Digital Communications 2

4Study of the Transmission of Digital Information on Two-wire Cables

4.1. Introduction

The electrical transmission of information in digital form is not a recent phenomenon and it has even preceded the transmission of information in analog form, however it is only recently that digital transmissions have grown considerably. They used and still use, as a transmission medium, the important and classical infrastructure provided by the telephone and telegraph network of Telecommunications, infrastructure that has the advantage of existing.

However, the switched telephone network (the telex network being limited to very low modulation speeds) designed for the analog transmission of speech, only allocates a 300 to 3,400 Hz frequency band. This 3,100 Hz bandwidth allows, at the current time and under normal conditions, the transmission of digital information from 9,600 bit/s up to roughly 38 or even 56 Kbit/s. These limitations were due to amplitude and phase distortions and to the noise present in the transmission channel.

Developments in teleinformatics and the Internet (transmission of data, documents, still images, digital video-phone or television programs), the difficulties of performing correct equalization encountered in analog transmissions, while the needs of the telephone and its applications are also increasing, have led to the study and development of a high-speed universal network of digital transmissions of analog and digital information.

The study and development ...