Different Kinds of Communication Devices

The telephone has been a fact of life for more than a hundred years. A magnetic sensor in the mouthpiece of the telephone moves in response to pressure waves — the sound of your voice. That moving magnetic sensor creates an electrical signal. In a traditional landline, the signal travels down wires to another telephone where the changing electrical signal creates a magnetic field which moves a diaphragm up and down. The diaphragm creates pressure waves — sound — and long-distance communication is possible.

Cellphones also use magnets to convert sound to an electrical signal, but instead of sending that signal along a wire, cellphones turn that sound into a radio signal. The radio signal is picked up by a radio transceiver on a cell tower. Each tower has its own coverage area — its "cell." As long as the phone is within range of a cell tower the signal can be picked up and sent to any other phone in the world.

To function, your cellphone needs to be in the vicinity of a transceiver. But there's another type of mobile phone that doesn't need to be near any towers, because it communicates directly with satellites in orbit above the Earth. There is more than one provider of satellite telephone service, but the industry leader is Iridium. The Iridium system consists of an interconnected network of 66 satellites that provide global service. That is, from the South Pole to the Amazon River basin, your Iridium phone will let you communicate with anyone around the world.

Each of the telephone types described above has its own particular method of transmitting and receiving signals, but behind each of them is an interconnected global network. Signals from cellphones, landlines, possibly even satellite phones, all end up travelling through the same network: a complex and sophisticated combination of wires, fiber optics, and microwave and radio transmitters. The network is just full of communication devices that you'll never see: routers, optical-to-electronic converters, add/drop multiplexers and many more. The key to global interconnectivity is that each different style of telephone can tap into the network, which allows billions of people to reach each other.

Computer communications — email, Twitter, Facebook, web searching — use the same kind of network. In fact, the networks are converging and merging into an interconnected monolith. That's one reason why the phone company now offers television service, the cable company offers telephone service, and your Internet provider wants you to buy television and telephone from them. Any device that can tap into that network — your cellphone, your tablet, your laptop — can conceivably offer all the services available through that network.

The previous sections outlined various options for telecommunications, reaching people well beyond your vicinity. There is an entire class of devices designed to help people communicate with others right next to them. These devices are called augmentative and alternative communication, or AAC, devices, and they're intended to help people with communication disabilities interact with the people around them. Some AAC devices simulate speech and some display messages, and different input styles fit different people's needs.