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Introduction to SONET (Synchronous Optical Networking)

The SONET standards were developed in the mid-1980s to take advantage of low-cost transmission over optical fibers. It defines a hierarchy of data rates, formats for framing and multiplexing the payload data, as well as optical signal specifications (wavelength and dispersion), allowing multi-vendor interoperability.

SONET is sometimes described as "T-1 on steroids". Why is that? As we know, the digital hierarchy (DS-0, DS-1, DS-2, DS-3 and more) was created to provide cost-effective multiplexed transport for voice and data traffic from one location in a network to another.

SONET (Synchronous Optical Networking, used in the United States) and SDH (Synchronous Digital Hierarchy, used in Europe and other parts of the world) are two equivalent multiplexing protocols for transferring multiple digital bit streams using lasers or LEDs (light-emitting diodes) over the same optical fiber.

SONET and SDH were designed to replace PDH (Plesiochronous Digital Hierarchy) system to eliminate the synchronization problems that PDH had. SONET is synchronous, which means that each connection achieves a constant bit rate and delay. For example, SDH or SONET may be utilized to allow several Internet Service Providers to share the same optical fiber, without being affected by each others traffic load, and without being able to temporarily borrow free capacity from each other.

SONET and SDH are considered to be physical layer protocols since they offer permanent connections and do not involve packet mode communication. Only certain integer multiples of 64kbits⁄s are possible bit rates.

SONET is actually TDM (time division multiplexing) based and this makes it readily supports fixed-rate services such as telephony. Its synchronous nature is designed to accept traffic at fixed multiples of a basic rate (64kbit⁄s), without requiring variable stuff bits or complex rate adaptation.

The SONET data transmission format is based on a 125us frame consisting of 810 octets, of which 36 are overhead and 774 are payload data. The basic SONET signal, whose electrical and optical versions are referred to as STS-1 and OC-1, respectively, is thus a 51.84Mb⁄s data stream that readily accommodates TDM channels in multiples of 8 kb⁄s.

SONET can be used to encapsulate PDH and other earlier digital transmission standards. It can also be used directly to support either ATM (Asynchronous Transfer Mode) or Packet over SONET⁄SDH (POS) networking. So SONET⁄SDH is actually a generic all-purpose transport container for moving both voice and data traffic. They in themselves are not communications protocols.

SONET brings with it a subset of advantages that makes it stand above competitive technologies. These include mid-span meet, improved operations, administration, maintenance, and provisioning (OAM&P), support for multipoint circuit configurations, non-intrusive facility monitoring, and the ability to deploy a variety of new services.

Improved OAM&P is one of the greatest contributions that SONET brings to the networking field. Element and network monitoring, management, and maintenance have always been something of a catch-as-catch-can effort because of the complexity and diversity of elements in a typical service provider's network. SONET overhead includes error-checking ability, bytes for network survivability, and a diverse set of clearly defined management messages.

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