Effective CISSP Questions

Satellite technologies provide various services, such as telecommunications, broadcasting, and data communications. The commercial offering of internet access through a constellation of satellites is emerging. Which of the following is not true?
A. Satellites can result in light pollution and space debris.
B. Satellites are deployed in geostationary orbit to reduce round-trip latency.
C. Satellites use transponders or transceivers to receive and retransmit signals.
D. Free-space optical communication (FSO) can be used for inter-satellite communication.


My suggested answer is B. Satellites are deployed in geostationary orbit to reduce round-trip latency.



A satellite is basically a self-contained communications system with the ability to receive signals from Earth and to retransmit those signals back with the use of a transponder—an integrated receiver and transmitter of radio signals.

Source: How Satellites Work

Satellite Orbits and Latency

Most of the satellites for data communication are deployed in the low Earth orbit (LEO) because of low altitude that results in short latency. Geostationary orbit produces the longest latency.

Satellites operate in three different orbits: low Earth orbit (LEO), medium Earth orbit (MEO), and geostationary or geosynchronous orbit (GEO).

  • LEO satellites are positioned at an altitude between 160 km and 1,600 km (100 and 1,000 miles) above Earth.
  • MEO satellites operate from 10,000 to 20,000 km (6,300 to 12,500 miles) from Earth. (Satellites do not operate between LEO and MEO because of the inhospitable environment for electronic components in that area, which is caused by the Van Allen radiation belt.)
  • GEO satellites are positioned 35,786 km (22,236 miles) above Earth, where they complete one orbit in 24 hours and thus remain fixed over one spot.

Source: How Satellites Work

Light Pollution

The planned large number of satellites has met with criticism from the astronomical community because of concerns for light pollution. Astronomers claim that the number of visible satellites will outnumber visible stars and that their brightness in both optical and radio wavelengths will severely impact scientific observations. Because the Starlink satellites can autonomously change their orbits, observations cannot be scheduled to avoid them. The International Astronomical Union (IAU) and National Radio Astronomy Observatory (NRAO) have released official statements expressing concern on the matter.

Source: Starlink (Wikipedia)

Space Debris

The maintenance of a satellite’s orbital position is called “station keeping,” and the corrections made by using the satellite’s thrusters are called “attitude control.” A satellite’s life span is determined by the amount of fuel it has to power these thrusters. Once the fuel runs out, the satellite eventually drifts into space and out of operation, becoming space debris.

Source: How Satellites Work

Transponder and Transceiver

Both a transponder and transceiver are functionally similar devices that receive and transmit signals, e.g., radiofrequency, optical, and electrical signals.

Fiber Optic Communication

A transponder and transceiver are both functionally similar devices that convert a full-duplex electrical signal in a full-duplex optical signal. The difference between them is that the fiber transceiver adopts a serial interface and it can transmit and receive signals in the same module, whereas the transponder uses a parallel interface and it needs to work with two fiber optic modules to achieve the whole transmission. That is to say, the transponder needs to transmit signals by a module on one side and then get response to this signal by a module on the other side.

Although the transponder can handle lower-rate parallel signals easily, it has a larger size and higher power consumption than transceivers. Additionally, optical modules can only provide an electrical-optical conversion, while transponders can achieve the optical-electrical-optical conversion from one wavelength to another wavelength. As such, transponders can be considered as two transceivers placed back-to-back and they are more likely to be applied for long-distance transmission in WDM systems that common optical transceivers fail to reach.

Source: FS

RFID Transponders

RFID uses transponders. Tags and smart cards are common applications of RFID. RFID transponders can be divided into two categories: active and passive, in terms of the communication between transponders and readers.

  • Active transponders have a built-in power source. 
  • Passive transponders have no power source and don’t transmit any signals independently. Instead, readers interrogate passive transponders by induction.

Free-space optical communication (FSO)

Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications or computer networking. “Free space” means air, outer space, vacuum, or something similar. This contrasts with using solids such as optical fiber cable.

Various satellite constellations such as SpaceX Starlink are intended to provide global broadband coverage employ laser communication for inter-satellite links between the several hundred to thousand satellites effectively creating a space-based optical mesh network.

Source: Wikipedia




衛星技術提供各種服務，例如電信，廣播和數據通信。通過衛星群(constellation)提供互聯網訪問的商業產品正在興起。 以下哪一項是不正確的？
A. 衛星會導致光污染和太空碎片。
B. 將衛星部署在地球靜止軌道(geostationary orbit)上，以減少往返延遲(latency)。
C. 衛星使用轉發器(transponders)或收發器(transceivers)來接收和轉發信號。
D. 自由空間光通信(free-space optical communication)可用於衛星間通信。

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