GEC ELECT 21.

LIVING IN THE IT ERA

CHAPTER 2: DATA COMMUNICATION AND INTERNET

Lesson 6: Types of Transmission Media

Transmission media can be defined as physical path between transmitter and receiver in a
data transmission system. And it may be classified into two types as shown in Figure 1.

• Guided: Transmission capacity depends critically on the medium, the length, and whether
the medium is point-to-point or multipoint (e.g. LAN). Examples are coaxial cable, twisted
pair, and optical fiber.

• Unguided: provides a means for transmitting electro-magnetic signals but do not guide
them. Example wireless transmission.

Characteristics and quality of data transmission are determined by medium and signal
characteristics. For guided media, the medium is more important in determining the limitations
of transmission. While in case of unguided media, the bandwidth of the signal produced by
the transmitting antenna and the size of the antenna is more important than the medium.
Signals at lower frequencies are omni-directional (propagate in all directions). For higher
frequencies, focusing the signals into a directional beam is possible. These properties determine
what kind of media one should use in a particular application. In this lesson we shall discuss the
characteristics of various transmission media, both guided and unguided.

Transmission media

Twisted- Fiber-optic cable

Unguided Atmosphere
pair cable (wireless)
Guided Coaxial cable
(wired)
Figure 1. Classification of the transmission media

GUIDED TRANSMISSION MEDIA

For guided transmission media, the transmission capacity, in terms of either data rate or
bandwidth, depends critically on the distance and on whether the medium is point-to-point or
multipoint. In this section we shall discuss about the most commonly used guided transmission
media such as twisted pair of cable, coaxial cable and optical fiber.
 TWISTED PAIR CABLE

Fi

gure 2. CAT6a Twisted Cable

A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A
wire pair acts as a single communication link. Typically, a number of these pairs are bundled
together into a cable by wrapping them in a tough protective sheath. Over longer distances,
cables may contain hundreds of pairs. The twisting tends to decrease the crosstalk
interference between adjacent pairs in a cable. Neighboring pairs in a bundle typically have
somewhat different twist lengths to reduce the crosstalk interference. On long-distance links,
the twist length typically varies from 5 to 15 cm. The wires in a pair have thicknesses of from
0.4 to 0.9 mm.

APPLICATIONS
The oldest and the most popular use of twisted pair are in telephone network. In LAN it is
commonly used for point-to-point short distance communication (say, 100m) within a
building or a room.

COAXIAL CABLE
Coaxial cable, like twisted pair, consists of two
conductors, but is constructed differently to permit
it to operate over a wider range of frequencies. It
consists of a hollow outer cylindrical conductor that
surrounds a single inner wire conductor. The inner
conductor is held in place by either regularly spaced
insulating rings or a solid dielectric material. The
outer conductor is covered with a jacket or shield.A
single coaxial cable has a diameter of from 1 to 2.5
cm. Coaxial cable can be used over longer
distances and support more stations on a shared
line than twisted pair.
APPLICATIONS
Coaxial cable is perhaps the most versatile transmission medium and is enjoying widespread use
in a wide variety of applications. The most important of these are

• Television distribution
• Long-distance telephone transmission
• Short-run computer system links
• Local area networks

Coaxial cable is widely used as a means of distributing TV signals to individual homes—cable TV.
From its modest beginnings as Community Antenna Television (CATV), designed to provide
service to remote areas, cable TV reaches almost as many homes and offices as the telephone.
 A cable TV system can carry dozens or even hundreds of TV channels at ranges up to a few
tens of kilometers

FIBER OPTIC
CABLE
An optical fiber is a thin (2 to
125 μm),
flexible medium capable of
guiding an
optical ray. Various glasses
and plastics
can be used to make optical fibers. The
lowest losses have been obtained using
fibers of ultrapure fused silica. Ultrapure
fiber is difficult to manufacture; higher
loss multicomponent glass fibers are more
economical and still provide good performance. Plastic fiber is even less costly and can be used for
short-haul links, for which moderately high losses are acceptable.

An optical fiber cable has a cylindrical shape and consists of three concentric sections: the core,
the cladding, and the jacket. The core is the innermost section and consists of one or more very
thin strands, or fibers, made of glass or plastic; the core has a diameter in the range of 8 to 100
μm. Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical
properties different from those of the core. The interface between the core and cladding acts as a
reflector to confine light that would otherwise escape the core. The outermost layer, surrounding
one or a bundle of cladded fibers, is the jacket. The jacket is composed of plastic and other
material
layered to protect against moisture, abrasion, crushing, and other environmental dangers.

APPLICATIONS
Because of greater bandwidth (2Gbps), smaller diameter, lighter weight, low attenuation,
immunity to electromagnetic interference and longer repeater spacing, optical fiber cables are
finding widespread use in long-distance telecommunications. Especially, the single mode fiber is
suitable for this purpose.

Fiber optic cables are also used in high-speed LAN applications. Multi-mode fiber is commonly
used in LAN.
• Long-haul trunks-increasingly common in telephone network (Sprint ads) • Metropolitan trunks-without
repeaters (average 8 miles in length)
• Rural exchange trunks-link towns and villages
• Local loops-direct from central exchange to a subscriber (business or home) • Local area networks-100Mbps to
10Gbps ring networks.

The advantages of optical fiber over twisted pair and coaxial cable become more compelling as the demand for
all types of information (voice, data, image, video) increases.

UNGUIDED TRANSMISSION MEDIA (WIRELESS)

Unguided transmission is used when running a physical cable (either fiber or copper) between two end points is
not possible. For example, running wires between buildings is probably not legal if the building is separated by a
public street

Three general ranges of frequencies are of interest in our discussion of wireless transmission. Frequencies in
the range of about 1 GHz (gigahertz) to 40 GHz are referred to as microwave frequencies. At these frequencies,
highly directional beams are possible, and microwave is quite suitable for point-to-point transmission.
Microwave is also used for satellite communications. Frequencies in the range of 30 MHz to 1 GHz are suitable
for omnidirectional applications. We refer to this range as the radio range.

Infrared is useful to local point-to-point and multipoint applications within confined areas, such as a single room.

Microwave signals commonly used for longer distances (10's of km). Sender and receiver use some sort of dish
antenna

DIFFICULTIES
• Weather interferes with signals. For instance, clouds, rain, lightning, etc. may adversely affect communication.
• Radio transmissions easy to tap. A big concern for companies worried about competitors stealing plans.
• Signals bouncing off of structures may lead to out-of-phase signals that the receiver must filter out.

ANTENNAS
An antenna can be defined as an electrical conductor or system of
conductors used either for radiating electromagnetic energy or for
collecting electromagnetic energy. For transmission of a signal,
electrical energy from the transmitter is converted into
electromagnetic energy by the antenna and radiated into the
surrounding environment (atmosphere, space, water). For reception
of a signal, electromagnetic energy impinging on the antenna is
converted into electrical energy and fed into the receiver.

TERRESTRIAL
MICROWAVE
The most common type of microwave antenna is the
parabolic “dish.” A typical size is about 3 m in diameter.
The antenna is fixed rigidly and focuses a narrow beam
to achieve line-of-sight transmission to the receiving
antenna. Microwave antennas are usually located at
substantial heights above ground level to extend the
range between antennas and to be able to transmit over
intervening obstacles. To achieve long- distance
transmission, a series of microwave relay towers is used,
and point-to-point microwave links are strung together
over the desired distance.
The primary use for terrestrial microwave systems is in long haul telecommunications service, as

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an alternative to coaxial cable or optical fiber. The microwave facility requires far fewer amplifiers or repeaters
than coaxial cable over the same distance but requires line-of-sight transmission. Microwave is commonly used
for both voice and television transmission.

SATELLITE MICROWAVE
Satellite communication is based on ideas similar to those used
for line-of-sight. A communication satellite is essentially a big
microwave repeater or relay station in the sky. Microwave
signals from a ground station is picked up by a transponder,
amplifies the signal and rebroadcasts it in another frequency,
which can be received by ground stations at long distances.

A communication satellite is, in effect, a microwave relay

station. It is used to link two or more ground- based microwave
transmitter/receivers, known as earth stations, or ground
stations. The satellite receives transmissions on one frequency
band (uplink), amplifies or repeats the signal, and transmits it
on another frequency (downlink). A single orbiting satellite will operate on a number of frequency bands, called
transponder channels, or simply transponders.

BROADCAST RADIO
The principal difference between broadcast radio and
microwave is that the former is omnidirectional, and the
latter is directional. Thus, broadcast radio does not
require dish-shaped antennas, and the antennas need
not be rigidly mounted to a precise alignment.

Radio is a general term used to encompass frequencies

in the range of 3 kHz to 300 GHz. We are using the
informal term broadcast radio to cover the VHF (very high
frequency) and part of the UHF (ultra-high frequency)
band: 30 MHz to 1 GHz. This range covers FM radio and
UHF and VHF television. This range is also used for a
number of data networking applications

INFRARED
Infrared communications is achieved using
transmitters/receivers
(transceivers) that modulate noncoherent infrared light.
Transceivers must be within the line of sight of each other either
directly or via reflection from a light-colored surface such as the
ceiling of a room.

One important difference between infrared and microwave

transmission is that the former does not penetrate walls. Thus the
security and interference problems encountered in microwave
systems are not present. Furthermore, there is no frequency
allocation issue with infrared, because no licensing is required.

LINK TO WATCH

Data Transmission (Wired)

https://www.youtube.com/watch?v=0rFZtCovx28
Data Transmission (Wirelesss)
https://www.youtube.com/watch?v=X2infMqOoEA
Transmission Media
https://www.youtube.com/watch?v=Rim2WQ8ryNs

SOURCES:
Khaaragpur. Data Communication Fundamentals

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