Telecom 4G, 5G and 6G: Generations, Spectrum, Fibre

The correct answer is C, 2022. Commercial 5G services began in India on 1 October 2022, when they were launched at the India Mobile Congress in New Delhi, following the spectrum auction held earlier that year in the 600 MHz, 700 MHz, 3300 MHz and 26 GHz bands among others. Option A is wrong because 2018 is the year of the National Digital Communications Policy, which set the goal of broadband for all but predates 5G service. Option B is wrong because 2020 saw trials and standards work, including the acceptance of the Indian 5Gi radio interface into the global standards, but no commercial service. Option D is wrong because 2024 is later than the launch; by then 5G had already spread to most districts. Fix the date 1 October 2022 together with the India Mobile Congress.

The correct answer is C, IMT-2020. The International Telecommunication Union names each generation of mobile telephony by a standard family, and 5G is IMT-2020, whose requirements include a peak downlink rate of 20 Gbps, a user experienced rate of 100 Mbps, one millisecond of air interface latency and a million connected devices in a square kilometre. Option A is wrong because IMT-2000 is 3G, the generation that first made mobile data and video calling practical. Option B is wrong because IMT-Advanced is 4G, met in practice by LTE and LTE-Advanced. Option D is wrong because IMT-2030 is the framework for 6G, which is still under development and is expected to appear commercially around 2030. The pattern is easy once seen: the number is the decade the generation was meant for.

The correct answer is A, Long Term Evolution. LTE is the radio technology through which 4G was delivered; the name was chosen because it was presented as the long-term evolution of the earlier 3G standards rather than a break with them. Its advanced version, LTE-Advanced, met the IMT-Advanced requirements that define true 4G. LTE made the network all-IP, so voice itself travels as packets, which is Voice over LTE or VoLTE. Option B is wrong because no standard uses the expansion Linear Transmission Encoding. Option C is wrong because Ethernet is a wired local area network technology and has nothing to do with the mobile radio standard. Option D is wrong because Local Transfer Exchange describes nothing in telecom; the exchange in a mobile network is the core, not an LTE.

The correct answer is C, 20 Gbps. Under IMT-2020 the peak downlink data rate for 5G is set at 20 gigabits a second, with 10 gigabits a second in the uplink; these are laboratory peaks under ideal conditions, not the speed a subscriber sees, and the separate target for the user experienced rate is 100 megabits a second downlink. Option A is wrong because 1 Gbps was the peak rate required of 4G under IMT-Advanced, which is exactly why it is offered as a distractor. Option B is the uplink figure for 5G, so it is right about the standard but wrong about the direction the question asks. Option D is wrong because 100 Gbps belongs to no mobile standard yet and is discussed only in the research literature on 6G and on fixed optical links.

The correct answer is D, circuit-switched voice bearer, which is not a 5G category at all; 5G is an all-IP system in which voice is carried as packets, and circuit switching belongs to the older 2G and 3G architecture. The three categories actually defined are given in the other options. Option A, enhanced mobile broadband, is the fast consumer internet with high data rates. Option B, ultra-reliable low-latency communication, is meant for driverless vehicles, factory automation and remote surgery, where a few milliseconds of delay matter. Option C, massive machine-type communication, is designed for very large numbers of cheap, low-power sensors, which is why the connection density target is a million devices per square kilometre. Remember the three by their initials, eMBB, URLLC and mMTC.

The correct answer is C, 1995. The first mobile telephone call in India was made on 31 July 1995 between Kolkata and New Delhi, a year after the National Telecom Policy of 1994 opened mobile services to private operators. Option A is wrong because 1985 is the year the Department of Telecommunications was formed, and 1986 saw the creation of MTNL and VSNL, but there was no mobile network then. Option B is wrong because 1991 is the year of the economic reforms, which made the later opening of telecom possible but carried no mobile call. Option D is wrong because 2000 is the year BSNL was formed out of the Department of Telecommunications. Fix the chain: policy in 1994, first call in 1995, TRAI in 1997.

The correct answer is B, 1997. TRAI was set up under the Telecom Regulatory Authority of India Act, 1997, to regulate telecom services, fix tariffs and protect the interests of consumers once private operators entered the field. An amendment in 2000 took the dispute settlement and appellate work away from TRAI and gave it to a new tribunal, the Telecom Disputes Settlement and Appellate Tribunal. Option A is wrong because 1994 is the year of the National Telecom Policy that opened the sector, not of the regulator. Option C is wrong because 2000 is the year of that amendment and of the formation of BSNL. Option D is wrong because 2012 is the year of the National Telecom Policy that set broadband goals, long after TRAI already existed.

The correct answer is A, IMT-2030 and the Bharat 6G Vision. The International Telecommunication Union has adopted the framework recommendation for IMT-2030, which describes what 6G should achieve: terahertz and other very high frequency bands, latency below a millisecond, artificial intelligence built into the network, communication combined with sensing, satellite-extended coverage and better energy efficiency. India released its Bharat 6G Vision document in 2023 and formed the Bharat 6G Alliance the same year. Option B is wrong because IMT-2020 is 5G and the National Broadband Mission is about fibre and broadband reach. Option C is wrong because IMT-Advanced is 4G and BharatNet is the rural fibre project. Option D is wrong because there is no IMT-2040, and Digital India is a wider governance programme. The stated aim of the Indian effort is that the country should contribute patents and standards to 6G instead of importing them, as it did in the earlier generations.

The correct answer is C, total internal reflection of light. In an optical fibre the glass core has a higher refractive index than the cladding around it, so light that meets the core-cladding boundary at an angle greater than the critical angle is reflected entirely back into the core instead of passing out, and it travels along the fibre in a zig-zag of such reflections with almost no loss. Option A and Option D are wrong because there is no process called total external refraction or total external reflection in optics. Option B is wrong because refraction means the bending of light as it passes into another medium, which is the opposite of what keeps the ray inside; total internal reflection is defined precisely by the ray not being refracted out. This is why the carrier of 5G backhaul is immune to electrical interference.

The correct answer is D, single mode. A fibre-to-the-home connection runs from the operator's equipment to the subscriber's premises over a distance of kilometres, and for such runs single-mode fibre is used: its core is only a few micrometres wide, so light travels along one path, there is no spreading of the pulse from different paths arriving at different times, and both loss and distortion stay very low. Options A and B are wrong because multimode fibre, whether of the graded-index or the step-index kind, has a wider core and carries many paths at once; it suffers modal dispersion and is therefore used only over short distances, such as within a building or a data centre. Option C is wrong because there is no category of fibre called multiaxis. Fibre of this kind is also the backhaul that 5G towers depend on.

The correct answer is C, photo diode. At the receiving end of an optical fibre link the light pulses must be turned back into an electrical signal, and that is the work of a photodiode, a reverse-biased junction in which incident photons generate charge carriers and so a current proportional to the light falling on it; PIN and avalanche photodiodes are the types used, because both respond fast enough for gigabit signalling. Option A is wrong because a light-emitting diode is a source, used at the transmitting end. Option B is wrong because an infrared diode is also an emitter, used in remote controls and short-range links. Option D is wrong because a light-dependent resistor changes its resistance slowly with illumination and is far too sluggish for data; it is used in street-light switches and light meters.

The correct answer is B, LED. The transmitter of an optical fibre link must be small enough to couple light into a core a few micrometres across, must switch on and off millions or billions of times a second, and must emit in a narrow band of wavelengths; a light-emitting diode meets these conditions, and for long-haul links a laser diode, which is the same family of semiconductor device, is used for its still narrower spectrum. Option A is wrong because an arc lamp is a bulky, broad-spectrum source that cannot be modulated at these speeds. Option C is wrong because a mercury vapour lamp is used for street lighting and in laboratories, not for signalling. Option D is wrong because an incandescent lamp has a glowing filament with great thermal inertia and cannot be switched fast at all.