Lecture in SONET/SDH

(Last Updated On: December 8, 2017)
Lecture for SONET/SDH

Definition of Terms

  • Synchronous Optical Network (SONET) is a standard developed by ANSI for fiber-optic networks: Synchronous Digital Hierarchy (SDH) is a similar standard developed by ITU-T.
  • SONET was developed by ANSI; SDH was developed by ITU-T.
  • Each synchronous transfer signal STS-n is composed of 8000 frames. Each frame is a two-dimensional matrix of bytes with 9 rows by 90 × n columns.
  • A SONET STS-n signal is transmitted at 8000 frames per second.
  • Each byte in a SONET frame can carry a digitized voice channel.
  • In SONET, the data rate of an STS-n signal is n times the data rate of an STS-1 signal.
  • STS multiplexers/demultiplexers mark the beginning points and endpoints of a SONET link.
  • An STS multiplexer multiplexes signals from multiple electrical sources and creates the corresponding optical signal.
  • An STS demultiplexer demultiplexes an optical signal into corresponding electric signals.
  • Add/drop multiplexers allow insertion and extraction of signals in an STS. An add/drop multiplexer can add an electrical signals into a given path or can remove a desired signal from a path.
  • SONET has defined a hierarchy of signals called synchronous transport signals (STSs). SDH has defined a similar hierarchy of signals callefd synchronous transfer modules (STMs). 
  • An OC-n signal is the optical modulation of an STS-n (or STM-n) signal.
  • Pointers are used to show the offset of the SPE in the frame or for justification.
  • SONET uses two pointers show the position of an SPE with respect to an STS.
  • SONET use the third pointer for rate adjustment between SPE and STS. 
  • A regenerator takes a received optical signal and regenerates it.
  • The SONET regenerator also replaces some of the existing overhead information with new information.
  • SONET defines four layers: path, line, section, and photonic.
  • The path layer is responsible for the movement of a signal from its source to its destination.
  • The line layer is responsible for the movement of a signal across a physical line.
  • The section layer is responsible for the movement of a signal across a physical section.
  • The photonic layer corresponds to the physical layer of the OSI model. It includes physical specifications for the optical fiber channel.
  • SONET uses NRZ encoding with the presence of light representing 1 and the absence of light representing 0.
  • SONET is a synchronous TDM system in which all clocks are locked to a master clock.
  • SONET sends 8000 frames per second; each frame lasts 125 µs. 
  • Section overhead is recalculated for each SONET device (regenerators and multiplexers).
  • Path overhead is only calculated for end-to-end (at STS multiplexers).
  • An STS-3c signal can carry 44 ATM cells as its SPE.
  • An STS-I frame is made of 9 rows and 90 columns; an STS-n frame is made of 9 rows and n x 90 columns.
  • STSs can be multiplexed to get a new STS with a higher data rate.
  • SONET network topologies can be linear, ring, or mesh.
  • A linear SONET network can be either point-to-point or multipoint.
  • A ring SONET network can be unidirectional or bidirectional.
  • To make SONET backward-compatible with the current hierarchy, its frame design includes a system of virtual tributaries (VTs).
  • SONET is designed to carry broadband payloads. Current digital hierarchy data rates, however, are lower than STS-1. To make SONET backward-compatible with the current hierarchy, its frame design includes a system of virtual tributaries (VTs). A virtual tributary is a partial payload that can be inserted into an STS-1.

A SONET system can use the following equipment:

  • 1.  STS multiplexers
  • 2.  STS demultiplexers
  • 3.  Regenerators
  • 4.  Add/drop multiplexers
  • 5.  Terminals

SONET/SDH rates

SONET/SDH rates

A simple network using SONET equipment

A simple network using SONET equipment

SONET layers compared with OSI or the Internet layers

SONET layers compared with OSI or the Internet layers

Taxonomy of SONET networks

Taxonomy of SONET networks

Virtual tributary types

Virtual tributary types

Note: You can proceed to take the multiple choice exam regarding this topic. SONET/SDH – Set 1 MCQs

List of Data Communications Lectures

credit: Behrouz A. Forouzan©2013 www.PinoyBIX.org
Lecture in SONET/SDH
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