SDH, Synchronous Digital Hierarchy
Synchronous optical networking, commonly known as SONET, is a standard for communicating digital information using lasers or light emitting diodes (LEDs) over optical fiber as defined by GR-253-CORE from Telcordia. It was developed to replace the PDH system for transporting large amounts of telephone and data traffic.
The more recent Synchronous Digital Hierarchy (SDH) standard developed by ITU (G.707 and its extension G.708) is built on experience in the development of SONET. Both SDH and SONET are widely used today; SONET in the U.S. and Canada, SDH in the rest of the world. SDH is growing in popularity and is currently the main concern with SONET now being considered as the variation.
SONET differs from PDH in that the exact rates that are used to transport the data are tightly synchronized to network based clocks. Thus the entire network operates synchronously. SDH was made possible by the existence of atomic clocks.
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 networking.
The basic SONET signal operates at 51.840 Mbit/s and is designated STS-1 (Synchronous Transport Signal one). The STS-1 frame is the basic unit of transmission in SONET.
The Synchronous Transport Module level 1 (STM-1) is the basic signal rate of SDH.
The two major components of the STS-1 frame are the transport overhead and the synchronous payload envelope (SPE). The transport overhead (27 bytes) comprises the section overhead and line overhead. These bytes are used for signalling and measuring transmission error rates. The SPE comprises the payload overhead (9 bytes, used for end to end signalling and error measurement) and the payload of 774 bytes. The STS-1 payload is designed to carry a full DS-3 frame.
The entire STS-1 frame is 810 bytes. The STS-1 frame is transmitted in exactly 125 microseconds on a fiber-optic circuit designated OC-1 (optical carrier one). In practice the terms STS-1 and OC-1 are used interchangeably.
Three OC-1 (STS-1) signals are multiplexed by time-division multiplexing to form the next level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbit/s. The multiplexing is performed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing 2430 bytes and transmitted in 125 microseconds. The STS-3 signal is also used as a basis for the SDH hierarchy, where it is designated STM-1.
Higher speed circuits are formed by successively aggregating multiples of slower circuits, their speed always being immediately apparent from their designation. For example, four OC-3 or STM-1 circuits can be aggregated to form a 622.08 Mbit/s circuit designated as OC-12 or STM-4.
The highest rate that is commonly deployed is the OC-192 or STM-64 circuit, which operates at rate of just under 10 Gbit/s. Speeds beyond 10 Gbit/s are technically viable and are under evaluation. Where fiber exhaust is a concern, multiple SONET signals can be transported over multiple wavelengths over a single fiber pair by means of Dense Wave Division Multiplexing (DWDM). Such circuits are the basis for all modern transatlantic cable systems and other long-haul circuits.
SONET/SDH was originally developed primarily to transport pulse-code modulated voice traffic in fixed rate 64kbit/s timeslots through a synchronous optical network. Therefore it was inefficient to transport the bursty packet traffic of the Ethernet world. By introducing virtual concatenation, SONET/SDH became capable of transmitting packet-sized data without bandwidth losses. The data payload like Ethernet is mapped to SDH/SONET using X.86 or Generic Framing Procedure(GFP) protocols. Also recent additions like Link Capacity Adjustment Scheme (LCAS) allows for dynamically changing the bandwidth.
From the Wikipedia article on SDH.