an 81c is a cs1000 if it is IP enabled. which doesnt make it a CS1000E, just CS1000. the raw definition is a 21-81 meridian option is a TDM switch.
a CS1000E is a new design rack mount MGC cabinate IP PBX that can still do TDM, but is geared for IP. The design difference in some parts are drastic.
Mato' Was'aka