tailieunhanh - Mạng và viễn thông P5

Digital Transmission and Pulse Code Modulation Following trials in the196Os, digital telecommunications systems were first widely deployed in the 1970s. Since then, the miniaturization and large scale integration of electronic components and the rapid advancein computer technology have made digital technology the obvious for all choice of most countries and with world wider international satellite and submarine networks, digital transmission has no rivals. . | Networks and Telecommunications Design and Operation Second Edition. Martin P. Clark Copyright 1991 1997 John Wiley Sons Ltd ISBNs 0-471-97346-7 Hardback 0-470-84158-3 Electronic 5 Digital Transmission and Pulse Code Modulation Following trials in the 1960s digital telecommunications systems were first widely deployed in the 1970s. Since then the miniaturization and large scale integration of electronic components and the rapid advance in computer technology have made digital technology the obvious choice for all new telecommunications transmission and switching systems. Now in the networks of most countries and with wider world international satellite and submarine networks digital transmission has no rivals. So what exactly is it and what can we gain by it DIGITAL TRANSMISSION In investigating analogue transmission we found a relationship between bandwidth and overall information carrying capacity and we described frequency division multiplexing FDM . This was a method of reducing the number of physical wires needed to carry a multitude of individual channels between two points and it worked by sharing out the overall bandwidth of a single set of four-wires transmit and receive pairs between all the channels to be carried. We now discuss digital transmission in detail how it works and the equivalents of analogue bandwidth and channel multiplexing. In contrast with analogue networks digital networks are ideal for the direct carriage of data because as the name suggests a digital transmission medium carries information in the form of individual digits. Not just any type of digits but binary digits bits in particular. The medium used in digital transmission systems is usually designed so that it is only electrically stable in one of two states equivalent to on binary value T or off binary value 0 . Thus a simple form of digital line system might use an electrical current as the conveying medium and control the current to fluctuate between two values current on .