Abstract:
A method of operating a wireless network comprising a plurality of wireless terminals and optionally a base station uses a transmission frame ( 300 ) including an association section ( 310 ) for use by wireless terminals wishing to join the network. A variable number of association sections ( 310 ) is scheduled in response to operating characteristics such as the load on the network or the presence of a high security application on the network.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to wireless networks for communication between a number of wireless terminals which are free to join and leave the network at any time. The network may be configured so that all the wireless terminals communicate through a base station, or so that the devices communicate directly with one another. 
     Wireless networks are intended to offer flexible and cost-effective alternatives to wired networks, for example as Local Area Networks (LANs). An advantage of wireless over fixed networks is the ease with which new networks can be created and with which new devices, known as wireless terminals, can be added to the network. Some wireless network standards exist, for example IEEE 802.11 in the USA and HIPERLAN in Europe, whilst others are under development, for example Wireless Asynchronous Transfer Mode (WATM). Such standards define the format of a transmission frame, within which control and data transfer functions can take place. The format and length of transmission frames may be fixed or dynamically variable. 
     Known wireless networks, for instance as disclosed in U.S. Pat. No. 5,274,841, typically operate in one of two different configurations: 
     1 A base station orientated configuration in which wireless terminals can only communicate with each other with the support of a base station, which is responsible for all wireless terminals within a certain radio coverage area. The base station itself may be connected to further wired networks for communications beyond the radio coverage area. The base station may be a separate unit, or it may be a wireless terminal with additional functionality. 
     2 A peer to peer configuration in which wireless terminals communicate directly with one another. One of the wireless terminals is typically chosen dynamically to perform the control function. 
     The first configuration is generally considered to be most suitable for networks with a relatively fixed structure, while the second is considered most suitable for the generation of dynamic networks, such as for document exchange at a meeting. 
     In order for a wireless network to realise the advantage of flexibility, the procedure for wireless terminals joining and leaving the network should be as simple as possible. The act of a wireless terminal joining a network is known as association and needs to be scheduled within the data transmission format defined for the wireless network. Examples of known techniques for association include dynamic framelength slotted ALOHA, described in GB-B-2 069 799, carrier sense multiple access and address based tree resolution algorithms. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the present invention is to improve the flexibility and efficiency of association to a wireless network. 
     According to a first aspect of the present invention there is provided a method of operating a wireless network, wherein transmission of data takes place in transmission frames of variable length subdivided into a plurality of time slots, the wireless network comprising a controller and a variable number of wireless terminals, the method comprising the controller maintaining a list of wireless terminals currently registered with the network, determining the lengths of successive transmission frames, and allocating time slots within said frame for various purposes, characterised by the controller scheduling a variable number n of association sections in each transmission frame, where n has a minimum value of zero, in response to the current operating characteristics of the network, by which association sections the wireless terminals can register with the controller. 
     If desired, the association sections may use a contention based protocol. 
     According to a second aspect of the present invention there is provided a controller for a wireless network having a plurality of wireless terminals, the controller comprising means for maintaining a list of wireless terminals currently registered with the network, means for monitoring the operating characteristics of the network, means for determining the lengths of successive transmission frames and means for allocating time slots within each of said frames, characterised by means responsive to the current operating characteristics of the network for scheduling a variable number n of association sections in each transmission frame, where n has a minimum value of zero. 
     According to a third aspect of the present invention there is provided a wireless terminal for use in a wireless network constituted by at least one wireless terminal and a controller, the wireless terminal comprising transceiver means for obtaining scheduling information relating to transmission frames in the network, characterised by means for determining whether an association section is available in a transmission frame and in that in response to said means the wireless terminal attempts association during the association section. 
     The present invention is based upon the recognition, not present in the prior art, that adding an explicit association section to a transmission frame format and enabling this association section to be scheduled more or less frequently depending on system load improves the efficiency and flexibility of operation of a wireless network. 
     By means of the present invention a wireless network can schedule a variable number of association sections dependent on considerations such as network loading and the number of active wireless terminals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a block schematic diagram of a wireless network made in accordance with the present invention, having a base station orientated configuration; 
     FIG. 2 is a block schematic diagram of a wireless network made in accordance with the present invention, having a peer to peer configuration; 
     FIG. 3 is a diagram of an example of a possible transmission frame format suitable for use in a wireless network configured with a base station of the type shown in FIG. 1; 
     FIG. 4 is a diagram of an example of a possible transmission frame format suitable for use in a peer to peer configured wireless network of the type shown in FIG. 2; 
     FIG. 5 shows in outline the process of a wireless terminal associating with the network; 
     FIG. 6 is a diagram showing an expanded view of the association section in a transmission frame; and 
     FIG. 7 in the lower half is a graph showing an example of the variation of network load with time (t) and in the upper half showing the corresponding scheduling of association sections. 
    
    
     In the drawings the same reference numerals have been used to indicate corresponding features. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The base station orientated wireless network shown in FIG. 1 comprises a number of wireless terminals  102  and a base station  108 , which base station is optionally connected to a fixed telecommunications network, for example the PSTN. The wireless terminals  102  comprise transceivers  106  which communicate with one another via the base station  108  which comprises a transceiver  106  and a controller  110 . The controller  110  may be at the same location as the transceiver  106  or remote from it and connected by land line connection means. A variety of devices may be configured to act as wireless terminals  102 , for example portable computers, printers and measurement equipment. In addition to the transceiver  106  each wireless terminal also comprises an application-specific block  104  which implements the required functionality. 
     The peer to peer wireless network shown in FIG. 2 comprises a number of wireless terminals  102  and an enhanced wireless terminal  202 , which comprises a transceiver  106  and application-specific block  104 , as for a standard wireless terminal, and further comprises a controller  110  to enable it to perform the control function in a wireless network if required. The wireless terminals  102  and enhanced wireless terminals  202  communicate directly with one another rather than via a base station. If there is only one enhanced wireless terminal  202  it performs the control function for the network, otherwise, one of the enhanced wireless terminals  202  is chosen dynamically to perform the control function. 
     The format of the transmission frame  300  shown in FIG. 3 is for a base station orientated configuration of the type shown in FIG.  1 . This particular format is intended for use with WATM and similar standards aimed at wireless local area networks. The format is a time division format comprising successive transmission frames  300 , where each transmission frame  300  is divided into a number of sections, each of which contains a number of slots in which data is transmitted. The lengths of the complete frames and the sections within them can be varied in response to changing requirements for data transmission to and from the wireless terminals  102 . 
     A control section  302  is used by the base station  108  to signal the allocation of slots to wireless terminals  102  in the sections comprising the remainder of the frame  300 . The allocation takes into consideration the number of slots requested by the wireless terminals  102 , the available bandwidth, restrictions on the length of transmission frames and any other relevant criteria. 
     A downlink section  304  is used for transfer of data from the base station  108  to wireless terminals  102 , the transfer of data to a particular wireless terminal  102  taking place in the slots allocated for it during the control section  302 . An uplink section  306  is used for transfer of data in the reverse direction, from wireless terminals  102  to the base station  108 . Again, the transfer of data from a particular wireless terminal  102  takes place in the slots allocated for it during the control section  302 . 
     An energy burst section  308  is provided for use by wireless terminals  102  that have associated with the network but are inactive. Each of these terminals is assigned a unique slot within the energy burst section  308  in which it may transmit to indicate that it requires an uplink slot to transmit data. The base station  108  will then take this request into account when scheduling the slots in the uplink section  306  of the next transmission frame  300 . 
     An association section  310  is provided for use by wireless terminals  102  that wish to join the network. The detailed working of this section is described below. The association section may not always be present, as it is scheduled under control of the base station  108 . The base station  108  may modify the scheduling of the association section  310  to: 
     Schedule an association section  310  when it is convenient. This minimises the risk of losing transmission packets due to peaks in transmission traffic. Under conditions of high network traffic the base station  108  may not schedule an association section  310  in a transmission frame. In order to avoid completely shutting out new wireless terminals  102  the base station  108  may be controlled so that there is a maximum time delay between one association section and the next. 
     Schedule association sections  310  more frequently during conditions of low network traffic. This allows faster association for waiting wireless terminals  102 . 
     Schedule multiple association sections  310 . This may be desirable when many wireless terminals  102  wish to associate, for example if no association sections  310  have been scheduled for some time. 
     Schedule no association sections  310 , which may be desirable in some special circumstances. One example when the network has no spare capacity, another example is when a high security application is running and wishes to prevent any further wireless terminals  102  joining the network. 
     It should be noted that the order of sections within the transmission frame illustrated in FIG. 3 may be varied, in particular it may be beneficial to schedule the energy burst section  308  and the association section  310  near the start of the frame to reduce transmission time delays. 
     The format of the transmission frame  400  shown in FIG. 4 is for a peer to peer configuration of the type shown in FIG. 2, although much of the format is the same as for the base station orientated frame format shown in FIG. 3. A control section  402  is used by the enhanced wireless terminal  202  acting as base station to allocate slots to wireless terminals  102  in the remainder of the frame. There are no separate downlink and uplink sections instead there is a data transfer section  405 , where transfer of data from one wireless terminal  102  to another occurs in the slots allocated to it, the reservation of which was detailed during the control section  402 . 
     The energy burst section  408  and association section  410  work in a similar manner to a base station orientated configuration. 
     Consider now in more detail the sequence of events occurring when a wireless terminal wishes to associate with a network, taking a base station orientated configuration as the example. FIG. 5 shows a sequence of transmission frames  502 ,  504 ,  506 ,  508 ,  510  and  512  transmitted over time t. Each frame has a format identical to or based on that shown in FIG. 3, although the total length of the transmission frame  300  and the sections within it may vary. At time t 1  a wireless terminal  102 , identified subsequently as WTA, is switched on and wishes to associate with the network. The control section  302  of frame  502  indicates that no association section  310  is scheduled in this frame. The control section  302  also indicates the length of the frame  502 , to enable WTA to turn off its radio transceiver for the duration of the frame to save battery power. 
     WTA then turns on its transceiver to receive the control section  302  of the next frame  504  which indicates that there is no association section  310  scheduled in this frame. WTA optionally turns off its receiver for the remainder of this frame to conserve power. The same procedure is followed for the subsequent frames  506 ,  508  and  510  which also have no association section  310  scheduled. Finally, scheduling information transmitted within the control section  514  of the next frame  512  indicates that an association section  516  is scheduled for the frame  512 . 
     An expanded view of the association section  516  of frame  512  is shown in FIG.  6 . In this example the association section  516  is subdivided into  20  slots, indicated by the tick marks in the figure. WTA has determined when the association section  516  will occur within the frame  512  from scheduling information transmitted in the control section  514  of frame  512 . WTA then proceeds to poll in this section, for example using a contention based protocol such as the slotted ALOHA format disclosed in GB-B-2 069 799, which requires WTA to choose at random a slot within the association section  516 . FIG. 6 shows WTA polling in the fourth slot  602 . The base station  108  listens to any polls occurring during the association section  516 , and notes the association request in the fourth slot  602 . 
     The base station  108  now schedules a transmission slot in the uplink section  306  of a subsequent transmission frame. At this point the base station  108  can only determine that a wireless terminal attempted association in slot  602  of the association section  516  in frame  512 . Hence in the control section  302  of the subsequent frame slots in the uplink section  306  are reserved for the wireless terminal  102  that attempted association in the fourth slot  602  of the association section  516 . The information transmitted by WTA during this uplink slot enables the base station  108  to decide whether to allow WTA to associate with the network. If WTA is allowed to associate with the network, the association proceeds in the manner defined for the particular network standard being used. 
     It is possible for two or more wireless terminals  102  to poll in the same slot during an association section  310 . This might not be detected by the base station  108  which would typically just be detecting whether any energy was transmitted in a particular slot. Hence the same wireless terminals  102  would then transmit during the transmission slot in the uplink section  306  reserved for the associating wireless terminal. If the base station  108  receives a corrupted signal during the scheduled transmission slot it therefore assumes that two or more wireless terminals  102  have transmitted at the same time, and takes appropriate actions. These actions may include not acknowledging the association requests, and scheduling another association section  310  as soon as convenient since it is known that there are wireless terminals  102  attempting association. The wireless terminals themselves know by not receiving an acknowledgement that their requests have not been successful, and will attempt association again. 
     A method of association in accordance with the present invention has the advantage of allowing the base station  108  the flexibility to determine when to schedule association sections  310 . This is useful during periods of high transmission activity, when an association section  310  may be postponed to maximise data throughput in the network. An upper limit to the length of time that an association section  310  can be postponed may be specified for the system. The flexibility in scheduling association sections  310  is also useful during periods of lower activity, when more association sections  310  may be scheduled to enable wireless terminals  102  to associate with small time delays. 
     A possible scenario showing the scheduling of association sections with varying network load is shown in FIG.  7 . The lower part of the figure is a plot of network activity A against time t, where A might be measured in transmitted bytes per second. The upper part of the figure shows the times at which association sections  310  are scheduled. In this example when the network load is high the frequency of association sections  310  is reduced, but the base station  108  also ensures that the time between association sections  310  never exceeds a maximum (for example at least one association section  310  every five seconds). 
     The approach described above is equally applicable to a peer to peer network using the transmission frame format shown in FIG.  4 . The main difference is that the control function for the network is performed by a dynamically chosen enhanced wireless terminal  202 . 
     From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of wireless networks and component parts thereof which may be used instead of or in addition to features already described herein.