Introduction
Mobile cellular networks and the Internet have become indispensable parts of our lives and broadband communications on infrastructure-based networks have found widespread use. There is, however a need for a large number of infrastructure-less applications such as group communication during multi-party disaster recovery operations after human-made or natural disasters. In this book, we cover ad hoc networks as a general passageway to deploying customized systems that do not, in general, have a pre-configured infrastructure, and we generally design such networks for a specific purpose or application.
Despite research for decades, we witness an evolution in ad hoc networks. The development of new hardware (drones, uncrewed ground /air vehicles, lightweight sensors/actuators) and embedded software facilitate the evolution characterized by many new challenges.
In this chapter, we define ad hoc networks and exhibit some typical applications thereof. There are many design challenges that we review in this chapter after discussing the characteristics that distinguish ad hoc networks from infrastructure-based networks. Finally, we present an overview of the book chapter-by-chapter.
Ad-hoc networks focus on multi-hop communication. We can track its history back to 500 BC, where Persian King Darius the First had used intermittently located archers for messaging instead of mounted scouts [@murthy2004ad]. The King decreased end-to-end delay, almost 95% using this method. After more than 1900 years when hop-to-hop communication is born, ad-hoc wireless networks come again into the picture in Hawaii, with the project ALOHANet. ALOHANet and its next packet radio network (PRNET) project aimed to design a multi-hop wireless communication infrastructure in relatively large areas. [ietf]{acronym-label="ietf" acronym-form="singular+full"} [manet]{acronym-label="manet" acronym-form="singular+full"} working group started to develop an open-source standard using the [ip]{acronym-label="ip" acronym-form="singular+full"}.
::: mdframed The word "ad hoc" means "not regular or planned, but happening only when necessary" in the Cambridge dictionary. :::
::: definition The word "ad hoc" means "not regular or planned, but happening only when necessary" in the Cambridge dictionary. :::
The word "ad hoc" means "not regular or planned, but happening only when necessary" in the Cambridge dictionary. We will follow this course of definition in this book. An ad hoc network is a networked system that is designed for a particular application to accomplish a definite goal. Usually, the topology of the network cannot be preplanned. It does not have a preplanned infrastructure but can organize and implement its infrastructure in a self-organized fashion. It can establish a multi-hop topology where nodes help each other route packets from source to destination.
There are many types of ad hoc networks, such as mesh networks, mobile ad hoc networks, or vehicular ad hoc networks. The applications of ad hoc networks can be civilian or military.
Nodes that constitute the network may be of various capabilities and may have various types and levels of resources. Resources are power sources, computational units, and networking interfaces.
In wireless communication, there are two major approaches: cellular networks and ad-hoc networks. The differences between cellular and ad-hoc networks are summarized in Table 1.1{reference-type="ref" reference="tab:diff"}.
::: {#tab:diff}
| Cellular networks | Ad-hoc networks |
|---|---|
| Fixed infrastructure | Infrastructureless |
| Single-hop links | Multi-hop links |
| Guaranteed bandwidth | Shared radio channel |
| Centralized routing | Distributed routing |
| Reliable connection | Frequent de-linking under mobility |
| High cost and time of deployment | Quick and cost-effective deployment |
| Spatial frequency reuse | Dynamic frequency reuse |
| Easy synchronization | Bandwidth-required synchronization |
| High cost of network maintenance | Self-organized networks |
| Scenario-specific architecture | Adaptable architecture |
: Differences between cellular and ad-hoc networks [@murthy2004ad]. :::
Ad-hoc networks are significantly used in the emergency, disaster and army-tactical scenarios. Packets in such networks are forwarded between nodes themselves due to multi-hop nature of the architecture. In ad-hoc networks with flat topology, scalability is one of the important problems considering such multi-hop communication. Organization and management of the unlimited number of nodes dynamically (i.e., without any central controller) are other major challenges.
For instance, the size of a routing table directly depends on the number of nodes in a network. The management of the tables is getting harder with the increasing population of the network and eventually control traffic outnumbers the actual data traffic. Similarly, resource allocation and link scheduling to orchestrate overall communication are not easy to handle in the absence of the centralized control mechanisms. A number of techniques and protocols has been proposed to overcome those challenges for years.
