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Chapter 7 Telecommunications, the Internet, and Wireless Technology 285

4. Network Interface layer. At the bottom of the reference model, the Network
Interface layer is responsible for placing packets on and receiving them from
the network medium, which could be any networking technology.
Two computers using TCP/IP are able to communicate even if they are
based on different hardware and software platforms. Data sent from one
computer to the other passes downward through all four layers, starting with
the sending computer’s Application layer and passing through the Network
Interface layer. After the data reach the recipient host computer, they travel
up the layers and are reassembled into a format the receiving computer can
use. If the receiving computer finds a damaged packet, it asks the sending
computer to retransmit it. This process is reversed when the receiving
computer responds.

7.2 COMMUNICATIONS NETWORKS

Let’s look more closely at alternative networking technologies available to
businesses.

SIGNALS: DIGITAL VS. ANALOG

There are two ways to communicate a message in a network: either using an
analog signal or a digital signal. An analog signal is represented by a continuous
waveform that passes through a communications medium and has been used
for voice communication. The most common analog devices are the telephone
handset, the speaker on your computer, or your iPod earphone, all of which
create analog waveforms that your ear can hear.
A digital signal is a discrete, binary waveform, rather than a continuous
waveform. Digital signals communicate information as strings of two discrete
states: one bit and zero bits, which are represented as on-off electrical pulses.
Computers use digital signals and require a modem to convert these digital signals
into analog signals that can be sent over (or received from) telephone lines, cable
lines, or wireless media that use analog signals (see Figure 7.5). Modem stands for
modulator-demodulator. Cable modems connect your computer to the Internet
using a cable network. DSL modems connect your computer to the Internet using a
telephone company’s landline network. Wireless modems perform the same func-
tion as traditional modems, connecting your computer to a wireless network that
could be a cell phone network, or a Wi-Fi network. Without modems, computers
could not communicate with one another using analog networks (which include
the telephone system and cable networks).

FIGURE 7.5 FUNCTIONS OF THE MODEM

A modem is a device that translates digital signals into analog form (and vice versa) so that computers
can transmit data over analog networks such as telephone and cable networks.

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