Mobile Subscriber Equipment
Mobile Subscriber Equipment (MSE) is the end user interface to an obsolescent U.S. Army communications system, TRI-TAC which interconnects substantial numbers of users by wired and wired links. Essentially, it is the end user equipment for the TRI-TAC architecture, begun in 1971 and intended for use at reasonably fixed units at corps and echelons above corps. A Marine officer called it "can best be described as the combination of a mobile cellular telephone and a commercial telephone."
Part of TRI-TAC Architecture
TRI-TAC's basic technology is of a digitally switched telephone network, which can support wireless as well as wired links. Its core is made up of interconnected AN/TTC-39 "intertoll" digital telephone switches, ruggedized for field use and designed to incorporate encryption devices for communications security. These toll switches, in turn, could connect to the "international long distance" of the Department of Defense core networks via the Army-managed Ground Mobile Forces (GMF) satellite radio systems.
As with commercial telephone switches of the time, it was expected that the TTC-39 switches would be interconnected with digitally multiplexed links, but tending to be wireless rather than the commercial T1 carrier.
MSE role in TRI-TAC
MSE development started in 1980, with the intention of providing higher headquarters communications to more forward units. It was an advance on pure telephony, as it included packet data service as well as voice. MSE did not include only the user equipment, but the equivalent of end office switches, to which residences and businesses are wired, in telephone networks, which treated the AN/TTC-39s as regional-level intertoll switches. These end office switches, in TRI-TAC terminology, were Unit Level Circuit Switch (ULCS), and were intended the division to the brigade/regimental levels. The ULCS were developed under the U.S. Marine Corps, while the MSE end user instruments were an Army responsibility. 
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- Program Manager, U.S. Army air and missile defense command and control system (AMDCCS), Forward Area Air Defense Command & Control (FAAD C2)