Modes and switching

Modes of communication describe who can send, and when:

• Simplex: data flows in one direction only. A keyboard sends data to a computer, but never receives any; an IoT controller sends commands to appliances.
• Half-duplex: data flows both ways, but only one way at a time. With a walkie-talkie, one person speaks while the other listens, then they switch.
• Full-duplex: data flows both ways at the same time. On a telephone call, both people can speak and hear at once.

Switching decides how data finds its way through a network of many paths.

• Circuit switching: before any data is sent, a dedicated path is set up between sender and receiver, and all the data follows it. The path stays reserved for the whole conversation, even when nobody is speaking. Old telephone calls worked this way.
• Packet switching: the data is split into small packets, each with a header giving its destination and its place in the sequence. The packets travel independently, perhaps by different routes, and are reassembled in the right order at the destination. A channel is used only while a packet passes through it, so it can be shared by many users. The Internet works this way.