OSI Reference Model

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