COMS 161 Introduction to Computing

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. Applications NetworkServices NetworkSystem PhysicalTransport 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)

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. HUB HUB 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 REPEATER

14. HUB HUB 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 BRIDGE

15. HUB 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 ROUTER

16. BRIDGE HUB HUB HUB ROUTER BRIDGE HUB Gateway • Generally used to connect LANs to WANs • Very effective at routing Internet traffic The Internet GATEWAY

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 .tvetc.

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 theInternet whenever you are connected • Information on yourcomputer 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 beshared 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 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 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