1. Open System Interconnection

2. ISO – ORG FOR STANDARDISATION
The International Organisation for Standardisation (ISO) is an International standards organisation responsible for a wide range of standards, including many that are relevant to networking
In 1984 in order to aid network interconnection without necessarily requiring complete redesign, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture

3. OSI Model
An ISO standard that covers all aspects of Network Communication is the Open System Interconnection
An open system allows two different systems to communicate with each other regardless of their underlying architecture
ISO is an Organization
OSI is a model

4. THE NEED FOR STANDARDS
Previously Networks used different Hardware & Software implementations
Resulting in incompatibilities
With different specifications nodes were unable to communicate with each other
To address problem ISO researched various network schemes
Need to create a NETWORK MODEL
Help vendors create interoperable network implementations

5. THE OSI REFERENCE MODEL
OSI is short for Open System Interconnection
Task : Moving info between computers over a network
Divide the task into SEVEN smaller / more manageable tasks
Separation into smaller more manageable functions is known as layering
WHAT needs to be done to send data from one computer to another and not HOW it should be done

6. OSI Model

7. A LAYERED NETWORK MODEL
Each of 7 layers has particular Network functions
Each layer provides a service to the layer above it
Each layer correspond with the layers above and below it and also the Peer layer.
Through agreed upon rules called Protocols
Peer to Peer process
Each layer add info to message it receives from layer above it called Header & Trailer (Data link layer only)
By breaking up tasks into layers reduces complexity
Enhance professionalism

8. Organization of Layers
Lower 3 layers are Network Support Layers
Concerned with flow of data from end to end through Network
Combination of HW & SW
Physical layer always implemented in Hardware
Upper 3 layers are User Support Layers
Always implemented in Software.
Layer 4 ensures end to end reliable data transmission
Layer 2 ensures reliable transmission on a single link
Layers glued together by interfaces
Each interface defines what info & services it must provide for the above layer
Headers & Trailers are added as data moves from Application to Physical and removed vice versa
Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.

9. Only last 3 layers involved as data moved through Intermediate nodes
OSI Layers
Only last 3 layers involved as data moved through Intermediate nodes

10. Exchange Using the OSI Model Header & Trailer being added & removed

11. LAYER 7: APPLICATION
The application layer is responsible for providing services to the user
Closest to the user and provides user interface
Establishes the availability of intended communication partners
Does not provide services to any other OSI layer
Examples: Spreadsheet programs, word processing programs, electronic mail, remote file access & transfer and share database management and other distributed information services
Examples of Application layer protocols are: Telnet, SMTP, FTP, SNMP

12. LAYER 6: PRESENTATION
Presentation layer is concerned with the syntax and semantics of the information exchanged between two systems
This layer is primarily responsible for the translation, encryption and compression of data
It ensures that the information that the application layer of one system sends out is readable by the application layer of another system. Defines coding and conversion functions
This layer also manages security issues by providing services such as data encryption and data compression
Examples of these formats and schemes are: MPEG, QuickTime, ASCII, EBCDIC, GIF, TIFF, JPEG

13. LAYER 5: SESSION
The session layer defines how to start, control and end conversations (called sessions) between applications
Establishes dialog control between the two computers in a session, regulating which side transmits, plus when and how long it transmits (Full duplex)
Synchronization: Allows processes to add check points. E.g. Insert check point at every 100 page of 2000 page file to ensure that each 100-page unit is received & acknowledged
Transmits Data

14. LAYER 4: TRANSPORT
It regulates information flow to ensure process-to- process connectivity between host applications reliably and accurately
Adds service point address or Port address
Segmentation & Re-assembly: SEGMENTS data from sending node and reassembles data on receiving node
Flow control / Error control at Source to destination level
Connection oriented transport service ensures that data is delivered error free, in sequence with no losses or duplications
Establishes, maintains and terminates virtual circuits
Connection oriented / Connectionless: TCP (Reliable, provides guaranteed delivery), UDP (Unreliable, less overhead, reliability can be provided by the Application layer)
Provides multiplexing; the support of different flows of data to different applications on the same host

15. LAYER 3: NETWORK
Defines source to destination delivery of packets across NWs
Defines logical addressing
No need if 2 systems on same Link
Treat each packet independently
Defines how routing works and how routes are learned
Converts frames to packets
Routed protocols ( encapsulate data into packets) and Routing protocols (create routing tables) work on this layer
Examples of Routed protocols are: IP, IPX, AppleTalk and Routing protocols are OSPF, IGRP/EIGRP, RIP, BGP
Relieves higher layers from knowing about underlying data transmission and switching technologies used to interconnect systems
May use packet switched / circuit switched networks
Routers operate at Layer 3.

16. LAYER 2: DATA LINK
Packages raw bits from the physical layer into FRAMES
The data link layer provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses
Source & Destination ( address of device that connects one Network to next) address
Flow Control: Prevent overwhelming of Receiving Node
Error Control: Through Trailer
Access Control: Which device to have control
Data Link LAN specifications: Fast Ethernet, Token Ring, FDDI
Data Link WAN specifications are: Frame Relay, PPP, X.25
Bridges and Switches operate at this layer

17. LAYER 1: PHYSICAL
Deals with the physical characteristics of the Transmission medium
Transmits bits from one computer to another
Electrical: Voltage levels / timing of voltage changes
Mechanical: Physical connectors (DB-25)
Procedural: Sequence of events by which bit stream is exchanged across Physical medium
Data rates / Maximum transmission distances
Physical Topology
Synchronization of bits: Tx & Rx sync
Line configuration: Pt –to –Pt / Multi Point
Transmission modes & Repeaters operate at this layer

18. Data Encapsulation
Data Encapsulation is the process of adding a header to wrap the data that flows down the OSI model.
The 5 Steps of Data Encapsulation are:
1. The Application, Presentation and Session layers create DATA from users' input.
2. The Transport layer converts the DATA to SEGMENTS
3. The NW layer converts the Segments to Packets (datagram)
4. The Data Link layer converts the PACKETS to FRAMES
5. The Physical layer converts the FRAMES to BITS.

19. Important Data-Link: Communication between nodes on same network
Network: Comm between nodes on possibly different NWs
Transport: Communication between processes (running on machines on possibly different networks)
Connecting Networks Devices
Repeater: Physical layer
Bridge: Data link layer
Router: Network layer
Gateway: Transport layer and above

20. SUMMARY No standard for networks in the early days
Difficult for Networks to communicate with each other
ISO recognised this and researched various network schemes, and in 1984 introduced OSI reference model
The OSI ensure greater compatibility and interoperability between various types of network technologies
Organizes network functions into seven numbered layers
Each layer provides a service to the layer above it and communicates with the same layer on other computers
Layers 1-4 are concerned with the flow of data from end to end through the network and Layers 5-7 are concerned with services to the applications