Transcript Network
Networks CS105 What is a computer network? • A computer network is a collection of computing devices that are connected in various ways so that they can communicate and share resources Purpose Computer networks are used for: 1. Facilitating communications: Email, instant messaging, chat rooms, telephone, video telephone calls, and video conferencing. 2. Sharing hardware: Hardware resources on the network – printing 3. Sharing files, data, and information: Access data and information stored on other computers on the network. 4. Sharing software: Users connected to a network may run application programs on remote computers. Terminology • • Any device/computer connected to a network is called a node or host Data transfer rate or bandwidth: The speed with which data travels from one node to another – • • Number of bits per unit time Protocols: Set of rules that defines how data is formatted and processed on a network Client-Server models: A distributed approach in which a client makes requests of a server and Response the server responds – File servers, Web Servers Server Client Request Types of networks The connections between computers in a network are either made using physical wires/cables or they can be wireless Networks can be classified into: • Local-Area network (LAN): A network connecting a small number of computers + devices in a close geographic area • Wide-Area network (WAN): A network connecting two or more LANS Types of networks LAN LAN Gateway Gateway Wide Area Network (WAN) Internet • An infrastructure for communication • Internet backbone: High capacity data routes that carry internet traffic – High performance computers whose only purpose is to route data – Provided by commercial & government agencies False! – No one owns the internet, no central administration – Anybody who connects to the net owns a slice of the net – Protocols to communicate Why can’t I connect to the backbone directly? Why do I have to pay for internet? In theory, you can. But you need a few million (perhaps billion) dollars to make a global network of cables and routers and get other ‘backbone’ agencies to network with you. How do we get internet in our computers? • Through an ISP (internet service provider): a company that provides other companies or individuals with access to the internet – ISPs connect directly to the internet backbone or to a larger ISP Internet Backbones ISPs Users Different colors represent different users How do we get internet in our computers? • Three most popular ways to connect to the net is: – Phone modem: A device converts computer data into an analog audio signal for transfer over a telephone line • Upper boundary on the amount of data transmitted per second (64K/sec) • Data is treated as a phone conversation • Phone cannot be used while you are on the net – Digital subscriber line: Uses regular phones to transmit digital data at a higher frequency • Phone line can be used for voice conversations and browsing the net at the same time – Cable modem: Data is transferred on the lines through cable TV signals are transmitted • Both DSL and cable modem are called Broadband because data speeds are greater than 128Kbits per second Data transmission Connectionless transmission • Data is sent over a network in packets: unit of data sent across a network Sent Message Message is divided into packets Packet 1 Packet 2 Packet 2 Packet 3 Packet 3 Packet 1 Packets are sent over the internet by the most expedient route Packet Switching Received Message Packets are reordered then assembled Packet Switching • Router: A network device that directs a packet between networks towards its final destination Host C Host D Host A Router 1 2 Router 3 Router 2 1 Router 4 1 9 6 5 7 Router 5 Host B 4 10 Host E 3 11 Router 7 Router 6 Packet Switching • Advantages: – More efficient – Data is sent immediately • Disadvantages: – Packets are rerouted so there might be delay – Packets may be lost or ‘dropped’ while en route