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An Equalizer Design Technique for the PCM Modem: A New Modem for the Digital Public Switched Network

Summary: An Equalizer Design Technique for the PCM Modem:
A New Modem for the Digital Public Switched Network
Ender Ayanoglu 1 , Nuri R. Dagdeviren 2 , Glenn D. Golden 3 , J. E. Mazo 4
Bell Laboratories, Lucent Technologies
Modems designed for the Public Switched Telephone Network (PSTN) have conventionally been
based on modeling assumptions which view the PSTN connection as an essentially analog medium.
However, as the PSTN evolves toward all digital transport and switching, and particularly as major
traffic sources, such as Internet service providers, increasingly have direct digital connections
to the PSTN, it is appropriate to revisit the model assumptions. Recently, several modem and
chipset manufacturers have announced ``56K'' modems based on an emerging system paradigm in
which one user (a residential Internet subscriber) has an analog connection to the PSTN, and the
other (an Internet service provider) has a digital one. ITU­T is expected to finalize details of a
corresponding Recommendation in early 1998. With this configuration, modem designs based on
signaling with the ¯­law alphabet become feasible, and the conventional Shannon limit disappears
as the quantization noise is avoided. Thus, the conventional Shannon limit of about 36 kbps can be
beaten, and it is possible to approach the digital transmission rate of 64 kbps. Modems employing
this general approach have become known as ¯­law or PCM modems. In this paper, we present
a signaling technique and the sampling theory based on this technique, display the structure and
operating principles of a PCM modem equalizer, and show how this equalizer problem can be cast


Source: Ayanoglu, Ender - Department of Electrical and Computer Engineering, University of California, Irvine


Collections: Engineering