PCM Transmission in the Exchange Plant

The choice of a modulation method or code for processing a signal preparatory to transmission over a communication channel depends on a multiplicity of conflicting parameters. All of the technical factors must be considered to arrive at a system that serves a need at a justifiable cost. Generally a few of the characteristics of the transmission medium stand out to limit performance. In the situation at hand -- pulse transmission at high rates in the exchange plant -- near-end crosstalk between cable pairs is the principal transmission deterrent. Examination of the factors that enter into the choice of a simple code and a repeater to reconstruct this code, to combat crosstalk interference, is the principal objective of this paper. Stated another way, we are seeking a transmission scheme that maximizes the number of pairs in a cable that can be used without pair selection. 99 100 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , J A N U A R Y 1962 1.1 Road Map Before we attack this objective it is profitable for later comparisons to provide some system background, to review the functions involved in a P C M reconstructive repeater, and to define terminology. Though the bulk of this introductory material has been covered in the literature, we introduce some new approaches and concepts. It is essential to a clear definition of the problem and our chosen solution. Most of this material is covered in Sections 1.2 through 2.4. Some preliminary comparisons are made in Section 2.5 that contribute to the choice of complete retiming and regeneration.