## 2 Low-Power Wireless Technologies
### DASH7 Alliance Mode  
DASH7 Alliance Mode (D7 AM) [10] is a new standard for low-power wireless communications developed by the DASH7 Alliance. Contrary to other existing low-power wireless technologies, D7 AM defines all the layers of the OSI (open source interconnect) model, from the physical layer to the application layer, and is targeted at asynchronous communications. The goal of D7 AM is to be simple, elegant and reliable for handling bursty, light data and asynchronous and transient usage models. This approach is referred to as BLAST (bursty, light, asynchronous, stealth and transitive), and this means that it is tightly tuned for dealing with inherently mobile devices that need to upload small bits of information reliably thanks to its excellent range, low-power and robustness features. A complete overview of D7 AM can be found in [22]. To maximize the range of one-hop communications [23], D7 AM operates at the 433 MHz band. The 433 MHz band, which provides 1.74 MHz of bandwidth, is available (almost) worldwide as an ISM band, thus not requiring a license to operate. The D7 AM physical layer is organized into fifteen 108 kHz basic channels, which are combined into four channel classes. The base class provides a single 432 kHz channel. The normal class provides eight 216 kHz non-overlapping channels. The hi-rate class provides four 432 kHz overlapping channels, and finally, the blink class provides a single 648 kHz channel. D7 AM uses a (G)FSK (gaussian frequency shift keying) modulation scheme, one narrowband (55 kbps) for the base and normal channels and one wideband (200 kbps) for the hi-rate and blink channels. The benefit of using FSK is that the modulated signal has a constant envelope and, thus, is less sensitive to amplitude distortion caused by the hardware or the channel. In addition, all channels are encoded to enhance the robustness of one-hop communications. The standard defines two encoding mechanisms, PN9 and FEC (forward error correction). The PN9 encoding scheme, implemented using a LSFR (linear feedback shift register), provides a statistically DC-free data stream, but introduces no processing gain. The FEC encoding scheme, implemented using a non-recursive convolutional code followed by a 4 \(\times\) 4 interleaving matrix is optional and focuses on providing data integrity in environments with bursty errors caused by shadowing. Regarding the data-link layer, D7 AM supports both synchronous and asynchronous communication models. To support both communication models, the data-link layer is based on two well-known techniques: preamble sampling (PS) [24] and carrier sense multiple access (CSMA) [25]. In addition, all motes in the network share a common knowledge of time, the tick (ti), which is equal to \(2^{-10}\) s or 0.976 ms and is the smallest amount of time at which events at the MAC layer can be resolved. However, there is no global network synchronization, as each clock may tick at a different rate, e.g., due to physical construction, aging or temperature drift. Using PS, a node can trigger communications with another node or a group of nodes asynchronously. Nodes execute the _channel scan series_, which is an ordered list of time events at which nodes wake up and turn on the radio to receive a background frame. In order to trigger communications, the standard defines the _beacon transmit series_. The beacon transmit series is an ordered list of time events at which the node is expected to wake up and turn on the radio to transmit a background frame. Background frames include information regarding the time that the node is expected to wake up and the channel that it has to listen to. Both the _channel scan series_ and the _background scan series_ can be configured depending on the application requirements, e.g., to minimize latency or maximize battery duration. Upon synchronization, nodes use foreground frames to exchange data using the two channel types available: non-guarded channels (NGC) and guarded channels (GC).