(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(19) World Intellectual Property
`Organization
`International Bureau
`
`(43) International Publication Date
`20 January 2005 (20.01.2005)
`
`
`
`(10) International Publication Number
`WO 2005/006659 A1
`
`(51) International Patent Classification’:
`H04Q 7/32, GO6F 1/32
`
`HO4L 12/28,
`
`TM,TN,TR, TT, TZ, UA, UG, US, UZ, VC, VN, YU, ZA,
`ZM, ZW.
`
`(21) International Application Number:
`PCT/FI2003/000559
`
`(22) International] Filing Date:
`
`11 July 2003 (11.07.2003)
`
`(25) Filing Language:
`
`(26) Publication Language:
`
`English
`
`English
`
`(71) Applicant (for all designated States except US): NOKIA
`CORPORATION [FI/FI]; Keilalahdentie 4, FIN-02150
`Espoo (FT).
`
`(72) Inventors; and
`SINIVAARA,
`(75) Inventors/Applicants (for US only):
`Hasse [FI/FI]; Tahkorinne 19 A 1, FIN-02760 Espoo (FD.
`KASSLIN, Mika [HI/LIJ; Seljamaki 6 A, VIN-02770
`Espoo (FI). JOKELA, Jari [FI/FI]; Makisenkentintie 6,
`FIN-33480 Yléjarvi (FI.
`
`(74) Agent: KOLSTER OY AB; Iso Roobertinkatu 23, P.O.
`Box 148, FIN-00121 Helsinki (FD.
`
`(34) Designated States (regional): ARIPO utility model (GH,
`GM,KE, LS, MW, MZ, SD, SL, SZ, TZ, UG, ZM, ZW),
`ARIPOpatent (GH, GM, KE, LS, MW, MZ, SD, SL, SZ,
`TZ, UG, ZM, ZW), Eurasian patent (AM, AZ, BY, KG, KZ,
`MD,RU, TJ, TM), Europeanpatent (AT, BE, BG, CH,CY,
`CZ, DE, DK, EE, ES, Tl, FR, GB, GR, HU,IE, IT, LU,
`MC, NL, PT, RO, SE, SI, SK, TR), OAPI patent (BF, BJ,
`CE, CG, CI, CM, GA, GN, GQ, GW, ML, MR, NE, SN,
`TD, TG).
`
`Declarations under Rule 4.17:
`as to applicant’s entitlement to apply for and be granted
`a patent (Rule 4.17(ii)) for the following designations AE,
`AG, AL, AM, AT, AU, AZ, BA, BB, BG, BR, BY, BZ, CA,
`CH, CN, CO, CR, CU, CZ, DE, DK, DM, DZ, EC, EE, ES,
`Fl, GB, GD, GE, GH, GM, HR, HU, 1D, LL, IN, LS, JP. KE,
`KG, KP. KR, KZ, LC, LK, LR, LS, LT, LU, LV, MA, MD,
`MG, MK, MN, MW, MX, MZ, NI, NO, NZ, OM, PG, PH,
`PL, PT, RO, RU, SC, SD, SE, SG, SK, SL, SY, TJ, TM, TN,
`TR, TT, TZ, UA, UG, UZ, VC, VN, YU, ZA, ZM, ZW, ARIPO
`patent (GI, GM, KE, LS, MW, MZ, SD, SL, SZ, TZ, UG,
`ZM, ZW), Eurasian patent (AM, AZ, BY, KG, KZ, MD, RU,
`TJ, TM), European patent (AT, BE, BG, CH, CY, CZ, DE,
`DK,EE, ES, FI, FR, GB, GR, HU, LE, IT, LU, MC, NL, PT,
`RO, SE, SL, SK, TR), OAPIpatent (BF. BJ, CF, CG, Cl, CM,
`GA, GN, GQ, GW, ML, MR, NE, SN, TD, TG)
`of inventorship (Rule 4.17(iv)) for US only
`
`Published:
`
`For two-leuer codes and other abbreviations, refer to the "Guid-
`ance Notes on Codes andAbbreviations" appearing at the begin-
`ning of each regular issue of the PCT Gazette.
`
`(81) Designated States (national): AE, AG, AL, AM,AT(util-
`ity model), AT, AU, AZ, BA, BB, BG, BR, BY, BZ, CA,
`CH, CN, CO, CR, CU, CZ (utility model), CZ, DE (util-
`ity model), DE, DK (utility model), DK, DM, DZ, EC, EE
`—_with international search report
`(utility model), BE, ES, FI (utility model), FI, GB, GD, GE,
`GH, GM,HR,HU,ID,IL,IN, IS, JP, KE, KG, KP, KR, KZ,
`LC, LK, LR, LS, LT, LU, LV, MA, MD, MG, MK, MN,
`MW, MX, MZ, NI, NO, NZ, OM, PG, PH, PL, PT, RO,
`RU, SC, SD, SE, SG, SK (utility model), SK, SL, SY, TJ,
`
`(57) Abstract: The invention relates to a
`method for decreasing power consumption in
`a wireless terminal intended for a short-range
`wireless communication system.
`In the
`method the
`standard MAC functionality
`of a wireless terminal
`is modified so that
`packet transaction times becomeshorter thus
`reducing the power consumption and giving
`the terminal more time for the power save
`mode. This is accomplished by increasing
`@\V the transmission rate of physical layer data units comprising a preamble, a header, and a payload portion so that the header is
`© transmitted at a transmission rate higher than what is standardized for the header. The transmission rate of the header is preferably
`increased to the highest transmission rate standardized for the payload portion. In further embodiments of the invention further
`modifications of the MAC functionality are introduced, the embodiments further decreasing the packet transaction times.
`
`(54) Title: REDUCTION OF POWER CONSUMPTION IN WIRELESS TERMINALS
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`REDUCTION OF POWER CONSUMPTIONIN WIRELESS TERMINALS
`
`Field of the Invention
`
`The invention relates generally to wireless terminals intended for short-range,
`beacon-based communication systems. More particularly,
`the present
`invention concerns a mechanism for decreasing power consumption in
`wireless terminals engaged in short-range, beacon-based communication.
`
`Background ofthe Invention
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`The current development towards truly mobile computing and networking has
`brought on the evolvement of various access technologies that also provide
`the users with access to the Internet when they are outside their own home
`network. At present, wireless Internet accessis typically based on either short-
`range wireless systems or mobile networks, or both.
`
`Short-range wireless systems have a typical range of one hundred meters or
`less. They often combine with systerns wired to the Internet
`to provide
`communication over long distances. The category of short-range wireless
`systems includes wireless personal area networks (PANs) and wireless local
`area networks (WLANs). They have the common feature of operating in
`unlicensed portions of the radio spectrum, usually either in the 2.4 GHz
`Industrial, Scientific, and Medical (ISM) band orin the 5 GHz unlicensed band.
`
`Wireless personal area networks use low cost, low power wireless devicesthat
`have a typical range of about ten meters. The best-known example of wireless
`personal area network technology is Bluetooth, which uses the 2.4 GHz ISM
`band. It provides a peakair link speed of one Mbps, and power consumption
`low enough for use in personal, portable electronics such as PDAs and mabile
`phones. Wireless local area networks generally operate at higher peak speeds
`of 10 to 100 Mbps and have a longer range, which requires greater power
`consumption.
`
`30
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`Wireless LAN systemsare typically extensions of a wired network, providing
`mobile users with wireless access to the wired network. Examples of wireless
`local area network technology include the IEEE 802.11a, which is designed for
`the 5 GHz unlicensed band, and uses orthogonal
`frequency division
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`multiplexing (OFDM) to deliver up to 54 Mbpsdata rates; the 802.11b, which is
`designed for the 2.4 GHz ISM band and uses direct sequence spread
`spectrum (DSSS) to deliver up to 11 Mbps data rates; and the HIPERLAN
`Standard, which is designed to operate in the 5 GHz unlicensed band.
`
`two basic network topologies are available for
`In wireless LAN technology,
`network configuration: an ad-hoc network and an infrastructure network. An
`ad-hoc network is formed by two or more independent mobile terminals
`without the services of a basestation, i.e. in an ad-hoc network the terminals
`communicate on a peer-to-peer basis. An ad-hoc network is normally formed
`for temporary purposes. The infrastructure network, in turn, comprises one or
`more wireless base stations, called access points, which form part of the wired
`infrastructure.
`In a typical network of this type, all traffic goes through the
`accesspoints, regardless of whetherthe traffic is between two terminals or a
`terminal and the wired network, i.e. the mobile terminals do not communicate
`
`on a peer-to-peer basis. The mobile terminals are provided with wireless LAN
`cards, whereby they can access the wired network or set up an ad-hoc
`network.
`
`So far, WLAN (wireless LAN) technology has been used mainly in laptop
`computers, which are typically AC powered, but which may also be used in
`battery mode that provides a fairly high battery capacity. To prolongthelife of
`the batteries, the WLAN standards define a specific power save mode into
`which the terminals may enter in order to decrease their power consumption.
`In this mode the WLAN-specific power consumption is very low, but the
`terminals have to wake up (i.e. enter the active state) periodically to receive
`regular beacon transmissions broadcast
`in
`the network. The beacon
`transmissions indicate,
`for example, whether there are incoming packets
`puffered for a terminal. If so, the terminal retrieves the packets, goes back to
`sleep, and wakes up again to listen to the next beacon transmission.
`
`The current WLAN power management has been designed assuming that the
`terminal devices are laptop type computers featuring a relatively high battery
`capacity. Along with the generalization of various other types of personal
`communication devices, such asintelligent phones, having a smaller size and
`thus also a lower battery capacity than laptop computers, power consumption
`has, however, becomea critical issue when new properties are designed for
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`wireless systems and terminals. This development work is complicated by the
`fact that the capabilities of legacy terminals must be taken into account, which
`often translates to some sort of compromise. Emerging ad-hoc mode
`applications in which power consumption mayberather high further aggravate
`the problem. Examplesof such applications are games played in small groups
`or business meetings in which large files may be shared (wirelessly) by the
`terminals.
`
`The presentinvention seeks to accomplish a solution by means of which the
`above drawbacks can bealleviated or eliminated.
`
`Summary of the Invention
`The present invention seeks to devise a new mechanism for decreasing the
`power consumption in wireless terminals operating in short-range, beacon-
`based communication systems. The invention also seeks to bring about a
`MAC (Media Access Control) layer functionality which is optimized in termsof
`power consumption and which is particularly suitable for close proximity
`communication in ad-hoc networks where beacon frames are broadcast.
`
`In the present invention, a wireless terminal is provided with a modified MAC
`layer functionality as compared to a standard WLANterminal. The starting
`point of the invention is thus a standard WLAN functionality. This functionality
`is modified by shortening the transmission intervals of the terminal. The
`transmission of the physical
`layer frame is accelerated, contrary to the
`standard WLAN operation, so thatthe frame headeris transmitted at the sarne
`high bit rate as the payload portion of the irarne. This modification translztes to
`shorter packet transaction intervals and to lower power consumption, while
`also allowing the terrninals more time for the power save mode.
`
`Thus one embodiment of the invention is the provision of a method for
`decreasing power consumption in a wireless terminal
`intended for a short-
`range communication system according to predefined system specifications,in
`which system beacon frames are broadcast at beaconintervals. The method
`includes the step of configuring the terminal
`to assemble data to be
`transmitted into data units including a preamble, a header and a payload
`portion, where the data units have a format according to the predefined system
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`least one
`specifications and where said system specifications include at
`dedicated transmission rate for
`the preamble, at
`least one dedicated
`transmission rate for the header, and at least one dedicated transmission rate
`for the payload portion. The method also includes configuring the terminal to
`transmit the header at a first transmission rate higher than the at least one
`dedicated transmission rate for
`the header,
`thereby to reduce power
`consumption in the terminal, where said beacon frames are transmitted in the
`payload portions of said data units.
`
`the invention provides a wireless terminal for a
`In another embodiment,
`wireless communication system. The wireless terminal
`includes MAC layer
`functionalities and physical layer (PHY) functionalities, the MAC layer being
`configured to pass data to the physical layer, and the physical layer being
`configured to form data units comprising a preamble, a header and a payload
`portion, where the data units have a format according to predefined system
`specifications, which include at least one dedicated transmission rate for the
`preamble, at least one dedicated transmission rate for the header, and at least
`one dedicated transmission rate for the payload portion. The terminal also
`includes a first operation mode in which the terminal is configured to transmit
`the headerat a first transmission rate higher than the at least one dedicated
`transmission rate for the header, thereby to reduce power consumption in the
`terminal, wherein said beacon frames are transmitted in the payload portions
`of said data units.
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`In further embodiments of the invention additional modifications are introduced
`
`into the MAC layer, the embodiments further shortening the transmission
`intervals and reducing the power consumption of the terminal.
`
`29
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`Other features and advantages of the invention will become apparent through
`reference to the following detailed description and accompanying drawings.
`
`Brief Description of the Drawings
`
`30
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`In the following, the invention and many of its embodiments are described
`more closely with reference to the examples shown in FIG. 1 to 9 in the
`appended drawings, wherein:
`
`FIG. 1 illustrates a typical communication system according to the invention;
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`FIG. 2 illustrates the MAC entity utilized in the present invention;
`
`FIG.3 illustrates a transmission frame according to IEEE 802.11b standard;
`
`FIG. 4 illustrates a dual MAC terminal according to the invention;
`
`FIG. 5 is a flow diagramillustrating an embodiment of a terminal transmitting
`beacon frames;
`
`FIG. 6 is a flow diagram illustrating an embodiment of the operation of a
`terminal listening to the beacon transmissions;
`
`FIG. 7 illustrated a further modification according to the invention;
`
`FIG. 8 is a block diagram illustrating the basic elements of the terminal; and
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`FIG. 9 illustrates a terminal intended for ad-hoc mode only.
`
`Detailed Description of the Invention
`
`The terminal of the invention is preferably based on the IEEE 802.11
`standards for wireless local area networking. Furthermore, the terminals of the
`invention are preferably such that they can operate both in the infrastructure
`mode and in the ad-hoc mode,although it is also possible that they are only
`ad-hoc capable devices, as discussed below.
`
`FIG. 1 illustrates a typical WLAN communication system. The system includes
`one or more WLAN networks 100, each connected by means of a gateway
`401 (a router) to another network, such as the Internet, which contains service
`providers 102. Each WLAN network comprises one or more access points
`1903, cach communicating wirelessly with the terminals within the coverage
`area, ie. the cell, of the access point and thus forming a bridge between the
`terminals and the wired network.
`
`In an infrastructure network an access point and at least ons terminal is said to
`form a Basic Serving Sei (BSS). A series of BSSs then forms an Extended
`Service Set (ESS). These BSSs are connected to each other by a Distribution
`System (DS), which can be a wired network, such as an Ethernet LAN, within
`which TCP/IP packets are transmitted, or a wireless network, or a combination
`of these two. However,
`the basic type of an IEEE 802.11 LAN is an
`Independent BSS (IBSS), which consists of two or more terminals. The
`terminals of an IBSS form an ad-hoc network 110. As the invention does not
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`relate to the architecture of the WLAN system,it is not discussed in more
`detail here.
`
`The terminals may be portable computers, PDA equipment, intelligent phones
`or other such mobile terminals 120.
`In the same way as an ordinary GSM
`telephone, the terminals can be made up of two parts: the actual subscriber
`device, e.g. a portable computer (with software), and a SIM (Subscriber
`Identity Module), whereby from the viewpoint of the network the subscriber
`device becomesa functioning terminal only when the SIM has been inserted
`into it. The SIM may be the subscriber identity module for use in the GSM
`(Global System of Mobile communications) network or in the UMTS (Universal
`Mobile Telecommunication System), for example. In the latter case it is termed
`the USIM (Universal Services Identity Module). However, the terminals may
`equally well be traditional WLANterminals in which no SIM is used.
`
`The system further typically contains an authentication server 130 of the
`WLAN network. The authentication server
`is connected to the above-
`mentioned gateway through a secured connection, which is typically a TCP/IP
`connection established through an operator network or through the Internet.
`As discussed below, in an infrastructure network the access points broadcast
`beacon messages 30, while in an ad-hoc network the terminals share this
`responsibility.
`
`As is known, the IEEE standard 802.11 defines the physical layer options and
`the MAC (Media Access Control) layer protocol for the wireless LAN.
`In the
`presentinvention, the MAClayeris modified to reduce the power consumption
`of the device and to obtain a solution that is particularly suitable for close
`proximity cornrnunication in ad-hoc rode.
`
`FIG. 2 illustrates the protocol architecture of the IEEE 802.11 standard. As
`shownin the figure, the actual MAC protocol operates in the lower subdayer of
`the second layer of the OS! layer model, which is the Data Link Layer (DLL).
`The MAC management
`layer
`supports
`the association and roaming
`functionalities and it
`further controls
`the power
`saving functions,
`the
`authentication and encryption mechanisms, and synchronization of
`the
`terminals, for example. The MAC managementlayer further maintains a MAC
`layer management database,i.e. the MIB (ManagementInformation Base) of
`the MAC layer. The MAG layer cooperates with the physical management
`layer to maintain the database. Examplesof the MAC layer attributes that can
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`(Received Signal Strength
`invention are RSSI
`be utilized in the present
`Indicator), which indicates the level of the received signal, and NF (Noise
`Floor), which indicates the interference level onthelink.
`
`The physicallayeris divided into two sub-layers, which are the PLCP (Physical
`Layer Convergence Protocol) sub-layer and the PMD (Physical Medium
`Dependent) sub-layer. The purpose of the PLCP is to provide minimum
`dependenceon the PMD in order to simplify the interface between the physical
`layer and the MAC layer. The PLCP sub-layer takes a frame that a terminal
`wishes to transmit and forms a PLCP Protocol Data Unit (PPDU), the format of
`whichisillustrated in FIG. 3. The PPDU comprises three successive parts: a
`PLCP preamble 300, a PLCP header 301, and a PLCP Service Data Unit
`(PSDU) 302. The PSDU containsat least the header of the MAC frame. The
`IEEE 802.11b standard defines two different preambles and headers for the
`PPDU: a long preamble and header and a short preamble and header.
`According to the standard, the long preamble and header comprise 144 and
`48 bits, respectively. As they are transmitted at 1 Mbit/s, the total processing
`time of the preamble and header is 196 ys. The short preamble and header
`comprise 72 and 48 bits, respectively. The preambleis transmitted at 1 Mbit/s
`and the header at 2 Mbit/s. Consequently, the total processing time of the
`preamble and headeris 96 ys. In both PPDU formats,the actual payload, i.e.
`the PSDU, is transmitted at 1, 2, 5.5 or 11 Mbit/s.
`
`In the present invention, both the long and short format may be used, although
`the short PPDU formatis preferable, as it has a better performancein terms of
`overhead and data throughput as compared to the long format.
`In one
`embodiment of the invention, the standard functionality described above is
`modified so that only the highest available daiarate, ie. 11 tibit/s in this case,
`is used for the header and the payload portion.
`In other words, unlike in
`standard IEEE 802.11b WLAN, the PLCP headers are also transmitted in the
`high bit rate mode. As the preamble serves to synchronize the receiver, it is
`transmitted at the standardized rate.
`
`In the invention the range is thus compromised (the coverage is better at a
`lower rate) in favor of power consumption. This can be done as the modified
`MAC functionality of the invention is mainly intended for close proximity
`communication. The modified MAC layer transmits the PLCP headers at an
`accelerated rate, which preferably corresponds to the highest rate available for
`the PSDUs. By accelerating the transmission of the header contrary to the
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`WLANspecifications, the processing time of the data packets can be made
`shorter than in the standard MAClayer. The shorter transmission intervals(i.e.
`shorter packet transaction intervals) obtained in this way translate to lower
`battery consumption, while also allowing the terminal more time for the sleep
`mode. The modification also simplifies the implementation of the MAC
`functionality, since a link quality analysis is no longer neededfor controlling the
`transmission rate.
`
`In one embodimentof the invention the WLANterminal is provided with a dual
`MAClayer asillustrated in FIG. 4. The first MAC layer 40 provides standard
`WLANoperation, while the second MAClayer 41 is the modified MAClayer of
`the invention. The modified MAC layer is in this context termed the Proximity
`MAC (PMAC). The standard MAClayeris used for accessing standard WLAN
`networks through access points and other standard WLAN devices, while the
`PMAC layer 41 is used in ad-hoc networking,
`i.e.
`in close proximity
`communication on a terminal-to-terminal basis. The entity controlling the
`operation of the (P)MAC and physical
`layers is in this context termed the
`WLAN engine. The term thus refers to an entity that includes WLAN-specific
`contro! information and controls the said two sub-layers. The two MAC layers
`preferably use the same RF resources. Consequently, both MAC layers
`cannot be operated at the sametime since in that case one layer would cause
`interference to the other. As discussed below,
`the user may select the
`operation mode by enabling/disabling one of the MAC layers at each time
`through the userinterface.
`
`is to be noted here that in this context the PMAC refers to the layer
`it
`comprising the modifications of the invention. Therefore, the interface between
`the PMAC, which elso includes MAC managernent functions, and the physical
`layer does not necessarily correspond to the interface between the standard
`MAC layer and the physical
`layer, as the modification may relate to the
`ohysical layer also. As shown above, the basic modification is such that the
`transmission rate is raised, although the physical layer frame structure remains
`in standard format.
`
`In some embodiments of the invention the above PMAC layer is further
`modified with respect to beacon broadcasts. As is known, in WLAN networks
`beacon frames are periodically broadcast to enable the terminals to establish
`and maintain communications in an orderly fashion.
`In the infrastructure
`networks, each access point sends beacon framesat regularintervals. In an
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`ad-hoc network, where no access points exist, one of the wireless terminals
`assumes the responsibility of sending the beacon frame. Having received a
`beacon frame, each terminal waits for the beaconinterval and then broadcasts
`a beacon frameif any other terminal does not do so after a random time delay
`calculated by the terminal. The purpose of the random time delay is to
`circulate the broadcast
`responsibility among the terminals of the ad-hoc
`network. In one embodiment of the modified MAC layer of the invention, the
`broadcasting of the beacon is changed so that the terminal that instantiates
`the network (other terminals will join) continues as the beacon broadcaster and
`maintains the beacon interval in the network. In other words, the responsibility
`of beacon broadcastingwill not circulate, but the initiator of the ad-hoc network
`remains as a centralized beacon broadcaster. By prohibiting the circulation of
`the broadcasting turns during the normal operation of the ad-hoc network,all
`the terminals, except the one broadcasting the beacon, may savetheir battery.
`The modification may also allow a decreasein the beacon transmission power,
`since the point of beacon transmission does not any morecirculate at the
`boundaries of the network, but movesonly if the relevant terminal moves.
`
`In this embodiment the power consumption of the initiating node is thus
`sacrificed to save the batteries of the other terminals. However, the initiating
`node may also reduce its power consumption, as compared to the use offixed
`beaconintervals, by using an adaptive beacon interval the length of which
`depends on the load detected by the terminal transmitting the beacon.
`
`The use of an adaptive beaconinterval is illustrated in FIG. 5, whichillustrates
`the control of the beacon interval during normal operation. The terminal
`transmitting the beacon frequently defines the current
`load (step 50),
`compares the defined load to the previously defined load (step $7), and
`changes the length of the beaconinterval if the load has changed enough.If
`the load has decreased enough, the beacon interval is decreased (step 54)
`and if the load has increased enough, the beacon interval is increased (step
`55). The load can be measured as the channelutilization level, for example.
`
`In another embodiment of the invention, the centralized beacon broadcasting
`is combined with the traditional distributed mode to form a “semi-centralized”
`
`is changed after every N (N>1)
`in which the broadcasting terminal
`mode,
`beacon frames. This mode converts to the centralized mode when N becomes
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`infinite.
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`10
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`is utilized in connection with centralized or
`If an adaptive beacon interval
`“semi-centralized” modes,
`the beacon interval may be decreased and
`increased by adding beacon transmission moments between fixed beacon
`transmission moments and by removing beacon transmission moments from
`betweenfixed transmission moments,respectively. The transmission moments
`are added and removed so that all the transmission moments are evenly
`distributed in the time domain. A beacon transmission method like this is
`disclosed in a co-pending U.S. Patent Application Serial No. 10/400 233,filed
`on March 25, 2003. However, the method disclosed in said U.S. Patent
`Application is intended for accesspoints.
`
`In case the terminal transmitting the beacon frames disappears from the ad-
`hoc network,
`rules are defined in the PMAC layer for
`transferring the
`responsibility to another terminal.
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`10
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`FIG.6illustrates one embodiment for the beacon monitoring operation of a
`terminal other than the current beacon broadcaster. In this embodiment each
`15
`of said other terminals monitors the beacon transmission moments (step 60)
`and if a beacon is not received at the end of the beaconinterval, the terminal
`starts a random time delay (step 62). During this delay the terminal monitors
`the traffic in the network in order to detect, whether another terminal with a
`shorter time delay starts to send the beacon or whetherthere is data traffic on
`the channel (step 63). When the random delay period elapses, the terminal
`starts to transmit the beacon signal provided that no other terminal starts the
`transmission earlier during the delay period, and provided that no traffic is
`received from the channel. The monitoring of the traffic is a precautionary
`measure to prevent an event in which the terminal starts to send a beacon
`when moving ternporarily so far from ihe beacon transmitting terrninal ihat it
`cannot receive the beacon (althoughit can hearthe traffic of the network).
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`The transier of the beacon transmission resoonsibility may also be based on a
`priority list, which is stored in each terminal. The terminal transmitting the
`beacon may compile and maintain the list, and transmit it to the other terminals
`in the beacon frame.
`If the terminal that has the highest priority in thelist
`detects that the beacon has disappeared from the network,it starts to transmit
`the beacon, after which the other terminals join the network. The list may
`include the MAC addressesof the terminals,for example.
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`In a still further embodiment of the invention, an additional rnodification is
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`made to further decrease the transmission intervals of the terminal. This is
`performed by increasing the proportion of payload data in the data to be
`transmitted. This is accomplished byutilizing layer 2 (data link layer) frames,
`i.e. Ethernet frames, without
`IP and TCP/UDP headers.
`in other words,
`application data is written directly to layer 2 frame,
`i.e. omitting the layers
`between the application layer and layer 2. This is illustrated in FIG. 7 showing
`a layer 2 frame. The frame is constructed by encapsulating an application level
`data unit including user data and an application header into an Ethernet frame
`by adding an Ethernet headerandtrailer to the application level data unit. The
`use of this embodiment thus means that the Ethernet frame does not carry
`network and transport layer headers (i.e. [P and TCP headers), whereby the
`proportion of payload data in the transmitted frame is increased.
`In this
`embodiment, the addressing is based on terminal-specific MAC addresses.
`The MAC addresses can be used, since no routing (i.e.
`IP addresses) is
`neededin the ad-hoc network, wherethe terminals communicate on a peer-to-
`peer basis. By decreasing the amount of overhead the throughput times can
`be made shorter, whereby power consumption is reduced and the terminals
`have more time for the sleep mode. The gain achieved by this modification
`depends greatly on the application used. The gain is greatest in applications
`that use a lot of short packets. Examples of such applications are network
`games, which senda lot of control packets of only 60 to 150 bytes.In this case
`the decrease in overhead, which is achieved by omitting the IP and TCP/UDP
`protocols,
`is
`significant,
`about
`half
`of
`the
`packet
`size.
`The
`transmission/reception times are thereby significantly shorter, which reduces
`the power consumption and gives the terminal more time for the power save
`mode,
`
`The drawbackrelated to the above modification is that an application modified
`in the above manner is compatible only with terminals modified similarly, i.e.
`not necessarily with conventional terminals.
`
`In a still further embodiment of the invention, dynarnic power controlis utilized
`to further reduce power consumption in the terminals. Each terminal may
`measure the powerreceived from the terminal it is communicating with, and
`adjust its own transmission power according to the power measured. This may
`be implemented, for example, by dividing the power range into successive
`sub-ranges, and adjusting the transmission power according to the sub-range
`to which the RSS] measured on the link concerned belongs.In this way, the
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`transmission power can be decreased when the measured RSS! value
`increases and increased when the measured RSSI value decreases. Since the
`PMACis typically used in an environment where the terminals are close to
`each other, the transmission powerwill be very low. As some terminals may
`also access through an accesspoint located in the neighborhood, the above-
`mentioned NF attribute may be used for evaluating the interference level and
`the signal-to-noise ratio on the channel. Based on the detected interference
`level, the transmission power maybe increased, or the terminals may switch to
`another channel having a lower interferencelevel.
`
`FIG. 8 illustrates the basic elements of the terminal. The mobile terminal 800
`com