1. Transmission Media
A transmission medium can be broadly defined as
anything that can carry information from a source to a
destination. For example, the transmission medium for
two people having a dinner conversation is the air. For a
written message, the transmission medium might be a
mail carrier, a truck, or an airplane.
2.
3. Guided Media (or Wired Media)
In this type of media, the signal energy is contained
and guided within a solid media. The guided media
is used for point-to-point communication. Example
of guided media are Twisted pair cable, Co-axial
cable and Fiber optic cable.
Unguided Media (or Wireless Media)
In the unguided media, the signal energy
propagates in the form of unguided electromagnetic
waves. The examples of unguided media are radio
and infrared light.
4. Twisted-pair cables
Twisted-pair cables are usually made of copper and a
pair of wires are twisted together to reduce interference
by adjacent wires as shown in fig. A twisted pair consists
of two conductors (normally copper), each with its own
plastic insulation, twisted together. One of the wires is
used to carry signals to the receiver, and the other is
used only as a ground reference.
5. This means, if two wires are parallel to each other, the
noise or cross talk may effect one wire and the
difference between two levels will vary.
When these wires are twisted, both wire have similar
effect of noise. This way, the receiver receives correct
signal.
The number of twists on the cable define the quality of
signals carried by them. So, more twists means better
quality signals.
6. Twisted pair wires are generally used in local
telephone communication and for digital data
transmission over short distance upto 1 km.
Twisted pairs are normally used to connect
terminals to the main computer up to short
distances.
Data transmission speeds of up to 9600 bits per
second (bps) are possible for a distance up to 100
meters.
For longer distance, speed of data transmission is
1200 bps.
7. Advantages
• It is easiest to install. manpower to repair and
service are easily available.
• In telephone system, signal can travel several
kilometers without amplification.
• Can be used for both analog and digital data
transmission.
• Least costly for short distances.
• It is flexible and easy to connect.
• It has low weight.
8. Disadvantages
1. Higher error rates when the line length is more
than 100 meters because it is easily affected by
more signals.
2. Being thin in size, likely to break easily.
3. It has low bandwidth.
4. It only supports data transfer rate up to 10 MBPS
(Megabytes per second).
9. Types of Twisted Pair Cables
(a) Unshielded Twisted Pair (UTP)
(b) Shielded Twisted Pair (STP)
UTP : A twisted pair consists of two insulated conductor
twisted together in the spiral form as shown in Fig. It
can be shielded with plastic cover. The UTP cables are
very cheap and easy to install. But they are badly
affected by the noise interference.
STP :STP cable has a metal foil to cover each pair of
insulating conductors. This is known as the metal shield.
It reduces the interference of the noise but makes the
cable bulky and expensive. So practically UTP is more
used than STP.
10.
11. Co-axial Cable
Co-axial cables consist of a central copper wire
surrounded by a PVC insulation over which a sleeve
of copper mesh is placed. The metal sleeve is again
shielded by an outer shield of thick PVC material as
shown in fig.
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PVC – Polyvinyl chloride
A polymer of vinyl chloride is used instead of
rubber in electric cables.
12.
13. The construction of the co-axial cable gives a good
combination of high bandwidth and excellent noise
immunity.
For 1 km cables, a data rate of 10 mbps is feasible.
Higher data rates are possible for shorter cables.
Co-axial cables are widely used for cable TV and
for long distance transmission within the telephone
system.
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Shielding : protection of circuit from outside noise or unwanted
signals.
Attenuation : Reduction of signal strength during transmission.
14.
15. Disadvantages
1. Coaxial cables are more expensive.
2. Not compatible to twisted pair cables.
3. Heavy as compared to twisted pair cables.
16. Applications of Co-axial Cables
1. Telephone networks
2. Cable TV
3. Traditional Ethernet LANs
4. Digital transmission
17. Fiber-Optic cable
A fiber-optic cable is made of glass or plastic and transmits
signals in the form of light signals. The construction of an
optical fiber cable is as shown in Fig. It consists of an inner
glass core surrounded by a glass cladding, that reflects the
light back into core. Each fiber is surrounded by a plastic
jacket.
18. In fibre optics, semiconductor lasers transmit information in the
form of light along hair-thin glass (optical) fibers at the speed
of light, (1,86,000 miles second) with no significant loss of
intensity over very long distances. As shown in fig. the system
basically consists of fiber optic cables that are made of tiny
threads of glass or plastic.
