Wireless technology is a key enabler for an expanding range of mobile devices, including not only smart phones and tablets, but also wearable sensors, industrial machinery, health-monitoring instruments and robotics. These devices play a key role in many new application domains that push wireless networks to dramatically improve quality-of-service properties such as throughput, timeliness, reliability, security, privacy, usability, and efficiency. Generically, due to their limited energy resources, these embedded devices can only support short-range wireless communication.

The communication between mobile nodes and a fixed infrastructure has been extensively studied in Cellular and WiFi networks, and it has been addressed through the use of hand-off mechanisms. However, these methods cannot be readily applied to low-power wireless networks. First, Cellular and WiFi networks have more sophisticated radios with more energy resources. This means that their wireless links are much longer and more reliable than those provided by low-power radios, and hence the thresholds and parameters associated to hand-off mechanisms need to be tuned accordingly. Second, base stations in cellular networks build on fixed wired infrastructures with strong processing and communication capabilities, which is not applicable in low-power networks. Third, mobile nodes in Cellular and WiFi networks are usually in the coverage range of several strong radios while the unreliable links of low-power wireless networks have little overlap.

A more efficient solution is for mobile nodes to select a single AP to transmit data at any given time. This alternative requires nodes to perform hand-offs between neighboring APs. Hand-off refers to the process where a mobile node disconnects from one AP and connects to another AP. Here, we provide the overall idea of smart-HOP, a hand-off process for low-power networks, and highlight the main parameters involved in this process:

Link monitoring. It determines how frequent the link monitoring should be. The link monitoring property is captured by the Window Size (WS) parameter which represents the number of packets required to estimate the link quality.

Handoff thresholds and hysteresis margin. In WSNs, the selection of thresholds and hysteresis margin is dictated by the characteristics of the transitional region and the variability of the wireless link. The lowest threshold, TH(low), has to consider the lower boundary of the transitional region. If the threshold is too high, the node could perform unnecessary handoffs (by being too selective). If the threshold is too low the node may use unreliable links. The hysteresis margin (HM) plays a central role in coping with the high variability of low-power wireless links. If the margin is too narrow, the mobile node may end up performing unnecessary and frequent handoffs between two APs (ping-pong effect). If the margin is too wide, the handoff may take too long which ends up increasing the delay and decreasing the delivery rate.

Stability monitoring. Due to the high variability of wireless links, the mobile node may detect an AP that is momentarily above TH(high), but the link quality may decrease shortly after being selected. In order to avoid this, it is important to assess the stability of the candidate AP. After detecting an AP with the link quality above TH(high), smart-HOP sends m further bursts of beacons to check the stability of that AP. The burst of beacons stands for the WS request beacons followed by the reply packets received from neighboring APs. Stability monitoring is tightly coupled to the hysteresis margin. A wide hysteresis margin requires a lower m, and vice-versa. We have shown that an appropriate tuning of the hysteresis margin can lead to m = 1.

The smart-HOP algorithm has two main phases: (i) data transmission and (ii) discovery. A timeline of the algorithm is depicted in the following figure.

For the sake of clarity let us assume that a node starts in the Data Transmission Phase. In this phase, the mobile node has a reliable link with an AP, defined as serving AP in the figure. The mobile node monitors the link quality by receiving reply packets from the serving AP. Upon receiving n data packets in a given window, WS, the serving AP replies with the average received signal strength (RSSI) of the n packets. If no packets are received, the AP takes no action. This may lead to disconnections, which are solved through the use of a time-out mechanism.

We performed a carefully designed set of experiments, based on IEEE 802.15.4 radios, to get a better insight on the settings of key parameters, namely, the lower link quality threshold level and the hysteresis margin. Calibrating the parameters of smart-HOP requires a test-bed that provides a significant degree of repeatability. A fair comparison of different parameters is only possible if all of them observe similar channel conditions. In order to achieve this, we deployed a model-train in a large room. The room was 7m x 7m and the locomotive followed a 3.5m x 3.5m square layout. The speed of the locomotive was approximately 1 m/s (average walking speed).

We implemented smart-HOP in TinyOS 2.0.2 and used telosB motes for the evaluation. The transmission period of the beacon and data packets was 10 ms. This value is close to the maximum rate possible considering the processing, propagation and communication delays. The idea behind choosing the maximum data rate was to evaluate smart-HOP for scenarios with demanding QoS requirements.

Four APs were located at each corner of the deployment, and up to six more APs were randomly placed to assess the impact of APs density. To test smart- HOP under demanding conditions, we identified a transmission power that provided minimum overlap among access points. For our settings, Pout = 20 dBm satisfied this condition. Then, we ran several laps with the mobile node broadcasting packets. The broadcast laps were run at different times of day, during several days and with different number of people in the room.

Detailed information on the algorithms performance is available HERE.