## 2 Related Work
BLR and BGR have to avoid duplications due to neighbors that cannot overhear broadcast message, so they exploit a restricted forwarding area for which only the mutually-communicable nodes can be the forwarding candidate nodes. However, this strategy is unnecessarily conservative; therefore, it might fail at finding a forwarding node and change the routing mode into the perimeter mode too fast, even if there exist appropriate forwarding candidates in the rest of the greedy forwarding area. Recently, [PERSON], _et al._ have introduced Link quality and Geographical-aware OR protocol (LINGO) [4] for video transmission, which exploits a cross-layer link quality and geographical-aware beaconless opportunistic routing protocol. LINGO combines link quality, geographic information and energy for routing decisions under dynamic topology changes. The extension version of LINGO, XLINGO [5], has been proposed by the same authors, in order to deal with node and link failures, as well as route failures by combining a set of cross-layer and human-related parameters for routing decisions, such as packet delivery ratio, queue length, link quality, geographic locations and residual energy. Both LINGO and XLINGO can provide realistic and practical dynamic forwarding delay (DFD) for beaconless routing, but they cannot be directly adopted to wireless sensor networks due to their strict route creation for the hidden terminal problem. The second one is a handshake-based beaconless geographic routing scheme [3, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. In this scheme, a sender currently holding data sends an Ready-to-Send(RTS) message (request) to get the location information of the neighbors. Neighbors that successfully receive the request packet send a Clear-to-Send (CTS) message (response) to the sender according to the distance-based timer. Finally, the sender selects an appropriate forwarding node and sends the data packet to the node. The contention-based forwarding [6] (CBF) protocol proposes three different suppression strategies, which vary with respect to forwarding efficiency and suppression characteristics. These strategies are able to make a sender select a next hop forwarding node without beacons by using the RTS/CTS/DATA approach. The Beacon-less On Demand Strategy (BOSS) [3] protocol includes a data packet in the RTS packet in order to improve the probability of data forwarding. Since a data packet has commonly a bigger packet size than other control packets, such as a traditional RTS/CTS packet, a sender firstly broadcasts an RTS packet, including the data packet, to all of its neighbors, and they respond with a CTS packet to the sender by the distance-based timer. After that, the sender sends only a small-sized ACK message to an appropriate forwarding node. This BOSS protocol was well analyzed and evaluated by carrying out an empirical evaluation in a real testbed in order to show the correctness of our simulation results in [7]. Based on these research results, the authors reviewed some of the latest proposals in the field of beaconless geographic routing and introduced the main design challenges and alternatives in [8]. These research efforts led the development of the self-protected beaconless geographic routing (SBGR) protocol [9]. The main objective of SBGR is to take malicious nodes into account and to analyze the effects of insider attacks in beaconless geographic routing protocols. Another research work for the handshake-based beaconless geographic routing protocol was conducted to meet special requirements. [PERSON] [10] has shown that delivery is guaranteed by beaconless forwarder planarization (BFP) and the new circluar neighborhood graph (CNG). Furthermore, simulation results were presented to show similar message complexities in the average case. This research is extended to include the recovery scheme using only three messages (RTS, CTS and DATA) per forwarding step in [11].Next, the cross-layer approach was taken for beaconless geographical routing.