Encoding refers to the method by which electrical signals are generated and decoded. There are two types of encoding in use on T1 links today: Alternate Mark Inversion (AMI), and Binary Eight Zero Substitution (B8ZS). Generally, AMI is used for voice circuits, and B8ZS is used for data. B8ZS can be used for voice, but AMI should not be used for data, for reasons noted next.
AMI is a method of encoding that inverts alternate marks. In T1 signaling, there are two possible states: mark and space. Simply put, a mark is a one and a space is a zero. On a T1, a space is 0V, and a mark is either +5V or −5V. AMI encodes the signal such that the polarity of each mark is the opposite of the one preceding it.
This allows for some interesting error-detection techniques. For example, Figure 21-2 shows two ones occurring in a row with the same polarity. This is considered an error (a bipolar violation, or BPV). If all ones were positive voltage, a voltage spike could be misconstrued as a valid one. As an added benefit, when the alternating marks are flipped, the average voltage of the physical line will always be 0V, making the physical T1 wires safe to handle.
Figure 21-2. T1 AMI signaling
T1s are asynchronous links, meaning that only one side of the link provides clocking. The far side of the link must rely on the signal itself to determine where bits begin and end. Because the duration ...