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WBBSE Madhyamik Telecom (Vocational) study materials
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WBBSE Madhyamik Telecom (Vocational) practice questions

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Question 1Introduction to Telecom and Communication Systems

In a basic telecommunication system, which element carries the signal from the transmitter to the receiver?

  1. The oscillator
  2. The encoder
  3. The transducer
  4. The channel Correct answer.

Explanation

A communication system consists of a source, transmitter, channel, receiver and destination. The channel is the physical medium, such as copper cable, optical fibre or free space, that carries the transmitted signal from sender to receiver. Encoders and transducers perform signal conversion rather than transmission.

Question 2Introduction to Telecom and Communication Systems

Which transmission mode sends signals in only one direction, as in a commercial radio broadcast?

  1. Full duplex
  2. Multiplex
  3. Half duplex
  4. Simplex Correct answer.

Explanation

Simplex transmission sends information in one direction only; a radio broadcast is a common example because listeners cannot transmit back over the same link. Half duplex and full duplex both allow two-way communication, differing in whether both directions can be used at the same time.

Question 3Introduction to Telecom and Communication Systems

The number of complete cycles of a periodic signal per second is measured in which unit?

  1. Hertz Correct answer.
  2. Ohm
  3. Volt
  4. Watt

Explanation

Frequency is the number of cycles completed in one second and is expressed in hertz (Hz). One hertz equals one cycle per second, while watt, volt and ohm measure power, potential difference and resistance respectively.

Question 4Introduction to Telecom and Communication Systems

Which property distinguishes a digital signal from an analog signal?

  1. It has infinite levels
  2. It varies continuously
  3. It uses discrete levels Correct answer.
  4. It changes smoothly

Explanation

A digital signal has a finite set of discrete amplitude levels, such as the two levels used in binary signalling. An analog signal, by contrast, varies continuously in both time and amplitude.

Question 5Introduction to Telecom and Communication Systems

In a communication channel, bandwidth refers to which quantity?

  1. Its cable length
  2. Its frequency range Correct answer.
  3. Its user count
  4. Its signal power

Explanation

Bandwidth is the width of the frequency range over which a channel can carry a signal, usually measured in hertz. It indicates the information-carrying capacity of the channel, not its physical length, number of users or transmitted power.

Question 6Introduction to Telecom and Communication Systems

In a twisted-pair cable, the two conductors are twisted around each other mainly to perform which function?

  1. Cancel induced noise Correct answer.
  2. Raise the signal voltage
  3. Increase tensile strength
  4. Reduce cable weight

Explanation

Twisting the two insulated conductors makes the interference currents induced in successive twists oppose and largely cancel each other. This reduces common-mode noise and crosstalk in the pair; mechanical strength, voltage level and weight are not the main purpose.

Question 7Introduction to Telecom and Communication Systems

Why is modulation used when a baseband audio signal is sent over a radio link?

  1. To increase the audio volume
  2. To remove the need for an antenna
  3. To convert digital to analog
  4. To shift the signal to a carrier Correct answer.

Explanation

Modulation places the information signal on a higher-frequency carrier so that it can be radiated efficiently by an antenna and can occupy an assigned frequency channel. It does not increase audio power or eliminate the antenna; those are separate system concerns.

Question 8Introduction to Telecom and Communication Systems

Which statement correctly describes circuit switching in a telephone network?

  1. No connection is established
  2. Packets are routed independently
  3. Bandwidth is shared randomly
  4. A dedicated path is reserved Correct answer.

Explanation

Circuit switching establishes a dedicated end-to-end path before communication begins, and that path remains reserved for the duration of the call. Packet switching instead divides data into packets that may follow different routes.

Question 9Introduction to Telecom and Communication Systems

A noiseless channel has a bandwidth of 4 kHz and uses 8 signal levels. According to the Nyquist formula, what is the maximum bit rate?

  1. 24 kbps Correct answer.
  2. 4 kbps
  3. 12 kbps
  4. 8 kbps

Explanation

Nyquist's formula for a noiseless channel is C = 2B log2 L, where B is the bandwidth and L is the number of signal levels. Here C = 2 × 4000 × log2 8 = 2 × 4000 × 3 = 24,000 bps, which is 24 kbps.

Question 10Introduction to Telecom and Communication Systems

An amplifier raises a signal from 10 mW to 100 mW. What is the power gain in decibels?

  1. 10 dB Correct answer.
  2. 1 dB
  3. 100 dB
  4. 20 dB

Explanation

Power gain in decibels is calculated as 10 log10(Pout/Pin). With 100 mW output and 10 mW input, the power ratio is 10, so the gain is 10 log10 10 = 10 dB.

Question 11Introduction to Telecom and Communication Systems

In a telephone handset, which component converts the user's spoken sound into an electrical signal for the local loop?

  1. Earpiece receiver
  2. Carbon microphone Correct answer.
  3. Electromagnetic ringer
  4. Numeric keypad

Explanation

A microphone is an input transducer: sound pressure waves strike its diaphragm and vary an electric current or voltage. The earpiece performs the reverse conversion, changing electrical signals back into sound. The ringer and keypad do not convert speech into a transmission signal.

Question 12Introduction to Telecom and Communication Systems

Which of the following is an unguided transmission medium used in telecommunication?

  1. Twisted-pair cable
  2. Free space Correct answer.
  3. Coaxial cable
  4. Optical fibre cable

Explanation

An unguided medium does not confine the signal to a physical conductor; radio waves travel through free space. Coaxial, twisted-pair and optical fibre cables all guide the signal inside a manufactured physical path.

Question 13Introduction to Telecom and Communication Systems

Which transmission medium carries information in the form of light pulses along a glass or plastic core?

  1. Shielded copper pair
  2. Coaxial cable
  3. Twisted-pair cable
  4. Optical fibre Correct answer.

Explanation

Optical fibre guides modulated light through a core surrounded by cladding, using total internal reflection. Copper media such as twisted pair and coaxial cable carry electrical current instead of light pulses.

Question 14Introduction to Telecom and Communication Systems

A signal launched at 5 mW arrives at the far end of a cable with only 0.5 mW remaining. Which term describes this loss of signal strength?

  1. Modulation
  2. Amplification
  3. Multiplexing
  4. Attenuation Correct answer.

Explanation

Attenuation is the reduction in signal power or amplitude as a signal travels along a medium; here the power falls to one-tenth of its original value. Modulation shifts information onto a carrier, multiplexing shares a channel, and amplification increases signal strength rather than reducing it.

Question 15Introduction to Telecom and Communication Systems

A walkie-talkie lets both operators speak, but only one can transmit at any instant. Which mode of communication does this describe?

  1. Half-duplex Correct answer.
  2. Multicast
  3. Simplex
  4. Full-duplex

Explanation

In half-duplex operation both stations can transmit and receive, but not simultaneously, so they must take turns. Simplex allows only one fixed direction, while full-duplex permits both directions at the same time.

Question 16Introduction to Telecom and Communication Systems

In a long digital transmission line, what is the main purpose of inserting a repeater?

  1. To regenerate the weakened digital pulse train Correct answer.
  2. To divide the channel among several subscribers
  3. To increase the analogue amplitude together with the noise
  4. To convert the signal from electrical to optical form

Explanation

A repeater receives a distorted or attenuated digital signal, decides the value of each bit, and retransmits a fresh pulse. This regeneration removes accumulated noise, unlike a simple analogue amplifier, which would boost noise along with the signal.

Question 17Introduction to Telecom and Communication Systems

Several voice conversations share one high-capacity link, and each conversation is assigned its own frequency band. Which technique is being used?

  1. Synchronous time-division multiplexing
  2. Frequency-division multiplexing Correct answer.
  3. Packet switching
  4. Time-division multiplexing

Explanation

Frequency-division multiplexing places each channel in a separate slice of the available frequency spectrum, so all channels can be sent at once. Time-division multiplexing would instead give each channel a recurring time slot on the same frequency band.

Question 18Introduction to Telecom and Communication Systems

A voice signal is band-limited to 3.4 kHz before sampling. According to the Nyquist sampling theorem, what is the minimum sampling rate needed to reconstruct it without aliasing?

  1. 3.4 kHz
  2. 1.7 kHz
  3. 6.8 kHz Correct answer.
  4. 5.1 kHz

Explanation

The Nyquist theorem requires sampling at least twice the highest frequency present in the signal. Twice 3.4 kHz is 6.8 kHz, so lower rates would fold high-frequency components back as aliasing.

Question 19Introduction to Telecom and Communication Systems

At a receiver the wanted signal power is 1 µW and the noise power is 1 nW. What is the signal-to-noise ratio in decibels?

  1. 20 dB
  2. 10 dB
  3. 30 dB Correct answer.
  4. 60 dB

Explanation

Both powers must use the same unit: 1 µW equals 1000 nW, so the power ratio is 1000:1. In decibels, SNR = 10 log10(1000) = 30 dB.

Question 20Introduction to Telecom and Communication Systems

Which statement best describes packet switching in a data network?

  1. The channel is divided into fixed frequency bands for each user
  2. A dedicated end-to-end path is held for the whole session
  3. Messages split into independently routed packets Correct answer.
  4. Only one direction of transmission is permitted

Explanation

Packet switching breaks a message into packets, each carrying address information and travelling independently through the network. Links are shared statistically rather than reserved, which differs from circuit switching and from frequency-division multiplexing.

Question 21Introduction to Telecom and Communication Systems

In a dial-up data link, which device converts the computer's digital data into an analog signal for an ordinary voice-grade telephone line and recovers the digital data at the receiving end?

  1. A modem Correct answer.
  2. A network switch
  3. A hub
  4. A router

Explanation

A modem performs modulation at the sending end to place digital data on an analog carrier and demodulation at the receiving end to recover the original bits. The word modem is itself a contraction of modulator and demodulator. Routers, switches and hubs forward data frames or packets but do not carry out this analog line conversion.

Question 22Introduction to Telecom and Communication Systems

Which guided transmission medium has a central copper conductor surrounded by a tubular metallic shield and is commonly used for cable television distribution?

  1. Coaxial cable Correct answer.
  2. Optical fibre
  3. Ribbon cable
  4. Unshielded twisted pair

Explanation

Coaxial cable consists of an inner copper conductor, a dielectric insulator, a braided metallic shield and an outer jacket. The shield confines the signal and reduces interference, which suits broadband distribution such as cable television. Twisted pair lacks the tubular shield, optical fibre carries light, and ribbon cable is a flat multiconductor cable.

Question 23Introduction to Telecom and Communication Systems

In amplitude modulation, which characteristic of the high-frequency carrier wave is changed in step with the message signal?

  1. Its phase
  2. Its frequency
  3. Its polarisation
  4. Its amplitude Correct answer.

Explanation

Amplitude modulation varies the instantaneous amplitude of the carrier while keeping its frequency and phase constant. The envelope of the modulated wave then follows the shape of the original message. Frequency modulation and phase modulation vary the other parameters instead.

Question 24Introduction to Telecom and Communication Systems

A periodic voltage completes 50 full cycles in one second. What is its time period?

  1. 50 s
  2. 5 s
  3. 0.02 s Correct answer.
  4. 0.5 s

Explanation

The frequency is 50 Hz, so the time period is T = 1/f = 1/50 second = 0.02 s, which equals 20 ms. The period is the duration of one complete cycle and must therefore be a small fraction of a second here. Multiplying the frequency or taking half of it gives the wrong values.

Question 25Introduction to Telecom and Communication Systems

In a public switched telephone network, what does the local loop connect?

  1. Two different telephone exchanges
  2. The subscriber's telephone to the local exchange Correct answer.
  3. The exchange to the international gateway
  4. The modem to the computer's serial port

Explanation

The local loop, also called the subscriber line, is the pair of copper wires running from the customer's premises to the nearest telephone exchange. It carries voice and signalling between the telephone instrument and the exchange. Links between exchanges are trunk circuits rather than local loops.

Question 26Introduction to Telecom and Communication Systems

Several digital signals share one high-capacity link by being allotted fixed, recurring time slots in turn. Which multiplexing technique is this?

  1. Time-division multiplexing Correct answer.
  2. Code-division multiplexing
  3. Wavelength-division multiplexing
  4. Frequency-division multiplexing

Explanation

Time-division multiplexing gives each input channel the whole link bandwidth for a short recurring interval, so the channels take turns. Frequency-division multiplexing instead assigns separate frequency bands simultaneously. Wavelength-division multiplexing applies frequency division to optical wavelengths, while code-division multiplexing separates users by distinct codes.

Question 27Introduction to Telecom and Communication Systems

Shannon's channel-capacity theorem states that the maximum error-free data rate of a noisy channel depends on its bandwidth and on which other quantity?

  1. The number of signal levels only
  2. The cable's physical length only
  3. The carrier frequency only
  4. The signal-to-noise ratio Correct answer.

Explanation

Shannon's formula C = B log2(1 + S/N) shows capacity rising with bandwidth B and with the signal-to-noise ratio S/N. It does not require the number of discrete signal levels, which belongs to the Nyquist formula for a noiseless channel. Cable length and carrier frequency affect practical performance but are not the second variable in the theorem.

Question 28Introduction to Telecom and Communication Systems

A push-button telephone keypad uses dual-tone multi-frequency signalling. How is each key represented?

  1. By two simultaneous tones of different frequencies Correct answer.
  2. By a train of direct-current dial pulses
  3. By a burst of broadband white noise
  4. By a single continuous tone whose frequency never changes

Explanation

DTMF assigns each key a unique combination of one low-frequency tone and one high-frequency tone chosen from two separate groups. The exchange detects the pair and identifies the digit. Single tones, DC pulses and noise bursts do not provide the same reliable two-out-of-eight identification.

Question 29Introduction to Telecom and Communication Systems

Which type of noise arises from the random motion of electrons in a conductor and increases as the conductor's absolute temperature rises?

  1. Thermal noise Correct answer.
  2. Quantisation noise
  3. Cross-talk
  4. Impulse noise

Explanation

Thermal noise, also called Johnson noise or white noise, is produced by randomised electron motion and has power proportional to absolute temperature and bandwidth. Impulse noise comes from sudden spikes such as switching transients, cross-talk is coupling from a neighbouring conductor, and quantisation noise arises during analog-to-digital conversion.

Question 30Introduction to Telecom and Communication Systems

Which transmission impairment occurs when different frequency components of a signal travel through a medium at different speeds and therefore arrive at different times?

  1. Echo
  2. Thermal noise
  3. Delay distortion Correct answer.
  4. Attenuation

Explanation

Delay distortion, also called phase distortion, results from non-uniform propagation delay across the signal's frequency band, so the received waveform is spread and smeared. Attenuation is a loss of amplitude, thermal noise is random added energy, and echo is a delayed reflection rather than differential delay within the signal band.

Question 31Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

Which switching technique reserves a dedicated end-to-end path for the entire duration of a call?

  1. Statistical multiplexing
  2. Packet switching
  3. Message switching
  4. Circuit switching Correct answer.

Explanation

Circuit switching sets up a continuous physical path before communication begins and holds it until the call ends. Packet and message switching forward data units without reserving a continuous path. Statistical multiplexing shares capacity on demand rather than dedicating a path.

Question 32Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

Which phenomenon keeps a light ray confined to the core of an optical fibre?

  1. Absorption by the outer jacket
  2. Dispersion across the core
  3. Diffraction at the core boundary
  4. Total internal reflection Correct answer.

Explanation

Light travelling in the core strikes the core–cladding boundary at an angle greater than the critical angle. The cladding has a lower refractive index, so the ray is reflected back into the core instead of refracting out. This repeated total internal reflection guides the signal along the fibre.

Question 33Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

Which transmission medium consists of a central copper conductor surrounded by a dielectric and a braided metallic shield?

  1. Optical fibre
  2. Microwave link
  3. Unshielded twisted pair
  4. Coaxial cable Correct answer.

Explanation

Coaxial cable has an inner conductor, an insulating dielectric, a metallic shield, and an outer jacket. The shield carries the return signal and reduces external interference. Twisted pair lacks the single central conductor and shield; fibre uses glass, not copper.

Question 34Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

In a packet-switched network, which device examines the destination IP address and forwards each packet towards its destination network?

  1. Router Correct answer.
  2. Hub
  3. Repeater
  4. Layer-2 switch

Explanation

A router operates at the network layer and uses the destination IP address to choose the next hop. A hub repeats bits to all ports without addressing, and a Layer-2 switch forwards frames using MAC addresses within a LAN. A repeater only regenerates the physical signal.

Question 35Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

Why is single-mode fibre preferred over multimode fibre for very long-distance trunk routes?

  1. Its plastic core is easier to splice and costs less in field installations.
  2. Its small core supports essentially one mode, minimising modal dispersion. Correct answer.
  3. Its large core allows many modes to arrive with equal delay, increasing reach.
  4. It converts optical signals to electrical pulses, which avoids optical loss.

Explanation

Single-mode fibre has a core diameter of about 8–10 µm, close to the wavelength, so only the fundamental mode propagates. With one mode there is no intermodal dispersion, which allows higher bandwidth over longer distances. Multimode fibre's many modes arrive at different times and limit reach.

Question 36Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

Which multiplexing method sends several independent signals through one optical fibre by assigning each to a different wavelength?

  1. Time-division multiplexing
  2. Wavelength-division multiplexing Correct answer.
  3. Pulse-code modulation
  4. Frequency-division multiplexing

Explanation

WDM uses an optical multiplexer to combine carriers of different wavelengths onto a single fibre. Each wavelength behaves as a separate channel and is separated at the receiver by a demultiplexer. TDM shares one wavelength by time slots, while PCM is a digitisation technique, not an optical multiplexing scheme.

Question 37Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

In a graded-index multimode fibre, how does the refractive index vary across the core?

  1. It increases steadily from the core centre to the cladding.
  2. It alternates between high and low values in concentric rings.
  3. It decreases gradually from the core centre to the cladding. Correct answer.
  4. It is constant in the core and abruptly lower in the cladding.

Explanation

A graded-index core has its highest refractive index at the centre and a gradually decreasing index towards the cladding. Rays travelling away from the axis bend back continuously, and outer rays travel faster in the lower-index region. This equalises arrival times and reduces modal dispersion compared with step-index fibre.

Question 38Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

What is the primary function of a telephone exchange in a switched telecom network?

  1. To convert every optical signal into an electrical signal only.
  2. To amplify the analogue voice signal before it reaches each subscriber.
  3. To store all call records permanently for billing purposes.
  4. To set up and release a temporary path between subscribers. Correct answer.

Explanation

An exchange performs switching: it analyses the dialled number and connects the caller to the required subscriber or trunk. The connection exists only for the duration of the call and is then released. Billing and signal conversion may occur, but they are not the exchange's primary switching function.

Question 39Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

A 10 km optical fibre link has a uniform attenuation of 0.2 dB/km. If the input power is 1 mW (0 dBm), what is the output power?

  1. 0 dBm
  2. -2 dBm Correct answer.
  3. -0.2 dBm
  4. 2 dBm

Explanation

Total loss is 0.2 dB/km × 10 km = 2 dB. Input power of 1 mW equals 0 dBm, and a loss of 2 dB gives 0 − 2 = −2 dBm. The output is therefore 2 dB below 1 mW, about 0.63 mW.

Question 40Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

Which characteristic makes circuit switching suitable for real-time voice calls?

  1. It reserves a fixed path with constant, low delay after setup. Correct answer.
  2. It stores each complete voice message before forwarding it onward.
  3. It shares bandwidth among all users on a best-effort basis.
  4. It retransmits every lost packet automatically until delivery succeeds.

Explanation

In circuit switching, the end-to-end path and bandwidth are reserved for the call, so delay stays nearly constant after setup. This suits voice, which is sensitive to jitter and variable delay. Packet switching is best-effort and can introduce variable delay, while message switching stores whole messages before forwarding.

Question 41Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

Which switching technique requires a node to receive and store an entire message before forwarding it towards the destination?

  1. Circuit switching
  2. Cell switching
  3. Packet switching
  4. Message switching Correct answer.

Explanation

Message switching is a store-and-forward technique in which the complete message is received at each node, stored, and then retransmitted to the next node. Because the whole message occupies the link at once, it suits telegraph and file traffic but is poor for interactive voice.

Question 42Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

Which multiplexing technique combines 30 digitised voice channels into a single 2.048 Mbit/s E1 carrier by giving each channel a recurring time slot?

  1. Code-division multiple access
  2. Time-division multiplexing Correct answer.
  3. Frequency-division multiplexing
  4. Wavelength-division multiplexing

Explanation

E1 is a TDM hierarchy: each of the 30 voice channels is allotted a fixed time slot in a repeating 125 microsecond frame, with two further time slots used for framing and signalling. This yields 32 time slots of 64 kbit/s each, giving 2.048 Mbit/s.

Question 43Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

At which operating wavelength does a standard silica single-mode fibre normally show its lowest attenuation, making it attractive for long-haul trunk routes?

  1. 1310 nm
  2. 1550 nm Correct answer.
  3. 850 nm
  4. 1490 nm

Explanation

The attenuation of silica fibre falls to a minimum near 1550 nm, about 0.2 dB/km, which is why long-haul and submarine systems operate in this window. The 1310 nm window has near-zero chromatic dispersion but higher loss, while 850 nm is used for short multimode links.

Question 44Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

A PCM voice channel uses 8000 samples per second, with each sample encoded into 8 bits. What is the resulting bit rate of one such channel?

  1. 128 kbit/s
  2. 2048 kbit/s
  3. 8 kbit/s
  4. 64 kbit/s Correct answer.

Explanation

Bit rate equals sampling rate multiplied by bits per sample: 8000 multiplied by 8 gives 64,000 bit/s, or 64 kbit/s. This 64 kbit/s channel is the basic building block of the E1 and T1 digital hierarchies.

Question 45Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

In an optical receiver specification, which term denotes the minimum optical power needed at the photodetector to achieve a stated bit error rate?

  1. Sensitivity Correct answer.
  2. Responsivity
  3. Dark current
  4. Quantum efficiency

Explanation

Receiver sensitivity is quoted in dBm and defines the weakest optical signal that still meets a specified bit error rate, such as 10 to the power minus 10. It is a key figure in the power budget of a fibre link because it fixes how much loss the link can tolerate.

Question 46Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

What is the main cause of chromatic dispersion in a single-mode optical fibre?

  1. Different modes travelling along paths of different lengths
  2. Different wavelengths propagating at slightly different speeds Correct answer.
  3. Absorption of light by hydroxyl ions in the glass
  4. Light escaping at a tight bend in the fibre

Explanation

Chromatic dispersion arises because the refractive index of silica varies with wavelength, so the spectral components of a modulated source travel at different group velocities and spread the pulse. It becomes the dominant dispersion limit on long single-mode links, especially at 1550 nm.

Question 47Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

Which test instrument launches a short optical pulse into a fibre and analyses the returning backscattered light to locate breaks and measure loss along the link?

  1. Fusion splicer
  2. Power meter
  3. Spectrum analyser
  4. OTDR Correct answer.

Explanation

An optical time-domain reflectometer sends a pulse and measures Rayleigh backscatter and Fresnel reflections as a function of time, which is converted into distance. The resulting trace shows splice and connector losses and pinpoints breaks or bends along the fibre.

Question 48Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

A 40 km single-mode link uses fibre with 0.22 dB/km attenuation, two connectors of 0.5 dB each and four splices of 0.1 dB each. What is the total link loss?

  1. 10.2 dB Correct answer.
  2. 11.2 dB
  3. 8.8 dB
  4. 9.8 dB

Explanation

Fibre loss is 40 multiplied by 0.22, which gives 8.8 dB; the connectors contribute 2 multiplied by 0.5, or 1.0 dB; the splices add 4 multiplied by 0.1, or 0.4 dB. Summing them gives 8.8 plus 1.0 plus 0.4, equal to 10.2 dB.

Question 49Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

A step-index fibre has a core refractive index of 1.48 and a cladding refractive index of 1.46. What is its numerical aperture, approximately?

  1. 0.02
  2. 1.47
  3. 0.48
  4. 0.24 Correct answer.

Explanation

The numerical aperture is the square root of n1 squared minus n2 squared. Substituting gives the square root of 2.1904 minus 2.1316, which is the square root of 0.0588, approximately 0.24. This value sets the acceptance cone of the fibre and hence how much light can be launched into the core.

Question 50Telecom Network Fundamentals: Switching, Transmission and Optical Fibre

Why does packet switching usually achieve higher link utilisation than circuit switching when the traffic is bursty data?

  1. It assigns a dedicated copper pair to each conversation
  2. It allocates capacity on demand instead of reserving it Correct answer.
  3. It guarantees a constant end-to-end delay for every packet
  4. It reserves a fixed time slot for each user throughout the call

Explanation

Packet switching uses statistical multiplexing: packets from many sources are queued and sent as capacity becomes free, so a silent source consumes no bandwidth. Circuit switching reserves capacity for the whole call even during silence, which wastes link time when the traffic is bursty.