Fiber Optic Interview Questions and Answers

This document provides a set of fiber optic interview questions and answers, designed to help candidates prepare for job interviews in the fiber optic domain. These questions are also useful as college viva questions.

Question 1: Explain two main components which form a fiber optic communication system.

Answer 1: At the transmit end, an LED (Light Emitting Diode) is commonly used, while at the receive end, a photodiode is used. The LED converts the electrical signal into light, and the photodiode performs the reverse conversion, turning light back into an electrical signal.

Question 2: What is FTTH? Explain different types of optical networks.

Answer 2: FTTH stands for Fiber To The Home. There are two main types of optical networks used in FTTH deployments:

• AON (Active Optical Network): An active optical network uses electrically powered switching equipment to manage signal distribution.
• PON (Passive Optical Network): A passive optical network uses optical splitters to distribute signals, requiring no electrical power in the distribution network.

Question 3: Explain the difference between SDH and PDH?

Answer 3: Synchronous Digital Hierarchy (SDH) and Plesiochronous Digital Hierarchy (PDH) are both digital transmission technologies used in telecommunications. SDH is a synchronous system where all network elements operate under a common clock, enabling precise multiplexing, easier management, and high-speed data transmission with rates starting from 155 Mbps (STM-1). In contrast, PDH is an older, asynchronous system where different network elements have slightly varying clocks, leading to complex multiplexing and demultiplexing processes. SDH offers better flexibility, higher data rates, efficient network management, and easier fault detection compared to PDH, which suffers from difficult scalability and limited bandwidth efficiency. Due to these advantages, SDH has largely replaced PDH in modern telecom networks.

Question 4: Explain the difference between SDH and SONET.

Answer 4: Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET) are both standardized fiber optic transmission systems used for high speed digital communication, but they differ mainly in origin and framing structure. SONET, developed in the U.S. by ANSI, uses a base transmission rate of 51.84 Mbps (STS-1/OC-1) and follows a hierarchical structure with STS/OC levels. SDH, standardized by ITU-T and widely used internationally, starts at 155.52 Mbps (STM-1) and follows a different multiplexing structure. While both technologies share similar principles and can interoperate, SDH uses STM frames, whereas SONET uses STS frames with slight differences in overhead structure and synchronization methods. Despite these differences, SDH and SONET are often considered compatible, with SDH being more widely adopted globally.

Question 5: What is WDM? Explain different types of WDM.

Answer 5: WDM stands for Wavelength Division Multiplexing. It’s a technology that multiplexes multiple optical carrier signals onto a single optical fiber by using different wavelengths (colors) of light to carry different signals.

There are different types of WDM, including:

• Narrowband WDM
• Wideband WDM
• Coarse WDM (CWDM)
• Dense WDM (DWDM)

Question 6: Explain Basics of Optical Amplifier. Explain EDFA.

Answer 6: An optical amplifier is used to amplify the optical signal directly without converting it to an electrical signal first. This is crucial for long-distance fiber optic communication.

EDFA stands for Erbium-Doped Fiber Amplifier. It’s a type of optical amplifier that uses a special optical fiber doped with erbium ions to amplify the signal.

Question 7: Explain the function of optical isolator and optical circulator in a fiber optic communication system.

Answer 7:

• Optical Isolator: An optical isolator allows an optical signal to pass in one direction while blocking signals traveling in the opposite direction. This prevents reflections from interfering with the source.
• Optical Circulator: An optical circulator couples the signal from one port to the next port in a specific direction (either clockwise or anti-clockwise). It redirects light from one port to another in a predetermined path.

Question 8: Explain SDH network and describe SDH modulation.

Answer 8:

• SDH Network: Synchronous Digital Hierarchy (SDH) is a standardized fiber-optic transmission system used in telecom networks to transport large volumes of data efficiently. It enables high-speed digital communication by synchronizing all network elements under a common clock, ensuring seamless multiplexing and transmission of lower-rate signals like PDH. SDH uses a layered structure with various network elements such as multiplexers, add-drop multiplexers (ADMs), and regenerators to ensure reliable and flexible data transmission. It supports multiple services, including voice, video, and data, making it essential for modern telecom infrastructure.
• SDH Modulation: SDH does not use traditional analog modulation but employs digital multiplexing and framing techniques to structure data streams. It organizes data into fixed-length frames (STM-N) and uses scrambling techniques to maintain signal integrity. The data is transmitted using NRZ (Non-Return-to-Zero) encoding over optical fibers, ensuring efficient and error-resistant transmission. The synchronous nature of SDH eliminates clock drift issues found in older PDH systems, providing robust and scalable communication.

Question 9: Explain SDH frame structure.

Answer 9:

The SDH frame structure is based on a fixed-length STM (Synchronous Transport Module) format, where STM-1 (155.52 Mbps) is the basic frame, and higher levels (STM-4, STM-16, etc.) are multiples of STM-1.

• Frame Size: 9 rows × 270 columns (2430 bytes per frame).
• Frame Duration: 125 µs (8000 frames per second).
• Sections:

1. Section Overhead (SOH) – First 9 columns contain management, synchronization, and error-checking data.
2. Pointer (H1, H2, H3) – Used for dynamic alignment of payload data.
3. Payload (SPE - Synchronous Payload Envelope) – Remaining 261 columns hold user data (e.g., voice, video, internet traffic).

The SDH frame is synchronous, meaning all elements follow a common clock, enabling efficient multiplexing, easy fault detection, and flexible bandwidth allocation.

Question 10: Explain PDH frame structure.

Answer 10:

The PDH frame structure is designed for digital transmission but operates asynchronously, meaning different network elements have slightly varying clock frequencies. It uses multiplexing to combine lower bit rate signals into higher bit rate streams. Key Features of PDH Frame Structure:

1. Basic Unit: E1 (2.048 Mbps) or T1 (1.544 Mbps) frame forms the base of PDH.
2. Frame Size: 32 time slots (TS) of 8 bits each in an E1 frame, repeating every 125 µs.
3. Time Slot Allocation:

• TS0 : Frame alignment and synchronization.
• TS1 to TS15, TS17 to TS31 – User data.
• TS16 : Used for signaling (in E1).

4. Higher Multiplexing Levels :

• Multiple E1s (2.048 Mbps) are combined into E2 (8.448 Mbps), E3 (34.368 Mbps), and E4 (139.264 Mbps).
• In the U.S., T1 (1.544 Mbps) is multiplexed into T2 (6.312 Mbps), T3 (44.736 Mbps), and T4 (274.176 Mbps). PDH’s asynchronous nature makes multiplexing and demultiplexing complex, requiring additional justification bits to handle timing variations, which SDH overcomes with a synchronous structure.

This set of fiber optic interview questions and answers is useful for both freshers and experienced professionals seeking jobs in the fiber optic industry.