# Analytical Model for the Duty Cycle in Solar-Based EH-WSN for Environmental Monitoring
## 2 Related Work  
Current MAC protocols for battery-powered sensor networks can be classified according to the type of duty cycle implementation. Accordingly, the following categories are devised:  
* Contention-free, schedule-based or synchronous MAC protocols. The idea of these protocols is to couple the duty cycles of sender and receiver, in such a way that both wake up at the same time in order to exchange one or several packets. To achieve this behavior, a predetermined or negotiated schedule is used, which requires tight synchronization among nodes. This represents a disadvantage from the implementation point of view, but even more severe is the fact that such a rigid schedule leads to limitations in terms of network scalability, protocol adaptability to spatial and temporal variations, robustness, etc. Altogether, these deficiencies make scheduled protocols less attractive in spite of their benefits: very low duty cycles and absence of collisions and packet overhearing. These protocols rely on the principles of TDMA (Time Division Multiple Access). Some examples are Self-Organizing MAC (SMACS) [10], Sensor-MAC (SMAC) [11], ReSync [12], TRaffic-Adaptive Medium Access protocol (TRAMA) [13] and Timeout-MAC (TMAC) [14]. * Contention-based, random access or asynchronous MAC protocols. In this case, clock synchronization is avoided and consequently the duty cycles of nodes are completely decoupled. Thus, in order to link a transmitter that has data to send to a receiver that is duty-cycling, many asynchronous protocols make use of LPL techniques. Essentially, as stated in [15], these techniques shift the burden of synchronization to the sender. In the most primitive form of LPL, a transmitter sends a sufficiently long preamble before the data packet, in such a way that, upon waking up, the receiver detects the preamble and stays awake for the time required to capture the packet. This implementation, known as preamble sampling (PS), represents a solution purely focused on the physical layer. Examples of protocols using preamble sampling are Aloha with preamble sampling [16] and B-MAC [17]. Later, more efficient implementations were developed, which basically substituted the long preamble by a repetitive sequence of a wake-up packet, which could be the data packet itself or an advertisement packet. The resulting protocols could then be categorized according to the way the receive check is performed: either at the MAC layer (packet header recognition), as in X-MAC [15], or at the physical layer (energy detection), as in BoX-MAC-1 and BoX-MAC-2 [18]. Finally, an alternative to LPL is a technique called Low Power Probing (LPP) [19]. In this case, it is the receiving node that periodically sends small packets called beacons or probes, to announce that it is awake and ready to receive data. A node willing to send a packet turns its radio on and waits for a probe. Upon receiving a probe from the intended destination, it sends an acknowledgement (ACK) and, subsequently, the data packet. The most representative LPP-based protocols are Receiver-Initiated MAC (RI-MAC) [20] and A-MAC [21]. An extensive survey on asynchronous MAC protocols for wireless sensor networks is provided in [22]. * Hybrid protocols. In general, asynchronous protocols offer better balance among multiple design criteria than their synchronous counterparts. Certainly, despite asynchronous protocols are less energy-efficient due to their contention nature, they exhibit higher levels of scalability, flexibility and robustness. In order to keep the benefits of the two categories, some hybrid solutions have also been proposed over the years, like WiseMAC [23] or Zebra MAC (Z-MAC) [24]. As stated in [18], these approaches attempt to combine TDMA with certain types of asynchronous support. However, these protocols have not led to standard implementations. * Adaptive protocols. Other protocols focus on dynamically setting the duty cycle of nodes so as to make it adaptive to changing traffic conditions.