# A Mobility-Aware Adaptive Duty Cycling Mechanism for Tracking Objects during Tunnel Excavation
## 2 Background and Related Work
### Synchronous and Asynchronous Duty Cycling  
Duty cycling mechanisms for static topology are classified into synchronous and asynchronous schemes as shown in Figure 1[11]. In the case of a synchronous scheme, entire nodes periodically synchronize their clock and transmit data among nodes according to a predetermined schedule. In the case of the asynchronous scheme, nodes do not have to keep a global clock for synchronizing and a sender repeatedly sends the same data to a receiver until the receiver replies with an acknowledgement message after receiving the data. Our duty cycling mechanism is a kind of the asynchronous scheme because objects are moving and a network topology is also changed frequently in tunnel construction site. For Sensor-MAC (S-MAC) [12], neighboring nodes form groups (i.e., virtual clusters), and synchronize the clock of member nodes within each virtual cluster because it takes high overhead to synchronize all the nodes with one global clock. Each node broadcasts its sleep schedule to the neighbors in the same virtual cluster and records its silent time in a variable called Network Allocation Vector (NAV). If NAV is not zero, the node determines that the wireless media is busy and increases the value of NAV to avoid  
Figure 1: Synchronous and asynchronous duty cycling mechanisms (redrawn from [11]). Timeout-MAC (T-MAC) [13] was proposed to improve S-MAC (with fixed active/sleep duty cycles) by dynamically adjusting the end of active periods. In order to determine an active period, T-MAC defined a threshold time interval (i.e., Time of Arrival or TA). In this system, an active period ends when no data transmission has occurred for time TA. Finding an optimal TA determines the performance of T-MAC because TA indicates the minimal amount of idle listening per frame. T-MAC has a problem called early sleep, which means that a receiver goes to sleep when a sender still has packets for the receiver. Both S-MAC and T-MAC are synchronous duty cycling mechanisms and not suitable for burst traffic and dynamic network topology. Various asynchronous mechanisms were proposed to support dynamic and mobile networks. Berkeley-MAC (B-MAC) [14] provides an initial idea of an asynchronous mechanism. B-MAC employs an adaptive preamble sampling scheme to reduce duty cycle and minimize idle listening. Every node goes to sleep asynchronously and wakes up periodically to check for channel activity. Whenever a node wakes up, the node turns on the radio and checks for channel activity. If activity is detected, the node powers up and stays awake long enough to complete data reception. If the channel becomes inactive, the node goes back to sleep. X-MAC [15] employs a short preamble instead of the long preamble used by B-MAC to reduce energy consumption and latency. X-MAC uses two methods, embedding a receiver's address and using a strobed preamble. The former is to embedded receiver's address information in the preamble so that non-target receivers go back to sleep shortly. This method can save the energy of non-target receivers but makes broadcasting messages to neighbor nodes difficult. The latter is to allow a target receiver to interrupt the long preamble as soon as the receiver wakes up. The short strobed preamble reduces the time and energy wasted while waiting for complete transmission of the entire preamble. ContikiMAC [16] was designed to maximize the radio off time. Receiver nodes periodically wake up to listen for packet transmissions from neighbors. If a packet transmission is detected during a wake up, the receiver stays awake to receive the packet. When the packet is completely received, the receiver sends an acknowledgment to the sender. A sender repeatedly sends its packet until the sender receives an acknowledgement from the receiver. ContikiMAC uses the RSSI of the radio transceiver to determine the active state of the channel. A sender only tries to send a packet when RSSI is below a given threshold indicating that the channel is clear to avoid collision.