Computer networks 2 A Informatica prof.sse ALLEMANI e CARUSO Yesterday…. Central architecture Before the coming of the PC, the common architecture was made up of a central computer - the only one with a processing capability – with some “stupid” terminals. Today… Several computers linked in a network, with indipendent processing capability, which share resources and provide services networks of networks What is a network? A computer network is a group of computers linked together by means of cables or aerials which communicate data and share resources through a series of protocols and rules. Why a network? It is possible: To share expensive peripherals, such as printers, scanner, plotter. To send data to users without any other support. Moreover there are fewer restrictions concerning the size of the file which can be transferred through a network. To centralize fundamental programs. Sometimes users must be allowed to access the same program, in order to work on it simultaneously. To create automatic backup systems of files. Network classification Typology ( distance among computers ) Topology (geometric shape used to create the network ) Network Typologies LAN (Local Area Network ) (1m – 2km) it is a network in a restricted area, such as an office room, a building or a campus MAN (Metropolitan Area Network ) ( 2km - 10Km ) it is a network made of groups of devices or LANs linked by a phone line or other types of cables in a large geographical area, such as a town. WAN (Wide Area Network ) it is a network made of the devices which let the connection of LANs and MANs over a large geographical area, or even around the world ( an example is the Internet ). Network Topologies Bus Ring Star BUS Topology Each node is connected one right after the other along the same backbone. Information sent from a node travels along the backbone until it reaches its destination node. All the computers can communicate with each other without having to go through the server. Advantages: A computer or device that has gone down does not affect the others. Disadvantages: It cannot connect a large number of computers. RING Topology It connects the computers and other devices one to the other in a circle. There is no central hub that holds all the data. Communication flows in one direction around the ring. Advantages: It requires less cabling and so it is less expensive. Disadvantages: If one node goes down, it takes down the whole network. STAR Topology The computers are connected to a central device called hub. The hub takes a signal that comes from any node and passes it along to all the other nodes in the network. Advantages: -It gives close control of data -Each PC sees all the data -If a computer (not the hub) fails, no other computer is affected Disadvantages: If the hub or its software goes down, the whole network is down. What is the Internet? The Internet is a network of networks, which links computers able to share information and resources with one another. The Internet can be defined as a “mean of transport” for the information and resources which are inside the computers. The Internet has been compared to a motorway. The Internet itself does not contain any information; it is wrong to say that a document has been found “in the Internet” or “on the Internet”. It would be better to say that it has been found “through the Internet” or “using the Internet”. Internet Services The computers connected to the Internet can take advantage of several services: E- MAIL – It allows you to send and receive e-mails and to access to a forum. FTP (File Transfer Protocol) – It allows you to have a quick access to a file in another computer, save on your computer and vice versa. WWW (World Wide Web) - You must not confuse the Internet with WWW; this is only a part of the Internet. What is the World Wide Web? The World Wide Web is a global network of computers. This means that all the computers which belong to this global network ( the web ) can share data and information. The computers must use a common language, in order to understand each other. The common language used by all the computers in the web is ruled by the standard protocol HTTP. How does the World Wide Web work? The information are stored in documents called web pages. The web pages are files placed into computers called web server. The computers which read the pages stored into web servers are called web client. The web clients show the user the web pages, by using a program called web browser. How does the browser show the web pages? The web pages stored into web servers are written in a standard language: the HTML. The pages written in html contain the instructions for the browser in order to be correctly shown.