Transcript 10/01/04

COMS 161
Introduction to Computing
Title: Local Area Networks
Date: October 1, 2004
Lecture Number: 16
1
Announcements
• This material is from chapter 17 in the
book
• Paper 1 due on Friday
• Homework 5
– Due next Wednesday 10/06/04
2
Review
• Connecting to the Digital Domain
3
Outline
• LANs
– Transmission media
• Bounded
• Unbounded
4
Media Access Control (MAC)
• Determines how nodes make use of the
underlying medium
• Not all nodes can talk at once! A protocol is
needed to act as the ‘traffic cop’
• Two MAC protocols are in common use in
LANs
– Ethernet MAC (“CSMA/CD” or “802.3”)
– Token passing MAC (“802.5”)
• These are both packet-switching protocols
– Data is broken into discrete packets which are
sent individually and reassembled at the
destination
5
Ethernet MAC
• Commonly used in star and bus topologies
• Much like a conversation at a dinner table
– Not everyone can talk at once
– If someone is talking, politely wait until they’re
done
– When there is a lull in the conversation, you can
speak
– You address you comments to one person, even
though everyone can hear you
– If two people happen to try to talk at the exact
same time (a ‘collision’), both stop and wait a
moment to see if they can talk (‘random backoff
strategy’)
6
Ethernet MAC
• Ethernet follows this paradigm
– Transmitters listen to the channel
• Wait until the channel is idle
• Transmits a packet
– Transmission may be successful
• Packet is received by destination node
– Transmission may not be successful
• Different transsmitters sent packets simultaneously
• Packets become garbled (packet collision)
• Random backoff strategy
• Leads to ‘orderly chaos’
– Very efficient for low to medium load networks
7
Token Passing MAC
• Typically used in a ring topology
• Very methodical (orderly) protocol
– A ‘token’ (a small data packet) is passed around the
ring continuously (like a baton in a relay race)
– When a node receives the token, it can attach some
data to it, and then pass the token to the next node
– When that node receives the token, it looks to see if
any attached data is addressed to it; if so, it keeps it
– Any attached data addressed to another node is
simply passed on with the token to the next node
– And so on, ad infinitum
• “And ya don’t stop, and ya don’t quit”
8
Layered Network Model
• Networks are created using layers of networking
software and hardware
– Consider two nodes communicating
• Use an application (e.g. send Email, get a file)
• The application uses the network services of the
operating system (provides high-level functions,
e.g. file sharing services)
• The operating system services make use of the
network system (provides basic connectivity tools
– ensuring that the nodes are communicating)
• The network system works through the physical
transport layer (the hardware and software of the
network – Ethernet, token ring, etc. – that actually
moves the data packets from node to node)
Applications
Network
Services
Network
System
Physical
Transport
9
Extending LANs
• LANs are great for local networks
• Need to be able to tie these local networks
together into larger groupings
• Connecting separate LANs (possibly of
different types) is called internetworking
• Eventually, want local networks to be part of
the global network – the Internet
• How do we interconnect these local networks?
10
Metropolitan Area Network (MAN)
•
•
•
•
Link two or more LANs in a city
Extends over a longer distance than a LAN
Each network site is a node on the network
Data is transmitted over common
“superhighways” called the backbone
11
Interconnecting LANs
• Different devices available for connecting
LANs together
– Repeaters
– Routers
– Bridges
– Gateways
12
Repeater
• Amplifies and repeats all signals
• Used to increase the size of a LAN
• Especially useful when the LAN must extend
to a distance longer than a single cable can
handle
HUB
REPEATER
HUB
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Bridge
• Connects two LANs of similar types
• Only data for the ‘other’ LAN is passed
through
• Lets LANs act together like a larger LAN while
still maintaining their individual autonomy
HUB
BRIDGE
HUB
14
Router
• Can connect LANs of similar or different
types
– Specially designed to manage data flow in connected
networks – knows which route to use to most
effectively get the data to the right destination
HUB
ROUTER
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Gateway
• Generally used to connect LANs to WANs
• Very effective at routing Internet traffic
BRIDGE
HUB
HUB
BRIDGE
HUB
The
Internet
ROUTER
HUB
GATEWAY
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Internet(work)
• Collection of
autonomous
networks
• The Internet
• Intranets
17
The Internet
• The Internet
– Really just a very loose collection of networks
– No single entity controls the Internet
– Many kinds of information fly through it constantly
•
•
•
•
•
•
Email, IM (instant messaging)
Web pages
Entertainment – files and streaming media
Commerce and business data
VOIP – Voice over Internet Protocol (telephone)
Etc., etc.
– No one validates this information
– No one directly polices this information
18
History of The Internet
• Originally a US military project from the late
60’s: ARPANET
– Designed to survive a nuclear attack
• Expanded into academics and research in the
70’s
• Separated from MILNET in early 80’s
• Decommissioned in 1990
• Originally only non-commercial uses allowed
– Peer-pressure only, since there is no central
control!
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History of The Internet
• The ARPANET in 1971 – 18 sites
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History of The Internet
• The ARPANET in 1980 – about 75 sites
21
History of The Internet
• Original top-level domains:
.edu .gov .org .net .mil .com .int
• International domains came later:
.us .uk .jp .de .tv etc.
22
The Web
• The World Wide Web (WWW)
– Just one part of the Internet
– Consists of all information on the Internet that has
been made available using a particular method
(HTML & HTTP)
– Your computer is part of the Internet whenever you
are connected
– Information on your computer will become part of
the Web during the first few labs
23
Web Browsers
• Web browsers are programs that make it easy for
anyone on the Internet to access information on the
Web
• Many to choose from; they all use common
techniques
–
–
–
–
–
–
–
–
Mozilla (www.mozilla.org)
Netscape Communicator (AOL Time Warner)
Internet Explorer (Microsoft)
Opera (www.opera.com)
MSN TV [formerly WebTV] (Microsoft)
Cell Phones
PDAs (e.g. Palm Pilots)
Lynx (text-only browser)
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History of The World Wide Web
• Invented by Tim Berners-Lee at CERN (1989)
• Originally designed for
distributing scientific
research results
– Text pages that can be
shared among different
computer systems
– Simple, text-based browsers
• Quickly adopted by other organizations
25
History of The World Wide Web
• First graphical browser (Mosaic) developed in 1993
by an undergrad at University of Illinois (Marc
Andreesen)
– (only 11 years ago!)
– Distributed freely
– Widely used in academics and government
• Mosaic expanded by Netscape
– Internally called “Mozilla”
– Originally still distributed without charge
• Microsoft’s Internet Explorer came later
• Netscape now commercialized (AOL Time/Warner)
• Mozilla still free (split off from Netscape)
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History of The World Wide Web
• “Ban” on commercial traffic on the Web lifted soon
after Mosaic released
• .com quickly becomes largest domain
• E-commerce explosion starts in the late 90’s
•
•
•
•
Amazon.com founded 1994 by Jeff Bezos
IPO 1997 @ $18/share
Each share bought then now worth $540 – 3000% in 7 yrs
First profit 2001; currently $6,000,000,000/yr sales
• New top-level domains recently added to the Internet
due to increased Web activity:
.biz .info .name etc.
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Internet Growth and the Web
Year
Internet Nodes1
Web Servers2
2004
2002
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1971
240,000,000
162,129,000
93,048,000
36,739,000
12,881,000
3,212,000
992,000
313,000
56,000
5,089
1,024
235
18
48,000,000
33,083,000
18,170,000
4,279,000
300,000
3,000
50
Sources: (1) www.isc.org/ds; (2) www.netcraft.com/survey
28
Client/Server
• CLIENT--(local)
system requesting
services
• SERVER--(remote)
system that
receives and
handles requests
from many clients
concurrently
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Web as Client/Server App
• Protocol--a set of rules that govern how
an activity takes place
• HTTP--(HyperText Transfer Protocol)
specifies how Web clients and servers
communicate
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Web as Client/Server App
• URL--(Uniform Resource Locator)
addressing for Web resources
• HTML--(HyperText Markup Language)
defines content and display of Web
pages
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URL Example
• URL for the book
• Protocol
• Domain Name
• Resource
location
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Web Browser (Client)
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Researching on the Web
• Search service--generates lists of other
Web sites containing information about
supplied topics
• Web directory--a search service
organized as a topical hierarchy and
compiled by (human) editors
• Search engine--search services whose
databases are compiled by automated
Web crawlers
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Keyword Searching
• A form of (automated) text matching
• Keyword--a word or phrase used as a
text pattern for matching
• Hits--matches with text patterns
• Misses--fails to match some or all of the
text pattern
• False positives--hits that are not related
to the desired topic
35
Boolean Expressions
• search engines
interpret multiple
keywords as Boolean
expressions
• AND--intersection,
i.e., references that
contain both
keywords
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Boolean Expressions
• OR--union, i.e.,
references that
contain either
keyword
37
Boolean Expressions
• NOT--negation,
i.e., references
that do not contain
the keyword
38
Mathematical Operators
Mathematical operators may be used to
signify Boolean expressions
Boolean Expression
Mathematical Operator
Labrador AND retrievers +Labrador +retrievers
Labrador AND NOT
retrievers
+Labrador -retrievers
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Hybrid Search Services
• Metasearch engines--consult other
search engines and directories and
summarize query data
• Niche services--commercial services
targeted at a specific audience or topic
• Portals--gateway Web site with
searching capabilities
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