The invention relates to a display system that is updated remotely using wireless communication and in particular, but not exclusively, to such systems comprising low power displays, in particular reflective, bi-stable or multi-stable displays, sometimes referred to as electronic paper.

Background to the invention

The use of electronic point of purchase (EPOP) display units is known for maintaining product information in retail environments such as hypermarkets, for example, see WO2010/004349. Such displays enable retailers to provide information to consumers, for example the price of a product and any associated details and offers, and to update the information using a remote control unit, without having to be physically present at the product. In environments such as hypermarkets where there are thousands of products, and accordingly thousands of EPOP units, this type of system is an excellent way to control the pricing of those products.

Typically, these systems work by having a number of communicators strategically positioned throughout the building in which the display system is located. These communicators transmit information, such as display information, to groups of remote EPOP display units that are adjacent to the products with which they are associated. The information that is sent to the communicators is controlled by a central unit, thereby allowing easier control of the data that is disseminated.

With the increasing size of retail outlets, more EPOP display units are required to accompany the increasing number of products that are housed in such outlets. Accordingly, in large stores such as hypermarkets which may have upwards of 100,000 different products for sale, several communicators are required in order to transmit updated display information to the display units across the entire store. The choice of communicator with which a EPOP display unit is associated, and receives its updated display information, is generally based on the strongest signal that the EPOP receives from the communicator.

In large retail outlets, a problem associated with the large size of such outlets that have many products is that when EPOP display units are relocated within the outlet (typically by staff when rearranging isles within the store) it is difficult to keep track of where the displays are in relation to the communicators. EPOP display units are generally passive devices that do not transmit unless a communication session is initiated by a communicator. This ensures that the EPOP display units consume less power.

In an example of the prior art, one or more communicators broadcast commands that tell the EPOP display units when they should 'wake up' from a low power mode and operate in a higher power mode, the duration of time spent operating at a higher power can thus be controlled in order to reduce overall power consumption. During the periods when the EPOP display units are awake (i.e. in higher power mode), synchronisation data packets are received from the communicators, which allow the EPOP display units to calibrate their internal clocks in order to ensure that all EPOP display units are synchronised.

It is known to send a command to an EPOP display unit in order to determine which communicator it is synchronised with. However, such a process is time consuming and typically will only be able to determine which communicator the EPOP is synchronised with for approximately 2000 EPOP display units per hour. In large stores this process make take over 2 days.

A further problem is that there is often an inhomogeneous distribution of products and hence EPOP display units throughout a retail outlet. For example, lower priced items such as baked beans may be stacked in relatively high densities, with many different branded products placed in close proximity to each other, and each different product requiring updated display information. Conversely, higher-priced items such as larger electrical goods may populate shelves with a sparser distribution, thus requiring fewer EPOP display units. As layouts of the stores are typically changed on a regular basis the density distribution of the products does not remain constant. In such examples, communicators which are placed near goods which are stacked at a high density will typically be assigned a much greater number of EPOP display units (as they would have the strongest transmission signal) compared with communicators which are placed near goods stacked at a low density.

When the process of updating the EPOP display units takes place, the time that it takes for the communicators with the higher number of associated EPOP display units to perform the operation will be longer than for those communicators with fewer associated display units.

In order to mitigate for at least one of the aforementioned problems, a method of optimising the number of electronic point of purchase display units assigned to a communicator in a display system, the display system comprising a plurality of communicators in communication with a plurality of display units, is provided, the method comprising the steps of: for each of a plurality of display units, identifying one or more preferred communicators, from the plurality of communicators, for the display unit based on a measure of the signal strength between the display unit and communicator, and assigning the display unit to one of the preferred communicators, calculating for each of the plurality of communicators the number of display units assigned to the communicator, and determining whether the number of display units assigned to the communicator falls within a predetermined range if the number of display units assigned to a communicator is outside of the predetermined range, reassigning one or more display units to a different preferred communicator for the display unit, until such time that the number of display units assigned for each of the plurality of communicators is within the predetermined range.

Beneficially such a method allows for the number of EPOP display units assigned to each communicator to be distributed evenly and therefore the computational load is distributed more evenly across the communicators, resulting in a more efficient updating of EPOP display units. Furthermore, in order to mitigate for at least one of the aforementioned problems, a method of determining the location of a plurality of electronic point of purchase display units which are in communication with a first communicator is provided, the method comprising steps of broadcasting on the first communicator a broadcast command message, the broadcast command message receivable by a display unit and comprising instructions causing the display unit to broadcast a response message at a calculated time, upon receipt of the broadcast command message at a first display unit, calculating a response time at which time the first display unit is to send the response message, the response time based on the RF address of the display unit, and the response message comprising information identifying the first display unit and the first display unit broadcasting the response message at the calculated response time. Advantageously, the method allows for more efficient determination of the location of EPOP display units in relation to the communicators. Such a method allows for a much higher number of EPOP display units to be located per hour. Therefore, the time taken to locate all EPOP display units in a store is reduced.

An aspect of the invention provides an improved method for determining the location of a plurality of EPOP display units which are in communication with communicators.

Another aspect of the invention provides an improved method for optimising the number of EPOP display units assigned to a communicator in a display system, Further aspects of the invention will be apparent from the description and the appended claim set.

Brief description of the Figures

Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawing in which:

Figure 1 is a schematic of a display system; Figure 2 is a flowchart showing a method of reassigning associations between display units and communicators according to one aspect of the invention; Figure 3 is a flowchart showing a method of communication between communicators and display units;

Figure 4 is an example of response timings; Figure 5 is an example of packet structures;

Figure 6 is a flowchart showing a method of display units responding to commands from communicators; Figure 7 is a flowchart showing a method of determining whether display units need to be reassigned to different communicators;

Figure 8 is flowchart showing a method for determining the reconfiguration of associations of display units with communicators;

Figure 9 is a flowchart showing synchronisation between EPOP display units and communicators;

Figure 10 is an example of reassigning associations between display units and communicators, according to one aspect of the invention; and

Figure 11 is an example of how the use of the method according to one aspect of the invention can reduce computational requirements.

Detailed description of an embodiment Figure 1 shows an overview of a display system 10 that is implemented in a retail outlet 14. Inside the retail outlet 14 there are three communicators CI, C2, C3, which are positioned within the retail outlet 14. There are electronic point of purchase (EPOP) display units 18 distributed throughout the retail outlet 14. Figure 1 shows nineteen EPOP display units 18. In stores there are typically tens of thousands of EPOP display units 18 though for the purpose of clarity only a small number are shown in Figure 1.

Each EPOP display unit 18 is assigned an individual RF address. The RF addresses in Figure 1 are labelled from RF2 to RF 20. The EPOP display units 18 are typically not distributed in sequential order in the retail outlet 14. Furthermore, as the layout of the store will often change any ordering of the EPOP display units 18 is prone to change over time. The communicators 16 are in communication with a central processing unit 12. The central processing unit 12 can communicate with the communicators 16 either wirelessly, and/or through wired connections. The communicators may be arranged in a star or mesh configuration.

In further examples, the EPOP display units 18 are used to convey information and are distributed in other environments that are not retail environments.

The communicator 16 and EPOP display unit 18 components of the display system 10 are now described in more detail.

The communicator 16 receives commands and data from a central processing unit 12 and processes these to output them over an RF transceiver using a communications (RF) protocol. Communicator 16 is also responsible for ensuring that synchronisation signals are periodically transmitted within a precise time slot making it possible for all EPOP display units 18 to remain in synchronisation.

A synchronisation signal 31 consists of multiple packets of data or synchronisation packets 40. Each synchronisation packet 40 contains an offset from the centre of the sync period to enable an EPOP display unit 18 to re-adjust its internal clock and take care of any drift that may have occurred since the last synchronisation signal 31 was received. The EPOP display unit 18 comprises low-power multidisplay (not shown), such as bistable LCD available from ZBD Displays Limited of Malvern, UK, and as described in US 6,249, 332 and US 6,456,348. Such bistable displays are preferred as they retain the image for long periods without any applied power. The display unit typically periodically wakes to listen for instructions sent by the communicator 16 to which it is synchronised. The instructions from the comunicator 16 may contain synchronisation information, or image display information to reflect a change in price of the goods, or for example, promotional material such as advertising. Such communications are described in WO 2010/004349. According to an aspect of the invention there is provided a method of balancing the number of EPOP display units 18 assigned to a plurality of communicators and of determining the location of EPOP display units 18 within a location, such as a store.

Figure 2 is a flowchart showing the steps that are involved in optimising the number of electronic point of purchase display units 18 assigned to a communicator 16 in a display system 10.

In the prior art, EPOP display units 18 are typically assigned to the communicator with the strongest signal. This may result in a first communicator CI having many more EPOP display units 18 assigned to it than the other communicators (C2 and C3). This may result in an unbalanced system where there is an inhomogeneous distribution of EPOP units through the store. The inhomogeneous distribution of EPOPs to the communicators results in an unbalanced system which suffers from the problems detailed above, such as an increased time to update the display units.

In Figure 2 there is shown the process of optimising load balancing across a plurality of communicators. At step SI 02 the process of optimisation of the number of EPOP display units 18 by load balancing is initiated. At step SI 04 search and respond commands are transmitted by a first communicator CI to all EPOP display units 18 which are in listening range. Preferably all other communicators C2, C3 are placed in listen mode and do not transmit information during this process. The search and respond commands involve the first communicator CI broadcasting a set of commands which are received at EPOP display units 18. The broadcast commands contain instructions which when processed by the EPOP display unit 18 cause the EPOP display unit 18 to transmit a response message identifying the EPOP display unit 18 at a designated timeslot. The EPOP display units 18 then respond to the broadcast commands with response packets 80, which are transmitted in allocated response slots 34. All communicators CI, C2, C3 are placed in a listen mode during the allocated timeslots and are configured to listen for the response messages broadcast by the EPOP display units 18. Therefore, it is possible for each communicator CI, C2, C3 to determine which display units 18 (as identified by the information contained in the response message broadcast by the EPOP display unit 18) can communicate with the communicator and furthermore can calculate the received signal strength (RSSI). This process will be described in further detail with reference to Figures 3, 4, 5 and 6.

Upon receipt of the responses from the EPOP display units 18 by the communicators 16, the results are analysed at step SI 06. This step of the process is where the initial distribution of associations between EPOP display units 18 and communicators 16 is established. At step SI 06, each EPOP display unit 18 is assigned to the communicator which has the highest RSSI.

At step SI 08, the balancing of the display system 10 is calculated. At step SI 08 the number of EPOP display units 18 assigned to each communicator 16 is determined and the results for each communicator are compared. If it is determined that there is an imbalance between the number of EPOP display units 18 assigned between the communicators 16, at step SI 08 the system calculates how to rebalance the communicators 16 at step S108. This involves a method for deciding how associations between EPOP display units 18 and communicators 16 could be rearranged so that any imbalance in distribution, which may result in some communicators 16 doing significantly more work than other communicators 16, can be addressed. This process is described in further detail with reference to Figure 7.

At step SI 10, dependent upon the results of the balancing calculated at step S108, there is a redistribution of association of EPOP display units. Therefore the number of EPOP display units 18 assigned to each communicator 16 is approximately equal across all communicators 16, and therefore each communicator 16 will do approximately the same amount of work, allowing system capacity to be increased. The process then finishes at step SI 12.

Figure 3 is a flowchart that describes the process of the issuance of the search and respond commands by the communicator 16, and the subsequent responses by the EPOP display units 18 that are performed at step SI 04 of Figure 1.

An initial search and respond process begins at step S202. Multiple search and respond commands are issued based on the number of RE addresses on the display system 10. For each communicator 16 (step S204), each respond group (step S206), and for each timeslot (step S208), a search and respond command is broadcast at step S210. The number of respond groups is determined by dividing the number of RF addresses on the display system 10 by the number of response slots 34. The response timings 20 for search and respond commands are shown in greater detail in Figure 4. Each timeslot 22 is subdivided into multiple response slots 34 and each response slot 34 corresponds to the period of time when an EPOP display unit 18 can send a response packet. The response slot 34 corresponding to an EPOP display unit 18 is calculated based on the modulo division of the RF address 88 of the EPOP display unit 18, by the number of response slots 34 available in the response period 28.

The same search and respond command is broadcast by an active first communicator 16 in the first respond group in all timeslots 22 during steps S204 to S210. Whilst the first communicator 16 is broadcasting search and respond commands, all of the communicators 16, including the second and subsequent communicators 16 are set to listen to responses on the same RF channel as the active first communicator 16 at step S212, so that they can receive response packets transmitted by EPOP display units 18. The search and respond command broadcast at S210 is subsequently received at a number of EPOP display units 18. The EPOP display units 18 then broadcast responses in their allotted response slots 34, in a process that will be described later, in further detail, with reference to Figure 4 and Figure 5.

The results are recorded at step S214, where the information contained in a response packet 80, as shown in Figure 5, is received. The information received includes the RF address 88 of the EPOP display unit 18 and the packet's received signal strength indication (RSSI) value. The information is stored in a single table at step S214. The process continues so that each timeslot (at step S216), each response group (at step S218) and each communicator (at step S222) cycles through the steps of S210 to S214. Once the search and respond command has been broadcast in all timeslots 22 in all respond groups for a first communicator 16, the process is repeated for a second communicator 16. The second communicator 16 becomes the active communicator 16 and all communicators 16, including the first communicator 16, are put into a listening mode to detect responses on the same RF channel that the second active communicator 16 is broadcasting on. The same search and respond commands are broadcast for each respond group and for each timeslot 22 by the second communicator 16 and the responses are recorded in a single table at step S214. The responses include the RF addresses 88 of the EPOP display units 18 and their RSSI values. The process is repeated for all communicators 16 and all the timeslots 22 in all of the respond groups for each communicator 16. A table of these results is built up at step S224 and the process finishes at step S226.

Figure 4 shows response timings 20 associated with the EPOP display units 18 and in particular how they respond to a search and respond command broadcast by a communicator 16 as per step S212 of Figure 3.

The upper part of the Figure shows a timing diagram of a command transmitted by a communicator 16 in a timeslot 22. During a timeslot 22 in which the communicator 16 is operating, there is a designated amount of available communication time 24. The available communication time 24 is the length of time available in a timeslot 20 minus the 'Listen Before Talk' LBT 30 time, which is a period of time allocated for a communicator 16 to deal with any RF interference on the same radio channel. If the communicator 16 detects interference on the channel that does not disappear within a pre-determined time, the communicator 16 skips that timeslot 22. This is to ensure that the remaining EPOP display units 18 stay in synchronisation. During the available communication time 24, a synchronisation signal 31 is broadcast by the communicator 16. The synchronisation signal 31 consists of multiple packets of data or synchronisation packets 40. Each sync packet 40 contains an offset from the centre of the sync period to enable an EPOP display unit 18 to re-adjust its clock and take care of any drift that may have occurred since the last synchronisation signal 31 was received. The structure of the sync packets 40 and subsequent correction is described in WO 2010/004349.

Following the synchronisation signal 31, a broadcast command 32 is broadcast by the communicator 16. The remainder of the available communication time 24 can be used as a response period 28, during which EPOP display units 18 can respond. In Figure 4, the lower two signal representations are examples of two signals 35, 37 from two EPOP display units 18. Both of the two EPOP display units 18 signals represent EPOP display unit 18 starting in a lower powered sleep mode. The EPOP display units 18 awaken in a timeslot 22 and synchronise their clocks based on the offset from the centre of the sync period broadcast by the communicator 16.

The EPOP displays units 18 then issue an EPOP response packet 36, 38 at determined time. The time the response is broadcast by the individual EPOP display unit 18 is based on the RF address 88 of the broadcasting EPOP display unit 18. The response period 28 of the timeslot 22 is subdivided into 320 response slots 34 of three milliseconds each. The time of the broadcast of the response packet 36, 38, is based on the modulo division of the RF address 88 of the EPOP display unit 18, by the number of response slots 34 available in the response period 28. For example, if the RF address 88 of response packet 36 was 10, the response slot 34 for that packet would be 10. If the RF address 88 of response packet 38 was 11682, the response slot 34 for that packet would be 162.

In further examples, the timeslot 22 is subdivided into a different number of response slots 34, of different lengths. The number of response slots 34 may be altered according to user preference and/or system requirements. In an example the format of the response packet 38 may be increased to provided further information thus reducing the amount of response slots available, in further examples the opposite is true. When a first communicator CI is active and broadcasting search and respond commands, the second and subsequent communicators C2, C3 listen for signals on a radio frequency that is the same radio frequency that the active communicator CI is broadcasting on. Once the first communicator CI has completed its search and respond command, a subsequent communicator C2 may become the active communicator 16 and all of the other communicators CI, C3, including the first communicator 16, listen for signals on a radio frequency that is the same radio frequency as the active communicator 16 is broadcasting on. This process is preferably repeated across all communicators. In an example of the invention the sequence of communicators of which are active and broadcast the search and respond commands is determined by the number of units already assigned to the communicator. The communicator with the highest number of units already assigned is the first communicator to broadcast the search and respond commands, with all other communicators listening for the response messages from the display units. Once completed the communicator with the second highest number of communicators already assigned is the active communicator. The process repeats with the selection of the active communicator being based on the number of units assigned to the communicator.

In a further example of the invention, the system may further comprise a manual override in which a system administrator may manually assign units to a communicator. Additionally, the system administrator may also initiate a rebalance command for one, or more, of the communicators.

Multiple broadcast command messages 32 are issued by a communicator 16, wherein each search and respond command relates to a radio frequency on the display system 10. In further examples, single broadcast command messages 32 are issued by a communicator 16, wherein each search and respond command relates to a radio frequency on the display system 10.

A broadcast command message 32 is broadcast for each response group. In further examples broadcast command messages are only broadcast occasionally. All communicators 16 broadcast search and respond commands. In further examples, a selected number of communicators 16 broadcast search and responds commands.

The communicators 16 that receive response packets 80 decode the response packets 80 and determine the RF address of the response packet 80. Furthermore, the communicators 16 that receive response packets 80 decode the packet and determine the packet's received signal strength indication (RSSI) value.

The search and respond command is issued in each timeslot 22 before incrementally moving on to the next response group and transmitting a new search and respond command in all timeslots 22. In further examples the timings may be altered to suit the needs of the system that is being used, including changing the number of response slots.

Figure 5 schematically shows the structure of the synchronisation packets 40, broadcast command packets 60 and response packets 80 that are communicated between the EPOP display units 18 and the communicators 16 used in an example of the invention. Other formats of the packets may be used in further examples. Each packet is broken down into discrete, sequential components of varying lengths.

The synchronisation packets 40 consist of data split into the following sections: synchronisation packet preamble 42, synchronisation packet synchronisation data 44, a synchronisation packet length 46, a synchronisation packet network ID 48, a number of remaining synchronisation packets 50, a data payload packet Len 52, a synchronisation packet broadcast address 54, a synchronisation packet group address 56 and a synchronisation packet Cyclic Redundancy Check CRC 58.

The broadcast command packets 60 consist of data split into the following sections: a broadcast command packet preamble 62, broadcast command packet synchronisation data 64, a broadcast command packet Len 66, a broadcast command packet Net ID 68, a broadcast command packet group address 70, a broadcast command packet command 72 and a broadcast command packet CRC 74. The Response packets 80 consist of data split into the following sections: a response packet preamble 82, response packet synchronisation data 84, a response packet timeslot 86, a response packet RF address 88 and a response packet CRC 90.

The values shown in the packet structure schematics represent the number of bits of data. The preamble data 42, 62, 82 is typically 32 bit, the synchronisation data 44, 64, 84 is typically 32 bit, Len data 46, 52, 66 is typically 8 bit, network ID data 48, 68 is typically 8 bit, as are remaining synchronisation packet data 50 and response packet timeslot data 86. Synchronisation packet broadcast address data 54, synchronisation packet group address data 56, CRC data 58, 74, 90, broadcast command packet command data 72 and response packet RF address data 88 are all typically 16 bit.

Synchronisation packets 40 typically last 3.744 milliseconds, broadcast command packets 60 typically last 15.39 milliseconds and response packets 80 typically last 2.5 milliseconds. In further examples, the size of the packets may change according to system requirements or user preferences. Figure 6 is a flowchart describing the steps performed by an EPOP display unit 18 upon receipt of a search and respond command at step S302.

The process first involves calculating the response group ID at step S304. The calculation to do this is shown at step S306. In an example the response group ID for an individual EPOP 18 is calculated as the integer of the RF address 88 of the individual EPOP 18 divided by the multiplication of the number of response slots 34 by the number of timeslots 22.

Once the response group ID has been determined, it is determined at step S308 whether the response group ID is in the current group. If it is not, the process moves to step S316, where the EPOP display unit 18 goes into sleep mode. Preferably, the EPOP display unit 18 wakes up when the response group is the current group. If the is response group ID is in the current group, the process moves to step S312, where the response position is calculated. This response position is given as the RF address 88 modulo response slots 34 and is shown at step S310. The process then moves to step S314, where the response packet 36 is issued at the appropriate time, as calculated in the previous step S312. The EPOP display unit then returns to sleep mode at step S316.

Therefore, the invention provides a method of rapidly locating EPOP display units 18 in a location. It is found that the above described method can locate approximately 130,000 display units per hour. Therefore, an entire store of over 100,000 units may be located in less than one hour, compared to approximately 50 hours using methods in the prior art.

A further aspect of the invention is the ability to balance the number of EPOP displays units 18 assigned to a plurality of communicators in order to optimally use the communicators and increase the capacity of the system.

Figure 7 is a flowchart detailing the process of load balancing of EPOP display units 18 with respect to communicators 16.

The process is initiated at step S402. The process then calculates the initial balancing based on the best received signal strength indication RSSI at step S406. This information is received from step S404. The information is received during the search and respond process, where each of the communicators 16 has issued a search and respond command, as described in Figure 3. During the process, a first communicator 16 broadcasts a search and respond command for all timeslots 22 in all respond groups. The EPOP display units 18 that receive the command respond based on their RF address 88. The information containing the RF address 88 is sent in a response packet 80 in the response slot 34 that is related to the RF address 88 of the EPOP display unit 18, as previously described. All communicators 16 listen for the responses of the EPOP display units 18 on the same RF channel as the active communicator 16. The communicators 16 record the RF address 88 of the response packets 80 and the RSSI values of the signals and build a table of the responses. The initial distribution, or balancing, of the display system 10 is determined based on the received RSSI values. The EPOP display units 18 are associated with the communicator 16 with which they are detected to have the first RSSI value above a minimum level. Therefore, each EPOP display unit 18 will be associated with a communicator 16. The process asks at step S408 whether the balancing is within pre-specified limits. The balancing limits are pre-determined based on a mathematical condition, such as whether the number of preferred EPOP display units 18 associated with a communicator 16 is more than eighty percent of the total number of EPOP display units 18 divided by the total number of communicators 16 and less than one-hundred and twenty percent of the total number of EPOP display units 18 divided by the total number of communicators 16. If the number of EPOP display units 18 assigned to each communicator 16 is within the pre-specified limits, the process finishes at step S416. If it is not, the process moves to step S410, whereby the donor communicators 16 and recipient communicators 16 are calculated in order to balance the number of EPOP display units 18 assigned to each communicator. A donor communicator is a communicator 16 that has more than the balanced count of EPOP display units 18 associated with it and a recipient communicator is a communicator 16 that has less than the balanced count of EPOP display units 18 associated with it, as calculated at step S406.

At step S412 it is determined whether there are EPOP display units 18 that can be reassigned to different communicators 16. An EPOP display unit 18 can be reassigned to a different communicator if the RSSI value at the different communicator 16 is above a minimum level. If there are not any units 18 which can be reassigned, the process finishes at step S416. If there are, the EPOP display units 18 are reassigned to communicators 16 at step S414. The minimum level may be set at a fixed predetermined level, e.g. a minimum received signal strength indication (RSSI) or similar, or it may be dynamic and vary according to any number of predetermined factors e.g. total number of units, average RSSI across the units, environmental conditions etc.

At step S414, the donors are ranked in order by the number of EPOP display units 18 that they can donate, and the recipients are ranked in order by the number of EPOP display units 18 that they can receive. The process of reassignment is that the donor communicator 16 donates EPOP display units 18 to each of the recipients in the list until either the communicator 16 has no more EPOP display units 18 to donate, or there are no donor EPOP display units 18 that match a recipient communicator 16, or all recipient communicators meet the balanced conditions.

The process returns to step S408, after the reassignment of an EPOP display unit 18 to a different communicator 16 at step S414, in order to determine whether the reassignment has sufficiently balanced the load of EPOP display units 18 to communicators 16. The process continues until either there are no more EPOP display units 18 that can be reassigned, or the load balancing is sufficiently within limits and the process finishes at step S416. Preferably, after each donation, the list is recalculated. The recalculation ensures that the distribution of EPOP display units 18 associated with communicators 16 remains equal and that a single recipient communicator 16 does not receive all EPOP display units 18 from all donor communicators 16. Therefore, by recalculating the list after each donation the optimal rebalancing of the system is ensured.

Figure 8 is a flowchart detailing the reconfiguration process that is involved in reassigning EPOP display units 18 to different communicators 16. The process begins at step S502, where the reconfiguration process is initiated. The next step is to build a list of EPOP display units 18 to reassign. This information is inputted from step S504, which refers to the calculations performed in the flowchart of Figure 7, where it has been determined whether the balancing of the EPOP display units 18 associated with communicators 16 is within pre-specified limits and further to this determination, which of the communicators 16 are donors and which of the communicators 16 are recipients, ranking them by the number of EPOP display units 18 that they can donate, or receive, respectively. It has also been determined whether there are EPOP display units 18 that can be reassigned and if there are, the balancing after each iteration of donation of the EPOP display units 18 that can be reassigned, producing an updated list.

Once the final list has been established, the process moves to step S508 where for each EPOP display unit 18 in the list it is asked if the preferred communicator is the current communicator. If it is the case that the preferred communicator is the current communicator, then there is no need to reassign the EPOP display unit 18 and the process moves to step S514, where it is asked whether there is a next EPOP display unit 18 in the list. If the current communicator is not the preferred communicator, then the process moves to step S512, where preferred communicator commands are issued to EPOP display units 18. The preferred communicator commands are contained in the command 72 portion of the broadcast command packet 60. These cause the EPOP display units 18 that receive the preferred communicator commands to switch association to the preferred communicator from the current communicator. The process then moves to step S514, where it is asked if there is a next EPOP display unit 18 in the list. If there is, the process returns to step S508 and the process of steps S508 to S514 are repeated for the next EPOP display unit 18 in the list. If there is not, the process finishes at step S516.

If an EPOP display unit 18 has not previously been sent a preferred communicator command, such a command is sent. If the preferred communicator has changed, a preferred communicator command is sent. A preferred communicator command can be sent as part of a standard EPOP display unit command. Figure 9 is a flowchart detailing the EPOP display unit 18 reassignment process, which begins by the EPOP display unit 18 receiving a preferred communicator command at step S602. The process for issuing preferred communicator commands is detailed in the flowchart of Figure 8. Upon receipt of the preferred communicator command the EPOP display unit 18's new communicator channel is placed at the head of the channel list at step S606. The channel at the head of the channel list is the one on which the communicator preferably operates. For each channel in the list (step S606), a synchronisation search is performed at S608. This process determines whether there is a synchronisation packet 40, containing information identifying the timeslot 22 in which the synchronisation packet 40 is being broadcast, as well as information enabling the EPOP display unit 18 to determine the offset from the centre of the synchronisation period, allowing the clock to be adjusted to take care of any drift that may have occurred.

At step S610 it is asked whether a synchronisation packet 40 is found. If it is found, the process moves to step S612, whereby the EPOP display unit 18 enters an in- synchronisation state. This state allows for the EPOP display unit 18 to wake up periodically in its allotted timeslot 22 and compare synchronisation data with the associated communicator 16, without having to be in an active state between times. If a synchronisation packet 40 is not found, it is asked at step S614 whether there is a next channel on which the EPOP display unit can communicate with a communicator 16. If there is, the process returns to S606 and searches for a synchronisation packet 40. The process is repeated, as necessary, and if no further channels are found, the EPOP display unit 18 enters a roaming state at step S616. This means that the EPOP display unit 18 searches for appropriate communicators 16 with which it can be associated.

Figure 10 shows a worked example of the load balancing process. The results are for example only and use a display system 10 with components as described in Figure 1.

There are three communicators in the example, CI, C2 and C3. There are nineteen EPOP display units 18 in the example, with RF addresses 88 from 2 to 20. The first table 700 on the left hand side of Figure 10 shows the results of a search and respond command as per step S104. The values in the table are RSSI values corresponding to each of the three communicators 16, for each of the EPOP display units 18. Therefore, the system determines for each EPOP display unit 18 what communicators 16 are within communication range, and the strength of the signal.

The second table 702 of Figure 10 illustrates preferred associations between EPOP display units 18 and communicator 16. The preferred communicator 16 is calculated by choosing the first communicator 16 with an RSSI value above a minimum predetermined value. In the example of Figure 10, the minimum RSSI value is -90. Hence, EPOP display unit 18 with RF address 88 2 is associated with communicator CI, EPOP display unit 18 with RF address 88 4 is associated with communicator C2 and so on. Using this technique, the second displayed table 702 in Figure 10 shows that communicator CI has four EPOP display units 18 associated with it, communicator C2 has five EPOP display units 18 associated with it and communicator C3 has ten EPOP display units 18 associated with it. In the example shown, the preferred communicator 16 being defined as the first one with a RSSI value above the minimum results in an unequal distribution of EPOP display units 18 per communicator 16. In the present example, communicator C3 will have an unequal burden of EPOP display units 18, with which to communicate and update. In order to assess whether the distribution of EPOP display units 18 associated with communicators 16 falls within acceptable limits, a calculation is performed. For example, the predefined range may be set such that the number of preferred EPOP display units 18 associated with any one communicator 16 is less than (1.2 multiplied by the maximum number of display units) divided by the number of communicators and more than (0.8 multiplied by the maximum number of EPOP display units) divided by the number of communicators. If the conditions are not met and the display system 10 is considered to be unbalanced and the EPOP display units 18 need to be moved from the current assigned communicator 16 to the communicator 16 with the next highest RSSI.

In the example of Figure 10, the preferred number of EPOP display units 18 per communicator 16 is less than eight and greater than 5, in accordance with the above calculation. From the second table 702 of Figure 10 it can be seen that communicator CI has to gain at least two EPOP display units 18 to meet the requirements of a balanced display system 10. Similarly communicator C2 has to gain at least one EPOP display unit 18. Communicator C3 has to lose at least three associated EPOP display units 18. In order to reassign EPOP display units 18 to alternate communicators 16, it is necessary to build a list of EPOP display units 18 that can be reassigned. This involves assessing which EPOP display units 18 have a minimum RSSI value on an alternative communicator 16 to the one with which they are currently assigned.

The upper third 704 and lower third 706 tables of Figure 10 show the donor and recipient communicators respectively. Communicator C3 is a donor, since it needs to disassociate with at least three EPOP display units 18. Communicators CI and C2 are recipients, since they both need to gain associated EPOP display units 18 in order to balance the display system 10. The donors are ranked in order by the number of EPOP display units 18 that they can donate, the recipients are ranked in order by the number of EPOP display units 18 that they can receive. The upper third table 704 of Figure 10 shows a list of the EPOP display units 18 initially associated with the donor communicator C3. It further shows the RSSI values corresponding to the other two communicators 16 in the display system 10. In the example, in table 704, the circled values 707 indicate which communicator 16 and EPOP display unit 18 associations do not meet the minimum predefined RSSI value of -90. In the example, EPOP display unit 18 with the RF address 88 of 11 is not suitable for reassignment. The process of reassignment is that the donor communicator 16 (in the example of Figure 10, C3) donates EPOP display units 18 to each of the recipients in the list until either the communicator 16 has no more EPOP display units 18 to donate, or there are no donor EPOP display units 18 that match a recipient communicator 16, or all recipient communicators meet the balanced conditions.

Preferably, after each donation, the list is recalculated. The recalculation ensures that the distribution of EPOP display units 18 associated with communicators 16 remains equal and that a single recipient communicator 16 does not receive all EPOP display units 18 from all donor communicators 16. Therefore, by recalculating the list after each donation the optimal rebalancing of the system is ensured.

Once the EPOP display units 18 have been reassigned from donor communicators 16 to recipient communicators 16, a table of the data relating to the final balanced conditions is produced. This is seen in the fourth table 708 on the right hand side of Figure 10. In the example of Figure 10, the display system 10 becomes well balanced. This is a simple display system 10 and in more complex ones, such balancing may not be possible. However, by following the procedures shown here, a more efficient distribution is produced.

Figure 11 is a table showing the typical time taken for search and respond commands to be performed using the methods described herein with particular reference to Figures 3 and 4.

In Figure 11 there is shown the results in an example display system 10 with two communicators 16 and 10,000 EPOP display units 18 in the upper table 800 and 8 communicators 16 and 65,000 EPOP display units 18 in the lower table 802. These operations would take 35.2 seconds and 30.6 minutes respectively. If a conventional ping command were to be used, the process would take over 60 minutes for the display system 10 with 2 communicators 16 and 10000 EPOP display units 18.

Therefore, the invention results in substantial time savings. Furthermore, the improved capacity of the communicators 16 allows for fewer communicators 16 to be required, which in turn can produce cost savings with respect to installation and maintenance of the communicators 16.