[snip]

Though Unix itself didn't get into switches, Unix people did
have a significant influence on the OS architecture for
ESS 5. Bob Morris, having observed some of the tribulations of
that project, suggested that CS Research build a demonstration
switch. Lee McMahon, Ken Thompson, and Joe Condon spearheaded
the effort and enlisted Gerard  Holzmann's help in verification
(ironically, the only application of Gerhard's methods to
software made in his own department). They called the system,
which was very different from Unix, TPC--The Phone Company. It
actually controlled many of our phones for some years. The
cleanliness of McMahon's architecture, which ran on a PDP-11,
caught the attention of Indian Hill and spurred a major
reworking of the ESS design.