System and method for allocating time slots for unicast use - Patents.com

Abstract

A system and method are described for reducing end-to-end latency in link-layer unicast transmissions by shifting time slots in a TDMA-mode superframe based on membership in a multicast group. In this way, transmissions to all multicast subscribers can occupy adjacent time slots, optimizing slot allocation and improving end-to-end latency for multicast communications.

Description

[Technical field]

[0001] The field of the invention is multicast communications, for example for use in in-flight wireless sensor networks, Internet of Things (IoT), home automation, and low power devices. [Background technology]

[0002] The following description contains information that may be useful in understanding the present invention. It is not admitted that any of the information provided herein is prior art or relevant to the present claimed invention, or that any publication specifically or implicitly referenced is prior art.

[0003] Currently, there is an increasing demand for new applications of wireless sensor networks (WSNs). One exemplary area of ​​application relates to in-flight communications, particularly for aircraft. As aircraft systems become more and more connected, additional sensors are utilized in aircraft to determine the status and health of seats, electronics, and other components. With the demand for these increasing new applications, it is useful to connect WSNs to the Internet by Internet Protocol (IP) addressing, and particularly by Internet Protocol version 6 (IPv6) and future protocols, to achieve all IP communications and thus interoperability.

[0004] However, current standards, including the Institute of Electrical and Electronics Engineers (IEEE) 802.15.4, do not natively support multicast. Instead, algorithms have been developed that utilize link-layer unicast transmission or link-layer broadcast transmission to achieve IP multicast. While link-layer unicast transmission provides high reliability due to acknowledgement-based retransmission, it does not provide good end-to-end latency because an individual transmission is required for each multicast subscriber and requires high energy consumption. In contrast, broadcast transmission provides low end-to-end latency and low energy consumption due to a single transmission to all multicast subscribers. However, it is less reliable due to the lack of retransmission and acknowledgement.

[0005] Accordingly, various techniques have been developed to attempt to address the problems identified above, but each has its own problems. For example, the IEEE 802.11aa standard identifies various mechanisms for multicast transmission. A study of the standard can be found in "IEEE 802.11aa" by BT Vijay and B. Malarkodi, published at the 2017 8th International Conference on Computer, Communication and Networking Technologies (ICCCNT), Delhi, 2017. The standard addresses methods for transmission of streams (e.g., voice, audio, and video) via multicast communication.

[0006] With this standard, the relationship between multiple access points can be improved by adding new control messages and mechanisms. In addition, wireless network performance can be better evaluated with interference more likely to be avoided through better channel selection. Load sharing can also be achieved between APs with additional techniques for scheduling and adding broadcast transmission opportunities between APs. Using the Groupcast with Retry (GCR) service, a multicast frame is sent several times without waiting for an acknowledgment, which increases the probability of delivery.

[0007] However, the 802.11aa standard is a non-deterministic scheme based on CSMA. The GCR service adds overhead depending on the number of retries, which is inefficient and unnecessary when the link quality is good.

[0008] As another example, the TRM-MAC protocol can be used to manage available slots for the current node density. If the node density is small, the smallest slots can be used. At a large node density, additional slots can be added. The TRM-MAC protocol obtains a higher successful transmission rate with minimal delay by providing a tradeoff between energy and latency. Additional details about the protocol can be found in "TRM-MAC: A TDMA-based reliable multicast MAC protocol for WSNs with flexibility to trade-off between latency and reliability" by Bhatia and Hansdah, published in Computer Networks in 2016. As shown in FIG. 1, the MAC frame is further divided into three parts: Contention Free Period 1 (CFP1), Contention Free Period 2 (CFP2), and Contention Access Period (CAP).

[0009] The channel access mechanism used in CFP1 of the MAC frame is TDMA, which is used by internal nodes to relay multicast data. The slot size in this part is typically 100 ms for transmitting multicast data (current data rate, t data ) The number of slots in the CFP1 portion depends on the number of interior nodes in the Multicast Spanning Tree (MST) and the algorithm used to do TDMA slot scheduling for the interior nodes.

[0010] The channel access mechanism used in CFP2 of the MAC frame is also TDMA and is used by leaf nodes using an ACK-based approach to transmit their acknowledgement (ACK) messages. The slot size in this part is typically 1 / 2 the length of the ACK message (t ack ), which is usually smaller than the slot size in CFP1. The number of slots in this part depends on the number of leaf nodes using the ACK-based approach in MST and the algorithm used to do TDMA slot scheduling.

[0011] The channel access mechanism used in the CAP portion of the MAC frame is (a) j-1-nRouters-nACK)*t cca A prioritized CSMA approach uses a NACK-based approach in which a child node with local id j sends a NACK message in the CAP portion of the MAC frame if it finds that the channel is idle for a period of time defined by t, and (b) if it has not received any data. cca is the time required for a clear channel assessment (CCA) by any sensor node to ensure that no other nodes are already transmitting. nRouters is the number of internal child nodes of its parent. The value of tcca is usually t data ,t ack and t nack This mechanism is called prioritized CSMA because nodes with smaller local ID values ​​have higher priority to access the channel compared to nodes with larger local ID values. The length of this portion is (α-1)*t cca +t nack where α is the maximum number of leaf child nodes using the NACK-based approach at any interior node in the network.

[0012] TRM-MAC may be disadvantageous because it does not provide a method to optimize slot allocation in the CFP. In addition, CCA operation may fail in the presence of hidden nodes. Thus, the proposed prioritized CSMA channel access mechanism does not completely avoid collisions between NACK messages.

[0013] As another example, optimization-based rate allocation and scheduling in TDMA-based wireless mesh networks can be used. This is further described in the article "Optimization based rate allocation and scheduling in TDMA based wireless mesh networks" by Wang, Mutka, and Torng, published in the 2008 IEEE International Conference on Network Protocols. In this article, the authors study optimizing multicast and unicast communications of TDMA protocols, taking into account delay and throughput. To achieve this result, the authors introduce a multi-transmission rate scheduling algorithm based on the construction of a perfect graph. The algorithm first constructs a spanning tree routed at the gateways, and then prunes the tree to accommodate all sessions to achieve a perfect graph.

[0014] Unfortunately, the algorithm uses broadcast slots for its multicast transmissions, which is unreliable, requires centralized control, and can reduce special reuse. In addition, the algorithm is quite complex to implement and requires some computational power. The algorithm may also require larger time slots, which adds latency to an entire slot frame.

[0015] All publications identified in this specification are incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. In the event that the definition and use of a term in an incorporated reference is inconsistent with or contrasts with the definition of the term provided herein, the definition of the term provided herein shall apply and the definition of the term in the reference shall not apply.

[0016] Therefore, there remains a need to reduce end-to-end latency in unicast transmissions. Summary of the Invention

[0017] The present subject matter provides an apparatus, system, and method for reducing end-to-end latency in low power wireless personal area networks (6LoWPANs) using link layer unicast mode, in which the protocol at the link layer (L2) is typically IEEE 802.15.4 and the protocol at the network layer (L3) employs IPv6.

[0018] IEEE 802.15.4 provides a low power, energy saving protocol. The IEEE 802.15.4 protocol has a wide range of applications including, for example, home automation, factory automation, predictive maintenance and live monitoring. IEEE 802.15.4 can also be deterministic when using a Time Division Multiple Access (TDMA) mode, where communication between nodes is divided into time slots and scheduled in superframes (SFs). However, any TDMA-based protocol can be used.

[0019] Although the systems and methods described herein are described in the context of the IEEE 802.15.4 protocol, it is understood that it is contemplated that the systems and methods described herein may be used with later evolved protocols in which relocation of transmissions to subscribers of a multicast group is possible to reduce end-to-end delay.

[0020] Latency in such networks can be reduced by shifting time slots in a superframe when in one of the TDMA modes. Time slot reallocation is preferably based on membership to a multicast group, such that transmissions to each of the group's multicast subscribers occupy adjacent time slots. This can advantageously result in optimized time slot allocation and improved end-to-end latency for multicast communications.

[0021] The subject matter described herein has a variety of applications. In an aircraft environment, for example, on-board sensors may be divided into separate multicast groups. In such an environment, the galleys may form a first multicast group, each seating class may form a separate multicast group, and on-board carts may be formed as separate multicast groups.

[0022] Multicast groups can also be formed based on application, for example oxygen mask release in a passenger service unit can form a separate group.

[0023] Various objects, features, aspects and advantages of the present subject matter will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings, in which like reference numerals represent like components. [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 is a prior art diagram showing the TRM-MAC protocol. [Diagram 2] 1 is a flow chart for a smart broadcast forwarding algorithm. [Diagram 3] 13 is an additional flowchart for the smart broadcast forwarding algorithm. [Figure 4] 1 illustrates dynamic allocation of time slots in a superframe. [Diagram 5] FIG. 1 illustrates shifting of time slots in a superframe. [Figure 6] FIG. 1 illustrates one embodiment of a method for reducing end-to-end latency. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Throughout the following description, reference may be made to an access point, router, server, service, interface, portal, platform, or other system formed from a computing device. It is recognized that use of such terms is intended to represent one or more computing devices having one or more processors configured to execute software instructions stored on a computer-readable tangible non-transitory medium. For example, an access point can be configured to perform pre-programmed roles, responsibilities, or functions.

[0026] The following description provides a number of exemplary embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus, if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, the inventive subject matter is considered to include any other remaining combinations of A, B, C, or D, even if not expressly disclosed.

[0027] 2 and 3 show a flow chart for one embodiment of the smart broadcast algorithm. Such concepts are further described in co-pending patent application PCT / US20 / 44318, filed July 30, 2020. As shown, the algorithm utilizes both link layer unicast and broadcast transmissions, introducing bidirectionality in multicast communication. In this approach, multicast packets can traverse both up and down the RPL tree, which represents a routing protocol for low power networks, without the multicast packets having to traverse all the way to the root.

[0028] An exemplary process for forwarding a packet is shown in Figure 2. As shown, a mote receives a packet whose destination is a multicast group, and it is determined whether the packet was received from a preferred parent.

[0029] If a packet is received from a preferred parent, the multicast group is checked to see if it is listed in the routing table. If so, the packet is forwarded downstream using link-layer unicast or broadcast transmission based on the number of children. Then the node (node ​​and mote are used interchangeably herein to represent devices in a wireless sensor network) is checked to see if it is a member of the multicast group. If the multicast group is not listed in the routing table, the packet is not forwarded and the mote is checked to see if it is a member of the multicast group.

[0030] If the mote is a member of the multicast group, the packet is delivered to the network stack and the packet is accepted, otherwise the packet is dropped.

[0031] If the packet was not received from the preferred parent, then a check is made to see if the packet arrived from below in the RPL tree (i.e., was it received from a link-layer unicast transmission). If so, then a check is made to see if the multicast group is listed in the routing table. If not, the packet is dropped.

[0032] If the multicast group is listed in the routing table, the packet is forwarded downstream using link-layer unicast, except to children (from which the packet was received). Then it is checked whether the mote is an RPL root. If so, it is checked whether the mote is a member of the multicast group. If it is not, the packet is forwarded upstream to the preferred parent.

[0033] If the mote is a member of the multicast group, the packet is delivered to the network stack and the packet is accepted, if not the packet is dropped.

[0034] Since all motes (sensors) in the network are aware of the topology, the multicast source can be anywhere in the network. The processor can decide whether to utilize unicast or broadcast at the link layer based on the number of interested children and the duty cycle ratio.

[0035] To improve the reliability of messages received during broadcast transmission, time synchronous channel hopping (TSCH) can be used to provide broadcast retransmission. As shown in FIG. 3, a packet can be retransmitted a predetermined number of times. This retransmission always occurs because no acknowledgement is received. The time slots in the TSCH can be classified according to function so that there are time slots for sending and receiving broadcast retransmissions. To reduce the receiver's energy needs during these retransmissions, once a packet is successfully received, any remaining time slots where a broadcast retransmission may be received are set to sleep mode. Thus, the receiver mote goes into sleep mode after receiving a packet, and subsequent retransmissions are ignored. Thus, the probability that a packet is received can be increased while reducing the receiver's energy needs.

[0036] In unicast transmissions, as the number of subscribers increases, the number of over-the-air transmissions increases due to the individual transmission of packets to every subscriber and the acknowledgements sent back. Timeslots for multicast communication may span a superframe, increasing the end-to-end latency.

[0037] This is caused, at least in part, by the inherent delays due to the TDMA mode. For example, Figure 4 shows an example timeslot schedule at mote B. As shown in the first superframe (SF1), a timeslot for transmission from mote B to subscriber 1 is followed by a timeslot for reception from subscriber 1, a timeslot for transmission from mote B to its parent mote A, a timeslot for reception from mote A, and a timeslot from mote B to subscriber 2. Assuming that each timeslot has a duration of 10 ms, the transmission to subscriber 2 occurs three timeslots after the transmission to subscriber 1, or 30 ms after the first transmission. These 30 ms add to the end-to-end latency. Thus, as the number of subscribers increases, the end-to-end latency may also increase.

[0038] To reduce such latency, the preferred system and method shifts time slots in a superframe based on membership to a multicast group, thus allowing all multicast subscribers to occupy adjacent time slots, thereby optimizing slot allocation and reducing end-to-end latency for multicast communications.

[0039] An example of this is shown in Figure 5. As shown, time slots in a superframe can be shifted (reallocated) based on the membership of subscribers (motes) to the multicast group, so that transmissions to multicast subscribers occupy adjacent time slots. In this example, motes 1 and 2 are subscribers to the multicast group. Therefore, the transmission in slot 5 is shifted to slot 2 so that it is adjacent to the transmission from mote B. This results in a savings of at least 30 ms in end-to-end latency. Where the number of subscribers is large, such as in a real-world environment like an aircraft, end-to-end latency can be dramatically reduced.

[0040] 6 illustrates an example method 600 for reducing end-to-end latency in link layer unicast transmissions using the IEEE 802.15.4 protocol in accordance with a disclosed embodiment. At step 610, a router having a processor and memory may be provided. In some embodiments, the router is a component of an access point (AP) in the network.

[0041] In step 620, time slots of a time division multiple access (TDMA) superframe may be dynamically allocated by the router for transmissions to and acknowledgments from a set of motes, which may include multiple motes including a first mote, a second mote, and a third mote. Transmissions from mote A to each of the motes may thereby be allocated in separate time slots.

[0042] In step 630, a router or other component may determine whether any of the set of motes are members of the multicast group. If the second mote is a member of the same multicast group as the first mote, the time slots allocated for transmission to the second mote may be shifted or reallocated in step 640 so that the time slots for transmission to the first and second motes are adjacent.

[0043] If it is determined that the third mote is also a member of the same multicast group as the first mote, the time slots assigned for transmission to the third mote can be shifted or reallocated in step 650 so that the time slots for transmission to the second and third motes are adjacent.

[0044] In one aspect, a system for reducing end-to-end latency in link layer unicast transmissions using the IEEE 802.15.4 protocol includes a router having a processor and a control thread stored in a non-transitory computer readable medium. The processor and the control thread can enable a transceiver module at the start of a time slot of a TDMA superframe. Although the IEEE 802.15.4 protocol is described above, any TDMA-based protocol can be used.

[0045] The processor can be configured to dynamically allocate a timeslot for transmission to each of a set of motes. The processor can then determine whether each of the motes in the set is a member of the first multicast group. If a mote in the set is a member of the first multicast group, the router can shift a timeslot for transmission to the mote such that the shifted timeslot is adjacent to another of the timeslots allocated for transmission to subscribers of the first multicast group. This process can continue until all motes in the set that are members of the first multicast group have timeslots allocated for transmission adjacent to another of the timeslots.

[0046] In some contemplated embodiments, the processor can be configured to determine whether each of the set of motes is a member of a second multicast group. The processor can then shift collocated time slots for transmission to each mote that is a member of the second multicast group such that each time slot for transmission to each of the members of the second multicast group is shifted adjacent to another one of the time slots allocated for transmission to members of the second multicast group.

[0047] Preferably, the processor defines a structure of an IEEE 802.15.4 based superframe that conforms to the topology of the wireless sensor network that comprises the set of motes. The systems and methods described herein may further be implemented by a program stored on a non-transitory computer-readable storage medium. A preferred program is configured to perform a set of operations for multicast communication over an IP network when the program is executed by one or more processors of the router.

[0048] The contemplated operation includes dynamically allocating time slots of a TDMA superframe for transmissions from a first mote to a second mote and from the first mote to a third mote, where each transmission is allocated to a separate time slot, for example, a transmission from a first mote to a second mote may be allocated to a first time slot.

[0049] Another operation can be to determine if a third mote is a member of the same multicast group as the second mote, and if so, the time slot allocated for transmission to the third mote can be relocated or shifted based on the third mote's membership in the multicast group such that the reallocated second time slot is adjacent to the first time slot.

[0050] Similarly, the third time slot of the TDMA superframe can be dynamically allocated for transmission from the first mote to the fourth mote. Another operation can be to determine if the fourth mote is a member of the same multicast group as the second mote, and if so, the time slot allocated for transmission to the fourth mote can be relocated or shifted based on the fourth mote's membership in the multicast group such that the reallocated third time slot is adjacent to the second time slot.

[0051] Preferably, the motes communicate wirelessly over a low-power wireless personal area network using the IEEE 802.15.4 protocol. While a central entity can distribute a fixed schedule (i.e., a list of time slots in a superframe) to each node in the network to improve determinism, the inventive concepts described herein address time slot allocation in an environment where time slots are dynamically allocated by nodes based on their distance from each other. In such an environment, time slots may be allocated randomly and may not be optimized for end-to-end communication, and more particularly for multicast communication where nodes use 802.15.4 unicast transmission to send multicast packets.

[0052] As used herein, unless the context dictates otherwise, the term "coupled to" is intended to include both direct coupling (the two elements that are coupled to each other are in contact with each other) and indirect coupling (one or more additional elements are positioned between the two elements). Thus, the terms "coupled to" and "coupled with" are used synonymously.

[0053] In some embodiments, the numbers expressing properties such as amounts of ingredients, concentrations, reaction conditions, and the like, used to describe and claim certain embodiments of the present invention are understood to be modified in some cases by the term "about". Thus, in some embodiments, the numerical parameters set forth in the specification and appended claims are approximations that may vary depending on the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters are interpreted in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the present invention are approximations, the numerical values ​​set forth in the specific examples are reported as precisely as practicable. The numerical values ​​presented in some embodiments of the present invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0054] Unless the context dictates to the contrary, all ranges set forth herein are to be understood to be inclusive of their endpoints, and open-ended ranges are to be understood to include only commercially feasible values. Similarly, all lists of values ​​are to be considered to include intermediate values, unless the context dictates to the contrary.

[0055] As used in the description and throughout the claims that follow, "a," "an," and "the" include plural references unless the context clearly indicates otherwise. Also, as used in the description, "in" includes "in" and "on," unless the context clearly indicates otherwise.

[0056] The recitation of ranges of values ​​herein is merely intended to serve as a shorthand method of individually referring to each separate value falling within the range. Unless otherwise indicated herein, each individual value with a range is incorporated herein as if it were individually set forth herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or made clear by context. Any and all examples provided for specific embodiments herein, or the use of exemplary language (e.g., "etc.") are intended only to better illustrate the invention, and are not intended to limit the scope of the invention as otherwise claimed. No language in this specification should be construed as indicating any non-claimed element necessary to the practice of the invention.

[0057] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limiting. Each group member may be referenced and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group may be included in or deleted from a group for reasons of convenience and / or patentability. When such inclusion or deletion occurs, the specification is deemed to include the modified group and thus fulfills all Markush-style group descriptions used in the appended claims.

[0058] It will be apparent to those skilled in the art that many more modifications beyond those already described are possible without departing from the inventive concepts herein. Accordingly, the inventive subject matter is not to be limited except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms shall be interpreted in the broadest possible manner consistent with the context. In particular, the terms "comprises" and "comprising" shall be construed to refer to elements, components, or steps in a non-exclusive manner, indicating that the referenced element, component, or step may be present, utilized, or combined with other elements, components, or steps not expressly referenced. When a claim of the specification refers to one or more selected from the group consisting of A, B, C.... and N, the sentence shall be construed to require only one element from the group, and not A and N, or B and N, etc.

Claims

1. 1. A method for reducing end-to-end latency using link layer unicast transmission in a network, comprising: providing a router having a processor and a memory, the role of the router being capable of being assumed by any mote that is part of a network topology; dynamically allocating a first time slot, a second time slot, and a third time slot of a time division multiple access (TDMA) superframe for transmission from a mote A to a set of motes, the set of motes comprising a first mote, a second mote, and a third mote, such that transmissions to each of the motes are allocated to separate time slots; the first mote, the second mote, and the third mote are each members of a first multicast group; mote A is allocated the first time slot for transmission to the first mote; the router reallocating the second time slot for transmission from mote A to the second mote based on the second mote's membership in the first multicast group, the reallocated second time slot being shifted so that the second time slot is adjacent to the first time slot; and the router reallocating the third time slot for transmission from mote A to the third mote based on the third mote's membership in the first multicast group, the reallocated third time slot being shifted so that the third time slot is adjacent to the second time slot.

2. 10. The method of claim 1, wherein the routers communicate wirelessly using a TDMA (Time Division Multiple Access) based protocol.

3. 2. The method of claim 1, wherein the router dynamically allocates the first time slot, the second time slot, and the third time slot, and reallocates the second time slot and the third time slot.

4. The method of claim 1 , wherein the router communicates wirelessly over a low power wireless personal area network.

5. receiving a packet at the router; determining, with the processor, if the packet has a multicast group address that is a preferred parent; If the source is the preferred parent, the router checks whether the multicast group address is listed in the router's routing table; dropping the packet if the source is not the preferred parent; the router forwarding the packet downstream using link-layer unicast or broadcast transmission based on the number of children only if the multicast group address is listed in the routing table; after determining whether the multicast group address is listed in the routing table, the router checks whether the first mote is a member of the first multicast group; 2. The method of claim 1, further comprising: if the router determines that the first mote is the member of the first multicast group, delivering the packet upstream.

6. if the source is not the preferred parent, the router checks whether the packet was received from below; if the router determines that the packet was received from below, the router determines whether the multicast group address is listed in the routing table; if the router determines that the packet was not received from below, dropping the packet; 6. The method of claim 5, further comprising the step of: if the multicast group address is listed in the routing table, the router forwards the packet downstream using link-layer unicast transmission, but not forwarding the packet to a child from which the packet was received.

7. A router, A processor; a control thread stored in a non-transitory computer readable medium; the processor dynamically allocates time slots of a time division multiple access (TDMA) superframe for transmission from mote A to each of a set of motes for link layer unicast transmissions using a first protocol, such that transmissions to each of the motes are allocated to separate time slots; The processor determines whether each of the set of motes is a member of a first multicast group; a router that shifts one or more of the allocated time slots for transmission to each of the set of motes that are members of the first multicast group such that each allocated time slot for transmission to one of the members of the first multicast group is adjacent to another of the allocated time slots.

8. The processor determines whether each of the set of motes is a member of a second multicast group; 8. The router of claim 7, wherein the router shifts the allocated time slots for transmission to each of the set of motes that are members of the second multicast group such that each time slot for transmission to each of the members of the second multicast group is shifted to be adjacent to another one of the time slots allocated for transmission to members of the second multicast group.

9. The router of claim 7 , wherein the first protocol comprises the IEEE 802.15.4 protocol.

10. 8. The router of claim 7, wherein said processor defines a structure of said TDMA superframe based on IEEE 802.15.4 according to a topology of a wireless sensor network comprising said set of motes.

11. 8. The router of claim 7, wherein the router communicates wirelessly over a low power wireless personal area network.

12. A program stored on a non-transitory computer-readable storage medium, which when executed by one or more processors of a router, for multicast communication over an IP network, comprises: dynamically allocating first and second time slots of a time division multiple access (TDMA) superframe for transmissions from a first mote to a second mote and from the first mote to a third mote, respectively, each transmission being allocated to a separate time slot; a transmission from the first mote to the second mote is allocated to the first time slot, the second mote being a member of a first multicast group; a transmission from the first mote to the third mote is allocated to the second time slot; determining if the third mote is a member of the first multicast group; and if the third mote is a member of the first multicast group, reallocating the second time slot allocated for transmission to the third mote based on the third mote's membership in the first multicast group, wherein the reallocated second time slot is shifted so that the second time slot is adjacent to the first time slot.

13. The operation includes: dynamically allocating a third time slot of a TDMA superframe for transmission from the first mote to a fourth mote; determining if the fourth mote is a member of the first multicast group; 13. The program of claim 12, further comprising: if the fourth mote is a member of the first multicast group, reallocating the third time slot based on the fourth mote's membership in the first multicast group, wherein the reallocated third time slot is shifted so that the third time slot is adjacent to the second time slot.

14. The motes communicate with each other using the Institute of Electrical and Electronics Engineers (IEEE) 802 13. The program of claim 12, wherein the motes communicate wirelessly using a .15.4 protocol, the motes communicate wirelessly over a low power wireless personal area network, or both.

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