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Synchronous optical networking, is a method for communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber. more...

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The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone and data traffic and to allow for interoperability between equipment from different vendors.

There are multiple, very closely related standards that describe synchronous optical networking:

SDH or synchronous digital hierarchy standard developed by the International Telecommunication Union (ITU), documented in standard G.707 and its extension G.708; SONET or synchronous optical networking standard as defined by GR-253-CORE from Telcordia;

Both SDH and SONET are widely used today; SONET in the U.S. and Canada, SDH in the rest of the world. Although the SONET standards were developed before SDH, their relative global prevalence means that SONET is now considered as the variation.

Synchronous networking differs from PDH in that the exact rates that are used to transport the data are tightly synchronized across the entire network, made possible by atomic clocks. This synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between each element in the network.

Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either ATM or so-called Packet over SONET/SDH (POS) networking. As such, it is inaccurate to think of SDH or SONET as a communications protocols in and of themselves, but rather as a generic and all-purpose transport container for moving both voice and data.

Structure of SONET/SDH signals

The basic unit of transmission

for SONET is a signal that operates at 51.840 Mbit/s, designated STS-1 (Synchronous Transport Signal - 1).; for SDH is a signal that operates at 155.52 Mbit/s, exactly three times that of STS-1, designated STM-1 (Synchronous Transport Module level - 1) .;

Framing

In standard packet or frame oriented data transmission, a frame usually consists of a header and a payload, with the header of the frame being transmitted first, followed by the payload (and possibly a trailer, such as a CRC). In synchronous optical networking, this is modified slightly. The header is termed the overhead and the payload still exists, but the overhead is not all transmitted before the payload - instead the transmission is interleaved, with part of the overhead being transmitted, then part of the payload, then the next part of the overhead, then the next part of the payload, until the entire frame has been transmitted. In the case of SONET, the frame is 810 octets in size; whereas in SDH the frame is 2430 octets in size. In the case of SONET, the frame is transmitted as 3 octets of overhead, followed by 87 octets of payload, nine times over until 810 octets have been transmitted, taking 125 microseconds. In the case of SDH, 9 octets of overhead are transmitted, followed by 261 octets of payload, also nine times over until 2430 octets have been transmitted, also taking 125 microseconds. For both SONET and SDH this is normally represented by the frame being displayed graphically as a block: of 90 columns and 9 rows for SONET; and 270 columns and 9 rows for SDH. This representation aligns all the overhead columns, so the overhead appears as a contiguous block, as does the payload.

Read more at Wikipedia.org