Breakdown: Network Layers

What this lesson covers

This lesson breaks down the most common problem a computer network solves: passing a message from computer A to computer B. The first layer is physical. With a mirror reflecting sunlight, a reflected ray is a one and a missed ray a zero, so the receiver needs a start and an end delimiter, an agreed time interval for each bit and, on a wire, a voltage convention, all agreed beforehand. The second layer is routing. It is not compulsory, since anyone listening on the medium hears the message, but it avoids broadcasting to everyone. Like a post box, a router reads the receiver's address and forwards the message. An address needs both an identity that never changes, which maps to the MAC address, and a current location, which maps to the IP address, and machines announce their IP address to nearby routers so messages can be forwarded hop by hop. The third layer is behaviour: once messages can be read and routed, A and B hold a conversation, which raises frequency, direction and shared context, as when a large file arrives in parts. The video maps these onto the OSI model: physical stays physical, routing spans data link, network and part of transport, and behaviour maps to the session layer.

• The core problem of networking is passing a message from computer A to computer B
• The physical layer needs start and end delimiters, an agreed time interval per bit and a voltage convention
• Routing is optional but useful: a router reads the receiver address and forwards the message, as a post box does
• An address needs an identity that never changes (MAC) and a location that can change (IP)
• The behavioural layer governs frequency, direction and context of a conversation and maps to the OSI session layer