On any network, the various entities must
communicate through some form of media.
Just as humans can communicate through telephone
wires or sound waves in the air, computers can
communicate through cables, light, and radio waves.
• Transmission media enables computers to send and
receive messages but do not guarantee that the
messages will be understood.
Communication is possible only if information is
encoded in a signal, and the signal is carried on a
transmission ,the characteristics of the signal and of
the medium both determine the quality of the
There are two main groups of transmission media,
namely the guided medium and the unguided
medium (wireless medium).
For the guided medium, there is a physical path (such
as a cable) for electromagnetic wave propagation.
For the unguided medium, however, the
electromagnetic wave is transmitted through air,
water, or vacuum.
A good transmission medium should provide
communication with good quality at long distance.
For voice communication, quality of communication is
determined by the voice quality.
For data communication, however, the quality of
communication is mainly determined by the effective
data rate of communication.
Communication bandwidth of the medium
The transmission impairments
The bandwidth of a medium determines the
signal frequencies that can be carried in the
A wide bandwidth, or broadband, usually
allows communication at a higher data rate.
Distortion during signal propagation
Guided media includes everything that
‘guides’ the transmission. That usually
takes the form of some sort of a wire.
Usually copper, but can also be optical
• A transmission medium consisting of pairs
of twisted copper wires.
• We can transmit 1 Mbps over short distances
(less than 100m).
• They are mainly used to transmit analog
signals, but they can be used for digital
The twisted pair cable can be broadly categorized
into the following two types
• Shielded Twisted Pair (STP) Cable
the pair is wrapped with metallic foil or braid to
insulate the pair from electromagnetic interference
• Unshielded Twisted Pair (UTP) Cable
each wire is insulated with plastic wrap, but the pair is
encased in an outer covering
• In its simplest form, coaxial consists of a
core made of solid copper surrounded by
insulation, a braided metal shielding, and
an outer cover.
• A transmission medium consisting of
thickly insulated copper wire, which can
transmit a large volume of data than twisted
• Optical fiber consists of a glass core,
surrounded by a glass cladding with
slightly lower refractive index.
• In most networks fiber-optic cable is
used as the high-speed backbone, and
twisted wire and coaxial cable are used to
connect the backbone to individual
Unguided media is still ‘media’ (stuff that
signal travels though). The trick is that the
media is usually not directional, like air,
space, etc. Because the effect is usually
much wider than with guided media, there
have been a lot of regulation, licensing,
and standardization of transmissions via
• Wireless transmission that sends signals
through air or space without any physical
• Common uses of wireless data
transmission include pagers, cellular
telephones, microwave transmissions,
communication satellites, mobile data
networks, personal digital assistants,
television remote controls.
Radio waves are used for multicast
communications, such as radio and television, and
paging systems. They can penetrate through walls.
Highly regulated. Use Omni directional antennas
Omni directional antenna
Microwaves are used for unicast communication
such as cellular telephones, satellite networks,
and wireless LANs.
Higher frequency ranges cannot penetrate walls.
Use directional antennas - point to point line of sight
Infrared signals can be used for shortrange communication in a closed area using
Key is the antenna
Role of antenna – conversion between electrical
signals and airborne signals
Transmission – antenna gets electrical signals, and
radiates airborne energy into the medium; i.e., air
Reception – antenna receives airborne waves from the
surrounding medium and converts them to electrical
Every wireless system MUST have antennas.
Antenna design is related to three major considerations:
•Frequency to be transmitted
•Direction of transmission
•Power needed for transmission
It is inexpensive and already is in use.
Twisted wires are slow, high-speed transmission causes
Limiting factors: skin effect & radiation effect.
It is often used in place of twisted wire for important
links in a network because it is a faster.
More interference-free transmission medium (speed:
200 megabits per second).
Coaxial cable is thick, is hard to wire in many
buildings. It does not support analog conversations.
Optical fibers are faster, lighter, and suitable for
transferring large amount of data.
Fiber-optic cable is more difficult to work with, more
Its difficult to install them.
• Optical fiber cable differs from both these transmission
media in that it carries the transmitted information in
the form of a fluctuating beam of light in a glass fiber.
• Light transmission has much wider bandwidth, thus
enabling the transmission rate of hundreds of
megabits per second.
• Optical transmission is immune to
electromagnetic interference and crosstalk.
While others are immune to interference.
• Optical fibers have less loss of signal
strength than copper, after every 30 miles we
need to use a repeater, whereas in copper, we
should insert repeaters at an interval of 2.8
• Optical fiber is more secure, no easy tapping
on the cable, like in copper.
• But optical fibers are more expensive than
other two cables.
Bandwidth: The difference between
the lowest and highest frequency signals
that can be transmitted across a
transmission line or communication system.
Interference: The distortion on the
signal in transmission due to unwanted
signals from outside sources.
Attenuation, distortion, or noises that will
degrade the signal quality during
Attenuation: The loss of a signal’s
energy when the signal is transmitted
across a medium.
Signal Distortion: Any change in a
signal during signal transmission.
Noises: Random signals that can be
picked up during signal transmission and
result in degradation or distortion of
Trunk Circuits: The
communication circuits between two
In summary, transmission media are essential for
communication systems. Signals that carry
information can be transmitted on a transmission
medium for communication purposes. The
transmission characteristics of the medium in use
are important because they directly affect the
communication quality. Since different types of
transmission media have different transmission
characteristics and costs, they are used in different
(and the most suited) applications.