While in wireless networks with infrastructure help a base station always encompasses all mobile nodes, this is not always the case in an ad hoc network. A destination not might be out of range of the source not transmitting packets.
Thus, routing is required to find a path between source and destination and to forward the packets properly. In wireless networks using an infrastructure, cells have been described. Within a cell, the base station can reach all mobile nodes without routing via broadcast. In the case of ad hoc networks, each mode must be able to forward data to other nodes.
Asymmetric links: If node A gets a signal from node B this does not tell anything about the quality of the link in a reverse way. B might receive nothing, have a low link, or even have a better link than the reverse way. Thus, routing data gathered for One Direction is of almost no use for the other direction. However, many routing algorithms for wired networks rely on the asymmetric scenario.
Redundant links: wired networks, too, have redundant links to survive link failures. However, that is only some redundancy in wired networks, which, additionally, is controlled by a network administrator. In ad hoc networks nobody controls redundancy, so there might be many redundant links up to the extreme of a completely meshed topology. Routing algorithms for wired networks can handle some redundancy, but a high redundancy can cause a large computational overhead for routing table updates.
Interference: In wired networks connections exist only when a wire exists, and connections are designed by network administrators. Links come and go depending on transmission features, one transmission might interfere with another one, and nodes might listen to transmissions of other nodes.
Interference thus produces new problems by unplanned links between nodes. If two close-by nodes send two transmissions, they might interfere and destroy each other. Interference might also help, routing on the other hand. A node can get the apology with the help of packets overheard.
Dynamic topology: the greatest problem for routing arises from the highly dynamic topology. In ad hoc networks, routing tables must somehow reflect these frequent changes in topology, and routing algorithms have to be adapted. Routing algorithms used in wired networks wood either react much too slowly or generate too many updates to reflect all changes in topology.
Routing table updates in fixed networks, for illustration, take a position every 30 seconds. This updating rate might be too weak to be beneficial for ad hoc networks. Furthermore, some algorithms rely on a complete picture of the whole network. Why this works in wired networks where changes are rare, it fails in the ad hoc network. The topology changes during the distribution of the current snapshot of the network, rendering the snapshot useless.
Considering all the additional difficulties in comparison to wired networks, the following observations concerning routing can be made for ad hoc networks with moving nodes:
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