The AN/TTC-42 is an S-280 shelterized telephone central office that provides secure automatic switching service (loop-to-loop, loop-to-trunk and trunk-to-trunk) and subscriber service functions such as loop and trunk hunting, precedence and preemption. These services are available to the TRI-TAC family of 4-wire, digital subscribers and non-secure voice terminals (DSVTs and DNVTs) and to 4-wire digital trunks, including both single channel and Time Division Multiplexed (TDM) groups. The apportionment of these channels among loops and trunks (collectively called lines) is variable. The AN/TTC-42 will also perform automatic switching for 4-wire analog loops and trunks. Communication security (ComSec) used in the AN/TTC-42 includes the HGF-93, HGF-94, KG-82, KG-93 and KG-94.