1. Ch 2. Network Models

2. 1. LAYERED TASKS Concept of layers – Consider two friends who communicate through mail – What happens when one sends a letter to the other? – Or when one calls to the other?

3. Tasks Involved in Sending a Letter

4. 2. The OSI Model International Standards Organization (ISO) – Established in 1947 – Multinational body dedicated to worldwide agreement on international standards – An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model – The model was first introduced in the late 1970s

5. OSI 7 Layer Model  OSI model is NOT a protocol. It aims to “show” how to facilitate communications between systems  Layered architecture  Well-defined interface between each pair of adjacent layers provide “modularity” to network system User support Network support Link

6. Interaction between Layers

7. Message Exchange in OSI Model Encapsulations

8. Physical (PHY) Layer Responsible for movements of individual bits from one node to the corresponding other(s) – Physical characteristics of interfaces and medium – Representation of bits (encoding) – Data rate – Synchronization – Line configuration (point-to-point, multipoint) – Topology (mesh, star, ring, bus, …) – Transmission mode (simplex, half-duplex, …) Application Presentation Session Transport Network Data Link Physical

9. Physical (PHY) Layer At sender At receiver

10. Data Link Layer Responsible for moving frames from one node to the corresponding other(s) – Framing (Divides the strep into data units) – Physical addressing (of the sender/receiver) – Flow control (avoid overwhelming) – Error control (detect damaged/lost frames) – Access control Application Presentation Session Transport Network Data Link Physical

11. Data Link Layer At sender At receiver

12. Hop-to-hop delivery PHY and data link layers responsible for one- hop (or hop-to-hop) delivery

13. Network Layer Responsible for the delivery of individual packets from the source node to the destination node(s) (but, it may not guarantee the delivery!) – Logical addressing (of the sender/receiver) – Routing Application Presentation Session Transport Network Data Link Physical

14. Network Layer At sender At receiver

15. Allow End-to-end Delivery

16. Transport Layer Responsible for reliable delivery from one process to another – Service-point (port) addressing – Segmentation and reassembly – Connection control Connectionless, or connection-oriented – End-to-end flow control – End-to-end error control Application Presentation Session Transport Network Data Link Physical

17. Transport Layer At senderAt receiver

18. Processes in a Host Node Process-to-process delivery – Multiplex (mux) and de-multiplex (demux)

19. Session Layer Responsible for dialog control and synchronization Application Presentation Session Transport Network Data Link Physical At sender At receiver

20. Presentation Layer Responsible for translation, compression and encryption Application Presentation Session Transport Network Data Link Physical At senderAt receiver

21. Application Layer Responsible for providing service to user Application Presentation Session Transport Network Data Link Physical At sender At receiver

22. Summary