Computer Networks

1
Computer Networks

• Getting started

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Who am I?

• Dr. Lillian N. Cassel
• Professor
• 161 Mendel Science Center
• 610 519 7341
• http//www.csc.villanova.edu/cassel
• cassel_at_acm.org

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What is a Network?

• What do you think of when network comes to
  mind?
• Connected machines?
• Facilitated communication?
• Resource sharing?
• Collaboration?
• Access to distributed resources?

4
Reasons for Installing Networks

• Communication
• Price/Performance Ratio
• Performance Quality
• Reliability
• Accessible Resources
• Incremental Growth of Computing Power

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Significance of Networks in the Computing
Environment

• Computing platform for many applications.
• Cooperate with other programs running on
  different systems.
• Simple file access involves a file server.
• Printing requires contact with the right printer.
• Performance varies with network load.
• Some resources may be inaccessible.
• Programs may be competing for resources.

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Networks, Data Communications, and Distributed
Systems

• Closely related topics
• Distributed Systems
• Applications that run on multiple computing
  platforms
• Issues of concurrency, data integrity, etc.
• Networks
• The systems that allow distributed applications
• Data Communications
• The components that allow message flow

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Network System Software

• Three major components to a network
• a transmission medium,
• an interface between the network station and the
  medium,
• and software to drive the network connection.
• Support similar to what the O S provides
• make life easier for the user
• protect the system

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Network Operating System

• A layer between the application and the real
  operating system
• interprets some system calls
• if the call involves network operation, capture
  and service it
• if the call does not involve network operation,
  pass the call on to the operating system

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Network message exchange

• Explicit message sending
• attach the identification of the sender and
  intended receiver
• pass it on to the appropriate network support
  facility for processing
• Receiving a message
• must be listening all the time
• look for messages that identify me as the
  intended recipient

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Protocols

• Agreements
• How to interpret a message received
• How to respond the the message content
• local actions
• returned messages
• What to send when initiating interaction

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Protocol Example
Tiger
Monet
Tiger, Monet, C, D, Reset Printer
Monet, Tiger, OK
Reset Printer
C confirm D Display
Monitor
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Another example protocol
Neptune
Jupiter
Neptune, Jupiter, S, Reset Printer
Jupiter, Neptune, OK
Reset Printer
Monitor
S show, (confirmation implicit)
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Duplicate protocols

• How to deal with proliferation of protocols that
  do not interact
• Drop one protocol entirely and use the other on
  all the systems
• Drop both protocols and develop a new one that
  all stations use
• Each station runs both protocols, using the
  correct one for each communication
• Keep the original protocol in each set of
  stations and also adapt the new global protocol
  to extend the range of stations each can work

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Network Protocols and Standards

• The concept of layering
• isolate specific functions
• easier to analyze performance, revise
• make upgrades easier
• The OSI Reference Model
• seven layers
• basis of most discussions of network operations
  

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OSI Reference Model

• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical

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Major Types of Networks

• Local Area Networks
• high speed
• high reliability
• Wide Area Networks
• speed limited by distance requirements
• reliability less dependable
• Metropolitan Area Networks
• high speed over wider spread than LANs
• Wireless Networks
• Speed and reliability issues
• Mobility usually

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A Brief Historical Perspective

• ARPAnet
• four operating nodes by the end of 1969
• a worldwide network of more than 60,000 nodes in
  1989
• succeeded by a combination of networks using its
  protocols and called the Internet.
• Alohanet
• began operating in June 1971
• the direct ancestor of the bus protocols in very
  common use today.
• 1.5 years after it began operating, the ALOHA
  system was connected to the ARPAnet.

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More history

• Aloha
• lead to the Ethernet (1973)
• In many ways relevant to the wireless networking
  issues
• Cambridge Ring
• developed at Cambridge University in the mid
  1970s
• similar to very modern approaches such as DQDB
• Token ring network
• released by IBM in 1985

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Internet Addresses and Host Names

• Two kinds of addresses
• local area network interface address
• Internet (and/or other) address
• LAN addresses
• Usually, 48 bits associated with the hardware
  interface unit
• Internet addresses
• 32 bits assigned by an authority
• dotted decimal notation

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Names and addresses

• LAN address is usually not of interest to the
  human user or to a program
• Internet addresses are often required for
  effective communication
• Names allow humans to remember machine
  identifiers
• Servers translate the names to numbers or vice
  versa as needed
• We will look at that service later

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IP addresses

• More detail will come when we look at the Network
  layer
• 32-bit addresses are represented in dotted
  decimal notation w.x.y.z
• each part represents 8 bits, so the possible
  values range from 0 to 255
• Blocks of addresses are assigned to organizations
• Network administrators then assign individual
  addresses to machines

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Types of IP addresses

• IP address structure (v4)
• Class A 0xxxxxxxyyyyyyyyyyyyyyyyyyyyyyyy
• Class B 10xxxxxxxxxxxxxxyyyyyyyyyyyyyyyy
• Class C 110xxxxxxxxxxxxxxxxxxxxxyyyyyyyy
• Multicast1110xxxxxxxxxxxxxxxxxxxxyyyyyyyy
• Reserved 1111xxxxxxxxxxxxxxxxxxxxyyyyyyyy
• Ranges Class A 0.0.0.0 to 127.255.255.255
• . Class B 128.0.0.0 to 191.255.255.255
• . Class C 192.0.0.0 to 223.255.255.255
• . Class D 224.0.0.0 to 239.255.255.255
• . Class E 240.0.0.0 to 247.255.255.255

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Class A network addresses

• Class A 0xxxxxxxyyyyyyyyyyyyyyyyyyyyyyyy
• 27 networks
• each with up to 224 hosts attached
• Not quite. Addresses of all 0 or all 1 are
  special cases and not permitted for general use

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Class B and Class C networks

• Class B 10xxxxxxxxxxxxxxyyyyyyyyyyyyyyyy
• 214 networks (16,384)
• each with up to 216 hosts (65,536)
• - again, not quite.
• Class C 110xxxxxxxxxxxxxxxxxxxxxyyyyyyyy
• 221 networks (2097152)
• each with up to 28 hosts (256) (approximately)

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The World Wide Web

• Joining network infrastructure with hypertext
  applications
• Easily accessible networked communication
• Easy entry into distributed applications

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Web basics

• Browser
• Presents a graphical display of a document,
  broadly defined
• Netscape, Mozilla, safari
• Internet Explorer
• Others
• Server
• Makes some part of the computers file system
  accessible to browsers
• Sends document files to the browsers

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A protocol and a language

• HTTP (Hyper text transport protocol)
• specifies the behavior of the sending and the
  receiving system
• HTML (Hypertext markup language)
• describes how a page should be presented
• originally intended for report formats, primarily
  text
• XML (EXtensible Markup Language)
• extend the paradigm beyond documents

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Basic web system
Browser
File System
Server
The server may fetch a file from a different
system or from its own file system
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Web-based applications

• Simplest entry into distributed computing
• Client and server side
• client side initiates activity
• server side responds to the request.

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Web Form Server
Data from the form is input to a program, which
may call other programs, interact with a
database, or do anything any other program could
do.
Server
Browser
Database
Program
Program
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Security

• A server responding to a web-based form is
  allowing strangers to execute code.
• Obviously could be dangerous
• Access is restricted to programs stored in
  particular directories
• If a problem arises, there is a restricted area
  to search
• Programs allowed into that area should be
  carefully reviewed for safety

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Week 1 Summary

• Networks include hardware and software
• hardware for physical connection
• software for meaningful interaction
• protocols allow communication between cooperating
  processes
• resources and applications give a reason for
  accessing one computer from another
• We will focus on some distributed computing
  issues and the network facilities that make those
  possible
• We will give little attention to the data
  communication details

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Summary continued

• The OSI Reference Model summarizes the layers of
  software protocols needed to make distributed
  applications and network accessible resources
• Network types
• local, wide area, metropolitan area
• characterized by speed and susceptibility to
  errors
• Historically, the ARPAnet is the parent of the
  Internet and Alohanet is the parent of local
  networks such as Ethernet