Plug-ins
Plug-ins connect to the system using the EPOC Component Object Model (ECOM). ECOM is based on client/server architecture and provides services to instantiate, resolve, and destroy instances of plug-ins at run time.
It is important to verify all plug-ins before loading them into the application, because a hostile or malfunctioning plug-in may cause crashes or security leaks, even on otherwise well-tested applications. Platform security takes care of the plug-in verification by using the capability model; processes cannot load plug-ins that are less trusted than the process itself, that is, have less capabilities. See ECom architecture in the Symbian OS Library for more information about the ECOM plug-in architecture. Examples are available at Forum Nokia's Symbian C++ code and examples page.
Communication plug-ins
It is possible to use plug-ins to implement additional functionality to Serial, Socket, and Messaging frameworks. These kinds of plug-ins are especially important for security because they act in the vicinity of communication facilities.
For serial communication there are communications modules (CSY), which are DLL plug-ins to the Comms Server and need to be loaded before data can be transferred.
For socket-based communication there are protocol modules (PRT) and an affiliated capability file (.esk). Generally, PRTs work in the same way as CSYs in serial communications.
For messaging there are Message Type Module (MTM) plug-ins. The main difference when compared to serial and socket plug-ins is that MTMs are a suite of DLLs rather than a single DLL. Also, MTMs include both client and server components.