Plain Old Telephone Service
Plain Old Telephone Service (POTS) refers to the basic voice service traditionally transmitted over the copper wire network. The sound waves of a caller’s voice are converted by the telephone handset into electrical signals that travel over the network. The copper network is prone to interference, and the signal may weaken over distance, thus requiring amplification along the way. See Chart 1.
The copper network originally carried only “analog” signals, which travel in a continuous stream and require a dedicated circuit. But the network has been upgraded also to carry “digital” signals, which do not require a continuously open and dedicated circuit, thereby increasing network transmission capacity.
Telephone numbers in the United States are organized according to the North American Numbering Plan. The numbering plan is administered by a private firm selected by the Federal Communications Commission through competitive bidding. The numbering plan is subject to directives from regulatory authorities in member countries.
The 10-digit numbers used in the United States consist of three separate codes that designate the route and billing of every call. Each number, when dialed or pressed, emits a tone deciphered by network computers. The first three digits, known as the area code (or Numbering Plan Area), identify a metropolitan area. The next three digits, known as the exchange (or Prefix), specify the central office from which the call is routed to a local destination. The last four digits (Station) represent the individual customer line.
Under federal law, a customer must be allowed to keep a telephone number when changing service providers within a local area. This “number portability” requires a master database to determine whether the customer line is maintained by the original service provider or assigned to a competitor.
Circuit-based technology, commonly referred to as “analog,” relies on a dedicated, continuous transmission path through the network. A dedicated circuit is among the most reliable technologies, although it is not the most efficient in terms of network capacity.
Packet-based technology, commonly referred to as “digital,” does not require a dedicated path through the network, but instead arranges data in fragmented “packets” to speed transmission. Each packet is routed using the best network connection available at a given time, and the packets are reassembled in their original order at the destination of the call.
Digital Subscriber Line (DSL) technology enables data to be transmitted at high speeds through the copper-wire telephone network. A “transceiver” linked to a personal computer connects to the network of an Internet Service Provider through the local telephone network. Data is compressed into digital packets and routed by the Internet Service Provider to the World Wide Web.
The Integrated Services Digital Network technology (ISDN) allows a single copper-wire telephone line to transmit both voice and data signals. Users must dial in to establish a network connection, and fees are typically assessed based on the duration of transmission. ISDN is only available within 3.4 miles of a service provider’s central office.
T1 (or DS1)
A T1 line is a high-speed digital circuit that provides the equivalent of 24 voice-grade lines (or channels) of transmission capacity. The line is leased as a direct connection to a computer system, an Internet Service Provider or a destination specified by the customer. A T1 line is capable of transmitting large text files, as well as graphics and audio.
T3 (or DS3)
A T3 line is a higher-speed digital circuit that provides the equivalent of 672 voice-grade lines (or channels) of transmission capacity. The T3 line serves as the principal artery for heavy volumes of Internet traffic, including transmissions generated by corporations, universities and Internet Service Providers. The T3 is capable of full-screen, full-motion video transmissions.
Fiber to the Home
Fiber to the Home (FTTH), also known as Fiber to the Premises (FTTP), entails replacing copper telephone lines with optical fiber cable at the user’s residence to increase transmission capacity. The hair-thin strands of glass fiber carry pulses of light that deliver volumes more data at higher speeds. Transmitters are needed to convert electrical impulses from a computer into light streams.
OCn, or Optical Carrier Networks, transmit large amounts of data as light signals. The networks vary in capacity. An OC1, for example, can carry the equivalent of a T3 line. Telephone companies use OC12 systems between central offices to carry some 8,000 simultaneous conversations on a single strand of fiber.
The coaxial cable through which television programming is delivered can also accommodate voice and high-speed data transmissions. Coaxial cable requires use of a modem to properly relay signals to the Internet and other network connections. Modem signals are first received by a neighborhood “node” that directs hundreds of such transmissions to network connections at the cable vendor’s facility. Amplifiers boost signal strength along the transmission route.
Voice Over Internet Protocol (VOIP) sometimes refers to private networks that use packet-based technology to transmit calls. The sound waves of a caller’s voice are digitally encoded and transmitted as packets of data. The message is decoded to voice at the destination of the call. Private networks allow users to prioritize call routing to ensure transmission speed and quality.
VOIP also refers to calls transmitted over the public Internet in order to bypass the local calling network. Unlike private networks, calls routed over the public Internet may be impacted by network congestion associated with multiple users transmitting large amounts of data simultaneously. However, these technical challenges are expected to be overcome as the technology continues to advance.
Cellular telephones essentially operate as two-way radios that are also capable of transmitting video and text data. Calls are transmitted as electrical signals within the radio-wave channels allocated to service providers. The signals are relayed between cellular towers that connect with switches to other networks, including the wireline network. Calls may be transmitted as analog or digital signals. See Chart 2 above.
Wireless Local Loop
Wireless Local Loops use rooftop antennas rather than copper wire or optical fiber to transmit telephone calls. Unlike cellular calling, wireless local loops only provide service between fixed points. The antennas relay the signals to “hub” receivers, which interconnect with the wire line network.
“Electromagnetic spectrum” is the scientific term for the full range of electric, magnetic and visible radiation in the universe. Waves within the spectrum vary in size, frequency and energy, and they are classified by their wavelength. The waves can extend from one-billionth of a meter, as in gamma rays, to centimeters and meters, as in radio waves. Waves of similar length are categorized into bands. Within bands, waves travel at various frequencies. The Federal Communications Commission allocates licenses for use of specific radio-wave frequencies. Click on "More Images..." above to see Chart 3.
Spectrum capacity continues to expand as technology improves at delineating new frequencies and reducing interference.
Wireless Fidelity, commonly referred to as “WiFi,” is a local computer or audio network that uses high-frequency radio signals to transmit and receive data over short distances.
Satellites operate as celestial antennas, relaying signals to and from computers to various Internet Service Providers. The transmissions are weather-sensitive and more prone to landscape interference than other technologies.
Broadband Over Power line (BPL)
A number of utilities are experimenting with using power lines to transmit voice and data signals. The existing wiring of homes and businesses presents opportunities for a variety of applications. Computer adapters are necessary to filter the various signals.