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Emerging Technology » SDH



Full Name

Synchronous Digital Hierarchy


SDH (Synchronous Digital Hierarchy) is a standard technology for synchronous data transmission on optical media. It is the international equivalent of Synchronous Optical Network. Both technologies provide faster and less expensive network interconnection than traditional PDH (Plesiochronous Digital Hierarchy) equipment. In digital telephone transmission, "synchronous" means the bits from one call are carried within one transmission frame. "Plesiochronous" means "almost (but not) synchronous," or a call that must be extracted from more than one transmission frame.


SDH was first introduced into the telecommunications network in 1992 and has been deployed at rapid rates since then. It's deployed at all levels of the network infrastructure. The original SDH standard defined the transport of 1.5/2/6/34/45/140 Mbps within a transmission rate of 155.52 Mbps and is being developed to carry other types of traffic, such as: Asynchronous transfer mode (ATM) and Internet protocol (IP). 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. The ability to transport ATM traffic was another early application. In order to support large ATM bandwidths, concatenation was developed, whereby smaller multiplexing containers (e.g., STS-1) are inversely multiplexed to build up a larger container (e.g., STS-3c) to support large data-oriented pipes. One problem with traditional concatenation, however, is inflexibility. 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. For example, fitting a 100 Mbit/s Fast Ethernet connection inside a 155 Mbit/s STS-3c container leads to considerable waste. More important is the need for all intermediate network elements to support newly introduced concatenation sizes. This problem was overcome with the introduction of Virtual Concatenation. Virtual concatenation (VCAT) allows for a more arbitrary assembly of lower-order multiplexing containers, building larger containers of fairly arbitrary size (e.g., 100 Mbit/s) without the need for intermediate network elements to support this particular form of concatenation. Virtual concatenation leverages the X.86 or Generic Framing Procedure (GFP) protocols in order to map payloads of arbitrary bandwidth into the virtually concatenated container. The Link Capacity Adjustment Scheme (LCAS) allows for dynamically changing the bandwidth via dynamic virtual concatenation, multiplexing containers based on the short-term bandwidth needs in the network. The set of next-generation SONET/SDH protocols that enable Ethernet transport is referred to as Ethernet over SONET/SDH (EoS).


The basic unit of SDH is STM-1.
STM-1 (Synchronous Transport Mode 1) has a line rate of 155.52 Mbps

Data Rates

Each rate is an exact multiple of the lower rate therefore the hierarchy is synchronous:
STM-1 = 155.52 Mbit/s
STM-4 = 622.08 Mbit/s
STM-16 = 2588.32 Mbit/s
STM-64 = 9953.28 Mbit/s

Services Offered

SDH service allows you to have your own transmission channel with defined capacity.

SDH provides transmission pipes that run between the network nodes.
SDH has dedicated Data Communication Channels (DCC)s for management traffic.



Can operate in frequency bands from 4 to 13 GHz for long-haul transmission.


Synchronous Digital Hierarchy is primarily used in the telecommunication industry to carry telephony traffic.
SDH is very suitable for leased lines.
The availability of a set of generic standards enables multi-vendor interoperability
SDH?s Flexible architecture is capable of accommodating future applications, with a variety of transmission rates

SDH is not suitable for fast switching applications.

Present/Future in SAMENA

Majority of the telecom operator in the SAMENA region are trying to add data handling capabilities to there networks in order to migrate from the traditional voice centric world to data centric world.Circuits and packets are converging at both equipment and management levels.
Next-generation SDH, with its data-specific features, provides a carrier-class platform to enable this convergence and represents the biggest sector for optical systems sales today.

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