1. Computer Networks 0010-ISO-OSI Gergely Windisch windisch.gergely@nik.uni-obuda.hu

2. How can we do effective communication? Take a look at the client-server image from the previous lecture Where should all the communication take place?

3. Protocol hierarchy

5. Layers, Layers everywhere

6. Why OSI? Networks were being developed at different places – universities, companies, research institutions – different technology, different naming conventions – how are they going to talk to each other? ISO – common standard model should be created – Open System Interconnect open means that the details of implementation are openly avaliable – not necessarily free and can also be tied to a vendor – To have a common base – same nomenclature so that people can collaborate

7. OSI – Open System Interconnect 7 layers have been defined – number of layers is arbitrary, implementations do not have to follow Having layers is important – inside is a black box – layer separation is important – layers should be as independent from each other as possible - make sure that they are separated well Layers are service providers

8. OSI model Learn tehese by heart. You’ll thank me later. source: http://www.infocellar.com/networks/osi-model.htm

9. Sidenote: mnemonics to help you remember All People Seem To Need Data Processing Princess Diana Never Tried Screwing Prince Andrew Please Do Not Take Sales People's Advice Philys Did Networking Till She Passed Away American Presidents Should Try New Dating Practices All Penguins Stand Too Near Deep Pools Angus Prefers Sausages To Nibbling Dried Pork A Pathetic Silly Trick Never Does Please People Don't Need This Stuff Presented Anyway Programmers Do Not Throw Sausage Pizza Away Arse Pounders Seem To Neglect Delicious Pussy All Parents Should Teach New Dads Parenting Apply Proper Sense To Network Data Path People Don't Need To Study Protocol Analysis Please Do Not Touch Steve's Pet Alligator Phil Donahue Never Televises Sick People Anymore Please Dear, Not Tonight, Soft Penis Already All People Studying This Need Drastic Psychotherapy People Design Networks To Send Packets Accurately Australian Post Sucks They Never Deliver Parcels Philosophers Deign Not To Solipstically Ponder Anachronisms Please Don't Network These Stupid People Again Any Person Standing Totally Nude Doesn't Perspire All Pussy Seems To Need Douching Periodically

10. Layer 1 - Physical Transfers bits to the other side Access medium is handled on the Physical layer – light – radio signals – electrical impulse Physical layer is not reliable, bits get lost, duplicated, corrupted

11. Layer 2 – Data link Uses the services of the dumb phyisical layer to send bits Provides the service of sending large chunks of bits to the other side reliably – creates frames, adds synchronizing bits to the beginning of the frame and crc to the end – CRC is used for error detection – Error correction has been used earlier, but physical layers are too reliable now – error correction became extinct 1 wrong bit in 1 billion

12. Layer 3 – Network Receives packets and sends them to any host in the world* Interesting questions that might arise – how do we know who we want to send it to? – how the hell do we get there? *: For a given value of any

13. Layer 3 – Network Receives packets and sends them to any host in the world* Interesting questions that might arise – how do we know who we want to send it to? – how the hell do we get there? Interesting answers to the interesting questions – logical addressing (IP) – routing protocols Routing protocols help find the destination – one hop at a time – how many hops does it take to get from this room to a computer lab in china? *: For a given value of any

14. Layer 4 – Transport Layer 3 can transport individual packets, but we are not in the business of transferring packets – we want to transfer data – documents, music files (only legal of course), cute pictures of cats, video files of our favorite Britney Spears concert and so on. The transport layer takes the data apart, creates datagrams and gives them to Layer 3 for transportation. Transport layer helps for the data to get to the given application – ports TCP and UDP are the most important protocols

15. Layer 5 – Session Session layer handles the connection properties between two computers – user names, passwords, whatnot

16. Layer 6 – Presentation Data conversion – byte order for example, code pages – encoding if necessary (session layer handles key excange, presentation layer uses them)

17. Layer 7 – Application The actual program that the user uses There are an infinite amount of application protocols – ftp, http, smtp and many others

18. Encapsulation source: http://www.infocellar.com/networks/osi-model.htm