(12) United States Patent
`US 6,570,857 B1
`(10) Patent N0.:
`Haartsen et al.
`
`(45) Date of Patent: May 27, 2003
`
`U8006570857B1
`
`(54) CENTRAL MULTIPLE ACCESS CONTROL
`FOR FREQUENCY HOPPING RADIO
`NETWORKS
`
`6,246,677 B1 *
`
`6/2001 Nap et al.
`
`FOREIGN PATENT DOCUMENTS
`
`WO
`
`97/48216
`
`12/1997
`
`(75)
`
`Inventors: Jacobus Haartsen, Borne (NL);
`Johannes Elg, Malmo (SE)
`
`* cited by examiner
`
`(73) Assignee: Telefonaktiebolaget L M Ericsson,
`Stockholm (SE)
`
`Primary Examiner—Melvin Marcelo
`Assistant Examiner—Inder Pal Mehra
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/210,594
`
`(22)
`
`Filed:
`
`Dec. 15, 1998
`
`(60)
`
`Related US. Application Data
`Provisional application No. 60/071,262, filed on Jan. 13,
`1998.
`
`Int. Cl.7 ................................................... H04J 3/24
`(51)
`(52) US. Cl.
`....................... 370/312; 370/328; 370/349;
`370/432
`
`(58) Field of Search ......................... 370/322, 328—329,
`370/337, 345—350, 431, 442, 449—450,
`312—313, 338, 475, 375, 443, 468, 432,
`321, 323
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`7/1987 Grover
`4,680,583 A
`6/1991 Klughart
`5,025,486 A *
`3/1994 Gilbert et al.
`5,297,144 A
`5,475,681 A * 12/1995 White et al.
`5,577,043 A
`11/1996 Guo et al.
`5,668,803 A *
`9/1997 Tymes et al.
`5,754,535 A *
`5/1998 Vandenabeele et al.
`6,236,674 B1 *
`5/2001 Morelli et al.
`6,246,670 B1 *
`6/2001 Karlsson et al.
`
`..... 370/321
`
`(74) Attorney, Agent, or Firm—Burns, Doane, Swecker &
`Mathis, L.L.P
`
`(57)
`
`ABSTRACT
`
`A system comprises a Wireless master unit and one or more
`Wireless slave units, each having a unique identifier. When
`a Wireless slave unit is active, it is addressed by one of a
`limited number of temporary addresses. A PARK mode
`enables a Wireless slave unit to be in an idle state during
`which its temporary address is deallocated, enabling that
`address to be assigned to another Wireless slave unit. To page
`a parked slave, a paging beacon packet is broadcast to, and
`received by, each of the Wireless slave units at fixed intervals
`during a master-to-slave time slot. Each Wireless slave unit
`determines Whether
`the received paging beacon packet
`includes its unique identifier. If so, the Wireless slave units
`retrieves a temporary address from the paging beacon
`packet. The Wireless unit transmits a response to the Wireless
`master unit during a subsequent slave-to-master time slot if
`the received paging beacon packet
`included the unique
`identifier belonging to the Wireless slave unit. Parked Wire-
`less slave units are also assigned a unique response number
`by the master. The master broadcasts a polling beacon
`packet during a master-to-slave time slot. If the parked slave
`unit desires access to the channel, it transmits a response in
`an N:th slave-to-master time slot following the polling
`beacon packet, Where N is a function of the slave’s unique
`response number.
`
`24 Claims, 5 Drawing Sheets
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`[301
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`UNIT IDENTITY
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`US. Patent
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`May 27, 2003
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`US 6,570,857 B1
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`US. Patent
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`May 27, 2003
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`US 6,570,857 B1
`
`1
`CENTRAL MULTIPLE ACCESS CONTROL
`FOR FREQUENCY HOPPING RADIO
`NETWORKS
`
`CROSS-REFERENCE TO CO-PENDING
`APPLICATION
`
`This application claims the benefit of US. Provisional
`Application No. 60/071,262, filed Jan. 13, 1998, which is
`hereby incorporated herein by reference in its entirety.
`BACKGROUND
`
`The invention relates to radio networks, that are networks
`in which units wirelessly exchange information by way of
`radio signals. In particular, radio networks in which the air
`interface applies frequency hopping to spread the signal over
`a wide spectrum are considered. The problem addressed is
`the multiple access of different units on a common, fre-
`quency hopping channel.
`The system considered is based on a frequency hopping
`(FH) system, different aspects of which are described in US.
`patent application Ser. Nos. 08/685,069; 08/932,911; and
`08/932,244; as well as US. Provisional Application No.
`60/109,692, filed on Nov. 24, 1998 in the name of J.
`Haartsen which are all hereby incorporated herein by refer-
`ence. In this system, a channel is defined as a frequency hop
`sequence which is a pseudo-random number (PN) sequence
`determined by the identity of one of the units participating
`on the channel, called the master. The phase in the sequence
`is determined by a master clock associated with the master.
`As the master clock progresses, the channel hops from radio
`frequency (RF) hop frequency to RF hop frequency at the
`clock rate. All other units participating on the channel, called
`slaves, are synchronized to the FH scheme by using the same
`FH sequence and same clock as used by the master. The
`channel shared between the master and the one or more
`
`slaves is called a piconet.
`the master parameters that are
`At connection setup,
`required to maintain FH synchronization are transferred
`from the master to the slave. A strict Time Division Duplex
`(TDD) scheme is adhered to: time slots (“slots”) in which
`traffic is transferred from master to slave and slots in which
`traffic is transferred from slave to master, alternate at the
`hopping rate. Preferably, a high hopping rate is used in order
`to obtain immunity against interferers that share the spec-
`trum. A high hopping rate results in short slots and small
`packets.
`The master controls the access on the channel. A distrib-
`
`uted access method, like carrier-sense multiple access, is not
`useable due to the fast hopping of the channel; the dwell
`time on a RF hop frequency is too short to carry out an
`effective contention-based access scheme. On the other
`hand, reserved access schemes like TDMA are not suitable
`for packet-switched data connections. Therefore, a polling
`scheme is used which is entirely controlled by the master of
`the piconet. At any moment in time, a master may select any
`of the slaves participating on the channel to send data to in
`the master-to-slave slot. However, only the slave addressed
`by the master in this master-to-slave slot may respond in the
`succeeding slave-to-master slot.
`In this scheme, the master selects a slave in the master-
`to-slave slot to send data to and from which it can receive
`data. As a result, collisions between slaves that want to send
`information to the master at the same time are prevented.
`When the master sends information to slave X, this implic-
`itly means that slave X may respond in the next slave-to-
`master slot. The slave is implicitly polled by the master. If
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`the master has no data to send, it may send a specific ‘poll’
`packet to give the slave a chance to respond. A poll packet
`is a very short packet carrying no data.
`The addressing scheme in the system is carried out as
`follows. Each unit has a unique identity which is, for
`example, derived from the 48-bit IEEE 802 addressing
`space. The identity of the master is used to form the FH
`sequence used by the channel in the piconet. Each packet is
`preceded by a preamble which is also derived from the
`master identity. This preamble is used by all
`the units
`participating in the piconet to identify whether there is a
`packet present in the slot, and if so whether the packet
`belongs to this piconet. Since many uncoordinated fre-
`quency hopping piconets may be co-located, occasionally
`they may happen to land on the same hop frequency. The
`preamble prevents the users in one piconet from accepting
`packets belonging to another piconet. The master address
`therefore identifies the piconet (or channel) and can be
`regarded as a channel identifier.
`To distinguish between the different participants on the
`piconet, a short
`length Medium Access Control (MAC)
`address is used which is temporarily allocated by the master
`to the slave when the slave is connected to the piconet. The
`MAC address is located in the header of the packet. The
`master uses the proper MAC address to address a slave. The
`size of the MAC address is preferably small in order to
`minimize the overhead in the packet header. As was men-
`tioned before, the system preferably uses a fast hopping rate.
`As a result, the packet can only be short and the amount of
`overhead (including the MAC address) must be minimized.
`However, the use of only a short-length MAC address limits
`the number of slaves that can simultaneously participate on
`the channel.
`
`Slaves that do not have to exchange a great deal of
`information can be placed in a low power mode called
`HOLD. When the slave is in the HOLD mode, it does not
`participate on the channel. It neither transmits nor receives
`data, but it does keep its clock running (so that it remains
`synchronized to the FH channel), and it retains its MAC
`address. At the conclusion of a HOLD interval (the duration
`of which is agreed upon by both the master and the slave
`prior to entering the HOLD mode),
`the slave leaves the
`HOLD mode and participates on the channel as before.
`Units that want to remain locked to the channel can enter
`
`the HOLD mode to save power consumption. However,
`since they keep their MAC addresses, units that rarely
`participate on the channel deny access to the channel to other
`units since the MAC address space is limited. This ineffi-
`cient use of the MAC addresses is more of a problem in
`those described FH systems in which the MAC address is
`short (to minimize overhead), resulting in only a few slaves
`being able to participate on the channel.
`SUMMARY
`
`It is therefore an object of the present invention to provide
`techniques for keeping units synchronized to the channel in
`a piconet without requiring them to retain their MAC
`addresses.
`
`The foregoing and other objects are achieved in appara-
`tuses and methods of operating a system comprising a
`wireless master unit and one or more wireless slave units,
`wherein each of the one or more wireless slave units has a
`
`In accordance with one aspect of the
`unique identifier.
`invention, a wireless slave unit may be in a so-called PARK
`mode, in which it is not associated with a temporary address
`(e.g., a MAC address described in the BACKGROUND
`7
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`7
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`

`

`US 6,570,857 B1
`
`3
`
`section). In order to page a parked wireless slave unit, a
`paging beacon packet is broadcast to, and received in, each
`of the one or more wireless slave units at fixed intervals
`during a master-to-slave time slot. Each wireless slave unit,
`determines whether the received paging beacon packet
`includes the unique identifier belonging to the wireless slave
`unit. If it does,
`then the wireless slave unit retrieves a
`temporary address from the paging beacon packet, and
`transmits a response to the wireless master unit during a
`subsequent slave-to-master time slot.
`In another aspect of the invention, the wireless slave unit
`can determine whether a subsequent traffic packet from the
`wireless master unit includes the temporary address and, if
`so, respond by transmitting a response to the wireless master
`unit during another subsequent slave-to-master time slot.
`In yet another aspect of the invention, the paging beacon
`packet is a type of beacon packet, wherein beacon packets
`have a header portion that includes a predefined temporary
`address that is never assigned to any of the one or more
`wireless slave units in the system.
`In still another aspect of the invention, parked wireless
`slave units are offered an opportunity to request access to the
`piconet. This is accomplished by defining a series of time
`slots comprising alternating occurrences of a master-to-slave
`time slot and a slave-to-master time slot, wherein each of the
`slave-to-master time slots comprises a plurality of slave-to-
`master sub-slots. Depending on the embodiment, the number
`of sub-slots per slave-to-master time slot may be any integer
`greater than or equal to 1. Furthermore, a unique response
`number is allocated to each of the one or more wireless slave
`
`units. A polling beacon packet is broadcast by the master
`unit to each of the one or more wireless slave units at fixed
`
`intervals during a master-to-slave time slot. Receipt of the
`polling beacon packet by a wireless unit indicates an oppor-
`tunity to request access to the piconet. Accordingly, if a
`wireless unit desires to access the piconet,
`it transmits a
`packet to the wireless master unit during a slave-to-master
`sub-slot that occurs N slave-to-master sub-slots after the
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`polling beacon packet, wherein N is a function of the unique
`response number of the at least one or more wireless slave
`units.
`
`40
`
`In yet another aspect of the invention, the master unit is
`not required to give each of the wireless units an opportunity
`to respond to the polling beacon packet. To accommodate
`this possibility, a slave unit detects whether any master
`activity occurred in the master-to-slave time slot immedi-
`ately preceding the slave-to-master sub-slot that occurs N
`slave-to master sub-slots after the polling beacon packet,
`and if so, transmits the packet to the wireless master unit
`only if no master activity was detected in the master-to-slave
`time slot immediately preceding the slave-to-master sub-slot
`that occurs N slave-to-master sub-slots after the polling
`beacon packet.
`In still another aspect of the invention the wireless master
`unit receives the response packet from the at least one of the
`one or more wireless slave units, and determines which of
`the one or more wireless slave units transmitted the packet
`by determining which slave-to-master sub-slot the packet
`was received in, relative to the master-to-slave time slot
`during which the polling beacon packet was broadcast.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The objects and advantages of the invention will be
`understood by reading the following detailed description in
`conjunction with the drawings in which:
`FIG. 1 is a timing diagram of an exemplary air interface
`in accordance with one aspect of the invention;
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`FIG. 2 is a diagram of an exemplary packet format for use
`on the air interface in accordance with one aspect of the
`invention;
`FIG. 3 schematically depicts an exemplary addressing
`scheme for use with the air interface in accordance with one
`
`aspect of the invention;
`FIG. 4 is a timing diagram illustrating beacon transmis-
`sion and park wake-up in accordance with one aspect of the
`invention;
`FIG. 5 is a timing diagram illustrating the paging of a
`parked slave A,
`in accordance with one aspect of the
`invention;
`FIG. 6 is a timing diagram illustrating a parked slave’s
`access resolution with immediate master action, in accor-
`dance with one aspect of the invention; and
`FIG. 7 is a timing diagram illustrating a parked slave’s
`access resolution with deferred master action, in accordance
`with one aspect of the invention.
`DETAILED DESCRIPTION
`
`An overview of various aspects of the invention will first
`be presented, followed by an even more detailed description.
`Overview
`
`A method is described in which units remain synchro-
`nized to the frequency hopping channel without owning a
`MAC address. These units are in a new mode referred to
`herein as PARK mode. The channel identifier is used to
`
`initially communicate between the piconet master and
`parked slaves. When a parked slave wants to become active,
`it indicates this to the master, at which time the master
`allocates this slave a free, temporary MAC address. Once
`active,
`the slave can participate in the piconet, and can
`occasionally be put on HOLD for short time periods keeping
`its MAC address. For longer periods of inactivity, the slave
`can enter the PARK mode, giving up its MAC address in the
`process, thereby freeing up the MAC address for use by a
`different slave.
`
`the master transmits a
`the PARK mode,
`To support
`broadcast packet at fixed intervals, which operates as a kind
`of beacon. The broadcast packet is identified by the all-zero
`MAC address. All slaves in PARK mode always wake up to
`read the beacon. If the master wants a parked slave to
`become active, it issues a paging message in the payload of
`the beacon packet. This paging message includes the full
`48-bit identity of the slave and the temporary MAC address
`to be used by this slave. Only the slave that was paged is
`allowed to respond in the next slave-to-master slot.
`A different scheme is used to permit parked slaves to
`access the channel without being paged. When the slave
`enters the PARK mode, it is allocated a response number that
`determines when the parked slave is allowed to respond
`without being explicitly/individually paged. The slaves are
`allowed in the slave-to-master slot which are preferably (but
`not required to be) divided into a plurality of slave-to-master
`sub-slots for this purpose. In the exemplary embodiment
`described herein, the number of slave-to-master sub-slots in
`each slave-to-master slot is two, and these slave-to-master
`sub-slots are therefore referred to herein as half-slots. It
`should be understood that alternative embodiments can
`
`easily be derived from the description of the exemplary
`embodiment by substituting the term “slave-to-master sub-
`slots” in place of the term “half slots”, thereby indicating
`that there may be more or even fewer than two of such slots.
`Continuing now with the exemplary embodiment,
`in
`which the slave-to-master sub-slots are half slots, a parked
`slave with response number N is allowed to send a message
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`US 6,570,857 B1
`
`5
`in the Nth slave-to-master half slot counted from the beacon
`packet, provided no master activity has been detected in the
`master-to-slave slot precisely preceding this response slot.
`The response message will consist of the channel identifier
`(preamble) only. The position of the response message with
`respect to the beacon packet tells the master which parked
`slave is requesting access. The master may grant the access
`by directly sending a (broadcast) paging message mentioned
`before in the next master-to-slave slot. Alternatively,
`the
`master may wait until all parked slaves have had a chance to
`respond, and then make a decision regarding which slave to
`address. Parked slaves that have requested access but are not
`granted access should keep listening to the channel for
`(broadcast) paging messages, because the master may grant
`access to the requesting slaves sequentially before the next
`beacon occasion. Parked slaves that have not issued an
`access request can enter the sleep mode until the next beacon
`occasion.
`the slave unit must first be
`To enter a PARK mode,
`registered with the piconet master. This registration couples
`the identity of the parked slave to the response number
`(which should always be kept as low as possible). The
`master should regularly do registration updates so that slaves
`that were formerly parked but have left the coverage area,
`can be de-registered so that the response number can be
`reused for another parked slave. Parked slaves with high
`priority should be issued lower response numbers than
`parked slaves having lower priority.
`If the master is already engaged with another (active)
`slave at the designated time for a beacon transmission, it
`does not have to abort its operations. Instead it may defer the
`beacon transmission to the next available master-to-slave
`
`slot. The parked units will wake up and read the channel
`identifier to adjust their clocks. Units not desiring access can
`then return to sleep until the next beacon event. Units that
`desire access remain awake and wait until the beacon packet
`indeed passes along.
`The invention distinguishes between active slaves in a
`high-power mode and inactive slaves in a low-power mode.
`By reserving the MAC address for the active slaves only, a
`large number of inactive slaves can be supported without
`much overhead on the channel. For bursty data traffic, active
`and inactive slaves can be swapped (reusing the MAC
`addresses) based on their traffic requirements. In this way,
`the number of slaves virtually connected to the channel can
`be much larger than indicated by the MAC address.
`More Detailed Description
`The various features of the invention will now be
`
`described in even greater detail with respect to the figures,
`in which like parts are identified with the same reference
`characters. In order to facilitate a better understanding of the
`invention, the focus of the discussion is on the air interface,
`the types of communications that take place between master
`and slave units, and on the various ways that master and
`slave units respond to receipt of various types of packets.
`Those having ordinary skill in this art should have no trouble
`designing and making operable systems based on the func-
`tional description presented herein. Such systems may
`include,
`for example, programmable equipment
`that
`executes program instructions created in accordance with
`the principles set forth herein, and stored in any of a variety
`of computer readable storage media,
`including but not
`limited to Random Access Memory (RAM), magnetic stor-
`age media (e.g., hard and/or floppy disks) and optical storage
`media (e.g., Compact Disc (CD) Read Only Storage
`(ROM)).
`Private radio communications require the deployment of
`unlicenced bands. Presently there is not much unlicenced
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`radio spectrum that is globally available. One band, the
`Industrial, Scientific, Medical (ISM) band at 2.4 Ghz is an
`exception;
`it is available worldwide although the precise
`operational channels may differ per country.
`The usage of the ISM band is restricted to radio systems
`applying signal spreading. In this way, uncoordinated sys-
`tems spread their interference. Each system is given a fair
`chance to make use of the spectrum and no single system can
`dominate the usage. Acost-effective spreading method is the
`use of frequency-hop spreading, that is the ISM band is
`divided into a number, M, of RF hop frequencies and the
`channel hops from one hop frequency to the next according
`to a pseudo-random hop sequence.
`The hopping rate is restricted to a minimum of 2.5 hops/s.
`The choice of the hopping rate depends on a number of
`criteria. To obtain the optimal
`interference immunity
`(through interference diversity and statistical multiplexing)
`a high hopping rate is desired. If a hop is lost due to
`interference, only a small burst of the communications is
`lost. This is especially advantageous for voice communica-
`tions which can overcome only short periods of high bit
`error
`rates without noticeable effects. For data
`
`the choice of a suitable hopping rate
`communications,
`depends on the choice of access scheme. For an ethernet-like
`access scheme, like carrier-sense multiple access (CSMA,
`also known as “listen before talk”), a slow hopping is
`desired for optimal contention resolution.
`If voice and data have to be combined, a high hopping rate
`must be used for voice transmission, requiring a different
`access scheme for the data. Instead of CSMA/CA (CSMA/
`collision avoidance), a polling scheme is used in which a
`central unit, the master, controls the access to the channel.
`The system has been designed in which all units are peer
`units in principle, but when establishing a connection
`between the units, one of the units will be the master
`whereas the other units will become slaves. The master-
`
`slave relation is only valid for the duration of the connection.
`The master can set up a piconet. The piconet uses a FH
`channel at a high hopping rate. A strict TDD scheme is used
`in which the master-to-slave and slave-to-master transmis-
`
`sions alternate at the hopping rate. The FH sequence is
`determined by the master identity, the phase in the sequence
`is determined by the master system clock. At connection
`setup, the master transfers its identity and clock to all slaves.
`By using this single identity and clock, all users (master and
`slaves) are synchronized and can follow the hopping chan-
`nel. FIG. 1 is a timing diagram of an exemplary FH-TDD
`channel as meant in this disclosure. Each packet 101 used on
`the channel is uniquely identified by a preamble. This is, for
`example, a 64-bit unique word with good cross- and auto-
`correlation properties. The preamble is derived from the
`master identity. A packet 101 has to have the proper pre-
`amble before it is accepted by the participating units. The
`preamble can be regarded as the channel identifier since it
`identifies the packets belonging to the channel. Apacket 101
`has a typical format as shown in FIG. 2. In the example, the
`preamble 201 is followed by a header 203 which is followed
`by a payload 205.
`Each unit has a unique identity, which is for example
`derived from the 48-bit IEEE 802 address space. This
`identity is only used at the time of call set up for the purpose
`of paging a unit. During the connection, a temporary MAC
`address is used. This can be a much smaller address, for
`example 3 bits, since it only has to distinguish between the
`participating units. This MAC address is part of the header
`203. The addressing is schematically shown in greater detail
`in FIG. 3. Each unit has a unique identity (wake-up
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`US 6,570,857 B1
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`identifier) 301 which is used during the paging process. A
`channel identifier 307 is derived from the master identity
`303. Finally, a MAC address 305 in the header identifies the
`units participating in the same piconet. The unit identities
`301 and the derived channel identifiers 307 are unique. The
`MAC address 305 is only allocated temporarily and is valid
`during the connection. The all-zero MAC address is reserved
`for broadcast messages. To avoid collisions on the channel,
`the master and slave strictly follow the TDD scheme: the
`master is only allowed to transmit in the master-to-slave slot,
`and the slaves are only allowed to transmit in the slave-to-
`master slot. In order to avoid collisions between slaves, the
`only slave that is allowed to transmit is that slave which was
`addressed with its MAC address 305 by the master in the
`preceding master-to-slave slot. This is called polling: a slave
`can only respond when polled/addressed by the master. This
`polling can occur implicitly, that is by sending a packet
`containing traffic in the payload addressed to the proper
`slave; or explicitly by using a special POLL packet without
`payload but addressed to the proper slave.
`The MAC address 305 is much smaller than the unit
`
`identity 301. This will reduce the overhead in the packet
`because the (Forward Error Correction (FEC) encoded)
`MAC address 305 is present
`in each packet header.
`However, this limits the amount of units that can participate
`in a piconet. In particular, units that want to be attached to
`a piconet but not actively involved in communications (like
`sleeping participants) need a MAC address 305 which is
`inefficiently used. Therefore, a method is now described that
`allows units to remain parked to the piconet channel, without
`their being assigned a MAC address 305.
`Units that have been in connection with the piconet have
`all the information needed to remain synchronized to the
`piconet, that is, they have the master identity 303 and the
`master clock. From the master identity 303, the FH sequence
`and the channel identifier 307 (packet preamble) can be
`derived; from the clock, the phase in the FH sequence can
`be derived. Occasionally, a unit has to listen to the master
`transmission to adjust its clock to account for clock drifts.
`We now distinguish between four different modes of opera-
`tion: STANDBY, ACTIVE, HOLD, and PARK.
`In
`STANDBY, a unit is not attached to any other device. It
`periodically wakes up to listen to page messages. The page
`message must include the unit’s identity. Aunit in ACTIVE
`mode uses the master identity 303 and clock to keep
`synchrony to the FH channel and to extract the proper
`packets by filtering the packets with the proper preamble. In
`addition, it has a MAC address 305 to be recognized by the
`master. Units that for a short moment can be put inactive will
`enter the HOLD mode. In this mode, the slave sleeps for a
`pre-determined period of time, after which it becomes active
`again. During the sleep mode, the slave cannot get access to
`the channel, nor can it be reached by the master. A slave in
`HOLD mode retains its MAC address 305. A slave that can
`
`be put inactive for a longer amount of time will enter the
`PARK mode. In this mode, a slave gives up the MAC
`address 305, thereby making that MAC address 305 avail-
`able for assignment to another slave unit. The slave in PARK
`mode wakes up periodically to listen for the channel iden-
`tifier 307 to adjust its clock to account for drifts.
`To let parked units participate again, a special access
`method has to be carried out. Amaster can activate a parked
`unit by paging it. To facilitate this paging,
`the master
`transmits a broadcast message at
`regular
`intervals
`(hereinafter also referred to as a beacon). During this beacon
`event,
`the parked unit can become active again, so in
`principle, the parked slave only has to wake up during the
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`beacons. The broadcast message is identified by a predefined
`MAC address that is never assigned to any of the slaves. In
`the exemplary embodiment, the predefined MAC address is
`the all-zero MAC address. To activate a parked slave, the
`master pages this slave by including the slave’s identity 301
`in the payload 205 of the broadcast packet 101. In addition,
`the payload 205 includes the (temporary) MAC address 305
`to be used by the parked slave. Aslave paged in this manner
`is allowed to respond directly in the slave-to-master slot
`following. The slave also retains the assigned MAC address
`305, so that it will recognize future packets directed to it by
`the master unit.
`
`For the parked slave to get access to the channel, a
`different approach is required. Again, collisions between
`different parked slaves that desire access simultaneously,
`must be avoided. The following scheme is used. When
`entering the PARK mode,
`the parked unit is allocated a
`response number by the master. This response number is
`used by the parked slave to determine when it is allowed to
`transmit a channel access request to the master. Channel
`requests are only allowed when the beacon indicates that
`requests can be sent. Instead of polling each parked slave
`separately, a broadcast poll is transmitted indicating to the
`parked units that
`they are allowed to request access.
`However, the response number determines in which slot a
`parked slave is allowed to transmit an access request. In
`another aspect of the invention, in order to speed up the
`request procedure, the slave-to-master slots may be divided
`into a plurality of slave-to-master sub-slots. For purposes of
`illustration,
`the exemplary embodiment utilizes two half
`slots per slave-to-master slot. However, alternative embodi-
`ments may use more or fewer than two half slots per
`slave-to-master slot.
`
`For a request, the parked slave only has to transmit the
`channel identifier 307 (which is just a preamble 201). The
`identifier and its position with respect to the broadcast poll
`indicates to the master which parked slave is requesting
`access.
`It can then send a page message to this slave
`(including the slave’s identity 301 and a MAC address 305)
`to activate the slave.
`
`An example will further clarify the PARK mode proce-
`dures. FIG. 4 is a timing diagram showing how a broadcast
`message 401 (e.g., packet that includes a header 203 con-
`taining a zero MAC address) is sent at regular intervals to act
`like a beacon. Units in PARK mode only wake up during the
`beacon (or alternatively during only every N beacons in
`order to reduce power consumption). Referring now to FIG.
`5, when a master wants to activate parked slave A, it sends
`a