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
A simple network using SONET equipment
SONET layers compared with OSI or the Internet layers
Taxonomy of SONET networks
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