1. 27-1 Operating Systems

2. 27-2 What is impact does the operating system have? So Operating Systems provide: Communications with the user Loading and executing user programs Memory Management Scheduling those programs to the CPU Communication between programs and the computer hardware A file system (organizing/manipulating files) Other useful application the vendor includes to help sell the operating system –So different operating systems provide different qualities of the above

3. 27-3 Comparing Operating Systems Not all Operating Systems run on every type of computer The OS contains hardware specific code –Microsoft Windows is written for generic (“x86”) PC hardware –Macintosh OSX requires Apple hardware... –Linux (a free UNIX-based variant) will run any architecture programs really run on top of the OS, not the hardware –There is no Microsoft Word for Linux, but there are Word Processors (free ones!) –There is Microsoft Word for OSX... How do you compare Operating Systems?

4. 27-4 Comparing Operating Systems Comparing: –Windows XP –Macintosh OS X 10.4 (Tiger) –Linux Price? Hardware platform? Included Software? Ease of Use? Pretty? Software Availability/Compatibility?

5. 27-5 Comparing Operating Systems Microsoft Windows XP Home – Based on Windows NT –Internally uses a modified version of Windows NT Crashes much less than Windows 95-98 (finally) Relatively easy to use –Price = ~$110 –Hardware platform? Generic PC hardware (which is cheap!) –Included Software Web browser, Basic text editing, Basic Image editing, Movie Playback (but not DVD), Windows Media Player music player –Ease of Use: Moderate Difficult to get started, but not too hard A lot of freedom  can be complicated User Interface –Includes Microsoft’s GUI –Also provides a limited command-line interface to old MS-DOS commands

6. 27-6 Comparing Operating Systems Apple OSX 10.4 – Based on UNIX (BSD) –Internally uses a modified version of UNIX, but presents an Apple created user interface Doesn’t crash (fail) very often (finally) Very easy to use –Price = ~$130 –Hardware platform? Apple hardware only Moderate negative, since Apple hardware is more expensive than generic PC hardware (though mac mini changes this proposition) –Included Software Web browser, Basic text editing, Basic Image editing, DVD Playback, iTunes music player –Ease of Use: Easy Considered to be the easiest Operating System to use So easy to use, can be limiting User Interface –Includes Apple’s own GUI, which is both easy to use and pretty –Internally runs a version of UNIX which makes it stable

7. 27-7 Comparing Operating Systems Linux – Based on UNIX: –Used by “high-powered computer” users in business, science, engineering and networking. Flexible in doing multiple computing jobs. Doesn’t crash (fail) very often (ever?). –Price = free (mostly) Open-source software: The original program source code is freely distributed. Changes can be made to suit computing needs. –Hardware platform? All (Mac, PC, you name it, there’s probably a version) –Included Software? Varies –Ease of Use: Generally difficult (Varies) Many distributions, each with varied ease of install and use Considered to be more difficult to use than others User Interface –Can use either command line or GUI –Popular command line: Korn shell, C shell, Bourne shell. –Examples of GUI to Linux: XWindows, Gnome and KDE A BU distribution of Linux is available: http://www.bu.edu/computing/linux/

8. 27-8 Networks With the explosion of the Internet –Many Operating Systems are considered “network Operating Systems” In addition to standard Operating System tasks they provide programs that allow communication with other computers on a network –As security precautions were important before to protect the operating system and other programs from malicious programs Now we have to consider network security We open your computer up to communication with the rest of the world –Why is network security a problem? »We now have a source of data (bits) that we may not be able to trust. We have to be extra careful about how we handle those bits (e.g., do not execute!) »Bugs that exist in OS programs might allow malicious users to access your computer remotely – without your knowledge

9. 27-9 Computer Networking

10. 27-10 Goal of a Network Network: A collection connected entities –Refer to each entity in the network by the name “node” generic name given to any device in a network. –The goal of a network is to allow nodes in the network to communicate in some way thinking concretely: –nodes  computers –communication  transfer of bits –If computers are networked, we can communicate anything that can be represented in binary Text, Images, Sound, Video, Programs, etc... –to facilitate such a network in abstract terms we need...

11. 27-11 Networking Requirements Specification/Naming of: –Nodes: Identify which entity you want to communicate with –Paths: Describe the route of communications from you to the target –Services: Identify where at the target this communication is headed i.e., what program/application Delivery Mechanism: How data is actually moved Content (the thing we are actually communicating)... consider some domain specific networks

12. 27-12 Postal Network Specification/Naming of: –Nodes: A mailing address. Street Address. –Paths: Not explicit by the user USPS does it for you, but you can request “faster” or “priority” paths –Services: A specific person at that address Delivery Mechanism: Local PO picks up letters... –The ZIP code breaks the country into regions, regional distribution... content: we send packages and letters – physical things All naming/addressing is on the outside of the envelope (public) Content is on the inside envelope (private)

13. 27-13 Telephone Network Specification/Naming of: –Nodes: Telephone numbers –Paths: Not specified by the user. No priority –Services: “Is John there?” “ For Susan, press 6 now ” Delivery Mechanism: You dial a number and –The area code breaks the country into regions, the prefix breaks regions into smaller distribution regions content: we send audio on a wire Everything is “private” -- unless it's a cell phone call

14. 27-14 The telephone network How do I place a call in the telephone network? –I pick up the phone –I dial your (unique) phone number –Your phone rings How does the call get to you?

15. 27-15 The telephone network Your phone is connected to the central office by a wire In olden days – an operator connected the wire to your phone to the wire to someone else’s phone...

16. 27-16 (switching) The telephone network So a central office has *lots* of wires headed in. - you pick up the phone and ring a bell - the operator asks who you want to talk to - the operator rings the person you want to call - the operator then runs a wire between the two of you How do we place longer distance calls? Central Office

17. 27-17 (switching) The telephone network As the telephone network was modernized … Connections of central offices to connect “long distance” calls telephone numbers – unique ways to identify every phone user. Automated switching to remove the need for operators to manually patch the connections The phone at the ends is stupid! The intelligence is in the network.

18. 27-18 Cable Network Specification/Naming of: –Nodes: Cable boxes? –Paths: Not specified by the user. –Services: Different channels ? Delivery Mechanism: They send you everything, the same everything they send to everyone else! –Your cable box decodes which channel you want to watch content: was audio/video Everything is public (but there's nothing that is private to protect)

19. 27-19 Let’s network computers What we would like: –Naming and Addressing; For computers this might be: The node will be some computer The service will be some program you are communicating with on that computer The path doesn't matter to the user –The content is bits (service specific) –The delivery mechanism? Wires?... It's not that simple.

20. 27-20 Send Locally, Scale Globally We want to build a facility by which we can send bits between a few computers that are connected to some shared wire. –Computers directly connected by a so-called: physical connection –the physical connections between computers create a local network Terminology: –internet: (lower case i) Any network connecting two or more computer networks connected via physical connection (LAN) –Internet: A world-wide network connecting millions of computer networks for the purpose of exchanging data and communications How do we build the Internet? –We build many internets and then connect them to form the Internet

21. 27-21 The Internet Hourglass Model The physical layer is how machines are physically connected to each other... FTP HTTPNVTFTP TCPUDP IP Token Ring Wi-fi (802.11 ) Ethern et InterNetwork Transport Application Physical

22. 27-22 Physical Layer We want to let a small number of computers communicate (and scale up from there) A physical connection between two or more computers (a wire?) Hardware in the computer that will know how to communicate across that wire –Network Interface: a physical device connected to the computer that connects the device to the network (network interface card or NIC) Programs: that use/control the NIC –Programs adhere to a Protocol: An agreement on how communication on this connection should take place

23. 27-23 Physical Layer –Networking at the physical layer, local-area-network or LAN –Our goal is to move bits (1’s and 0’s) between two nodes/computers: A E B C D –We need: All network interfaces on the LAN will be connected to the same wire must agree on a Protocol Each network interface should have a unique “name” (i.e., for each computer) –Must include the ability to label messages with a source address and a destination address