1. 1 COMS 161 Introduction to Computing Title: Local Area Networks Date: September 24, 2004 Lecture Number: 13

2. 2 Announcements This material is from chapter 4 and 17 in the book

3. 3 Review Connecting to the Digital Domain

4. 4 Outline Connecting to the Digital Domain

5. 5 Benefits and Costs of LANs Benefits –Communications Direct communications Client/server architectures Distributed computing –Management control Centralized mass storage Centralized backups Roll-out of upgrades Software license control –Cost-effectiveness Resource sharing Downsized, distributed computing Costs –Need for additional equipment/software –Maintenance requirements Standardization of hardware and software across nodes to avoid incompatibilities –Support staff

6. 6 Differentiating LANs Transmission media –What are the actual hardware connections between nodes (computers) made from? Topologies –In what way are the various nodes arranged and interconnected?

7. 7 Transmission Media Bounded media –Coaxial cable (like a TV cable) –“Twisted-pair” cable (copper wires) –Optical fiber cable Unbounded media (wireless networking) –RF (radio frequency) –IR (infrared) –Cellular modem

8. 8 Transmission Media Bounded media –Coaxial cable (like a TV cable) Original LAN installations were coax Now almost never used (cost) –“Twisted-pair” cable (copper wires) Generally limited to about 100 meters max (330 ft) Telephone wire (“CAT-3”, or “category 3”) CAT-5 wiring (up to 10 Mbps – 10 million bits per second) CAT-5e wiring (up to 100 Mbps) CAT-6 wiring (emerging “gigabit” standard – up to 1 Gbps)

9. 9 Transmission Media Bounded media (cont’d) –Optical fiber cable Signal is composed of pulses of laser light, not electricity Extremely thin glass strand transmits the light pulse Lower error rates and high data bandwidth (>2 Gbps) Becoming very cost-effective for high speed data needs

10. 10 Transmission Media Unbounded media (wireless networking) –RF (radio frequency) Becoming very common Speeds of 11 Mbps now common (“802.11b”) Faster speeds becoming available (54 Mbps, 108 Mbps) Public access points (“hotspots”) becoming common –Various areas on campus –Downtown areas, such as Manhattan –Airports, hotels, coffee shops, etc. (free or pay)

11. 11 Transmission Media Unbounded media (cont’d) –IR (infrared) An early wireless technique, now mostly unused as RF wireless has advanced Required line-of-sight – works well only within enclosed spaces Still used for some simple ad hoc networking tasks, such as –Laptop-to-PDA –PDA-to-printer –Digital camera uploads –Etc.

12. 12 Transmission Media Unbounded media (cont’d) –Cellular modem Wireless connectivity anywhere that there is appropriate digital cellular service –Various cellular companies are competing –Coverage is still spotty, but improving Up to 120 Kbps – better than dialup

13. 13 Transmission Media NETWORKS ARE BUILT ON PHYSICAL MEDIA TypeUses Maximum Operating Principal Distance (without amplification) Cost Twisted pairSmall LANs300 feetLow Coaxial cableLarge LANs600–2,500 feetMedium Fiber opticNetwork backbones; WANs1–25 milesHigh Wireless/infraredLANs3–1,000 feet (line of sight)Medium Wireless/radioConnecting things that moveVaries considerablyHigh

14. 14 LAN Topologies Topology –The logical layout or geometric organization of a network –Topology indicates potential paths for communications between nodes –Many topologies possible, with pros and cons Point-to-point Star Bus Ring

15. 15 Point-to-Point Topology Point-to-point is the simplest topology –Each node connected to some of its neighbors –Needs a control mechanism The Internet uses TCP/IP P2P file-sharing programs (Napster, Kazaa, etc.) use centralized directory servers –While this works for the Internet, it requires too much overhead for a successful LAN implementation

16. 16 Star Topology All nodes are connected to a single hub HUB

17. 17 Star Topology Advantages –Simple to implement –Centralized management –Easy to add new nodes –Network can expand by ‘daisy-chaining’ hubs –Not subject to failure due to a single node or cable failure Disadvantages –Number of nodes limited to size of hub –Cabling must all feed back to the hub –Hub failure is catastrophic –Hub can be a bottleneck for data throughput

18. 18 Bus Topology Single transmission medium (‘bus’ or ‘backbone’) Nodes connected to the bus by ‘taps’

19. 19 Bus Topology Advantages –Simple to implement –Shorter cabling –Easy to add new nodes –Not subject to failure due to a single node failure Disadvantages –Length of backbone limited –Failure of the backbone cable is catastrophic –Centralized management difficult –Cannot expand network through daisy-chaining

20. 20 Ring Topology All nodes connected in a ring (‘token ring’) Once heavily promoted by IBM, now not used much Nodes have a specified order on the ring 1 3 6 4 25

21. 21 Ring Topology Advantages –Originally higher speed than possible with other types (first to 10 Mbps –Exactly predictable delay rate Disadvantages –Size of ring limited –Adding or removing nodes is difficult –Cannot expand network through daisy-chaining –Failure of the backbone cable is catastrophic –Failure of any single node is also catastrophic –No centralized management

22. 22 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

23. 23 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’) Ethernet follows this paradigm Leads to ‘orderly chaos’ – very efficient for low to medium load networks

24. 24 Token Passing MAC Typically used in a ring topology Very methodical 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 the token, and then pass it 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 someone else 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”

25. 25 Layered Network Model Networks are created by 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

26. 26 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?

27. 27 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

28. 28 Interconnecting LANs Different devices available for connecting LANs together –Repeaters –Routers –Bridges –Gateways

29. 29 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

30. 30 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

31. 31 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

32. 32 The Internet Gateway Generally used to connect LANs to WANs Very effective at routing Internet traffic HUB BRIDGE HUB BRIDGE ROUTER GATEWAY

33. 33 Internet(work) Collection of autonomous networks The Internet Intranets

34. 34 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

35. 35 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!

36. 36 History of The Internet The ARPANET in 1971 – 18 sites

37. 37 History of The Internet The ARPANET in 1980 – about 75 sites

38. 38 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..tv

39. 39 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

40. 40 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)www.mozilla.org –Netscape Communicator (AOL Time Warner)AOL Time Warner –Internet Explorer (Microsoft) –Opera (www.opera.com)www.opera.com –MSN TV [formerly WebTV] (Microsoft)Microsoft –Cell Phones –PDAs (e.g. Palm Pilots) –Lynx (text-only browser)

41. 41 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

42. 42 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)

43. 43 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 BezosAmazon.com 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.

44. 44 Internet Growth and the Web YearInternet Nodes 1 Web Servers 2 2004240,000,00048,000,000 2002162,129,00033,083,000 200093,048,00018,170,000 199836,739,0004,279,000 199612,881,000300,000 19943,212,0003,000 1992992,00050 1990313,000 198856,000 19865,089 19841,024 1982235 197118 Sources: (1) www.isc.org/ds ; (2) www.netcraft.com/survey www.isc.org/ds www.netcraft.com/survey

45. 45 Client/Server CLIENT--(local) system requesting services SERVER--(remote) system that receives and handles requests from many clients concurrently

46. 46 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

47. 47 Web as Client/Server App URL--(Uniform Resource Locator) addressing for Web resources HTML--(HyperText Markup Language) defines content and display of Web pages

48. 48 URL Example Protocol Domain Name Resource location URL for the book

49. 49 Web Browser (Client)

50. 50 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

51. 51 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

52. 52 Boolean Expressions search engines interpret multiple keywords as Boolean expressions AND--intersection, i.e., references that contain both keywords

53. 53 Boolean Expressions OR--union, i.e., references that contain either keyword

54. 54 Boolean Expressions NOT--negation, i.e., references that do not contain the keyword

55. 55 Mathematical Operators Boolean ExpressionMathematical Operator Labrador AND retrievers+Labrador +retrievers Labrador AND NOT retrievers +Labrador -retrievers Mathematical operators may be used to signify Boolean expressions

56. 56 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