Geir Egeland

The availability and reliability of wireless multi-hop networks with stochastic link failures

The network reliability and availability in wireless multi-hop networks can be inadequate due to radio induced interference. It is therefore common to introduce redundant nodes. This paper provides a method to forecast how the introduction of redundant nodes increases the reliability and availability of such networks. For simplicity, it is assumed that link failures are stochastic and independent, and the network can be modelled as a random graph. First, the network reliability and availability of a static network with a planned topology is explored. This analysis is relevant to mesh networks for public access, but also provides insight into the reliability and availability behaviour of other categories of wireless multi-hop networks. Then, by extending the analysis to also consider random geometric graphs, networks with nodes that are randomly distributed in a metric space are also investigated. Unlike many other random graph analyses, our approach allows for advanced link models where the link failure probability is continuously decreasing with an increasing distance between the two nodes of the link. In addition to analysing the steady-state availability, the transient reliability behaviour of wireless multi-hop networks is also found. These results are supported by simulations.

Internet connectivity for multi-homed proactive ad hoc networks

A prerequisite for a widespread and successful deployment of proactive ad-hoc networking technology is its ability to provide easy access to the Internet. Normally, proactive routing protocols provide routing messages that establish default routes to ensure connectivity for outgoing IPv4 packets destined for the Internet. However, mechanisms to ensure connectivity for incoming IPv4 packets from the Internet are yet poorly documented in published material. Possible solutions include implementing a modified mobile IPv4 foreign agent (MIP-FA) or network address translation (NAT) on each Internet gateway node in the ad hoc network. In this paper we discuss different strategies for providing Internet access to proactive ad hoc networks. We also describe problems experienced in our lab test-bed with default routes under the condition of site multi-homing. Based on this experience, we propose working solutions for Internet access from proactive ad hoc networks.

Name resolution in on-demand MANETs and over external IP networks

Common user applications cannot run in mobile ad-hoc networks (MANETs) before a method for name resolution is in place. While the domain name system (DNS) works well on the fixed Internet, it represents a centralized approach to name resolution, which is not suitable for MANETs. This article refines the framework developed for name resolution in MANETs and shows a mechanism for local name resolution can be implemented transparently to the application. It also proposes methods to align local name resolution on the MANET with DNS over external IP networks, forming one integrated method for name resolution. Finally, different strategies for how to store name-to-address mappings on the MANETs are discussed.

The reliability and availability of wireless backhaul mesh networks

When planning the structure of a backhaul mesh network for public access, it is common to introduce redundant nodes in the mesh network. These are called redundant, because they do not increase the network capacity under normal operation, due to the shortest-path metric of the routing protocol. Instead, their sole purpose is to increase the network reliability by providing failover links when a link in the shortest-path fails. This paper shows how to estimate the additional network reliability that results from introducing a redundant node. The best location of a redundant node, i.e. where to locate the node in order to maximise the additional network reliability, can also be found.

NAT-Based Internet Connectivity for On-Demand Ad Hoc Networks

A prerequisite for widespread and successful deployment of on-demand ad-hoc networking technology is its ability to provide easy access to the Internet. Existing solutions for Internet access are mainly based on modifying Mobile IPv4 (MIPv4). An easier approach, yet poorly documented in published material, is to implement Network Address Translation (NAT) on Internet Gateway nodes in the ad hoc network. In this paper we describe problems experienced in our lab test-bed with NAT-based solutions under the condition of site multi-homing. Based on this experience, we propose a working solution for multi-homed ad hoc networks.

A Model for the Loss of Hello-Messages in a Wireless Mesh Network

The links in an ad hoc or wireless mesh network are normally kept alive by the exchange of Hello-messages between neighbouring nodes. These Hello-messages are prone to collisions with traffic from hidden nodes. If several Hello-messages are lost due to overlapping transmissions, the node expecting the Hello-messages erroneously assumes that the link is down. This is called an apparent link failure. This paper provides an analytical model for the loss of Hello-messages in a mesh network. Knowing the probability of a lost Hello-message, the probability of apparent link failures can easily be found, which can further be used in reliability analysis of wireless mesh networks.

The Economy of Redundancy in Wireless Multi-Hop Networks

Due to frequent link failures in multi-hop wireless networks, redundancy can be an important feature. For example, when planning the structure of a backhaul mesh network for public access, it is common to introduce redundant nodes in the mesh network. These are called redundant, because they do not increase the network capacity under normal operation, due to the shortest-path metric of the routing protocol. Instead, their sole purpose is to increase the network reliability by providing failover links when a link in the shortest-path fails. This paper shows how to estimate the optimal number of redundant nodes for a given topology. In order to do so, a method to calculate the additional network reliability that results from introducing a redundant node to a given topology is also proposed.

Could Proactive Link-State Routed Wireless Networks Benefit from Local Fast Reroute?

The communication performance in wireless networks is often heavily influenced by failures caused by node mobility and radio disturbance. Proactive link-state routing protocols like OLSR and OSPFv3 with wireless extensions (WOSPF) are currently relying on slow re-convergence to restore successful packet forwarding. This may not be appropriate for particular applications, and it may cause instability during transient failures. Support for fast and local reroute should therefore be added as a feature to proactive link-state based routing protocols. In this paper, we discuss the challenges and requirements that are associated with such solutions. In addition, we evaluate and discuss to what extent recent developments for fixed IP routing can serve as solutions.

Investigating race conditions in multi-homed on demand ad hoc networks

A prerequisite for a widespread and successful deployment of on-demand ad-hoc networking technology is its ability to provide easy access to the Internet. Existing solutions for Internet access are mainly based on modifying mobile IPv4 (MIPv4). An easier approach, yet poorly documented in published material, is to implement network address translation (NAT) on Internet gateway nodes in the ad hoc network. In this paper we describe race conditions experienced by simulations of common NAT-based solutions when the network is multi-homed. Based on our results, we propose a working alternative solution for multi-homed ad hoc networks.

The Performance of Wireless Mesh Networks with Apparent Link Failures

A wireless multi-hop network is a network consisting of a group of nodes interconnected by the means of wireless links. The nodes in such a network, which are often self-configured and self-organized, communicate with each other over multiple hops through a routing protocol. Examples of such networks include Wireless Mesh Networks (WMNs) IEEE802.11s (2010), Mobile Ad Hoc Networks (MANETs) Chlamtac et al. (2003) and Wireless Sensor Networks (WSNs) Gharavi & Kumar (2003). The performance and the reliability of these networks depend heavily on the routing protocol’s capability to detect link failures between neighboring nodes as well as its link-maintenance mechanism to recover a path from source to destination when a link-failure happens. While MANETs generally appear more dynamic due to node mobility, the network topology for WMNs and WSNs remains comparatively stable. No matter which network form is concerned, however, these networks exhibit ad hoc features since wireless links are intrinsically unreliable. In the majority of cases, link failures are present in a multi-hop network regardless of the use of link-maintenance mechanisms. Sometimes link failures are unavoidable, such as when a mobile node deliberately leaves a network or is subject to the exhaustion of its battery power. In another case a link would cease to be operative when two nodes move outside each others’ radio transmission range. In addition to these, a set of link failures which we refer to as apparent link-failures exist. They are primarily caused by radio links being vulnerable to radio induced interference, but also appear when a link-maintenance mechanism erroneously assumes a link to be inoperable due to loss of beacons. A beacon is a short packet transmitted periodically to a node’s one-hop neighbors and its purpose is to detect neighbors and to keep links alive. Beacons are normally broadcast, and are thus not acknowledged, i.e. they are unreliable and vulnerable to overlapping transmissions from hidden nodes Tobagi & Kleinrock (1975). Moreover, common protection mechanisms against hidden nodes (such as RTS/CTS of the IEEE 802.11 MAC protocol IEEE802.11 (1997)) are not applicable, since unicast data transmission using RTS/CTS will only provide protection for packet reception at the node that issued the CTS.