From the view of routing protocols in underwater sensor networks (UWSNs), the presence of communication void, where the packet cannot be forwarded further using the greedy mode, is perhaps the most challenging issue. In this paper, we review the state of the art of void-handling techniques proposed by underwater geographic greedy routing protocols. To this, we first review the void problem and its negative impact on the category of the geographic greedy routing protocols, which does not entail any void recovery technique. It is followed by a discussion about the constraints, challenges, and features associated with the design of void-handling techniques in UWSNs. Afterward, currently available void-handling techniques in UWSNs are classified and investigated. They can be classified into two main categories: 1) location-based and 2) depth-based techniques. The advantages and disadvantages of each technique along with the recent advances are then presented. Finally, we present a qualitative comparison of these techniques and also discuss some possible future directions.

The mobility of nodes in mobile ad hoc networks (MANETs) results in frequent changes of network topology making routing in MANETs a challenging task. Some studies have been reported in the literature to evaluate the performance of the proposed routing algorithms. However, since the publication of experimental standards for some routing protocols by IETF, little activity has been done to contrast the performance of reactive against proactive protocols. This paper evaluates the performance of reactive (AODV, DSR) and proactive (OLSR) routing protocols in MANETs under CBR traffic with different network conditions. Our results, contrarily to previously reported studies conducted on the same routing protocols, show the superiority of proactive over reactive protocols in routing such traffic at the cost of a higher routing load.

Increasing attention has recently been devoted to underwater sensor networks (UWSNs) because of their capabilities in the ocean monitoring and resource discovery. UWSNs are faced with different challenges, the most notable of which is perhaps how to efficiently deliver packets taking into account all of the constraints of the available acoustic communication channel. The opportunistic routing provides a reliable solution with the aid of intermediate nodes’ collaboration to relay a packet toward the destination. In this paper, we propose a new routing protocol, called opportunistic void avoidance routing (OVAR), to address the void problem and also the energy-reliability trade-off in the forwarding set selection. OVAR takes advantage of distributed beaconing, constructs the adjacency graph at each hop and selects a forwarding set that holds the best trade-off between reliability and energy efficiency. The unique features of OVAR in selecting the candidate nodes in the vicinity of each other leads to the resolution of the hidden node problem. OVAR is also able to select the forwarding set in any direction from the sender, which increases its flexibility to bypass any kind of void area with the minimum deviation from the optimal path. The results of our extensive simulation study show that OVAR outperforms other protocols in terms of the packet delivery ratio, energy consumption, end-to-end delay, hop count and traversed distance.

Employing contention-based medium access control (MAC) protocols in underwater sensor networks (UWSNs) is typically costly. This is due to the unique characteristics of underwater acoustic channels such as long propagation delay, limited bandwidth, and high bit error rate. As a consequence, the contention-based (handshaking and random access-based) MAC protocols do not perform as efficiently as expected. The collision-free approach is therefore considered to achieve a high performance by avoiding the collisions at MAC layer in order to improve the energy efficiency, throughput, and fairness. In this paper, we propose, inspired by graph colouring techniques, a novel energy-conserving and collision-free reservation-based MAC protocol, called GC-MAC, for UWSNs. GC-MAC employs a TDMA-like approach by assigning separate time-slots, colours, to every individual sensor node in every two-hop neighbourhood. Sensors with the same colours can thus transmit at the same time with no chance of collision. GCMAC is also able to address the near-far effect, spatial-temporal uncertainty, and hidden/exposed node problems, without requiring CDMA or power adjustment for collision avoidance. The network coverage and connectivity is then discussed to show the effectiveness of using cubes to cover a three-dimensional (3D) underwater environment. Our extensive performance study shows that GC-MAC performs well by avoiding collisions achieve better throughput and energy-efficiency performance compared to those of contentionbased protocols. There is also a significant improvement in terms of packet delivery ratio and fairness among the nodes under different operational conditions.

Underwater Sensor Network (UWSN) has recently attracted a considerable attention due to its ability to discover and monitor the aquatic environment. However, its acoustic communication has posed several inherent characteristics such as high latency, low available bandwidth, and high bit error rate. These unique characteristics have made contention-based Medium Access Control (MAC) protocols inefficient for UWSNs. They are mostly expensive and are not as effective as they are in terrestrial networks. Through this principle, a contention-free MAC protocol is therefore considered to be more reliable and flexible to overcome the consequences of applying acoustic signals, and also to achieve a high performance (improving the energy efficiency and throughput across the network) by eliminating the chance of collision. In this paper, we propose a novel energy-conserving and collision-free Depth-based Layering MAC protocol (DLMAC) for UWSNs. DL-MAC is able to deal with the underwater MAC challenges such as the near-far effect, spatial-temporal uncertainty, and hidden/exposed terminal problems. It is able to efficiently schedule the transmission and receptions operations in each side by using the concept of layering and a distributed clustering algorithm. By using a TDMA-based principle, DL-MAC can assign separate time slots to every sensor node individually to access the medium without any possibility of collision. Our extensive simulation study shows that DL-MAC outperforms other protocols in terms of throughput, packet delivery ratio, energy consumption, and packets lost under varying traffic rates and numbers of nodes.

Networks on chip (NoC) emerged as a promising alternative to bus-based interconnect networks to handle the increasing communication requirements of the large systems on chip. Employing an appropriate topology for a NoC is of high importance mainly because it typically trade-offs between cross-cutting concerns such as performance and cost. The spidergon topology is a novel architecture which is proposed recently for NoC domain. The objective of the spidergon NoC has been addressing the need for a fixed and optimized topology to realize cost effective multi-processor SoC (MPSoC) development [7]. In this paper we analyze the traffic behavior in the spidergon scheme and present an analytical evaluation of the average message latency in the architecture. We prove the validity of the analysis by comparing the model against the results produced by a discrete- event simulator.