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.
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.
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).
-
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 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.
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.
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