COMS 161 Introduction to Computing Title Local Area

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

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

Review • Connecting to the Digital Domain 3

Outline • Connecting to the Digital Domain 4

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 • Costs – Need for additional equipment/software – Maintenance requirements • Standardization of hardware and software across nodes to avoid incompatibilities – Support staff – Cost-effectiveness • Resource sharing • Downsized, distributed computing 5

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? 6

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 7

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-5 e wiring (up to 100 Mbps) • CAT-6 wiring (emerging “gigabit” standard – up to 1 Gbps) 8

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 9

Transmission Media • Unbounded media (wireless networking) – RF (radio frequency) • Becoming very common • Speeds of 11 Mbps now common (“ 802. 11 b”) • 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) 10

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. 11

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 12

Transmission Media NETWORKS ARE BUILT ON PHYSICAL MEDIA Type Uses Maximum Operating Principal Distance (without amplification) Cost Twisted pair Small LANs 300 feet Low Coaxial cable Large LANs 600– 2, 500 feet Medium Fiber optic Network backbones; WANs 1– 25 miles High Wireless/infrared LANs 3– 1, 000 feet (line of sight) Medium Wireless/radio Connecting things that move Varies considerably High 13

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 14

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 • P 2 P 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 15

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

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 17

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

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 19

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 6 2 5 3 4 20

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 21

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 22

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 23

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” 24

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 25

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? 26

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 27

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

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 29

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 30

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 31

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

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

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 34

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! 35

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

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

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. 38

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 39

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 Web. TV] (Microsoft) Cell Phones PDAs (e. g. Palm Pilots) Lynx (text-only browser) 40

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 41

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) 42

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/yr sales • New top-level domains recently added to the Internet due to increased Web activity: . biz. info. name etc. 43

Internet Growth and the Web Year Internet Nodes 1 Web Servers 2 2004 2002 2000 1998 1996 1994 1992 1990 1988 1986 1984 1982 1971 240, 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 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 44

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

Web as Client/Server App • Protocol--a set of rules that govern how an activity takes place • HTTP--(Hyper. Text Transfer Protocol) specifies how Web clients and servers communicate 46

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

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

Web Browser (Client) 49

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 50

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 51

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

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

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

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 55

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 56