(SEM VIII) THEORY EXAMINATION 2017-18 OPTICAL NETWORKS

SECTION A (Brief Explanations)

SDH Frame Structure
The SDH (Synchronous Digital Hierarchy) frame structure is a standardized digital transmission format used in optical networks. It organizes data into frames of 125 microseconds duration. The basic unit is STM-1, which consists of overhead bytes and payload. Overhead is used for management, synchronization, and error monitoring, while the payload carries actual user data.

Advantages of PIN Photodiode
A PIN photodiode offers high speed response and good sensitivity for detecting optical signals. It has low noise and is suitable for high-frequency communication systems.

Features of Time-Slotted Optical TDM Network
Time-slotted optical TDM divides transmission time into fixed slots, allowing multiple users to share the same optical fiber. Each user transmits data in assigned time slots, ensuring efficient bandwidth utilization and synchronized communication.

OLT (Optical Line Terminal)
OLT is a central device in fiber optic networks, especially in FTTH systems. It connects the service provider’s network to multiple optical network units (ONUs) and manages data transmission between them.

Topologies Used in Fiber Optical Network
Fiber optical networks commonly use star, ring, and bus topologies. Ring topology provides better fault tolerance, while star topology is simple and widely used in access networks.

CLOS Architecture Switches
In CLOS architecture, switches are arranged in multiple stages to reduce the number of crosspoints while maintaining connectivity. It is used in large switching systems to improve scalability.

Difference Between Isolator and Circulator
An isolator allows light to pass in one direction only and blocks reverse signals to protect equipment. A circulator directs light sequentially between multiple ports and is used in bidirectional communication systems.

HDLC (High-Level Data Link Control)
HDLC is a bit-oriented data link layer protocol used for reliable communication. It provides framing, error detection, and flow control in synchronous communication systems.

SECTION B (Brief, 10-Mark Style Answers)

SONET/SDH Multiplexing and Framing

SONET/SDH is a standardized optical transmission system used for high-speed communication. It multiplexes lower data rate signals into higher-rate frames using synchronous time division multiplexing. The basic SDH unit is STM-1, and higher levels like STM-4 and STM-16 are formed by byte interleaving. Framing includes overhead bytes used for synchronization, error detection, and network management, while the payload carries user data. This structure ensures reliable and efficient long-distance transmission.

Functional Blocks of FTTH

FTTH (Fiber to the Home) consists of an Optical Line Terminal at the service provider side, optical fiber distribution network, splitters, and Optical Network Units at the customer premises. The OLT manages traffic and distributes signals through passive optical splitters to multiple users. The ONU converts optical signals into electrical form for user devices. This structure enables high-speed broadband services like internet, IPTV, and VoIP.

OTDM and Burst Switching

Optical Time Division Multiplexing (OTDM) increases capacity by transmitting multiple optical signals in different time slots at very high speeds. It requires precise synchronization and ultrafast optical switches. Burst switching is a technique where data is transmitted in bursts instead of continuous streams. It combines advantages of circuit and packet switching, reducing delay and improving bandwidth efficiency in optical networks.

SECTION C (Brief, 10-Mark Style Answers)

Wavelength Assignment in WDM Networks

In Wavelength Division Multiplexing (WDM), multiple signals are transmitted over the same fiber using different wavelengths. Wavelength assignment is the process of allocating wavelengths to different connections without interference. It must satisfy wavelength continuity constraints unless wavelength converters are used. Efficient assignment improves network utilization and reduces blocking probability.

Optical Network Protection and Restoration

Optical networks use protection techniques such as 1+1 protection, ring protection, and mesh protection to ensure reliability. In 1+1 protection, data is sent simultaneously on two paths, and the receiver selects the best signal. Ring protection quickly reroutes traffic in case of failure. These mechanisms reduce downtime and ensure uninterrupted communication.

CLOS Architecture in Optical Switching

CLOS architecture is a multistage switching structure used to reduce hardware complexity while maintaining high connectivity. It consists of input, middle, and output switching stages. This architecture is scalable and suitable for large optical cross-connect systems.

Optical Burst Switching (OBS)

Optical Burst Switching transmits large data bursts after sending a control packet to reserve resources. It reduces electronic processing in core networks and improves bandwidth utilization. OBS is considered a promising technique for next-generation high-speed optical networks.