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Communication – Transmission Media
This unit of work addresses aspects of the following syllabus outcomes:
A student:
H1.2 differentiates between properties of materials and justifies the selection of materials, components and processes in engineering.
H6.2 demonstrates skills in analysis, synthesis and experimentation related to engineering.
Extract from Engineering Studies Stage 6 Syllabus © Board of Studies NSW 1999.
Introduction
Communication involves the transfer of information in the form of text, graphic images, sound or moving pictures from one place to another. The medium through which this transfer takes place—the transmission media—determines the effectiveness of such communication. It is important to understand the types of media available, their method of construction and their advantages and disadvantages in different situations.
Transmission media refers to the physical means by which information is transferred. It does not include books, compact disks, tapes, floppy disks, photographs and slides which are themselves moved from one site to another. Transmission media, then, includes electrical wires, coaxial cables, optical fibres and electromagnetic waves. Besides microwaves, there are two other commonly used wireless media, namely radio and infra-red optical systems.
Communication requires an encoding process to enable data to be transported along a transmission media. On arrival at its destination, the data needs to be decoded to restore it to its original form. For example, a television transmitter encodes television pictures into electrical signals suitable for transmission These then need to be decoded to produce a picture on the television screen.
Coding may be done in analogue or digital formsIn recent years there has been a world trend toward digital encoding. Here, data is encoded into a binary digital form for transmission. Once we have digitised the information we want to send, we must decide whether to transmit it as parallel or series data. If a single byte of data, such as {01100110}, is to be transmitted then it could be sent along eight wires (one bit per wire) as parallel data transmission, see figure 1, or it could be sent along a single wire as a series of data, see figure 2. The parallel method is faster, since all bits are transferred simultaneously but eight separate channels are required. Serial transfer requires only one channel, but it does take longer. The final choice will inevitably depend on cost and efficiency.
Figure 1: Representation of a single byte {01100110} to be transmitted as parallel binary data.
Figure 2: Representation of a single byte {01100110} to be transmitted as series binary data.
The transmission media often introduces unwanted changes to a signal, which limit the speed and frequency range of the signals that can be transmitted by that media. These unwanted changes are often referred to as noise.Lets look at a few of the transmission media available to the modern communicator and discover some of the factors that determine the choices.
Open wires
Although not often seen in recent years, having been replaced by microwave systems or newer technologies, open wires were used extensively in rural areas of Australia to provide twelve speech channels over a single wire strung between telephone poles(see the photograph below). The system provided a cheap but effective means of communication that was predominant despite many problems of security and line maintenance.
Figure 3 Telephone wires on telegraph
posts
Source: Fraser, D (1989) Sydney: from
settlement to city. Engineers Australia: Sydney
Untwisted pairs
Two insulated wires moulded into a straight, flat, parallel pair cable (see figure 3) can be used for short distance or low bit rate communication. This is a cheap method but it is subject to cross talk and spurious noise pick up. The wires are easy to terminate.
Figure 4: Untwisted pairs consist of two wires moulded into a flat, straight wire.
Twisted pairs
Two insulated wires are twisted around each other to form a twisted pair. This system leads to better electrical performance and significantly higher bit rates than untwisted pairs. These pairs are often then, in turn, twisted with other pairs to form a cable that is capable of high-speed communication.
The twisting of wires around each other helps to reduce the noise by cancelling unwanted induced electrical signals and each pair usually carries a single communication. Twisted pairs have become the most common form of transmission media cable used today having applications in telephone and computer networks. By adding a ground shield signal noise from external sources is further reduced. Systems using balanced signals obtain the highest bit rates. The ground shield makes termination a little more difficult.
Figure 5: Cable consists of three twisted pairs that have been, in turn, twisted and insulated.
Carefully dissect an off-cut of twin speaker wire, documenting your results for further reference. Then consider the method of manufacture of the cable you have dissected. How was the copper wire formed into a wire strand and how might the outer shield have been fitted. Thirdly, justify why the cable needs to have the configuration it has. What advantages does this configuration provide for the intended end use of the cable?
Obtain an off-cut of telephone cable and, with a sharp trimming knife, remove the outer shield to expose the several coloured, insulated wires. Carefully remove the insulation from these wires to determine the nature of the wire inside. Document your finding for later reference.Secondly consider the method of manufacture of the cable you have dissected and thirdly, justify why the cable needs to have the configuration it has. What advantages does this configuration provide for the intended end use of the cable?
Coaxial cable
Coaxial cable is a two-wire conductor with a larger bandwidth than twisted pair cable. It is used in television, radio and Ethernet connections where each coaxial cable supports about 60 speech channels. The inner cable is heavily insulated so it is safe from bending and crushing and separated from the braided outer conductor as can be seen in figure 5. This outer cable is in turn insulated. The resultant electrical field occurs outside the coaxial cable, thus reducing the level of noise making the medium suitable for conducting high bit rates over longer distances than twisted pairs.
Figure 6: Coaxial cable.
Take an off-cut of coaxial cable and, with a sharp trimming knife, remove the outer shield for about 40 mm length to expose the braided outer conductor. Push back the braiding (or unravel it) to expose the insulator and bare about 20 mm of the inner cable. Document your finding for later reference.Secondly, consider the method of manufacture of the cable you have dissected. How was the copper wire formed into a wire strand, how was the insulation formed and how might the outer shield been fitted.Thirdly, justify why the cable needs to have the configuration it has. What advantages does this configuration provide for the intended end use of the cable?
Optical fibres
Optical fibre consists of a glass or plastic core surrounded by a cladding with a higher refreactive index. Data is carried as pulses of light from a laser or high-power light emitting diode (LED). The light pulses are contained within the core as a result of internal reflection. The core/cladding interface may be stepped or graded, each producing a different waveform. In a step index fibre, there is a sudden transition in refractive index. A graded index fibre has a gradual transition from high to low index, and much higher performance.
Optical fibre is non-electrical, and therefore completely immune from electrical radiation and interference problems. It has the highest bit rate of all media. Each fibre optic strand can support thousands of speech channels and multiple television channels simultaneously. High bandwidths give enormous transmission capacity for national and intercontinental links and greater distances can be tolerated between repeaters. The small physical size, non-corrosive construction and immunity from electrical interference make optical fibre a highly attractive form of transmission media.
There is a need in optical fibre communication for semiconductors for both the emitter (usually gallium arsenide laser) and the photodetector. Communication signals are converted to light by the emitter and reconverted by the receiver.
Microwaves
An extremely high frequency (1 GHz or higher) "line of sight" radio link carries a point-to-point signal, which is accurately focused using dish antennas. Since it is a line of sight transmission, repeater towers need to be located about every 35 km. In a microwave radio system, telecommunications traffic is transmitted in the form of directed beams of microwaves. Microwaves are a kind of electromagnetic radiation like light or like the radio waves used in ordinary broadcasting, but of a frequency intermediate between these.
Microwave systems have sufficient bandwidth capacity to support a large number of voice channels and one or two television channels and can achieve high bit rates over moderately long distances. They have been used extensively in inner city data systems where cabling would be very expensive.
Satellites are a special application of a microwave system. Here, ground stations with large dishes direct information to a communications satellite in geostationary orbit above the Equator. Originally these satellites were passive objects but the modern satellite captures the signal, amplifies it using a transponder, and relays it back to Earth or on to another satellite. Each channel is managed by a transponder, which can support thousands of speech channels and about four television channels simultaneously. Despite their high cost of construction and launch, satellites have become part of our way of life for live television, international telephone and data links.
Useful web links
http://www.informit.com/articles/article.aspx?p=683070 
http://www.csse.monash.edu.au/~timf/cse3020/transmedia.pdf
http://en.wikipedia.org/wiki/Transmissin_medium
Telstra Information Kit No. 2, "This Busy Ray"
Other Useful Resources
Copeland, PL 2001, Engineering
Studies The Definitive Guide Vol 2, Star Printery, Sydney.
Fraser, D (1989) Sydney: from
settlement to city. Engineers Australia: Sydney
Ivanoff, V 1997, Engineering
Mechanics, The McGraw-Hill Companies, Inc.
Schlenker, B R & McKern, D 1994,
Introduction to Engineering Mechanics, Jacaranda Press.
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