Defining Some Terms

Before we move on, let’s define some terms:
Protocols. Protocols are the rules used by different layers in a protocol stack to
talk to and negotiate with one another.
Protocol stack. The protocol stack is the list of protocols used in the system.
The higher up you are in the protocol stack, the more likely you are to be using
software—partly because of the need for flexibility, partly because speed of
execution is less critical. Things tend to need to move faster as you move down
the protocol stack.

Peers. The machines in each layer are described as peers.
Entities. Peers are entities, self-contained objects that can talk to each other.
Entities are active elements that can be hardware or software.
Network architecture. A set of layers and protocols make up a network
architecture.
Real-time operating system. We have already, rather loosely, used the term realtime
operating system (RTOS). What do we mean by real time? The IEEE definition
of a real-time operating system is a system that responds to external
asynchronous events in a predictable amount of time. Real time, therefore, does
not mean instantaneous real time but predictable real time.