A Time Synchronized Wireless Sensor Tree Network using SimpliciTI

Abstract

Abstract-SimpliciTI TM is a low power radio frequency protocol from Texas Instruments (TI), which supports peer to peer and star topologies. The system uses Access point (AP) as data hubs, and end devices (ED) to realize the sensor functionality. APs need to be always ON to ensure they can respond to join requests from new EDs and collect data from existing EDs in the network. This is not practical with long-running battery operated APs. We can reduce the energy requirements in the network by putting APs to sleep, but this requires careful synchronization so that the APs will be ready to receive data when the EDs transmit. We implement a time synchronization algorithm to ensure that APs are awake whenever an ED sends data. We also propose a simpliciTI based tree topology where one ED (named PCED, personal computer end device) is made common to the network of multiple APs and now PCED acts as the data hub. To manage the data traffic at the PCED we implement time division multiplexing along with store and forward function on EDs and APs.Index Terms-Star and tree topology, time division multiplexing in wireless sensor networks, time synchronization in SimplicTI, wireless sensor networks. I. INTRODUCTIONSimpliciTI, a radio frequency (RF) protocol by TI, is used to setup wireless sensor networks [1]. It is suitable for battery operated devices working at low data rates of up to 250 kbps [2]. It supports peer to peer and star topologies [2]. The basic device types available in the protocol are access point (AP), range extender (RE) & end device (ED) [2]. A. End Device D. SimpliciTI ArchitectureIt has three layers as shown in Fig. 1, the minimal RF interface layer (MRFI), the network layer and the application layer. The transceiver frames the data and thus functions as the physical and data link layer. SimpliciTI has a set of application programmable interfaces (APIs) at the application layer which allow the user to setup bidirectional links, send messages from one device to another etc [4]. The application layer is blind to the transceiver and network configurations. All this makes it very easy for the user to setup a network and modify its operation according to the application. The supported frequencies are 480, 868, 915, 955 MHz and 2.4 GHz [2].The protocol runs on Texas Instruments' ultra-low power microcontrollers (µC) and transceivers. One such device is the eZ430-RF2500. It is shown in Fig. 2. It has a MSP430F2274 µC interfaced to the CC2500, RF transceiver (TxRx, 2.4 GHz), through a serial peripheral interface (SPI) [3]. Both the µC and transceiver have sleep modes which reduce power consumption.TI describes a "wireless sensor monitor" application using the eZ430-RF2500 [1]. The application uses the SimpliciTI based star topology to setup the network as shown in Fig. 3. In the application the EDs act as the sensor terminals (field devices) and send the battery voltages and temperature readings to the AP and go to sleep. AP sends this data to the PC com port over universal asynchronous receiver transmitter (...