Page "Modified AMI code" Paragraph 2

T-carrier was originally developed for voice applications.

When voice signals are digitized for transmission via T-carrier, the data stream always includes ample 1 bits to maintain synchronization.

( To help this, the μ-law algorithm for digitizing voice signals encodes silence as a continuous stream of 1 bits.

) However, when used for the transmission of digital data, the conventional AMI line code may fail to have sufficient marks to permit recovery of the incoming clock, and synchronization is lost.

This happens when there are too many consecutive zeros in the user data being transported.