Optical Fiber & Telecom Network Fundamentals

Posted on Aug 17, 2025 in Business Administration and Innovation Management

Optical Transmission Systems

Optical Fibers: Types, Numerical Aperture, and Structure

Optical fibers are fundamental components in transmission systems. They are categorized into two main types:

  • Single-Mode (SM) Fibers:
    • Offer the highest transmission rate (up to 1 Gbps/km).
    • Are generally more expensive.
    • Support distances up to 100 km.
    • Are excited only by laser diodes.
    • Support only one mode of light propagation without reflections.
  • Multi-Mode (MM) Fibers:
    • Are less expensive.
    • Offer easier connection.
    • Have a high Numerical Aperture (NA).
    • Can also be excited by LEDs.
    • Come in two main profiles: Step-Index (SI), typically used for homes, or Graded-Index (GI), often used for LANs.

The angle that meets the condition for guiding the beam through a fiber defines the Numerical Aperture (NA). The formula for NA is: NA = n0 * sin(θm) * (n1^2 – n2^2)^0.5. Here, n0 is the refractive index of the outer environment, n1 is the core’s refractive index, and n2 is the cladding’s refractive index.

The basic structure of an optical fiber typically consists of:

  • Core
  • Cladding (glass)
  • Primary Protection
  • Secondary Protection

Optical Fiber Transmission Parameters

Key parameters influencing optical fiber transmission include:

  • Attenuation (α) [dB/km]: The power signal decreases with distance from the source. Attenuation is influenced by:
    • Intrinsic Absorption: Losses due to the optical material itself.
    • Extrinsic Absorption: Losses due to contaminant molecules.
    • Linear Scattering: Occurs when the core and cladding are not ideally homogeneous.
    • Nonlinear Scattering: A portion of the spectrum changes its wavelength after transmission.
    • Micro-bending: Bends in the fiber on the order of millimeters.
    • Macro-bending: Bends in the fiber on the order of tens of millimeters.
  • Dispersion: The optical pulse shape changes along the path; its peak value decreases, and its width increases. At the end of the path, the signal can be degraded to be unusable. There are two main types:
    • Mode Dispersion (only in MM fibers): Each optical beam reaches the fiber output at a different time due to differences in path length.
    • Chromatic Dispersion: Caused by different propagation speeds of particular beams (different wavelengths).

Wavelength Division Multiplexing (WDM) Systems

WDM systems multiplex several optical channels into one fiber using different wavelengths. There are two primary types:

  • Dense Wavelength Division Multiplexing (DWDM, ITU-T G.692):
    • Acceptable carrier frequencies are integral multiples of 50 (or 100) GHz, with a bandwidth of 192.1 THz up to 196.1 THz.
    • Typically supports 80 (or 40) channels (bandwidth 1528.77 nm up to 1560.61 nm) separated by 0.4 (or 0.8) nm.
    • Transmission rates: 40-80 Gbit/s.
    • Can transmit up to 500 km without amplification.
  • Coarse Wavelength Division Multiplexing (CWDM, ITU-T G.694-95):
    • Primarily used for fibers with a bandwidth of 1270 nm up to 1610 nm.
    • Supports 18 channels separated by 20 nm.
    • Transmission rate up to 2.5 Gb/s.

Telecommunication Network Structure

Characteristics of Backbone Networks

Backbone networks form the core of telecommunication infrastructure, characterized by:

  • Transport of payload between specific network nodes.
  • Transfer rates up to hundreds of Gbit/s.
  • Transport over long distances.
  • Transmission equipment generally located in provider premises.
  • Typically based on SDH (Synchronous Digital Hierarchy) systems, utilizing optical fibers with transmission rates ranging from 2.5 Gbit/s (STM-16) to 40 Gbit/s (STM-256).

Characteristics of Access Networks

Access networks connect end-users to the core network, featuring:

  • Concentration of traffic from a defined area at a service node of the telecommunication network.
  • Transmission rates from tens of kbit/s (e.g., phone calls) to hundreds of Mbit/s (e.g., multimedia services).
  • Transport over short to medium distances.
  • Transmission equipment located on the subscriber’s side (potential problems with power supply and unauthorized access).

Principle of PSDN and CSDN Networks

Two fundamental network types based on switching principles are:

  • Circuit Switched Data Network (CSDN):
    • Uses switched data circuits.
    • Provides a lower bit error rate.
    • Offers higher transmission efficiency.
    • Ensures higher channel capacity through circuit switching.
  • Packet Switched Data Network (PSDN):
    • Based on principles of time-division multiplexing.
    • Utilizes temporary storage (Store-and-Forward method).