Radio Mesh Network Configuration

BACKGROUND OF THE INVENTION

This invention relates to radio devices for use in radio mesh networks.

Wireless mesh networks allow radio devices, arranged as nodes of the network, to communicate with each in a decentralised ad hoc manner. Messages can be relayed between device over multiple hops, allowing two devices to communicate even when they are located beyond radio range of each other. Mesh networks allow systems to be deployed flexibly and scalably, with less need for fixed infrastructure such as cabling compared with traditional planned networks. They are well suited for efficiently deploying networks of smart devices in a residential or industrial building, such as wireless light switches and wireless luminaires, or other types of wireless sensors and appliances. Radio devices may be fixed or mobile. They may be powered by an external electrical supply or by an internal source such as a battery and/or photovoltaic cells.

One example of a protocol for radio mesh networking is ETSI’s DECT-2020 New Radio (NR) standard, which operates in a license-exempt 1.9 GHz frequency band.

In a DECT-2020 radio mesh network, radio devices (RDs) can detect and associate with each to establish radio connections between pairs of radio devices. For each connection, one device operates in a “fixed terminal” (FT) mode and the other operates in in “portable terminal” (PT) mode. Note that these terms do not describe whether the devices are physically fixed or portable. A device operating in FT mode coordinates local radio resources and provides information as to how other radio devices may connect and communicate with it. A device operating in PT mode acts based on information provided to it by the associated FT device. A device in FT mode may be associated with multiple PT-mode devices, whereas a PT-mode device selects a single FT-mode device to associate with (based on signal strength and route cost). A single radio device can be associated with multiple other radio devices and can operate both in FT and PT modes simultaneously. In this way, the mesh network can form a clustered tree topology, in which each device operating in FT mode defines a respective cluster consisting of itself and the PT-mode devices with which it is associated. A radio device that has Internet connectivity can act as a gateway or “sink’ device and provide the root node of the tree topology.

Radio devices in FT mode transmit periodic beacon messages that enable other devices to identify and initiate an association with them. Every radio device is required to maintain reception of beacon messages from a FT-mode radio device with which it is associated. Two types of beacon message are transmitted periodically by device operating in FT mode: network beacon messages and cluster beacon messages. The transmission periods of the network and cluster beacon messages can be different.

Network beacon messages allow other devices to find the mesh network rapidly. They may be transmitted on a limited set of channels and announce the presence of a network. They indicate the operating channel of the FT radio device, as well as indicating the periodicity of its network and cluster beacon transmissions.

Cluster beacon messages provide frame and slot timing for a cluster, as well as announcing radio parameters and radio resources so that other radio devices can associate with the FT device transmitting the cluster beacon.

Battery-powered radio devices operating in FT mode may be set to use a relatively long beacon transmission interval (up to several seconds, depending on the actual use case), whereas mains-powered FT-mode radio devices may be set to use a more frequent beaconing period. However, while this may improve the battery-life of battery- powered devices, it can negatively impact the performance of the mesh network.

Embodiments of the present invention seek to provide a better approach to the transmission of radio beacons in mesh networks.

SUMMARY OF THE INVENTION

From a first aspect, the invention provides a radio device for use in a DECT-2020 mesh network, wherein the radio device is configured to: transmit DECT-2020 radio beacons of a predetermined type periodically with a first beacon period; and determine that a predetermined condition is met, and, in response to determining that the predetermined condition is met, transmit DECT-2020 radio beacons of the predetermined type with a second beacon period, different from the first beacon period.

From a second aspect, the invention provides a system of radio devices configured to provide a DECT-2020 mesh network, the system comprising a plurality of radio devices, each configured to: transmit DECT-2020 radio beacons of a predetermined type periodically with a respective first beacon period; and determine that the respective predetermined condition is met, and, in response to determining that the respective predetermined condition is met, transmit DECT-2020 radio beacons of the predetermined type with a respective second beacon period, different from the respective first beacon period.

From a third aspect, the invention provides a method of operating a radio device in a DECT-2020 mesh network, the method comprising the radio device: transmitting DECT-2020 radio beacons of a predetermined type periodically with a first beacon period; and determining that a predetermined condition is met, and, in response to determining that the predetermined condition is met, transmitting DECT-2020 radio beacons of the predetermined type periodically with a second beacon period, different from the first beacon period.

Thus it will be seen that, in accordance with embodiments of the invention, the same DECT-2020 router radio device (i.e. a radio device operating in fixed-terminal (FT) mode) can automatically respond to changing conditions by sending beacons more frequently at certain times (e.g. when it is desirable for mesh network formation to occur quickly) and less frequently at other times (e.g. when it is desirable to converse electrical power and/or radio bandwidth). A system comprising one or more such radio devices may thus be able to support fast network formation while also being able to be efficient at other times with regard to power and/or bandwidth utilisation.

The type of the DECT-2020 radio beacons may be network beacons or cluster beacons. In some embodiments, the radio device may be configured to transmit network beacons periodically with a network-beacon period and also to transmit cluster beacons periodically with a cluster-beacon period (which may be different from the network-beacon period). The radio device may, in response to determining that the predetermined condition is met, change both the network-beacon period and the cluster-beacon period, although this is not necessarily the case.

The radio device may change one or more further parameters in response to determining that the predetermined condition is met. It may change a random-access resource allocation, or a transmission or reception bitrate, or a size of transmission data packet. Changing these further parameters may allow the radio device to provide a lower-latency higher-throughput “high-power” cluster at times, and a higher-latency lower-throughput “low-power” cluster at other times.

In particular, the radio device may be configured to allocate resources for random access transmission. It may broadcast random-access allocation information, e.g. encoded within a cluster beacon. The radio device may, in response to determining that the predetermined condition is met, additionally increase or decrease the resources allocated for random access transmission. It may decrease the beacon period and increase a random-access allocation. Alternatively, it may increase the beacon period and decrease a random-access allocation. In this way, the radio device may change a cluster between a relatively high-power routing state and a relatively low-power routing state.

The radio device may transmit the DECT-2020 radio beacons periodically with the first beacon period while the predetermined condition is not met. The radio device may be configured to check, at intervals (e.g. regular intervals), whether the predetermined condition is met. It may detect, e.g. as an event or interrupt, when the predetermined condition changes from being not met to being met.

The predetermined condition may relate to a state of the radio device (e.g. whether the radio device is being powered by an external power source) and/or to a state of the mesh network (e.g. whether the radio device is able to route messages to a sink node). The predetermined condition may depend on a combination of two or more conditions (i.e. sub-conditions). It may be a Boolean function of one or more sub-conditions — e.g. binary sub-conditions combined using a combination logical OR and/or AND and/or NOT operators.

The predetermined condition may depend on any one or more of: an initial network formation process being completed; the radio device is associated with a parent radio device; the radio device being associated with at least a predetermined minimum number of child radio devices (where the predetermined minimum number may be one, two, three or more); the radio device being associated with at least one child radio device that is a router radio device (i.e. that is associated with its own child devices); the radio device receiving data at more than a threshold rate; the radio device transmitting data at more than a threshold rate; the radio device needing to transmit data at more than a threshold rate at a future time; the radio device needing to receive data at more than a threshold rate at a future time; the radio device being powered by an external power source; the radio device detecting a power outage; and the radio device detecting one or at least a threshold number of high-priority messages.

Each condition may be associated with a respective time period, and may be such that it has to have occurred during the time period, or for some or all of the time period, for the radio device to determine the condition as having been met.

A power outage may be detected in any appropriate way. A power outage may, in certain mesh installations, cause a large number of alert messages to be sent through a high-priority messaging channel, e.g. to a cloud service. This may be detected by the radio device. A parent device may be any associated radio device that is operating in DECT-2020 fixed-terminal (FT) mode, while a child device may be any associated radio device that is operating in DECT-2020 portable-terminal (PT) mode.

The radio device may increase or decrease the beacon period in response to determining that the predetermined condition is met — i.e. the second beacon period may be longer or shorter than the first beacon period.

The radio device may be configured to transmit beacons with the first beacon period initially after the radio device is powered on, wherein the first beacon period is shorter than the second beacon period. This can allow the radio device to form network associations more quickly when it is first powered on, and then reduce its power consumption subsequently.

In some embodiments, the radio device may be configured to increase the beacon period in response to determining that an initial network formation process is completed. In particular, it may do this for network beacon transmissions. The radio device may determine that initial network formation process is completed by determining any one or more of: a predetermined time having elapsed after the radio device was associated with a parent radio device; a data packet having been successfully routed to and/or from a sink node of the mesh network; a predetermined time having elapsed since a latest child radio device was associated with the radio device; or the mesh network having operated without the radio device detecting an error for longer than a predetermined time.

In some embodiments, the radio device may be configured to increase the beacon period in response to determining that the radio device is not associated with any child radio device that is operating as a router device. In particular, it may do this for cluster beacon transmissions. It may additionally decrease a random-access resource allocation in response to determining this. This can allow the radio device to switch from a network-discovery state or high-power routing state to a low-power routing state, for greater power and/or bandwidth efficiency, when it has only leaf nodes downstream of it. In some embodiments, the radio device may be configured to decrease the beacon period in response to determining that the radio device is associated with a child radio device that is operating as a router device. In particular, it may do this for cluster beacon transmissions. It may additionally increase a random-access resource allocation in response to determining this. This can allow the radio device to enter a high-power routing state to provide greater responsiveness to downstream devices when it has at least one further cluster downstream of it.

In some embodiments, the radio device may be configured to increase the beacon period in response to determining that the radio device is not associated with any parent device, or in response to determining that the radio device has below a predetermined threshold number of child device associations (e.g. below one, two or more). In particular, it may do this for network beacon transmissions. These conditions may be indicative of a power or network disruption that could have affected multiple or all radio devices in the system; the network may be reformed quicker by the radio device(s) increasing their beacon periods.

Because the radio devices of the system can change their beacon periods (for network and/or cluster beacons), the mesh network can contain different radio devices, operating as routers, that are transmitting network beacons and/or cluster beacons at different periods from other routers of the same mesh network.

Thus, in some embodiments the system may comprise: a first radio device configured to transmit DECT-2020 radio beacons of a predetermined type periodically with a first-device beacon period; and a second radio device communicably coupled with the first radio device by the DECT-2020 mesh network and configured to transmit DECT-2020 radio beacons of the predetermined type periodically with a second-device beacon period, different from the first-device beacon period, at the same time as the first radio device transmits DECT-2020 radio beacons of the predetermined type periodically with the first-device beacon period.

The first and second radio devices may be radio devices having any of the further features disclosed herein. Supporting different beacon periods in the same mesh network can allow the network to offers radio devices (e.g. leaf devices, operating only in portable-terminal mode) a choice of network cluster to join in order to optimise their battery life and/or latency and/or data bitrate, as appropriate for their needs. A radio device may thus be configured to select which router radio device to associate with (i.e. which cluster to join) in dependence on a latency and/or power requirement of the radio device.

In any of the embodiments disclosed herein, the radio device may be further configured to: transmit DECT-2020 radio beacons of the predetermined type periodically with the second beacon period; and determine that a second predetermined condition is met, and, in response to determining that the second predetermined condition is met, transmit DECT-2020 radio beacons of the predetermined type with the first beacon period, or with a third beacon period, different from the first and second beacon periods.

The second predetermined condition may be the logical inverse of the (first) predetermined condition, or it may be an independent condition. It may comprise any one or a logical combination of the conditions or logical inverses thereof disclosed herein.

More generally, radio devices may change beacon period for the transmission of cluster and/or network beacons in response to any of a plurality of different conditions being met.

The idea of changing beacon period in response to changing conditions is not limited only to DECT-2020 networks but may also be beneficially applied to other types of mesh network.

More generally, from a further aspect, the invention provides a radio device for use in a radio mesh network, wherein the radio device is configured to: transmit radio beacons periodically with a first beacon period; and determine that the predetermined condition is met, and, in response to determining that the predetermined condition is met, transmit radio beacons with a second beacon period, different from the first beacon period. From a further aspect, the invention provides a system of radio devices configured to provide a radio mesh network, the system comprising a plurality of radio devices, each configured to: transmit radio beacons periodically with a respective first beacon period; and determine that the respective predetermined condition is met, and, in response to determining that the respective predetermined condition is met, transmit radio beacons with a respective second beacon period, different from the respective first beacon period.

From a further aspect, the invention provides a method of operating a radio device in a radio mesh network, the method comprising the radio device: transmitting radio beacons periodically with a first beacon period; and determining that a predetermined condition is met, and, in response to determining that the predetermined condition is met, transmitting radio beacons periodically with a second beacon period, different from the first beacon period.

The radio device may be a router radio device. It may be configured to transmit invitations for other radio devices to associate with it. The radio beacons may be, or may comprise, invitations for other radio devices to associate with the radio device transmitting the radio beacon. However, the radio device need not necessarily be acting as a router for any other radio devices. In some situations, it may indicate a higher route cost than one or more other router radio devices of the radio mesh network, thereby discouraging other radio devices from associating with it.

The plurality of radio devices may all be configured to implement a common routing protocol. More generally, they may all be configured to transmit and receive messages according to a common radio communication protocol, which may be a proprietary or standardised protocol. This could be any protocol that supports mesh networking. However, in some embodiments, all the radio devices implement at least the MAC layer specification, or all parts, of a current or future version of the DECT-2020 New Radio (NR) standard. If the radio device is capable of transmitting radio beacons of different types, the first and second beacon periods may relate to the same type of beacon — i.e. such the period changes at a moment in time. Any of the features of the preceding aspects may be features of these aspects also.

In any of the aspects disclosed herein, each radio device may comprise radio transceiver circuitry for transmitting and receiving messages. Each may comprise or may be an integrated-circuit radio transceiver — e.g., a silicon chip. Each may comprise, or be connectable to, one or more off-chip components, such as a power supply, antenna, crystal, discrete capacitors, discrete resistors, etc. Each may comprise one or more processors, DSPs, logic gates, amplifiers, filters, digital components, analog components, non-volatile memories (e.g., for storing software instructions), volatile memories, memory buses, peripherals, inputs, outputs, and any other relevant electronic components or features. Each radio device may comprise a memory storing software instructions for execution by a processing system of the radio device. The software instructions may instruct the device for performing any of the steps or operations disclosed herein. Each device may comprise a DSP and/or a general purpose processor, such an Arm™ Cortex-M™ processor. Any of the processing steps disclosed herein may be performed wholly in software, or wholly by hardwired circuitry (e.g., using digital logic gates), or by a combination or software and hardware.

Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a set of radio devices embodying the invention;

Figure 2 is a schematic diagram of an exemplary radio device of the mesh network;

Figure 3 is a schematic diagram of the set of radio devices in a first phase of forming a mesh network;

Figure 4 is a schematic diagram of the set of radio devices in a second phase of forming the mesh network; Figure 5 is a schematic diagram of the set of radio devices in a third phase of forming the mesh network;

Figure 6 is flow chart of state changes in a first radio device embodying the invention; and

Figure 7 is flow chart of state changes in a second radio device embodying the invention.

DETAILED DESCRIPTION

Figure 1 shows a system 100 of ten radio devices, RD0-RD9, configured for short- range or medium-range radio communication and able to route messages between each other as a single radio mesh network (i.e. all being communicably coupled to each other through use of the same short- or medium-range radio protocol). Each radio device (RD) can be associated with one or more other RDs by performing a predetermined association protocol. The RDs may implement any standardised or proprietary radio communication protocol that supports mesh networking, but in these example embodiments, the RDs of the system 100 are configured to implement a current or future version of the DECT-2020 New Radio (NR) standard.

One of the nodes, RD0, is a special gateway or “sink” node. It has a connection to the Internet 106 and can act as a bridge between the mesh network and the Internet 106.

It can therefore allow some or all of the RDs to communicate with a server 108, accessed through the Internet 106, which may be remote from the system 100, e.g. being in a different building, town, state or country.

In some embodiments, the RDs may be smart devices in a residential or industrial building — e.g. with some or all of them being either a wireless light switch or a wireless luminaire, or any other sensor or control devices. The RDs may be Internet-of-Things (loT) devices, and may be accessible from the Internet 106 through the gateway RD0.

Although Figure 1 shows the system 100 as having ten RDs, it will be appreciated that it could have any number of RDs, which may be larger or smaller than ten. The number of RDs associated as part of a single mesh network may also change dynamically over time. Figure 2 schematically shows a representative radio device 200 of the system 100. Some or all of the ten RDs shown in Figure 1 may be devices similar to the RD 200 shown in Figure 2, although this is not essential.

The RD 200 comprises radio transceiver circuitry 204 for performing full-duplex or halfduplex digital radio communication according to a standardised or proprietary radio protocol — e.g. DECT-2020. This circuitry 204 is coupled to a radio antenna 206. The RD 200 also contains a processor 208 (e.g. an Arm™ processor) which is coupled to the radio transceiver circuitry 204 as well as to a memory 210 and a set of peripherals 212. The radio transceiver circuitry 204, processor 208 and memory 210 may all be integrated on a single System on Chip (SoC) although this is not essential. Some of the peripherals 212 may also be at least partly integrated with the processor 208. The peripherals 212 may include communication ports, analog circuitry, digital circuitry, etc. They may include interfaces to physical features of the RD 200 such as a switch (e.g. a light switch), a lamp, an LED, etc. The RD 200 could be an appliance such as a washing machine, or a manufacturing robot in a factory, or it could be a module for incorporation within such an appliance or a larger device. The RD 200 may be powered externally (e.g. mains powered), but in the example shown in Figure 2 it is powered by an internal battery 214.

The memory 210 may comprise volatile (e.g. SRAM) and/or non-volatile (e.g. flash) memory regions. The memory 210 has space for storing firmware 220 comprising software instructions for execution by the processor 208. It also has space for storing data 222 including configuration data for configuring the behaviour of the radio transceiver circuitry 204.

Each RD 200 stores its own unique ID (e.g. a serial number), as well as being able to store the ID of a parent RD with which it has been associated. If the RD 200 is capable of acting as a router, it can also store the IDs of one or more child RDs with which it has been associated. The values of these respective parent and any child IDs stored by each RD define a tree of associations which can be used for efficiently routing messages through the system 100.

Figures 3, 4 and 5 show how the RDs of the system 100 can progressively associate with each other in order to build up a mesh network 102 for routing messages between RDs of the system 100. Associations between pairs of RDs are represented by solid arrows.

In Figure 3, in a first phase of network formation, none of the RDs are yet associated with each other to form communication links for routing messages through. The sink device RDO is operating in FT mode and transmits network beacons and cluster beacons periodically. These are received by three RDs — RD1 , RD2, RD3 — which are operating in PT mode, and which each select the sink device RDO for association. The sink RDO and RD1-3 thus form a first cluster, Cluster 1.

Figure 4 shows a second phase of the network formation, sometime after the first phase shown in Figure 3. The devices RD1 & RD2 are now also operating as routers in FT mode (while still also operating in PT mode with regard to the sink node RDO). They also transmit periodic network beacons, advertising the mesh network 102, as well as periodic cluster beacons. In this example, two devices RD4, RD5, operating in PT mode, have associated with RD1 to form part of a second cluster, Cluster 2.

A further device RD6 has selected RD2 to associate with, thereby joining a third cluster, Cluster 3, provided by the device RD2. The device RD6 operates in PT mode with regard to the router RD2, but is also operating as a router (i.e. in FT mode) and is therefore periodically transmitting its own network and cluster beacons.

Figure 5 shows a third phase, by which time two further devices RD7, RD8 have received the beacons transmitted by RD6 and have selected RD6 as a router to which to associate, thereby forming a fourth cluster, Cluster 4. One of these devices, RD8, is also operating as a router, and a tenth device RD9 has joined the cluster, Cluster 5, defined by RD8.

In this way, the mesh network 102 has been generated with a clustered tree topology, which can be used to route data packets between RDs and also to router data to and from the Internet 106 through the sink node RDO. In this example, four of the devices — RD3, RD4, RD5, RD7, RD9 — are leaf nodes of the mesh network 102, operating only in PT mode, and being associated with only one other RD (i.e. their respective parent RD). The remaining RDs — in this example, RD1 , RD2, RD6, RD8 — are branch nodes, being routers that operate both in PT and FT modes, and associated with a parent RD and at least one child RD.

Any of the radio devices may be battery-powered. In particular, at least some of the leaf devices RD3, RD4, RD5, RD7, RD9 may be battery-powered.

Significantly, the router RDs — RDO, RD1 , RD2, RD6, RD8 — are all configured to control the periodicity of their beacon transmissions dynamically based on one or more conditions. They are each able to provide a high-speed cluster at times, during which they transmit beacons at short intervals, and a low-speed cluster at other times, during which they transmit beacons at longer intervals. They may also allocate more resources for random access transmission when operating in a high-speed routing state, and fewer resources when operating in a low-speed routing state. RDs that are not currently acting as routers, but which have the capability of doing so, may also be configured to support these different states when they are operating in FT mode.

Which state each router operates in is determined according to rules encoded in the firmware of the radio devices, as explained below. In the example topology shown in Figure 5, these rules have the effect that, once network formation has been completed and for as long as the network topology remains stable, the Clusters 1, 3, 4 will operate as high-speed clusters while the Clusters 2 & 5 will operate as low-speed clusters. This allows Clusters 1 , 3, 4 to provide high bandwidth links for routing data efficiently to the clusters that are downstream of them, while Clusters 2 & 5 (which have no downstream clusters) can allow radio devices to conserve battery power by transmitting and receiving beacons less often, and also to free up channel capacity for the high-power clusters.

Some or all of the following states may be supported by radio devices when operating in FT mode (i.e. when acting as routers), for deciding when to change the period with which they transmit network and/or cluster beacons.

Router devices may support a high-speed routing state in which they transmit cluster beacons with relatively high frequency, and in which they set a relatively high Random Access Channel (RACH) allocation, e.g. when connected to external power supply, so as to provide fast and responsive clusters that allow effective routing through the mesh network 102.

Router devices (e.g. battery-powered sensor devices with display screens) may support a low-power routing state in which they transmit network and cluster beacons with relatively low frequency and set a relatively low RACH allocation, e.g. when powered by internal power. They may be allowed to sleep at times, to conserve power, but may still be required to support downlink data with specific minimum latencies. For example, they might transmit infrequent beacons (e.g. every 32 seconds) for sending data and for receiving metering information to display on their screens.

Router devices may support a network-discovery state in which they transmit network and cluster beacons with high frequency and set a relatively high RACH allocation, e.g. during a network formation phase, so that new network connections can be established quickly.

Non-router devices (i.e. leaf nodes of the mesh network 102) may be battery-operated sensors with only uplink connectivity and so desire extremely low power consumption, or they may be actuators or other devices that are controlled remotely and have a requirement for low latency (e.g. battery-operated window shutters, door locks, etc.). Uplink-only devices could, for example, be utility meters that do not need to receive downlink data and therefore don’t need to listen to frequent cluster beacons in order to receive downlink message indications. Instead, they may process a cluster beacon only occasionally, e.g. in order to resynchronize shortly before sending uplink data. Uplink-only devices may be able to sleep for 15 minutes or longer and then wake to send sensor information, and so can be associated with a cluster that transmits relatively infrequent beacon intervals (e.g. 32 seconds, or longer). By contrast, actuators may be optimally joined to a cluster that has more frequent beacon periods, e.g. 200 ms periods, so as to be sufficiently responsive to actuator commands. In some embodiments, some or all the router devices may be capable of supporting the following three different states of operation, with indicative values given by way of example only.

1) “High-power routing” state

■ Network beacon period: long, e.g. 4 seconds

■ Cluster beacon period: short, e.g. 50ms to 200ms

■ Random Access Channel (RACH) allocation: high, e.g. 50%

2) “Low-power routing” state

■ Network beacon period: long, e.g. 4 seconds

■ Cluster beacon period: long, e.g. 8 to 32 seconds

(this reduces power consumption by increasing the sleep time for both the router device and its child devices)

■ Random Access Channel (RACH) allocation: low, e.g. 10%

(this frees bandwidth for other clusters and also reduces the power consumption by the router device, which can sleep when RACH is not available,)

3) “Network-discovery” state

■ Network beacon period: short, e.g. 500 ms

■ Cluster beacon period: short, e.g. 50ms

■ Random Access Channel (RACH) allocation: high, e.g. 50%

To help mesh performance and battery optimization, different types of clusters can be created dynamically during the operation of the system 100, e.g. based on changing network conditions and the mesh topology.

A radio device may provide a “low-power routing” cluster (i.e. operating in a low-power routing state, such as described above) if no router devices are connected to it — e.g. if it has only leaf nodes as its child (i.e. immediate downstream) nodes. When such clusters are created, they send beacons rarely and have small channel allocation. Battery-operated devices, operating in PT mode, may select these clusters and thereby reduce their own power consumption (compared with associating with a higher-power cluster), as they only need to wake in the beacon periods. More generally, cluster selection may be based on any combination of beacon period, RACH allocation, route cost, and a received signal strength indicator (RSSI).

However, if a new router device associates with (i.e. joins) a low-power cluster, the radio device providing the cluster modifies the cluster so that it becomes a “high-power routing” cluster (i.e. operating in a high-power routing state, such as described above). When a router device selects a parent cluster it may be configured to select the fastest cluster available, and if it selects an unsuitable cluster this may provide an indication to the parent that change is needed. After a cluster changes to a routing cluster, any battery-powered leaf devices in the cluster may try to move to better-suited (e.g. low- power) cluster if available.

A “routing” cluster is optimized for high data rate and low latency. Battery-operated actuators and displays may reduce their power usage a lot when using a low-power cluster, but at the cost of latency. If a radio device requires low latency, it can therefore select a routing cluster so as to better support its required application use.

If all child routers leave a “routing” cluster, the FT device changes the cluster back to being a low-power cluster.

When devices notices that the mesh network 102 needs to be formed, they will form “network-discovery” clusters that are optimized for speed of network formation (i.e. operating in a network-discovery state, such as described above). This change may be triggered by a router device detecting any one or more of the following conditions: if it loses its parent or multiple of its children; if it has just been powered on; if it has had to select a new parent because of connection problems. In a network-discovery cluster, cluster beacons are sent frequently to speed up parent discovery, and network beacons are also sent frequently to speed up network discovery by new devices. When a router device in network-discovery state notices that network formation is done, it returns to its previous or default cluster configuration (e.g. to high-power or low-power routing state). It may determine this by any one or more of the following conditions: a predetermined time after a functioning connection has been established to its parent and to the sink RDO; after it determines that the mesh network 102 has been successfully routing packets; or after it has spent a predetermined time in network-discovery state without any changes to its network associations. A radio device may determine that the mesh network 102 has been successfully routing packets by monitoring the type of network traffic it is receiving and routing. For instance, after power on there will typically be lots of association type link local communication; after a while there will be a large amount of routing traffic to the backbone (e.g. authenticate to cloud) and/or to other devices (e.g. discovery); after this, a more stable phase will be reached during which there should be less traffic overall. The device may detect such changes in traffic pattern and respond accordingly. For example, when lots of bandwidth is used for application, it may move away from the network-discovery configuration to the high-power routing configuration. After application traffic starts to reduce, it may move to the low-power routing configuration (or remain at high-power routing if configured so).

Figure 6 illustrates the behaviour of an exemplary radio device, operating in FT mode, that supports two different beacon transmission periods. It may implement this behaviour with regard to network beacons or cluster beacons or both. At a first time, the device transmits 600 radio beacons periodically with a first period (e.g. transmitting cluster beacons every 50 ms). At intervals it checks 601 if a first predetermined statechange condition is met. This could be any single condition or logical combination of conditions disclosed herein. If it is not met, it continues transmitting the beacons with the first period. If it determines the condition is met, it changes to transmitting 602 the beacons at a second, different period (e.g. transmitting cluster beacons every 8 seconds). At intervals it then checks 603 if a second predetermined state-change condition is met. This could be any single condition or logical combination of conditions disclosed herein; it could simply be the reversal of the first condition. If it is not met, it continues transmitting the beacons with the second period. If it determines the condition is met, it changes to transmitting 600 the beacons at the first period (e.g. transmitting cluster beacons every 50 ms).

Figure 7 illustrates the behaviour of an exemplary radio device, operating in FT mode, that supports three different states, at least two of which specify different beacon transmission periods for at least one type of beacon (e.g. for network beacons or cluster beacons or both). After the device is powered on 700, it enters a networkdiscovery state 701 , in which it transmits network beacons at a first rate. At intervals it checks 702 if network formation has finished (e.g. based on one or more of the criteria described above). If not, it remains in the network-discovery state 701. If so, it changes to a low-power routing state 703, in which it transmits network beacons less frequently. It may also transmit cluster beacons less frequently, and set a lower RACH allocation. At intervals it checks 704 for any connection problems, e.g. the loss of its parent or the loss of two or more of its child devices. If so, it returns to the network-discover state 701. If no change is detected, it checks 705 if any of its child devices are routers (i.e. operating in FT mode). If not, it remains in the low-power routing state 703. If so, it changes to a high-power routing state 706, in which it transmits cluster beacons more frequency and sets a higher RACH allocation. At intervals it continues to check for any losses of its connection 704 and for if it no longer has any downstream routers 705; if the first is detected, it re-enters the network-discover state 701; if the second is detected, it changes to the low-power routing state 703.

It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims.