WO2009024925A2

WO2009024925A2 - Method and system for implementing low duty cycle wireless systems

Info

Publication number: WO2009024925A2
Application number: PCT/IB2008/053319
Authority: WO
Inventors: Dave A.T. Cavalcanti; Maulin D. Patel; Monisha Ghosh
Original assignee: Koninklijke Philips Electronics, N.V.
Priority date: 2007-08-20
Filing date: 2008-08-19
Publication date: 2009-02-26
Also published as: WO2009024925A3; TW200917761A

Abstract

A remote monitoring system includes a monitor and multiple sensors configured to collect sensor data and to send at least a portion of the sensor data and an alarm to the monitor over a wireless communications link. Each sensor receives reservation information from the monitor in a beacon frame or a control frame of a broadcast signaling window in a super-frame. The reservation information identifies data allocation slots of the super- frame for sending sensor data to the monitor, alarm slots of the super-frame for sending the alarm to the monitor in response to an alarm trigger, and a duty cycle schedule. Each sensor periodically awakens from an inactive state according to the duty cycle schedule to send the sensor data to the monitor. Each sensor also awakens in response to the alarm trigger, regardless of the duty cycle schedule, to send the alarm indication to the monitor.

Description

METHOD AND SYSTEM FOR IMPLEMENTING LOW DUTY CYCLE

WIRELESS SYSTEMS

Recently, there has been increasing interest in wireless systems for various monitoring, automation and control applications. Typically, these systems consist of nodes or sensors and one or more monitoring units which communicate wirelessly. For example, in the medical field, wireless monitoring of a patient may include a series of sensors attached to a patient that send and receive signals from a centralized monitoring device over a wireless local area network (WLAN), a wireless personal area network (WPAN), a wireless body area network (WBAN) or the like, such as an IEEE 802.11 network (Wi-Fi), a WiMedia Ultra-Wideband (UWB) network, or an IEEE 802.15.4 network ( ZigBee). The signals include data collected by the sensors and control signaling provided by the monitoring device. One example is the Sensor Tape monitoring system proposed by the Defense Advanced Research Projects Agency (DARPA) in BAA07-44, Proposer Information Pamphlet for DARPA Strategic Technology Office, Sensor Tape (August 13, 2007).

Typically, the sensors of a wireless monitoring network have very low duty cycles and limited power availability and/or lifespan. Therefore, minimizing power consumption to the extent possible, without compromising effectiveness of the monitoring, is desirable. Physical and medium access control (MAC) layers for wireless monitoring networks are thus typically designed to reduce power consumption. For example, in order to reduce power consumption, the sensors may temporarily power down ("hibernate," or enter an inactive state or "sleep mode") at periodic intervals in order to conserve energy, and then power up again to enable communications ("wake up," or enter an active state or "awake mode"). However, such systems increase response latency whenever events trigger alarms and commands that need to be wirelessly transmitted to/from a monitoring unit.

Existing wireless communication protocols do not meet all the requirements for very low duty cycle monitoring systems. For example, IEEE 802.11 and WiMedia protocols are designed for high throughput applications and are too complex to be efficiently implemented in a simple, low-cost sensor node. Also, although the IEEE 802.15.4 protocol has been designed for low duty cycle sensors, it does not provide the reliability and Quality of Service (QoS) required by critical applications (e.g. patient monitoring), and is not sufficient energy efficient. In one aspect of the present teachings, a method is provided for implementing a low duty cycle monitoring system. The method includes receiving reservation information from a master device over a selected channel of a wireless network; identifying at least one allocated data slot of a super- frame, based on the reservation information, for periodically sending data to the master device; identifying an alarm signaling window, including at least one alarm slot of the super- frame, based on the reservation information, for sending an alarm to the master device in response to an alarm trigger event; and identifying predetermined data transmission time periods during which to send the data to the master device based on the reservation information. The data is sent to the master device over the wireless network in the at least one allocated data slot during the predetermined time periods. A sleep mode is entered when not sending the data during the predetermined time periods.

The alarm may be sent in the alarm signaling window at a time other than the predetermined data transmission time periods in response to the alarm trigger. In addition, entering the sleep mode may include powering off a transceiver used for sending the data and/or the alarms to the master device over the wireless network.

The method may further include resending the alarm when an acknowledgement of the alarm is not received from the master device. Also, a request from the master device may be received in response to the alarm for additional information relating to the alarm. The additional information may be sent to the master device in at least one response slot, identified in a frame which includes the request for additional information. The frame which includes the request for additional information may be a subsequently received beacon frame or request frame included in a broadcast signaling window.

The method may further include performing a contention-based access protocol to access the alarm signaling window when the alarm signaling window is shared. For example, the contention-based access protocol may be a Carrier Sense Multiple Access (CSMA) protocol, an ALOHA protocol or a slotted ALOHA protocol.

The reservation information may be included in a broadcast signaling window of the super- frame, where the broadcast signaling window may include at least one slot of the super-frame. Alternatively, the reservation information may be included in a beacon frame of the super-frame, where the beacon frame includes at least one slot of the super-frame. Also, the wireless network may be discovered based on the beacon frame sent from the master device. In another aspect of the present teachings, a low duty cycle wireless device provides data over a wireless network to a monitor. The wireless device includes a processor for controlling the wireless device to collect the data and to communicate with the monitor, a memory for storing the collected data and a transceiver. The transceiver is configured to receive reservation information from the monitor in at least one reservation slot of a super- frame, to send at least a portion of the collected data to the monitor in at least one data allocation slot of the super- frame, and to send an alarm to the monitor in an alarm signaling window of the super-frame. Slots of the at least one data allocation slot and the alarm signaling window are identified based on the reservation information. The transceiver periodically awakens from an inactive state to send the at least a portion of the collected data in accordance with a transmission schedule provided in the reservation information. The transceiver also awakens from the inactive state to send the alarm in response to an alarm trigger provided by the processor, regardless of the transmission schedule. The transceiver may awaken from the sleep state by powering on. Also, the at least one reservation slot may be a beacon frame of the super- frame or a broadcast signaling window, including multiple consecutive slots of the super-frame. The transceiver may resend the alarm when an acknowledgement of the alarm is not received from the monitor. Further, the transceiver may receive a request from the monitor for additional information relating to the alarm in a subsequent beacon frame of the super- frame in response to the alarm. The transceiver may send the additional information, provided by the processor, to the monitor in at least one response slot of the super-frame identified in the subsequent beacon frame.

In another aspect of the present teachings, a remote monitoring system includes at least one monitor connected to a wireless communications link and multiple sensors configured to collect sensor data and to send at least a portion of the sensor data and an alarm to the at least one monitor over the wireless communications link. Each sensor receives reservation information from the at least one monitor in a beacon frame or in a frame transmitted in the broadcast signaling window of a super-frame. The reservation information identifies at least one data allocation slot of the super-frame for sending the at least a portion of the sensor data to the monitor, at least one alarm slot of the super-frame for sending an indication of the alarm to the monitor in response to a corresponding alarm trigger, and a duty cycle schedule. Each sensor periodically awakens from an inactive state according to the duty cycle schedule to send the at least a portion of the sensor data to the monitor. Each sensor also awakens from the inactive state in response to the alarm trigger, regardless of the duty cycle schedule, to send the alarm indication to the monitor.

The alarm trigger may be activated based on at least one of the collected sensor data or an indication of remaining battery power of the corresponding sensor. In addition, when the at least one monitor shares the wireless channel with a second monitor, the reservation information may further identify coexistence time slots of the super-frame for including beacons from the first and second monitors, respectively. Each of the sensors associates with one of the first monitor or the second monitor based on the corresponding beacons.

FIG. 1 is a block diagram of a communications system having a monitor device and multiple sensor devices according to one embodiment.

FIG. 2 is a block diagram of a super-frame structure used within a network according to one embodiment. FIG. 3 is a flow diagram of a process of initializing the network and receiving association requests from sensor devices according to one embodiment.

FIG. 4 is a flow diagram of a process of receiving alarms from sensor devices according to one embodiment.

FIG. 5 is a flow diagram of a process of associating with a monitor device and providing data to a monitor device according to one embodiment.

FIG. 6 is a flow diagram of a process of providing alarms to a monitor device according to one embodiment.

In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of an embodiment according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known devices and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and devices are clearly within the scope of the present teachings.

In various embodiments, in order to reduce power consumption and guarantee reliability, the monitoring may include, for example, periodically receiving data and/or asynchronous alarms from multiple sensors over a WLAN, WPAN or WBAN. The alarms may be triggered by certain predetermined events, such as when measured parameters exceed prescribed values or available power drops below a predetermined threshold. The monitoring may further include asynchronous request/response data exchange in which the monitoring unit makes unscheduled requests for data to the sensor(s). The monitoring system is implemented with a MAC protocol that effectively shifts all complex operations to the monitoring unit. The MAC protocol according to the various embodiments is relatively simple, reliable, fault-tolerant and energy efficient. Energy efficiency is particularly significant since the energy consumed during wireless communications is the largest portion of total energy consumed by a sensor, which may have a limited power supply.

The MAC protocol provides a master- slave arrangement between the monitor and the sensor(s), respectively. More particularly, the MAC protocol includes reservation- based channel access controlled by the master, and supports periodic data transfer, asynchronous alarms and request/response applications through deterministic signaling windows. There may be multiple monitors, in which case the MAC protocol supports seamless handover among the various monitors, enabling coexistence with other systems sharing the frequency spectrum.

FIG. 1 is a functional block diagram of one embodiment of a monitoring communications system 100. As will be appreciated by those skilled in the art, the various functions and components shown in FIG. 1 may be physically implemented using a software-controlled microprocessor, hard- wired logic circuits, or a combination thereof. Also, while the functional blocks are illustrated as being segregated in FIG. 1 for explanation purposes, they may be combined variously in any physical implementation. The monitoring communications system 100 includes a monitor 110 (master) and multiple sensors 120, 130 (slaves), which communicate with the monitor 110 over a shared channel of a wireless network or communications link 140. The wireless communications link 140 may be a WLAN, WPAN, or WBAN, for example, and may operate in any appropriate frequency range, including unlicensed industrial, scientific and medical (ISM) bands, such as the 2.4GHz band used by Bluetooth. Although FIG. 1 depicts only one monitor 110 and two sensors 120, 130, it is understood that various implementations may include multiple monitoring units and any number of sensors, each of which may be configured to communicate with one or many monitoring units. Each sensor 120, 130 may include substantially the same components, used to collect, process and/or store a variety of electronic data. For example, the sensor 120 includes antenna system 122, transceiver 124, microprocessor 126 and memory 128.

The transceiver 124 includes receiver 123 and transmitter 125, and provides functionality for the sensor 120 to communicate with other wireless devices (e.g., the monitor 110) in a wireless communication network. The processor 126 is configured to execute one or more software algorithms in conjunction with memory 128 to provide the functionality of the sensor 120. Beneficially, the processor 126 includes its own memory (e.g., nonvolatile memory) for storing executable software code that allows it to perform the various functions of the sensor 120. Alternatively, the executable code may be stored in designated memory locations within memory 128.

In one embodiment, the antenna system 122 may include a directional antenna system, which provides a capability for the sensor 120 to select from multiple antenna beams for communicating with other wireless devices in multiple directions. For example, the antenna system 122 may include multiple antennas, each corresponding to one antenna beam, or the antenna system 122 may include a steerable antenna that can combine a multiple different antenna elements to form a beam in different directions. Alternatively, antenna system 122 may be a non-directional or omnidirectional antenna system.

The monitor 110 is likewise a processing device capable of wireless communications, as well as receiving, processing and storing electronic data. Further, the monitor 110 remotely controls various functions of the sensors 120, 130, discussed below. The monitor 110 includes antenna system 112, transceiver 114, microprocessor 116 and memory 118.

The transceiver 114 includes receiver 113 and transmitter 115, and provides functionality for the monitor 110 to communicate with other wireless devices (e.g., the sensors 120, 130). The processor 116 is configured to execute one or more software algorithms in conjunction with memory 118 to provide the functionality of the monitor 110. Beneficially, the processor 116 may include its own memory (e.g., nonvolatile memory) for storing executable software code that allows it to perform the various functions of the monitor 110. Alternatively, the executable code may be stored in designated memory locations within the memory 118.

As discussed above with respect to the sensor 120, the antenna system 112 of the monitor 110 may include a directional antenna system, which provides the capability for the monitor 110 to select from multiple antenna beams for communicating with other wireless devices in multiple directions. For example, the antenna system 112 may include multiple antennas, each corresponding to one antenna beam, or the antenna system 112 may include a steerable antenna that can combine a multiple different antenna elements to form a beam in different directions. Alternatively, the antenna system 112 may be a non- directional or omnidirectional antenna system.

In an embodiment, the MAC protocol of the present embodiment, enabling communications between the monitor 110 and the sensors 120, 130 over wireless communications link 140, is based on Time Division Multiple Access (TDMA) scheme, and avoids collisions, minimizes idle listening and minimizes overhearing, thereby improving the energy efficiency and reliability. As stated above, the network architecture is master-slave, where the monitor 110 operates as a master, and the sensors 120, 130 operate as slaves. Each sensor (slave) 120, 130 associates with the monitor (master) 110, which controls access to the wireless communications link 140 and grants transmission opportunities. Accordingly, efficient ultra low-power operation of the sensors 120, 130 is achieved by turning off at least the transceiver 124 (entering a sleep mode) in each of the sensors 120, 130 when the sensors 120, 130 are not transmitting or receiving data, respectively.

Also, by centralizing processing and control in the monitor 110, power consumption and physical attributes of the sensors 120, 130, such as size, weight, rigidity and the like, may be reduced. For example, in an embodiment, the sensors 120, 130 may be flexible sensor patches that attach to a patient's body in a medical setting. Such sensor patches may monitor and store information detected from the patient, including vital signs, such as temperature, respiratory rate, heart rate, blood oxygen level, pulse wave form, and the like. Also, multiple sensors in a sensor patch may share a single transceiver, in which case a network may be formed by multiple patches, each of which includes multiple sensors and a single transceiver. Thus, each patch may act as a single sensor node with respect to the MAC layer protocol.

The sensors 120, 130 then send data to the monitor 110 at periodic intervals or in response to various alarm triggers and asynchronous requests, as discussed below. The monitor 110 may be any type of device configured to receive, process, store and/or display such information. For example, the monitor 110 may be a handheld, mobile device carried by medical personal, a centralized computer or server (e.g., at a nurses' station), or the like. However, embodiments of the present teachings are not limited to implementations involving patient monitoring. Other illustrative embodiments may include collecting ambient weather related data, such as temperature, humidity and atmospheric pressure, collecting manufacturing data in a plant environment, or collecting premises data in a building security environment, such as motion and sound detection.

FIG. 2 is a block diagram depicting a structure of super-frame structure 200 of an illustrative MAC protocol, according to one embodiment. According to the MAC protocol, the monitor 110 transmits periodic beacon frames, e.g., beacon frames 210, 270, which carry information enabling network communications, such as network discovery, network identification, node identification, network- wide synchronization, and the like. As shown in the illustrative embodiment of FIG. 2, super- frame 200 is divided into 100 time slots of 6 msec, each, such that one super- frame 200 is 600 msec. The beacon frames 210, 270 occupy the first time slot (e.g., slot 0) in consecutive super-frames 200. (The remainder of the super-frame incorporating beacon frame 270 is not shown.) The specific duration of super- frame 200, and the number and duration of the time slots within the super- frame 200 are related to duty cycle, power-consumption and QoS requirements. Therefore, the values shown in FIG. 2 are merely examples, which may differ to optimize various configurations, without departing from the spirit and scope of the present teachings.

Super-frame 200 includes time slots reserved as signaling windows, during which the monitor 110 and/or the sensors 120, 130 are able to access the medium (e.g., wireless communications link 140). Three types of signaling windows are defined at fixed time slots, which help simplify the design of the sensors 120, 130. A Broadcast Signaling Window (BSW) 221 is used by the monitor 110 to transmit broadcast frames with control and reservation information to the sensors 120, 130. For example, the monitor 110 may provide slot allocations for the sensors 120, 130 in BSW 221. The monitor 110 may indicate in beacon frame 210 of super-frame 200 an upcoming broadcast frame, which will be transmitted in BSW 221. In the illustrative super- frame 200, BSW 221 occupies four slots, slots 1 to 4, immediately following the beacon frame 210, although the size and position of BSW 221 may vary. Super-frame 200 includes an Alarm Signaling Window (ASW) 253, occupying for slots (e.g., slots 92-95), for example, located near the end of super-frame 200. ASW 253 is used by the sensors 120, 130 to communicate with the monitor 110, e.g., including transmitting alarms, and sending association request frames. In the depicted embodiment, ASW 253 is shared by the sensors 120, 130, which may employ a contention-based access protocol, such as Carrier Sense Multiple Access (CSMA) with random backoff, ALOHA or slotted ALOHA contention based protocols, in order to access the shared channel during the ASW 253. Accordingly, the sensors 120, 130 may verify the absence of other traffic prior to transmission.

In an embodiment, BSW 221 may not be used, in which case the slots for BSW 221, e.g., slots 1 through 4 immediately following the beacon frame 210 in super-frame 200, may be used an ASW 223. ASW 223 may be used to provide the same types of communication discussed above with respect to ASW 253, either in addition to ASW 253 or in place of ASW 253. When multiple ASWs 223, 253 are available, different sensors may use the different ASWs 223, 253 to avoid sharing the same slots.

Super-frame 200 also includes a Coexistence Signaling Window (CSW) 260, which consists of the last 4 slots (e.g., slots 96-99), for example, of the super-frame 200. CSW 260 enables coexistence of multiple, neighboring master devices (e.g., monitor 110) operating in the same wireless channel. The slots in CSW 260 are used by additional monitors (not pictured) to join the network and to synchronize their respective super- frames with the super-frames (e.g., super-frame 200) of the existing monitors (e.g., monitor 110). The size or number of slots in CSW 260 determines the maximum number of monitors operating in the same wireless channel. For example, CSW 260 of super-frame 200 depicted in FIG. 2 includes four slots and thus enables up to four additional simultaneously functioning monitors to merge operations. The merged monitors use their respective slots in CSW 260 to transmit their corresponding beacon frames. The size of CSW 260 is also a system parameter that may be adjusted according to the capacity of the wireless channel and application requirements. The remaining slots of super-frame 200 are used for data communication between the sensors 120, 130 and the monitor 110. Access to these slots is controlled by the monitor 110, which assigns non-overlapping slots of super- frame 200 to each of the sensors 120, 130 for data transmission. For example, sensor slot allocation 230 (e.g., slots 5-10) is allocated by the monitor 110 to the sensor 120. The sensors 120, 130 may receive the slot allocation information in beacon frame 210 and/or BSW 221. When there are multiple masters sharing the channel, each master is responsible for allocating data communication slots to each of its corresponding slaves. In the depicted embodiment, the MAC protocol supports efficient application data transfers between the sensors 120, 130 and the monitor 110. For example, data is transferred only periodically (e.g., every five minutes), in the course of normal communications, e.g., in which no alarms are triggered. The sensors 120, 130 can transmit alarm messages to the monitor 110, as well as association request frames when first associating with the monitor 110, within the reserved ASWs, ASW 223 and/or ASW 253. The monitor 110 acknowledges successful reception of an alarm in the next beacon frame (e.g., beacon frame 270). When additional sensor data relating to the alarm message is required, the monitor 110 may use the asynchronous request/response feature, described below. Accordingly, alarms from the sensors 120, 130 are delivered to the monitor 110 with less than a one second delay, for example, even though data is routinely transferred at a much slower rate (e.g., every five minutes).

In an embodiment, the asynchronous request/response feature of the MAC protocol enables the monitor 110 to interrogate sensors 120, 130 at any time by indicating the request in a beacon frame 210, 270, which also identifies the slots to be used by the respective sensor 120, 130 for transmitting the response back to the monitor 110. For example, a request in beacon frame 210 may identify response allocation 240 (e.g., slots 86-91) as the portion of super- frame 200 in which the information from the responding sensor 120, 130 is to be included. FIG. 3 is a flow diagram of a process of initializing the network and receiving association requests from sensor devices according to one embodiment. The master device (e.g., monitor 110) is responsible for network initialization, and access control and authentication of corresponding slave devices (e.g., sensors 120, 130). In order to perform initialization, the monitor 110 scans available channels for communications link 140 at step S310, depending on the frequency band used, and determines whether other monitors are operating on the channels at step S312. When other monitors are present (step S312: YES), the monitor 110 coordinates its operation with the other monitors at step S314. For example, the CSW 260 can be used by multiple monitors operating in the same channel to align their respective super-frame structures and enable intelligent scheduling of the slots. Therefore, the monitor 110 obtains the other monitor's slot reservations and designs non- overlapping schedules. In an embodiment, each of the multiple monitors is responsible for coordinating communications with other monitors operating in the same channel. Alternatively, subsequent monitors may be required to avoid conflicts with the monitors already operating in the channel.

Further, in an embodiment, the MAC protocol supports seamless handover of sensors between two monitors. For example, a first monitor (e.g., monitor 110) may instruct a set of sensors 120, 130 to switch to a new monitor. The instruction is sent in the beacon frame 210, 270 and/or BSW 221, for example. The sensors 120, 130 accordingly adjust their operation merely by beginning to listen for beacons from the new monitor, which must occupy one of the slots in CSW 260 of the super-frame structure 200. Thus, the MAC protocol enables coexistence of multiple monitors without adding additional complexity to the sensors.

At step S316, the monitor 110 selects an operation channel, as well as various network parameters. The monitor 110 then begins transmission of beacon frame 210 at step S318, using available slots of CSW 260 if other monitors have been detected in the operating channels. At step S320, the monitor 110 receives an association request for each sensor (e.g., sensor 120, 130) that desires to associate with the monitor based on the beacon frame 210. The monitor 110 verifies the association request at step S322. When the monitor 110 determines that a requesting sensor is not part of its group or should otherwise be prevented from communicating with the monitor 110 (step S322: NO), the monitor 110 simply continues to transmit the beacon frame 210, e.g., until all sensors are joined. When the monitor 110 determines that a requesting sensor is part of its group (step S322: YES), the monitor 110 validates the sensor and begins periodically receiving data from each of the verified sensors (e.g., the sensors 120, 130) at step S324. The data is received according to the predetermined duty cycle of the sensors, which cycle between awake and sleep modes to conserve energy.

Throughout the course of the association, the monitor 110 continues to monitor the sensors 120, 130 for alarm signals at step S326. As long as no alarm signal is received (step S326: NO), the process of receiving data (step S324) and monitoring for alarm signals (step S326) simply continues until it is determined that the communications session is complete at steps S328 (step S328: YES).

When an alarm signal is received (step S326: YES), the process proceeds to FIG. 4, which is a flow diagram of a process of receiving alarm signals according to one embodiment. An alarm identifier is received at step S410, which identifies the specific type of alarm. For example, the alarm identifier may indicate that the sensor sending the alarm (e.g., sensor 120) has critically low battery power, or that a parameter being monitored by the sensor 120 (e.g., temperature or blood pressure) has exceeded a predetermined threshold. The monitor 110 sends an acknowledgement of the alarm at step S412.

At step S414, the monitor 110 determines whether it needs additional data relating to the conditions of the alarm. For example, the monitor 110 may request the actual value of the blood pressure or the blood pressure readings over the preceding two minutes. When addition information is desired (step S414: YES), the monitor 110 may formulate and send an asynchronous request to the sensor 120 at step S416, using beacon frame 210, for example. The asynchronous request may be formulated and sent automatically or through human intervention at a user interface. The requested information relating to the alarm is received by the monitor 110 at step S418, using the response allocation 240, the slots of which may have been identified in beacon frame 210, as previously discussed. After the desired information is received at step S418, or when no addition information relating to the alarm is requested (step S414: NO), the process returns to step S328 in FIG. 3. The process continues until the session ends.

FIG. 5 is a flow diagram of a process of providing data to a monitoring device according to one embodiment. Initially, a slave device (e.g., sensor 120) scans the various channels of the RF spectrum band used in the communications system 100 and listens for beacon frames at step S510 to locate its corresponding master device. The sensor 120 receives a beacon frame (e.g., beacon frame 210) at step S512. Based on the received beacon frame 210, the sensor 120 selects the channel and master device (e.g., monitor 110) with which to associate at step S514, and synchronizes with the super- frame timing of the monitor 110 at step S515.

At step S516, the sensor 120 transmits association request frames to the monitor 110, for example, in slots of ASW 223 and/or ASW 253, and waits for an association response in a subsequent beacon frame (e.g., beacon frame 270) at step S518. When the association response is not received (step S518: NO), the process returns to step S510 for the sensor 120 to continue to scan the available channels. In an embodiment, the sensor 120 may make a predetermined number of attempts to obtain an association response before returning to step S510. When an association response is received (step S518: YES), the association is successful and normal operation begins. At steps S520, the sensor 120 identifies its assigned slots from reservation information, which the monitor 110 advertises in beacon frames (e.g., beacon frame 210) and/or broadcast frames (e.g., BSW 221). For example, the reservation information includes an initial super-frame number, a start slot number (e.g., slot number 5 of sensor slot allocation 230) and duration (e.g., the number of slots assigned per reservation or six slots).

At step S522, the sensor 120 also identifies the timing according to which it will transmit sensor data to the monitor 110. In addition, the sensor may receive additional timing information according to which it will receive subsequent beacon frames, although the same timing information may apply to sending sensor data and receiving beacon frames. The timing information may also be included in the reservation information provided by the monitor 110. For example, the reservation information may include a repetition period identifying the interval of super-frames in which the sensor 120 should include its sensor data. For example, the timing information may require the sensor 120 to send its sensor data every 500 super-frames, which equates to every five minutes, assuming the super- frames are 600 msec, in length, as shown in the illustrative super-frame structure 200 of FIG. 2. With this information, the sensor 120 knows when it must transmit data to the monitor 110, and can thus optimize its power-save operation by remaining in the sleep mode unit the next transmission time.

At step S524, the sensor 120 turns off its transceiver 124 (either before or after initially transmitting data) and enters the sleep mode to converse power. Even when in the sleep mode, the sensor 120 monitors data relating to various alarms at step S526 and keeps track of the timing at step S528. When no alarms have been triggered (step S526: NO), the sensor 120 waits for the next transmission period (step S528: YES). At step S530, the sensor 120 awakens at the prescheduled time, for example, by turning on its transceiver 124, and transmits the sensor data to the monitor 110. The sensor 120 may likewise receive data from the monitor 110 while in its awake mode. At step S532, the sensor 120 may determine that it must resend the transmitted sensor data (step S532: YES), for example, in response to an asynchronous request from the monitor 110. When there is no need for retransmission (step S532: NO), the sensor 120 returns to the sleep mode at step S524 when the communications session is not complete (step S538: NO). The sensor 120 continues to revive from sleep mode to periodically send its sensor data until the session ends (step S538: YES).

When sensing data is detected that triggers an alarm (step S526: YES), the sensor 120 signals the alarm and provides related information to the monitor 110, as requested, according to FIG. 6, which is a flow diagram of a process of providing alarm signals according to one embodiment. Because alarm occurrences are unlikely to coincide with the predetermined wake-up schedule, the sensor 120 must first awaken from the sleep mode in step S608 in response to an alarm trigger in order to communicate over the communications link 140. In step S610, the sensor 120 sends an alarm identifier in ASW 223 and/or ASW 253 to the monitor 110 and determines whether an acknowledgement of receipt is received at step S612. The sensor 120 may remain in the active state until the acknowledgement is received, for example, in the next beacon frame 210. Alternatively, the sensor 120 may re-enter the inactive state until the time for the next beacon frame 210 arrives. For example, if the alarm identifier is sent to the monitor 110 in ASW 253, which is near the end of the current super-frame 200, and the monitor is programmed to respond in the next consecutive beacon frame, then the sensor 120 may stay awake until the acknowledgement is received due to the relatively short period of time. However, if the alarm identifier is sent to the monitor 110 in ASW 223, which is near the beginning of the current super-frame 200, or the monitor is programmed to respond in some later beacon frame 2110, then the sensor 120 may re-enter the sleep mode to conserve energy until the expected time of acknowledgement, at which point the sensor 120 awakes again.

When there is no acknowledgment (S612: NO), the sensor 120 sends the alarm identifier again, for example, in the next ASW 223 and/or ASW 253 until an acknowledgement is received (S612: YES). As discussed above, a contention-based protocol for accessing the shared slots of ASW 223, 253, as well as the automatic resending, enables reliable and timely deliver of the alarms to the monitor 110.

In addition, the sensor 120 determines whether the monitor 110 has requested additional information relating to the alarm at step S614, as previously discussed. For example, the sensor 120 may receive an asynchronous request from the monitor 110 in a subsequent beacon frame 210. As discussed above with respect to acknowledgements, the sensor 120 may stay awake until the next beacon 210 to find out whether the monitor 110 requests additional information, or it may re-enter the sleep mode and re-awaken in time for the next beacon(s) 210 to find out whether the monitor 110 requests additional information, e.g., depending on use of ASW 223 or ASW 253, or the predetermined timing of the response. When additional information is requested (step S614: YES), the sensor 120 collects (when necessary) and transmits the requested information in response allocation 240, e.g., identified in beacon frame 210, which is included the asynchronous request. After the requested information is sent at step S616, or when no addition information relating to the alarm is requested (step S614: NO), the process returns to step S526 in FIG. 5. The sensor 120 then continues to periodically transmit data and monitor for alarm triggers until the session ends (step S538: YES).

The MAC protocol according to various embodiments avoids collisions, idle listening and overhearing, and enables ultra low duty cycle operation of slave devices, e.g., sensors 120, 130, thereby saving energy. The scheduling algorithms maximize channel usage to provide required QoS for applications, while minimizing power usage. The sensors 120, 130 wake up to receive beacons and/or requests, to transmit data at pre- scheduled times, and to transmit alarms in the reserved ASWs 123, 153, when needed. At all other times, each sensor 120, 130 switches off its transceiver 124 to enter the sleep mode. Estimates indicate that the duty cycle of the sensors 120, 130 operating according to embodiments of the MAC protocol discussed above are approximately one percent.

While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the present teachings. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.

Claims

1. A method for implementing a low duty cycle monitoring system, comprising: receiving reservation information from a master device over a selected channel of a wireless network; identifying at least one allocated data slot of a super- frame, based on the reservation information, for periodically sending data to the master device; identifying an alarm signaling window, comprising at least one alarm slot of the super-frame, based on the reservation information, for sending an alarm to the master device in response to an alarm trigger; identifying a predetermined plurality of data transmission time periods during which to send the data to the master device based on the reservation information; sending the data to the master device over the wireless network in the at least one allocated data slot during the predetermined plurality of time periods; and entering a sleep mode when not sending the data during the predetermined plurality of time periods.

2. The method of claim 1, wherein the alarm is sent in the alarm signaling window at a time other than the predetermined plurality of data transmission time periods in response to the alarm trigger.

3. The method of claim 2, wherein entering the sleep mode comprises: powering off a transceiver used for sending the data and the alarm to the master device over the wireless network.

4. The method of claim 2, further comprising: resending the alarm when an acknowledgement of the alarm is not received from the master device.

5. The method of claim 4, further comprising: receiving a request from the master device in response to the alarm for additional information relating to the alarm; and sending the additional information in at least one response slot, the at least one response slot being identified in a frame which includes the request for additional information.

6. The method of claim 5, wherein the frame which includes the request for additional information comprises a subsequently received beacon frame.

7. The method of claim 2, further comprising: performing a contention-based access protocol to access the alarm signaling window when the alarm signaling window is shared.

8. The method of claim 7, wherein the contention-based access protocol comprises at least one of a Carrier Sense Multiple Access (CSMA) protocol, an ALOHA protocol or a slotted ALOHA protocol.

9. The method of claim 1, wherein the reservation information is included in a frame transmitted in a broadcast signaling window of the super-frame, the broadcast signaling window comprising at least one slot of the super-frame.

10. The method of claim 1, wherein the reservation information is included in a beacon frame of the super- frame, the beacon frame comprising at least one slot of the super- frame.

11. The method of claim 10, wherein wireless network is discovered based on the beacon frame sent from the master device.

12. A low duty cycle wireless device for providing data over a wireless network to a monitor, the wireless device comprising: a processor for controlling the wireless device to collect the data and to communicate with the monitor; a memory for storing the collected data; and a transceiver for receiving reservation information from the monitor in at least one reservation slot of a super-frame, for sending at least a portion of the collected data to the monitor in at least one data allocation slot of the super-frame, and for sending an alarm to the monitor in an alarm signaling window of the super- frame, slots of the at least one data allocation slot and the alarm signaling window being identified based on the reservation information; wherein the transceiver periodically awakens from an inactive state to send the at least a portion of the collected data in accordance with a transmission schedule provided in the reservation information, and wherein the transceiver awakens from the inactive state to send the alarm in response to an alarm trigger provided by the processor, regardless of the transmission schedule.

13. The device of claim 12, wherein the transceiver awakens from the sleep state by powering on.

14. The device of claim 12, wherein the at least one reservation slot is a beacon frame of the super-frame.

15. The device of claim 12, wherein the at least one reservation slot is a broadcast signaling window comprising a plurality of consecutive slots of the super-frame.

16. The device of claim 12, wherein the transceiver resends the alarm when an acknowledgement of the alarm is not received from the monitor.

17. The device of claim 12, wherein the transceiver receives a request from the monitor for additional information relating to the alarm in a subsequent beacon frame of the super-frame in response the alarm, and wherein the transceiver sends the additional information, provided by the processor, to the monitor in at least one response slot of the super-frame identified in the subsequent beacon frame.

18. A remote monitoring system, comprising: at least one monitor connected to a wireless communications link; and a plurality of sensors configured to collect sensor data and to send at least a portion of the sensor data and an alarm to the at least one monitor over the wireless communications link, each sensor of the plurality of sensors receiving reservation information from the at least one monitor in a beacon frame of a super-frame, the reservation information identifying at least one data allocation slot of the super-frame for sending the at least a portion of the sensor data to the monitor, at least one alarm slot of the super-frame for sending an indication of the alarm to the monitor in response to a corresponding alarm trigger, and a duty cycle schedule; wherein each sensor periodically awakens from an inactive state according to the duty cycle schedule to send the at least a portion of the sensor data to the monitor, and wherein each sensor awakens from the inactive state in response to the alarm trigger, regardless of the duty cycle schedule, to send the alarm indication to the monitor.

19. The remote monitoring system of claim 18, wherein the alarm trigger is activated based on at least one of the collected sensor data or an indication of remaining battery power of the corresponding sensor.

20. The remote monitoring system of claim 18, wherein when the at least one monitor comprises a first monitor and a second monitor, the reservation information further identifies coexistence time slots of the super- frame for including beacons from the first and second monitors, respectively, each of the plurality of sensors associating with one of the first monitor or the second monitor based on the corresponding beacons.