SONET/SDH

1. The ANSI standard is called the Synchronous Optical Network (SONET), and the ITU-T standard is called the Synchronous Digital Hierarchy (SDH). These two standards are nearly identical.

2. SONET/SDH is a synchronous network.

3. SONET/SDH contains recommendations for the standardization of fiber-optic transmission system (FOTS) equipment sold by different manufacturers.

4. The SONET/SDH physical specifications and frame design include mechanisms that allow it to carry signals from incompatible tributary systems.

5. SONET defines a hierarchy of signaling levels called synchronous transport signals (STSs). The physical links defined to carry each level of STS are called optical carriers (OCs). Currently the most popular implementations are OC-1, OC-3, OC-12, and OC-48.

6. SDH specifies a similar system called synchronous transport module (STM). STM is intended to be compatible with existing European hierarchies, such as E lines, and with STS levels.

7. SONET devices
   • An STS multiplexer/demultiplexer either multiplexes signals from multiple sources into an STS signal or demultiplexes an STS signal into different destination signals.

   • AN STS regenerator is a repeater that takes a received optical signal and regenerates it.

   • An add/drop multiplexer can add signals coming from different sources into a given path or remove a desired signal from a path and redirect it without demultiplexing the entire signal.

8. Sections, Lines, and Paths
   • A section is the optical link connecting two neighbor devices.

   • A line is the portion of the network between two multiplexers.

   • A path is the end-to-end portion of the network between two STS multiplexers.

9. SONET Layers
   • The photonic layer corresponds to the physical layer of the OSI model. SONET uses NRZ encoding.

   • The section layer is responsible for the movement of a signal across a physical section. It handles framing, scrambling, and error control.

   • The line layer is responsible for the movement of a signal across a physical line. STS multiplexers and add/drop multiplexers provide line layer functions.

   • The path layer is responsible for the movement of a signal from its optical source to its optical destination. STS multiplexers provide path layer functions. Data received from an electronic interface, such as a T-1 line, is encapsulated in a frame and overhead is added.

10. Each frame contains 6480 bits (810 octets).

11. 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 and combined with other partial payloads to fill out the frame.

12. The most important phyiscal carrier for ATM is projected to be SONET's ST-3 service.

13. Applications
    • SONET can replace existing T-1 or T-3 lines. A T-1 load can easily be carried in a VT1.5 tributary and a T-3 load can easily be carried in a full SPE of an STS-1 frame.

    • SONET can be a carrier for ISDN and B-ISDN.

    • SONET can be a carrier for ATM cells.

    • SONET can support bandwidth on demand.

    • SONET can replace the fiber-optic cables used in cable TV networks.

    • SONET can be used as a backbone or totally replace other networking protocols such as SMDS or FDDI.