Peer to Peer IS NOT A MESH!

Misconceptions are my new obsession, and there are loads in the book.

The biggest misconception in the Networks topic is confusion between topology and “service model”. Here are some typical questions from Facebook that illustrate the confusion:

  • what is the difference is between a mesh network and peer to peer network?
  • Would you give marks for star topology advantages being central backup and security. Or is that not right because it is not referring to the actual cable layout?
  • It seems to be common to conflate Star with Client Server and Mesh with Peer to Peer, is this OCR stance?

Peer to Peer and Client-Server are two service models which are unrelated to the topology (star, mesh, ring, bus etc.). These service models are a logical description of the network, while topology is the physical description.

A network has a physical arrangement of devices and connections, and this is its topology. Independent of the topology is the way files and programs are shared, and this is the service model, or sharing model, which can be peer-to-peer or client-server.

The topology matters when we are discussing how packets travel around the network, which devices they pass through and how they get to their destination. But this is irrelevant when we are talking about how files are shared, because the user does not care about the journey the packets take when copying a file from another device or saving a file on the server. So a Star topology can support a peer-to-peer model of network, just as a mesh topology can support a client-server network (or vice versa).

The confusion arises partly because of unclear images in textbooks, which appear to show a mesh network described as a peer-to-peer model, and a star network as client-server, and we can see this on the Wikipedia page for P2P here. Confusing as these images may be, the authors are trying to illustrate the logical relations between devices rather than their physical connections, so the images are not wrong. However, learners (and indeed teachers) may be more used to seeing diagrams of the physical connections – the topology – of a network, and therefore unused to seeing the logical relationships described the same way. In short, the diagrams look exactly like topology diagrams, even though they are intended to convey the logical service model.

I discuss this misconception and many more in “How to Teach Computer Science”, out in June 2021 from John Catt Ed.

By mraharrisoncs

Head of Computing, Manchester.

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