Computer Networks
A computer network consists of two or more computing or other devices connected by a
communications medium, such as a wireless signal or a cable. A computer network provides a way to
connect with others and share files and resources such as printers or an Internet connection.
In business settings, networks allow you to communicate with employees, suppliers, vendors,
customers, and government agencies. Many companies have their own network, called an intranet,
which is essentially a private Internet within the company’s corporate “walls.” Some companies also
offer an extension of their internal network, called an extranet, to suppliers and customers. For
example, a supplier might be allowed to access inventory information on a company’s internal network
to make sure the company does not run short of a vital part for its manufacturing process. In your home,
networks are useful for sharing resources among members of your family. For example, using a home
network, you might share one printer or fax machine among three or four computers.
The Internet is a global network made up of several networks linked together. If you consider all the
applications, services, and tools the Internet allows you to access, you can begin to understand the
power of networking and how it opens up a new world of sharing and functionality.
Communications Systems
A computer network is one kind of communications system. This system includes sending and receiving
hardware, transmission and relay systems, common sets of standards so all the equipment can “talk” to
each other, and communications software.
You use such a networked communications system whenever you send/receive IM or email messages,
pay a bill online, shop at an Internet store, send a document to a shared printer at work or at home, or
download a file.
The world of computer network communications systems is made up of:
Transmission media upon which the data travels to/from its destination.
A set of standards and network protocols (rules for how data is handled as it travels along a
communications channel). Devices use these to send and receive data to and from each other.
Hardware and software to connect to a communications pathway from the sending and receiving ends.
The first step in understanding a communications system is to learn the basics about transmission
signals and transmission speeds when communicating over a network.
Types of Signals
Two types of signals are used to transmit voices and other sounds over a computer network: analog and
digital. An analog signal is formed by continuous sound waves that fluctuate from high to low. Your
voice is transmitted as an analog signal over traditional telephone lines at a certain frequency. A digital
signal uses a discrete signal that is either high or low. In computer terms, high represents the digital bit
1, and low represents the digital bit 0. These are the only two states for digital data.
Telephone lines carry your voice using an analog signal. However, computers don’t “speak” analog;
rather, they use a binary system of 1s and 0s to turn analog data into digital signals. If you send data
between computers using an analog medium such as a phone line, the signal has to be transformed
from digital to analog (modulated) and back again to digital (demodulated) to be understood by the
computer on the receiving end. The piece of hardware that sends and receives data from a transmission
source such as your telephone line or cable television connection is a modem. The word modem comes
from the combination of the words modulate and demodulate.
Today, most new communications technologies simply use a digital signal, saving the trouble of
converting transmissions. An example of this trend is the demise in 2009 of analog television
transmissions as the industry switched to digital signals. Many people were sent scrambling to either
buy a more recent television set or buy a converter to convert digital transmissions back to analog to
work with their older equipment. More recent computer networks, too, use a pure digital signal method
of sending and receiving data over a network.
Transmission Speed
If you’ve ever been frustrated with how long it takes to download a file from a website, you are familiar
with the fact that, in a communications system, data moves from one computer to another at different
speeds. The speed of transmission is determined by a few key factors.
The first factor is the speed at which a signal can change from high to low, which is called frequency. A
signal sent at a faster frequency provides faster transmission (Table 1). The other factor contributing to
the speed of data transmission is bandwidth. On a computer network, the term bandwidth refers to the
number of bits (pieces of data) per second that can be transmitted over a communications medium.
Think of bandwidth as being like a highway. At rush hour, with the same amount of cars, a two-lane
highway accommodates less traffic and everybody moves at a slower speed than on a four-lane
highway, where much more traffic can travel at a faster speed.
Table 1: Bandwidth Measurements
Term
1 kilobit per second
1 megabit per second
1 gigabit per second
1 terabit per second
1 petabit per second
Abbreviation
1 Kbps
1 Mbps
1 Gbps
1 Tbps
1 Pbps
Meaning
1 thousand bits per second
1 million bits per second
1 billion bits per second
1 trillion bits per second
1 quadrillion bits per second
If you have plenty of bandwidth and your data is transmitted at a high frequency, you get faster
transmission speeds. Any communications medium that is capable of carrying a large amount of data at
a fast speed is known as broadband.
Though transmission speeds at any moment in time may vary depending on network traffic and other
factors, each of the common communications media has a typical speed (Table 2). These speeds are
constantly being improved upon. In fact, some very high-powered connections provide transmission
speeds of as much as 100 gigabits (one billion bits) per second, which allows you to download a highdefinition DVD movie in two seconds.
Table 2: Average Network Connection Speeds
Type of Connection
56 K dial-up
satellite
DSL
fiber-optic
cable TV
Typical Speed
56 Kbps
1.5 Mbps
7 Mbps
25 Mbps
50 Mbps