Tài liệu Dịch vụ mạng thế hệ kế tiếp P2 doc

2

The Transmission

Infrastructure

2.1 INTRODUCTION

We pass by transmission on the way to the Intelligent Network (IN)

because it is important to understand how all the switching and signalling

nodes (not to mention voice links) are connected together. Like the last

chapter, we will pass briefly through analogue transmission first as a

means of exposing the desires for digital transmission. Then move on to

digitisation of speech and look at how large volumes of calls are econom￾ically carried across the world.

Analogue voice signals can suffer from a number of interference

problems ranging from degradation due to distance through to external

signalling inducing noise into transmission. One of the big culprits for

‘noise’ on the line is cross talk. Cross talk occurs when a number of

transmission systems are carried through the cabling trunk in close proxi￾mity. External noise in the form of interference from electricity mains

cabling and other electronic equipment conspire to reduce the speech to

an unintelligible hiss. It is this reason as well as economies of scale in

amalgamating voice connections together into large transmission systems

that has brought about the desire to digitise the analogue signals

produced by the human voice. Digital signals are less prone to interfer￾ence as interference only indirectly affects the signal. The original analo￾gue signal must be decoded from the binary representation, as long as the

accuracy of the binary representation of the signal is maintained the

original signal can be regenerated immune from noise.

Next Generation Network Services

Neill Wilkinson

Copyright q 2002 John Wiley & Sons, Ltd

ISBNs: 0-471-48667-1 (Hardback); 0-470-84603-8 (Electronic)