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MINNESOTA TELECOM ALLIANCE Telecommunications Guide July 2000
How Do Telephones Work? Ever wonder how cell phones work? What those different ways to connect to the Internet are? What all the acronyms and terms in telecommunications mean? Well, there isn't space here for the textbooks needed to explain everything, but there is space to begin answering some basic questions about telecommunications.
How Phones Work Despite the fact that you can connect with almost any phone on or off the planet, the telephone is actually one of the simplest home appliances. And it hasn't changed that much over the years. A phone made decades ago still works on today's network! Today's phone still has the three essentials any phone needs - a speaker, a microphone and a switch to connect to the phone network. Phones also need a duplex coil that allows you to hear your own voice (as a reduced transmission to provide a more natural sound to the speaker); a bell (it's now a circuit generating a ringing tone through a speaker); and a touch-tone keypad and frequency generator (because each number is a combination of two frequencies). Basically, a phone turns sound into electric current - and vice versa - for transmission over a pair of copper wires. Phones once connected manually through switchboards now use sophisticated, computerized electronic switches that can determine the best route to use to connect a caller to the number called. Computers also allow changes to be made to software only and more convenient switch locations that reduce cabling. Generally, a phone is connected to the world through copper wires on a pole or in an underground cable containing many copper wires. These run directly to a phone company's area switch or to a digital concentrator, where hundreds of voices then travel on a coaxial or fiber optic line to the phone company's central office. The voice/data is then digitized and sent, ultimately, with millions of other voices and data which may go out over fiber optic cable. The wireless network (cellular, PCS, satellites, microwave) plays an increasingly expanding role in today's telecommunications as well. And cordless phones? They have a small radio transmitter with a limited range that sends a signal to a receiver in the phone's base. From there, calls go on the same "landline"/wireless system.
How Wireless Phones Work Wireless - both cellular and PCS (Personal Communications Services) - is actually radio. This is important to remember because analog wireless conversations are vulnerable to interception by third parties. Newer digital PCS (and digital cellular) are very secure, however. Cellular and PCS are both duplex systems, which means people on two phones can talk at the same time because there are separate frequencies for talking and listening. (CB radio is a simplex system because there's only one frequency and only one person can talk at a time.) In 1983, the first analog cellular system, using the AMPS (Advanced Mobile Phone System) standard, went on line. Each system is designed to have A and B carriers, both of which divide a service area into "cells." Each carrier has a Mobile Telephone Switching Office (MTSO) to connect with the land-based phone system and control base stations. Each cell has a base station (tower and radio equipment) that allows 59 phones to talk in that cell at any one time. If there are any more phones, a user may temporarily be unable to access the network, or a disconnection or "drop-out" results. Cells can be sub-divided if this becomes a big problem. Cell phones and base stations transmit at low power, allowing for small batteries in phones, small phones and the reuse of the same frequencies in different cells. Modern cell phones listen for a System ID on a control channel to see if there's a call. The phones also send a registration request, which a computer database stores so the network knows which cell a phone is in. As you move between cells, the phone detects changes in the control channel's signal strength and registers itself with the new cell. If the phone finds no control channels, it's "out of range." The MTSO decides which frequency your cell phone should use and arranges a "handoff" to the next base station each time you "roam" to a new cell. The base station automatically transfers your phone to an available frequency in that cell - you don't know the difference. Digital is the latest advance in cellular. It expands capacity by allowing 3-10 times as many calls on the same bandwidth by converting voices into ones and zeros. PCS, a term sometimes used in place of digital cellular, is actually a digital wireless phone service. PCS spreads all signals across the entire bandwidth, each with a code that the receiver uses to recover the signal. Because PCS requires smaller cells and more antennas, it has been implemented first in urban areas. In fact, it may never be economically feasible for PCS to move out of those areas, requiring cell/PCS users to have "dual" mode phones for urban/non-urban use.
Copper, Coaxial, Fiber Optic & More Copper wire was the first standard transmission medium for phone calls. Starting in 1944, coaxial cable became a reliable and easy-to-install substitute. Microwave radio transmission began in the 1950s and a vast network of towers was in place by 1980. With the launch of Telstar in 1962, satellite transmission joined the mix. Fiber optic cable came on the scene in the early '80s. Using laser light to carry sound in digital form on hair-thin strands of ultra-pure glass, fiber optic cable has advantages over all other transmission media. It's faster and can carry more calls and data at higher quality than the other media. A single pair of optical fibers can carry hundreds of thousands of two-way conversations. A 4.5-pound spool of optical fiber has the same capacity as 200 reels of copper wire weighing over 1,600 pounds. That makes it easier and cheaper to install. Fiber optic cable transmission isn't affected by atmospheric conditions that can trouble satellite or microwave and requires fewer signal repeaters than coaxial. Fiber (and other media) aren't limited to uninterrupted line-of-sight transmissions as microwave is. And today's fiber optic is more than 100 times faster than its original versions.
Connecting to the Internet The Internet is a network connecting computers all over the world that allows electronic mail and the exchange of computer files and data. The World Wide Web is accessed through the Internet. Making the Internet work involves very complex systems. But that's of little interest to the average Internet user. He or she just wants to log on and navigate as quickly as possible. Technology has obliged with faster connections. Here's a summary of what's what in Internet connections: Dial-up - Using standard phone lines, it's the most common way to connect and still the least expensive. It's also the slowest. It's still OK for most individuals, but businesses transferring huge amounts of data find it too slow. ISDN (Integrated Services Digital Network) - About twice as fast as the fastest dial-up, it requires a special direct phone line and equipment that integrates with a user's current network. Data is in a continuous stream. DSL (Digital Subscriber Line) - Copper wire is being replaced by fiber optic cable, but there's a lot of copper still around and DSL puts it to good use. Digitized data travels over copper at speeds up to 1 megabyte per second (mbps), almost 8 times faster than ISDN. T-1 - A step up from DSL, a T-1 line has a top speed of 1.5 mbps and can be configured as a higher grade ISDN. Fiber optic cable - Because it sends digitized data as light pulses over glass wires, fiber optics is the fastest way to send data. Speeds of 100 mbps are possible, making fiber optic "future-proof," at least until something better comes along that's also practical. Fiber is typically used as a backbone for voice and data. It is not designed for average home use.
Glossary of Some Common Terms Analog - an electric signal in the form of continuously variable waves that has the same characteristics, in electrical form, as a voice's sound waves. Bandwidth - the range of frequencies an analog transmission medium can carry. It's measured in Hertz (cycles per second). Broadband - the capacity of a network. It can be delivered by DSL over copper wire, by fiber optics or by wireless. CDMA - a "spread spectrum" wireless transmission technology that transmits data from calls across the entire bandwidth with codes for proper signal recovery at the other end. Used by PCS. Coaxial (or coax) cable - an insulated sheath surrounding a metal core conveying information in electric current. Coax carries more information faster and with better quality than copper wire. Copper wire - a transmission medium where signals travel in the form of electric current. Data transmission - computers and other devices communicating on a network. Examples include e-mail, the Internet, ATMs. Digital - information carried as a stream of binary "1s" and "0s" instead of as an analog electric signal. DSL - Digital Subscriber Line, in either ASDL (Asymmetrical) or VDSL (Very High Speed) form, sends data over copper wires much faster than normal (allowing transmission of cable TV, for example, without installing a new network of cables). Fiber optic cable - sheathed strands of very thin glass transmitting great amounts of digital information on pulses of light at very high speed and with high quality. Frame relay - a data network that sends data in "frames." Similar to "packets," only with higher capacities. ISDN - Integrated Services Digital Networks can carry voice and data simultaneously. MTSO - a Mobile Telephone Switching Office controls cellular base stations and handles phone connections to the public switched network. Packet-switched networks - digital data sent in quantities called "packets" can follow different routes and arrive out of sequence but can be reassembled in original form. Private network - a network of lines leased by a private customer. Public switched network - the "landline" network all wired phones are connected to. T-1 - a high-speed line for data transmission. TDMA - Time Division Multiple Access is a digital cellular wireless transmission technology that divides cell channels into three time slots so more calls can be carried.
Fun Facts - The dial tone is a combination of two tones, one at 350 Hertz, the other at 440 Hertz. - The busy signal is also two tones (480 and 620 Hertz), on for 1/2 of every second. - To allow more calls, voices are limited. All tones below 400 Hertz and above 3,400 Hertz are "cut off." (The human ear is capable of hearing sounds in the range of 20-20,000 Hertz.) - Without a switch or exchange, every phone would need a separate connection to every other phone. - Cable TV and satellite TV services, as well as the satellite distribution of programs by TV stations, use telecommunications networks.
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