Lecture in SONET/SDH | Forouzan

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

A simple network using SONET equipment

SONET layers compared with OSI or the Internet layers

Taxonomy of SONET networks

Virtual tributary types