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multi-layer system comparison table

Subject	be	have interaction	involve	have clients	have problem	incorporate	divide up into	use	contain	have server
communication system	forward-compatible and backward-compatible with other versions of clients and servers by designing the client-server protocols to be very general and flexible	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	at least one server and one client	a program that allows users to send a message or maintain a conversation with users on another computer	developers of clients are frequently forced to upgrade their clients whenever the server is changed if clients and servers are developed by different organizations	encryption, firewalls and similar methods of ensuring security	clients and servers	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	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	a program that routes messages and can have features such as 'forwarding' that people are familiar with from the telephone network
operating system	better at error handling if it effectively prevents the user from making errors, detects errors, and helps the user to correct errors						subsystems	multi-layer architectural pattern	a kernel layer and higher level layers dealing with such functions as user account management, screen display, etc.

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