## 3 Developed UWSN Ecosystem
### Underwater Wake-Up Modern Model  
An underwater modem with wake-up capabilities enables the modem to be in an ultra-low power state, while being able to recognize certain stimuli sent prior to the current data packet and, then, wake the main circuitry for reception. The presented wake-up model is based on the ITACA modem [8] and was previously introduced in [9]. It has been designed as an energy-efficient architecture for small/medium range networks with  
Figure 2: Simplified class diagram of the ns-3 energy model. low-power UWSN consumption restraints. Its architecture is based on a microcontroller (MCU) that only consumes \(24\,mW\) in reception and \(3\,\mu W\) in the sleep state. It is capable of transmitting up to \(100\,m\) using a frequency-shift keying (FSK) modulation at \(1\,kbps\), while consuming \(120\,mW\). The acoustic wake-up signal is transmitted using an on-off keying (OOK), which is compatible (without additional hardware) with the FSK modulation used for regular transmissions. As a result, this modem also transmits the wake-up signal in the same frequency band as the regular signal. To be able to handle acoustic wake-up signals, the ITACA modem includes an off-the-shelf commercial peripheral, the AS3933 from Austria Microsystems [10]. Since this circuit is intended to be triggered using magnetic coupled signals, a net was specifically designed to adapt the acoustic incoming signals to the RFID-based wake-up circuit. This integrated circuit with the adaptation net consumes \(8.1\,\mu W\). In addition to the wake-up tone capability, this modem can, with the same energy consumption, be programmed to send various wake-up patterns for selective wake-ups and only activate the receiving node. Figure 3 depicts a simplified component diagram of the wake-up modem model, and further details can be found in [9]. As in the regular model, there is one UANetDevice acting as a network interface card (NIC) card and one UANChannel to emulate the underwater channel. To model the regular modem and the wake-up system, two UANPhys and two UANacs are introduced to model both systems. The two UANphy modules are equal in functionality, and the only differences are the consumption parameters (set to match the consumption parameters of the regular modem and the wake-up system). In turn, the UANacWU module is responsible for channel assessment and sending the wake-up packet before the packet that the UANac module sends. The UANac module is an abstract module. Any class implementing this module should implement the medium access algorithm. The module must encapsulate the data to be sent (namely, the data from  
Figure 3: Simplified component diagram of the proposed underwater wake-up model. the upper layers or the control packets, such as RTS or CTS) into an UanPacket and must ask the UanMacWU module if the channel is free. If it is free, the UanMac module sends the packet to the UanMacWU module. If it is not free, the UanMac module must decide what to do with this packet: discard it, back-off, _etc._  
When a packet is received from another node, it will always be preceded by an UanPacketWU. Therefore, after reception, the UanMacWU will check if the packet destination is correct. If it is not correct, it will discard the packet. If it is correct, it will wake the UanPhy running the regular radio, so it can receive the packet. Figure 4 shows an interaction diagram between the regular radio model and the wake-up model. In Figure 4(a), the interaction diagram for the sending sequence is depicted. It can be seen how the UanMac module asks the UanMacWU for the current channel status, and if the channel is free, it sends its UanPacket with the data from the upper layers, or its own control packets, to the UanMacWU. The UanMacWU then sends the wake-up packet, immediately followed by the UanMac packet. Figure 4: Interaction diagram between the radio model and the wake-up model. (**a**) Sending sequence; (**b**) Receiving sequence. The receiving sequence is shown in Figure 4(b).