Computer Systems: Hardware, Software, and Networks

1. Networks Week 2 LBSC 690 Information Technology

2. Computer Systems • Hardware • Types of hardware • Storage hierarchy • Moore’s law • Software • Types of software • Types of interfaces

3. Types of Software • Application programs (e.g., Internet Explorer) • What you normally think of as a “program” • Compilers and interpreters (e.g., JavaScript) • Allow programmers to create new behavior • Operating system (e.g., Windows XP) • Moves data between disk and RAM (+lots more!) • Embedded program (e.g., BIOS) • Permanent software inside some device

4. Installing Applications • Copy to a permanent place on your hard drive • From a CD, the Internet, … • Installs any other required programs • “DLL” files can be shared by several applications • Register the program’s location • Associates icons/start menu items with it • Configures the uninstaller for later removal • Configure it for your system • Where to find data files and other programs

5. Discussion Point: What’s a Virus? • Characteristics • Initiation • Behavior • Propagation • Spyware • Detection

6. Graphical User Interfaces • Easy way to perform simple tasks • Used to start programs, manage files, … • Relies on a physical metaphor (e.g., a desktop) • Built into most modern operating systems • Windows XP, Mac System 10, Unix X-windows • Application programs include similar ideas • Point-and-click, drag and drop, …

7. Cursor-based Interfaces • Useful for specifying complex operations • Available in most operating systems • SSH connection to WAM • Command window in Windows XP • Used when graphical display is difficult • Dial-in access from older computers

8. Network • Computers and devices connected via • Communication devices • Transmission media

9. Why Network? • Sharing data • Sharing information • Sharing hardware • Sharing software • Increasing robustness • Facilitating communications • Facilitating commerce

10. Packet vs. Circuit Networks • Telephone system (“circuit-switched”) • Fixed connection between caller and called • High network load results in busy signals • Internet (“packet-switched”) • Each transmission is routed separately • High network load results in long delays

11. Packet Switching • Break long messages into short “packets” • Keeps one user from hogging a line • Route each packet separately • Number them for easy reconstruction • Request retransmission for lost packets • Unless the first packet is lost!

12. Networks of Networks • Local Area Networks (LAN) • Connections within a room, or perhaps a building • Wide Area Networks (WAN) • Provide connections between LANs • Internet • Collection of WANs across multiple organizations

13. Local Area Networks • Within a campus or an office complex • Short-distance lines are fast and cheap • Fast communications makes routing simple • Ethernet is a common LAN technology • All computers are connected to the same cable • Ordinary phone lines can carry 10 Mb/sec • Every host broadcasts everything to all others • Collisions limit throughput to about 50% utilization

14. Shared Network • All attach to the same cable • Ethernet and “cable modems” • Transmit anytime • Collision detection • Automatic retransmission • Inexpensive and flexible • Easy to add new machines • Robust to computer failure • Practical for short distances • Half the bandwidth is wasted

15. Switched (“Star”) Network • All attach directly to a hub • Switched Ethernet • Digital Subscriber Lines (DSL) • Higher cost • Line from hub to each machine • Hub must handle every packet • Hub requires backup power • Much higher bandwidth • No sharing, no collisions • Allows disks to be centralized

16. Local Area Networks CSS www rac2 rac3 rac4 ttclass PLS sam kim raven ann dove joe HBK

17. Wireless Networks • Radio-based Ethernet • Effective for a few rooms within buildings • “Access Point” gateways to wired networks • Available throughout most of the Maryland campus • Commercial providers offer “hot spots” in airports, etc. • Available in two speeds • IEEE 802.11b: 10Mbps (most common) • IEEE 802.11g: 54Mbps (now becoming available)

18. http://www.tss.northwestern.edu/wireless/faq_wir.html

19. Wide Area Networks • Campus, regional, national, or global scale • Expensive communications must be used well • Limiting to two hosts allows 100% utilization • Routing is complex with point-to-point circuits • Which path is shortest? Which is least busy? … • Internet routers exchange “routing tables” • Which routes seem fast, which seem slow?

20. An example of WAN

21. Ring Network • Unidirectional transmission • Used mostly for WANs • Very high bandwidth • No collisions • Simple routing policies • Complex management • Changes must be coordinated

22. Maryland’s Campus Network CSS www rac2 rac3 rac4 ttclass PLS sam kim raven ann dove joe HBK

23. The Internet • Global collection of public “IP” networks • Private networks are often called “intranets” • Independent • Each organization maintains its own network • Cooperating • Internet Protocol (IP) address blocks • Domain names • World-Wide Web Consortium (W3C) • Computer Emergency Response Team (CERT)

24. A Short History of the Internet • 1969: Origins in government research • Advanced Research Projects Agency (ARPAnet) • Key standards: UDP, TCP, DNS • 1983: Design adopted by other agencies • Created a need for inter-network connections • Key standards: IP • 1991: World-Wide Web added point-and-click • Now 150 million Internet “hosts” • Key standards: HTTP, URL, HTML, XML

25. Overview

26. http://www.geog.ucl.ac.uk/casa/martin/atlas/isp_maps.html

27. Types of Internet “Nodes” • Hosts • Computers that use the network to do something • Routers • Specialized computers that route packets • Gateway • Routers that connect two networks • Firewall • Gateways that pass packets selectively

28. An Internet Protocol (IP) Address Identifies a LAN IP address:216.183.103.150 Identifies a specific computer

29. Routing Tables 120.0.0.0 45.0.2.10 121.0.0.0

30. Domain Name Service (DNS) • “Domain names” improve usability • Easier to remember than numeric IP addresses • DNS coverts between names and numbers • Written like a postal address: general-to-specific • Each name server knows one level of names • “Top level” name server knows .edu, .com, .mil, … • .edu name server knows umd, umbc, stanford, … • .umd.edu name server knows wam, glue, ttclass, … • .wam.umd.edu name server knows rac1, rac2, …

31. IP Addresses and Domain Names IP address:216.183.103.150 Domain Name:www.howstuffworks.com

32. Hands-on: Learn About Your IP Address • Start a command window • Select “start” on the taskbar, then “Run” • Type in “cmd” and click “OK” • Find your IP address • Type “ipconfig /all” (and press enter) • See who “owns” that address • Use http://www.checkdomain.com • See how packets get to your computer • Use http://www.traceroute.org

33. The TCP/IP “Protocol Stack” • Link layer moves bits • Ethernet, cable modem, DSL • Network layer moves packets • IP • Transport layer provides services to applications • UDP, TCP • Application layer uses those services • DNS, FTP, SSH, …

34. User Datagram Protocol (UDP) • The Internet’s basic transport service • Sends every packet immediately • Passes received packets to the application • No delivery guarantee • Collisions can result in packet loss • Example: sending clicks on web browser

35. Transmission Control Protocol (TCP) • Built on the network-layer version of UDP • Guarantees delivery all data • Retransmits missing data • Guarantees data will be delivered in order • “Buffers” subsequent packets if necessary • No guarantee of delivery time • Long delays may occur without warning

36. Telnet • Simulates a dial-up connection • Read data from another machine • VT-100 protocol allows only text • The pine email program is designed for VT-100 • X Windows extension adds graphics

37. File Transfer Program (FTP) • Used to move files between machines • Upload (put) moves from client to server • Download (get) moves files from server to client • Available using command line and GUI interfaces • Normally requires an account on the server • Userid “anonymous” provides public access • Web browsers incorporate anonymous FTP • Automatically converts end-of-line conventions • Unless you select “binary”

38. Hands On: FTP • Start a cmd window • Type “ftp ftp.umiacs.umd.edu” • Login in anonymously with • User: anonymous • Password: your email address • Go to directory lbsc690 • Type “cd pub/gina/lbsc690/” • Get file “hwOne.ppt” • Type “get hwOne.ppt” • Exit • Type “quit”

39. HyperText Transfer Protocol (HTTP) • Send request GET /path/file.html HTTP/1.0 From: someuser@jmarshall.com User-Agent: HTTPTool/1.0 • Server response HTTP/1.0 200 OK Date: Fri, 31 Dec 1999 23:59:59 GMT Content-Type: text/html Content-Length: 1354 <html><body> <h1>Happy New Millennium!</h1> … </body> </html>

40. Network Abuse • Flooding • Excessive activity, intended to prevent valid activity • Worms • Like a virus, but self-propagating • Sniffing • Monitoring network traffic (e.g., for passwords)

41. Encryption • Secret-key systems (e.g., DES) • Use the same key to encrypt and decrypt • Public-key systems (e.g., PGP) • Public key: open, for encryption • Private key: secret, for decryption • Digital signatures • Encrypt with private key, decrypt with public key

42. Encrypted Standards • Secure Shell (SSH) • Replaces Telnet • Secure FTP (SFTP)/Secure Copy (SCP) • Replaces FTP • Secure HTTP (HTTPS) • Used for financial and other private data • Wired Equivalent Protocol (WEP) • Used on wireless networks

43. Before You Go On a sheet of paper, answer the following (ungraded) question (no names, please): What was the muddiest point in today’s class?