## 4 Stationary Sensor Network
### Wireless Sensor Networks  
Some notable studies have explored the use of multihop WSNs for purposes of long-term persistent sensing. A number of such systems are time division multiple access (TDMA) based networks which support both time and frequency division to facilitate robust communications while permitting for low battery consumption [13, 17]. Successful applications have ranged from industrial control [18], military applications [19], and habitat monitoring [20]. The majority of these studies was built upon the IEEE 802.15.4 stack [21], which specifies hardware and software requirements to be met to facilitate interoperability between low-power wireless devices. A number of key factors emerge when considering the needs imposed on the stationary communications infrastructure:  
* Reliability: Guarantees must be given to ensure reliable data packet delivery, and the network should have methods to mitigate the effects external radio interference. * Low power consumption: Stationary nodes have to meet long-term deployment goals while running only on batteries. * Scalability: The network should support an arbitrary number of stationary or mobile nodes. * Security: Communication should be encrypted to protect data integrity and mitigate malicious attacks. * Flexibility: The communication protocol has to dynamically allocate resources to meet the needs imposed by high data throughput requirement of MAVs with those of the low throughput stationary sensing nodes. To meet these requirements for the stationary network backbone, we propose an architecture built upon the time synchronized mesh protocol (TSMP) [13]. This protocol has also been standardized under the IEEE 802.15.4E workgroup [22], as well as the Wireless Hart Foundation [23], and ISA100 [24]. At its core, it ensures reliable, secure, and scalable wireless communications by combining very tight time synchronization with frequency channel hopping and routing diversity. These protocols also specify authentication and encryption routines. TSMP relies on a number of MAC-layer enhancements (and thus larger implementation overhead) to facilitate synchronization and frequency channel diversity. It is a networking technique that relies on an agreed upon transmission schedule between network nodes (Figure 3). Time is sliced up into time slots of equal length (10 ms according to the IEEE 802.15.4e standard); a constant number of slots make up a slot frame which repeats indefinitely over time. Once synchronized, network node pairs are scheduled to exchange communications at a specified time slot and channel offset (carrier frequency, 16 of which are specified by IEEE 802.15.4) within the repeating slot frame. A major requirement stipulates that no two node pairs can communicate during the same time slot and on the channel offset, thus ensuring collision-free communication. It has been shown that multipath fading and external interference can drastically impede network connectivity if a single frequency channel is used [7]. Since nodes only communicate when scheduled, they keep their radios powered off most of the time. This low duty cycle plays a critical role in keeping battery consumption to a minimum, theoretically permitting one of our nodes to last years on a standard AA battery. Such a TDMA-based network can significantly increase the throughput of a WSN, while reducing energy consumption and packet collision rates [13]. The workgroup IEEE 802.15.4E [22] focuses on enhancing the MAC protocol proposed in IEEE 802.15.4 while keeping the same physical layer. In its current proposal, nodes can switch between different hopping sequences. As with TSMP, slots can be added/removed during the lifetime of the network. In the stationary setting, a TSMP network is centrally scheduled by a network manager. When a node first joins a network, it leaves its radio on and listens for advertisements (ADV packets, see Figure 3) from its neighbors. Upon hearing an ADV packet, the node synchronizes to the network by the adjusting its slot frame to reflect that of its time parent.