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OSI Reference Model

OSI Reference Model. Open Systems Interconnection Model. The Layers. (7) Application (6) Presentation (5) Session (4) Transport (3) Network (2) Data Link (1) Physical. Mnemonic. All People Seem To Need Digital Pizza Make your own!. Application Layer. Closest to user

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OSI Reference Model

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  1. OSI Reference Model Open Systems Interconnection Model

  2. The Layers • (7) Application • (6) Presentation • (5) Session • (4) Transport • (3) Network • (2) Data Link • (1) Physical

  3. Mnemonic • All People Seem To Need Digital Pizza • Make your own!

  4. Application Layer • Closest to user • Communicates with user applications and selects appropriate network applications for those user applications • Identifies and establishes availability of application resources in order to synch apps, negotiate error recovery, and provide data integrity. • Ex: Email, WWW, FTP, Gopher, MS Word, Telnet

  5. Presentation Layer • Ensures that information received from the Application layer is readable by the Application layer of the destination peer • Operations such as encryption, decryption, compression, and decompression are part of this layer • Ex: ASCII, EBCDIC, MIDI, MPEG, QuickTime, PICT, TIFF, GIF, JPEG

  6. Session Layer • Establishes, manages, and terminates sessions between applications • A “session” consists of a dialogue between Presentation layers on two or more systems • Ex: Network Filing System (NFS), SQL, RPC

  7. Transport Layer • Segments upper-layer application data • Establishes end-to-end connection • Sends segments from one host to another • Ensures reliable data transport • Key word associated with the Transport layer: segment

  8. Transport Layer Flow Control • Buffering – each system has a certain amount of memory available for buffering information • Multiplexing – consists of interweaving packets from different segments and transmitting them. • Parallelization – if the upper layers require faster service than a channel can provide, the Transport may combine channels

  9. Connection-Oriented • Transport layer involves reliable transport of segments • Sends “acks” for each segment • Retransmits any segments not acknowledged by receiving system • Discard any duplicate segments • Re-orders segments • Provides congestion avoidance & control

  10. Connection vs. Connectionless = Telephone vs. Post Office *Connectionless Protocol Ex: UDP

  11. Transport Layer Examples • TCP, NBP, SPX

  12. Network Layer • Primary purpose is to determine the best path from one network to another and to *route* messages in that direction • The process of routing includes path determination and packet switching • Creates “packets” from segments • Ex: Border Gateway Protocol (BGP), Open Shortest Path First (OSPF, Routing Information Protocol (RIP)

  13. Data Link Layer • Responsible for translating messages into “frames” • Switches & bridges • Divided into two sublayers: • The LLC (Logical Link Control) • The MAC (Media Access Control)

  14. Data Link Layer (LLC Sublayer) • Allows upper layers to perform independently of the LAN/WAN protocol or physical media • Performs flow control for upper-layer protocols • Performs sequences of frames

  15. DLL (Media Access Control) • Maintains the physical address of a device • The physical address is also referred to as the “MAC address” • MAC address is a 48-bit address expressed as 12 hexadecimal digits • First six digits comprise manufacturer • Last six digits assigned *by* the manufacturer • Ex: FF34.2345.12AB

  16. Data Link Layer Examples: • Frame Relay; Link Access Procedure, Balanced (LAPB); Synchronious Data Link Control (SDLC); Point-to-Point Protocol (PPP) • Device: Bridge

  17. Physical Layer • Performs the mechanical and electrical functions of the OSI model • Specifies the conversion of 1s and 0s into an electrical current or pulse of light • Transmission of binary data over a communications network • Ex: Ethernet, FDDI, Token Ring • Device: Repeater

  18. Encapsulation • Protocol stack – starts at Application Layer (PDUs) • Essential to understanding how the OSI model operates • Encapsulation • De-encapsulation

  19. Summary OSI ISO

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