Synchronous Digital Hierarchy (SDH)

Abstract — The deployment of  has nowadays gained worldwide attention as one of the most fiber optic transmission system due to its flexibility, interoperability with other vendors, low operating cost and many services with high qua lity, that are provided by SDH. This paper describes SDH network elements and its structure and defines its features provided by this system comparing with other transmission systems and explores the disadvantages. The paper further discusses the future of this technology . Keywords — SDH, PDH, SONET, fiber optic transmission system .

SDH is a technology and stands for synchronous digital hierarchy, which is used to transport multiple digital bit streams synchronously over optical fiber by using either highly coherent light generated by light -emitting diodes (LEDs) or lasers. In this context there are two main concepts which first should be explained, namely Synchronous and Plesiochronous. “Synchronous” means the bits that belong to one call are transported in one transmission frame. “Plesiochronous” means “approximately” but not completely synchronous. In other words, a call should be taken out from many transmission frames, as is used in Plesiochronous Digital Hierarchy (PDH) system, which will b e illustrated later in this paper.

SDH appeared as an equivalent technology of Synchronous optical networking (SONET), which has been created in the early 1980s by Bellcore in the USA and then suggested to the American National Standards Institute (ANSI) i n early 1985s, whereas SDH was defined by Consultative Committee for International Telegraphy and Telephony (CCITT) to carry the European hierarchy [1]. The original design of the SDH and SONET was to transmit circuit mode communications from a diversity o f several sources, but in fact the main purpose of these two protocols, is to assist real – time, uncompressed and circuit -switched voice encoded in PCM form.

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The main obstacle to fulfill this, before the appearing of SDH and SONET, was the variation of the synchronously sources of these different circuits. In other words, the operation of every circuit was at a little different rate and different phase.

By utilizing SDH and SONET the simultaneous transmitting of several different circuits of various origins within individual framing protocol was made possible.  SDH/SONET is not considered as a communication protocol in itself but as a transport protocol .  SDH Network Elements SDH network contains the following various elements: 1) Synchr onous multiplexer : The synchronous multiplexer has two functions, which are multiplexing and live line transmitting, so the equipment of the line transmitting and Plesiochrounous multiplexers are exchanged. Synchronous multiplexers are divided into two typ es: • Add and Drop Multiplexer (ADM) : Its main purpose is to make it possible to “Add” or “Drop” channels from the “Through Channels”. ADM is considered as SDH structure for local access to synchronous networks. •

Terminal Multiplexer (TM) : TM takes several branch signals and collects them into aggregate signals. Regenerators : As the name indicates, the main function of the regenerators is to regenerate the signal. The main use of the regenerators is to transmit the date for a long distanc Synchronous Digital cross Connect : The equipment of the cross connect can be considered as a switch, which can select specific lower order channels without the using of a transport channel. B. Synchronous Transport Modules of SDH SDH net work includes a hierarchy for various date transport rate. Five standardized transport modules can be classified in SDH network, where N equals. The value of N is proportional to the transmission rate. That means the bigger the value of N, the bigger the transmission rate. As shown in the table 1 below the date speed can be calculated by multiplying the base transmission ratewith the number of N.

The recommendation is to use the lower transfer rate in smaller or local n etworks and the higher transfer rate in large or trunk networks. SDH hierarchy levels and speed C. SDH Frame Structure SDH frame has a repeated format with a duration of 125 µs, which equals to 0.125kHz and contains nine equal length segments. The basic synchronous transport module of the SDH SDH Level Line rate. At the overall transmit rate of 155.52 Mbit /s for this transport module, there is a burst of nine bytes at the beginning of every segment. Synchronous multiplexing components are structured fixed -size group of bytes, which are either mapped one – into -the -other or byte -interleaved to the finally form STM -N frames. D. SDH Layers SDH layer are divided into four sub -layers, which are multiplex section, regenerator section, path and photonic layer.

Overhead operation is defined by the SDH framing structure at these four sub -layers in order to provide some features such as communicate alarm condition, evaluate error rate and grant maintenance support. E. SDH Network Topology SDH multiplexers consi sts of two interface groups, which are the aggregate interface and the user interface. The purpose of the aggregate interface is to make a linear inter -switch connection possible, while the purpose of the user interface is to connect users with each other within the network. These Interfaces allow to make the following topologies possible: Ring Topology : The ring topology is a path back to itself which assists to detect the error or failure in the connection, set up the connection back rapidly and to ease t he development of protocols.

At every node in this topology an ADM and two or four fibers can be used. Ring topology is the most popular topology. Star Topology : In this topology the traffic makes its way through a Synchronous Digital Cross Connect, wh ich can be considered as a central hub.  Point -Point Link : This topology is derived from PDH systems that provide Point -Point connections, but SDH will exchange these systems with STM  line systems. Regenerators could be used to avoid the transmission problems.  Linear Bus Topology : This topology is used when protection is required because this topology is flexible. F. SDH Virtual Container Structure Virtual Containers (VCs) can be defined as the structure block, that maps the payload. Mapping is an operation, by which sub signals like ATM and PDH can be packed into the SDH transport modules STM -N. For ATM signals or 140 Mbit/s VC -4 mapping can be utilized and for 2 Mbit/s signals VC -12 can be utilized.

VCs can be accessed separately and independentl y within SDH frames through pointer information that is directly related with VCs by multiplexing . G. Structure of SDH Overhead SDH overhead has a very important role in SDH network because it provides many significant functions, such as switching control, messaging, labeling and monitoring. Specific bytes are assigned per frame or multi -frame in each layer. By using overheads, the observation of two ends from one end, for sector management (transmit traffic) and central management via Dat a Communications Channel. SDH comparing to other technologies Many technologies have been designed to transmit digital bit streams synchronously over optical fiber, one of these technologies is Plesiochronous Digital Hierarchy (PDH).

PDH is a technology that has been designed to transmit huge data over fiber optic or microwave radio system and to be the original standard for telephone networks. Time Division Multiplexing (TDM) is used in PDH. Since PDH uses var ious standards, which makes the connection to various networks quite hard, PDH is being replaced by either SDH or SONET in many telecommunication systems. The essential difference between SDH and PDH is that the accurate rate, which is used to transmit the data on SDH/SONET is tightly synchronized across the whole network by using atomic clocks. Therefore, the whole inter -country network operates synchronously with less amount of buffering needed between components in the network.

A useful characteristi c in SDH/SONET is that the prior digital transport standards, such as PDH standard, can be encapsulated by using SDH/SONET, or it can make use of them to give the required support to either packet over SDH/SONET (POS) or Asynchronous Transfer Modul (ATM) n etworking. Thus, it would be inexact to consider the SDH as a communication protocol in itself; it is general, its serves as transport containers to move the voice and date. Due to the bandwidth flexibility of the SDH signal, various services can be carrie d in Virtual Containers. B. Advantages of SDH The transmission rate of SDH comparing to PDH can reach up to 10 Gbit/s and it would be easy to take out and add low bit rate channels to high bit streams.

In case of failure SDH network contains restore /repair and backup technique, furthermore when an error occurs, it does not affect the whole network. SDH system has the following advantages: • It is capable to transport existing PDH signals. • It is friendly to other vendors and can support various operation. • Multiplexing and demultiplexing processes are more simplified. • It supports services such as interactive multimedia and video conferencing. • Quick error correction. • Ring topology provides switching protection to traffic. • There is no limitation for the optical fiber bandwidth increment. • The capability of transmit broadband signals. • Easy expansion to higher bit rate, which improves the maintenance and administration process.

Disadvantages of SDH Although SDH technology ha s many advantages but like any other transmission system, SDH has some weaknesses. The disadvantages of SDH include: • The efficiency of used bandwidth is low comparing to PDH: 155 Mbit/s transfers 63 ? 2 and 140 Mbit/s transfers 64 ? 2 Mbit/s. • SDH does not support directly PDH 8 Mbit/s. • Complex SDH network components are needed due to the diversity of options and management traffic types. • Because of SDH is dependent on Operation Administration and Maintenance (OAM), which is protocol for monitor ing, installing and troubleshooting Ethernet networks, security in network is absence. OAM could be attacked by trojans or viruses [4]. • Pointers is used to fulfill the direct adding or dropping of lower -rate signals, which makes the system more complicated .

Future of SDH Nowadays SDH technology is applied in many fiber optic transmission systems due to the high capacity and interoperability, which are provided by SDH system. As PDH dominated transmission for more than 20 years, SDH is expected to dominate transmission for the next years. SDH development makes the implementation of internet services and application quite easy, such as high -volume data backup and online gaming. In order to stay one of the most relevant transmission technologies in the world, SDH system elements must be developed constantly to meet the needs of the customer, which the customer expects from the operators.

SDH handles many problems of its predecessor PDH such as the complexity use of Multiplexing and Demultiple xing processor, limits of the bandwidth of the optical fiber, error correction and many other problems, which can have a negative effect on the performance of the telecommunication systems. A transmission system with many improved features like SDH systems can provide and ensure a high -performance level which customer expects from the telecommunication operators. Because of SDH is interoperability and can support different vendors, SDH system allows the independent manufactures to develop more services, whi ch require higher bandwidth capacity without any restriction or any additional requirements with SDH.

References

1. Freeman, “Digital transmission system” in Telecommunication System Engineering, 4th ed.
2. New Jersey: John Wiley & Sons, Inc., 2004, ch.8, sec.13, pp. 305 -307. [5] F. Neri and G. Reti, “SONET -SDH,” Politec.
3. Di Torino, Dep. Electr. Eng., 2007. [6] N. Jyothirmai, R. M. Valli, and A. R. Krishna, “SDH and Its Future Trends,” Int. J. Innov.
4. Technol. Explor. Eng., vol. 1, no. 6, pp. 74 –78, 201 2. [7] M.Rautio, “Synchronous digital hierarchy implementation to a radio transmission node”, M.S. thesis, Dep. Elect. Eng., Lappeenranta Univ. Tech., Espoo, Finland, 2006.