what is a DCS????

There are good articles on this subject in Wikipedia.

Hi Shameem - DCS = "Digital Control System" (broad category of devices in a system); PLC = "Programmable Logic Controller", one element of a DCS. PLC's are made by manufacturers like Allen-Bradley, Siemens, Omron, Mitsubishi, Schneider, and GE Fanuc, and are dedicated controllers for critical process applications like putting bottle caps onto glass bottles at the beer factory or controlling the customer entrance/exit doors at your local supermarket. Wiki's a good source for more details; I would also check out/have faith in unbiased definitions/details from the non-profit trade organization "Instrumentation Society of America" (ISA), at http://www.isa.org. - april05

Digital Control System

Actually, DCS is acronym for "distributed control system" such as those furnished by ABB and others.

If DCS and DDCMIS implemented same plant what protocol required for both network implementation

DCS means Distributed Control system. As the name indicates it implies integration of several control systems. For eg. In a power plant, there will be many systems like Boiler system, steam flow system, Ash handling system, Water treatment system etc., Each will have seperate PLC's to automate and monitor those systems. These PLC's will be connected in a common network and all the systems are monitored and controlled from a common area through a high end PC. This whole system is DCS.