RT-Link: A Time-Synchronized Link Protocol for Energy ConstrainedMulti-hop Wireless NetworksAnthony Rowe, Rahul Mangharam, Raj RajkumarElectrical and Computer Engineering Department, Carnegie Mellon University, PittsburghTechnical Report CMU-ECE-TR05-08AbstractMulti-hop wireless networks of embedded nodes facilitateapplications in industrial control, surveillance and inventorytracking. Our focus is on low-cost large-scale deploymentswhere nodes need to be battery-powered with predictablenetwork lifetimes and applications require bounded end-to-end delay. An effective approach to such energy-constrainednetworks is to operate at low duty cycles and maximizethe shutdown interval between packet exchanges. The pri-mary challenge is in coordinating transmissions so they arecollision-free while minimizing the duration the nodes areactive. RT-Link is a time-synchronized link protocol for fixedand mobile embedded radios. We identify three key observa-tions in the design and deployment of RT-Link: (a) RT-Linkoffers predictable network lifetime with bounded end-to-enddelay. (b) Achieving global time synchronization is both eco-nomical and convenient for indoor and outdoor deployments.(c) Due to interference between nodes, our experiments con-firm that nodes with the same schedule must be spaced by aminimum of 3 hops. Furthermore, to minimize end-to-end de-lay, it is more important to order time slots than to minimizethe number of time slots. RT-Link has been deployed on net-works with more than 30 custom embedded nodes and usesthe IEEE 802.14.5 physical layer. It outperforms energy-efficient protocols such as B-MAC and S-MAC in throughput,energy consumption and end-to-end delay.1. IntroductionNetworks of embedded wireless nodes provide a versatileplatform for applications in industrial control, surveillanceand inventory tracking. The purpose of such networks is togather data and deliver it across one or more hops to at leastone gateway. The principal requirements are low-cost batterypowered radios, minimal configuration on set up with simpleand scalable energy-efficient protocols for predictable net-work lifetime and bounded end-to-end message delay. Thefollowing deployment scenarios motivate the need for net-works of embedded nodes with such requirements:• Industrial Control Networks: In chemical and automo-bile plants, remote control of machinery and access toperformance data requires reliable real-time communi-cation. In such environments, it is necessary that nodesdo not require infrastructure for data and power as suchprovisioning may be both impractical and expensive.• Surveillance and Monitoring Networks: Networks ofembedded cameras for monitoring motion and intrusionrequire bounded end-to-end delay to the gateway anddeterministic peak throughput for intermittent transferof captured images.• Inventory Tracking and Reporting: Networks to classifyand locate assets need to be scalable and must operatein a variety of multi-hop wireless topologies.An effective approach to energy-efficient service for applica-tions with periodic and aperiodic flows is to operate all nodesat low duty cycles so as to maximize the shutdown intervalsbetween packet exchanges. The two fundamental challengesin delivering delay-bounded service in such networks are (a)coordinating transmissions so that all active nodes commu-nicate in a tightly synchronized manner and (b) ensuring alltransmissions are collision-free. Time synchronization is im-portant because it tightly packs the activity of all nodes sothat they may maximize a common sleep interval betweenactivities. Furthermore, it provides guarantees on timeliness,throughput and network lifetime for end-to-end communica-tion. Such assurances are only possible when the link is re-liable and collision-free. It is therefore the responsibility ofthe link layer protocol to provide exclusive and interference-free access to the shared wireless channel and a mechanismto coordinate sleep intervals of all nodes. The focus of thispaper is on RT-Link, a time-synchronized link layer proto-col for collision-free and energy-efficient real-time serviceover multi-hop wireless networks. RT-Link facilitates dy-namic admission of both fixed and mobile nodes into a tightlysynchronized regime. It schedules nodes in time slots suchthat concurrent transmitters do not interfere with each otherand the activity of all nodes are coordinated to maximize thesleep duration. Finally, RT-Link maintains contention-freeoperation by employing an online and automatic link conflictdetection and resolution scheme. Such a scheme is usefulCarnegie Mellon University Department of Electrical and Computer Engineering Technical Report August 2005when topology or environment changes cause interference tonodes between concurrent transmitters. RT-Link has beenimplemented as a link layer protocol in low-cost and low-power embedded nodes developed by us. Each node includesa short-range 2.4GHz IEEE 802.15.4 [1] physical layer forradio communication. Through the design and deploymentof RT-Link, we identify the following four observations:1. RT-Link offers predictable network lifetime withbounded end-to-end delay for packets between the gate-way and every node.2. Provision of global time synchronization for embed-ded multi-hop wireless networks is both economical andconvenient. We achieve this by employing an Ampli-tude Modulation (AM) based carrier-current method in-doors and with atomic clock receivers for the outdoors.3. Our experiments show that due to interference acrossthe shared channel, nodes with the same schedule (i.e.concurrent transmitters) must be spaced by a minimum3-hop distance.4. In high throughput networks, the scheduling objectiveis to maximize the number of concurrent transmitters[2]. In contrast, in energy-efficient sensor networks, theordering of the time slots is more important than thenumber of time slots.The paper is organized as follows: we address related work inthe next section followed by a description of the RT-Link pro-tocol in Section 3. We study the timeliness, robustness andefficiency of the protocol in Section 4. Section 5 providesan overview of our implementation platform and deploymentexperiences. This is followed by a comparative evaluation ofRT-Link in Section 6 and our concluding remarks.2. Related WorkSeveral MAC protocols have been proposed for low-power and distributed operation for single and multi-hopwireless mesh networks. Such protocols may be catego-rized by