communication transmission media

COMMUNICATION TRANSMISSION MEDIA:CHARACTERISTICTS, PROS AND CONS

MARTIN MATOVU

06/U/5761/PSA 206006175BSc. In Telecommunications Eng, Department of Electrical Engineering

Faculty of Technology, Makerere UniversityKampala, Uganda

MARTIN MATOVU BSTE IV

Abstract - Inorder to carry a communication signal from one end to another, we need a physical system which is the transmission medium. A transmission medium is a material substance which can propagate energy waves. [1] Take for example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media for sound. This paper deals with the different transmission media: their characteristics, comparative advantages as well as disadvantages.

I. INTRODUCTION

In telecommunications, transmission is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired or wireless. [2] A transmission may be simplex (one station is a transmitter and the other is the receiver), Half-duplex (both stations may transmit, but only one at a time), Duplex (both stations transmit simultaneously).Many transmission media are used as communication

Figure 1: Operating frequencies of different transmission media [3]

channels thus the term, Communication transmission media. The quality of transmission is determined by both the characteristics of the medium and the characteristics of the signal. There are two basic categories of Transmission Media: Guided media and unguided media

Basically, the guided transmission media uses a "cabling" system that guides the data signals along a specific path. The signals are bound by the "cabling" system. Guided media is also known as bound media.And the unguided transmission media consists of a means for the signals to propagate but nothing to guide them along a specific path and as such are often called unbound media.

II. TRANSMISSION MEDIA

There are design factors considered when using transmission media: Bandwidth- higher bandwidth gives higher data rates, Transmission impairments like attenuation, interference and the number of receivers- in guided media, more receivers (multi-point) introduce more attenuation.

For the guided media, the medium itself is more important in determining the limitations of transmission while for unguided media, the bandwidth of the signal is more important than the medium because the medium is shared by many applications. [4]

Figure 2: Categories of transmission media [5]

A. GUIDED MEDIA

The three basic types of guided media include; Twisted pair, Coaxial cable, Optical fiber as shown in the figure 2. TWISTED PAIR

They are


often twisted together, bundled into cables and insulated. The nearby pairs have different twist length inorder to reduce crosstalk. The twist length varies from 5 to 15cm, and wire thickness 0.4 to 0.9mm.

(a) Twisted pair

Being the most common medium, twisted pair has wide applications- telephone networks, Private branch exchange (PBX) within buildings, Local area networks (LAN) usually 10Mbps or 100Mbps.

The transmission characteristics exhibited by the twisted pair include:- Analog signal transmission requires amplifiers for

every 5 to 6km.- Digital signal transmission requires repeaters every

few hundred meters- Has the highest attenuation compared to other media- Limited bandwidth (1MHz) and data rates (100MHz)- Susceptible to interference and noise.On the other hand, twisted pair is cheap and easy to work with as well.There are two categories of the twisted pair- Unshielded twisted pair (UTP) and Shielded twisted pair (STP)

COAXIAL CABLES

These basically have a braided shield as an outer conductor and the inner conductor is a solid metal. The

two conductors are separated by an insulating material and are covered by padding.

Coaxial cables allow operation for a wider range of frequencies, the conductor diameter range from 1 to 2.5 cm. Due to the shielding; coaxial cables are much less susceptible to interference and crosstalk.

Coaxial cables various applications due to their versatility. They are used for TV distribution, Long distance telephone transmission using FDM, Short distance computer systems links and Local area networks as well. Common to coaxial cables, are the following characteristics:- Analog, amplifiers needed for every few km- Frequency up to 500MHz- Digital, repeaters needed every 1km and closer for

higher data rates- Performance is limited by attenuation, thermal noise,

and intermodulation noise

OPTICAL FIBER

Made of a core with one or more thin strands or fibers (8 to 100 mm), Cladding: glass or plastic coating with different optical properties than the core, the -Core/Cladding interface: reflector –confine ray and the jacket for protection against moisture, abrasion and

crushing.


The ultrapure fiber is used for lowest losses though difficult to manufacture and the higher loss mutlicomponent glass fibers are more economical. However, the plastic fiber is the cheapest –used for short-haul links. Fiber transmission has three modes as shown below;

(a) Step-index multimode

(b) Graded-index multimode

(c) Single mode

The benefits of using fiber include: unlimited capacity up to hundreds of Gbps, small size and weight, lower attenuation, electromagnetic isolation as well as achieving greater repeater spacing up to tens of kms at least. The applications of fiber range from being used in Long-haul trunks, metropolitan trunks, rural exchange trunks, subscriber loops and LANs.

Optical fiber has the following characteristics:- They act as wave guide for 1014 to 1015 Hz- Utilize Light Emitting Diode (LED) which are

Cheaper, have wider operating temp range and last longer

- Operate with the Injection Laser Diode (ILD) which are more efficient and have higher data rate

- Utilizes Wavelength Division Multiplexing (WDM)

Figure 3: Attenuation of typical guided media [3]

B. UNGUIDED MEDIA

The unguided media are classified by the type of wave propagation; Radio, microwave and satellite also shown in figure 2.

RADIO FREQUENCY PROPAGATION [6]

Radio Frequencies are in the range of 300 kHz to 10 GHz. There are 3 types of Radio Frequency propagation: Ground Wave, Ionospheric and Line of Sight (LOS) Propagation.

Ground Wave Propagation follows the curvature of the Earth. Ground Waves have carrier frequencies up to 2 MHz. AM radio is an example of Ground Wave Propagation.

Ionospheric Propagation bounces off of the Earths Ionospheric Layer in the upper atmosphere. It is sometimes called Double Hop Propagation. It operates in the frequency range of 30 - 85 MHz. Because it depends on the Earth's ionosphere, it changes with weather and time of day. The signal bounces off of the ionosphere and back to earth. Ham radios operate in this range.

Ionospheric Propagation

Line of Sight Propagation transmits exactly in the line of sight. The receive station must be in the view of the transmit station. It is sometimes called Space Waves or Tropospheric


Propagation. It is limited by the curvature of the Earth for ground based stations (100 km: horizon to horizon). Reflected waves can cause problems. Examples of Line of Sight Propagation are: FM Radio, Microwave and Satellite.

Table showing operating Radio frequencies

MICROWAVE [6]

Microwave transmission is line of sight transmission. The Transmit station must be in visible contact with the receive station. This sets a limit on the distance between stations depending on the local geography. Typically the line of sight due to the Earth's curvature is only 50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across the country.

Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry large quantities of data due to the large bandwidth. They have the following advantages:

a. Require no right of way acquisition between towers. b. Carry high quantities of information due to

their high operating frequencies.

c. Low cost land purchase: each tower occupies small area.

d. High frequency/short wavelength signals require small antenna.

Microwave propagation has the following disadvantages: a. Attenuation by solid objects: birds, rain, snow and

fog. b. Reflected from flat surfaces like water and metal. c. Diffracted (split) around solid objects d. Refracted by atmosphere, thus causing beam to be

projected away from receiver.

SATELLITE [6]

Satellites are transponders that are set in a geostationary orbit directly over the equator. A transponder is a unit that receives on one frequency and retransmits on another. The geostationary orbit is 36,000 km from the Earth's surface. At this point, the gravitational pull of the Earth and the centrifugal force of Earths rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to fling it out to space.


The types of satellites include: Geosynchronous Earth Orbit (GEO) Medium Earth Orbit (MEO) Low Earth Orbit (LEO)

Satellite communication finds applications in the telecommunications sector especially for inter-continental communication, TV broadcast where usage is increasing, Navigation and other similar local applications GPS.

The uplink is the transmitter of data to the satellite. The downlink is the receiver of data. Uplinks and downlinks are also called Earth stations due to be located on the Earth. The footprint is the "shadow" that the satellite can transmit to. The shadow being the area that can receive the satellite's transmitted signal.

REFERENCES

1. http://en.wikipedia.org/wiki/Transmission_medium2. http://en.wikipedia.org/wiki/

Transmission_(telecommunications)3. William Stallings, Data & Computer communications, 6th

Edition , November, 2, 19994. http://arquiredes.unsl.googlepages.com/

TransmissionMedia.pdf5. Transmission media- guided and unguided by Norrima

Mokhtar6. http://www.techbooksforfree.com/intro_to_data_com/

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