Telemarketing (known as telesales in the UK and Ireland) is a method of direct marketing in which a salesperson solicits prospective customers to buy products or services, either over the phone or through a subsequent face to face or Web conferencing appointment scheduled during the call. Telemarketing can also include recorded sales pitches programmed to be played over the phone via automatic dialing. Telemarketing has come under fire in recent years, being viewed as an annoyance by many.
The public switched telephone network (PSTN) also referred to as the Plain Old Telephone Service (POTS) is the network of the world’s public circuit-switched telephone networks. It is a worldwide net of telephone lines, fiberoptic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables connected by switching centers, which allows any telephone in the world to communicate with any other. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed telephones.
The technical operation of the PSTN utilises standards created by the ITU-T. These standards allow different networks in different countries to interconnect seamlessly. There is also a single global address space for telephone numbers based on the E. 163 and E. 164 standards. The combination of the interconnected networks and the single numbering plan make it possible for any phone in the world to dial any other phone. A public switched data network (PSDN) is a publicly-available packet-switched network, distinct from the PSTN. Originally this term referred only to Packet Switch Stream (PSS), an X. 5-based packet-switched network, mostly used to provide leased-line connections between local area networks and the Internet using permanent virtual circuits (PVCs). Today, the term may refer not only to Frame Relay and Asynchronous Transfer Mode (ATM), both providing PVCs, but also to Internet Protocol (IP), GPRS, and other packet-switching techniques. Whilst there are several technologies that are superficially similar to the PSDN, such as Integrated Services Digital Network (ISDN) and the Digital Subscriber Line (DSL) technologies, they are not examples of it.
ISDN utilizes the PSTN circuit-switched network, and DSL uses point-to-point circuit switching communications overlaid on the PSTN local loop (copper wires), usually utilized for access to a packet-switched broadband IP network. Asymmetric Digital Subscriber Line (ADSL) is one form of the Digital Subscriber Line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice telephone call. 1] A splitter, or DSL filter, allows a single telephone connection to be used for both ADSL service and voice calls at the same time. ADSL can generally only be distributed over short distances from the central office, typically less than 4 kilometres (2 mi), but has been known to exceed 8 kilometres (5 mi) if the originally laid wire gauge allows for farther distribution. At the telephone exchange the line generally terminates at a Digital Subscriber Line Access Multiplexer (DSLAM) where another frequency splitter separates the voice band signal for the conventional phone network.
Data carried by the ADSL are typically routed over the telephone company’s data network and eventually reach a conventional Internet Protocol network. Digital Subscriber Line (DSL) is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term Digital Subscriber Line is widely understood to mean Asymmetric Digital Subscriber Line (ADSL), the most commonly installed technical variety of DSL.
DSL service is delivered simultaneously with regular telephone on the same telephone line. This is possible because DSL uses a higher frequency. These frequency bands are subsequently separated by filtering. The data throughput of consumer DSL services typically ranges from 384 KB/s to 20 Mbit/s in the direction to the customer (downstream), depending on DSL technology, line conditions, and service-level implementation. In ADSL, the data throughput in the upstream direction, (i. e. in the direction to the service provider) is lower, hence the designation of asymmetric service.
In Symmetric Digital Subscriber Line (SDSL) service, the downstream and upstream data rates are equal. Contents[hide]| An Analog or analogue signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i. e. , analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are meaningful. Analog is usually thought of in an electrical context; however, mechanical, pneumatic, hydraulic, and other systems may also convey analog signals.
An analog signal uses some property of the medium to convey the signal’s information. For example, an aneroid barometer uses rotary position as the signal to convey pressure information. Electrically, the property most commonly used is voltage followed closely by frequency, current, and charge. Any information may be conveyed by an analog signal; often such a signal is a measured response to changes in physical phenomena, such as sound, light, temperature, position, or pressure, and is achieved using a transducer.
For example, in sound recording, fluctuations in air pressure (that is to say, sound) strike the diaphragm of a microphone which induces corresponding fluctuations in the current produced by a coil in an electromagnetic microphone, or the voltage produced by a condensor microphone. The voltage or the current is said to be an “analog” of the sound. An analog signal has a theoretically infinite resolution. In practice an analog signal is subject to noise and a finite slew rate. Therefore, both analog and digital systems are subject to limitations in resolution and bandwidth.
As analog systems become more complex, effects such as non-linearity and noise ultimately degrade analog resolution to such an extent that the performance of digital systems may surpass it. Similarly, as digital systems become more complex, errors can occur in the digital data stream. A comparable performing digital system is more complex and requires more bandwidth than its analog counterpart.  In analog systems, it is difficult to detect when such degradation occurs.
However, in digital systems, degradation can not only be detected but corrected as well. A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio.