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Cross-Layer design is a model that allows the flow of information, interaction, coordination and joint optimization across the different layers of a protocol stack in a non-trivial way. Because of the various limitations of the protocol stack (TCP/IP and OSI Models) especially in terms of wireless network performance, cross-layer design is assumed to improve the performance of wireless communications systems (Fotis, F.; Vangelis, G.; and Nancy, A.; 2008).
Though, there are many approaches to cross-layer design, they can be classified into strong and weak cross layering. A weak cross-layering, otherwise known as evolutionary approach allows interaction among different layers of a protocol stack while strong cross-layering (also called revolutionary approach) on the other hand, allows joint design of the algorithm implemented with any entity at any level of the protocol stack.
Cross-Layer Signalling Architectures
Researchers have proposed several Cross-Layer Signalling architectures though very few are prototyped and included into current operating systems (Kliazovich, D.; Michael, D.; and Fabriozio, G.; 2009). These architectures can be group under the following:
Inter-layer Signalling Pipe: this architecture allows the flow of data in the protocol stack in a bottom-up or top-down manner. This can be achieved via two methods: Packet header which carries the information to the IPv6 header and; packet structure which carries the information into a specific section of the packet structure.
Direct Inter-layer Communication: it introduces short-cuts out of band performance. For example, CLASS.
Central Cross-Layer Plane: the signalling information in this architecture can be accessed by all the layers of the protocol stack. For example, ÉCLAIR
Network-wide Cross-Layer Signalling: this method aim at defining cross-layer signalling at different layers of a protocol stack of a system terminal (Kliazovich, D.; Michael, D.; and Fabriozio, G; 2009).
Similarly, cross-layer design can be performed in either a loosely coupled or tightly-coupled cross-layer design. In a loosely-coupled cross-layer design, the optimization is done on one layer without crossing the layers. This can be improved by taking into consideration all the parameters in the protocol layer and transferring the information to the other layers. The information can be utilized in two ways: the algorithm must be changed based on the information from the other layers and; information from the other layers is just as the one in the protocol layer.
In the tightly-coupled cross-layer design in which the information sharing between the layer is not enough and thus the algorithms in different layers are optimized together as one optimization problem (Akyildiz, I.; and Wang, X.; 2009).
Cross-Layer design in 3G (UMTS)
Wang, Q.; and Yuan, D.; (2010) proposes an active ACK control strategy with cross-layer information interaction (CL-AACS) in order to solve the problem of multi-user contention brought by packet scheduling in MAC layer and to improve the performance of TCP in UMTS. The scheme is embedded and placed in the base station to control and return an ACK for the information transferred between the MAC-layer and transport-layer so that all users can have fair TCP throughput and optimized utilization of wireless link resources.
The simulated result shows that the scheme provides fair and effective trade-off for TCP over UMTS.
Cross-Layer design in WiMAX
In order to close one of the major gaps of inter-layer connectivity between the network and link layer, Neves, P.; Sargento, S.; Pentikousis, K.; and Fontes, F.; (2012) proposes the WiMAX Cross-Layer (WXL) system which is responsible for providing all the required cross-layer services between the WiMAX between the WiMAX MAC layer and the network layers. The system is made up of the middleware layer which is located between the WiMAX technology and the network layer, thus hiding the specific functions of the WiMAX technology from the control plane of the network layer. Interactions with the network layers in the WXL system is provided by a set of dedicated interfaces which consists of the upper and lower interfaces and comprises support for QoS mobility and interactions with link layer technologies respectively.
The test result performed on the test bed indicates a small processing time for QoS reservations, modification and deletions.

Cross-Layer design in in Ad hoc Network
Ghafur, M.A.; Upadhayaya, N.; and Sayyed, S.A.; (2013) presents an Enhanced Virtual Carrier Mechanism with a controlled exchange of RTS/CTS for an effective interaction among the protocol layers by sharing valuable information for making the right decision for a realistic response in mobile ad hoc network by enhancing the MAC and network layers. This is done in a way that the elements in one network can be used to improve the other. The scheme will also repair the route within the shortest possible time with less overhead.
Though the simulation result indicates that the scheme overtakes the standard one in different scenarios, but “if the traffic is above a pre-defined threshold no attempt is made for local repairing” (Ghafur, M.A.; Upadhayaya, N.; and Sayyed, S.A.; 2013).
2.5 Cross-Layer Mobility Management
To provide a Cross-Layer Mobility Management, Wang, Q. and Abu-Rgheff, M.A.; (2012) proposes the Cross-Layer Signalling Shortcuts (CLASS) scheme to enable direct communications between arbitrary layers which can be applied in QoS management, power, radio resources, mobility, and in the IP-based next-generation wireless network.
Though CLASS has significant advantages when compared with other methods through a qualitative evaluation, it is yet to be introduced to solve real-world problems and future research is underway to simulate it in a software simulator and implement on I on Linux with kernel modified operating system (Wang, Q. and Abu-Rgheff, M.A.; 2012).
Hung, W-K.; Dai, H-J.; and Luo, C.; (2012) proposes the media-independent pre-authentication redirect tunneling (MPA-RT)which is integrated with media- independent handover (MIH) and session initiation protocol (SIP) to provide a seamless cross-layer handover for SIP applications.
Simulation results indicate that the framework is more efficient to MPA-DB in terms of packet transmission delay and buffer utilization.
Wang, D-C.; He, W.; and Chen, I-R; (2012) proposes and analyses a cross-layer integrated mobility and service management scheme called DMAPwSR in IPv6 environments with the aim of minimizing the overall cost of mobility and service management. The framework uses smart routers which act as access routers for the MIPv6 systems except that they are capable of processing binding messages from the mobile terminal and storing their location updates in the routing table. The DMAPwSR which is analyzed based on stochastic Petri net performs better HMIPv6 in terms of network communication overhead which resulted in saving cost of communication per time unit per user because of the selection of best DMAP service area.
The limitations of the DMAPwSR include the minimal functionality of the smart router hence the need for making it feasible for all-IPv6 DMAP- compliant.
Crismani, A.; Babich, F.; and Hanzo, L.; (2010) designs a MAC scheme which adopts a cooperative physical layer aided cross-layer techniques based on the CoopMAC techniques of Liu et al., “which is improved by improved by facilitating cooperative signal combining at the destination and and employing two relays in the context of a successive relaying technique” (Crismani, A.; Babich, F.; and Hanzo, L.; 2010).
The algorithm for the selection of transmission rate which provided a fixed block error ratio (BLER) when decoding the frame received both at the source and the relay was amalgamated with the efficient likelihood ratio (LLR) combination at the destination of the direct and relayed components. This in turn decreases the probability of outage and raises the throughput of the network.
The framework which is evaluated by the Monte-Carlo simulations investigated the selection and activation of the two relays by the protocol, mitigated the multiplexing loss imposed by the half-duplex constraint of 802.11 stations using a successive relaying protocol relying the two relays. The result indicates that “invoking two relays has the potential of further increasing the network’s throughput gain” (Crismani, A., Babich, F.; and Hanzo, L.; 2010).
Di Caro, G.A.; et al; (2010) presents a novel design approach based on autonomic components and cross-layer monitoring and control for performance maximization of the WiOptiMo framework which provides a seamless roaming between networks by handling application layer mobility.
The framework which is in its introductory stage is yet to be concluded but it is aimed at presenting an innovative cross-layering and autonomic design for the forth-coming release of next WiOptiMo. From the preliminary results, the researchers intend to include the use of different type of traffic in the wireless network; to monitor other parameters to further boost the response of the scheme and to investigate the use of techniques for efficient smoothing the noisy RSS and to boost the robustness of handover initiation decisions by calculating measures to be used in cross-validation with the RSS.
Wang, Q.; and Abu-Rgheff, M.A.; (2010) presents IP-based multi-layer advanced mobility management architecture to take advantage of the various contributions of the single and cross-combined TCP/IP layers by identifying and abstracting the contribution of each layer to the various functions supporting mobility management. The architecture though supports co-ordinated mobility management of different levels for various types of mobility for heterogeneous networks and also facilitates such functions as fast/seamless handoffs, QoS adaptation due to changes in contexts as a result of mobility; it requires active cross-layer interactions to improve its performance.

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