COMPUTER NETWORK AND SECURITY

TRANSMISSION MEDIA 

These are the means by which a communication signal is carried from one system to another. These media can carry information from a source to a destination.  The transmission media can usually be free space such as: satellite, microwave, radio and infrared systems, metallic cables such as: twisted pair, or coaxial cable, or fiber-optic cable. 

In telecommunication, transmission media can be divided into two broad categories: 

1. Guided transmission media 

2. Unguided transmission media 

 

 

 

Guided Transmission Media 

Guided Transmission media uses a cabling system that guides the data signals along a specific path. They provide the physical path way for the transmission of the data from the source to the destination. The data signals travelling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductor that accept and transport signals in the form of electric current. Optical fiber is a cable that accepts and transport signals in the form of light.  Guided Media are also known as Bound media or wired media. 

 

1. Twisted Pair Cable 

 

Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources; for instance, electromagnetic radiation from unshielded twisted pair (UTP) cables, and crosstalk between neighboring pairs. 

This cable is the most commonly used and is cheaper than others. It is lightweight, cheap, can be installed easily, and they support many different types of network. Some important points: 

• Its frequency range is 0 to 3.5 kHz.  
• Typical attenuation is 0.2 dB/Km @ 1kHz.  
• Typical delay is 50 µs/km.  
• Repeater spacing is 2km.  

 

 

 

 

 

Twisted Pair is of two types : 

• Unshielded Twisted Pair (UTP)  
• Shielded Twisted Pair (STP)  

Unshielded Twisted Pair Cable 

It is the most common type of telecommunication when compared with Shielded Twisted Pair Cable which consists of two conductors usually copper, each with its own colour plastic insulator. Identification is the reason behind colored plast

ic insulation. 

Shielded Twisted Pair Cable 

This cable has a metal foil or braided-mesh covering which encases each pair of insulated conductors. Electromagnetic noise penetration is prevented by metal casing. Shielding also eliminates crosstalk . 

It has same attenuation as unshielded twisted pair. It is faster the unshielded and coaxial cable. It is more expensive than coaxial and unshielded twisted pair. 

 

2. Coaxial Cable 

 

Coaxial is called by this name because it contains two conductors that are parallel to each other. Copper is used in this as centre conductor which can be a solid wire or a standard one. It is surrounded by PVC installation, a sheath which is encased in an outer conductor of metal foil, braid or both. 

Outer metallic wrapping is used as a shield against noise and as the second conductor which completes the circuit. The outer conductor is also encased in an insulating sheath. The outermost part is the plastic cover which protects the whole cable. 

Here the most common coaxial standards. 

• 50-Ohm RG-7 or RG-11 : used with thick Ethernet.  
• 50-Ohm RG-58 : used with thin Ethernet  
• 75-Ohm RG-59 : used with cable television  

• 93-Ohm RG-62 : used with ARCNET.  

RG- Radio guide

 

Advantages: 

• Bandwidth is high  
• Used in long distance telephone lines.  
• Transmits digital signals at a very high rate of 10Mbps.  

• Much higher noise immunity  
• Data transmission without distortion.  
• They can span to longer distance at higher speeds as they have better shielding when compared to twisted pair cable  

Disadvantages: 

• Single cable failure can fail the entire network.  

• Difficult to install and expensive when compared with twisted pair.  

 

 

 

C) Fiber Optic Cable 

These are similar to coaxial cable. It uses electric signals to transmit data. At the centre is the glass core through which light propagates.  

In multimode fibers, the core is 50microns, and In single mode fibers, the thickness is 8 to 10 microns. 

The core in fiber optic cable is surrounded by glass cladding with lower index of refraction as compared to core to keep all the light in core. This is covered with a thin plastic jacket to protect the cladding. The fibers are grouped together in bundles protected by an outer shield. 

Fiber optic cable has bandwidth more than 2 gbps (Gigabytes per Second) 

Advantages: 

• Provides high quality transmission of signals at very high speed.  

• These are not affected by electromagnetic interference, so noise and distortion is very less.  
• Used for both analog and digital signals.  

Disadvantages: 

• It is expensive  
• Difficult to install.  

• Maintenance is expensive and difficult.  

 

 

 

 

1. Unguided Media:

It is also referred to as Wireless or Unbounded transmission media.No physical medium is required for the transmission of electromagnetic signals.

Features:

• Signal is broadcasted through air
• Less Secure
• Used for larger distances

There are 3 major types of Unguided Media:

1. Radiowaves –

 

These are easy to generate and can penetrate through buildings. The sending and receiving antennas need not be aligned. Frequency Range:3KHz – 1GHz. AM and FM radios and cordless phones use Radiowaves for transmission.

Further Categorized as (i) Terrestrial and (ii) Satellite.

2. Microwaves –

 

It is a line of sight transmission i.e. the sending and receiving antennas need to be properly aligned with each other. The distance covered by the signal is directly proportional to the height of the antenna. Frequency Range:1GHz – 300GHz. These are majorly used for mobile phone communication and television distribution.

3. Infrared –

 

Infrared waves are used for very short distance communication. They cannot penetrate through obstacles. This prevents interference between systems. Frequency Range:300GHz – 400THz. It is used in TV remotes, wireless mouse, keyboard, printer, etc.

 

Satellites:

A satellite is an object that revolves around another object. For example, earth is a satellite of The Sun, and moon is a satellite of earth.

A communication satellite is a microwave repeater station in a space that is used for telecommunication, radio and television signals.

How a Satellite Works

Uplink frequency is the frequency at which a ground station is communicating with a satellite. The satellite transponder converts the signal and sends it down to the second earth station, and this is called Downlink frequency.

Satellite Communication Basics

The satellites receive and retransmit the signals back to earth where they are received by other earth stations in the coverage area of the satellite. Satellite's footprint is the area which receives a signal of useful strength from the satellite.

Satellite Frequency Bands

The satellite frequency bands which are commonly used for communication are the C band, Ku-band, and Ka- band.

Earth Orbits

A satellite, when launched into space, needs to be placed in a certain orbit to provide a particular way for its revolution, so as to maintain accessibility and serve its purpose whether scientific, military or commercial. Such orbits which are assigned to satellites, with respect to earth, are called Earth Orbits.

The important kinds of Earth Orbits are −

 

• Geo-synchronous Earth Orbit
• Geo-stationary Earth Orbit
• Medium Earth Orbit
• Low Earth Orbit

 

 

 

Satellite Network Segments

Satellite networks have three major segments:

1. Space segment—The space segment is the actual design of the satellite and the orbit in which it operates. Satellites are launched into specific orbits to cover the parts of the earth for which coverage is desired.
2. Control segment—The control segment defines the frequency spectrum over which satellites operate and the types of signaling techniques used between the ground station and the satellite to control those communications.