Rotary and Dual Tone Multi-Frequency

Explain the difference between:

1. Rotary and Dual Tone Multi-frequency (DTMF)

Phone key pads are actually a somewhat recent invention. Until the 1970's, the rotary dial was in widespread use. Numbers were dialed by turning and then releasing a disk on the front of the phone. When the disk was released, a spring mechanism inside the phone returned the disk to its original position while generating a certain number of pulses at the proper rate.

Since then, rotary dial phones have largely been replaced by touch dial phones which have calculator-like keypads. To dial a number, buttons are pressed on the keypad and the phone then sends the corresponding DTMF tones to the phone company. DTFM tones are a series of sounds, which consist of two "pure" tones and convey the phone number to the telephone network. They consist of two pure tones. The sounds are sent to the phone company the same way the phone conversation would be.

2. Wide Area Telecommunications Service (WATS) and leased lines

WATS (wide-area telephone service) is a specialized form of fixed-rate long-distance telecommunication service. WATS lines are commonly used by businesses and government agencies. It seems that WATS is basically a toll service for dial-type telecommunications between a given customer [user] station and stations within specific geographic rate areas using a single access line between the customer [user] location and the serving central office. Each access line can be configured for either outward (OUT-WATS) or inward (IN-WATS) service, or both.

With most WATS lines, calling-zone restrictions apply. For example, it might not be possible to make or accept WATS calls to or from locations within the state where the subscriber is located, or to or from locations outside the country where the subscriber is located. In other words, with "outward WATS" it is possible to make an unlimited number of long distance calls (toll calls), for a fixed price, within pre-determined time and distance constraints. With "inward WATS," subscribers are issued a toll-free telephone number, typically beginning with a designated toll-free area code such as "800" or "888," and anyone may call that number without having to pay a toll charge. The recipient pays for the calls at a flat rate or other predetermined rate. "Inward WATS" service is available with Automatic Number Identification, a form of super-Caller ID.

Conversely, a leased line connects two locations for private voice and/or data telecommunication service. It is not a dedicated cable, but actually a reserved circuit between two points. Leased lines can span short or long distances. They maintain a single open circuit at all times, as opposed to traditional telephone services that reuse the same lines for many different conversations through the process of "switching." Leased lines are most commonly used by businesses to connect branch offices, because these lines guarantee bandwidth for network traffic.

3. Private Branch eXchange (PBX) and Centrex

A PBX (Private Branch Exchange) is a telephone switch owned by a company or organization. These organizations purchase PBX's to reduce the total number of telephone lines they need to lease from the telephone company. Without a PBX, a company will need to lease one telephone line for every employee with a telephone. With a PBX system, the company only needs to lease as many lines from the telephone company as the maximum number of employees that will be making outside calls at one time. In a PBX system, every telephone is wired to the PBX. When an employee picks up the telephone and dials the outside access code (usually 9), the PBX connects the employee to an outside line.

On the other hand, Centrex (central office exchange service) is a service from local telephone companies in the United States in which phone facilities at the phone company's central (local) office are offered to business users so that they don't need to purchase their own facilities. The Centrex service effectively partitions part of its own centralized capabilities among its business customers. The customer is spared the expense of having to keep up with fast-moving technology changes, for example, having to continually update their private branch exchange infrastructure. It appears to me that for the most part Centrex has now replaced the private branch exchange.

4. The layers