USC/ISI TECHNICAL REPORT ISI-TR-567, JANUARY 2003 1Medium Access Control with Coordinated,Adaptive Sleeping for Wireless Sensor NetworksWei Ye, John Heidemann, Deborah EstrinAbstract—This paper proposes S-MAC, a medium-access control (MAC)protocol designed for wireless sensor networks. Wireless sensor networksuse battery-operated computing and sensing devices. A network of thesedevices will collaborate for a common application such as environmentalmonitoring. We expect sensor networks to be deployed in an ad hoc fash-ion, with individual nodes remaining largely inactive for long periods oftime, but then becoming suddenly active when something is detected. Thesecharacteristics of sensor networks and applications motivate a MAC that isdifferent from traditional wireless MACs such as IEEE 802.11 in severalways: energy conservation and self-configuration are primary goals, whileper-node fairness and latency are less important. S-MAC uses three noveltechniques to reduce energy consumption and support self-configuration.To reduce energy consumption in listening to an idle channel, nodes period-ically sleep. Neighboring nodes form virtual clusters to auto-synchronizeon sleep schedules. Inspired by PAMAS, S-MAC also sets the radio tosleep during transmissions of other nodes. Unlike PAMAS, it only usesin-channel signaling. Finally, S-MAC applies message passing to reducecontention latency for sensor-network applications that require store-and-forward processing as data move through the network. We evaluate ourimplementation of S-MAC over a sample sensor node, the UCB Mote. Theexperimental results show that, on a source node, an 802.11-like MAC con-sumes 2–6 times more energy than S-MAC for traffic load with messagessent every 1–10s.Keywords—Medium access control, Sensor network, Wireless network,Energy efficiencyI. INTRODUCTIONWIRELESS sensor networking is an emerging technologythat has a wide range of potential applications includ-ing environment monitoring, smart spaces, medical systems androbotic exploration. Such networks will consist of large num-bers of distributed nodes that organize themselves into a multi-hop wireless network. Each node has one or more sensors, em-bedded processors and low-power radios, and is normally bat-tery operated. Typically, these nodes coordinate to perform acommon task.Like in all shared-medium networks, medium access control(MAC) is an important technique that enables the successful op-eration of the network. One fundamental task of the MAC pro-tocol is to avoid collisions so that two interfering nodes do nottransmit at the same time. There are many MAC protocols thathave been developed for wireless voice and data communicationnetworks. Typical examples include the time division multipleaccess (TDMA), code division multiple access (CDMA), andcontention-based protocols like IEEE 802.11 [1].This work is in part supported by NSF under grant ANI-0220026 as theMACSS project and under grant ANI-9979457 as the SCOWR project, and byDARPA under grant DABT63-99-1-0011 as the SCADDS project and undercontract N66001-00-C-8066 as the SAMAN project. The work is also supportby the Center for Embedded Networked Sensing and a grant from the Intel Cor-poration. W. Ye ([email protected]) and J. Heidemann ([email protected]) are withtheInformation Sciences Institute (ISI), University of Southern California (USC).D. Estrin ([email protected]) is with the Center for Embedded NetworkedSensing and the Computer Science Department, University of California at LosAngeles and USC/ISI.To design a good MAC protocol for the wireless sensor net-works, we have considered the following attributes. The first isthe energy efficiency. As stated above, sensor nodes are likelyto be battery powered, and it is often very difficult to change orrecharge batteries for these nodes. In fact, someday we expectsome nodes to be cheap enough that they are discarded ratherthan recharged. Prolonging network lifetime for these nodes isa critical issue. Another important attribute is scalability andadaptivity to changes in network size, node density and topol-ogy. Some nodes may die over time; some new nodes may joinlater; some nodes may move to different locations. A goodMAC protocol should gracefully accommodate such networkchanges. Other typically important attributes including fairness,latency, throughput and bandwidth utilization may be secondaryin sensor networks.This paper presents sensor-MAC (S-MAC), a MAC protocolexplicitly designed for wireless sensor networks. While reduc-ing energy consumption is the primary goal in our design, ourprotocol has also achieved good scalability and collision avoid-ance by utilizing a combined scheduling and contention scheme.To achieve the primary goal of energy efficiency, we need toidentify what are the main sources that cause inefficient use ofenergy as well as what trade-offs we can make to reduce energyconsumption.We have identified the following major sources of energywaste. The first one is collision. When a transmitted packetis corrupted it has to be discarded, and the follow-on re-transmissions increase energy consumption. Collision increaseslatency as well. The second source is overhearing, meaningthat a node picks up packets that are destined to other nodes.The third source is control packet overhead. Sending and re-ceiving control packets consumes energy too. The last majorsource of inefficiency is idle listening, i.e., listening to receivepossible traffic that is not sent. This is especially true in manysensor network applications. If nothing is sensed, nodes are inidle mode for most of the time. However, in many MAC proto-cols such as IEEE 802.11 ad hoc mode or CDMA nodes have tolisten to the channel to receive possible traffic. Measurementshave shown that idle listening consumes 50–100% of the energyrequired for receiving. For example, Stemm and Katz measurethat the idle:receive:send ratios are 1:1.05:1.4 [2], while the Dig-itan wireless LAN module (IEEE 802.11/2Mbps) specificationshows idle:receive:send ratios is 1:2:2.5 [3]. Most sensor net-works are designed to operate for long time, and nodes will bein idle state for long time. Thus, idle listening is a dominantfactor of energy waste in such cases.S-MAC tries to reduce energy waste from all the abovesources. In exchange it accepts some reduction in both per-hopUSC/ISI TECHNICAL REPORT ISI-TR-567, JANUARY 2003 2fairness and latency. The first technique of S-MAC is to estab-lish the