## 2 Related work  
In this section, we present some MAC protocols for intermittent asynchronous transmission and demonstrate the essential differences between sender-driven MAC and receiver-driven MAC. There are various approaches to media access control for intermittent asynchronous transmission. **B-MAC**[4] is the basis of LPL protocols as presented in Figure 1(a). In LPL, receiver nodes intermittently probe the state of the channel. As mentioned above, there are various problems associated with this LPL protocol; for instance, when the intermittent interval is comparatively long, each sender node occupies the channel by transmitting preamble packets for a period of time which is longer than the interval, thus interfering with any transmission from neighboring nodes. Moreover, the preamble packets transmitted from the sender consume the energy of unrelated receivers, which is known as \"overhearing problem\". Another problem is that each sender node has only one specific receiver. Energy-aware adaptive low power listening (**EA-ALPL**[9]) is based on B-MAC. The procedure followed by receivers and senders in EA-ALPL is the same as the one shown in Figure 1(a), however, each node reconfigures its intermittent interval and adapts it to changes in traffic in order to attain higher energy efficiency. For high energy efficiency, the next hop selected by a sender node is the receiver which has the minimum hop count from the sink node. When there are multiple receiver candidates with minimum hop count, a sender node selects one of the most preferable nodes in accordance with the cost function of the intermittent interval and the sensing activity of neighboring nodes. The sensing activity is a Boolean variable, and it is determined by the sensing frequency of a node. In order to select a receiver, nodes regularly exchange information regarding the sensing activity and their own intermittent interval. **X-MAC**[8] was designed to solve the overhearing problem of B-MAC. In order to prevent the preamble packets of the sender in B-MAC from occupying the channel, X-MAC continuously transmits short preamble packets to which the ID of the receiver is appended. The operation of X-MAC is shown in Figure 2(a). The receiver node replies with an early acknowledge (early ACK) packet when the ID added to the short preamble corresponds to its own ID. The sender node transmits a data packet after receiving this early ACK and waits for the acknowledge packet for the data. Receivers that detect unrelated short preambles can resume their state of sleep soon after the end of the reception. Thus, the overhearing problem generated by continuous transmission of preambles during intermittent intervals in B-MAC can be solved. Although various receiver-driven asynchronous MAC protocols have also been proposed, most of them either assume that all nodes are active and can receive packets at any time, or that they use multi-channel access for transmitting packets [10, 13, 14]. In [13], receiver-driven media access control with a single channel, named \"receiver initiated multiple access\" (**RIMA**), is proposed. RIMA employs a collision avoidance handshake mechanism with CSMA/CA and obtains a reasonable throughput; however, this protocol does not use intermittent operation since it does not consider energy consumption. In [10], two generic intermittent asynchronous MAC protocols are proposed, namely, Transmitter Initiated CyclEd Receiver (**TICER**) and Receiver Initiated CyclEd Receiver (**RICER**). The procedure of sending and receiving data in RICER is similar to that in IRDT, where receiver nodes periodically transmit ID packets. However, unlike the procedure in IRDT described in Section 3, a sender node in RICER transmits a data packet after obtaining an ID packet. Furthermore, two channels are used for communication, and a sender uses only one receiver in RICER. In contrast, IRDT uses a single channel, which simplifies the implementation and ensures a highly reliable system. However, single-channel access causes control packet collision.