What is transmission media? define.

Transmission Media

 

For any networking to be effective, raw stream of data is to be transported from one device to

other over some medium. Various transmission media can be used for transfer of data. These

transmission media may be of two types −

• Guided −

fixed path. For example, copper wires, fibre optic wires, etc.

• Unguided −

electromagnetic signal. For example, radio waves, lasers, etc.

Each transmission media has its own advantages and disadvantages in terms of

bandwidth, speed, delay, cost per bit, ease of installation and maintenance, etc. Let’s discuss

some of the most commonly used media in detail.

Twisted Pair Cable

 

Copper wires are the most common wires used for transmitting signals because of good

performance at low costs. They are most commonly used in telephone lines. However, if two

or more wires are lying together, they can interfere with each other’s signals. To reduce this

electromagnetic interference, pair of copper wires are twisted together in helical shape like a

DNA molecule. Such twisted copper wires are called twisted pair. To reduce interference

between nearby twisted pairs, the twist rates are different for each pair.

 

Up to 25 twisted pair are put together in a protective covering to form twisted pair cables that

are the backbone of telephone systems and Ethernet networks.

Advantages of twisted pair cable

Twisted pair cable are the oldest and most popular cables all over the world. This is due to the

many advantages that they offer −

• Trained personnel easily available due to shallow learning curve

• Can be used for both analog and digital transmissions

• Least expensive for short distances

• Entire network does not go down if a part of network is damaged

Disadvantages of twisted pair cable

With its many advantages, twisted pair cables offer some disadvantages too −

• Signal cannot travel long distances without repeaters

• High error rate for distances greater than 100m

• Very thin and hence breaks easily

• Not suitable for broadband connections

 

Shielding twisted pair cable

 

To counter the tendency of twisted pair cables to pick up noise signals, wires are shielded in

the following three ways −

• Each twisted pair is shielded.

• Set of multiple twisted pairs in the cable is shielded.

• Each twisted pair and then all the pairs are shielded.

Such twisted pairs are called shielded twisted pair (STP) cables.

 

UTP

 

The wires that are not shielded but simply bundled together in a protective sheath are

called unshielded twisted pair (UTP) cables. These cables can have maximum length of 100

metres.

Shielding makes the cable bulky, so UTP are more popular than STP. UTP cables are used as the

last mile network connection in homes and offices.

Coaxial Cable

Coaxial cables are copper cables with better shielding than twisted pair cables, so that

transmitted signals may travel longer distances at higher speeds. A coaxial cable consists of

these layers, starting from the innermost −

• Stiff copper wire as core

• Insulating material surrounding the core

• Closely woven braided mesh of conducting material surrounding the insulator

• Protective plastic sheath encasing the wire

Coaxial cables are widely used for cable TV connections and LANs.

 

Advantages of Coaxial Cables

These are the advantages of coaxial cables −

• Excellent noise immunity

• Signals can travel longer distances at higher speeds, e.g. 1 to 2 Gbps for 1 Km cable

• Can be used for both analog and digital signals

• Inexpensive as compared to fiber optic cables

• Easy to install and maintain

Disadvantages of Coaxial Cables

These are some of the disadvantages of coaxial cables −

• Expensive as compared to twisted pair cables

• Not compatible with twisted pair cables

Optical Fiber

 

 

Thin glass or plastic threads used to transmit data using light waves are called optical fiber.

Light Emitting Diodes (LEDs) or Laser Diodes (LDs) emit light waves at the source, which is read

by a detector at the other end. Optical fiber cable has a bundle of such threads or fibres

bundled together in a protective covering. Each fiber is made up of these three layers, starting

with the innermost layer −

• Core made of high quality silica glass or plastic

• Cladding made of high quality silica glass or plastic, with a lower refractive index than

the core

• Protective outer covering called buffer

Note that both core and cladding are made of similar material. However, as refractive index of

the cladding is lower, any stray light wave trying to escape the core is reflected back due

to total internal reflection.

 

Optical fiber is rapidly replacing copper wires in telephone lines, internet communication and

even cable TV connections because transmitted data can travel very long distances without

weakening. Single node fiber optic cable can have maximum segment length of 2 kms and

bandwidth of up to 100 Mbps. Multi-node fibre optic cable can have maximum segment

length of 100 kms and bandwidth up to 2 Gbps.

Advantages of Optical fiber

Optical fibre is fast replacing copper wires because of these advantages that it offers −

• High bandwidth

• Immune to electromagnetic interference

• Suitable for industrial and noisy areas

• Signals carrying data can travel long distances without weakening

 

Disadvantages of Optical fiber

Despite long segment lengths and high bandwidth, using optical fiber may not be a viable option for every one due to these

disadvantages −

• Optical fiber cables are expensive

• Sophisticated technology required for manufacturing, installing and maintaining optical fiber cables

• Light waves are unidirectional, so two frequencies are required for full duplex transmission