Standard network topologies rely on centralized devices, such as small cells, to establish a network within a fixed perimeter. These devices, or base stations, are then connected via a backhaul to the rest of the world. Figure 1 shows a generic client-server network architecture.
Figure 1: A client-server network
Ad hoc networks are different. They do not require a central access point and are based on a peer-to-peer architecture. Each device is at the same level in the network. All the devices in an ad hoc network communicate directly with each other, as shown in Figure 2.
Figure 2: A peer-to-peer network (ad hoc network)
For an ad hoc network to work, the devices must be close to each other. The network performance typically decreases as the number of users grows. Large ad hoc networks can also be difficult to manage.
Ad hoc networks are useful, however, in situations where a rapid and cheap deployment is needed (for example, in emergency situations where the wired backbone may have been destroyed by forces of nature). They are also very useful in situations like military operations, where troops in movement need to have a quickly deployed communication network. In fact, “ad hoc” is a Latin phrase meaning “for this” and can be interpreted as “for a specific purpose”. Ad hoc networks exist for a specific purpose and can disappear and restructure as needed, where needed. The fact that the network does not rely on a central device makes it versatile and resistant.
MANET stands for mobile ad hoc network and fits with our description of ad hoc networks except that the network elements are mobile. Take wirelessly connected cars for example, where cars moving on a freeway are networked together for security purposes such as alerting other cars if an accident is detected. Vehicular ad hoc networks (VANETs) are a subcategory of MANETs and are associated with the particular case of communications between vehicles. MANETs can be isolated – without connectivity to the external world – or connected to the Internet. Hybrids between dynamic and static members are also frequent, like in the case of cars connected to an immobile freeway component that is in turn connected to the Internet and relays traffic information to the rest of the world.
Ad hoc networks are not protocol-specific and may be deployed using standard protocols such as the 802.11 family.
Ad hoc networks are networks that can be deployed as needed and do not require an existing infrastructure. For this reason, they are very useful in contexts where networks need to be rapidly deployed without costs. Two downsides of ad hoc networks are that the devices must be in close range and are limited in number.