Tecnologia sdh

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Vol. 1


Pocket Guide revised version

Wavetek Wandel Goltermann is ITU-T sector member.

Pocket Guide to Synchronous Communications Systems Publisher : Acterna Eningen GmbH Postfach 12 62 72795 Eningen u. A. Germany E-mail: stephan.schultz@acterna.com http://www.acterna.com Stephan Schultz

Author :

The sun is made of copper . . .

Anyone making a statement like that thesedays would likely be considered as quite mad, yet with these words, spoken back in 1861, Johann Philipp Reis began something that has completely changed the world. This meaningless message, just spoken by Reis into his new invention, was clearly heard by the receiver. The telephone was born. Despite this, the first usable telephone (A.G. Bell, 1876: Patent for electrical and magnetic transmissionof sounds) was thought of as little more than a toy. Today, it would be difficult for us to imagine life without the telephone. World-wide, there are some 750 million telephone connections in use and the number of Internet users has exploded in the last few years. By the year 2000, according to a forecast from Nortel, there will be almost 475 million Internet users and the number of servicesprovided will also grow rapidly. Right from the start, network providers have been faced with coping with a steadily increasing number of users and thus of telephone traffic. This led to the development of various methods and technologies, designed on the one hand to meet the market needs and on the other hand to be as economical as possible. In the field of communications engineering, this resulted inthe introduction of frequency division multiplex (FDM) systems which enabled several telephone connections to be transmitted over a single cable. The idea was to modulate each telephone channel with a different carrier frequency to shift the signals into different frequency ranges. 1

With the advent of semiconductor circuits and the ever-increasing demand for telephone capacity, a new type oftransmission method called pulse code modulation (PCM) made an appearance in the 1960s. PCM allows multiple use of a single line by means of digital time-domain multiplexing. The analog telephone signal is sampled at a bandwidth of 3.1 kHz, quantized and encoded and then transmitted at a bit rate of 64 kbit/s. A transmission rate of 2048 kbit/s results when 30 such coded channels are collectedtogether into a frame along with the necessary signaling information. This so-called primary rate is used throughout the world. Only the USA, Canada and Japan use a primary rate of 1544 kbit/s, formed by combining 24 channels instead of 30. The growing demand for more bandwidth meant that more stages of multiplexing were needed throughout the world. A practically synchronous (or, to give it itsproper name: plesiochronous) digital hierarchy is the result. Slight differences in timing signals mean that justification or stuffing is necessary when forming the multiplexed signals. Inserting or dropping an individual 64 kbit/s channel to or from a higher digital hierarchy requires a considerable amount of complex multiplexer equipment. Towards the end of the 1980s, the so-called synchronousdigital hierarchy (SDH) was introduced. This paved the way for a unified network structure on a worldwide scale, resulting in a means of efficient and economical network management for network providers. The networks can easily be adapted to meet the ever-growing demand for ªbandwidth-hungryº applications and services. 2

The aim of this booklet is to provide an introduction to synchronouscommunications without going into details about ªbits and bytesº. The following section looks at the current trends and changes in the transmission marketplace.

Japanese standard 5. 64 4. 63 3. 65 2. order 64 primary rate

North American standard

European standard

64 66 63 64

67 63



624 Figure 1: Summary of plesiochronous transmission rates



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