By: Faiza Tariq
Classes of transmission media
Conducted or guided media
– use a conductor such as a wire or a fiber optic
cable to move the signal from sender to receiver
Wireless or unguided media
– use radio waves of different frequencies and do not
need a wire or cable conductor to transmit signals
for Transmission Media
Bandwidth: All other factors remaining constant,
the greater the band-width of a signal, the higher
the data rate that can be achieved.
Transmission impairments. Limit the distance a
signal can travel.
Interference: Competing signals in overlapping
frequency bands can distort or wipe out a signal.
Number of receivers: Each attachment
introduces some attenuation and distortion,
limiting distance and/or data rate.
Guided Media (Physical Media)
Guided Transmission Media
Transmission capacity depends on the
distance and on whether the medium is
point-to-point or multipoint
twisted pair wires
Twisted Pair Wire
Two or more pairs of single conductor wires that
have been twisted around each other.
Twisted pair wire is classified by category. Twisted
pair wire is currently Category 1 through Category 5e.
Twisting the wires helps to eliminate
electromagnetic interference between the two wires.
Shielding can further help to eliminate interference.
Twisted Pair Wires
Consists of two insulated with insulating material (like
plastic) copper wires arranged in a regular spiral
If you wrap one good conductor around another one
they make a field that protects the conducting wires
from RF noise. i.e. one approach taken by the twisted
The simple twist change the properties of the wire and
help make it suitable for network
They limit the electromagnetic energy the wire emits
or to minimize the electromagnetic interference
between adjacent pairs
Twists also prevent help in preventing signals on other
wires from interfering with the pair
Often used at customer facilities and also over
distances to carry voice as well as data
Low frequency transmission medium
UTP (unshielded twisted pair)
– UTP has four pairs of wire
– each wire is insulated with plastic wrap, but the pair is
encased in an outer covering
– It is used in Ether Networks (10 BaseT and 100 BaseT)
Easy to install
Least expensive of all media
Small diameter of cable
Proper termination procedures insures reliable
Shielded Twisted Pair (STP)
STP has two pairs of wires
– the pair is wrapped with metallic shield or foil to insulate
the pair from electromagnetic interference (extra
– It is commonly used in Token Ring network
Foil shields provide greater protection against EMI & RFI.
Increased cost of cable
UTP and STP
This does not necessarily imply that STP is
always better protected from RF noise than
UTP, but only that the two cable types take
Theory with UTP is that the two wires wrapped
around each other individually conduct noise but
cancel out each other’s noise.
The theory with STP is that the conductors are
best protected with a layer of conducting wires
rather than the two conductors being wrapped
around each other.
Ratings of Twisted Pair
Category 3 UTP
– data rates of up to 10mbps are achievable with atleast three
twists per foot.
Category 5 UTP
– data rates of up to 100mbps are achievable
– Operates at a maximum frequency of 100 MHz, but to
support speeds of 100 Mbp a frequency of only 62.5 MHz is
required. That is why?
– How fast cable could transmit the data under ideal conditions
– more tightly twisted than Category 3 cables
– more expensive, but better performance
– More expensive, harder to work with
Table 7.1 Categories of unshielded twisted-pair cables
Longer cable runs than UTP & STP (up to 500m)
Cheaper than using fiber for your backbone
Technology is well known (Cable TV)
Better at reducing EMI than UTP or STP
A single wire surrounded by the braided metal shield
is very good at blocking electromagnetic signals from
entering the cable and producing noise.
Base band co-axial
– Uses digital signalling in which cable carries only
one channel of digital signals
– Transmits analog signals & is capable of supporting
multiple channels of data simultaneously
– If each channel occupies a bandwidth of
approximately 6 MHz. When 50 channels are
transmitted together, the cable is supporting a
composite signal = 50 X 6 MHz = 300 MHz
Coaxial Cable (or Coax)
Used for cable television, LANs, telephony
Has an inner conductor surrounded by a a heavier
The heavy metal shield in a coaxial cable forms a
flexible cylinder around the inner wire and provides a
barrier to electromagnetic radiation.
This barrier isolates the inner wire in two ways
– It protects the wire from incoming electromagnetic energy
that could cause interference
– Keeps signal on the inner wire from radiating
electromagnetic energy that could affect other wires.
Both conductors share a common center axial, hence
the term “co-axial”
copper or aluminum
Can transmit signal perfectly up to 185
metre, then you need a repeater
PC’s can be connected directly without
High attenuation rate makes it expensive
over long distance
Thick Coaxial Cable
– Approximately 6 – 10 mm in diameter
– Used as broadband
Thin Coaxial Cable
– Approximately 4 mm in diameter
– Carries base band signals
than any other
rates than any
Fiber Optic Cable
Relatively new transmission medium used by
telephone companies in place of long-distance trunk
Also used by private companies in implementing local
data communications networks
This medium uses light to transport data
A transmitter at one end of a fiber (light source) uses a
light emitting diode (LED) or laser (Injection Laser
Diode – ILD) to send pulses of light down the fiber.
A receiver at the other end (detector) is a photodiode
which uses a light sensitive transistor to detect the
pulses and generates electrical pulse when light fall
fiber is used
Fiber Optic Layers
Data Transmission is unidirectional.
consists of three concentric sections
glass or plastic fiber core
Fiber Optic Cable
Fibre Optic can transmit data at 1000 Mbp for
Powerful lasers can drive a fiber 100 km long
without repeaters, although at much lower speed.
Fibre optic cable may have more than one fibre in
it meaning multiple pathway exist
Like different motorway’s lanes
One method used to send data through the optic
fibre cables is simply turning light on and off.
When light is on a 1 is transmitted and when light
is off a 0 is transmitted
Side View Of A Fibre Optic Cable
Cross Section Of A Fibre Optic Cable
Fiber Optic Cable
There are two types of fiber optic cable
– It sends transmission along a single path
– The beam of light is very intense, so it can
carry more data for longer distance (interbuilding)
– Suitable either for applications that are very
traffic-intensive or need to travel for long
Allow multiple modes to pass through the cable
at once. Suitable for within the building use.
Multiple channels, each on a different color of
light can be used to increase capacity. This is
called WDM (Wave Division Multiplexing) and it
allows many separate channels of data each
carried by different color of light in an optic fibre
Two kinds of multimode
A mode is a ray of light entering a fibre at a
Fiber Optic Types
multimode step-index fiber
– The light beam bounce around inside the cable in
– the reflective walls of the fiber move the light pulses
to the receiver
multimode graded-index fiber
– Rounded pattern of the light movement like a sine
– acts to refract the light toward the center of the fiber
by variations in the density
single mode fiber
– the light is guided down the center of an extremely
Fiber Optic Advantages
Greater capacity (bandwidth of up to 2 Gbps)
smaller size and lighter weight
As light is used which neither cause electrical
interference in other cables nor are they
susceptible to electrical interference
i.e. immunity to environmental interference
highly secure due to tap difficulty and lack of
Pairs of wires not required
Greater Repeater Spacing
Fiber Optic Disadvantages
Installing fiber requires special equipment that
polishes the ends to allow light to pass through
If fiber breaks inside the plastic jacket (e.g.
being bent at a right angle).
Repairing is difficult as special equipment is
needed to join two fiber so that light can pass
through the joint.
expensive over short distance
requires highly skilled installers
adding additional nodes is difficult
7-2 UNGUIDED MEDIA: WIRELESS
Unguided media transport electromagnetic waves
without using a physical conductor. This type of
communication is often referred to as wireless
Topics discussed in this section:
Wireless or Unguided Media
Within buildings the LAN may use copper wire or
fibre optic but connecting buildings may require
digging up the streets to lay a cable, which may
be an expensive task (it may also be illegal, if it
is a public road)
On the other hand putting a laser or infrared
transmitter and receiver on the roof of each
building is inexpensive, easy to do and nearly
Wireless communication does not require direct
physical connection between computers.
Radio, satellite transmissions, and
infrared light are all different forms of
electromagnetic waves that are used to
Note in the following figure how each
source occupies a different set of
Radio waves are used for multicast
communications, such as radio and
television, and paging systems
Microwaves are used for unicast
communication such as cellular
telephones, satellite networks, and
Infrared signals can be used for shortrange communication in a closed area
using line-of-sight propagation.
Radio waves are used for multicast communications, such as
radio and television, and paging systems.
Microwaves are used for unicast communication such as cellular
telephones, satellite networks,
and wireless LANs.
Infrared signals can be used for short-range communication in a
closed area using line-of-sight propagation.
Wireless (Unguided Media)
Transmission and reception are achieved
by means of an antenna (TX and RX)
Directional (Line-of-Sight Transmission)
– Transmitting antenna puts out focused beam
– Transmitter and receiver must be aligned
Signal spreads out in all directions
Can be received by many antennas
Higher frequency required
Microwave Radio Transmission:
Electromagnetic radiation beyond the frequency used
for radio and television can also be used to transport
information i.e. higher frequency version of radio
Many telephone companies use microwave
transmissions to carry telephone conversations.
Unlike radio broadcast in all directions, a microwave
can be aimed in a single direction, preventing others
from intercepting the signal.
Microwave transmission can carry more information
than lower frequency RF transmission
Microwave can not penetrate metal structures.
Suitable for long distance communication, widely used
in network as an alternative of cables.
Microwave Radio Transmission
The higher the tower, the greater the range.
With a 100 – meter distance of 100 km
between towers are feasible. Therefore
much cheaper rather than digging 100 km
No need of expensive repeaters
It may be affected by thunderstorms.
Microwave occur at frequencies between 2
and 40 GHz and these frequencies have
been divided into bands for common carrier,
government military and other users.
parabolic dish transmitter, mounted high
used by common carriers as well as
requires unobstructed line of sight
between source and receiver
curvature of the earth requires stations
(repeaters) ~30 miles apart
Line of sight requirement
Expensive towers and repeaters
Subject to interference such as passing
airplanes and rain
– Easy implementation
– Interference from other radio waves
– Limited by "line of sight" distance (30 miles
– Insecure (easily intercepted)
– Affected by environmental factors (such as
The wireless remote controls used electrical
appliances like television, stereo and air conditioner
communicate with infrared transmission.
Infrared is limited to a small area (e.g. a single room),
and usually requires that transmitter be pointed toward
Infrared is inexpensive as compared to other wireless
means and do not require antenna.
Computer networks can use infrared technology for
Notebook computers offer wireless communication
It is fully digital communication and is immune of
tapping or jamming as it is highly directional.
Uses transmitters/receivers (transceivers)
that modulate infrared light.
Transceivers must be within line of sight of
each other (directly or via reflection ).
Unlike microwaves, infrared does not
A beam of light can be used to carry data
through air (especially between buildings).
A laser communication system, like microwave
consists of two sites that each have transmitter
The equipment is mounted in a fixed position
often on a tower and aligned so the transmitter at
one location sends its beam of light directly to
Unfortunately laser beam can not penetrate
weather conditions like fog, rain and snow,
therefore laser transmission has limited use.
It is fully digital communication and is immune of
tapping or jamming as it is highly directional.
RF Technology can be combined with satellites to
provide communication along the longer distances.
The satellite contains transponder that consists of a
radio receiver and transmitter.
The transponder accepts an incoming radio
transmission, amplifies it and transmits amplified
signal to ground station at some different angle.
A single satellite uses multiple transponders (as it is
expensive to place satellite in orbit) to work
Each transponder uses different frequency (Channel)
Satellite transmission can be heard by any one with a
dish & tuning to a right frequency causing potential
Similar to terrestrial microwave except the
signal travels from a ground station on
earth to a satellite and back to another
Satellites can be classified by how far out
into orbit each one is (LEO, MEO, GEO,
LEO - Low Earth Orbit - 100 miles to 1000
miles. Used for pagers, wireless e-mail, special
mobile telephones, spying, videoconferencing.
MEO - Middle Earth Orbit - 1000 to 22,300
miles. Used for GPS and government.
GEO - Geosynchronous Orbit - 22,300 miles.
Used for weather, television, and government
HEO – Highly Elliptical Orbit
A fourth type of orbit used by the military for
spying and by scientific organizations for
photographing celestial bodies.
When satellite is far out into space, it takes
photos. When satellite is close to earth, it
Satellite microwave can also be classified by its
Bulk carrier configuration
Single-user earth station configuration (e.g. VSAT)
Satellite transmissions are subject to subject to noise
and long delay. Since every transmission must go
from the ground to the satellite and then from satellite
to ground, each transmission approximately takes a
45,000 miles trip.
Transmission occurs at almost speed of light, so trip
takes a quarter of a second.
Sending a message and receiving a response means
at least half second delay for the round trip
In this situation computer could transmit 4800 bits
during that time.
long delays in satellite transmission can cause “tunnel
effects.” where annoying echoes are heard during long
distance calls. Although echoes can be removed by
A satellite can act as a big microwave repeater
in the sky.
a microwave relay station in space
can relay signals over long distances
– remain above the equator at a height of 22,300
miles or 36,000 Km (geosynchronous orbit)
– When viewed from the ground, the satellite appears
to remain at exactly the same point in the sky at all
– travel around the earth in exactly the time the earth
takes to rotate, the satellite period is 24 Hours
There is limited space available in
geosynchronous orbit above the equator,
because communication satellites using a given
frequency must be separated to avoid
The minimum separation depends on the
power of transmitters, generally angular
separation of 4 and 8 degrees. Bearing this fact
in mind, how many satellites can be
accommodated in the orbit?
Entire 3600 circle above the equator can hold
45 – 90 satellites
Satellite Transmission Links
Earth stations communicate by sending signals
to the satellite on an uplink
the satellite then repeats those signals on a
the broadcast nature of the downlink makes it
attractive for services such as the distribution of
It contains one or more transponders, each of
which listens to incoming signals, amplifies it
and then re-broadcasts it at another frequency
to avoid interference with the incoming signal.
The downward beams can be broad covering
the substantial surface of the earth or narrow
covering hundreds of kilometres.
Satellite Transmission Process
– A network provides programming from a central
– Direct broadcast satellite (DBS)
– High-usage international trunks
Private business networks
Principal Satellite Transmission Bands
band: 4(downlink) - 6(uplink) GHz
the first to be designated
Reserved for telcos
band: 12(downlink) -14(uplink) GHz
– rain interference is the major problem, as water is
excellent absorber of short microwave
– Several downlink stations can overcome this
band: 19(downlink) - 29(uplink) GHz
– equipment needed to use the band is still very
Radio is omni directional and microwave
Radio is a general term often used to
encompass frequencies in the range 3
KHz to 300 GHz.
Mobile telephony occupies several
frequency bands just under 1 GHz.
Frequency Spectrum Classification
VHF TV (high band)
VHF TV (low band)
V. Good V.Low
LAN, BN Low
Media - Transmission
300, 1200, Speed 9600, 19200, 38400,
56000, 64000, 80000
300, 1200, 2400, 4800, 9600, 19200, 38400
2400, 4800, 9600, 19200, 56000, 64000
1M, 10M, 16M, 100M
1M, 10M, 16M, 100M
1M, 2M, 10M, 50M, 100M
up to 45M
up to 50M
Media Selection Criteria
Important factors to consider when choosing a
Speed (or Capacity)
Data Communication and Networking
– Chapter 7 page 187 – 211
Satellites from Tanenbaum’s Text Book