CSCI-235Micro-Computers in Science

The Network

• 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

• A Communications ModelSource

• 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

• Simplified Communications Model - Diagram

• Modems: From Digital to Analog and BackModems are devices that transform signals when sending and receiving transmissions

• Modulation – Transforming digital signals to analog

• Demodulation – Transforming analog signals to digital

Modulation

DigitalAnalog

Analog Digital

Demodulation

• BandwidthBandwidth 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

• NetworkingPoint 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

• History of InternetThe 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

By 1980, close to 100 sites were connected to the ARPANet

• Satellite connections provided links to select cities outside the continental U.S.

• Internet Growth

• Distributed Networks

• Packet SwitchingMessages to be sent over the network are first broken into small pieces known as packets

• These packets are sent independently to their final destination

• Advantages of PacketsSending 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)

• Protocols and AddressesThe 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/

• TCP/IPTransmission 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

• 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

• A typical URL