(SEM VII) THEORY EXAMINATION 2022-23 OPTICAL NETWORK

SECTION A – Short Answers (2 Marks Each)

(a) Full form of SDH

SDH stands for Synchronous Digital Hierarchy.

(b) Central frequency used in long-haul fiber optic communication

The central frequency lies around 193.1 THz, corresponding to the 1550 nm wavelength window.

(c) Time to process 70-byte Ethernet packet at 10 Gb/s

70 bytes = 560 bits
Time = 560 / (10 × 10⁹) = 56 nanoseconds

(d) Scattering matrix for 2×2 coupler

A 2×2 coupler is represented by a unitary scattering matrix that distributes power between output ports with defined coupling ratio.

(e) Dedicated protection

Dedicated protection is a protection scheme where backup resources are reserved exclusively for a single working path.

(f) Optical line amplifier

An optical line amplifier boosts optical signals directly in the optical domain without electrical conversion, commonly using EDFA.

(g) Spectral efficiency

Spectral efficiency is the data rate transmitted per unit bandwidth, measured in bits/s/Hz.

(h) Two classes of statistical traffic models

Poisson models

Self-similar (burst traffic) models

(i) Burst switching

Burst switching is a technique where data packets are aggregated into bursts and transmitted optically without per-packet processing.

(j) Optical fiber used in long-haul communication

Single-mode optical fiber (SMF) is used for long-haul communication.

SECTION B – Long Answers (10 Marks Each)

(Attempt any three in exam; answers provided for all)

(a) Linear vs Non-linear scattering

Linear scattering includes Rayleigh scattering caused by density fluctuations in fiber and is independent of signal power.
Non-linear scattering depends on optical power and includes Stimulated Brillouin Scattering (SBS) and Stimulated Raman Scattering (SRS), which limit high-power transmission.

(b) Working principle of optical grating

An optical grating consists of periodic variations in refractive index. When light passes through it, constructive and destructive interference occurs, causing wavelength-dependent diffraction. Gratings are widely used in WDM systems for multiplexing and demultiplexing.

(c) Header of ATM layer

The ATM cell consists of a 5-byte header and 48-byte payload.
Header fields include:

Virtual Path Identifier (VPI)

Virtual Channel Identifier (VCI)

Payload Type Indicator (PTI)

Cell Loss Priority (CLP)

Header Error Control (HEC)

(d) Network survivability vs Network protection

Network protection refers to pre-planned mechanisms such as backup paths for failures.
Network survivability is a broader concept that includes protection, restoration, and resilience strategies to maintain service during failures.

(e) Optical Time Division Multiplexing (OTDM)

OTDM multiplexes multiple low-speed optical signals into a single high-speed stream by allocating different time slots to each channel. It enables ultra-high data rates beyond electronic limits.

SECTION C – Long Answers (10 Marks Each)

3(a) Non-linear effects in optical fiber

Major non-linear effects include:

Self-Phase Modulation (SPM)

Cross-Phase Modulation (XPM)

Four-Wave Mixing (FWM)

Stimulated Brillouin Scattering (SBS)

Stimulated Raman Scattering (SRS)

These effects distort signals and limit system capacity in WDM networks.

3(b) Relation between wavelength separation and frequency separation

Frequency and wavelength are related as:

f=cλf = \frac{c}{\lambda}f=λc​

Differentiating:

Δf=cλ2Δλ\Delta f = \frac{c}{\lambda^2} \Delta \lambdaΔf=λ2c​Δλ

This relation is important in DWDM channel spacing design.

4(a) Working principle of Fiber Bragg Grating (FBG)

FBG reflects a specific wavelength (Bragg wavelength) while transmitting others.

λB=2nΛ\lambda_B = 2n\LambdaλB​=2nΛ

It is widely used in wavelength filtering and sensing applications.

4(b) Inter-channel vs Intra-channel crosstalk

Inter-channel crosstalk occurs between different wavelength channels due to imperfect filtering.
Intra-channel crosstalk occurs within the same channel due to dispersion and nonlinear effects.

5(a) OADM architectures

Serial OADM: Channels are dropped sequentially

Parallel OADM: Multiple wavelengths processed simultaneously

Band-drop OADM: A group of wavelengths is dropped together

Used in WDM ring networks.

5(b) Services of Optical Cross Connect (OXC)

OXC provides:

Wavelength routing

Traffic grooming

Protection switching

Network reconfiguration

Scalability and flexibility

6(a) Cost trade-off: Ring, Hub, Mesh topology

Ring: Moderate cost, good protection

Hub: Low cost, poor survivability

Mesh: High cost, maximum reliability

Mesh is preferred for core networks, ring for metro networks.

6(b) Efficient WDM network design (A-B-C)

Each router interface operates at 10 Gbps.
Required capacity = 50 Gbps → 5 wavelengths per link.
A full-mesh WDM design with wavelength reuse ensures efficient bandwidth utilization and scalability.

7(a) Protection techniques

1+1 Protection: Dedicated backup always active

1:1 Protection: Backup used only on failure

1:N Protection: One backup protects multiple working paths

7(b) Node failure handling in BLSR

In Bidirectional Line Switched Ring, traffic is automatically rerouted in the opposite direction around the ring when a node or link fails, ensuring fast restoration.