From Academic Kids

The term Appliance has several different areas of meaning, all usually referring to a device with a narrow function:

  • One class of objects includes items that are custom-fitted to an individual for the purpose of correction of a physical problem, such as prosthetic and orthotic appliances.
  • A certain class of computer products, where the device is has a specific function, and limited ability to configure.

Networking of Home Appliances

There is an increasing trend to network home appliances together, and combine their controls and key functions. For instance, energy distribution can be managed more evenly so that when the washing machine is on, the oven can ge into a delayed start mode, or vice versa. Or, a washing machine and dryer may share information about load characteristics (gentle/normal, light/full), and synchronize their finish times so the wet laundry does not have to wait before being put in the dryer.

Computer Appliances

Traditionally, all computing functions were written as software applications running on top of a general-purpose operating system. The consumer (whether home computer user or the IT department of a company) bought a computer, installed the operating system or configured a pre-installed operating system, and then installed one or more applications on top of the operating system. An e-Mail server was just an e-Mail application running on top of Linux, Unix, Microsoft Windows, or some other opearting system, on a computer that was not designed specifically for that application.

Specialized applications have recently started to use a different model. Instead of installing firewall software on top of a general purpose computer/operating system, the engineers have built computers that are designed specifically for the task. This has taken three forms:

1) The vendor builds an ASIC, so there is no separate "software" or operating system. The device has a limited interface, usually terminal console or web-based, to allow some basic configuration by the consulmer. The manufacturer often has some way of accessing deeper configuration mechanisms.

2) The vendor uses or creates a general-purpose computer, and designs a new operating system that integrates the application into the operating system. Cisco's IOS is an example; the Unix-like operating system has firewall functions and network/firewall configuration commands built into it. Sometimes, the device is also sealed, so the consumer has no access to reinstall the operating system or replace it with another operaing system. The consumer may also be restricted to a small group of configuration commands, while the more deatailed and lower level functions of the operating system are only available to the vendor. The more this "locked down" approach is carried out, the closer this type of device comes to appearing like an ASIC device.

3) Off the shelf computers and operating systems are used, but the user interface and "box" are designed so the user cannot access anything on the computer, except for the application interface that the vendor has created. Since the underlying computing architecture is locked down and essentially invisible, it becomes difficult to discern that the device really functions on top of general purpose hardware and operating systems. Linux, and BSD to a lesser degree, has become the operating system of choice for this type of appliance.

Sometimes, these three techniques are mixed. For example, a VPN appliance might contain a limited access software firewall running on Linux, with an encryption ASIC to speed up VPN access.

Some appliances are solid state, while others use a hard drive to load an operating system. Again, the two methods might be mixed -- an ASIC print server might allow an optional hard drive for job queueing, or a Linux-based device may encode Linux in firmware, so that a hard drive is not needed to load the operating system.

The term "appliance" came to be applied to these devices because of their similarity to home appliances. Home appliances are generally "closed and sealed" -- not serviceable by the owner; in computer appliances, the hardware box is usually sealed and not repaiarable or upgradable to the user. Home appliances usually have a button or dial interface designed to allow the user to adjust its functions within a limited set of parameters; computing appliances have a limited user interface to configure the device within parameters allowed by the vendor, while underlying aspects of the device that support its function are only configurable by vendor technicians. A home applicance may have a motor, but the motor can only be used within the appliance's function, and not for other purposes; computer appliances that use a general purpose computer platform hide the operating system commands and functions from the user, and only expose the application interface.

Some examples of computer functions that are often available as appliances are:

  • firewall
  • encryption
  • VPN
  • e-Mail filters for spam and viruses, or S/MIME message management
  • file servers (network attached storage, or NAS)
  • GPS map/driving direction devices, especially when installed in a car
  • search engines (e.g., Google sells a search appliance incorpoarting its search technology for use with corporate computing networks and databases)

Some consider the PDA to be a form of appliance, since most consumers do not make use of them as genetral purpose computing platforms.


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