The system hardware consists of adapter cards or other devices that provide a path or interface between the system software and the physical network. Other devices, such as modems, may attach to one of the standard ports on the computer. If the adapter does not contain software support, then this support must be provided by the adapter's device driver. Protocols All communications software uses protocols , sets of semantic and syntactic rules that determine the behavior of functional units in achieving communication. Protocols define how information is delivered, how it is enclosed to reach its destination safely, and what path it should follow.
Protocols also coordinate the flow of messages and their acknowledgments.
Protocols exist at different levels within the kernel and cannot be manipulated directly. The choices a user makes when invoking file transfer, remote login, or terminal emulation programs define the protocols used in the execution of those programs. A third feature common to communications networks is addresses . Addresses are associated with both software and hardware. The address is the means by which the sending or control station selects the station to which it sends data.