1. The OSI 7 Layer Model

2. What is a model? A model is… simplified representation or description
simplified framework designed to illuminate complex processes
exemplar: something to be imitated

3. What is the OSI 7 Layer Model?
Before OSI model computer systems had different incompatible networking software, so only computers made by the same company could communicate.
OSI – Open System Interconnect reference model in use and being developed since 1977
Allows computers of different types with different hardware, running different software, to communicate - Windows NT, 2000 and XP are all examples designed according to these standard models

4. Why have layered model? Network software is highly complex.
Rather than create one enormous software program the functions required are spilt up into different layers.
Makes writing, updating and debugging complex software easier if it is split into layers.
Makes networking easier to learn

5. What are the 7 layers?

6. Encryption/decryption
OSI 7 layer model 7 6 5 4 3 2 1
Application
Presentation
Session
Transport
Network
Datalink
Physical
POP3, FTP
Encryption/decryption
E-commerce
TCP, SPX
IP, IPX, Router
Ethernet, Switch
10BaseT,Hub
Layer No.
Name
Example
Software
Hardware

7. OSI Layers Upper (application) layers Lower (physical) layers
7 Application -> network services
6 Presentation -> common data format
5 Session -> start/stop sessions
Lower (physical) layers
4 Transport -> Error correction
3 Network -> Logical addressing
2 Data Link -> MAC, frames
1 Physical -> bits, voltages

8. Data encapsulation

9. Data encapsulation
As the data passes down through the layers it is broken down into smaller “chunks”
Extra information is added to each chunk to identify it and allow them to be reassembled at the other end
This extra information is wrapped round the existing data by the sender as it is sent then stripped off again by the receiver
Look at the “James Bond” scenario for an example

10. Extra detail on each of the 7 layers
Layer Detail
Extra detail on each of the 7 layers

11. Application layer (layer 7)
Represents the level at which applications access network services.
Represents the services that directly support applications such as software for file transfers, database access, and electronic mail.

12. Presentation layer (layer 6)
Translates data from the Application layer into an intermediary format.
Deals with the way data is represented.
Formats and structures data so it can be read - compresses data so that fewer bits need to be transferred on the network.
Manages security issues by providing services such as data encryption

13. Session (layer 5)
Establishes, manages and terminates sessions between applications on different computers.
A session is a bit like a telephone conversation. It establishes the rules of conversation between the two network applications.
Establishes dialog control between the two computers in a session, regulating which side transmits, plus when and how long it transmits.

14. Transport (layer 4) Provides delivery services for the session.
Deals with fault detection and recovery.
Flow Control Aspects -
repackages long messages when necessary into small packets for transmission and, at the receiving end, rebuilds packets into the original message.
The receiving Transport layer also sends receipt acknowledgments.

15. Network (layer 3)
Translates logical addresses and names into physical address recognition.
Selects the best path between two systems.
Determines the route from the source to the destination computer and manages traffic problems, such as switching, routing, and controlling the congestion of data packets.

16. Data Link (layer 2) Defines how data is organised into frames.
Packages raw bits from the Physical layer into frames (logical, structured packets for data).
Deals with physical addressing
Responsible for transferring frames from one computer to another, without errors.
After sending a frame, it waits for an acknowledgment from the receiving computer.

17. Physical (layer 1)
Deals with how data is converted (encoded) onto the physical media (e.g. cable, radio waves etc.).
Defines physical aspects of the media, such a connectors and cable types.