[attachment=11709]
A Bidirectional Routing Abstraction for Asymmetric Mobile Ad Hoc Networks
Abstract
Wireless links are often asymmetric due to heterogeneity in the transmission power of devices, non-uniform environmental noise, and other signal propagation phenomenon. Unfortunately, routing protocols for mobile ad hoc networks typically work well only in bidirectional networks. It presents a framework called BRA that provides a bidirectional abstraction of the asymmetric network to routing protocols. BRA works by maintaining multi-hop reverse routes for unidirectional links and provides three new abilities: improved connectivity by taking advantage of the unidirectional links, reverse route forwarding of control packets to enable off-the-shelf routing protocols, and detection packet loss on unidirectional links.
Introduction:
About Asymmetry & Routing Protocols:
A fundamental problem in mobile ad hoc networks is asymmetry. Asymmetric or unidirectional links arise in the network for several reasons: Devices transmitting with different powers explicitly cause unidirectional links. Even when the devices are transmitting at the same power, noise sources near a device that affects packet reception at that device more than others may create unidirectional links. Finally, other intractable factors such as barriers and environmental conditions that affect signal propagation also lead to asymmetry.
Asymmetric networks have fundamentally different connectivity than bidirectional networks. Two nodes may be connected through one or more unidirectional links requiring an alternative path in the reverse direction. Or worse, they may be connected in only one direction with no route in the reverse direction. Ignoring the unidirectional links, and routing solely on the bidirectional links, as many conventional routing protocols do, mitigates the problem but may instead prevent several connected nodes from communicating with each other.
Standard routing protocols often fail to function or function inefficiently in an asymmetric network. Some routing protocols (e.g., TORA [12]) were primarily designed for bidirectional networks and hence break down in the presence of unidirectional links. Several others (e.g., AODV [14]) function by avoiding the unidirectional links and routing data only along the bidirectional links. A few other protocols (e.g., DSR [8]) have the capability to include unidirectional links in their routes through expensive mechanisms that provide significantly decreased throughput in asymmetric networks.
In addition to the routing layer, unidirectional links also pose several problems at the lower layers such as the data page link and the MAC layers. Common MAC-level schemes for congestion avoidance (RTS-CTS) and packet loss recovery (ACKs) fail for unidirectional links. Moreover, other useful services such as detection of page link breaks and discovery of new neighbors provided by some MAC protocols become unavailable to the routing protocols.
Why Multi-hop Routing:
Routing solely on bidirectional links is highly unreliable; while bidirectional connectivity can often be quite good, it may deteriorate suddenly and cut-off several bidirectional routes leading to a poorly-connected network. Second, a substantial percentage of unidirectional links have short (one to three hops) paths connecting them in the reverse direction. Finally, inclusion of such unidirectional links with short reverse paths significantly increases the stability of the routes and leads to better connectivity overall, without significant overhead.