Download Computer Networks and the Internet

Computer Networks
and 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
H
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2
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P
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1
“HELP”
(to 8)
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3
L
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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
Client
Client
Client
Server
Client
Client
Client
A Ring Topology
Node
Node
Node
Messages are
passed in this
direction
Node
Node
Node
Node
Node
Node
A Bus Topology
Node
Node
Node
Server
Node
Node
Node
Node
Node
Backbone
Node
Node
Server
Node
Node
Mesh Topology
Node
Node
Node
Node
Node
Node
Node
Node
Node
Node
Node
Node
MSU’s Local Area Network
• Essentially a bus topology, with several subnetworks. Machines communicate using
the Ethernet protocol.
• The PCs in RI 105 and RI 108 -- together
with the server in RI 109 -- form one subnetwork. 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.