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communication system
subject			fact

communication system	contains layers in order from bottom to top facilities for transmitting and receiving signals is a layer that deals with splitting messages into packets and reconstructing messages that are received a layer that deals with handling ongoing connections with a remote host a layer that handles various protocols used by application programs such as http
	has clients a program that allows users to send a message or maintain a conversation with users on another computer
	has server a program that routes messages and can have features such as 'forwarding' that people are familiar with from the telephone network
	is a subtopic of 3.4 - The Client-Server Architecture
	is a subtopic of 9.5 - Architectural Patterns
	is a kind of client-server system
	is a kind of multi-layer system
multi-layer system	has part layer
	uses multi-layer architectural pattern
client-server system	generally uses multiple threads of control that can be concurrently executed to implement concurrent operations, otherwise when the client is waiting for one kind of input, it will not be able to respond to the other kind of input
	has interaction The server starts running The server listens for clients wanting to connect Clients start running and perform various operations, some of which require connecting to the server to request a service When a client attempts to connect, the server accepts the connection if it is willing The server waits for messages to arrive from connected clients When a message from a client arrives, the server takes some action in response, such as sending a message back to the client, then resumes waiting Clients and servers continue functioning in this manner until one of them decides to shut down
	has problem developers of clients are frequently forced to upgrade their clients whenever the server is changed if clients and servers are developed by different organizations
	involves at least one server and one client
	is inherently concurrent because the server runs concurrently with the clients, normally (but not necessarily) on different computers
	is prone to security violations, due to the fact that information is transmitted over a network
	is vulnerable to interruptions in communication or denial-of-service attacks
	should be forward-compatible and backward-compatible with other versions of clients and servers by designing the client-server protocols to be very general and flexible
	should incorporate encryption, firewalls and similar methods of ensuring security
software system	becomes complex because it is easy to add new features and because software engineers typically add features without fully understanding a system, which may not have been originally designed to accommodate the features
	can automate business process
	can be divided in many ways: A distributed system is divided up into clients and servers A system is divided up into subsystems A class is divided up into methods
	is typically initially developed as a prototype
	must have well-described requirements if other systems or subsystems are going to use its services or communicate with it
	should be designed for flexibility right from the start
	undergoes evolution over its life-span
distributed system	is divided up into clients and servers
system	can have specification which is then implemented by a collection of components
	exists even if its components change over the course of time, or are replaced by equivalent components
	has scope

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