Answer 7—In North America and other countries using the T1 standard, each voice channel actually has 64 kbps of data (8 ksps × 8 bits per sample) devoted to it.

However, the multiplexing equipment for voice calls (channel banks) uses in-band signaling and “robs” (overwrites) bit 7 once every six samples in order to indicate ringing/off-hook status at each end (E & M signaling). At first glance, this would appear to delete only 1333 bps of the available bandwidth, leaving 62.67 kbps for use by a modem.

But there’s an additional twist: a signal that passes through multiple such links in tandem may lose bit 7 in additional samples because tandem links do not necessarily synchronize at the “multiframe” level, which would also synchronize the bit robbing. As a result, a modem can only assume that it has only seven usable bits per sample, or 56 kbps.

The actual peak rate of a modem is further limited (to approximately 53 kbps) by an unrelated problem. If a modem were to use all possible data patterns at 56 kbps, the peak power of the analog signal in the phone line would exceed certain limits, creating problems with linearity and crosstalk with other lines. Therefore, certain data patterns are ruled out, limiting the usable bandwidth.

Contributor: David Tweed

Published March 2004