Communications MCQ Set 1
1. Optical switching can be classified into ________ categories.
Answer: a [Reason:] Optical switching is classified into two categories same as that of electronic switching.
The two categories are circuit switching and packet switching.
2. ___________________ are the array of switches which forms circuit switching fabrics.
a) Packet arrays
b) Optical cross connects
c) Circuit arrays
d) Optical networks
Answer: b [Reason:] Optical cross-connects incorporate switching connections or light paths. These larger arrays can switch signals from one port to another.
3. ___________ is an example of a static circuit-switched network.
b) Circuit regenerator
c) Packet resolver
Answer: d [Reason:] The circuit is said to be static when the network resources remain dedicated to the circuit connection. This should be followed during the entire transfer and the complete message follows the same path.
4. What is the main disadvantage of OCS?
a) Regenerating mechanism
b) Optical session
c) Time permit
d) Disability to handle burst traffic
Answer: d [Reason:] In traffic conditions, data is sent in the form of bursts of different lengths. Thus, the resources cannot be readily assigned. The OCS cannot efficiently handle burst traffic.
5. Optical electro-conversions takes place in _________________ networks.
b) Optical packet-switched
c) Optical circuit-switched
Answer: c [Reason:] In an optical packet-switched network, data is transported in the optical domain. This is done without intermediate optic-electrical conversions. Optical electro-conversions takes place in circuit-switched networks.
6. How many functions are performed by an optical packet switch?
Answer: a [Reason:] An optical packet switch performs four basic functions. These include routing, forwarding, switching and buffering.
7. ____________ provides data storage for packets to resolve contention problems.
Answer: c [Reason:] Switching involves directing the packets. Routing provides network connectivity while forwarding and reversing involves defining a packet. Buffering usually provides data storage for packets.
8. What is usually required by a packet to ensure that the data is not overwritten?
c) Guard band
Answer: c [Reason:] A packet consists of a header and the payload. The label points to an entry in the lookup table. A guard band is usually included to ensure the data is not overwritten.
9. Routing technique is faster than the labeling technique. State whether the given statement is true or false.
Answer: a [Reason:] Labeling suggests where the packet should be directed. Routing routes the data in the given direction. Thus, labeling technique is efficient and faster than the routing technique.
10. ______________ provides efficient designation, routing, forwarding, switching of traffic through an optical packet-switched network.
a) Label correlation
b) Multiprotocol label switching
c) Optical correlation
Answer: b [Reason:] Multiprotocol label switching (MPLS) was first proposed by CISCO systems. Earlier, it was called as tag switching. MPLS uses labels to forward, switch, designate the traffic.
11. MPLS is independent of layer 2 and 3 in the OSI model. State whether the given statement is true or false.
Answer: a [Reason:] MPLS is flexible in the current protocol landscape. It supports Ethernet, frame relay as a data link layer but is independent of layer 2 and 3 in the OSI model.
12. Which of the following service is provided by Multiprotocol label switching (MPLS)?
a) Data forwarding
Answer: c [Reason:] One of the important services provided by MPLS is IP virtual private networks. All others are provided by packet switched networks. These VPN’s provide a secure, dedicated wide area network (WAN) in order to connect the offices all over the world.
13. Burst header cell is also known as _____________
a) Burst channel
b) Burst header circuit
c) Burst regenerator
d) Burst header packet
Answer: d [Reason:] Burst header cell consists of information regarding switching and destination address. It works with the use of transmission units called as data bursts.
Communications MCQ Set 2
1. _____________ must be operated in stimulated emission region.
a) Injection laser
Answer: a [Reason:] Injection laser is a threshold device. In stimulated emission region, continuous optical output power levels are in the range of 1 to 10mW.
2. Coherent radiation is relatively __________
Answer: c [Reason:] Most of the light output is coupled into optical fibre. This is because of the isotropic distribution of narrow-line width, coherent radiation is directional.
3. _____________ are capable of launching powers between 0.5 and several mW.
b) Injection laser
Answer: b [Reason:] Coupling efficiency up to 30% may be obtained by placing a fiber close to laser mirror. These can approach 90% with suitable lens and optical coupling arrangements. So they can launch 0.5 to several mW of optical power into fiber.
4. LED’s display good linearity. State whether the given statement is true or false.
Answer: a [Reason:] LED’s appear to be suited to analog transmission. This is because of its output which is directly proportional to the drive current.
5. Which behaviour may prove as a limitation for injection lasers and LED’s?
Answer: c [Reason:] The thermal behaviour of the injection lasers and the LED’s limits their operation within the optical transmitter. The main problem is caused by the variation of injection laser threshold current.
6. Optical output power from an LED is directly proportional to the device junction temperature. State whether the given statement is true or false.
Answer: b [Reason:] Output power is dependent on the junction temperature in case of LED’s. Most LED’s exhibit a decrease in the optical output power following an increase in junction temperature.
7. _____________ from the LED is dependent on the effective minority carrier lifetime in the semiconductor material.
a) Spontaneous emission
b) Stimulated emission
Answer: a [Reason:] The speed of the response of the LED is dictated by the respective emission mechanism. Spontaneous emission is related to the carrier lifetime and hence dictating the speed of response.
8. The _________ of the LED is twice that of the effective minority carrier lifetime.
a) Dwell time
b) Reflection scatters
d) Rise time
Answer: d [Reason:] The response of the optical fiber source is specified in terms of the rise time. This rise time is reciprocally related to the device frequency response.
9. The finite spectral width of the optical source causes ___________
b) Frequency burst
c) Pulse broadening
d) Efficient reflection
Answer: c [Reason:] The finite spectral width causes pulse broadening due to material dispersion on an optical fiber communication link. This results in a limitation on the bandwidth-length product.
10. The coherent emission from an injection laser has a line width of ________
a) 2 nm
c) 8 nm
Answer: d [Reason:] An optical source such as injection laser is a narrow line width device as compared to the LED. It has a narrow line width of 1 nm or less.
11. Extinction ratio is denoted by symbol __________
Answer: a [Reason:] Extinction ratio is defined as the ratio of the optical energy emitted in the 0 bit period to that emitted during the 1 bit period. It is denoted by ε.
12. The use of low impedance driving circuit may increase _____________
d) Switching speed
Answer: d [Reason:] Pulse shaping is usually required to increase the switching speed. However, increased switching speed may be obtained from an LED without speed-up element by use of a low-impedance driving circuit.
Communications MCQ Set 3
1. Photonic crystal fibers also called as
a) Conventional fibers
b) Dotted fibers
c) Stripped fibers
d) Holey fibers
Answer: d [Reason:] Photonic crystal fibers contain a fine array of air holes running longitudinally down the fiber cladding. The microstructure within the fiber is highly periodic.
2. Conventional optical fibers has more transmission losses than photonic crystal fibers. State whether the statement is true or false.
Answer: a [Reason:] Conventional optical fibers have several hundreds of losses in transmission. Photonic crystal fibers have resulted in reduction in overall transmission losses.
3. Losses in photonic crystal fibers are reduced to a level of
Answer: c [Reason:] Conventional fibers have losses of several hundred decibels per km. The invention of photonic crystal tubes has reduced the losses by hundreds of decibels.
4. The high index contrast enables the PCF core to be reduced from around 8 μmin conventional fiber to
a) Less than 1μm
b) More than 5μm
c) More than 3μm
d) More than 2μm
Answer: a [Reason:] PCF’s have a wider range of optical properties in comparison with standard fibers. The lesser the core, more is the intensity of light in the core and enhances the non-linear effects.
5. The periodic arrangement of cladding air holes in photonic band gap fibers provides for the formation of a photonic band gap in the
a) H-plane of fiber
b) E-plane of fiber
c) E-H-plane of fiber
d) Transverse plane of fiber
Answer: d [Reason:] Photonic band gap fibers are a class of micro structured fiber in which periodic arrangement of air holes is required. As a PBG fiber exhibits a 2-dimensional band gap, than the wavelengths within this band gap cannot propagate perpendicular to the fiber axis.
6. In index-guided photonic crystal fiber structure, the dark areas are air holes. What does white areas suggests?
Answer: d [Reason:] Index-guided photonic crystal fibers have greater index contrast because the cladding contains air-holes having refractive index 1. Both index guided and conventional fibers arises from the manner in which guided mode interacts with the cladding region.
7. The unit of measurement of attenuation in optical fibers is
Answer: c [Reason:] Attenuation is also referred to as transmission loss. Channel attenuation helped to determine the maximum transmission distance prior to signal restoration. Attenuation is usually expressed in logarithmic unit of decibel. It is given by
αdBL = 10 log10Pi / Po Where αdB= signal attenuation per unit length
Pi & Po = Input and output power
8. The optical fiber incurs a loss in signal power as light travels down the fiber which is called as
Answer: b [Reason:] When the light is passed through the fiber, it travels a large amount of distance before it starts fading. It needs restoration in the path. This loss or fading is called as Attenuation.
9. If the input power 100μW is launched into 6 km of fiber, the mean optical power at the fiber output is 2μW. What is the overall signal attenuation through the fiber assuming there are no connectors or splices?
Answer: b [Reason:] Signal attenuation is usually expressed in decibels. It is given by
Signal attenuation=10 log10Pi / Po Where, Pi & Po = Input and output power.
10. A device which reduces the intensity of light in optical fiber communications is
b) Optical attenuator
Answer: b [Reason:] A compressor compresses the signal before transmission. It does not affect the intensity of light. Optical attenuator is a device that affects the intensity of light and incurs a loss in transmission.
11. A decibel may be defined as the ratio of input and output optical power for a particular optical wavelength. State whether the following statement is true or false.
Answer: a [Reason:] Signal attenuation refers to the loss in transmission and it needs a logarithmic unit to express. Decibel is mainly used for comparing two power levels. It has the advantage that the operations of multiplication and division reduce to addition and subtraction.
12. When the input and output power in an optical fiber is 120μW & 3μW respectively and the length of the fiber is 8 km. What is the signal attenuation per km for the fiber?
Answer: b [Reason:] Signal attenuation per unit length is given by
αdBL = 10 log10Pi / Po αdBL = 16 dB
αdB= 16 dB/L= 2dB/km.
Communications MCQ Set 4
1. For many applications which involve optical fiber transmission, an intensity modulation optical source is not required. State whether the given statement is true or false.
Answer: b [Reason:] In many optical fibers transmission, the cylindrical fibers used generally do not maintain polarization state of light input source not more than a few meters. So for this reason, optical sources intensity modulation is required.
2. The optical source used for detection of optical signal is
a) IR sensors
c) Zener diodes
Answer: b [Reason:] Optical signal is generally detected by photodiodes because photodiode is generally insensitive to optical polarization or phase of light with the fiber.
3. An optical fiber behaves as a birefringence medium due to differences in
a) Effective R-I and core geometry
b) Core-cladding symmetry
c) Transmission/propagation time of waves
d) Refractive indices of glass and silica
Answer: a [Reason:] In an optical fiber with ideal optically circulatory symmetric core, both polarization modes propagate with same velocities. These fibers have variations in internal and external stress; fiber bending and so exhibit some birefringence.
4. The beat length in a single mode optical fiber is 8 cm, when light from a laser with a peak wavelength 0.6μm is launched into it. Estimate the modal birefringence.
Answer: a [Reason:] Modal birefringence can be obtained by-
BF= λ/LB= 0.8×10-6/ 0.08
= 1×10-5 Where
λ= peak wavelength
LB= beat length.
5. Beat length of a single mode optical fiber is 0.6cm. Calculate the difference between propagation constants for the orthogonal modes
Answer: d [Reason:] The difference between the propagation constant for two orthogonal modes can be obtained by:
βx – βy= 2Π/LB= 2×3.14/ 0.06
βx & βy are propagation constants for slow & fast modes resp.
LB = beat length.
6. A polarization maintaining fiber operates at a wavelength 1.2μm and have a modal birefringence of 1.8*10-3. Calculate the period of perturbation.
a) 0.7 seconds
b) 0.6 seconds
c) 0.23 seconds
d) 0.5 seconds
Answer: b [Reason:] The period of perturbation is given by-
T= λ/BF Where λ is operating wavelength, BF = Birefringence, T= period of perturbation.
7. When two components are equally excited at the fiber input, then for polarization maintaining fibers δΓgshould be around
b) 1 ns/km
Answer: b [Reason:] The differential group delay δΓg is related to polarization mode dispersion (PMD) of fiber. This linear relationship to fiber length however applies only to short fiber-lengths in which birefringence are uniform.
8. Polarization modal noise can _________ the performance of communication system.
Answer: a [Reason:] Polarization modal noise is generally of larger amplitude than modal noise. It is obtained within multimode fibers and so it degrades the performance of communication system and prevents transmission of analog signals.
Communications MCQ Set 5
1. Which of these factors are critical in affecting the system performance in the case of coherent optical fiber transmission?
a) Laser line-width and stability
b) Refractive index and index difference
c) Core cladding diameter
Answer: a [Reason:] The system employing intensity modulation does not consider line-width and stability as the factors of utmost importance. In coherent optical source transmission, laser line-width and stability are critical factors. These factors affect the system performance and are in the range of 0.5-1 Megahertz.
2. _______________ occurs as a result of the change in lasing frequency with gain
a) Frequency multiplication
d) Line-width broadening
Answer: d [Reason:] Line-width broadening is a fundamental consequence of spontaneous emission process. It is related to the fluctuations in the phase of the optical fields. These phase fluctuations are due to the phase noises associated with the spontaneous emission process.
3. Laser cavity length can be extended by
a) Increasing the refractive index
b) Reducing frequency
c) Introduction of external feedback
d) Using GRIN-rod lenses
Answer: c [Reason:] the lasers having long external cavity are referred to as LEC lasers. The extension of the laser cavity length by introduction of external feedback can be achieved by using an external cavity with a wavelength dispersive element.
4. What is the purpose of wavelength dispersive element is LEC lasers?
a) Wavelength selectivity
b) Reduction of line-width
c) Frequency multiplication
d) Avalanche multiplication
Answer: a [Reason:] A wavelength dispersive element is a part of the laser cavity. It is required because the long resonator structure has very closely spaced longitudinal modes which necessitates additional wavelength selectivity.
5. An effective method to reduce the line-width is to make the cavity longer. State whether the following statement is true or false.
Answer: a [Reason:] As the laser power increases, the device line-width decreases. The output power f laser cannot be mode arbitrarily large. Thus, the line-width is reduced by making the cavity longer. Longer cavity also enables increased wavelength selectivity.
6. Which devices are used to modulate the external cavity in order to achieve the higher switching speeds?
Answer: b [Reason:] The devices are tuned mechanically to extend the cavity of laser. The disadvantage of using mechanically tuned devices is low. Thus, electro-optic devices are used to modulate the external cavity in order to achieve higher switching speeds.
7. How many techniques are used to tune monolithic integrated devices (lasers)?
Answer: c [Reason:] There are two techniques which can be employed to tune monolithic integrated devices. In the first method, the mode selectivity of a coupled cavity structure is used. Other method is used to a refractive index change in the device cavity provided by application of an electric field.
8. _________________ laser can be produced when a coupler section is introduced between the amplifier and phase sections of a structure
c) Y 4-shifted
Answer: b [Reason:] DBR lasers are capable of wavelength tuning. Grating assisted co-directional coupler with sampled reflector (GCSR). Laser is capable of a tuning range greater than 40 nm. It consists of a co-directional coupler between the amplifier and the phase section.
9. The rare-earth-doped fiber lasers have spectral line-width in the range of _________________
a) 0.1 to 1 nm
b) 1.2 to 1.5 nm
c) 6 to 10 nm
d) 2 to 2.3 nm
Answer: a [Reason:] The rare-earth-doped fiber lasers have spectral line-width in the range of 0.1 to 1 nm. These line-widths are too long for high speed transmission is possible in this range.
10. The lasing line-width of Fox-smith resonator is ____________________
a) Less than 1 MHz
b) 1 MHz
c) 2 MHz
d) Greater than 3 MHz
Answer: a [Reason:] Fox-smith resonator employs a fused coupled fabricated from erbium-doped fiber. Narrower spectral line-width can be obtained using a resonator. It provides favorable line-widths than semiconductor laser.
11. What is the widest tuning range obtained in optical fiber laser structure?
a) 60 nm
b) 80 nm
c) More than 100 nm
d) 100 nm
Answer: c [Reason:] A tuning range greater than 100 nm by using an erbium-doped photonic crystal fiber. A wider tuning range greater than 100 nm is obtained at wavelength 1.55 nm.
12. How many techniques can be used to increase the injection cavity length?
Answer: Two techniques can be used to increase the injection laser cavity length. These are using laser chips and by extending a cavity with a passive medium such as air, glass etc.
13. The mechanism which results from a refractive index change in the passive waveguide layer is called as
b) Spontaneous emission
c) Monolithic inversion
d) Bragg wavelength control
Answer: d [Reason:] A wider wavelength tuning length is obtained by separating the Bragg region in the passive waveguide and by introducing a phase region within a waveguide control mechanism provides phase control. It takes place by some changes in a passive waveguide layer.
14. How many sections are included in a sampling grating distributed Bragg-reflector laser (SG-DBR)?
Answer: b [Reason:] In SG-DBR laser, five sections are longitudinally integrated together on a semiconductor substrate. These five sections include two diffraction Bragg grating sections, a gain, a phase and an amplifier section.
15. Fiber based lasers provide diffraction-limited power at higher levels than solid-state laser. State whether the given statement is true or false
Answer: a [Reason:] In fiber lasers, the active gain medium is doped with rare earth elements. These lasers have active regions several kilometers long and thus provide high optical gain. Solid-state lasers, on the other hand, provide diffraction limited power at lower levels.