Alternate Mark Inversion (AMI)

Alternate Mark Inversion (AMI) is a type of line coding technique used in digital communication systems to transmit binary data over physical media such as copper wires or optical links. It improves synchronization and reduces certain transmission problems compared to simple encoding schemes.

What Is Line Coding?

Line coding is the process of converting binary data (0s and 1s) into electrical signals that can be transmitted over a communication channel. Different encoding techniques improve signal quality, timing recovery, and error detection.

How Alternate Mark Inversion Works

In AMI encoding:

• A binary 0 is represented by no voltage (0 volts).
• A binary 1 is represented by a non-zero voltage, but the polarity alternates between positive and negative.

This means:

• The first 1 → +V
• The next 1 → –V
• The next 1 → +V
• And so on...

The term “mark” refers to binary 1, and “inversion” refers to alternating the voltage polarity.

Example

Binary data:

1 0 1 1 0 0 1 0

AMI encoded signal:

+V 0 -V +V 0 0 -V 0

Notice how each successive 1 changes polarity.

Advantages of AMI

1. No DC Component: Since positive and negative pulses alternate, the average voltage over time is close to zero. This reduces DC buildup, making it suitable for transformer-coupled systems.
2. Better Synchronization: The presence of alternating pulses helps receivers maintain timing.
3. Simple Error Detection: If two consecutive pulses have the same polarity, the receiver can detect a violation (bipolar violation).
4. Efficient Bandwidth Usage: Compared to other coding schemes, AMI uses bandwidth more efficiently.

Disadvantages of AMI

1. Long Strings of Zeros: Since 0 is represented by no signal, long sequences of zeros can cause synchronization problems.
2. Requires Three Voltage Levels: Needs positive, zero, and negative voltage levels, making implementation slightly more complex than simple NRZ encoding.

AMI Variations

• HDB3 (High-Density Bipolar 3 Zeros)
• B8ZS (Bipolar with 8-Zero Substitution)

Applications of AMI

• T-carrier systems (like T1 lines)
• ISDN systems
• Digital telephony networks

Conclusion

Alternate Mark Inversion (AMI) is an effective bipolar line coding technique that alternates the polarity of binary 1s while keeping 0s at zero voltage. Its ability to reduce DC components and provide simple error detection made it a popular choice in early digital telecommunication systems. Although modern systems often use more advanced encoding methods, AMI remains an important foundational concept in digital communications.