Cordless telephone

Cordless telephones, invented by Teri Pall in 1965, consist of a base unit that connects to the land-line system and also communicates with remote handsets by low power radio. This permits use of the handset from any location within range of the base. Because of the power required to transmit to the handset, the base station is powered with an electronic power supply. Thus, cordless phones typically do not function during power outages. Initially, cordless phones used the 1.7 MHz frequency range to communicate between base and handset. Because of quality and range problems, these units were soon superseded by systems that used frequency modulation (FM) at higher frequency ranges (49 MHz, 900 MHz, 2.4 GHz, and 5.8 GHz). The 2.4 GHz cordless phones can interfere with certain wireless LAN protocols (802.11b/g) due to the usage of the same frequencies. On the 2.4 GHz band, several "channels" are utilized in an attempt to guard against degradation in the quality of the voice signal due to crowding. The range of modern cordless phones is normally on the order of a few hundred meters.

Digital Telephony

The Public Switched Telephone Network (PSTN) has gradually evolved towards digital telephony which has improved the capacity and quality of the network. End-to-end analog telephone networks were first modified in the 1970s by upgrading long-haul transmission networks with SONET technology and fiber optic transmission methods. Digital transmission made it possible to carry multiple digitized switched circuits on a single transmission medium (known as multiplexing). While today the end instrument remains analog, the analog signals reaching the aggregation point (Serving Area Interface (SAI) or the central office (CO) ) are typically converted to digital signals. Digital loop carriers (DLC) are often used, placing the digital network ever closer to the customer premises, relegating the analog local loop to legacy status.

Telephony

The telephone or phone is a telecommunications device which is used to transmit and receive sound (most commonly voice and speech) across distance. Most telephones operate through transmission of electric signals over a complex telephone network which allows almost any phone user to communicate with almost any other.