Linjë e nivelit të veçantë, linjë lidhëse e veçuar are some other choices.
Single tier only means "one level". The "line" refers to several units that work together to produce something, such as cars being manufactured along an assembly "line". So, in the context you've given , I am assuming the "line" of equipment needed to say, relay electrical impulses are indeed in a sequence that ends without branching off into several different levels. A goes to B which goes to C which is relayed to D. A could also go G which goes to L which goes to M. A could also branch out to P and then to R. BUT, in each case, the "lines" are TERMINATING at a SINGLE UNIT.
Good luck.