Many cross-layer solutions have been considered in literature, so that a complete classification results out of the scopes of this discussion. In the next section we follow the approach presented in [RI04], by providing some examples of cross-layer solutions involving Physical (PHY), Medium Access Control (MAC), Network (NET) and Transport (TRA) layers. In most of them, cross-layer feedbacks are used to enable
Chapter 2. Cross-Layer Design in Wireless Ad Hoc Networks 21
state information flow from upper to lower layers or viceversa, while the traditional layered structure is preserved.
PHY+MAC Cross-Layer Solutions. In a wireless network, each device has a transmission radius and an interference radius. The relation between the transmis- sion and interference radius depends on the underlying physical layer, and affects the contention level perceived at MAC level. In [NR03], the authors analyze the impact of the physical layer on the performance of MAC protocols, by demonstrat- ing the importance of physical layer parameters (SNR) in designing efficient MAC protocols. The authors of [CL06] propose a carrier-sensing scheme that use MAC state information to alleviate the problem of Hidden Terminals (HT) and Exposed Terminals (ET): the addresses of transmitter and receiver of a packet are incorpo- rated into the PHY header. Making use of address information for its carrier-sensing operation, a node can declare the channel busy or idle on the basis of transmitter- receiver pairs. The scheme described in [GCA06] attempts to mitigate the effect of EN and HN by using directional antenna: a novel MAC protocol (MAC-EDAMA) is proposed to exploit the directionality of the communication. A complete analysis of protocol harmonization between MAC and physical layer is investigated in [EW99], by focusing on the effects of packet length, transmit power and bit-error rate on system performance. The results shown in [EW99] demonstrate that the optimal transmit power is proportional to the packet length. Moreover, if the length of the packet is varied at MAC Layer accordingly with the current BER level, the energy consumption may be drastically reduced.
PHY+NET Cross-Layer Solutions. In [CP02], the authors evaluate the im- pact of the physical layer on the performance of five different routing protocols for MANETs. The results in [CP02] demonstrate that the performance obtained when physical layer properties such as path loss and shadowing are considered are drasti- cally different when compared with the results provided by a simple free propagation model. For these reasons, the authors conclude that the hop-count may not be an
22 Chapter 2. Cross-Layer Design in Wireless Ad Hoc Networks
Physical
Layer
Routing
Layer
MAC
Layer
Transport
Layer
MA C P roto col T unin g MA C/P HY Leve l Har mon izat ion Expl icit Link Faillu reN otifi catio ns A da ptive R ate a nd P ow er C ontro l M ultih op pe rfo rm ance o ptim iza tions B E R -a w a re ro u ti n gMAC/TCP Retransmission Enhancements
Figure 2.2: Cross-layer optimizations involving Physical, MAC, Routing and Transport Layers
optimal metrics for the routing process and that the routing metrics for MANETs should take into account the current state of the channel as well as the quality of each link. In [FMBT05], the AODV protocol is extended by considering the Bit Error Rate (BER) of each link in the route selection process: the resulting proto- col (named MAODV) leads to the selection of the route minimizing the end-to-end BER.
PHY+TRA Cross Layer Solutions. Power control can often influence the trans- mission rate of mobile nodes. In [Chi05], the authors examine the possibility to
Chapter 2. Cross-Layer Design in Wireless Ad Hoc Networks 23
enhance multi-hop communication in wireless ad hoc networks by balancing power control in the physical layer and congestion control in the transport layer. The authors present a distributive power control algorithm (JOCP) that couples with the original TCP protocols to increase the end-to-end throughput and energy effi- ciency of the network. The key idea of JOCP is that, during congestion periods, nodes will try to transmit packets faster at the bottleneck links by updating their transmission power. More specifically, at each time slot the transmission power at a transmitter i will increase proportionally to its packet queuing delay λ and will decrease proportionally to its current power level Pi. The analytical model described
in [Chi05] proves the convergence of this coupled system to the global optimum of
joint power control and congestion control, for both synchronized and asynchronous implementations.
MAC+NET Cross Layer Solutions. In [BDM02], the authors analyze the inter- action of the routing and MAC layer protocols on multi-hop MANETs topologies. Simulation results obtained in different scenarios confirm that routing protocols can significantly affect the performance of MAC protocols and viceversa. For exam- ple, the paths selected by the routing protocol directly affect the spatial contention among the involved nodes at the MAC layer. At the same time, the contention at the MAC layer can cause routing protocol to respond by initiating new route queries and route table updates. The authors conclude that it is not meaningful to con- sider MAC or routing protocols in isolation, and suggest that a cross-layer approach may produce effective enhancement to multi-hop communication in a MANET. Some recent works addresses the joint design of MAC and routing solutions for MANETs. A cross-layer design is investigated in [RS03]: a combined MAC and routing solution is illustrated aiming to obtain effective load balancing using max- imally node-disjoint routes and directional antennas. In [FB06, FB07b], a novel multipath routing scheme -called Concurrent Separate AOMDV protocol - for mo- bile ad hoc networks is proposed. The CS-AOMDV scheme extends the Ad Hoc On-demand Multipath Distance Vector (AOMDV) routing protocol, by introducing
24 Chapter 2. Cross-Layer Design in Wireless Ad Hoc Networks
a load-balancing scheme to concurrently distribute the traffic among the available paths. The traffic-path allocation scheme is based on cross-layer measurements of path statistics reflecting the congestion level of each path.
MAC+TRA Cross Layer Solutions. The inability of TCP to distinguish between packet loss caused by congestion and packet loss caused by other factors (mobility of nodes, wireless link fluctuations) is the main cause of the poor performance of TCP in multihop ad hoc networks [HAN03]. While several proposals in literature attempt to solve the problem by modifying the MAC level or the TCP in isolations, some other solutions explore joint strategies with a vertical calibration of MAC and TCP layers. In [MD02], the authors show that increasing the number of MAC retransmissions decreases the risk of TCP timeout, and improves the overall performance of the network.
NET+TRA Cross Layer Solutions. The Explicit Link Failure Notification (ELFN) scheme [HV02] is a a feedback based approach to handle route failures in MANETs. The ELFN technique is based on the interaction between the routing and transport layers: when a link failure is detected by a mobile router, an ELFN message is notified to the TCP sender, which responds by disabling its retransmission timers and enters a ”standby” mode. Ad hoc TCP (ATCP) [LS01] utilizes network layer feedback too. In addition to route failure notification, ATCP exploits Explicit Con- gestion Notification (ECN) messages from network layer which notify the occurence of congestion: upon reception of ECN, TCP congestion control is invoked normally without waiting for a timeout event. The ATCP protocol monitors the received ACKs to detect packet losses due channel errors. When three duplicate ACKs have been received, the ATCP protocol does not forward the third duplicate ACK but puts TCP in the ”persistent” state and quickly retransmits the lost packet from the TCP buffer. After receiving the next ACK, ATCP will resume TCP to the normal state [LS01].
Chapter 2. Cross-Layer Design in Wireless Ad Hoc Networks 25
Figure 2.3: Cross-layer architectures proposals in literature