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OSI Reference Model

OSI Reference Model. Agenda. The Layered Model Layers 1 & 2: Physical & Data Link Layers Layer 3: Network Layer Layers 4–7: Transport, Session, Presentation, and Application Layers. The Layered Model. Layered Communication. Location A. I like rabbits. Message.

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OSI Reference Model

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  1. OSI Reference Model

  2. Agenda • The Layered Model • Layers 1 & 2: Physical & Data Link Layers • Layer 3: Network Layer • Layers 4–7: Transport, Session, Presentation, and Application Layers

  3. The Layered Model

  4. Layered Communication Location A I like rabbits Message Information for the Remote Translator L: Dutch Ik hou van konijnen Fax #:--- L: Dutch Ik hou van konijnen Information for the Remote Secretary Source: Tanenbaum, 1996

  5. Layered Communication Location B Location A J’aime les lapins I like rabbits Message Information for the Remote Translator L: Dutch Ik hou van konijnen L: Dutch Ik hou van konijnen Fax #:--- L: Dutch Ik hou van konijnen Fax #:--- L: Dutch Ik hou van konijnen Information for the Remote Secretary

  6. Layered Communication Location B Location A Layers J’aime les lapins I like rabbits 3 Message Information for the remote translator L: Dutch Ik hou van konijnen L: Dutch Ik hou van konijnen 2 Fax #:--- L: Dutch Ik hou van konijnen Fax #:--- L: Dutch Ik hou van konijnen Information for the remote secretary 1

  7. Why a Layered Network Model? 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical • Reduces complexity (one big problem to seven smaller ones) • Standardizes interfaces • Facilitates modular engineering • Assures interoperable technology • Accelerates evolution • Simplifies teaching and learning

  8. Devices Function at Layers 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical NIC Card

  9. Host & Media Layers } 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical Host layers:Provide accurate data delivery between computers } Media layers:Control physical delivery of messages over the network

  10. Layer Functions Application 7 Provides network services to application processes (such as electronic mail, file transfer, and terminal emulation)

  11. Layer Functions Application 7 Network services to applications Presentation 6 Data representation • Ensures data is readable by receiving system • Format of data • Data structures • Negotiates data transfer syntax for application layer

  12. Layer Functions Application 7 Network services to applications Presentation 6 Data representation Session 5 • Inter-host communication • Establishes, manages, and terminates sessions between applications

  13. Layer Functions Application 7 Network services to applications Presentation 6 Data representation Session 5 Inter-host communication 4 Transport • End-to-end connection reliability • Concerned with data transport issues between hosts • Data transport reliability • Establishes, maintains, and terminates virtual circuits • Fault detection and recovery • Information flow control

  14. Layer Functions Application 7 Network services to applications Presentation 6 Data representation Session 5 Inter-host communication 4 Transport End-to-end connection reliability 3 Network • Addresses and best path • Provides connectivity and path selection between two end systems • Domain of routing

  15. Layer Functions Application 7 Network services to applications Presentation 6 Data representation Session 5 Inter-host communication 4 Transport End-to-end connection reliability 3 Network Addresses and best path 2 Data Link • Access to media • Provides reliable transfer of data across media • Physical addressing, network topology, error notification, flow control

  16. Layer Functions Application 7 Network services to applications Presentation 6 Data representation Session 5 Inter-host communication 4 Transport End-to-end connection reliability 3 Network Addresses and best path 2 Data Link Access to media 1 Physical • Binary transmission • Wires, connectors, voltages, data rates

  17. Peer-to-Peer Communications Host A Host B Application Presentation Session Transport Network Data Link Physical 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical Segments Packets Frames Bits

  18. Data Encapsulation } { Host A Host B Application Application Presentation Presentation Data Session Session Transport Transport Network Data Header Network Network Frame Frame Network Data Data Link Data Link Header Header Trailer Physical Physical 0101101010110001

  19. Layers 1 & 2: Physical & Data Link Layers

  20. 0000.0c12.3456 Physical and Logical Addressing

  21. MAC Address 24 bits 24 bits Serial Number Vendor Code 0000.0c12. 3456 ROM RAM • MAC address is burned into ROM on a network interface card

  22. Layer 3: Network Layer

  23. Network Layer: Path Determination Which Path? Which Path? • Layer 3 functions to find the best path through the internetwork

  24. Network Layer: Communicate Path • Addresses represent the path of media connections • Routing helps contain broadcasts 5 2 9 6 8 4 10 11 1 3 7

  25. Addressing—Network and Node Network Node 1 1 2.1 2 3 1.2 3.1 2 1 1.1 1.3 1 3 • Network address—Path part used by the router • Node address—Specific port or device on the network

  26. Protocol Addressing Variations Network Node General Example 1 1 Network Host TCP/IP Example 10. 8.2.48 (Mask 255.0.0.0) Network Node Novell IPX Example 1aceb0b. 0000.0c00.6e25

  27. X Y C C A A Network Layer Protocol Operations • Each router provides its services to support upper layer functions

  28. X Y C C A A Application Session Transport Network Data Link Physical Network Layer Protocol Operations • Each router provides its services to support upper layer functions B B Host Y Host X Application Presentation Presentation Router B Router A Router C Session Transport Network Network Network Network Data Link Data Link Data Link Data Link Physical Physical Physical Physical

  29. Routed Versus Routing Protocol • Routed protocol used between routers to direct user traffic Examples: IP, IPX, AppleTalk, DECnet Network Destination Exit Port Protocol Network to Use Protocol Name 1.0 1.1 2.0 2.1 3.0 3.1

  30. Routed Versus Routing Protocol • Routed protocol used between routers to direct user traffic Examples: IP, IPX, AppleTalk, DECnet • Routing protocol used only between routers to maintain routing tables Examples: RIP, IGRP, OSPF

  31. Static Versus Dynamic Routes Static Route Uses a protocol route that a network administrator enters into the router Dynamic Route Uses a route that a network protocol adjusts automatically for topology or traffic changes

  32. Static Route Example • Fixed route to address reflects administrator’s knowledge Point-to-point or A A circuit-switched connection Only a single network connection with no need B B for routing updates “Stub” network

  33. B A B A D C D C Adapting to Topology Change • Can an alternate route substitute for a failed route?

  34. B A B A D C D C Adapting to Topology Change • Can an alternate route substitute for a failed route? Yes—With dynamic routing enabled X X

  35. Net 2, Host 5 LAN-to-LAN Routing Example Network 2 Network 3 Host 4 Host 5 Token E1 Ring Network 1 To0 E0 802.3 Routing Table Destination Outgoing Network Interface 1 E0 2 To0 3 E1

  36. Net 2, Host 5 LAN-to-LAN Routing From LAN to LAN Network 2 Network 3 Host 4 Host 5 Token E1 Ring Network 1 To0 E0 Net 2, Host 5 802.5 802.3 Routing Table Destination Outgoing Network Interface 1 E0 2 To0 3 E1

  37. Data 1.3 2.4 Data 1.3 2.4 Data 1.3 2.4 Data Data LAN-to-WAN Routing 1.3 2.4 Data From 1.3 LAN Token Token Ring Ring 1.3 2.4 Data A A To Frame Relay WAN Frame Relay 1.3 2.4 Data B B 2.4 To Ethernet LAN 1.3 2.4 Data

  38. Layers 4–7: Transport, Session, Presentation, and Application Layers

  39. Transport Layer • Segments upper-layer applications • Establishes an end-to-end connection • Sends segments from one end host to another • Optionally, ensures data reliability

  40. Transport Layer— Segments Upper-Layer Applications Application Electronic File Terminal Mail Transfer Session Presentation Session Application Application Transport Data Data Port Port Segments

  41. Transport Layer— Establishes Connection Sender Receiver Synchronize Negotiate Connection Synchronize Acknowledge Connection Established Data Transfer (Send Segments)

  42. Transport Layer— Sends Segments with Flow Control Transmit Sender Receiver Buffer Full Not Ready Stop Process Segments Ready Go Buffer OK Resume Transmission

  43. Transport Layer— Reliability with Windowing • Window Size = 1 Send 1 Receive 1 Ack 2 Receive 2 Sender Receiver Send 2 Ack 3 • Window Size = 3 Send 1 Receive 1 Send 2 Receive 2 Receive 3 Sender Send 3 Receiver Ack 4 Send 4

  44. Transport Layer— An Acknowledgement Technique Sender Receiver 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Send 1 Send 2 Send 3 Ack 4 Send 4 Send 5 Send 6 Ack 5 Send 5 Ack 7

  45. Transport to Network Layer End-to-End Segments Routed Packets

  46. Session Layer • Network File System (NFS) • Structured Query Language (SQL) • Remote-Procedure Call (RPC) • X Window System • AppleTalk Session Protocol (ASP) • DEC Session Control Protocol (SCP) • Coordinates applications as they interact on different hosts Service Request Service Reply

  47. login: Presentation Layer Graphics • Text • • Provides code formatting and conversion for applications Visual images • Data • PICT ASCII TIFF EBCDIC JPEG Encrypted • Sound GIF MIDI • Video MPEG QuickTime

  48. Application Layer COMPUTER APPLICATIONS NETWORK APPLICATIONS INTERNETWORK APPLICATIONS Word Processor Presentation Graphics Spreadsheet Database Design/Manufacturing Project Planning Others Electronic Mail File Transfer Remote Access Client-Server Process Information Location Network Management Others Electronic Data Interchange World Wide Web E-Mail Gateways Special-Interest Bulletin Boards Financial Transaction Services Internet Navigation Utilities Conferencing (Voice, Video, Data) Others

  49. Summary • OSI reference model describes building blocks of functions for program-to-program communications between similar or dissimilar hosts • Layers 4–7 (host layers) provide accurate data delivery between computers • Layers 1–3 (media layers) control physical delivery of data over the network

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