Ad hoc network
An ad hoc network ( Latin ad hoc , meaning “made for this moment”) is a radio network with radio nodes and radio links that connect the radio nodes in a meshed network .
Networks that build up automatically and configure, is also called autonomous ad hoc networks ( english mobile ad hoc network , MANet ) or mesh networks (Engl. Mesh [ mɛʃ ] "scam" or "network").
functionality
Ad-hoc networks connect fixed or mobile devices ( network nodes ) such as
- Sensors and actuators ,
- Cell phones and smartphones
- Personal digital assistants and notebooks
with one another and with other networks, and - unlike infrastructure networks - without an administrative infrastructure. Data are also passed on from network node to network node via several stations until they have reached their recipient, which means that the data load is distributed more advantageously than in star-shaped networks with a central node . Scarce resources such as computing time , energy and data rate require the network nodes to interact effectively.
An autonomous radio network usually does not use any wireless access points based on the WLAN standard. In contrast, networks that conform to common standards can be connected to other network types via gateways .
A modern ad hoc network always configures itself automatically and then works autonomously without an entity for network management . Special routing procedures ensure that the network always adapts when nodes move, join or fail. The company uses special network protocols for the respective network type . The nodes of the network can be integrated in devices that are connected by means of the network protocol, or they are designed as independent devices that are connected to the data sources and data sinks by cables .
Advantages and disadvantages
Advantages:
- if a terminal device fails, data communication is still possible by rerouting
- very powerful
- good load distribution
- low network costs
- no central administration
Disadvantage:
- comparatively complex routing necessary
- Storage of routing tables in each end device
- each end device works as a router and is therefore often active
- the end devices should remain switched on if possible
- higher power consumption in the end device.
For more details see meshed network .
Topologies
The structure of an ad-hoc network is specifically determined according to topologies that support the throughput in the network and the availability of the network.
Direct connection
The individual nodes see each other and can communicate with each other.
Indirect connection
The individual nodes are sometimes so far apart that they cannot communicate directly with one another. The nodes in between forward the data. In this way, a self-managing network of almost any size can arise.
Routing protocols
There are more than 70 schemes for routing packets through a mobile ad hoc or mesh network. Some of these are historically significant or compete with corresponding components for the network nodes from different manufacturers.
There is no uniform classification of the protocols, but a classification into reactive and proactive protocols is widespread in the specialist literature , which distinguishes how paths are found and exchanged in the network. Alternative classifications differentiate whether the geodetic position is used or how many recipients are addressed. Details on this in the following.
Routing procedure in general
Special routing protocols are used to enable targeted forwarding of data in a mobile ad-hoc network. These have the task of determining a path from the source to the destination node. Depending on the metric used , this should e.g. B. be as short as possible or use the least polluted regions of the network. Further requirements for the protocols are routing tables that are as small as possible, which have to be constantly updated when nodes disappear, move or new ones appear. The time and number of messages needed to find a route should be as short as possible.
Due to the special conditions in a mobile ad hoc network, the routing algorithms usually used on the Internet cannot be used. The main reasons for this are:
- Nodes have no prior knowledge of the topology of the network; they have to explore this themselves;
- no central instances for storing routing information;
- Mobility of the nodes and the associated constant topology change ;
- changing metrics of the transmission lines z. B. by interference ;
- Limited resources of the nodes (e.g. system performance, energy consumption).
Topology-based routing methods
The topology-based routing processes manage without geodetic information about the positions of the nodes in the mobile ad hoc network. Logical information about the neighborhood relationships of the nodes is sufficient for them, i.e. which nodes have a direct connection or can connect via one or more intermediate nodes (hops). These neighboring nodes can communicate with one another. The topological information is usually obtained by sending so-called HELLO packages. Depending on the time at which the topology database was set up, it is either proactive or reactive routing . An example of a protocol from this class is the Neighborhood Discovery Protocol (NHDP), which uses elements of the Optimized Link State Routing Protocol (OLSR).
Proactive procedures
Proactive routing processes determine the paths to be used between two nodes before they are required for the transmission of user data. If user data are then to be sent, there is no need to wait for the path to the destination node to be determined. However, it is disadvantageous that these methods send many control packets even without traffic of user data in order to determine paths that may not be required later. An example of a protocol from this class is the Optimized Link State Routing Protocol (OLSR).
Reactive process
In contrast to the proactive method, reactive routing methods for mobile ad hoc networks only determine the required paths between two nodes when user data is to be transmitted. This means that the first data packet of a connection can only be sent with a delay, since it is first necessary to wait for the route determination to be completed. For this, however, control packets are only sent when user data is sent and this is necessary for route determination. This has a positive effect on the energy consumption of the nodes. The Ad-hoc On-demand Distance Vector (AODV) protocol is an example of a protocol in this category.
Hybrid processes
Hybrid methods combine proactive and reactive routing methods. The aim is to achieve the goal of combining the advantages of both approaches in a new routing protocol. For example, a proactive approach can be used in a localized area while a reactive approach can be used for more distant targets. This reduces the load on the network from control packets that would be sent over the entire network in a purely proactive procedure. In spite of this, paths are immediately available for local targets without having to wait for them to be determined as with a purely reactive method. Zone Routing Protocol (ZRP) is a routing protocol that implements this approach.
Position-based routing methods
Position-based routing methods use geodetic information about the exact positions of the nodes. This information is z. B. obtained via GPS receiver. This location information can be used to determine the shortest or otherwise best path between the source and destination nodes. An example of a position-based routing protocol is LAR .
Commercial importance
The market for components for ad-hoc networks is very large. It includes special component families for the industrial standards of the associated network protocol families, for example from
- Bluetooth of the Bluetooth Special Interest Group (SIG)
- ANT of the ANT Alliance
and many more.
standardization
The Internet Engineering Task Force (IETF) has a working group for the diversity of the proposed concepts , the MANET Working Group . This controls the publication of standards of the protocol families for such ad-hoc networks that are connected to the Internet. There is a special wiki to get you started.
history
In 2004, the topic of ad hoc networks was still academic; there were almost exclusively pilot projects . The takeover of MeshNetworks , a leading provider in the field of mobile ad hoc networks, by Motorola at the end of 2004 showed that the industry was expecting strong growth in markets.
Application examples
There are many uses for ad hoc networks. They are mostly operated in the license-free 2.4 GHz ISM band .
Roadcasting
In roadcasting , a mobile ad hoc network is used to broadcast an individualized radio program, but this software is still only a prototype.
On a large scale, as part of the non-profit project 100-Dollar-Laptop, the specially designed student laptops will be networked automatically via a mesh network. The Massachusetts Institute of Technology has installed a new type of protocol on mobile computers for this purpose.
See also
Web links
- Wireless LAN Mesh Whitepaper (PDF; 319 kB)
- IETF MANET group (English)
- Openwireless, open source mesh router project in Switzerland
- Freifunk, open source mesh router project in Germany
- FunkFeuer - free, experimental network in Austria
- Commotion Wireless - Encrypted Open Source Mesh Network
- Choice between infrastructure and ad hoc network mode