Basic Computer

 Computer networks This part of the book is dedicated to computer networks from a user’s perspective. Nowadays a computer is very likely to belong to some company’s network, or to be connected to the Internet via an Internet provider, and is therefore exposed to all the typical network problems. Without entering into technical details, this section will explore the situations in which a novice user can find himself in troubles and how he can try to survive dialoguing with network administrators in their own strange technical language. 3.1. Technical aspects A computer network is a set of devices which communicate and share resources. These devices are mostly computers, and sometimes stand-alone hard disks, telephones, printers and terminals (processorless computers which must rely on other computers to work). 3.1.1. Server and client A computer network interaction is based on the client server architecture. When considering a single interaction, one computer is the server and the other one is the client. The server is the computer which is offering its resource, usually programmed to wait until someone asks for its resource. The client is the computer which uses the resource, which sends the request to a waiting server. For example, when sending a document to the printer, the user’s computer is the client while the printer is the server; when retrieving personal emails, the user’s computer is the client which connects to the mailserver asking for available emails. When talking to a friend on an Internet chat, the interaction is composed of two different interactions: the user’s computer as a client is connected to the chat room’s computer acting as a server, and the friend’s computer does the same interaction. The same computer may be the client for a service and the server for another service. For example, a library computer may have a CD inside its reader shared to the network (server for the CD) and may be at the same time used by a user to print his own documents (client for the printer). 3.1.2. Areas Computer networks are commonly divided into three categories:  Local Area Network (LAN or Intranet), usually the network of computers in the same building or belonging to the same owner. Inside the LAN every computer is well identified and usually every user is known. It is considered a trusted area.  Wide Area Network (WAN or Internet), which is everything which connects LANs. Computers’ and users’ identification is very hard and anonymity is possible. It is considered a dangerous area.  Virtual Private Network (VPN) is a way to recognize a computer outside the LAN as a trusted computer: the user is identified with a password and his computer, even though connected via Internet, will be considered as part of the LAN, for as long as it remains connected. VPN is typically required to identify portable computers connected via wireless connection. 3.1.3. Transfer speed The network connecting components are the cables, which determine the speed of the LAN. Cables have a speed measure in bps (bits per second) which indicates how many bits can flow through the cable in one