After the data is transformed into a series of bits, it must be converted into signals that can be sent across the network media to its destination. Media refers to the physical medium on which the signals are transmitted. Examples of media are copper wire, fiber-optic cable, and electromagnetic waves through the air. A signal consists of electrical or optical patterns that are transmitted from one connected device to another. These patterns represent the digital bits (i.e. the data) and travel across the media from source to destination as either a series of pulses of electricity, pulses of light, or radio waves. Signals may be converted many times before ultimately reaching the destination, as corresponding media changes between source and destination.
There are three common methods of signal transmission used in networks, as shown in Figure 1-18:
• Electrical signals — Transmission is achieved by representing data as electrical pulses on copper wire.
• Optical signals — Transmission is achieved by converting the electrical signals into light pulses.
• Wireless signals — Transmission is achieved by using infrared, microwave, or radio waves through the air.
In most homes and small businesses, network signals are transmitted across copper wires (cables) or Wi-Fi enabled wireless connections. Larger networks employ fiber-optic cables in order to reliably carry signals for longer distances.
Figure 1-18 Three Types of Signal Transmissions
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Bandwidth and Throughput (1.3)
Network performance at times is measured by the speed in which users are sending or receiving information. Bandwidth and throughput are two ways of measuring how much data is transferred from source to destination.
Streaming a movie or playing a multiplayer game requires reliable, fast connections. To support these “high bandwidth” applications, networks have to be capable of transmitting and receiving bits at a very high rate.
Different physical media support the transfer of bits at different speeds. The rate of data transfer is usually discussed in terms of bandwidth and throughput.
Bandwidth is the capacity of a medium to carry data. Digital bandwidth measures the amount of data that can flow from one place to another in a given amount of time. Bandwidth is typically measured in the number of bits that (theoretically) can be sent across the media in a second. Common bandwidth measurements are as follows:
• Thousands of bits per second (Kbps or Kb/s)
• Millions of bits per second (Mbps or Mb/s)
• Billions of bits per second (Gbps or Gb/s)
Physical media properties, current technologies, and the laws of physics all play a role in determining available bandwidth.
Table 1-1 shows the commonly used units of measure for bandwidth.
