Alternate Mark Inversion

Alternate Mark Inversion (AMI) is a line encoding technique (line code) for T1s. This three-level system uses positive, negative, and grounded pulses (e.g. -5V, 0V, 5V) to represent logical values. A logical 0 is represented with a grounded or absent pulse, and a logical 1 by pulses of alternating polarity. The alternating coding prevents the build-up of a DC voltage in the cable. This is considered an advantage since the cable may be used to carry an additional small DC current to power intermediate equipment such as line repeaters. This DC component may be removed without interfering with this encoded information.

AMI coding was used extensively in first-generation PCM networks, but suffers the drawback that a long run of 0's produces no transitions in the data stream (and therefore does not contain sufficient transitions to guarantee lock of a DPLL). Successful transmission therefore relies on the user not wishing to send long runs of 0's and this type of encoding is not therefore transparent to the sequence of bits being sent. For voice traffic the least significant bit of each frame (every eighth bit) is always set to '1', thereby generating enough transitions on the line. This is acceptable for voice, but not for data.

Other T1 encoding schemes ensure regular transitions regardless of the data being carried. B8ZS is a newer format for North America, where HDB3 is the original line coding type used in Europe and Japan.

Example of AMI encoding: The pattern of bits " 1 0 0 0 0 1 1 0 " encodes to " + 0 0 0 0 - + 0 ".