 Data communications are the exchange of data between two devices via some
form of transmission media.
 A data communications system has five components.
Sender, Receiver, Message, Transmission medium, Protocols
 A network is the interconnection of a set of devices capable of communication.
A device can be a host such as a large computer, desktop, laptop, workstation,
cellular phone, or security system. It can also be a connecting device such as a
router, a switch, a modem that changes the form of data, and so on.
 The criteria of distinguishing one type of network from another is difficult and
sometimes confusing.
We use a few criteria to make this distinction such as –
size, geographical coverage, and ownership.
 An internet is a switched network in which a switch connects at least two links
together. A switch needs to forward data from a network to another network when
required.
The two most common types of switched networks are circuit-switched and packet-switched networks.
 An internet (note the lowercase i) is two or more networks that can communicate
with each other.
 The most notable internet is called the Internet (uppercase I), and is composed of
thousands of interconnected networks.
The Internet today is an internetwork that allows any user to become part of it.
The user, however, needs to be physically connected to an ISP.
The physical connection is normally done through a point-to-point WAN.
 An Internet standard is a thoroughly tested specification that is useful to and
adhered to by those who work with the Internet.
 It is a formalized regulation that must be followed.
 There is a strict procedure by which a specification attains Internet standard status.
 A specification begins as an Internet draft.
 An Internet draft is a working document (a work in progress) with no official status
and a six-month lifetime.
 A protocol is a set of rules that govern data communication; the key elements of a
protocol are syntax, semantics, and timing.
 A modulator-demodulator or modem is a computer hardware device that
converts data from a digital format into a format suitable for an
analog transmission medium such as telephone or radio. A modem transmits data
by modulating one or more carrier wave signals to encode digital information,
while the receiver demodulates the signal to recreate the original digital
information. The goal is to produce a signal that can be transmitted easily and
decoded reliably. Modems can be used with almost any means of transmitting
analog signals, from light-emitting diodes to radio.
 Modem Communication – POTS (Plain Old Telephone System), MODEM
(Modulator – Demodulator) – [56kbps & V9.2]
Modem types: Internal, External, Voice, Fax
 Digital Data Connections: Digital Phone Lines, Broadband Connection, ISDN
(Integrated Services Digital Network) – [3 & 24 data channels], T Lines, DSL (Digital
Subscriber Line) – requires DSL Modem, Cable Modem Connections – requires
Cable Modem, ATM (Asynchronous Transfer Mode) – efficient for videos and
audios
 Wireless Networks: Wireless 802.11 is Wi-Fi (Wireless Fidelity), WAP (Wireless
Access Point) – determines bandwidth, Wireless Adapters – equipped with signal
strength software
 The term analog data refers to information that is continuous; digital data refers to
information that has discrete states.
(As the wave moves from value A to value B) An analog signal has infinitely many
levels of intensity over a period of time. A digital signal, on the other hand, can
have only a limited number of defined values.
 A periodic signal completes a pattern within a measurable time frame, called a
period, and repeats that pattern over subsequent identical periods. The
completion of one full pattern is called a cycle.
A nonperiodic signal changes without exhibiting a pattern or cycle that repeats over
time.
 Periodic analog signals can be classified as simple or composite.
A simple periodic analog signal, a sine wave, cannot be decomposed into simpler
signals.
A composite periodic analog signal is composed of multiple sine waves.
 The term phase, or phase shift, describes the position of the waveform relative to
time 0.
If we think of the wave as something that can be shifted backward or forward along
the time axis, phase describes the amount of that shift.
It indicates the status of the first cycle.
 Wavelength is another characteristic of a signal traveling through a transmission
medium.
Wavelength binds the period or the frequency of a simple sine wave to the
propagation speed of the medium.