Synchronous Optical Network or simply SONET is the protocol to transfer multiple digital bit streams synchronously over optical fibers, which has been evolved and developed in North American countries (United States and Canada). This protocol allows use of electrical interface at low data transmission rates. Laser or coherent light sources are used for transmission of optical signals over optical fibers. SONET was developed to transport huge amounts of voice and data signals through the same media (optical fiber) without synchronization problems. SONET was introduced to replace PDH systems, the earlier forms of telecommunication protocol.
In Contrast, Synchronous Digital Hierarchy or SDH has been evolved in Europe and now widely used all over the world. Both SONET and SDH are used globally, but SDH has become popular protocol. Historically, SONET was the first protocol developed before SDH. Most often SONET is considered as a variation of SDH because of it’s popularity and similarity with the former. ITU-T G. 707, G.783, G.784 and G.803 recommendations define the SDH requirements. These requirements were detailed in ETSI (European Telecommunications Standards Institute), which is now adopted to ITU-T recommendations. On the other hand SONET was defined by Telcordia and ANSI T1.105 (American National Standards Institute). GR-253-CORE, the standard from Telcordia Technologies details the Generic Requirements for SONET.
SONET has standards for a number of data rates up to the maximum data rate of 39.808 gigabits per second, which is usually denoted as 40 Gbps. SONET can support higher data rates than 40 Gbps. SONET is considered to be the foundation for the physical layer of the broadband ISDN (B-ISDN). ATM (Asynchronous transfer mode) runs as a layer on top of SONET as well as on top of other technologies. SONET defines optical carrier levels and their electrical equivalents, called synchronous transport signals (STS) for fiber optic transmission. The first step in the process involves multiplexing many optical signals by generating the lowest level or base signal, called STS-1, the optical carrier counterpart of which is OC-1 (Optical Carrier). The data transmission rate of OC-1 or STS-1 is 51,480 Mb/s.
Both SONET and SDH protocols are essentially same and were originally designed to transport circuit mode communications from different sources. They were designed to support real-time, uncompressed, circuit-switched voice encoded in PCM format. In PDH systems, the synchronization sources of various circuits were different, result of which was operation of circuits at slightly different rates and with different phases. SONET/SDH allowed simultaneous transportation of many different circuits of differing origin within a single framing protocol. SONET/SDH is not basically a transport protocol rather than communications protocol. SONET/SDH has protocol neutrality and transport-oriented feature and hence they were the obvious choice for transporting the fixed length Asynchronous Transfer Mode (ATM) frames.
A basic Synchronous Optical Network includes the terminal multiplexer (PTE), a regenerator (which is required for long distance transmissions), an Add-Drop Multiplexer (ADM), Wideband digital cross-connects (W-DCS), Broadband Digital Cross-Connects, and the Digital Loop Carrier. Together, these elements may be used in a point-to-point, point-to-multipoint (hub), or ring network configuration.
SONET offers many benefits over asynchronous systems. Multiplexing technique in SONET allows simplified synchronous clocking and reduced back-to-back multiplexing that helps to reduce circuit complexity and cost. SONET’s optical interconnections meet a number of vendor specifications thereby allowing multi-vendor compatibility. The hub configuration adds greater flexibility to the system, allowing the convergence of a number of types of network protocols, ATM, Internet protocol, etc.