to give each TLF access to more junctors. The first expanded version allowed each office to have 20x40, and in the 1960s the maximum reached 30x60. Development stopped at that point because the four-stage layout was becoming progressively less efficient at greater sizes, and because the 1ESS switch with eight stages was under development.

A channel from a line to a trunk consisted of three links of switching fabric: line link, junctor, and trunk link. In a 10x20 or larger office, ten channels, numbered