OSI Layers

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This study guide is intended to provide those pursuing the CCNA certification with a framework of what concepts need to be studied. This is not a comprehensive document containing all the secrets of the CCNA, nor is it a “braindump” of questions and answers.

I sincerely hope that this document provides some assistance and clarity in your studies.

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  • OSI Layers

    1. 1. 1
    2. 2. OSI Model • OSI means Open System Interconnect model. • Developed by the International Organization for Standardization in 1974. • It consists of seven layers. • Each layer has a different but specific processing function. 2
    3. 3. OSI Model Layers Layer - 7 Application Application Layer - 6 Presentation Presentation Layer - 5 Session Session Layer - 4 Transport Transport Layer - 3 Network Network Layer - 2 Data Link Data Link Layer - 1 Physical Physical Upper Layer or Software Layer Heart of OSI Lower Layer or Hardware Layer 3
    4. 4. Application Layer Application Application Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Application Layer is responsible for providing Networking Services to the user. It is also known as Desktop Layer. Identification of Services is done using Port Numbers. Ports are Entry and Exit Points to the Layer Total No. Ports 0 – 65535 Reserved Ports 0 – 1023 Open Client Ports 1024 – 65535 4
    5. 5. Example of HTTP request http://www.zoomgroup.com Client Web Server 5
    6. 6. Example of HTTP request HTTP HTTP Reply Received Request http:// www.zoomgroup.com http://www.zoomgroup.com Webpage Client HTTP Request Listen on Port 80 Sending HTTP Reply Webpage Web Server 6
    7. 7. Example of FTP request ftp://ftp.microsoft.com Client FTP Server 7
    8. 8. Example of FTP request FTP Request Received FTP Reply ftp://ftp.microsoft.com ftp://ftp.microsoft.com Client FTP Request Listen on Port 21 Sending FTP Reply FTP Server 8
    9. 9. Examples of Networking Services Service Port No. HTTP 80 FTP 21 SMTP 25 TELNET 23 TFTP 69 9
    10. 10. Data flow from Application Layer Application Application 80 21 25 Data 53 67 69 Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical 10
    11. 11. Presentation Layer Application Application Presentation Presentation Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Presentation Layer is responsible for converting data into standard format. Examples : ASCII, EBCDIC, JPEG, MPEG, BMP, MIDI, WAV, MP3 Following tasks are perform at Presentation layer : Encoding – Decoding Encryption – Decryption Compression – Decompression 11
    12. 12. Data flow from Presentation Layer Application Application Data Presentation Presentation Data Session Session Transport Transport Network Network Data Link Data Link Physical Physical 12
    13. 13. Session Layer Application Application Presentation Presentation Session Session Session Transport Transport Network Network Data Link Data Link Physical Physical Session Layer is responsible for establishing, maintaining and terminating session. Session ID works at Session Layer. Examples : RPC  Remote Procedure Call SQL  Structured Query Language NFS  Network File System 13
    14. 14. Data flow from Session Layer Application Application Data Presentation Presentation Data Session Session Data Transport Transport Network Network Data Link Data Link Physical Physical 14
    15. 15. Transport Layer Application Application Presentation Presentation Session Session Transport Layer is responsible for end-to-end connectivity. It is also known as the heart of OSI Layers. Following tasks are performed at the Transport Layer : - Transport Transport Transport Transport • Identifying Service Network Network • Multiplexing & De-multiplexing Data Link Data Link Physical Physical • Segmentation • Sequencing & Reassembling • Error Correction • Flow Control 15
    16. 16. Identifying Service TCP • Transmission Control Protocol UDP • User Datagram Protocol • Connection Oriented • Connection Less • Acknowledgement • No Acknowledgement • Reliable • Unreliable • Slower • Faster • Port No. 6 • Port No. 17 • e.g. HTTP, FTP, SMTP • e.g. DNS, DHCP, TFTP 16
    17. 17. Multiplexing & De-multiplexing Application Application Presentation Presentation Session Session 80 21 25 53 67 69 Transport Transport TCP - 6 UDP - 17 Network Network Data Link Data Link Physical Physical 17
    18. 18. Segmentation Hello! How are you ? Hello! How are you ?Hello! How Are You ? A B 18
    19. 19. Sequencing & Reassembling you Hello! Hello! HowHow ? Are You Hello! are you ? Hello! How are How ? you ? You Hello! How ? Are Hello! How Are You ? are A B 19
    20. 20. Sequencing & Reassembling Hello! Hello! How are you ? Hello! How are How are you ? Hello! How Are You ? you ? 1/5 You 4/5 A 2/5 3/5 Hello! How 1/5 2/5 4/5 ? 5/5 5/5 Are 3/5 B 20
    21. 21. Error Correction Segment Missing Hello! Hello! How are you ? Hello! How are How are you ? Hello! How Are You ? you ? 1/5 You 4/5 A 2/5 3/5 Hello! How 1/5 2/5 4/5 ? 5/5 5/5 Are 3/5 B 21
    22. 22. Flow Control - Windowing Ack-RecAck. Sending 3 Received3-window Sending 4 5-window A PC-A PC-A Received can send 4 Segment can send 4 Segment 3-window 4-window at a time to at a time to PC-B PC-B B 22
    23. 23. Data flow from Transport Layer Application Application Data Presentation Presentation Data Session Session Data Transport Transport TH Segment Data Network Network Data Link Data Link Physical Physical 23
    24. 24. Network Layer Application Application Presentation Presentation Session Session Transport Transport Network Network Network Network Data Link Data Link Physical Physical Network Layer is responsible for providing best path for data to reach the destination. Logical Addressing works on this layer. Router is a Network Layer device. It is divided into two parts • Routed Protocols e.g. IP, IPX, Apple Talk. • Routing Protocols e.g. RIP, IGRP, OSPF, EIGRP 24
    25. 25. Routed Protocols Segment Hello! How are you ? Source IP 192.168.1.1 Hello! How are you ? Destination IP Segment 192.168.1.2 A B 192.168.1.1 192.168.1.2 25
    26. 26. Routing Protocols www.zoomgroup.com A 26
    27. 27. Data flow from Network Layer Application Application Presentation Presentation Data Session Session e.g. Router Data Data Transport Transport Segment Network Network Packet NH Segment Data Link Data Link Physical Physical 27
    28. 28. Datalink Layer Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Data Link Data Link Physical Physical Datalink Layer is divided into two Sub Layers : • LLC – Logical Link Control It talks about Wan protocols e.g. PPP, HDLC, Frame-relay • MAC – Media Access Control It talks about Physical Address. It is a 48 bit address i.e. 12 digit Hexadecimal Number. It is also responsible for Error Detection Devices working on Data Link Layer are Switch, Bridge, NIC. 28
    29. 29. Error Detection – CRC Check Packet Hello! How are you ? A PC-A PC-A CRC No. 33333 CRC No. 33333 PC-B PC-B Source MAC 00-20-18-C0-07-71 CRC No. 11114 CRC Detected Error No. 11114 33333 11114 46323 99434 43434 99323 55434 89696 22222 32434 00000 Destination MAC Packet Error Detected 00-20-18-C0-07-72 Error Detected For Error Correction For Error Correction Contact Source Transport layer Contact Source Transport layer 192.168.1.1 00-20-18-C0-07-71 B 192.168.1.2 00-20-18-C0-07-72 29
    30. 30. Error Detection – CRC Check Packet Source MAC 00-20-18-C0-07-71 Hello! How are you ? 20202 11114 46323 99434 43434 99323 55434 89696 22222 32434 00000 Destination MAC Packet 00-20-18-C0-07-72 A B 192.168.1.1 00-20-18-C0-07-71 192.168.1.2 00-20-18-C0-07-72 30
    31. 31. Data flow from Data Link Layer Application Application Data Presentation Presentation Data Session Session Data Transport Transport e.g. Switch Segment Network Network Packet Data Link Data Link Packet DH Frame DT Physical Physical 31
    32. 32. Physical Layer Application Application Presentation Presentation Session Session Transport Transport Physical Layer is responsible for electrical, mechanical and procedural checks. Data will be converted into Binary (i.e) 0’s & 1’s. Data will be in the form of electrical pulses if it is Coaxial or Twisted Pair cable and in the form of Light if it is Fiber Optic Cable. Network Network Data Link Data Link Physical Physical Physical Physical Devices working at Physical Layer are Hubs, Repeaters, Cables, Modems etc. 32
    33. 33. Physical Layer Example Frame Hello! How are you ? 1010101010101010 Frame A 0101011010101010101010101 1010101101010101010101010 0101011010101010101010101 B 33
    34. 34. Data flow from Physical Layer Application Application Data Presentation Presentation Data Session Session Data Transport Transport Network Network e.g. Hub Segment Packet Data Link Data Link Frame Physical Physical Bits 34
    35. 35. Data Encapsulation & De-capsulation A B Application Application Data Data Application Application Presentation Presentation Data Data Presentation Presentation Session Session Data Data Session Session Transport Transport TH Data Segment Segment Data TH TH Transport Transport Network Network Packet NH Segment Packet NH Segment Network Network NH Data Link Data Link Packet DHFrameDT Packet Packet DTFrameDH DH Data Link Data LinkDT Physical Physical Bits Bits Physical Physical 35
    36. 36. Comparing OSI with TCP/IP Layers OSI Layers TCP/IP Layers Application Application Presentation Presentation Application Application Session Session Transport Transport Network Network Data Link Data Link Physical Physical Transport Transport Internet Internet Network Network Access Access 36
    37. 37. 37
    38. 38. 38
    39. 39. Example of HTTP request http://www.zoomgroup.com 39
    40. 40. ftp://ftp.microsoft.com 40
    41. 41. Microsoft Windows 2000 [Version 5.00.2195] (C) Copyright 1985-2000 Microsoft Corp. C:> telnet 192.168.1.150 Connecting ..... ================================ Welcome to Hyderabad Router ================================ User Access Verification password : 41
    42. 42. http://www.microsoft.com 42
    43. 43. http://mail.yahoo.com xyz@yahoo.com xyz ****** 43

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