Communications = sending/transferring information
Telecommunications (or Telecom for short) is a very generic term for communicating over a distance. This may be one-way (example: watching TV) or two-way (ex: phone call).
The very basic aspect of telecom is known as "POTS" or Plain Old Telephone Service (or Plan Ordinary Telephone Service). First patented & promoted by Alexander Graham Bell in the late 1800s, it has grown into a complex world-wide network. POTS is still around today and is probably the most common interconnection between people. In the US, the vast majority of people have acess to a telephone, with most having telephone service where they live.
Basic POTS means you have a telephone that is connected to two thin, insulated metal wires twisted together (a "twisted pair") running through the walls to some point outside the residence. The telephone converts your voice into an electrical current that is modulated by the various frequencies in your speech and put onto the wires. This modulation is analog in nature and varies the electrical current with the intensity (strength) and pitch (frequency) of your voice.
That twisted pair (or simply a "pair" in common telco-speak) is then connected in some way to another set of wires that leads back to the telephone company (more generically, the "service provider"). In its simplest form, pairs from a group of telephone customers, or "subscribers," runs out to a single point from each customer premise (house, apartment, or other type of residence). From this single congregation point (a plastic "pedestal" or metal cabinet), these pairs are bundled together to form a larger cable that is buried or hung on poles that goes for some miles back to the Service Provider's (SP) facility. For telephone companies, this facility is known as a "Central Office" or CO. From the CO, these pairs -- each of which lead to a single subscriber -- are connected to electronic equipment known as "switches" which can connect one pair to any other pair coming in to it. Modern switches are computer-controlled electronic devices which provide not only the connection to other subscribers, but also provide services such as Caller ID (CID), voice mail, special ringing tones, and many other special features. There are many switches in a CO, depending on how many subscribers are in the area. The switches in a CO are all connected to each other so calling between the subscribers in the area served by that CO is accomplished simply by connecting one line to another.
To complete a call outside of the serving CO, there is another cable or set of cables that connects the various COs in a Service Provider's area. These are called "trunks" or "interoffice cables." Rather than having large bundles of pairs, these trunk lines carry "multiplexed" phone calls, with one twisted pair carrying 24 or more calls simultaneously by combining the signals from individual pairs in digital form. More on digitizing and multiplexing in a different section.
For some images on what a Central Office looks like, check out this link: http://davewhitmore.net/Images/central_office.htm
A typical telco pedestal.
Stay tuned for more...
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