Information Signals
Data is a type of information that the network stores in a computer or retrieves from it. As a result, wireless networks transfer data from one computer to another. This data can include e-mail messages, files, web pages, video, music, and voice conversations.
Communications systems—such as a wireless network— symbolize data using codes that electrical, radio, and light signals efficiently represent. The signals carry the information through the system from one point to another. The signals are either digital or analog, depending on their location within the system.
Digital Signals
Digital signals, which are found inside computers, vary in amplitude steps as time advances. (See Figure 2-7.) Digital signals are usually binary (two-state); therefore, it is common to refer to the signal as a string of binary digits (bits) or binary data. Digital circuitry inside the computer easily stores and processes these digital signals in binary form.
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2-7 Digital Signals Are Ideal for Use in Computers
Binary is a system that only uses 0s and 1s to represent the numbers. Conversions are easy from the more familiar decimal numbering system to binary, and computers can readily store binary numbers. With some protocols, the binary values within a data frame represent specific protocol information.
One of the advantages of digital signals is easy signal regeneration. As a signal propagates through the air medium, it might encounter noise or interference that changes the appearance of the signal's waveform. To clean up and regenerate the signal, digital circuitry can detect if a digital pulse is present at a certain period of time and create a new pulse that is exactly equal to the one originally sent. As a result, a digital signal can be sent over vast distances through periodic repeaters while preserving the integrity of the information. This is not possible with analog signals.
For security purposes, it is often necessary to encrypt and later decode a signal at the destination. This process is simple with digital signals because all that is necessary is to rearrange the bits using some type of secret keying process. When the destination receives the data, a device can use the same key and decrypt the data.
The following defines important characteristics of digital signals:
Data rate—The data rate corresponds to the speed that a digital signal transfers data across a wireless network. As a result, the data rate of a digital signal gives some insight on how long it will take to send data from one point to another, as well as identify the amount of bandwidth that the medium must supply to effectively support the signal.
Throughput—Throughput is similar to data rate; however, throughput calculations generally exclude the bits that correspond to the overhead that communications protocols include. There are no standards for representing throughput, but it usually includes only the actual information being sent across the network. As a result, throughput gives a more accurate way of representing the true performance and efficiency of a network. This makes throughput important when comparing wireless networks because it's directly related to performance. The higher the throughput, the higher the performance.
The data rate of a signal is equal to the total number of bits transmitted in relation to the time it takes to send them. The common unit of measure for bit rate is bits per second (bps). As an example, consider a signal that moves 1,000,000 bits in 1 second. The data rate is 1,000,000/1 = 1,000,000 bps (or 1 Mbps).
The data rate of a wireless LAN, for example, might be 11 Mbps, but the throughput might be only 5 Mbps. After removing the overhead—frame headers, error checking fields, acknowledgement frames, and retransmissions because of errors—the resulting information transfer is considerably lower. As the number of users increases, contention for the shared medium increases, which drives throughput even lower because computer devices (wireless NICs, to be more precise) must wait longer before sending data. This delay, which is a form of overhead, can significantly lower the throughput.
With wireless networks, it is common to say that the system sends data bits. In reality, a wireless network converts the binary digital signals into analog before transmitting the signal through the air medium.
Analog Signals
An analog signal, shown in Figure 2-8, is one where the amplitude of the signal varies continuously as time progresses. Much of the natural environment produces signals that are analog in form. Examples of this are light and the human voice. Man-made signals, such as radio waves, are also analog in form.
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2-8 Analog Signals Carry Information Through the Air Medium
In the early days of electronic communication, most systems processed signals in analog form, mainly because their inputs were information coming from humans. An analog signal has amplitude, in units of voltage or power, and a frequency (having a specific number of cycles per second often referred to as Hertz). Wireless networks generally use analog signals at 2.4 GHz, which is in a band of frequencies referred to as radio waves. There are several different methods for describing the amplitude of wireless signals. Refer to Chapter 3 for details on wireless analog signals.