Computer Network | Synchronous Optical Network (SONET)

SONET stands for Synchronous Optical Network. SONET is a communication protocol, developed by Bellcore – that is used to transmit a lage amount of data over relatively large distances using optical fibre. With SONET, multiple digital data streams are transfered at the same time over the optical fibre.

Key Points:

  • Developed by Bellcore
  • Used in North America
  • Standardized by ANSI (American National Standards Institute)
  • Similar to SDH (Synchronous Digital Hierarchy) which is used in Europe and Japan.

Why SONET is called a Synchronous Network?
A single clock (Primary Reference Clock, PRC) handles the timing of transmission of signals & equipments across the entire network.

SONET Network Elements:

  1. STS Multiplexer:
    • Performs multipleximg of signals
    • Converts electrical signal to optical signal
  2. STS Demultiplexer:
    • Performs demultiplexing of signals
    • Converts optical signal to electrical signal
  3. Regenerator:
      It is a repeater, that takes an optical signal and regenerates (increases the strength) it.
  4. Add/Drop Multiplexer:
      It allows to add signals coming from different sources into a given path or remove a signal.

Why SONET is used?
SONET is used to convert electrical signal into optical signal so that it can travel longer distances.

SONET Connections:

  • Section: Portion of network connecting two neighbouring devices.
  • Line: Portion of network connecting two neighbouring multiplexers.
  • Path: End-to-end portion of the network.

SONET Layers:

SONET includes four functional layers:

  1. Path Layer:
    • It is responsible for the movement of signal from its optical source to its optical destination.
    • STS Mux/Demux provides path layer functions.
  2. Line Layer:
    • It is responsible for the movement of signal across a physical line.
    • STS Mux/Demux and Add/Drop Mux provides Line layer functions.
  3. Section Layer:
    • It is responsible for the movement of signal across a physical section.
    • Each device of network provides section layer functions.
  4. Photonic Layer:
    • It corresponds to the physical layer of the OSI model.
    • It includes physical specifications for the optical fibre channel (presence of light = 1 and absence of light = 0).

Advantages of SONET:

  • Transmits data to large distances
  • Low electromagnetic interference
  • High data rates
  • Large Bandwidth


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