Transmission of Digital Information over Telephone Circuits

Transmission of digital information between central offices in the same or different cities is a basic process necessary to establishing a connection between subscribers. The existing methods of employing conventional dial pulses at the rate of about 10 pulses per second, or multifrequency pulsing at about 8 digits per second are adequate when used between existing central offices. When consideration is given to the application of electronic techniques to central office switching, it becomes apparent that higher signaling speeds are desirable if not necessary. The work reported here was carried out several years ago with the objective of selecting a method for transmitting and receiving a short coded message about 10 decimal digits in length over commercial telephone lines. The equipment should be simple and reliable, and capable of application to any commercial circuit without adjustment of the terminal equipment or the circuit itself. Transmission of digits should be at the highest speed consistent with reliable operation. In the following pages we consider the limitations imposed by the characteristics of telephone circuits and how they influence the choice of a signaling method. We then describe a reliable system which is compatible with these limita511