T1: A Survival Guide

Alternate Mark Inversion

In common encoding schemes, ones are represented by voltage pulses and zeros are represented by the lack of a voltage pulse. Each pulse is approximately 3 volts in amplitude and has a 50% duty cycle, meaning it takes up half of the time slot for pulse transmission. Pulses have a tendency to spread out in the time domain as they travel down a line, as illustrated in Figure 3-1. Restricting the initial transmission to occupy half the time slot helps the repeaters, and the receiving end, find the middle of each time slot and stay synchronized.

Figure 3-1. Time domain spreading

Commonly, a scheme called bipolar return to zero or Alternate Mark Inversion (AMI) is used. One and zero are sometimes referred to as mark and space, respectively, in communications jargon. AMI gets its name from the fact that only ones, or marks, result in pulses on the line. Successive pulses are encoded as positive and negative voltages, as shown in Figure 3-2.

Figure 3-2. AMI encoding

Alternating pulse polarity enables a quick-and-dirty form of error detection. AMI specifies that polarity must alternate, so two successive pulses of the same polarity, such as the sequence in Figure 3-3, might be an error. (Bipolar schemes can catch many errors, but not all of them. ...