An Adaptive MAC Protocol for Efficient Group Communications in Sensor Networks

An Adaptive MAC Protocol for Efficient Group Communications in Sensor Networks Turkmen Canli, Zhihui Chen, Ashfaq Khokhar University of Illinois at Chicago Ajay Gupta Western Michigan University Abstract-This paper proposes a novel adaptive energy efficient MAC protocol for wireless sensor networks, referred to as TDMA-WA. The proposed protocol is contention free and is based on the well known timedivision multiple access (TDMA) principle. However, the frame length is variable corresponding to the degree of 2- hop neighborhood for each node. This allows the frame length to adapt to the varying density of the network. Unlike most of the existing protocols that are based on the CSMA principle, the proposed protocol guarantees bounded end-to-end delay. Furthermore, unlike other TDMA based protocols, TDMA-WA does not require separate channel for signaling or slot reservation. The protocol is designed to facilitate efficient realization of group communication primitives that frequently arise in collaborative computations over sensor networks. The performance results of realizing group communication based on this protocol are presented and compared with adaptive S-MAC. Our proposed protocol provides significantly superior performance in terms of communication time and energy usage. For example, we show improvement in routing time between 25% to 75% over 50%-SMAC for routing linear permutations. Keywords: Sensor Network, Group Communications, MAC protocol, TDMA. I. Introduction Currently wireless sensors networks are gaining significant research interest due to their pervasive use in defense, commercial, industrial, and scientific applications [1,8,15-18]. Wireless sensor networks are composed of tiny smart sensors with limited energy and computation resources and are connected via wireless channels. Some of the example of smart sensors include: Berkeley s SmartDust [11] UCLA s WINS [12] and Rockwell s WINS and HiDRA [13]. Wireless sensor networks have some distinguished features that are quite different from existing wireless ad hoc networks. These features include low traffic rate, restricted memory, limited computing ability and extremely expensive battery power. Sensor nodes also coordinate with each other to implement a certain function, so traffic is not randomly generated as in mobile ad hoc networks. Due to the above mentioned characteristics, existing wireless communication protocols such as IEEE 802.11 [9] are not suitable for sensor networks. New protocols need to be designed to fit the special requirements of sensor networks. Since in sensor networks, traffic rate is low and power resource is extremely expensive, the main objective to be optimized is network lifetime instead of the channel utilization. Recently, several energy-efficient MAC protocols for wireless sensor networks have been proposed in the literature [2,3,5,7,9,14]. One common observation in all these protocols is that the main source of power consumption is idle listening, therefore most of these protocols try to let nodes turn off their RF circuit to save energy when nodes are not involved in transmission/receiving mode. (In [4] it is shown that idle listening accounts for more than 90% power consumption.) For example in S-MAC [9] all nodes listen and sleep periodically. Traffic is sent out to a destination only during its listening period. This scheme lets sparse traffic concentrate into a specific time-period so that in other times nodes can go to sleep and save energy by shutting down the radio. Most of the existing protocols use either contention based CSMA principle or employ separate radio channels for scheduling and reservation. We believe that such protocols are not well suited for group communications that frequently arise in distributed applications. Particularly contention based protocols do not guarantee delivery of message within a bounded time. This paper proposes a novel adaptive energy efficient MAC protocol for wireless sensor networks, referred to as TDMA-WA. The proposed protocol is contention free and is based on the well known timedivision multiple access (TDMA) principle. However, the frame length is variable corresponding to the degree of two-hop neighborhood for each node. This allows the frame length to adapt to the varying density of the network thereby efficiently utilizing the time slots within a frame. The sensor nodes agree upon a conflict free slot assignment among its twohop neighborhood nodes during a self organizing

procedure. The protocol is designed to facilitate efficient realization of group communication primitives. The performance results of realizing group communication based on this protocol are presented and compared with adaptive S-MAC. The proposed work is significantly different from our earlier work in [14] where we presented a contentionfree, TDMA based energy efficient MAC protocol called TDMA-W. In [14], we assumed that sensors in the network are uniformly distributed, so every node has similar degree of neighborhood, thereby using a fixed size frame whose length is proportional to the number of nodes in the network. This not only resulted in a large frame length but also introduced a larger average end-to-end delay, compared to S-MAC [2] and TRAMA [4] protocols. While the protocol is [14] was significantly superior in terms of energy usage, for group communication primitives such as permutations, scatter, gather, and all-to-all, it also introduced long delays. In this paper, we present an improved TDMA-W protocol (referred to as TDMA-WA) in which frame length is adapted to the varying density of the network. Since the proposed protocol adapts the frame length corresponding to the neighborhood density and uses wake-up slots in a TDMA framework we refer to it as TDMA-WA, where WA implies to the wake-up and adaptive concepts used. This novel protocol allows sensor nodes to choose among a set of fixed variable length frames resulting in significant reduction in transmission delays (as shown in Figure 1). The basic idea of the TDMA-WA is that time is divided into unit frames and each frame is divided into time slots. A node is assigned two slots in each frame. One is the Transmission or Send slot (T-SLOT) and the other is the Wakeup slot (W- SLOT). A node always listens to the channel during its W-SLOT and transmits during its T-SLOT. Only neighbors that are in the destination list of the outgoing traffic need to listen to that slot. Other neighbors can shut down their RF circuits to save energy. To activate a node, the source needs to send a Wakeup frame to the destination in its associated W- SLOT. After receiving the wakeup frame, the destination identifies the sender node and starts listening to the channel during the T-SLOT associated with the sender node. Our proposed protocol provides significantly superior performance in terms of communication time and energy usage. For example, we show improvement in routing time between 25% to 75% over 50%-SMAC for routing linear permutations. The rest of the paper is organized as follows: Section II describes the self-organization procedure and proposed TDMA-WA medium access protocol. Simulation results are given in Section III. Section IV concludes the paper. II. Self-Organization and TDMA-WA MAC Protocol In order to simplify communication analysis a simple single channel model is assumed. We assume that all nodes share a single wireless channel and both the data and control frames are sent and received through this channel. All the nodes are equipped with identical radios therefore have same communication ranges. All the nodes within the communication range of a node can receive and decode the frame without any error (i.e. channel is always good), provided there is only one frame being communicated at any point in time. The wireless channel is organized as TDMA frames. Instead of applying uniform fixed length frame, varying length frames are used for different nodes depending on the density of the network in their neighborhood. These lengths are integer multiples of the shortest frame, set apriori at the time of the network deployment. Furthermore the length of the shortest frame corresponds to the minimum two-hop degree of the network. Two-hop degree of a node V is defined as the number of nodes within two hops distance of V. The motivation for choosing different length frames is to make the channel access delay at each hop only proportional to the neighborhood density. In order to clarify the presentation, in this paper we use two types of frame lengths (short and long), thus assuming that there are only two types of neighborhood present in the network: sparse and dense neighborhoods. The protocol can be easily generalized to finer granularity of density. Figure 1 shows the structure of the short and long frames. In order to avoid collisions and conflicts in the transmission slots, the length of the long frame is twice that of the short frame. In a generalized case, the frame lengths must be integer multiples of the shortest frame length.

At the deployment stage, all nodes use the shortest frame length and randomly select a T-SLOT. Nodes then broadcast their T-SLOT selection and listen to their neighbors. If a node finds that there are too many nodes in its two-hop neighborhood, i.e. the number of nodes in the two-hop neighborhood is greater than the available number of T-SLOTS by a given threshold, it changes its frame length to be long. In network with non-uniform density, nodes using short frames and having a neighbor with long frames always occupy two transmission slots in the longer frame. In this way, nodes can adapt the frame length to their two-hop neighborhood density and avoid unnecessary delay introduced by long frame duration, especially for multi-hop traffic. W-SLOTS W-SLOTS Short Frame T-SLOTS T-SLOTS Long Frame W-SLOTS T-SLOTS Figure 1: Short Frame and Long Frames in TDMA-WA. W-SLOT is used for wake up packets and T-SLOT is used for transmission of data. SELF-ORGANIZATION ALGORITHM: The proposed self-organization procedure that determines the slot assignments is described below. 1. Each node starts with a short frame and randomly selects a slot with uniform probability among the available T-SLOTS as its transmission slot. 2. While not transmitting, a node turns to the listening mode and receives the slot selection information from the neighbors. Node should record all neighbor information of two-hop neighbors and their transmission slots. A collision 1 is defined when two nodes select the same T-SLOT. 3. If a node finds that the total number of its two-hop neighbors is more than the total number of T-SLOTS available, or too many collisions occur, the node switches to the next longer frame length. After switching to long frame, the node updates its record by marking the T-SLOTs in the second half as occupied for its neighbors who are still using short frame. 4. During its T-SLOT, a node sends its node ID, its T-SLOT number, its one-hop neighbors, and their T-SLOT numbers to all its neighbors. Slots numbers in which collisions are detected are also sent. 5. Nodes involved in a collision need to randomly reselect a slot as its new T-SLOT. Goto step 4. 6. Repeat steps 4 and 5 for a fixed number of iterations. If no new nodes join, no transmission slot changes occur, or no collisions are detected for a fixed period, it implies that all neighboring nodes have been identified and every node in a two hop neighborhood has a distinct T-SLOT assigned. 7. After T-SLOT assignment, each node broadcasts its two-hop neighbor information and finds out an unused slot in the frame and uses it as its W-SLOT. W-SLOT assignment need not be unique. 8. Each node broadcast its W-SLOT and the self-organization is complete. The self-organization protocol uses the same scheme as described in [14] to recover from one-hop collisions or slot assignment deadlock. Note that the slot assignment problem is very similar to the graph coloring problem in a two-hop neighbor hood. Since we assume more number of slots than the two-hop degree of the nodes, a unique coloring assignment can always be found and self stabilizing arguments can be used to prove the convergence and correctness. TDMA-WA PROTOCOL: After the network is successfully set up, the TDMA- WA communication protocol can be described as follows: 1 Generally, a collision is detected in wireless channels when there is chatter in the channel.

1. Each node maintains a pair of counters, outgoing counter and incoming counter, for every neighboring node. 2. If no outgoing data is sent to a neighbor in a round, the corresponding outgoing counter is decreased by one; otherwise it is reset to its initial value. 3. If no incoming data is received from a neighbor, the corresponding incoming counter is decreased by one. If the counter is less than or equal to zero, the RF circuit is shut down in that slot. 4. If an outgoing data transmission request arrives, the node first checks the outgoing counter, if the counter is greater than zero, then the link is active and the frame can be sent out directly. If the counter is less than or equal to zero, a wakeup frame has to be sent out prior to the data transmission. If there are two w-slots for the destination node in the long frame, the first available W-SLOT is used to send out the wake up message. 5. In the W-SLOT if a node receives a wakeup packet, it turns itself ON during the T- SLOT corresponding to the source ID contained in the packet. If a collision is detected during the W-SLOT, it means more than one node wants to send data, so the node listens to all the T-SLOTs associated with its immediate neighbors. The wakeup packets only contain the source and the destination information. The data packet can only contain the destination information and omit source ID since the source ID is determined by the transmission slot. For group communication patterns such as All-Broadcast data frame should set its destination field to be a special broadcast address. Also before sending out the broadcast data packet, the node should wakeup all the neighbors that intend to receive this frame. In the case of multiple users sharing the same wakeup slot, the destination field of the wakeup message should also be set to broadcast. Since the proposed protocol adapts the frame length corresponding to the neighborhood density and uses wake-up slots in a TDMA framework we refer to it as TDMA-WA, where WA implies to the wake-up and adaptive concepts used. III. Performance Results To verify the performance, we have simulated the proposed protocol using MATLAB communication toolbox. Networks consisting of 50, 100 and 200 nodes are deployed randomly in a 500 feet by 500 feet square area. The communication range is assumed to be 100 feet for all nodes. We assume a basic IEEE 802.11 data rate of 1M bps for all transmissions. The slot length is set to be 4 milliseconds, which is long enough for transmitting a 512-byte packet. Therefore, for a TDMA-WA frame of one second, there are 250 slots in one frame. The results are reported by averaging data from 10 different deployments and 10 runs on each deployment, thus all together 100 experiments for each data point. Figure 2 and table 1 show a sample deployment of 100 sensor nodes in a 500x500 sq. ft area and average one-hop and two-hop neighborhood degrees. Distance in feet 500 450 400 350 300 250 200 150 100 50 0 0 50 100 150 200 250 300 350 400 450 500 Distance in feet Figure 2: Sample deployment of 100 nodes in a square area of 500 x 500 sq. ft. Number of Nodes Average Number of One-hop neighbors Average number of Two-hop neighbors 50 5.12 10.84 100 10.41 26.13 200 20.87 58.47 Table 1: Sample one hop and two-hop neighborhood degrees in different size network deployments. We evaluate the proposed TDMA-WA protocol with three different protocols, including:

50%-S-MAC (the cycle consists of 50% sleep and 50% wake-up durations), and TDMA-W [14] with frame length proportional to the entire network size. As shown in Table 2, we compare the execution time of the self-organization procedures of the proposed TDMA-WA protocols with the TDMA-W of [14]. Network Size Proposed TDMA-WA Original TDMA-W [14] 50 nodes 5.80 4.12 100 nodes 5.51 3.80 200 nodes 5.46 3.39 Table 2: Self-Organization Time (in seconds). As expected, the adaptive TDMA-WA is taking longer time to converge due to smaller and variable frame lengths. For example 24 and 48 slot-frames are used in the TDMA-WA whereas 256 slot-frame is used in [14] for a network of 200 nodes. However, the execution time is relatively independent of the network size in all the protocols. Nonetheless, for a smaller network size, the network is sparse because deployment is in a fixed size area resulting in a smaller frame length and thus creating more conflicts. More conflicts imply more number of rounds to converge. Note that self-organization needs to be performed only once after the deployment or when there is a significant change in the network topology. While the self organization is taking relatively longer in the adaptive TDMA-WA protocol, we expect significant gains in the overall routing times for group communication operations due to varying length frames along a route and thus decreasing transmission time. We show performance results of realizing a one-toone routing operation between two arbitrary neighbors. We randomly select two nodes to communicate with each other and determine the shortest path between the two in terms of number of hops by diffusion [9] techniques. We compare the results with 50%-SMACPerformance results using one-to-one routing capture the ability of a protocol to route messages without any congestion in the network. Therefore it captures pure routing delay inherent in the protocol. Table 3 shows the execution times (in msec) of routing one-to-one communication primitive using different protocols on different size networks. In this table, comparing entries within the column makes more sense as the sources and destinations differ from one network size to the next and they are randomly chosen. For one-to-one communication, as expected the SMAC protocol is superior to the TDMA-W protocols in terms of total delay. This is primarily due to the fact that in TDMA based protocols, a message travel only one hop in one frame. Whereas in S-MAC it can travel multiple hops in the absence of channel collisions, which is the case when we route one-to-one communication. However, the energy consumption of the TDMA-W protocol for one-to-one routing is on the average 6 times lower than that of SMAC. However, the TDMA-WA significantly improves the routing time when compared with the original TDMA-W [14] protocol. This is primarily due to the small and adaptive frame lengths used in the protocol. Network Size (Nodes) S-MAC Proposed TDMA-WA Original TDMA-W [14] 50 10.36 118 210 100 8.54 210 326 200 4 114 185 Table 3: Performance of different protocols (in terms of execution time in msec) for routing one-to-one communication in different size networks. Figures 3 to 5 show the performance results of realizing linear permutations (node I sends to node I + k for some fixed k for all values of I). Such permutations routinely occur in signal/image processing applications such as FFT [19]. For routing a permutation each node determines shortest path to its destination and all the nodes route the data simultaneously along the shortest routes to their destinations. This operation also captures the performance of protocols in the presence of congestion. In order to show the raw performance of the protocols we are not employing any optimization (e.g. aggregation or coalescing along common routes) in the group communications routings. The figures show the routing time experienced by each node of the network and we have also plotted the average time (straight lines) for a permutation on each network. The gain in routing time experienced by the proposed TDMA-WA over 50%-SMAC [2] ranges from 16% to 75%, while the gains compared to the original TDMA-W [14] range from 25% to 50%, depending on the network size. The performance of the S-MAC protocol improves with the size of the network as the duty cycle gets more efficiently utilized.. We expect significantly better performance of the proposed TDMA-WA protocol for lower duty cycle SMAC

realizations, such as 10% S-MAC. This is due to the fact that with lower duty cycle, nodes sleep more often and thus will incur more number of cycles to realize a routing. In the full version of the paper, we will present those results in detail. We have already shown in [14] that the TDMA-W protocol consumes only 1.5% to 15% of the energy consumed by 10%-SMAC and 4 to 9 times less energy than the adaptive S-MAC depending on the traffic rate, yielding 6 to 67 folds enhancement in battery life. In terms of the energy usage, both TDMA-W and TDMA-WA give similar performance results. IV. Conclusions Figure 3: Performance of different protocols for routing linear permutations in a 50 node sensor network. Figure 4: Performance of different protocols for routing linear permutations in a 100 node sensor network. The design of TDMA-WA is motivated by the desire to efficiently realize group communications. Programming and networking tools for sensor networks must efficiently support group communications to facilitate realization of collaborative applications. This would allow denser and deeper deployments of smaller and cheap smart sensors. In this paper, we proposed a novel self-organizing and adaptive MAC protocol called TDMA-WA for efficient group communications over sensor networks. We verified our protocol with extensive simulations and found that the proposed protocol provides significantly superior performance in terms of communication time and energy usage. For example, we showed improvement in routing time between 25% to 75% over 50%-SMAC for routing linear permutations. Furthermore, the TDMA-WA protocol guarantees bounded delay for group communications such as broadcast, whereas collision-based protocols can incur infinite delay in the presence of network traffic. Also, as shown in Table 3, protocols that exhibit superior performance for lighter traffic patterns may not necessarily perform better in the presence of heavy network traffic loads. Our future work involves comparing the proposed protocol with other well known MAC protocols such as TRAMA and adaptive S-MAC and incorporating routing optimization techniques. References [1] Akyildiz, W. Su, Y. Sankarasubramaniam and E. Cayirci, A Survey on Sensor Networks, IEEE Communication Magazine, Aug. 2002 p.p. 102-114 Figure 5: Performance of different protocols for routing linear permutations in a 200 node sensor network. [2] T. Dam and K. Langendoen An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor

Networks Proceedings of the first international conference on Embedded networked sensor systems, Nov. 2003, p.p. 171-180 [3] V. Rajendran, K. Obraczka and J.J. Garcia-Luna- Aceves Energy-Efficeint, Collision-Free Medium Access Control for Wireless Sensor Networks Proceedings of the first international conference on Embedded networked sensor systems, Nov. 2003, p.p. 181-192 [4] J. Reason and J. M. Rabaey A Study of Energy Consumption and Reliability in a Multi-Hop Sensor Network ACM SIGMOBILE Mobile Computing and Communications Review, volume 8 Issue 1, Jan. 2004, p.p. 84-97 [5] S., H. Hassanein and H. Mouftah A MAC- Based Performance Study of Energy-Aware Routing Schemes in Wireless Ad hoc Networks GLOBECOM 02, Nov. 2002, p.p. 47-51 [6] S., H. Hassanein and H. Mouftah Optimal Cross- Layer Designs for Energy-Efficient Wireless Ad hoc and Sensor Networks Performance, Computing, and Communications Conference, April 2003, p.p. 123-128 [7] K. Sohrabi, J. Gao, V. Ailawadhi and G. J. Pottie, Protocols for Self-Organization of a Wireless Sensor Network, IEEE Personal Communications volume 7, issue 5, Oct. 2000, p.p. 16-27 [13] http://wins.rsc.rockwell.com/ [14] Zhihui Chen and Ashfaq Khokhar, Self Organization and Energy Efficient TDMA MAC Protocol by Wake Up For Wireless Sensor Networks, first international conference on Sensor and Ad Hoc Communications and Networks (SECON 04), Oct. 2004. [15] Special Issue on Wireless Sensor Networks, CACM, Vol. 47, No. 6, June 2004. [16] A. Mainwaring, R. Szewczyk, D. Culler, J. Anderson "Wireless Sensor Networks for Habitat Monitoring" ACM International Workshop on Wireless Sensor Networks and Applications (WSNA), 2002. [17] D. Estrin, David Culler, and Kris Pister, "Connecting the Physical World with Pervasive Networks," IEEE Pervasive Computing, 1,1 (Jan.- March 2002). [18] IEEE Computer, Special Issue on Wireless Sensor Networks, Vol. 37, No. 8, 2004. [19] T. Canli, M. Terwilliger, A. Gupta, A. Khokhar, Power-Time Efficient Algorithm for Computing FFT in Sensor Networks, The ACM Conference on Embedded Networked Sensor Systems (SenSys), November 2004. (Poster Paper). [8] N. Xu A Survey of Sensor Network Applications http://enl.usc.edu/~ningxu/papers/survey.pdf [9] W. Ye, J. Heidemann and D. Estrin An Energy- Efficient MAC Protrocol for Wireless Sensor Networks, INFOCOM 2002, June 2002, Vol. 2, p.p. 1567-1576 [10] IEEE802.11 Wireless LAN Medium Access Control (MAC) and Physical (PHY) layer specifications, 1999 [11] http://wwwbsac.eecs.berkeley.edu/archive/users/warnekebrett/pubs/hotchips12/tsld001.htm [12] http://www.janet.ucla.edu/wins/