The challenges of disconnected delay-tolerant MANETs

Abstract

This article is concerned with the challenges associated with supporting communication in disconnected MANETs with such a sparse population of nodes and so little (or no) fixed infrastructure that the network graph is rarely, if ever, connected. The networks considered are autonomous and do not depend on established infrastructure. The disconnected nature and the lack of end-to-end connectivity between nodes mean that the communication must be delay-tolerant. We refer to such a networks as Disconnected Delay-Tolerant MANETs (DDTMs). In general the challenges associated with mobile computing are not new. However, issues in wireless communication such as low bandwidth, disconnections and high bandwidth variability are problematic and further exacerbated in DDTMs by little or no infrastructure, variable node population and lossy links. DDTMs additionally face challenges of mobility which is frequent and uncontrolled resulting in a highly dynamic topology and disconnected network graph. In addition, portability remains a challenge, where battery power, memory and processing power are limited. The challenges are presented using a two-tier classification scheme that allows their causes and inter-dependencies to be mapped.

Introduction

The increased popularity of mobile computing and communication devices, such as cell phones, laptops and handheld digital devices such as Personal Digital Assistants (PDAs), means that wireless networks are increasingly the most convenient solution for interconnection in many usage scenarios. Since the early 2000s mobile devices have been getting smaller, cheaper and more convenient to carry, with the ability to run applications and connect to network services [20]. Currently, most of the connections among wireless devices are achieved through fixed infrastructure service providers or private networks. For example, since the 1980s mobile phones have been connected by cellular networks, and the connection of laptops to the Internet via wireless access points has grown rapidly in popularity in the early 2000s [20]. Current developments, such as 3G and 4G phones, show little signs of change in this trend. While infrastructure-based networks provide an effective mechanism for mobile devices to get network connectivity, setting up the necessary infrastructure can be time consuming and incurs potentially high costs. There are situations where networking connections are not available in a given geographic area, and providing connectivity and network services in these situations becomes a real challenge. Examples range from wildlife tracking and habitat monitoring sensor networks, military networks, inter-vehicle communication, disaster response networks, inter-planetary networks to nomadic community networks. For this reason, alternative ways to deliver services in disconnected environments have been emerging. Two such areas include MANETs which arose in the 1990s, and more recently Delay-Tolerant Networks (DTNs) which were first introduced in 2001.