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Tramsmission Media

Data can be transmitted using two mediums: Guided and Unguided. The quality of transmission is determined by both the characteristics of the medium and the characteristics of the signal.

• For guided media, the medium itself is more important in determining the limitations of transmission

• For unguided media, the bandwidth of the signal is more important than the medium because the medium is shared by many applications.

Guided Transmission Media

Guided Transmission Media uses a "cabling" system that guides the data signals along a specific path. The data signals are bound by the "cabling" system. Guided Media is also known as Bound Media. Cabling is meant in a generic sense in the previous sentences and is not meant to be interpreted as copper wire cabling only. Conducted or guided media use a conductor such as a wire or a fibre optic cable to move the signal from sender to receiver. There are four types of Guided media but this paper is only focusing on Coaxial Cable and Optical Fibre.


Coaxial cable

This consists of two conductors. The inner conductor is held inside an insulator with the other conductor woven around it providing a shield. An insulating protective coating called a jacket covers the outer conductor .

The outer shield protects the inner conductor from outside electrical signals. The distance between the outer conductor (shield) and inner conductor plus the type of material used for insulating the inner conductor determine the cable properties or impedance. Typical impedances for coaxial cables are 75 ohms for Cable TV, 50 ohms for Ethernet Thinnet and Thicknet. The excellent control of the impedance characteristics of the cable allow higher data rates to be transferred than with twisted pair cable.

Coaxial cable can be used for:

Television distribution (cable TV system can carry dozens or even hundreds of TV channels at ranges up to a few tens of miles)

Long-distance Telephone Transmission (using frequency-division multiplexing (FDM), a coaxial cable can carry over 10000 voice channels simultaneously)

And Local Area Networks

Transmits both analogue and digital signals (Because of its shielded, concentric construction, coaxial cable is much less susceptible to interference and crosstalk than twisted pair). The principle constraints on performance are attenuation, thermal noise, and intermodulation noise.

Optical fiber

This consists of thin glass fibers that can carry information at frequencies in the visible light spectrum and beyond. The typical optical fiber consists of a very narrow strand of glass called the core. Around the core is a concentric layer of glass called the cladding. A typical core diameter is 62.5 microns (1 micron = 10 -6 meters). Typically Cladding has a diameter of 125 microns. Coating the cladding is a protective coating consisting of plastic, it is called the Jacket.

An important characteristic of fiber optics is refraction. Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium, it "bends" as it passes from one medium to the other. An example of this is when we look into a pond of water. If the angle of incidence is small, the light rays are reflected and do not pass into the water. If the angle of incident is great, light passes through the media but is bent or refracted.

Optical fibers work on the principle that the core refracts the light and the cladding reflects the light. The core refracts the light and guides the light along its path. The cladding reflects any light back into the core and stops light from escaping through it - it bounds the medium!

Optical fibers can be used for:

Long-haul trunks which are increasingly common in the telephone network. Routes average about 900 miles in length and have high capacity (typically 20000 to 60000 voice channels)

Metropolitan trunks which join telephone exchanges in a metropolitan or city area. They have an average length of 7.8 miles and can handle as many as 100000 voice channels.

Rural-exchange trunks that can link towns and villages. Circuit lengths ranging from 25 to 100 miles but most have fewer than 5000 voice channels

Local area networks with a total capacity of 100 Mbps

And Subscriber loops where fibers run directly from the central exchange to a subscriber.

Optical fibers have caused telephone networks to evolve into full-service networks capable of handling not only voice and data, but also image and video.

Advantages of Optical Fiber:

•  Noise immunity: RFI and EMI immune (RFI - Radio Frequency Interference, EMI -Electromagnetic Interference)

•  Security: cannot tap into cable.

•  Large Capacity due to BW (bandwidth)

•  No corrosion

•  Longer distances than copper wire

•  Smaller and lighter than copper wire

•  Faster transmission rate

Disadvantages of optical fiber:

•  Physical vibration will show up as signal noise!

•  Limited physical arc of cable. Bend it too much and it will break!

•  Difficult to splice

The cost of optical fiber is a trade-off between capacity and cost. At higher transmission capacity, it is cheaper than copper. At lower transmission capacity, it is more expensive.

Unguided Transmission Media

Unguided transmission media is data signals that flow through the air. They are not guided or bound to a channel to follow. Wireless or unguided media use radio waves of different frequencies and do not need a wire or cable conductor to transmit signals.

There are three types of RF (radio frequency) propagation:

Ground Wave

Ionospheric

Line of Sight (LOS)

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 Earth's 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 the weather and time of day. The signal bounces off of the ionosphere and back to earth. Ham radios operate in this range.

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, from horizon to horizon). Reflected waves can cause problems. Examples of line of sight propagation are: FM radio, microwave and satellite.

Radio Frequencies

The frequency spectrum operates from 0 Hz (DC) to gamma rays (1019 Hz).

Radio frequencies are in the range of 300 kHz to 10 GHz. We are seeing an emerging technology called wireless LANs. Some use radio frequencies to connect the workstations together, some use infrared technology.

 

Microwave

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 their large bandwidth.

Advantages:

•  They require no right of way acquisition between towers.

•  They can carry high quantities of information due to their high operating frequencies.

•  Low cost land purchase: each tower occupies only a small area.

•  High frequency/short wavelength signals require small antennae.

Disadvantages:

•  Attenuation by solid objects: birds, rain, snow and fog.

•  Reflected from flat surfaces like water and metal.

•  Diffracted (split) around solid objects.

•  Refracted by atmosphere, thus causing beam to be projected away from receiver.

 

Infrared

Transmission uses low frequency light waves to carry data through the air on a direct line-of-sight path between two points.

Uses transmitters/receivers (transceivers) that modulate non-coherent infrared light.

Transceivers must be within line of sight of each other (directly or via reflection ).

Unlike microwaves, infrared does not penetrate walls.

• no security or interference problems

no frequency allocation issue

• no licensing is required

Infrared is used for short-range communication. remote controls used for TV, VCR, stereo systems. They are directional, cheap and easy to build.

 

Bibliography:

Hart-Davis, Guy and Lee, Linda. “Networking Complete”

http://www.techbooksforfree.com/intro_to_data_com/page37.html

http://www.eie.polyu.edu.hk/~em/dtss05pdf/00b%20Guided%20media.pdf

http://duck.cs.und.ac.za/~tanja/Webpage/Networks/part4.pdf

 

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