of mobile users that
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`will receive data in the current frame. That is, it is a
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`designation of which mobile users are to communicate
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`with the base station during this frame. All the mobiles
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`listen to this designation or indexing message and all
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`Receive-Inactive users (i.e., users who do not have a
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`message coming in from the base station in the current
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`frame) can simply turn their receiver power OFF until
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`the beginning of Header BH for Period B. At that time
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`all the mobiles turn their receivers ON and listen to
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`Transmitting Users Index in the Header BH section of
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`the frame. The receivers are turned OFF until the be-
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`ginning of Header CH for Period C. The Transmitting
`Users Index is a coded desi