This page will explain the differences between analog and digital signals.
2.2.7 Radio, television, and telephone transmissions have, until recently, been sent through the air and over wires using electromagnetic waves. These waves are called analog because they have the same shapes as the light and sound waves produced by the transmitters. As light and sound waves change size and shape, the electrical signal that carries the transmission changes proportionately. In other words, the electromagnetic waves are analogous to the light and sound waves.
Analog bandwidth is measured by how much of the electromagnetic spectrum is occupied by each signal. The basic unit of analog bandwidth is hertz (Hz), or cycles per second. Typically, multiples of this basic unit of analog bandwidth are used, just as with digital bandwidth. Units of measurement that are commonly seen are kilohertz (KHz), megahertz (MHz), and gigahertz (GHz). These are the units used to describe the frequency of cordless telephones, which usually operate at either 900 MHz or 2.4 GHz. These are also the units used to describe the frequencies of 802.11a and 802.11b wireless networks, which operate at 5 GHz and 2.4 GHz.
While analog signals are capable of carrying a variety of information, they have some significant disadvantages in comparison to digital transmissions. The analog video signal that requires a wide frequency range for transmission cannot be squeezed into a smaller band. Therefore, if the necessary analog bandwidth is not available, the signal cannot be sent.
In digital signaling all information is sent as bits, regardless of the kind of information it is. Voice, video, and data all become streams of bits when they are prepared for transmission over digital media. This type of transmission gives digital bandwidth an important advantage over analog bandwidth. Unlimited amounts of information can be sent over the smallest or lowest bandwidth digital channel. Regardless of how long it takes for the digital information to arrive at its destination and be reassembled, it can be viewed, listened to, read, or processed in its original form.
It is important to understand the differences and similarities between digital and analog bandwidth. Both types of bandwidth are regularly encountered in the field of information technology. However, because this course is concerned primarily with digital networking, the term ‘bandwidth’ will refer to digital bandwidth.
This page concludes this lesson. The next lesson will discuss networking models. The first page will discuss the concept of layers.
2.3.1 This page explains how layers are used to describe communications between computers.
The concept of layers is used to describe communication from one computer to another. Figure shows a set of questions that are related to flow, which is defined as the motion through a system of either physical or logical objects. These questions show how the concept of layers helps describe the details of the flow process. This process could be any kind of flow, from the flow of traffic on a highway system to the flow of data through a network. Figure shows several examples of flow and ways that the flow process can be broken down into details or layers.
A conversation between two people provides a good opportunity to use a layered approach to analyze information flow. In a conversation, each person wishing to communicate begins by creating an idea. Then a decision is made on how to properly communicate the idea. For example, a person could decide to speak, sing or shout, and what language to use. Finally the idea is delivered. For example, the person creates the sound which carries the message.
This process can be broken into separate layers that may be applied to all conversations. The top layer is the idea that will be communicated. The middle layer is the decision on how the idea is to be communicated. The bottom layer is the creation of sound to carry the communication.
The same method of layering explains how a computer network distributes information from a source to a destination. When computers send information through a network, all communications originate at a source then travel to a destination.
The information that travels on a network is generally referred to as data or a packet. A packet is a logically grouped unit of information that moves between computer systems. As the data passes between layers, each layer adds additional information that enables effective communication with the corresponding layer on the other computer.
The OSI and TCP/IP models have layers that explain how data is communicated from one computer to another. The models differ in the number and function of the layers. However, each model can be used to help describe and provide details about the flow of information from a source to a destination.
The next page will explain how layers are used to describe data communication.
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