Transmission Media - ERNETisg/NETWORKS/SLIDES/TransmissionMe… · • subject to external electromagnetic interference ... – Smaller size and lighter weight ... Dr. Indranil Sen

Transmission Media

Dr. Indranil Sen Gupta Transmission Media Slide 1

Introduction

• Physical path between transmitter and receiver in a

data transmission system.

• May be classified into two types:


– Guided:- waves are guided along a solid medium, such

as copper twisted pair, copper coaxial cable, or optical

fiber.

– Unguided:- provides a means for transmitting electro-

magnetic signals but do not guide them.

• Wireless transmission

Contd.

• 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.


– For unguided media, the bandwidth of the signal produced by the

transmitting antenna is more important than the medium.

• Signals at lower frequencies are omnidirectional

– propagate in all directions

• For higher frequencies, focussing the signals into a directional beam is

possible.

Guided Transmission Media

• Transmission capacity depends critically on the

distance, and whether the medium is point-to-point

or multipoint (e.g. LAN).

• Point-to-point transmission performances:


• Point-to-point transmission performances:

Transmission

Medium

Total Data

Rate

Bandwidth Repeater

Spacing

Twisted pair 4 Mbps 3 MHz 2-10 Km

Coaxial cable 500 Mbps 350 MHz 1-10 Km

Optical fiber 2 Gbps 2 GHz 10-100 Km

Figure 3.2 from the book by Stallings



Guided Media: Twisted Pair

• A twisted pair consists of two insulated copper wires

arranged in a regular spiral pattern.

– Twisting decreases the crosstalk interference between adjacent pairs

in a cable.


– Typically, a number of pairs are bundled together into a cable by

wrapping them in a tough protective sheath.

• Most common transmission media for both analog and

digital signals.

– Telephone network

– Connecting computers in a LAN (upto 100 Mbps)

Twisted Pair (contd.)

• Two common types:

– Unshielded Twisted Pair (UTP)

• ordinary telephone wire

• subject to external electromagnetic interference

• Two commonly used categories (vide Electronic Industries

Association standard EIA-568)


Association standard EIA-568)

– Category 3: upto 16 MHz

– Category 5: upto 100 MHz

• A key difference is the number of twists in the cable per

unit distance.

– Cat-3 ==> 3-4 twists per foot; Cat-5 ==> 3-4 twists per inch

– Tighter twisting of Cat-5 provides much better performance, but

also increases the cost.

Guided Media: Coaxial Cable

• 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.


• Due to its shielding, coaxial cables are much less

susceptible to interference or crosstalk than twisted pair.

• Used in a variety of applications:

– Television distribution (cable TV)

– Long-distance telephone transmission (10,000 voice channels per

cable)

– Local Area Networks

Guided Media: Optical Fiber

• An optical fiber is a thin (2-125 micrometer), flexible

medium capable of conducting an optical ray.

– Made of ultrapure fused silica, glass fiber or even plastic.

• It has a cylindrical shape and consists of three concentric


• It has a cylindrical shape and consists of three concentric

sections: the core, the cladding, and the jacket.

– The core consists of one or more very thin strands, or fibers, made

of glass or plastic.

– The cladding is a glass or plastic coating that has optical properties

different from that of the core.

– The jacket surrounds one or a bundle of cladded fibers.

Optical Fiber (contd.)

• Main advantages of optical fiber:

– Greater capacity (2Gbps over tens of kilometers)

– Smaller size and lighter weight

– Lower attenuation


– Lower attenuation

– Electromagnetic isolation

– Greater repeater spacing

• Long-haul fiber transmission is becoming increasingly

common in the telephone network.

– About 900 miles with 20,000 to 60,000 voice channels

Optical Fiber (contd.)

• Optical fiber systems operate in the range of 10^14 to

10^15 Hz.

– Light from a source enters the cylindrical glass or plastic core.

– Rays at shallow angles are reflected and propagated along the

fiber; other rays are absorbed by the surrounding material.

• Three types of communication:


• Three types of communication:

– Multimode: Refers to the variety of angles that will reflect.

Multiple propagation path exists, signal elements spread out in

time, and hence limits the data rate.

– Single-mode:- When the fiber code radius is reduced, fewer angles

will reflect. By reducing the radius of the core to the order of a

wavelength, only a single angle or mode can pass (the axial ray).

– Multimode graded index: By varying the refractive index of the

core, rays may be focussed more efficiently than multimode.

Attenuation of Typical Guided Media



Wireless Transmission

• Transmission and reception are achieved by means of

antenna.

– For transmission, the antenna radiated electromagnetic radiation in

the air.


– For reception, the antenna picks up electromagnetic waves from the

surrounding medium.

• Basically two types of configuration:

– Transmitting antenna puts out a focussed electromagnetic beam.

• Transmitter & receiver must be carefully aligned.

– Transmitted signal spreads in all directions.

• Can be received by many antennas.

Contd.

• Three general ranges of frequencies:

– 2-40 GHz

• microwave frequencies

• highly directional beams are possible


• suitable for point-to-point and satellite transmission

– 30 MHz - 1 GHz

• broadcast radio range

• suitable for omnidirectional applications

– 0.3 - 200 THz

• infrared range

• useful for point-to-point and multipoint applications within confined

areas, such as a single room.

Wireless: Terrestial Microwave

• Most common type of microwave antenna is the parabolic

dish.

– Antenna is fixed rigidly

– Focuses a narrow beam to achieve line-of-sight transmission


– Focuses a narrow beam to achieve line-of-sight transmission

• Primarily used in long-haul communication, as an

alternative to coaxial cable or optical fiber.

• Maximum distance between antennas

d = 7.14 sqrt(kh)

where d is the distance in Km, h is the antenna height in

meters, and k is an adjustment factor (= 1.33 approx.)

Wireless: Satellite Microwave

• A communication satellite is basically a microwave relay

station.

– Used to link two or more earth stations.

– Satellite receives transmission on one frequency band (uplink),


– Satellite receives transmission on one frequency band (uplink),

amplifies or repeats the signal, and transmits it on another frequency

(downlink).

– A single orbiting satellites operate on a number of frequency bands,

called transponders.

• Geostationary orbit

– often necessary for it to function effectively

– height of 35,784 Km

Satellite Communication

Configurations

Point-to-point linkBroadcast link


Transmitter

Multiple

Receivers

Multiple

Receivers

Satellite (contd.)

• Two satellites using the same frequency band, if close

enough, will interfere with each other.

– Minimum 4 degrees spacing in the 4/6 GHz band.




• Most important applications of satellite:

– Television distribution

– Long-distance telephone transmission

– Private business network (using VSATs).

Very Small Aperture Terminal

(VSAT)

• A relatively low-cost solution.

• A number of subscriber stations are equipped with low-cost

VSAT antennas.

– Using some protocol, these stations share a satellite channel for


– Using some protocol, these stations share a satellite channel for

transmission to a hub station.

– The hub station can exchange messages with each of the subscribers

as well as relay messages between subscribers.

VSAT Configuration

256 Kbps

256 Kbps

56 Kbps


56 Kbps

HUB

Wireless: Broadcast Radio

• Omnidirectional

• Covers VHF and part of UHF band

– 30 MHz -- 1 GHz

• Apart from FM radio as well as UHF and VHF TV, also


• Apart from FM radio as well as UHF and VHF TV, also

used in data networking applications.

Wireless: Infrared

• Uses transceivers that modulate non-coherent infrared light.

• Must be in light of sight, either directly or via reflection

from a light-colored surface such as the ceiling of a room.

• Does not penetrate walls.


• Does not penetrate walls.

• No frequency allocation issue.

– No licensing required.

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Transmission Media - ERNETisg/NETWORKS/SLIDES/TransmissionMe… · • subject to external electromagnetic interference ... – Smaller size and lighter weight ... Dr. Indranil Sen