Abstract— One of the important aspects in wireless sensor networks is an energy efficient communication protocol to allow battery powered devices with a lifetime of several of years. The most important step therefor is to maximize the time the network nodes staying in power down modes. The possibility to measure distances with an easy-to-use and low cost phase difference of arrival method between network nodes enables an accurate positioning of these nodes. Because this technology needs additional requirements on synchronization and channel access, this paper describes a centralized multi-hop beaconing protocol to synchronize all the devices and to schedule the guaranteed timeslots for the ranging procedures. This yields to an increase of time in power down mode and an increase of the maximum rate of distance measurements per second. Measurements were made at the implemented system at company ZIGPOS GmbH. Index Terms—beacon-enabled, timeslot, multi-hop, ranging, phase-of-arrival, positioning I. INTRODUCTION IRELESS sensor networks are used in a big range of applications like temperature/humidity measurement for climate control till complete home automation systems. It is also used in industrial and commercial scenarios, like production control systems and warehouse management. The usage of a positioning functionality of the sensors extends the usability of this networks such as creates a complete new range of applications. With the use of the phase of arrival distance measurement technology, to get high accurate positions, it is possible to establish an accurate indoor navigation system, warehouse management systems or an easy to install home automation system. It is ideally suited for local positioning in traffic applications like in [1]. But this technology depends special requirements at the sensor network protocol. Like described in the next chapter special This work was supported in part by the ZIGPOS GmbH and financially supported by the ESF “Europäischer Sozialfonds”. Frank Stephan is with University of Applied Sciences Dresden (HTW Dresden), 01069 Dresden, Germany. Working with the department of Telecommunications Technology (e-mail: stephan@htw-dresden.de). Sven Zeisberg is with University of Applied Sciences Dresden (HTW Dresden), 01069 Dresden, Germany. Working with the department of Telecommunications Technology (e-mail: zeisberg@htw-dresden.de). Oliver Michler is with the Dresden University of Technology (TU Dresden), "Friedrich List" Faculty of Transportation and Traffic Sciences, 01062 Dresden (e-mail: oliver.michler@tu-dresden.de). synchronization methods are required, what lead to a higher energy consumption in standard sensor network protocols. Therefor a new network synchronization and ranging timeslot (ts) distribution protocol will be introduced. II. NETWORKING BASICS A. Sensor network In wireless sensor networks lifetime of battery powered device is a major issue. That means it is necessary to maximize the time the network nodes stays in power down modes with the microcontroller, the RF-path and mostly all the other board periphery shut down or in deep sleep. The usage of a time synchronized network protocol to enable the devices and their RF-path only at dedicated timeslots is usual practice. The IEEE802.15.4 beacon-enabled mode described in [2] provides the possibility to synchronize devices to the coordinator in a star topology network and to split the available medium access time in 16 timeslots. The timeslots can be shared between multiple devices or assigned to one. With the two parameter Superframeorder (SO) and Beaconorder (BO) several timing schemes are accessible. This means the beacon-interval (BI), the time between two synchronization beacons, can be adjustable, such as the length of the superframe. The superframe describes the active communication part of a BI which can have also an additional inactive period. This results in different inactive/sleep time durations, latencies, throughputs and network sizes. To extend the range of the sensor network, router can be added. This yields to a multi-hop capable sensor network which can be structured in different topologies. The network described in this paper is tree structured. It consists of three device types. This is first the coordinator of the network to start and control the network. It provides the basic network structure and parameters, like PAN ID and frequency channel and new network devices can associate to it. Like mentioned above the second device type is the router, which expands the network functionality to a multi-hop network. The third type are enddevices at the leaf of the tree topology, used for the positioning application. The IEEE802.15.4 standard describes a possibility to extend the beacon-enabled mode of operation to use it in a multi-hop network based on a time division approach. In this case every router send its own beacon and start a superframe in the inactive period of the coordinators and the other routers BI. As an extension a frequency multiplexing can be used, like the in [3] described Multi-hop synchronization and timeslot distribution method for IEEE 802.15.4 based positioning networks Frank Stephan, Sven Zeisberg, Oliver Michler W 978-1-4799-4671-6/14/$31.00 ©2014 IEEE