Alternate Mark Inversion (AMI) is a line coding technique used in telecommunications to transmit digital signals over communication channels. It's commonly used in technologies such as T1 and E1 digital transmission systems.
In AMI encoding, each binary 1 is represented by alternating positive and negative voltage levels (marks), while binary 0 is represented by no voltage change (a space). The alternating voltage levels help in maintaining the DC balance in the transmitted signal, which is important for systems that are sensitive to DC components.
Here's how AMI encoding works:
1. Polarity Inversion: Each binary 1 is represented by alternating positive and negative voltage levels. The first binary 1 is represented by a positive voltage level, the next binary 1 by a negative voltage level, and so on. Binary 0s do not cause a change in voltage.
2. Zero Stuffing: To prevent long sequences of zeros without transitions, a technique called zero stuffing is used in AMI encoding. After a predefined number of consecutive zeros, an additional pulse (violation mark) is inserted to ensure that the polarity remains alternating.
3. DC Balance: AMI encoding ensures that the average DC voltage level over time remains close to zero. This is achieved by balancing the number of positive and negative voltage levels used to represent binary 1s.
AMI encoding offers several advantages:
- Efficient use of bandwidth: It requires only two voltage levels, which allows for efficient use of the available bandwidth. - DC balance: It maintains a balance of positive and negative voltage levels, making it suitable for transmission systems that are sensitive to DC components. - Long-distance transmission: It's suitable for long-distance transmission because it reduces signal attenuation and distortion.
However, AMI encoding has limitations, such as the need for clock recovery at the receiver end and the potential for errors if the receiver misinterprets the signal due to synchronization issues or signal distortion. Despite these limitations, AMI remains widely used in telecommunications for digital signal transmission.