1
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- Week 2
- LBSC 690
- Information Technology
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2
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- Hardware
- Types of hardware
- Storage hierarchy
- Moore’s law
- Software
- Types of software
- Types of interfaces
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3
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- 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
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4
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- 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
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5
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- Characteristics
- Initiation
- Behavior
- Propagation
- Spyware
- Detection
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6
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- Computers and devices connected via
- Communication devices
- Transmission media
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7
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- Sharing data
- Sharing information
- Sharing hardware
- Sharing software
- Increasing robustness
- Facilitating communications
- Facilitating commerce
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8
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- 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
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9
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- 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!
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10
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- 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
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11
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- 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
- 100 Mb/s connections require special cables
- 1 Gb/s connections require special switches
- Every host broadcasts everything to all others
- Collisions limit throughput to about 50% utilization
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12
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- 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
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13
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- 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
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14
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15
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- 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 (good enough for most uses)
- IEEE 802.11g: 54Mbps (required for wireless video)
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16
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17
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- 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?
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18
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19
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- Unidirectional transmission
- Very high bandwidth
- No collisions
- Simple routing policies
- Complex management
- Changes must be coordinated
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20
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21
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- 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)
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22
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- 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
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23
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24
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25
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- 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
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26
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27
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28
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- “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, …
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29
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30
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- Find your IP address
- Select “start” on the taskbar, then “Run”=
li>
- Type in “cmd” and click “OK”
- Type “ipconfig /all” (and press enter)
- See who “owns” that address
- Use http://whois.godaddy.com
- See how packets get to your computer
- Use http://www.traceroute.org
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31
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- Link layer moves bits
- Ethernet, cable modem, DSL
- Network layer moves packets
- Transport layer provides services to applications
- Application layer uses those services
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32
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- 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
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- Built on the network-layer version of UDP
- Guarantees delivery all data
- Guarantees data will be delivered in order
- “Buffers” subsequent packets if necessary
- No guarantee of delivery time
- Long delays may occur without warning
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34
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- 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
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35
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- 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”
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36
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- Start a cmd window
- Type “ftp ftp.umiacs.umd.edu”
- Login in anonymously with
- User: anonymous
- Password: your email address
- Go download a file
- Type “cd pub/gina/lbsc690/”
- Type “binary”
- Type “get hwOne.ppt”
- Exit
- Try it again with a graphical FTP program
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37
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- 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>
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38
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- 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
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39
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- Secure Shell (SSH)
- Secure FTP (SFTP)/Secure Copy (SCP)
- Secure HTTP (HTTPS)
- Used for financial and other private data
- Wired Equivalent Protocol (WEP)
- Used on wireless networks
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40
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- Flooding
- Excessive activity, intended to prevent valid activity
- Worms
- Like a virus, but self-propagating
- Sniffing
- Monitoring network traffic (e.g., for passwords)
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41
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- On a sheet of paper, answer the following (ungraded) question (no
names, please):
- What was the muddiest point in today’s class?
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