# Real-Time Communication Support for Underwater Acoustic Sensor Networks +
## 2 Related Work
In order to preserve the nodes' energy, the authors propose a CDMA protocol based on a repeated cycle of sleep/wake periods, reducing the power consumption of idle listening. The network topological restrictions were relaxed in [27], proposing a solution suitable for different network sizes and architectures. A protocol named cascading multi-hop reservation and transmission (CMRT) proposes that intermediate nodes between a source and a destination may start handshaking in advance for the next-hop relaying before handshaking for the previous node is completed [23]. By this concurrentrelaying, control packet exchange and data delivery cascade down to the destination. In [28] the authors compare analytically the slotted protocol FAMA with four way handshaking and the contention resolution Tone-Lohi protocol for different performance parameters, like energy consumption and throughput. In case of TDMA, it divides a time interval (called a frame) into time slots instead of dividing the frequency band. Each slot is assigned to an individual node or a set of nodes whose transmissions do not interfere with each other. Collisions of packets from adjacent time slots are prevented by adding guard times. Large propagation delays, delay variance over the acoustic channels and the nodes clocks drift, all require the addition of guard time periods to minimize the probability of collisions in data transmissions. There are many TDMA-based MAC protocols proposed in the literature to overcome the shortcomings of the TDMA, which is considered one of the best multiple access techniques for UW-ASNs, due to its simplicity and flexibility [20]. In order to cover large areas, underwater applications require a multi-hop network. Many routing protocols have been proposed for UW-ASNs, which may include suitable media access control, network and transport protocols. These protocols implement two classes of routing: localization-based and localization-free. The first one can take advantage of the absolute and relative localization of nodes; however, the nodes' position are not ease to know and maintain, since some nodes are fixed in depth and other are mobile, and unfortunately, GPS services are unavailable for underwater nodes. The second type of routing protocols is highly considered by the research community, and most of the proposals are oriented to communicate underwater nodes to a surface node (e.g., a sonobuoy), which is in charge of forwarding packets to an onshore station via RF transmissions [14; 29; 30; 31; 32; 33]. In [34] the authors propose a routing protocol called DBR (Depth-Based Routing) that uses the depth sensed by each node, as a routing metric. DBR suffers from redundant packet transmissions, therefore it is energy consuming. An alternative dynamic routing protocol is presented in [35], based on a node address called HopID, which is related to the hop count from the source to destination node. In both protocols, some nodes could be used to relay messages more frequently than others, making their batteries get exhausted and creating thus blank zones and routing holes. In critical applications based on UW-ASNs, like those used to deliver early warnings in hazardous events (e.g., tsunamis), a maximum end-to-end communication delay must be guaranteed. This requirement could be more critical than the energy consumption, therefore a real-time protocol must be used. Many proposals are trying to deal with this problem from different perspectives, for instance trying to minimize the end-to-end latency, increase the throughput, increase the link reliability, optimize synchronization, and increase the network efficiency. Although all these results are useful to improve the network performance, none of them ensure an upper-bounded message delay [36; 37; 38; 39; 40; 41]. In [42] the authors addressed the problem of data transmission with an associated end-to-end packet deadline.