Ad Hoc Networks

Wireless communication enables information transfer among a network of disconnected, and often mobile, users. Popular wireless networks such as mobile phone networks and wireless LANs are traditionally infrastructure-based, i.e. base stations, access points and servers are deployed before the network can be used. In contrast, ad hoc networks are dynamically formed amongst a group of wireless users and require no existing infrastructure or pre-configuration.

Infrastructure-based wireless network example Ad hoc network example
Infrastructure-based wireless network Ad hoc network

The dynamic and self-organizing nature of ad hoc networks makes them particular useful in situations where rapid network deployments are required or it is prohibitively costly to deploy and manage network infrastructure. Some example applications include:

  • Attendees in a conference room sharing documents and other information via their laptops and handheld computer;
  • Armed forces creating a tactical network in unfamiliar territory for communications and distribution of situational awareness information;
  • Small sensor devices located in animals and other strategic locations that collectively, monitor habitats and environmental conditions;
  • Emergency services communicating in a disaster area and sharing video updates of specific locations among workers in the field, and back to headquarters.

Unfortunately, the ad hoc nature that makes these networks attractive also introduces many complex communication problems. Although some of the first ad hoc networks were deployed in the early 1970's, significant research problems remain unanswered.

Evolution of Ad Hoc Networks

The Early Influence of Military Applications

The initial development of ad hoc networks was primarily driven by military applications, where rapid network formation and survivability are key requirements. Relying on a system centralized around base stations is simply not an option because the base stations must first be deployed in the correct location (almost impossible in a hostile environment) and the network is subject to failure if one or several base stations are destroyed. On the other hand, a distributed network architecture, with all nodes having equal responsibility and using broadcast radio, is ideally suited to the military requirements. To overcome the limited radio transmission ranges (i.e. not all nodes are within the range of every other node), nodes are equipped with the ability to forward information on behalf of others, i.e. multi-hop communications. Combined with packet switching technology (hence the term packet radio networks, which is often used interchangeably with ad hoc networks; other synonyms and related terms include: mobile ad hoc networks, wireless ad hoc networks, self-organizing networks, multi-hop wireless networks and mesh networks) and suitable medium access control protocols, multi-hop communication provides the basis for resilient, large-scale military ad hoc networks.

The US Department of Defence, in particular DARPA, played a key role in the development of, and hence fostering research in ad hoc networks, with the Packet Radio Network (PRnet) being deployed in 1972, followed by an updated network, Survivable Radio Network (SURAN), developed in 1983 and several ad hoc networks developed under the Global Mobile (GloMo) Information Systems program in 1994. Recent demonstration and production networks include the US Army Tactical Internet (TI) in 1997 and the Extending the Littoral Battlespace Advanced Concept Technology Demonstration (ELB ACTD) used by the US Marines in 1999.

In Australia, DSTO has led major research and development projects, in conjunction with local universities, on ad hoc networks, including the Packet Structures Research for Radios project in 1991, Self-Organising and Adaptive Links and Networks in 1993 and, more recently, Military Ad Hoc Wireless Networks (2001) and Routing in Military Ad Hoc Networks (2003).

Ad Hoc Networks and the Internet

The growth of the Internet in the early 1990's, combined with cheaper computing and radio equipment (e.g. IEEE 802.11 wireless LAN and Bluetooth devices), saw an increasing focus of the research and engineering community on ad hoc networks. In 1997, the Internet Engineering Task Force (IETF) established the Mobile Ad Hoc Networks (MANET) Working Group to create and standardize new routing protocols (i.e. rules for finding a path between two Internet nodes) that cope with the dynamics and multi-hop paths present in ad hoc networks. Today, not only are there the handful of routing protocols published by the MANET WG, but more than 50 protocols proposed for varying scenarios.

With publicly available routing protocols and nearly all new laptops and handheld computers being equipped with wireless capabilities, businesses are starting to realise the potential of commercial ad hoc network applications. Companies such as Mesh Networks, Green Packet, PacketHop and Firetide are offering products and solutions based on ad hoc networking technology, with applications such as Law Enforcement, Intelligent Transport Systems, Community Networking and Home Networks in mind.

However, the commercial technology available today is still a long way from the full potential of ad hoc networks. Fundamental problems must still be solved before ad hoc networks can fully enable a ubiquitous computing and communications environment.

Key Research Challenges

From a communications perspective, the main characteristics of ad hoc networks include:

  1. Lack of pre-configuration, meaning network configuration and management must be automatic and dynamic.
  2. Node mobility, resulting in constantly changing network topologies.
  3. Multi-hop routing.
  4. Resource limited devices, e.g. laptops, PDAs and mobile phones have power and CPU processing constraints.
  5. Resource limited wireless communications, e.g. reduced to 10's of kilobits per second by the fact that many nodes must share the radio medium.
  6. Potentially large networks, e.g. a network of sensors may comprise thousands or even tens of thousands of mobile nodes.

A key research challenge in ad hoc networks is to increase the efficiency of information transfer, while handling the harsh environmental conditions such as energy constrained and highly mobile devices. Advances in wireless communications technology are required to overcome the limitations inherent of broadcast radio networks. In addition, routing and transport protocols (e.g. TCP/IP) must be made more intelligent such that communication paths avoid nodes low on resources (e.g. low battery power).

A second challenge is enhancing the usability of ad hoc networks to support future commercial applications. With no prior configuration of network services, nor any central authority, basic tasks expected of a computer communications network become more complicated. Securing the network is perhaps the most difficult task. Supporting interactive voice and video applications will only be possible if some control of service quality is available. Finally, it is necessary to develop middleware services that hide the complexities of the ad hoc network from application programmers.

Specific projects may include:

  • Smart antennas and diversity techniques for ad hoc networks
  • Fundamental limits of ad hoc networking performance
  • Power aware networking protocols
  • Reliable multipath transport protocols
  • Middleware services for minimal configuration ad hoc networks
  • Quality of service control for multimedia applications
  • Traffic security and user access control in ad hoc networks

More Information

Australian Ad Hoc Network Researchers

Abbosh, Amin M
Abolhasan, Mehran
Bhaskaran Pillai, Sibi Raj
Blackmore, Kim Louise
Boulis, Athanassios
Chan, Terence Ho Leung
Chiera, Belinda Ann
Conder, Phillip
Dadej, Arkadiusz (Arek) J
Gao, Jason
Gondal, Iqbal
Herborn, Stephen Robert
Jamalipour, Abbas
Jayasuriya, Aruna U
Jones, Haley M
Karmakar, Nemai
Khan, Jamil Yusuf
Kibria, M. Rubaiyat
Lee, Ivan
Li, Yonghui
Lin, Zihuai
Mao, Guoqiang
Perreau, Sylvie L
Portmann, Marius
Rakotoarivelo, Thierry
Ros, Montserrat Beverley
Sakhaee, Ehssan
Sekercioglu, Ahmet
Seneviratne, Aruna
Shu, Feng
Tang, Zhongwei
Thanabalasingham, Thayaparan
Yi, Xun
Yu, Limin
Zaidi, Zainab Razia
Zhang, Jian Andrew

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