DMS switch -- is Nortel's acronym for "Digital Multiplex System" -- Nortel's digital telephone switches were given identifiers with a DMS prefix such as DMS 100 or DMS 500.
Nortel's DMS switches were used in both telephone company central office applications and also as private branch exchange (PBX) switches. Nortel's smaller PBX offerings also had names, such as Meridian, although the Meridian was, as I understand it, based on scaled down DMS technology.
Before Nortel went spectacularly bankrupt, it had a solid reputation in the PBX market, as well as for telephone central office and cellular radio switching (with the DMS platform). My clients bought hundreds of millions of dollars of Nortel's DMS 100 switches and radio equipment over the years and were pleased with the performance.
As I understand it, Nortel's interface between the digital PBX and the telephone instruments is TDM, but it's proprietary, not the standard telephony 64 kb/s version, or even the 2B+D ISDN flavor of TDM, but I could well be mistaken here as all my dealings with Nortel were in the cellular radio switching and base station fields and I never had reason to dig into the PBX technology.