Computer Networksand the Internet

CMPT 109

Montclair State University

Computer Data Communication• Computer Data communications involves

transmitting information on the form of coded bit patterns over appropriate channels.

– Download: The action of retrieving data into your computer.

– Upload: The action of sending data into another computer (you need privileges to access the other computer).

Data Communication

• Data Communication can be established using analog or digital signal.

• Analog signal is a continuous wave pattern. The telephone system uses analog signals

• Digital signal consists of a series of 1s and 0s. HDTV uses digital signals.– Signal can be of any kind: Electric current,

electromagnetic waves, light pulses, etc.

Telephone Network: Circuit Switching

•When you make a telephone call…•The telephone switching system creates an electrical circuit between two telephones

•The switching system maintains this circuit until you hang up.

Circuit Switching (continued)

• Appropriate for telephone networks– Analog data is degraded at each station– Telephone conversations require relatively

continuous flow of data across circuit

• Not appropriate for computer networks– Digital data does not degrade at links, so more

intermediate nodes are possible.– Computers generate data in bursts, with long

intervening periods of silence, wasting “bandwidth”

Computer Networks

• It normally consists of tow or more independent computer connected together with communication links and equipment, to share resources

• There are two types of networks: WAN (Wide Area Network) and LAN (Local Area Network).

– WANs may span great geographic distances.

– LANs connect computers located not far away.

Computer Networks– WANs share software and information

– LANs share devices, software and information

• The communication links are full-duplex, i.e both computers can communicate simultaneously. A half-duplex communication only allows one computer to transmit at any given time.

Computer Networks: Packet Switching

• Connect computers in a network, often with redundant links. Each computer is a node in the network.

• Source computer splits message into one or more packets, each with part of message.

• Each packet also contains address of destination computer.

• Source computer sends packets to an adjacent node in the network.

Packet Switching (continued)

• When a networked computer receives a packet, it examines the packet’s address.

• If the computer is not the packet’s final destination, it forwards the packet to an adjacent node that’s closer to the packet’s destination.

• The destination computer collects the packets and re-assembles them into the original message.

Packet Switching Illustration

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(to 8)

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Some Remarks about Packet Switching

• Not all the packets in a message need to take the same route between source and destination.

• The packets in the message do not need to arrive at the destination in proper order.

• Messages transmitted via a packet-switched network are inherently vulnerable to unauthorized interception.

Network Protocols

• A protocol is a set of rules for exchange of data among two or more parties (e.g., among computers in a network).

• A packet-switched network protocol needs to specify the following (at a minimum):– The structure of a packet– Format for source and destination address– Routing of packets through the network– Splitting and reassembling of messages

Types of Protocols

A general communication protocol often contains specific protocols for different types of messages.

For example, there may be protocols for

• Remote logins

• Transfer (actually copying) of data files

• E-mail messages

WANs

• They need hardware to connect the computers. It could be a modem and the POTS, or a modem and a ADSL connection.

• They also need software to synchronize the transmission of the data between the two computers.

LANs

• They need hardware to connect the computers: A NIC (Network Interface Card), and some type of cabling (twisted pair, coaxial, fiber optic).

• They also need the software, that must correspond to the network topology that is being used.

Network Topologies• A topology is the physical arrangement of

the computers in a network– In a star topology, each computer is linked to a

central computer.– In a ring topology, the computers are arranged

in a circle.– In a bus topology, computers (or sub-networks)

are linked to a backbone.– In a mesh topology, computers a linked in a

(rectangular) grid.

A Star Topology

Server

Client

Client

Client

Client Client

Client

A Ring Topology

Node

Node

Node

Node

NodeNode

Node

NodeMessages are passed in this direction

A Bus Topology

Node

Node

Node

Node

Node

Node

Node

Server

Node

Node

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BackboneServer

Node Node

Node

Mesh Topology

Node

Node Node

Node

Node

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NodeNode

Node

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Node

Node

MSU’s Local Area Network

• Essentially a bus topology, with several sub-networks. Machines communicate using the Ethernet protocol.

• The PCs in RI 105 and RI 108 -- together with the server in RI 109 -- form one sub-network. The PCs are clients.

• Most of the other PCs on campus are parts of different sub-networks

The Internet

• The Internet is really a network of networks, with no specified topology.

• The LAN at MSU is one of the networks in the Internet.

• The protocol for communication over the Internet is TCP/IP (Transmission Control Protocol/Internet Protocol).

Internet Protocols

• telnet -- allows remote logins

• ftp -- file transfer protocol

• mailto -- protocol for sending e-mail

• http -- hypertext transfer protocol. This protocol makes the World Wide Web possible.

The World Wide Web

• The part of the Internet that uses hypertext.

• Web pages written in hypertext markup language (HTML) may contain embedded hypertext links to other pages. This other page may be anywhere on the Internet.

• A hypertext link to a page makes use of that page’s Uniform Resource Locator (URL).

WWW (continued)

• A web browser is a program that allows one to view documents written in HTML and to traverse the embedded hypertext links.

• The two major web browsers are Netscape Navigator and Microsoft’s Internet Explorer. Other browsers exist (e.g. Lynx).

• Hypertext links allow one to access information without having to know its physical location.– Instead, the URL for the physical location is embedded

in the HTML document.