Performance Analysis of Slotted Carrier Sense IEEE 802.15.4 Acknowledged Uplink Transmissions Sofie Pollin 1,2 , Mustafa Ergen 2 , Sinem Coleri Ergen 2 , Bruno Bougard 1 , Francky Catthoor 1,3 , Ahmad Bahai 2 , Pravin Varaiya 2 1 Interuniversity Micro-Electronics Center (IMEC); E-mail : pollins@imec.be 2 University of Califiornia Berkeley; 3 Katholieke Universiteit Leuven; Abstract—Advances in low-power and low-cost sensor net- works have led to solutions mature enough for use in a broad range of applications, requiring various degrees of reliability. To facilitate this, a broad range of options are possible to tune reliability, throughput or energy cost in the IEEE 802.15.4 standard defining the medium access control (MAC) and physical layer for sensor networks. Knowing how to tune those knobs however requires detailed models of the protocol behavior under different conditions. In our earlier work, we have proposed a very accurate model for the slotted Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) access scheme of the IEEE 802.15.4 standard for the unacknowledged transmission mode. Because of the design of the 802.15.4 carrier sensing mechanism, modeling the performance of the network in case of acknowl- edged transmissions is not a trivial extension. In this paper, we hence derive such model and illustrate through simulations that it is extremely accurate. Next, using the model, guidelines are derived to optimize the energy or throughput performance of sensor networks using the IEEE 802.15.4 standard. I. I NTRODUCTION Wireless sensor networks are autonomous networks for monitoring purpose. Monitoring tasks vary however signif- icantly in terms of delay and reliability requirements, e.g. monitoring the temperature in a building for data collection is less critical then monitoring the temperature for fire warning. To address these requirements, the IEEE 802.15.4 standard specifying a medium access control (MAC) and physical (PHY) layer, has been developed [1], [2]. Despite the huge variety of wireless sensor applications, all sensor networks are severely constrained in terms of power consumption. As a result, many of the design choices for 802.15.4 networks focus on reducing the energy consumption, while allowing larger delays or less reliability when that is acceptable. E.g., as illustrated in Fig. 1, the use of the acknowledgement for uplink transmissions is optional, which means that nodes can switch to the low-power sleep mode immediately after transmitting the packet. The standard supports three networking topologies relevant to sensor networking applications: star, peer-to-peer and cluster-tree. Since most sensor network appliations involve monitoring tasks and reporting towards a central sink, and since the focus of this paper is on the 802.15.4. medium access control analysis, we focus on a one-hop star network as illustrated in Fig. 1. In this paper, we propose a model that accurately captures the behavior of the network with and without acknowledged traffic and hence allows to reliably predict the performance and energy consumption of an 802.15.4 network. We focus on Data PAN coordinator Full Function Device Reduced Function Device Acknowledgment Coordinator Network Device Beacon Downlink Uplink Data Request Acknowledgment Data Acknowledgment Coordinator Network Device Beacon (Optional) Fig. 1. Transmission schemes in a one-hop star 802.15.4 network. In the case of uplink transmission (monitoring scenario), the use of the acknowl- edgement is optional. In this paper, we model the network with and without acknowledgement to be able to address the performance of both possibilities. the uplink scenario, since that is most relevant for monitoring in sensor networks. Also, we assume the network operates in the Beacon-enable slotted carrier sense multiple access mecha- nism with collision avoidance (CSMA/CA), which means that the slot boundaries of each device are aligned with the slot boundaries of the coordinator. This mode is indeed the most energy-efficient since it allows for nodes to sleep in between beacons. While contending for the channel, nodes delay their carrier sensing by a random backoff delay. Only after that random delay, the contending node wakes-up to listen to the channel during maximally two backoff slots. As a result, the power consumption during channel listening is minimized. However, due to this sleeping, nodes can potentialy wake up and sense the channel idle in between the data transmission and its acknowledgement. It is exactly the interplay between the idle slot in between data and acknowledgement and the two-slot sensing scheme that will make the performance modeling non-trivial. The performance of the IEEE 802.15.4 protocol has been evaluated by simulation for small and low load networks in [3] and for dense networks in [4]. In contrast, in [5], we propose an analytical Markov model that predicts the performance and detailed behavior of the 802.15.4 slotted CSMA/CA mechanism with unacknowledged uplink traffic very accurately. The form of the analysis is similar to that of Bianchi for IEEE 802.11 DCF [6] only in the use of a per user Markov model to capture the state of each user at each moment in time. The assumptions to enable this important simplification and the coupling of the per user models are however different, as a result of the very different design of the 802.11 carrier sensing mechanism where nodes monitor the channel continously and are hence continously aware of the 1525-3511/08/$25.00 ©2008 IEEE This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2008 proceedings. 1559 Authorized licensed use limited to: Univ of Calif Berkeley. Downloaded on March 11, 2009 at 19:50 from IEEE Xplore. Restrictions apply.