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: , 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 the server responds – File servers, Web Servers ServerClient Request Response

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 Gateway LAN 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 – 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. False!

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 BackbonesISPs 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 Packet 1 Packet 2 Packet 3 Packet 2 Packet 3 Packet 1 Received Message Message is divided into packets Packets are sent over the internet by the most expedient route Packets are reordered then assembled Packet Switching

Router: A network device that directs a packet between networks towards its final destination Host A Host B Host E Host D Host C Router 1 Router 2 Router 3 Router 4 Router 5 Router 6 Router 7

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