Data Communication and Transmission Media: Notes

The correct answer is D, transparent plastic. An optical fibre carries information as pulses of light, not as an electric current, so its core and cladding must be transparent and not conducting. The two materials used are ultra-pure silica glass for long-distance cables and transparent plastic, usually an acrylic polymer, for short cheap links; among the four options only transparent plastic can pass light. Option A, copper, is the conductor of twisted pair and coaxial cable, which carry electrical signals and are exactly what fibre replaced. Option B, aluminium, is used for overhead power lines and for shielding, and like any metal it is opaque. Option C, steel, appears only as strength members in the outer sheath of some cables, never as the light-carrying core. The examinable point is that an optical fibre is a dielectric, not a metal.

The correct answer is B, sources. At the transmitting end of a fibre link the electrical signal must be turned into light, and the two devices that do it are the light-emitting diode and the laser diode. The light-emitting diode is cheap, long-lived and spreads its light over a range of wavelengths, so it suits short multi-mode links; the laser diode gives a narrow, coherent beam that couples into a thin single-mode core and so suits long high-speed routes. Option A, detectors, is the job of the photodiode at the far end, which turns the arriving light back into current. Option C, repeaters, describes a complete unit placed along a long route to receive, clean and resend the signal; a laser may sit inside one, but a repeater is not what the diode itself is called. Option D, amplifiers, strengthen a signal, and in fibre that work is done by an erbium-doped fibre amplifier, not by the source.

The correct answer is B, 540 kHz. In frequency division multiplexing each channel is given its own slice of the frequency band, and a small unused strip called a guard band is left between two neighbouring channels so that one does not spill into the next. Five channels of 100 kHz each need 500 kHz. Guard bands are needed only in the gaps between channels, and five channels placed side by side have four gaps, so four guard bands of 10 kHz add 40 kHz. The total is 500 plus 40, that is 540 kHz. Option A, 500 kHz, is the answer of a candidate who forgot the guard bands altogether. Option D, 560 kHz, comes from using five guard bands instead of four, the classic fencepost mistake of counting gaps as if they equalled the number of items. Option C, 460 kHz, wrongly subtracts the guard bands instead of adding them. Remember the rule: for n channels there are n minus 1 guard bands.

The correct answer is B, total internal reflection. The fibre has a core of higher refractive index surrounded by a cladding of lower refractive index. Light that enters the core and meets the boundary at an angle greater than the critical angle is reflected entirely back into the core, with none of it refracted away into the cladding. Repeating this thousands of times, the ray travels along the fibre even around bends. Option A, diffraction, is the bending of a wave around an obstacle or through a narrow slit and plays no part here. Option C, polarisation, is the restriction of a wave vibration to one plane, used in sunglasses and in liquid crystal displays, not in guiding light along a fibre. Option D, dispersion, is the splitting of light into its colours and is in fact an unwanted effect inside a fibre, since it spreads a pulse out and limits the data rate.

The correct answer is C, optical fibre. Electromagnetic interference acts on moving charge, so any medium that carries a signal as an electric current can pick up noise from motors, fluorescent lamps, lightning and nearby cables. An optical fibre carries the signal as light through glass or plastic and no current flows in it at all, so stray fields cannot add anything to the signal. Option A, unshielded twisted pair, is the most vulnerable of all the guided media; twisting the pair cancels much of the crosstalk but leaves it open to outside fields. Option B, coaxial cable, is better protected because of its braided metal shield, yet it still carries current and is not immune. Option D, radio waves, is the opposite case altogether, since the signal travels through open space and shares the band with every other transmitter. This immunity, with its huge bandwidth, is why fibre carries the trunk routes.

The correct answer is B, half-duplex. A walkie-talkie allows both parties to speak and both to listen, so communication is two-way, but the set has a single push-to-talk switch and one person must release it before the other can be heard. Transmission in both directions is therefore possible but not at the same instant, which is exactly the definition of half-duplex. Option A, simplex, is wrong because in simplex the data can travel only one way, as from a radio station to a receiver or from a keyboard to a computer; a walkie-talkie user can both send and receive. Option C, full-duplex, is wrong because in full-duplex both ends transmit simultaneously, as in an ordinary telephone call where both people can speak together. Option D, parallel, is not a direction of flow at all but a way of sending the bits of a byte together over several wires.

The correct answer is B, RJ-45. The RJ-45 is the eight-position modular plug crimped on to the end of an unshielded twisted pair cable of category 5e or 6 and pushed into the network port of a computer, switch or router. Its smaller cousin, the RJ-11, has fewer positions and terminates the twisted pair that carries an ordinary telephone line. Option A, BNC, is the bayonet connector used on coaxial cable, in early Ethernet and in test instruments. Option C, VGA, is the fifteen-pin plug of an analog monitor cable and has nothing to do with networking. Option D, SC, is one of the standard connectors of an optical fibre, along with the ST and FC types. The pairs worth memorising together are twisted pair with RJ-45, coaxial cable with BNC and optical fibre with SC, ST or FC.

The correct answer is A, modulator and demodulator. A computer works with digital signals, a series of ones and zeros, while an ordinary telephone line was built to carry an analog voice signal. At the sending end the modem modulates, that is, it rides the digital data on an analog carrier wave by varying its amplitude, frequency or phase. At the receiving end another modem demodulates, recovering the ones and zeros from the analog wave. The device therefore does both jobs and is named from the first syllables of the two. Option B invents two words that do not describe any function. Option C, mode and medium, is a plausible-looking guess but is not the expansion. Option D is meaningless in this context. Note that the modem works at the physical layer and that a broadband or cable modem does the same job over a different kind of line.

The correct answer is C, microwave. Transmission media fall into two families. A guided or bounded medium confines the signal to a solid conductor or light pipe, and the three guided media are twisted pair, coaxial cable and optical fibre. An unguided or unbounded medium sends the signal through air or space with nothing to confine it, and the three unguided media are radio waves, microwaves and infrared. A microwave link therefore belongs to the second family: it is directional, needs a clear line of sight between dish antennas on towers, and is used for long-distance telephone trunks and satellite communication. Option A, coaxial cable, option B, optical fibre, and option D, shielded twisted pair, are all guided media, each with a physical path along which the signal must travel. The quick test is whether a cable has to be laid.

The correct answer is C, infrared. Infrared lies just below visible light in frequency, and like visible light it is stopped by an ordinary wall. Its range is only a few metres and it needs a nearly clear path between the transmitter and the receiver, which is exactly the arrangement in a remote control pointed at a television set. That same limitation is an advantage inside a room, because a neighbour s remote cannot interfere with yours and the signal cannot be intercepted from outside. Option A, radio waves, pass easily through walls and travel far, which is why they serve broadcasting and mobile telephony. Option B, microwaves, need line of sight over long distances and are used in tower-to-tower and satellite links, not in remotes. Option D, very low frequency waves, have enormous wavelengths, penetrate sea water and are used for submarine communication.

The correct answer is B, copper conductor, insulator, braided shield, plastic jacket. A coaxial cable is built around a single solid or stranded copper wire at the axis, which carries the signal. Around it lies a dielectric insulator that holds the conductor exactly in the centre and sets the electrical properties of the cable. Over that comes a braided or foil metal shield, which acts as the return path and keeps external interference out. The outermost layer is a tough plastic jacket that protects everything from moisture and damage. The name coaxial comes from the conductor and the shield sharing one axis. Option A puts the shield at the centre, option C puts the insulator first and leaves the shield outside the jacket, and option D is simply the correct order reversed, describing the cable from the outside inwards. The usual connector for this cable is the BNC.