In North American T1 carrier systems, which line coding method is used for the transmitted signal on copper pairs?

Introduction:
T1 (DS1) systems historically carry 24 voice channels over copper using a standardized framing and line coding. The question focuses on the specific line code used for T1 on the physical circuit.

Given Data / Assumptions:

• We refer to classic T1 over copper (1.544 Mb/s).
• Early T1 employed Alternate Mark Inversion (AMI); later enhancements introduced B8ZS for long runs of zeros.
• We distinguish line coding from higher-layer multiplexing or framing.

Concept / Approach:
AMI is a bipolar code in which logical “1” pulses alternate in polarity (+, −, +, − …) and logical “0” is represented by no pulse. B8ZS (Bipolar with 8-Zero Substitution) deliberately injects bipolar violations to maintain timing when long zero sequences occur.

Step-by-Step Solution:

Verification / Alternative check:
Telephony references consistently list AMI and B8ZS for T1, while E1 often uses HDB3—another bipolar code—illustrating regional standards.

Why Other Options Are Wrong:

• NRZ: used in some links but not the T1 copper line code.
• Manchester: common in older Ethernet (10BASE-T coax variants), not T1.
• Binary (unipolar): not applicable to T1, which uses bipolar signaling.
• None of the above: incorrect because bipolar/AMI is correct.

Common Pitfalls:
Confusing framing with line coding; assuming Manchester applies broadly beyond Ethernet; overlooking B8ZS as an enhancement rather than a different family.

Final Answer:
Bipolar (AMI, later with B8ZS).