Abstract: An interface is disclosed that formats a first link (a source link--e.g., a T1 link) having N channels to fit into a second link (a source link--e.g., a PCM-30 or a doubled T1 link) having N+M channels. Each of the first and second links are implemented with primary rate carrier links, and have a standard payload portion and a standard overhead portion. A mechanism is disclosed for routing information between the first link and the second link, so that information being transported over the first link can also be transported over the second link. An adding mechanism is also disclosed for routing supplemental overhead information to at least one added overhead channel within the second link, wherein the added overhead channel occupies a standard payload portion of the second link. An overhead information bit is disclosed, which transmits information within a (k)th bit of an (l)th channel of each frame of the second link. An information bit signal is formed with at least five consecutive of the (k)th bits.