Synchronous optical networking
The method was developed to replace the plesiochronous digital hierarchy (PDH) system for transporting large amounts of telephone calls and data traffic over the same fiber without the problems of synchronization.
SONET and SDH, which are essentially the same, were originally designed to transport circuit mode communications, e.g. DS1, DS3, from a variety of different sources.
Due to SONET/SDH's essential protocol neutrality and transport-oriented features, SONET/SDH was the choice for transporting the fixed length Asynchronous Transfer Mode (ATM) frames also known as cells.
SDH differs from Plesiochronous Digital Hierarchy (PDH) in that the exact rates that are used to transport the data on SONET/SDH are tightly synchronized across the entire network, using atomic clocks.
Therefore, it is inaccurate to think of SDH or SONET as communications protocols in and of themselves; they are generic, all-purpose transport containers for moving both voice and data.
The basic format of a SONET/SDH signal allows it to carry many different services in its virtual container (VC), because it is bandwidth-flexible.
SONET is a set of transport containers that allow for delivery of a variety of protocols, including traditional telephony, ATM, Ethernet, and TCP/IP traffic.
The protocol is made more complex by the decision to permit this padding at most levels of the multiplexing structure, but it improves all-around performance.
The basic unit of framing in SDH is a STM-1 (Synchronous Transport Module, level 1), which operates at 155.520 megabits per second (Mbit/s).
SONET offers an additional basic unit of transmission, the STS-1 (Synchronous Transport Signal 1) or OC-1, operating at 51.84 Mbit/s—exactly one third of an STM-1/STS-3c/OC-3c carrier.
Their standards are extremely similar in implementation, making it easy to interoperate between SDH and SONET at any given bandwidth.
[nb 2] Carried within the information payload, which has its own frame structure of nine rows and 261 columns, are administrative units identified by pointers.
[13] The WAN PHY variant encapsulates Ethernet data using a lightweight SDH/SONET frame, so as to be compatible at a low level with equipment designed to carry SDH/SONET signals, whereas the LAN PHY variant encapsulates Ethernet data using 64B/66B line coding.
[17] The physical layer is modeled on three major entities: transmission path, digital line and the regenerator section.
It takes data to be transmitted and transforms them into signals required by the line layer, and adds or modifies the path overhead bits for performance monitoring and protection switching.
[19][20] Network management systems are used to configure and monitor SDH and SONET equipment either locally or remotely.
The systems consist of three essential parts, covered later in more detail: The main functions of network management thereby include: Consider the three parts defined above: This will often consist of software running on a Workstation covering a number of SDH/SONET network elements SONET equipment is often managed with the TL1 protocol.
The command language used by a SONET network element, such as TL1, must be carried by other management protocols, such as SNMP, CORBA, or XML.
With the convergence of SONET and SDH on switching matrix and network elements architecture, newer implementations have also offered TL1.
With advances in SONET and SDH chipsets, the traditional categories of network elements are no longer distinct.
Recent digital cross connect systems (DCSs or DXCs) support numerous high-speed signals, and allow for cross-connection of DS1s, DS3s and even STS-3s/12c and so on, from any input to any output.
BLSRs trade cost and complexity for bandwidth efficiency, as well as the ability to support "extra traffic" that can be pre-empted when a protection switching event occurs.
Clock sources used for synchronization in telecommunications networks are rated by quality, commonly called a stratum.
[25] Typically, a network element uses the highest quality stratum available to it, which can be determined by monitoring the synchronization status messages (SSM) of selected clock sources.
[26] In general, a network that has been properly configured should never find itself in a timing loop, but some classes of silent failures could nevertheless cause this issue.
SONET/SDH development was originally driven by the need to transport multiple PDH signals—like DS1, E1, DS3, and E3—along with other groups of multiplexed 64 kbit/s pulse-code modulated voice traffic.
Depending on the data and voice traffic mix that must be carried, there can be a large amount of unused bandwidth left over, due to the fixed sizes of concatenated containers.
More important is the need for all intermediate network elements to support newly introduced concatenation sizes.
Development has stagnated for the last decade (2020) and both suppliers of equipment and operators of SONET/SDH networks are migrating to other technologies such as OTN and wide area Ethernet.
British Telecom has recently (March 2020) closed down their KiloStream and Mega Stream products which were the last large scale uses of the BT SDH.
