Kinds of network
The same mathematics is applied to very different systems. What differs is what the nodes are, what flows along the links, and how quickly the network itself changes.
| Kind | Nodes and links | What is studied |
|---|---|---|
| Communication networks | Routers, switches and the cables or radio links between them | Routing, congestion, delay, survival of failures |
| Transport and infrastructure | Junctions and roads; stations and track; airports and routes; substations and power lines | Flow, bottlenecks, cascading failure |
| Social networks | People, and the ties of kinship, friendship or work between them | Influence, the spread of ideas and habits, communities |
| Information networks | Documents and the references between them: web pages and hyperlinks, papers and citations | Ranking, search, how knowledge accumulates |
| Biological networks | Neurons and synapses; proteins and their interactions; species and who eats whom | How function and stability arise from wiring |
| Economic and financial networks | Firms, banks and countries, linked by supply, lending or trade | How shocks and defaults propagate |
| Contact networks | People and their physical encounters | Outbreaks, and whom to vaccinate first |
| Peer-to-peer networks | Equal participants that connect directly, with no central server | Finding data with no directory; agreement among strangers |
Why one subject can cover all of these
A road map, a food web and the wiring of a worm’s nervous system can be written down the same way, as a list of nodes and a list of links, and the same theorems then apply to each. The similarity has limits: what flows differs — cars queue, electricity obeys physical law, rumours change as they travel — and conclusions reached on one kind of network have to be checked again on another.