BOX 3.1
Layers of Communications

1. Language Layer

• Natural language: complex syntax, complex semantics (whatever a human can say)

• Restricted verbal language (e.g., operating systems command language, air traffic control language): limited syntax, constrained semantics

• Direct manipulation languages: objects are "noun-like," get "verb equivalents" from manipulations (e.g., drag file X to Trash means "erase X"; drag message onto Outgoing Mailbox means "send message"; draw circle around object Y and click means "I'm referring to Y, so I can say something about it.")

2. Expression Layer

Most of these types of realization can be used to express statements in most of the above types of languages. For instance, one can speak or write natural language; one can say or write a restricted language, such as a command-line interface; and one can say or write/draw a direct manipulation language.

• Speaking: continuous speech recognition, isolated-word speech recognition

• Writing: typing on a keyboard, handwriting

• Drawing

• Gesturing (American Sign Language provides an example of gesture as the realization (expression layer choice) for a full-scale natural language.)

• Pick-from-set: various forms of menus

• Pointing, clicking, dragging

• Various three-dimensional manipulations—stretching, rotating, etc.

• Manipulations within a virtual reality environment—same range of speech, gesture, point, click, drag, etc., as above, but with three dimensions and broader field of view

• Manipulation unique to virtual reality environment—locomotion (flying through/over things as a means of manipulating them or at least looking at them)

3. Devices

Hardware mechanisms (and associated device-specific software) that provide a way to express a statement. Again, more than one technology at this layer can be used to implement items at the layer above.

• Keyboards (many different kinds of typing)

• Microphones

• Light pen/drawing pads, touch-sensitive screens, whiteboards

• Video display screen and mouse

• Video display screen and keypad (e.g., automated teller machine)

• Touch-sensitive screen (touch with pen; touch with finger)

• Telephone (audible menu with keypad and/or speech input)

• Push-button interface, with different button for each choice (like big buttons on an appliance)

• Joystick

• Virtual reality input gear—glove, helmet, suit, etc.; also body position detectors