An intercom system is a two-way communication electronic device that contains circuitry for the purpose of transmitting and receiving audio and/or video transmissions.
With the advent of cellular communication, digital telephone systems, and other communication devices, the use of intercoms has declined. However, there are still lots of applications where these devices still work better than anything else. There are a very wide variety of systems available as defined below:
source Wireless Intercoms:
In applications where wires cannot be run, a wireless system is used.
There are a broad spectrum of two-way wireless communication devices
that include such devices as handheld radios, outdoor wireless callboxes,
as well as traditional desktop devices.
The range these units will transmit varies widely based on the wireless technology used.
When you can run wires between locations,
a wired system can give you a little more privacy and eliminate
possible interference from neighboring systems.
There are different wire types such as 2-wire shielded or Cat-5/6
that can be used with these systems.
If you are considering a video system, a wired unit may be the best solution.
http://oranltd.com/abstract-algebra-homework-solutions/ Video Intercom:
These typically have a unit that has a camera,speaker,
and a push button that goes at an entrance door, and an internal monitor unit
that can communicate with the entrance unit.
Some systems can have multiple video monitors internally.
Given the difficulty of sending video over the airwaves at very much
distance without interference, most available units are wired.
This type of system has a door unit at an entrance door
and then each apartment or flat has either one
or multiple units that can speak to the door and possibly see the visitor,
or the door unit rings a landline or cellular telephone number.
A visitor simply presses the correct button for the apartment
they want to speak to, and the tenant can speak back and then
press a button to unlock the door.