USOO6993049B2
`
`(12)Un1ted States Patent
`(10) Patent N0.:
`US 6,993,049 132
`
`Davies
`(45) Date of Patent:
`*Jan. 31, 2006
`
`(54) COMMUNICATION SYSTEM
`
`(75)
`
`Inventor: Robert J- Davies, Horley (GB)
`.
`(73) A551gnee: Koninklijke Philips Electronics N.V.,
`Eindhoven (NL)
`
`(52) us. Cl.
`
`...................... 370/528; 370/346; 370/449;
`370/465
`(58) Field of Classification Search ................ 370/311,
`370/312; 328—330; 336; 337; 346; 347; 350;
`370/390 465 470 473 476 491 528 449
`See application file for complete search history.
`
`(*) Notice:
`
`This patent issued on a continued pros-
`ecution application filed under 37 CFR
`1.53(d); and is subject to the twenty year
`patent term prov151ons of 35 U.S.C. 154
`(20(2)
`
`Subject to any disclaimer; the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 635 days.
`
`This patent is subject to a terminal dis-
`claimer.
`
`(21) Appl. No.: 09/876,514
`
`(22)
`
`Filed:
`
`Jun. 7, 2001
`
`(65)
`
`Prlor Publlcatlon Data
`US 2002/0028657 A1
`Mar. 7; 2002
`
`Foreign Application Priority Data
`(30)
`Jun. 26 2000
`(GB)
`.................................... 0015454
`Aug. 15 2000
`(GB)
`.................................... 0020076
`’
`Int. Cl.
`H04] 3/12
`H04] 3/16
`H04L 12/403
`
`(200601)
`(2006.01)
`(2006.01)
`
`(51)
`
`(56)
`
`References Cited
`
`US. PATENT DOCUMENTS
`
`8/1988 Matai et a1.
`............... 340/755
`4,766,434 A *
`
`6/2003 Haartsen ............... 375/133
`6,574,266 B1 *
`............... 340/505
`6,664,891 B2 * 12/2003 Davies et al.
`
`* cited by examiner
`
`Primary Examiner—Chi Pham
`Assistant Examiner—Ronald Abelson
`
`(57)
`
`ABSTRACT
`
`Acommunications system comprises a primary station (100)
`and at least one secondary station (101). The primary station
`(100) is arranged to broadcast a series of inquiry messages;
`each in the form of a plurality of predetermined data fields
`arranged according to a first communications protocol such
`as Bluetooth. In addition the primary station (100) adds to
`some or all of the inquiry messages an additional data field
`fer Pelhhg ehe er mere eeeehdary Stahehe Whieh eah
`reSPehd to the P011 if they haVe data for hahShhSSieh- This
`system provides secondary stations (101) With a rapid
`response time Without the need for a permanently active
`communication link.
`
`12 Claims, 3 Drawing Sheets
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`LG 1001
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`FIG. 4
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`502
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`504
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`FIG. 5
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`Sheet 3 0f3
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`RX DATA
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`606@
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`US 6,993,049 B2
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`1
`COMMUNICATION SYSTEM
`
`The present invention relates to a communication system
`and further relates to primary and secondary stations for use
`in such a system and to a method of operating such a system.
`Although the present invention is described with particular
`reference to a Bluetooth system, it is applicable to a range
`of other communication systems.
`in
`In recent years there has been increasing interest
`enabling devices to interact via wireless communication
`links, thereby avoiding the need for extensive cabling. An
`example of a communication system which may be used for
`such wireless links is a Bluetooth network, operating
`according to the specification defined by the Bluetooth
`Special Interest Group. Such a network is intended to
`provide low-cost, short range radio links between mobile
`PCs, mobile phones and other devices, whether portable or
`not.
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`Communication in a Bluetooth network takes place in the
`unlicensed ISM band at around 2.45 GHZ. Stations form
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`ad-hoc networks which are known as piconets, each com-
`prising a master station and up to seven slave stations. All
`stations are identical and capable of acting as master or slave
`as required. A station can take part in more than one piconet,
`thereby linking piconets and enabling communication over
`an extended range.
`One application for which use of Bluetooth is proposed is
`the connection of controller devices to host systems. A
`controller device, also known as a Human/machine Interface
`Device (HID), is an input device such as a keyboard, mouse,
`games controller, graphics pad or the like. Such HIDs do not
`typically require a link having high data throughput, but do
`require a very responsive link.
`A Bluetooth system is more than capable of supporting
`the throughput requirements of HIDs. However, the degree
`of responsiveness required can be more difficult to achieve.
`An active Bluetooth link can offer a reasonably responsive
`service, but this requires both setting up of the link and its
`maintenance even during periods of inactivity.
`Two types of communication link supported in a Blue-
`tooth network are Asynchronous ConnectionLess (ACL)
`links and Synchronous Connection Oriented (SCO) links.
`ACL links allow slaves to enter a ‘park’ mode and cease
`active communications, which also allows a number of other
`HIDs to maintain links with the master station without
`
`violating the Bluetooth rule that no more than seven slaves
`can be active at any one time. A slave has to be polled before
`it can submit a request to leave park mode and become
`active. SCO links require continuous operation by a slave,
`but
`there are only a limited number of SCO channels
`available.
`
`Setting up a link requires a HID to join, as a slave, the
`piconet including the host system (which will typically act
`as piconet master, i.e. a base station). Joining the piconet
`requires two sets of procedures, namely ‘inquiry’ and ‘page’.
`Inquiry allows a would-be slave to find a base station and
`issue a request to join the piconet. Page allows a base station
`to invite slaves of its choice to join the net. Analysis of these
`procedures indicates that the time taken to join a piconet and
`then to be in a position to transfer user input to the master
`could be several tens of seconds.
`
`It is possible for this procedure to be carried out once and
`for all when the host system is turned on. However, HIDs
`will normally be battery operated and it is therefore not
`acceptable for them to have to remain permanently switched
`on.
`In particular,
`for a HID to sign on to the piconet
`automatically when the host system is turned on it will either
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`have to be regularly waking up to look for Bluetooth inquiry
`bursts,
`thereby consuming power, or it will need to be
`manually woken up by the user.
`It is therefore more likely that a HID will remain inactive
`until it is woken up, either by being explicitly switched on
`or by a user attempting some form of input. Hence, the host
`system’s Bluetooth master will need to run inquiry cycles
`periodically, which has two implications. The first is that
`because the inquiry phase is periodic rather than continuous,
`initial access time could be several tens of seconds. This
`could mean that it could take half a minute or more from the
`
`time a user moves a mouse to a cursor moving on a screen.
`Secondly, the fact that an inquiry cycle takes place at all
`means that ACL links will be suspended during this cycle,
`for up to 10.24 seconds at a time. Although SCO links could
`be used, a HID using such a link could not cease transmis-
`sions during inactive periods.
`It is therefore an object of the invention to address the
`problem of providing a responsive link between a HID and
`a host system which allows the HID to go to sleep during
`periods of inactivity.
`According to a first aspect of the present invention there
`is provided a communications system comprising a primary
`station and at
`least one secondary station, wherein the
`primary station has means for broadcasting a series of
`inquiry messages, each in the form of a plurality of prede-
`termined data fields arranged according to a first communi-
`cations protocol, and means for adding to an inquiry mes-
`sage prior to transmission an additional data field for polling
`at least one secondary station, and wherein the or each
`polled secondary station has means for determining when an
`additional data field has been added to the plurality of data
`fields, for determining whether it has been polled from the
`additional data field and for responding to a poll when it has
`data for transmission to the primary station.
`According to a second aspect of the present invention
`there is provided a primary station for use in a communi-
`cations system comprising at least one secondary station,
`wherein means are provided for broadcasting a series of
`inquiry messages, each in the form of a plurality of prede-
`termined data fields arranged according to a first communi-
`cations protocol, and for adding to each inquiry message
`prior to transmission an additional data field for polling at
`least one secondary station.
`According to a third aspect of the present invention there
`is provided a secondary station for use in a communications
`system comprising a primary station, wherein means are
`provided for receiving an inquiry message broadcast by the
`primary station, the message being in the form of a plurality
`of predetermined data fields arranged according to a first
`communications protocol and having added to it an addi-
`tional data field for polling at least one secondary station,
`and wherein means are provided for determining when an
`additional data field has been added to the plurality of data
`fields, for determining whether it has been polled from the
`additional data field and for responding to a poll when it has
`data for transmission to the primary station.
`According to a fourth aspect of the present invention there
`is provided a method of operating a communication system
`comprising a primary station and at least one secondary
`station, the method comprising the primary station broad-
`casting a series of inquiry messages, each in the form of a
`plurality of predetermined data fields arranged according to
`a first communications protocol, and adding to an inquiry
`message prior to transmission an additional data field for
`polling at least one secondary station, and further compris-
`ing the or each polled secondary station determining when
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`US 6,993,049 B2
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`3
`an additional data field has been added to the plurality of
`data fields, determining whether it has been polled from the
`additional data field and responding to a poll when it has
`data for transmission to the primary station.
`Embodiments of the present
`invention will now be
`described, by way of example, with reference to the accom-
`panying drawings, wherein:
`FIG. 1 is a diagram of an ad-hoc wireless network
`comprising two linked piconets;
`FIG. 2 is a block schematic diagram of a typical Bluetooth
`station;
`FIG. 3 is a chart illustrating the transmission of a train of
`inquiry access codes centred on a given frequency;
`FIG. 4 illustrates alternation between trains of inquiry
`messages over the duration of an inquiry broadcast;
`FIG. 5 illustrates the insertion of a packet of broadcast
`data within an existing transmission slot; and
`FIG. 6 is a flow chart illustrating a method of polling a
`HID in accordance with the present invention.
`In the drawings the same reference numerals have been
`used to indicate corresponding features.
`In the following description we consider particularly a
`system which utilises Bluetooth protocols for communica-
`tion of messages between stations. As will be recognised, the
`general invention concept of polling HIDs via a broadcast
`channel used as part of the inquiry procedure is not restricted
`to Bluetooth devices and is applicable to other communica-
`tions arrangements, in particular frequency hopping sys-
`tems.
`
`A basic Bluetooth network configuration is illustrated in
`FIG. 1. Such a configuration would typically begin with two
`connected host devices, for example a portable PC and a
`cellular phone, and grow to include additional connected
`devices. A wide range of additional host devices may be
`included, for example wireless headsets, personal organisers
`and home entertainment equipment. The network comprises
`a plurality of stations 100, 101 each included in such a host
`device, formed into two piconets 102a, 102b. In general the
`networking components (i.e. the Bluetooth chip for a Blue-
`tooth network) of all stations 100, 101 will be implemented
`identically. However, it is only necessary that all stations
`100, 101 comprising the network are able to operate accord-
`ing to a compatible protocol.
`The first piconet 102a is a point-to-multipoint network
`comprising four stations 100, a master 100(A) and three
`slaves 101(A1,A2,A3), with bidirectional communication
`channels 104 between the master 100 and each of the slaves
`
`101. The second piconet 102b is a point-to-point network
`comprising a master 100(B) and a slave 101(B1). Commu-
`nication between the piconets 102a, 102b is enabled by the
`master Am the first piconet 102a also acting as a slave in the
`second piconet 102b and vice versa. It is not necessary for
`the link between the piconets 102a, 102b to be between
`masters:
`it would be equally possible for the link to be
`between stations A3 and B1 or between A and B1, for
`example.
`An example of a station 100 for use in a Bluetooth system
`is illustrated in more detail in FIG. 2, and comprises two
`main sections. The first section is an analogue unit compris-
`ing a radio (RF) 202 having an antenna 204 for transmitting
`and receiving radio signals on the communication channel
`104. The second section is a digital controller unit 206,
`further comprising a link baseband controller (LC) 208, a
`microprocessor (,uP) 210 and an interface unit (INT) 212.
`The link controller 208 comprises means for performing
`baseband processing and execution of basic protocols close
`to the physical layer, for example implementing error cor-
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`rection coding, generating Automatic Repeat
`(ARQ) and performing audio coding. The microprocessor
`210 manages the station 100, controlling data transfer
`between the interface unit 212 and the link controller 208.
`
`The interface unit 212 comprises hardware and software for
`interfacing the station 100 to a host device such as a portable
`PC or a cellular phone. The interfacing is performed via a
`link 214, which might include interfaces to a USB (Univer-
`sal Serial Bus), external memory and other items as appro-
`priate for the particular application.
`The Bluetooth inquiry procedure allows a would-be slave
`101 to find a base station and issue a request to join its
`piconet.
`It has been proposed specifically to overcome
`problems caused by the frequency-hopping nature of Blue-
`tooth and similar systems. The applicants have recognised
`that it is possible to piggy-back a broadcast channel on the
`inquiry messages issued by the master 100. The broadcast
`channel can be used to poll HIDs at regular intervals.
`However, at the air interface,
`the mechanism is entirely
`compatible with conventional Bluetooth systems.
`To illustrate how this is implemented, we first consider
`how the Inquiry procedures themselves operate, with refer-
`ence to FIGS. 3 and 4. When a Bluetooth unit wants to
`discover other Bluetooth devices,
`it enters a so-called
`inquiry substate. In this mode, it issues an inquiry message
`containing a General Inquiry Access Code (GIAC) or a
`number of optional Dedicated Inquiry Access Codes
`(DIAC). This message is repeated at several levels; first, it
`is repeated in a train A of 16 frequencies from a total of 32
`making up the inquiry hopping sequence. The message is
`sent twice on two frequencies in even timeslots with the
`following, odd timeslots used to listen for replies on the two
`corresponding inquiry response hopping frequencies. Six-
`teen frequencies and their response counterparts can there-
`fore be covered in 16 timeslots, or 10 ms. The chart of FIG.
`3 illustrates the transmission of a single train on sixteen
`frequencies centred around f{k}, where f{k} represents the
`inquiry hopping sequence.
`The next step is the repetition of the train a plurality of
`times. At the very least, as presently specified, this means
`256 repetitions of the entire train. Finally, the train A is
`swapped for the train B consisting of the remaining 16
`frequencies and the cycle repeated. As shown by FIG. 4, the
`specification states that this switch must occur at least three
`times to ensure the collection of all responses in an error-free
`environment. This means that an inquiry broadcast could
`take at least 10.24 seconds.
`
`A portable device that wants to be discovered by a
`Bluetooth master 100 enters the inquiry scan substate. Here,
`it listens for a message containing the GIAC or DIACs of
`interest. It, too, operates in a cyclic way. It listens on a single
`hop frequency for an inquiry scan period which is long
`enough to cover the 16 inquiry frequencies used by the
`inquiry. On hearing an inquiry containing an appropriate
`IAC, the portable device enters a so-called inquiry response
`substate and issues a number of inquiry response messages
`to the master 100. The master 100 will then page the portable
`device, inviting it to join the piconet.
`As mentioned above and shown in FIG. 5, the applicants
`propose that the inquiry messages issued by the base station
`have an extra field 504 appended to them, capable of
`carrying a HID poll message. The extended field 504 may
`carry a header that signifies a HID poll to distinguish it from
`other applications of extended field information, such as
`context-aware services or broadcast audio (as disclosed in
`our
`co-pending United Kingdom patent
`applications
`00154542 (applicant’s
`reference PHGB 000084)
`and
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`US 6,993,049 B2
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`00154534 (applicant’s reference PHGB 000085) respec-
`tively). It will also carry the address of the HID being polled,
`and may also carry a small amount of information to the HID
`which might be used to provide supplementary information
`to a user (such as text on an LCD screen) or feedback (for
`example, motional feedback in games controllers). By add-
`ing the field to the end of the inquiry message, it will be
`appreciated that non-HID receivers can ignore it without
`modification. In addition, by using a special DIAC to signify
`a HID poll, HID devices can be alerted to the presence of the
`forthcoming poll.
`The presence of the extra data field 504 means that the
`guard space conventionally allowed at the end of a Blue-
`tooth inquiry packet
`is reduced. However,
`this space is
`provided to give a frequency synthesiser time to change to
`a new hop frequency and will be generally unused other-
`wise, as current frequency synthesisers are capable of
`switching at speeds which do not need extension into the
`extra guard space. The standard inquiry packet is an ID
`packet (ID PKT) 502 of length 68 bits. Since it is sent in a
`half-slot, starting either on a slot boundary (SB) 506 or a
`half-slot boundary (HSB) 508, the guard space allocated is
`(625/2—68)=244.5 us (625 as slot period, 1 Mbit/s signalling
`rate). Modern synthesisers can switch in much less time with
`figures of 100 as or lower considered routine by experts in
`the field. Hence a suitable size for the extra data field 504
`could be 100 bits.
`
`In a typical embodiment, four of the 100 bits will be lost
`as trailer bits for the ID packet 502: this is a consequence of
`it being read by a correlator. Of the 96 bits remaining,
`applicant’s preferred allocation is for 64 bits to be used as
`data and 32 bits as a 2/3 FEC (Forward Error Correction)
`checksum. Each inquiry burst
`thus contains 8 bytes of
`broadcast data, allowing space for several channels of key
`coded or digitised analogue inputs.
`In order
`to achieve the desired responsiveness, and
`because the HID has been specifically addressed, the HID is
`allowed to respond, if desired, in the next-but-one half-slot
`with a packet of similar format, containing information
`corresponding to the user’s input. As described above, the
`inquiry procedure involves the transmission of two sets of
`sixteen frequencies in trains of inquiry transmissions. The 16
`frequencies used within a train can be considered as 16
`polling channels, and therefore 16 devices can be polled
`every 10 ms if desired. Other arrangements are possible, for
`example polling up to 32 devices every 20 ms or up to 8
`devices every 5 ms. The arrangement of polling channels
`could also be flexible, with more rapid polling provided for
`devices which need a faster response time and vice versa.
`Each device need only monitor a single frequency within
`a train, but must be able to track the train switches and
`frequency changes due to changing clock phase.
`It
`is
`assumed that an initial set up procedure synchronises the
`HID Bluetooth slave 101 to the Bluetooth master 100, as
`well as establishing the nature of the HID and the format it
`uses for uplink and downlink transmissions. At this time the
`HID is allocated a device address and a channel number
`
`corresponding to one of the sixteen channels within a train.
`For fast polling, it is necessary for the Bluetooth master
`100 to operate continuously. This interferes with the con-
`ventional mechanism for setting up two-way links. How-
`ever, use of two radios operating in tandem, as disclosed in
`our
`co-pending United Kingdom patent
`application
`0015452.6 (applicant’s reference PHGB 000086), avoids
`this problem, thereby enabling the provision of fast access to
`the piconet and an unlimited two-way throughput capacity.
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`By polling every 10 ms, with eight bytes per poll, a
`capacity of 800 bytes per second for sixteen devices is
`provided. In variations on the basic scheme described above
`this capacity could be lowered to permit the operation of
`conventional inquiry procedures, or to increase the number
`of HIDs that a host system 100 can support beyond sixteen.
`In order to minimise their power consumption, HIDs are
`not obliged to respond to every poll if they have no infor-
`mation to offer. A watchdog timer could be provided in a
`HID to make it transmit at least once in a given period whilst
`it
`is nominally active. The period could for example be
`predetermined, determined by the host system or determined
`by negotiation between the host and each HID. HIDs failing
`to transmit within the determined period would then be
`removed from the master’s list of active devices.
`
`A method of polling a HID in accordance with the present
`invention is summarised in FIG. 6. The method starts, at step
`602, when the HID has data to transmit to the host system.
`The HID receives, at step 604, data from the extra field 504
`then tests, at step 606, whether it has been polled by the host
`system. If it has not been polled, the HID returns to step 604
`to receive the next extra field 504. If the HID has been
`
`polled, it transmits its data in the next-but-one half-slot, at
`step 608.
`Since an inquiry mode is necessary to permit access to the
`host system’s piconet, it must be provided in the conven-
`tional manner for at least some of the time. There are a range
`of strategies which may be employed.
`A first strategy involves the operation of one radio in two
`modes, namely set up and polling. In set up mode the inquiry
`procedure operates as normal and the HIDs can establish
`contact with the host master 100 in the conventional manner.
`Once all HIDs have established themselves, the master radio
`switches to polling mode, in which the inquiry procedure
`now operates in polling mode only. This strategy is ideal for
`games machines since it achieves the fastest response when
`polling mode is operational, while opportunities for other
`controllers to join the piconet occur at well-defined times,
`such as between games.
`A second strategy is more appropriate when the host
`system is a general purpose device, such as a PC, in which
`case opportunities must be provided for new devices to join
`at any time. In this case the master’s radio can operate in
`modeless fashion, devoting some of its time to fast polling
`and other times to conventional inquiry operations. Alternate
`10 ms periods could be devoted to each operation to achieve
`a 50:50 ratio for example, with the ratio being able to be
`modified as desired. Such a system would still have a quick
`response to HIDs and the general
`inquiry operation,
`although possibly slower, would still operate as normal. The
`use of a special DIAC in a polling message should ensure
`that a slave 101 going through normal inquiry response
`procedures will not send an inquiry response packet in the
`space reserved for a fast poll response.
`Allowing conventional
`inquiries in parallel with fast
`polling implies that, occasionally, there will be a slight pause
`in inquiry or fast polling to allow a new HID access to the
`host. This will probably not matter to the user since he will
`no longer be using old HIDs and will therefore not be aware
`of the temporary loss in responsiveness.
`A third strategy is required when conventional Bluetooth
`data (or other) links are required, so as to support both a fast
`response for the polling mechanism and the data carrying
`capacity of conventional Bluetooth. This requires the use of
`two radios, and the approach disclosed in our co-pending
`United Kingdom patent application 0015452.6 (applicant’s
`reference PHGB 000086) can also be used here. Alterna-
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`US 6,993,049 B2
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`7
`tively, one radio could operate as a conventional Bluetooth
`radio with the other permanently dedicated to fast polling.
`From reading the present disclosure, other modifications
`will be apparent to persons skilled in the art. Such modifi-
`cations may involve other features which are already known
`in the design, manufacture and use of fixed and portable
`communications systems and component parts thereof, and
`which may be used instead of or in addition to features
`already described herein. Although claims have been for-
`mulated in this application to particular combinations of
`features,
`it should be understood that
`the scope of the
`disclosure of the present application also includes any novel
`feature or any novel combination of features disclosed
`herein either explicitly or implicitly or any generalisation
`thereof, whether or not it relates to the same invention as
`presently claimed in any claim and whether or not it miti-
`gates any or all of the same technical problems as does the
`present invention. The applicants hereby give notice that
`new claims may be formulated to such features and/or
`combinations of features during the prosecution of the
`present application or of any further application derived
`therefrom.
`
`In the present specification and claims the word “a” or
`“an” preceding an element does not exclude the presence of
`a plurality of such elements. Further, the word “comprising”
`does not exclude the presence of other elements or steps than
`those listed.
`What is claimed is:
`
`1. Acommunications system comprising a primary station
`and at least one secondary station, wherein the primary
`station has means for broadcasting a series of inquiry
`messages, each in the form of a plurality of predetermined
`data fields arranged according to a first communications
`protocol, and means for adding to an inquiry message prior
`to transmission an additional data field for polling at least
`one secondary station, and wherein the at least one polled
`secondary station has means for determining when an addi-
`tional data field has been added to the plurality of data fields,
`for determining whether it has been polled from the addi-
`tional data field and for responding to a poll when it has data
`for transmission to the primary station.
`2. A primary station for use in a communications system
`comprising at least one secondary station, wherein means
`are provided for broadcasting a series of inquiry messages,
`each in the form of a plurality of predetermined data fields
`arranged according to a first communications protocol, and
`for adding to each inquiry message prior to transmission an
`additional data field for polling at
`least one secondary
`station.
`
`3. The primary station of claim 2, wherein means are
`provided for adding the additional data field at the end of a
`respective inquiry message.
`
`8
`4. The primary station of claim 2, wherein means are
`provided for including an indication in one of the predeter-
`mined data fields, the indication denoting the presence of the
`additional data field.
`
`5. The primary station of claim 2, wherein the first
`communications protocol comprises Bluetooth messaging.
`6. The primary station of claim 2, wherein the additional
`data field comprises at least 64 bits of data.
`7. The primary station of claim 2, wherein first radio
`means are provided for broadcasting substantially continu-
`ously inquiry messages having the additional data field and
`second radio means are provided for handling other aspects
`of communication links with secondary stations.
`8. Asecondary station for use in a communications system
`comprising a primary station, wherein means are provided
`for receiving an inquiry message broadcast by the primary
`station,
`the message being in the form of a plurality of
`predetermined data fields arranged according to a first
`communications protocol and having added to it an addi-
`tional data field for polling at least one secondary station,
`and wherein means are provided for determining when an
`additional data field has been added to the plurality of data
`fields, for determining whether it has been polled from the
`additional data field and for responding to a poll when it has
`data for transmission to the primary station.
`9. The secondary station of claim 8, wherein the first
`communications protocol comprises Bluetooth messaging.
`10. The secondary station of claim 8, wherein means are
`provided for responding to a poll after a predetermined
`interval has passed without transmission of data, whether or
`not the secondary station has data for transmission.
`11. A method of operating a communication system
`comprising a primary station and at least one secondary
`station, the method comprising the primary station broad-
`casting a series of inquiry messages, each in the form of a
`plurality of predetermined data fields arranged according to
`a first communications protocol, and adding to an inquiry
`message prior to transmission an additional data field for
`polling at least one secondary station, and further compris-
`ing the at least one polled secondary station determining
`when an additional data field has been added to the plurality
`of data fields, determining whether it has been polled from
`the additional data field and responding to a poll when it has
`data for transmission to the primary station.
`12. The method of claim 11, wherein not all inquiry
`messages have an additional data field for polling a second-
`ary station added to them.
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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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`40
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`45
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`50
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`8
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