Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or light-emitting diodes (LEDs). Lower rates can also be transferred via an electrical interface. The method was developed to replace the plesiochronous digital hierarchy (PDH) system for transporting larger amounts of telephone calls and data traffic over the same fibre wire without synchronization problems. SONET generic criteria are detailed in Telcordia Technologies Generic Requirements document GR-253-CORE. Generic criteria applicable to SONET and other transmission systems (e.g., asynchronous fiber optic systems or digital radio systems) are found in Telcordia GR-499-CORE.

SONET and SDH were originally designed to transport circuit mode communications (e.g., T1, T3) from a variety of different sources. The primary difficulty in doing this prior to SONET was that the synchronization sources of these different circuits were different. This meant each circuit was actually operating at a slightly different rate and with different phase. SONET allowed for the simultaneous transport of many different circuits of differing origin within one single framing protocol. In a sense, then, SONET is not itself a communications protocol per se, but a transport protocol.

Due to SONET's essential protocol neutrality and transport-oriented features, SONET was the obvious choice for transporting asynchronous transfer mode (ATM) frames. It quickly evolved mapping structures and concatenated payload containers to transport ATM connections. In other words, for ATM (and eventually other protocols such as TCP/IP and Ethernet), the internal complex structure previously used to transport circuit-oriented connections is removed and replaced with a large and concatenated frame (such as STS-3c) into which ATM frames, IP packets, or Ethernet are placed.
A rack of Alcatel STM-16 SDH add-drop multiplexersBoth SDH and SONET are widely used today. SONET in the U.S. and Canada and SDH in the rest of the world. Although the SONET standards were developed before SDH, their relative penetrations in the worldwide market dictate that SONET is considered the variation.

The two protocols are standardized according to the following:
SDH or Synchronous Digital Hierarchy standard was originally defined by the ETSI or European Telecommunications Standards Institute

SONET or synchronous optical networking standard as defined by GR-253-CORE from Telcordia and T1.105 from American National Standards Institute

Synchronous networking differs from PDH in that the exact rates that are used to transport the data are tightly synchronized across the entire network, using atomic clocks. This synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between elements 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 asynchronous transfer mode (ATM) or so-called packet over SONET/SDH (POS) networking. As such, it is inaccurate to think of SDH or SONET as communications protocols in and of themselves, but rather as generic and all-purpose transport containers for moving both voice and data. The basic format of an SDH signal allows it to carry many different services in its virtual container (VC) because it is bandwidth-flexible.

SONET and SDH often use different terms to describe identical features or functions. This can cause confusion and exaggerate their differences. With a few exceptions, SDH can be thought of as a superset of SONET.

The protocol is an extremely heavily multiplexed structure, with the header interleaved between the data in a complex way. This is intended to permit the encapsulated data to have its own frame rate and to be able to float around relative to the SDH/SONET frame structure and rate. This interleaving permits a very low latency for the encapsulated data. Data passing through equipment can be delayed by at most 32 microseconds, compared to a frame rate of 125 microseconds and many competing protocols buffer the data for at least one frame or packet before sending it on. Extra padding is allowed for the multiplexed data to move within the overall framing due to it being on a different clock than the frame rate. The decision to allow this at most of the levels of the multiplexing structure makes the protocol complex, but gives high all-around performance. SONET is the standard defined by the ANSI T1 for synchronous operation used in North America.

The basic unit of framing in SDH is a STM-1 (synchronous transport module level - 1), which operates at 155.52 Mbit/s. SONET refers to this basic unit as an STS-3c (synchronous transport signal - 3, concatenated), but its high-level functionality, frame size, and bit-rate are the same as STM-1.

SONET offers an additional basic unit of transmission, the STS-1 (synchronous transport signal - 1), operating at 51.84 Mbit/s - exactly one third of an STM-1/STS-3c. That is, in SONET the associated OC-3 signal will be composed of three STS-1s (or, more recently in packet transport, the OC-3 signal will carry a single concatenated STS-3c.) Some manufacturers also support the SDH equivalent: STM-0.

In packet-oriented data transmission such as Ethernet, a packet frame usually consists of a header and a payload. The header is 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 instead of being transmitted before the payload, is interleaved with it during transmission. Part of the overhead is 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 an STS-1, the frame is 810 octets in size while the STM-1/STS-3c frame is 2430 octets in size. For STS-1, the frame is transmitted as 3 octets of overhead, followed by 87 octets of payload. This is repeated nine times over until 810 octets have been transmitted, taking 125 microseconds. In the case of an STS-3c/STM-1 which operates three times faster than STS-1, 9 octets of overhead are transmitted, followed by 261 octets of payload. This is also repeated nine times over until 2,430 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 STS-1; and 270 columns and 9 rows for STM1/STS-3c. This representation aligns all the overhead columns, so the overhead appears as a contiguous block, as does the payload.
The internal structure of the overhead and payload within the frame differs slightly between SONET and SDH, and different terms are used in the standards to describe these structures. Their standards are extremely similar in implementation making it easy to interoperate between SDH and SONET at particular bandwidths.

It is worth noting that the choice of a 125-microsecond interval is not an arbitrary one. If one octet is extracted from the bitstream every 125 microseconds, this produces a data rate of 8 bits per 125 microseconds - or 64 kbit/s, the basic digital signaling rate for telecommunication systems world wide. This allows an extremely useful technique to be used in synchronous optical networking. The low data-rate channels or streams of data can be extracted from high data-rate streams by simply extracting octets at regular time intervals—there is no need to understand or decode the entire frame. This is not possible in PDH networking. It shows that a relatively simple device is all that is needed to extract a datastream from an SDH-framed connection and insert it into a SONET-framed connection and vice versa.

In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC-N format refers to the signal in its optical form. It is therefore incorrect to say that an OC-3 contains 3 OC-1s: an OC-3 can be said to contain 3 STS-1s.

A STM-1 Frame. The first 9 columns contain the overhead and the pointers. For the sake of simplicity, the frame is shown as a rectangular structure of 270 columns and 9 rows but, in practice, the protocol does not transmit the bytes in this order.

For the sake of simplicity, the frame is shown as a rectangular structure of 270 columns and 9 rows. The first 3 rows and 9 columns contain regenerator section overhead (RSOH) and the last 5 rows and 9 columns contain multiplex section overhead (MSOH). The 4th row from the top contains pointersThe STM-1 (synchronous transport module level - 1) frame is the basic transmission format for SDH or the fundamental frame or the first level of the synchronous digital hierarchy. The STM-1 frame is transmitted in exactly 125 microseconds, therefore there are 8000 frames per second on a fiber-optic circuit designated OC-3 (optical carrier three). The STM-1 frame consists of overhead and pointers plus information payload. The first 9 columns of each frame make up the Section Overhead and Administrative Unit Pointers, and the last 261 columns make up the Information Payload. The pointers (H1, H2, H3 bytes) identify administrative units (AU) within the information payload.

Carried within the information payload, which has its own frame structure of 9 rows and 261 columns, are administrative units identified within the information payload by pointers. Within the administrative unit is one or more virtual containers (VC). VCs contain path overhead and VC payload. The first column is for path overhead; it’s followed by the payload container, which can itself carry other containers. Administrative units can have any phase alignment within the STM frame, and this alignment is indicated by the pointer in row four,

The section overhead of an STM-1 signal (SOH) is divided into two parts: the regenerator section overhead (RSOH) and the multiplex section overhead (MSOH). The overheads contain information from the system itself, which is used for a wide range of management functions, such as monitoring transmission quality, detecting failures, managing alarms, data communication channels, service channels, etc.
The STM frame is continuous and is transmitted in a serial fashion, byte-by-byte, row-by-row.