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There is a growing demand for modern communication systems capable of transmitting high‐speed data signals over Bell System facilities designed primarily to provide telephone service. Much of this data traffic can be handled within telephone channels in the switched network. Somewhat higher data, rates are feasible on private lines by selection of facilities and special treatment of the voice‐grade circuits. The need for still higher data bit rates requires bandwidths equivalent to many message channels, e.g., the 12‐channel group, the 60‐channel supergroup, and the 600‐channel mastergroup. Presentation of basic measured data to statistically characterize transmission in the broader bands in terms of frequency domain specifications is the object of this paper. Data reduction is covered in detail. Characteristics of built‐up connections are predicted from knowledge of the characteristics of subunits, including inherent variability. Such variability is a basic limitation on the degree of equalization that can be achieved with a small set of fixed networks.

The problem of pulse code modulation (PCM) bit errors causing voice frequency (VF) modem errors has been studied in detail. The error mechanism consists of the addition of an impulse response type error signal added to the reconstructed VF data signal waveform at the digital‐to‐analogue (D/A) output of the PCM decoder. This error signal may cause a burst of errors registering in the VF modem receiver, depending on which PCM bit is in error. Numerous data have been collected in a laboratory experiment and analysed in various ways. The average bit error rate (BER) enhancement factor of VF data over PCM is between 10 and 20. For each PCM bit in error, an average of two VF data bits are in error. The analysis leads to suggestions for possible solutions to the problem.