Bipolar schemes are a type of line coding that use three voltage levels to represent binary data, namely positive, negative, and zero. There are two main bipolar encoding schemes: Alternate-Mark Inversion (AMI) and Bipolar Pseudoternary, which used exactly the opposite logic for the transitions. In this section, we will focus on the first one, the Bipolar Alternate-Mark Inversion (AMI) scheme, but both share the same basic coding principle: one of the possible bit values is represented by a null voltage, and the other one by a non-null voltage level that alternates between positive and negative voltage levels.
The Alternate-Mark Inversion (AMI) encodes binary data according to the following logic:
- Bit 0: voltage is zero (0V).
- Bit 1: voltage alternates between positive and negative (+V and -V).
Despite its simplicity, the AMI scheme has been used widely in long distance telecommunication systems (such as the T1 and E1 transmission systems) due to its ability to maintain clock synchronization (since, whenever a 1 is transmitted, the voltage alternates, helping the receiver to keep track of the timing). As a drawback to using this coding scheme, one must consider the case where long sequences of 0 bits are transmitted. In such cases, issues with synchronization may arise, just as it occurs in other schemes that aren't fully based on transitions (e.g., the Unipolar schemes).
Here are some examples of the Bipolar AMI scheme.
