1. CSCI-235 Micro-Computers in Science The Network

2. Network Fundamentals A computer network consists of two or more computers linked together to exchange data and share resources Communication is the process of sending and receiving messages Communication channels are the paths through which messages are passed Communication devices transform electronic signals

3. A Communications Model Source Generates data to be transmitted Transmitter Converts data into transmittable signals Transmission System Carries data Receiver Converts received signal into data Destination Takes incoming data

4. Simplified Communications Model - Diagram

5. Modems: From Digital to Analog and Back Modems are devices that transform signals when sending and receiving transmissions Modulation – Transforming digital signals to analog Demodulation – Transforming analog signals to digital Modulation Digital Analog Digital Demodulation

6. Bandwidth Bandwidth is usually used to refer to the data rate (i.e., the amount of data that can be transmitted through a communications channel) Digital bandwidth is measured in bits per second (bps), kilobits per second (Kbps), megabits per second (Mbps), or gigabits per second (Gbps) Low bandwidth is 56 Kbps and high bandwidth is 622 Mbps

7. Networking Point to point communication not usually practical Devices are too far apart Large set of devices would need impractical number of connections Solution is a communications network The Internet is a global, interconnected computer network in which every computer connected to it can exchange data with any other connected computer

8. History of Internet The ARPANet (precursor to the Internet) became a reality in 1969 Advanced Research Project Agency (ARPA) Intended to connect only military installations and universities participating in government projects It employed dedicated cables, buried underground The data transfer was 56K bits/sec, roughly the same as dial- up services today

9. By 1980, close to 100 sites were connected to the ARPANet Satellite connections provided links to select cities outside the continental U.S.

10. Internet Growth

11. Distributed Networks

12. Packet Switching Messages to be sent over the network are first broken into small pieces known as packets These packets are sent independently to their final destination

13. Advantages of Packets Sending information in smaller units increases the efficient use of connections Large messages can't monopolize the connection Analogy: limiting call lengths at a pay phone to limit waiting Transmitting packets independently allows the network to react to failures or network congestion Routers (special-purpose computers that direct the flow of messages) can recognize failures or congestion and reroute the packet around trouble areas Breaking the message into packets can improve reliability Since the packets are transmitted independently, it is likely that at least part of the message will arrive (even if some failures occur within the network)

14. Protocols and Addresses The Internet allows different types of computers from around the world to communicate This is possible because the computing community agreed upon common protocols (sets of rules that describe how communication takes place) The two central protocols that control Internet communication are Transmission Protocol (TCP) Internet Protocol (IP) These protocols rely on each computer having a unique identifier (known as an IP address) An IP address is a number, written as a dotted sequence such as 147.134.2.84 Each computer is assigned an IP address by its Internet Service Provider (ISP) http://www.whatismyip.com/

15. TCP/IP Transmission Control Protocol (TCP) Controls method by which messages are broken down into packets and then reassembled when they reach destination Internet Protocol (IP) Concerned with labeling the packets for delivery and controlling the packets’ paths from sender to recipient

16. The Internet and Web: What’s the Difference? The Internet is the physical connection of millions of networks. The Web uses the Internet for its existence The Web consists of hypertext embedded on Web pages that are hosted on Web sites Web browsers display a Web document and enable users to link to other Web pages Web servers respond to the requests of browsers. They find and send requested resources back to the browser

17. A typical URL