COMS 161 Introduction to Computing

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

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

Review • Connecting to the Digital Domain

Outline • Connecting to the Digital Domain

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 Benefits and Costs of LANs

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?

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

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)

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

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)

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.

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

Transmission Media

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

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 theInternet, it requires too much overhead for a successful LAN implementation

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

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 Star Topology

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

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 Bus Topology

1 6 2 5 3 4 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

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 Ring Topology

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

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

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”

Applications NetworkServices NetworkSystem PhysicalTransport 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 actuallymoves the data packets from node to node)

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?

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

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

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

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

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

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

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

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

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!

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

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

History of The Internet • Original top-level domains: .edu .gov .org .net .mil .com .int • International domains came later: .us .uk .jp .de .tvetc.

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

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)

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

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)

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.

Internet Growth and the Web Sources: (1) www.isc.org/ds; (2) www.netcraft.com/survey

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

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

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

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

Web Browser (Client)

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

COMS 161 Introduction to Computing