1 COMS 161 Introduction to Computing Title: Local Area Networks Date: October 1, 2004 Lecture Number: 16.

1

COMS 161Introduction to Computing

Title: Local Area Networks

Date: October 1, 2004

Lecture Number: 16

2

Announcements

• This material is from chapter 17 in the book

• Paper 1 due on Friday

• Homework 5– Due next Wednesday 10/06/04

3

Review

• Connecting to the Digital Domain

4

Outline

• LANs– Transmission media

• Bounded• Unbounded

5

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

6

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’)

7

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

8

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”

9

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 actuallymoves the data packets from node to node)

ApplicationsApplications

NetworkNetworkServicesServices

NetworkNetworkSystemSystem

PhysicalPhysicalTransportTransport

10

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?

11

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

12

Interconnecting LANs

• Different devices available for connecting LANs together– Repeaters– Routers– Bridges– Gateways

13

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

HUBHUB HUBHUBREPEATERREPEATER

14

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

HUBHUB HUBHUBBRIDGEBRIDGE

15

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

HUBHUB ROUTERROUTER

16

The InternetThe InternetThe InternetThe Internet

Gateway

• Generally used to connect LANs to WANs

• Very effective at routing Internet traffic

HUBHUB HUBHUBBRIDGEBRIDGE

HUBHUB HUBHUBBRIDGEBRIDGE

RO

UTER

RO

UTER

GATEWAYGATEWAY

17

Internet(work)

• Collection of autonomous networks

• The Internet

• Intranets

18

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

19

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!

20

History of The Internet

• The ARPANET in 1971 – 18 sites

21

History of The Internet

• The ARPANET in 1980 – about 75 sites

22

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.

23

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

24

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)

25

History of The World Wide Web

• Invented by Tim Berners-Lee at CERN (1989)

• Originally designed for distributing scientificresearch results– Text pages that can be

shared among different computer systems

– Simple, text-based browsers

• Quickly adopted by other organizations

26

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)

27

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.

28

Internet Growth and the Web

Year Internet Nodes1 Web Servers2

2004 240,000,000 48,000,0002002 162,129,000 33,083,0002000 93,048,000 18,170,0001998 36,739,000 4,279,0001996 12,881,000 300,0001994 3,212,000 3,0001992 992,000 501990 313,0001988 56,0001986 5,0891984 1,0241982 2351971 18

Sources: (1) www.isc.org/ds; (2) www.netcraft.com/survey

29

Client/Server

• CLIENT--(local) system requesting services

• SERVER--(remote) system that receives and handles requests from many clients concurrently

30

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

31

Web as Client/Server App

• URL--(Uniform Resource Locator) addressing for Web resources

• HTML--(HyperText Markup Language) defines content and display of Web pages

32

URL Example

• Protocol

• Domain Name

• Resource location

• URL for the book

33

Web Browser (Client)

34

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

35

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

36

Boolean Expressions

• search engines interpret multiple keywords as Boolean expressions

• AND--intersection, i.e., references that contain both keywords

37

Boolean Expressions

• OR--union, i.e., references that contain either keyword

38

Boolean Expressions

• NOT--negation, i.e., references that do not contain the keyword

39

Mathematical Operators

Boolean Expression Mathematical Operator

Labrador AND retrievers +Labrador +retrievers

Labrador AND NOT

retrievers

+Labrador -retrievers

Mathematical operators may be used to signify Boolean expressions

40

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