Analog and Digital Signals
Networks transmit information with two basic types of signals, analog and digital. Analog signals are continuous waves that transmit information by altering the characteristics of the waves. Analog signals have two parameters, amplitude and frequency. For example, all sounds including the human voice are analog, traveling to human ears in the form of waves. The higher the waves (or amplitude), the louder the sound; the more closely packed the waves, the higher the frequency or pitch. In contrast, digital signals are discrete pulses that are either on or off, representing a series of bits (0s and 1s). This quality allows digital signals to convey information in a binary form that can be interpreted by computers. Figure 6.3 illustrates both analog and digital signals. The function of a modem is to convert digital signals to analog signals a process called modulation and analog signals to digital signals a process called demodulation. (The name
Networks transmit information with two basic types of signals, analog and digital. Analog signals are continuous waves that transmit information by altering the characteristics of the waves. Analog signals have two parameters, amplitude and frequency. For example, all sounds including the human voice are analog, traveling to human ears in the form of waves. The higher the waves (or amplitude), the louder the sound; the more closely packed the waves, the higher the frequency or pitch. In contrast, digital signals are discrete pulses that are either on or off, representing a series of bits (0s and 1s). This quality allows digital signals to convey information in a binary form that can be interpreted by computers. Figure 6.3 illustrates both analog and digital signals. The function of a modem is to convert digital signals to analog signals a process called modulation and analog signals to digital signals a process called demodulation. (The name
modem is a contraction of modulator-demodulator.) Modems are used in pairs. The modem at the sending end converts a computer’s digital information into analog signals for transmission over analog lines, such as telephone lines. At the receiving end, another modem converts the analog signal back into digital signals for the receiving computer. There are three types of modems: dial-up modems, cable modems, and DSL modems. The U.S. public telephone system was originally designed as an analog network to carry voice signals or sounds in an analog wave format. In order for this type of circuit to carry digital information, that information must be converted into an analog wave pattern by a dial-up modem. Dial-up modems have transmission speeds of up to 56 kilobytes per second (Kbps) and are almost extinct in most parts of the developed world. Cable modems are modems that operate over coaxial cable for example, cable TV. They offer broadband access to the Internet or corporate intranets. Cable modem speeds vary widely. Most providers offer bandwidth between 1 and 6 million bits per second (Mbps) for downloads (from the Internet to a computer) and between 128 and 768 thousand bits per second (Kbps) for uploads. Cable modem services share bandwidth among subscribers in a locality. That is, the same cable line connects to many households. Therefore, when large numbers of neighbors access the Internet at the same time, cable speeds can decrease signifi cantly during those times. DSL (discussed later in this chapter) modems operate on the same lines as voice telephones and dial-up modems. DSL modems always maintain a connection, so an Internet connection is immediately available.
Communications Media and Channels
Communicating data from one location to another requires some form of pathway or medium. A communications channel is such a pathway. It is comprised of two types of media: cable (twisted-pair wire, cable, or fi ber-optic cable) and broadcast (microwave, satellite, radio, or infrared). Cable media or wireline media use physical wires or cables to transmit data and information. Twisted-pair wire and coaxial cables are made of copper, and fi ber-optic cable is made of glass. The alternative is communication over broadcast media or wireless media. The key to mobile communications in today’s rapidly moving society is data transmissions over electromagnetic media the “airwaves.” In this section you will study the three wireline channels. Table 6.1 summarizes the advantages and disadvantages of each of these channels.
Communications Media and Channels
Communicating data from one location to another requires some form of pathway or medium. A communications channel is such a pathway. It is comprised of two types of media: cable (twisted-pair wire, cable, or fi ber-optic cable) and broadcast (microwave, satellite, radio, or infrared). Cable media or wireline media use physical wires or cables to transmit data and information. Twisted-pair wire and coaxial cables are made of copper, and fi ber-optic cable is made of glass. The alternative is communication over broadcast media or wireless media. The key to mobile communications in today’s rapidly moving society is data transmissions over electromagnetic media the “airwaves.” In this section you will study the three wireline channels. Table 6.1 summarizes the advantages and disadvantages of each of these channels.
Twisted-Pair Wire. Twisted-pair wire is the most prevalent form of communications wiring; it is used for almost all business telephone wiring. As the name suggests, it consists of strands of copper wire twisted in pairs (see Figure 6.4). Twisted-pair wire is relatively inexpensive to purchase, widely available, and easy to work with. However, it also has some signifi cant disadvantages. Specifi cally, it is relatively slow for transmitting data, it is subject to interference
from other electrical sources, and it can be easily tapped by unintended receivers to gain unauthorized access to data. Coaxial Cable. Coaxial cable (Figure 6.5) consists of insulated copper wire. Compared to twisted-pair wire, it is much less susceptible to electrical interference, and it can carry much more data. For these reasons, it is commonly used to carry high-speed data traffi c as well as television signals (thus the term cable TV). However, coaxial cable is more expensive and more diffi cult to work with than twistedpair wire. It is also somewhat infl exible.
Fiber Optics. Fiber-optic cable (Figure 6.6) consists of thousands of very thin fi laments of glass fi bers that transmit information via light pulses generated by lasers. The fi ber-optic cable is surrounded by cladding, a coating that prevents the light from leaking out of the fi ber. Fiber-optic cables are signifi cantly smaller and lighter than traditional cable media. They also can transmit far more data, and they provide greater security from interference and tapping. As of mid-2013, optical fi ber had reached data transmission rates of more than 50 trillion bits (terabits) per second in laboratory experiments. Fiber-optic cable is typically used as the backbone for a network, whereas twistedpair wire and coaxial cable connect the backbone to individual devices on the network.
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