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

PurposeComputer 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 the server responds– File servers, Web Servers

Server Client

Request

Response

Types of networksThe 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

Gateway

LANLAN

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

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

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

Packet Switching

• Router: A network device that directs a packet between networks towards its final destination

Host A

Host BHost E

Host D

Host C

4 3

11

107

6

12

19

5

Router 1

Router 2Router 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