1. Open System Interconnection (OSI) Model

2. Objectives
Explain the OSI reference model, which sets standards for LAN and WAN communications
Discuss communication between OSI stacks when two computers are linked through a network
Apply the OSI model to realistic networking situations

3. OSI Reference Model
Foundation that brings continuity to LAN and WAN communications
Product of two standards organizations:
ISO
ANSI
Developed in 1974
Set of communication guidelines for hardware and software design

4. OSI Guidelines Specify…
How network devices contact each other; how devices using different protocols communicate
How a network device knows when to transmit and not transmit data
How physical network network devices are arranged and connected

5. OSI Guidelines Specify…
Methods to ensure that network transmissions are received correctly
How network devices maintain a consistent rate of data flow
How electronic data is represented on network media

6. OSI Layers

7. OSI Layers Bottom layers Middle layers Uppermost layers
Support for physical connectivity, frame formation, encoding, and signal transmission
Middle layers
Establish and maintain a communication session between two network nodes
Monitor for error conditions
Uppermost layers
Application/software support for encrypting data and assuring interpretation/presentation of data

8. Physical Layer Functions
Provides transfer medium (eg, cable)
Translates data into a transmission signal
Sends signal along the transfer medium
Includes physical layout of network
Monitors for transmission errors
Determines voltage levels for data signal transmissions and to synchronize transmissions
Determines signal type (eg, digital or analog)

9. Analog Signals

10. Digital Signals

11. Data Link Layer Functions
Constructs data frames
Creates CRC information; checks for errors
Retransmits data if there is an error
Initiates communications link; makes sure it is not interrupted (ensures node-to-node physical reliability)
Examines device addresses
Acknowledges receipt of a frame

12. Data Link Layer
Data link frame contains fields consisting of address and control information
Two important sublayers
Logical link control (LLC)
Media access control (MAC)
Connectionless service versus connection-oriented service

13. Network Layer Functions
Determines network path for routing packets
Helps reduce network congestion
Establishes virtual circuits
Routes packets to other networks, resequencing packet transmissions when needed
Translates between protocols

14. Transport Layer Functions
Ensures reliability of packet transmissions
Ensures data is sent and received in the same order
Sends acknowledgement when packet is received
Monitors for packet transmission errors and resends bad packets
Breaks large data units into smaller ones and reconstructs them at the receiving end for networks using different protocols

15. Session Layer Functions
Establishes and maintains communications link
Determines which node transmits at any point in time
Disconnects when communication session is over
Translates node addresses

16. Presentation Layer Functions
Translates data to a format the receiving node understands (eg, from EBCDIC to ASCII)
Performs data encryption
Performs data compression

17. Application Layer Functions
Enables sharing remote drivers and printers
Handles messages
Provides file transfer services
Provides file management services
Provides terminal emulation services

18. Communicating Between Stacks
OSI model provides standards for:
Communicating on a LAN
Communicating between LANs
Internetworking between LANs and WANs and between WANs and WANs

20. Peer Protocols

21. Applying the OSI Model