1. Unit 7 – Chapter 23

2.  Lesson 23-1  The Telephone System  Lesson 23-2  Using Telephone Communications  Lesson 23-3  Exploring High-Speed Telecommunications

3.  Communicating by Telephone  The first telephone message was sent in 1876 over a line connecting two rooms. Eventually telephone cables were connected to a central office. Operators in the office could connect calls to anyone on the network. Today’s equipment can connect telephones anywhere in the world.

4.  Telecommunications Formats  Sending information over a telephone network is called telecommunications. Telecommunications has grown due to the demand for instant communication. Today, people use many forms of telecommunications to rapidly relay infromation: ▪ Telephones ▪ Cell phones ▪ Pagers ▪ E-mail ▪ Internet and fax machines.

5.  Making telephone calls.  In the past, only a few companies provided telephone services in the United States. Today, people can choose from among many phone companies that offer both local and long distance service.

6.  Public Switched Telephone Network  Many phone calls are made through the Public Switched Telephone Network (PSTN). This network is built from copper wires and other cables and forms a circuit between the caller’s telephone and another telephone.

7.  Local Calls  For local calls, your phone company provides directly wired services between the homes and businesses that belong to the local network. Within your neighborhood, telephones connect to a common network for telephone service. This common network, called the local loop, connects to the phone company’s central office. Much of the local loop is an analog system. An analog system sends electrical signals that carry voice and other sounds.

8.  Long-Distant Calls  Outside the local loop, the long-distance telephone system today is mostly digital. Digital connections use computer code and can carry voice, data, and video on a single line. When you dial a long-distance number, computers figure out how to complete your call. To connect analog and digital networks, special equipment changes analog signals into digital signals.

9.  Contrasting Analog and Digital Communications.  People often confuse the terms, “analog” and “digital” when they are talking about communications or computers. The difference is important but easy to understand. In analog communications, sounds (such as a person’s voice or music) start as waves or vibrations in the air. The vibrating air varies in frequency or pitch (how high or deep the sound is) and strength or loudness. A small microphone in the telephone converts the sound waves into varying patters of electrical signals or radio waves.

10.  Contrasting Analog and Digital Communications.  The patter of electrical signals or radio waves is similar to the pattern of the sound waves. These signals are converted back into sound waves by a small loudspeaker in the receiver. In digital communications, sounds are converted into binary data (a series of 1’s and 0’s) at the caller’s end. The stream of 1’s and 0’s is transmitted without any variation in the pattern of electrical or radio waves. The receiver converts the binary data back into sound waves.

11.  The Wired – and the Wireless – World  Wires connect the phone jack in your wall to an interface box outside. Outside wires may be above or below ground. Wires also connect your local loop with distant places.

12.  Twisted Pair  At first, the entire telephone system depended on twisted pair technology. Twisted pair refers to a pair of copper wires that are twisted together to reduce interference, or outside noise. In the United States today, most homes and business buildings still have twisted pair wiring.

13.  Fiber-Optic Cables  Fiber-optic cables are strands of fiberglass that transmit digital data by pulses of light. These cables can carry large quantities of information. They work faster and more efficiently than copper wires. As they get lower in price, fiber optic cables will eventually replace copper.

14.  Wireless  Wireless communication frees users from traditional telephone lines. Messages are sent on radio or infrared signals. Cell phones use radio signals. Infrared signals are light waves that cannot be seen by the human eye. Remote control devices for TV’s, VCR’s, and DVD players use infrared signals.

15.  Wireless  Before fiber-optic cables, high-frequency radio waves called microwave signals were used to relay long-distance telecommunications. Microwaves are broadcast from repeater tower to repeater tower in a straight line.  Satellites orbiting Earth also transfer voice and data. Satellites provide an efficient means to handle large amounts of phone calls and data.

16.  Using Modems  Back in the early 1990’s when the Internet was still “new,” most people used phone lines to connect to it. Your computer is a digital device. The local loop that connects you to the telephone system, however, is analog. A device called a modem makes it possible for your computer and telephone lines to communicate, and for your computer and TV cables to communicate.

17.  Using Modems  The word modem actually names the work the device does: modulation and demodulation. Through modulation, the modem changes the digital signal of the computer to the analog sounds used by telephones. Then, the data-in the form of pictures, audio, or video-can travel over the telephone wires. When the data gets to it destination, the receiving modem change the analog signals back to digital. This process is called demodulation. The early dial-up modems were usually internal, housed inside the computer. Today’s modems for use with DSL and cable (discussed later) are usually external.

18.  Drawbacks to Dial-up  Today only 10% of U.S. households connect to the computer using the local phone line loop, but dial-up service is still a worthwhile backup. One problem with using dial-up was that you could not be on your computer and use your telephone simultaneously, so the computer modem required a dedicated phone line. Another larger drawback was slow speed.  Modem speed is measured in bits per second, or bps, which is the amount of data that can be sent in one second. Dial-up modems can only transmit 56,600 bps, which has been surpassed by newer, speedier technologies.

19.  Sending Faxes  While dial-up is usually a thing of the past, there is one machine that still used telephone lines to send printed messages or visual images. A facsimile machine, or fax machine, is a device that allows you to send pages of information to a fax machine anywhere in the world. Fax machines can send hand- written documents, printed text, pictures, blueprints, or anything else on a page. Yet, even these machines are largely being replaced by e-mail, by which you can send attachments over the Internet.

20.  How Fax Machines Work  As a document enters a fax machine, a sensor scans it. The data becomes a digital signal. An internal modem in the fax machine (or in an All-in- One printer with fax capability) changes the digital signal to an analog signal. The receiving fax machine accepts the analog signal, changes it back to digital, and prints a copy of the original document.

21.  Introducing Bandwidth  People always want faster, better, and less expensive telecommunication choices. Twisted pair copper wiring in many homes and businesses in the United States at first made it hard to increase bandwidth. Then telephone, cable, and satellite television companies began to compete in offering higher speed communications. Now, more than eight out of ten homes and businesses are using high-speed connections for Internet access.

22.  Understanding Bandwidth  Bandwidth is the amount of data that can be sent through a modem or network connection. The more bandwidth, the faster the connection. It is usually measured in bits per second (bps) or in Megabits per second (Mbps). The more bandwidth, the more information can be transferred in a given amount of time.

23.  Understanding Bandwidth  Imagine several people on different computers connecting to the Internet to visit Web pages, participate in video chats, or send e-mail. These users need a lot of bandwidth. That’s why there is always a race to find a technology that offers services that transmit data faster. Currently the fastest technology is broadband transmission.

24.  Working with Broadband Transmission  Broadband is the general term for all high-speed digital connections that transmit at least 1.5 megabits per second (Mbps), though current broadband services transmit between 10 and 30 Mbps. Several broadband technologies are available, and more are always on the drawing board. This high transmission speed is required for videoconferencing, video-on-demand, digital television services, and high-speed Internet connectivity in general.

25.  DSL  Digital Subscriber Line, or DSL, uses the same copper wires telephones use, but it transmits data in digital form rather than analog. Voice calls and DSL can exist simultaneously on copper lines, because each services has its own frequency band. Unlike the old dial-up, DSL allows for very fast connections to the Internet and features an “always-on” connection. DSL service also requires a modem, which translates the computer’s digital signals into voltage sent across phone lines to a central hub. There is one drawback to DSL: A user must be within a few miles of a local telephone switching station for a connection to be made.

26.  DSL  Different companies offer DSL at different levels of service and price. For instance, asymmetric DSL (ADSL) allows download speeds of up to 1.5 Mbps and upload speeds of 126 kilobits per second (Kbps). This means you can receive data (download) faster than you can send it (upload) with ADSL. Subscribers to a Symmetric Digital Subscriber Line (SDSL) can send data at the same speed at which they receive it. These are used usually by businesses that need to send large files, data, and programs.

27.  SONET  Telephone companies that offer DSL and other Internet connection methods rely on a digital network called SONET. SONET stands for Synchronous Optical Network. It uses fiver optics to provide faster connections and greater bandwidth—from 52 Mbps to up to a whopping 40 gigabits per second (Gbps).

28.  Cable and Satellite TV Connections  Most cable and digital television companies offer high-speed Internet connection through a cable modem. Satellite television companies also offer a similar service with signals sent through the customer’s satellite dish. A cable modem connects your computer to the local cable TV line and supports data transfer rates of up to 30 Mbps—over 500 times faster than the old dial-up modem.

29.  Cable and Satellite TV Connections  However, this number can be misleading, because most Internet Service Providers (ISPs) cap subscribers’ transfer rates to less than 6 Mbps to conserve bandwidth. However, this is more than enough for the average home computer user.

30.  Voice over Internet Protocol  Just as you can use your telephone to send and receive messages over the Internet, you can use the Internet to send and receive messages over the telephone. Voice over Internet Protocol (VoIP) technology allows you to have a telephone connection over the Internet. VoIP uses data sent digitally, with the Internet Protocol (IP), instead of analog telephone lines. People use VoIP to talk to each other from across the globe, sometimes without having to pay a cent. With Web-cams callers can also see each other during their calls.

31.  The Future of Bandwidth  The demand for bandwidth is growing. People want increased bandwidth for video-on-demand, meetings via the Internet, and Web-based learning. Thus, telephone and other high-tech companies continue to look for new ways to improve telecommunications services and data transmission.