An Introduction to SDH and SONET

SONET (Synchronous Optical Network): The standard for fiber optic transport

In brief, SONET defines optical carrier (OC) levels and electrically equivalent synchronous transport signals (STSs) for the fiber-optic based transmission hierarchy.  The standard SONET line rates and STS-equivalent formats are shown

SONET Heirarchy and Multiplexing

The next figure  illustrates the basic multiplexing structure of SONET.  Any type of service, ranging from voice to high-speed data and video, can be accepted by various types of service adapters.  A service adapter maps the signal into the payload envelope of the STS-1 or virtual tributary (VT). New services and signals can be transported by adding new service adapters at the edge of the SONET network.  All inputs are eventually converted to a base format of a synchronous STS-1 signal (51.84 Mbps or higher). Lower speed inputs such as DS-1s are first bit- or byte-multiplexed into virtual tributaries. Several synchronous STS-1s are then multiplexed together in either a single- or two-stage process to form an electrical STS-n signal (n = one or more).

Supplementary Slide

Supplementary Slide

  STS-1 Frame Structure

STS-1 is a specific sequence of 810 bytes (6,480 bits), which includes various overhead bytes and an envelope capacity for transporting payloads.  It can be depicted as a 90-column by 9-row structure.  With a total frame length of 125 µs (8,000 frames per second), STS-1 has a bit rate of 51.840 Mbps -- viz.

(9) x (90 bytes/frame) x (8 bits/byte) x (8,000 frames/s) = 51,840,000 bps = 51.840 Mbps

The order of transmission of bytes is row-by-row from top to bottom and from left to right (most significant bit first).   As shown in the figure below, the first three columns of the STS-1 frame are for the transport overhead. The three columns contain 9 bytes. Of these, 9 bytes are overhead for the section layer (for example, each section overhead), and 18 bytes are overhead for the line layer (for example, line overhead).   The remaining 87 columns constitute the STS-1 envelope capacity (payload and POH).

Supplementary Slide


Virtual Tributaries

In addition to the STS-1 base format, SONET also defines synchronous formats at sub-STS-1 levels.  The STS-1 payload may be subdivided into virtual tributaries, which are synchronous signals used to transport lower-speed transmissions.  There are three sizes of VTs:

Supplementary Slide


SONET uses a concept called “pointers” to compensate for frequency and phase variations. Pointers allow the transparent transport of synchronous payload envelopes (either STS or VT) across plesiochronous boundaries, i.e., between nodes with separate network clocks having almost the same timing. The use of pointers avoids the delays and loss of data associated with the use of large (125-microsecond frame) slip buffers for synchronization.

Concatenated Payloads

For future services, the STS-1 may not have enough capacity to carry some services.  SONET offers the flexibility of concatenating STS-1s to provide the necessary bandwidth.  The next illustrates SONET flexibility by concatenating three STS-1s to 155.52 Mbps to provide the capacity to transport an H4 digital-video channel.  STS-1s can be concatenated up to STS-3c. Beyond STS-3, concatenation is done in multiples of STS-3c.  For ATM services, concatenation is done in multiples of STS-12c.

SONET Network Elements

Why and How Synchronous (vs.Asynchronous or Plesiochronous ) Networks?

A Simple Asynchronous Signal -- terminal serial communication.
A Basic Synchronous (or Plesiochronous) Signal -- High-Level Data Link Control (HDLC) frame structure.

Synchronous Digital Hierarchy (SDH) -- International super standard

This page was prepared and is maintained by R. Victor Jones
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Last updated May 5, 2004