The document discusses the OSI model, which is a conceptual model that characterizes and standardizes the communication functions of a networking system without regard to their underlying internal structure and technology. It describes the seven layers of the OSI model from physical layer to application layer, and explains the functions of each layer, including physical addressing, framing, routing, error control, and providing services to applications. The layered architecture allows complete interoperability between different systems.

1. DATADATA
COMMUNICATIONCOMMUNICATION
LeCTUre # 6 & 7LeCTUre # 6 & 7

2. reCAp Of LeCTUre 05reCAp Of LeCTUre 05
• Protocols
• Elementsof Protocols
• Standards
• De-Jure
• De-Facto
• Standard organizations
• Internet Standards

3. OvervIew Of LeCTUre 6 & 7OvervIew Of LeCTUre 6 & 7
• TheOSI Model
• Layered task
• Peer-to-Peer communication
• OSI Layers

4. THe OSI MODeLTHe OSI MODeL
• The layered model that dominated data communications and networking
literaturebefore1990 wastheOpen SystemsInterconnection (OSI) model.
• Everyone believed that the OSI model would become the ultimate standard for
datacommunications, but thisdid not happen.
• The TCPIIP protocol suite became the dominant commercial architecture
because it was used and tested extensively in the Internet; the OSI model was
never fully implemented.
• Established in 1947 by International StandardsOrganization (ISO)
• An ISO standard, that covers all aspects of network communications is the
Open SystemsInterconnection (OSI) model.
• It wasfirst introduced in thelate1970s.
• No physical existence, but it is conceptual that helps to understand the
complex methodsof datatransmission among variousdevices.

5. THe OSI MODeL HeLp INTHe OSI MODeL HeLp IN
UNDerSTANDINgUNDerSTANDINg
• How physically computerscommunicateand connect.
• How bitsarepresented in anetwork medium.
• How thisensurethat datawill reach on thetarget error freeand it
will bereceived by correct computer.
• How network devices communicate, and how data is converted
from oneformat to another.
• How computer comes to know that when it should transmit the
dataand when not.
• How network devicesmaintain proper rateof dataflow.

6. LAyereD TASkSLAyereD TASkS
• Sender, Receiver and Carrier

7. Seven layers of the OSI model
LAyereD ArCHITeCTUreLAyereD ArCHITeCTUre

8. wHy LAyerS ?wHy LAyerS ?
• Designer identified which networking functions had related
uses and collected those functions into discrete groups that
becamethelayers.
• The OSI model allows complete interoperability between
otherwiseincompatiblesystems.
• Each layer uses the services of the layer immediately below it
and providesservicesto itsabovelayer.

9. peer-TO-peer prOCeSSeSpeer-TO-peer prOCeSSeS
•Layer x on one machine communicates with layer x on another
machine- called Peer-to-Peer Processes.
• Interfacesbetween Layers
• Each interface defines what information and services a
layer must providefor thelayer aboveit.
• Well defined interfaces and layer functions provide
modularity to anetwork

10. The interaction between layers in the OSI model
peer-TO-peer prOCeSSeS (CONT’D)peer-TO-peer prOCeSSeS (CONT’D)

11. OrganizatiOns Of the layersOrganizatiOns Of the layers
1. Network support layers: Layers1, 2, 3
2. User support layer : Layer 5, 6, 7
• It allows interoperability among unrelated software
systems
1. Transport layer (Layer 4) : links the two subgroups and
ensures that what the lower layers have transmitted is in a
form that theupper layerscan use.

12. An exchange using the OSI model

13. Osi layersOsi layers

14. applicatiOn layerapplicatiOn layer
• It enables the user, whether human or software, to access the
network.
• It providesuser interfacesand support for servicessuch as
• Electronic mail,
• FileTransfer, Accessand management (FTAM),
• Shared databasemanagement,
• Network virtual terminal
• Directory services(X.500)
• and other typesof distributed information services.

15. applicatiOn layerapplicatiOn layer

16. presentatiOn layerpresentatiOn layer
• The presentation layer is concerned with the syntax and
semanticsof theinformation exchanged between two systems.
• Thepresentation layer isresponsiblefor;
• Translation;
• At the sender end it changes the information from its sender-dependent
format into acommon format.
• At the receiving machine it changes the common format into its receiver-
dependent format.
• Encryption; (forprivacy)
• The sender transforms the original information to another form and sends
theresulting messageout over thenetwork.
• Decryption reversestheoriginal processto transform themessageback to its
original form.
• Compression;
• Datacompression reducesthenumber of bitscontained in theinformation.
• Important in thetransmission of multimediasuch astext, audio, and video.

17. presentatiOn layerpresentatiOn layer

18. sessiOn layersessiOn layer
• The session layer is responsible for dialog control and
synchronization.
• Responsiblefor;
1. Session Creation,
2. Session Maintenanceand
3. Synchronization.

19. sessiOn layersessiOn layer
• Allows two applications on different computers to establish,
useand end aconnection called asession.
• Provides synchronization between user tasks by placing
checkpointsin thedatastream.
• Also implements dialog control between communicating
processes, regulating which sidetransmits, when, for how long,
and so on.

20. sessiOn layersessiOn layer

21. TransporT LayerTransporT Layer
 The transport layer is responsible for the delivery
of amessagefrom oneprocess(application) to another.
• This layer ensures that packets are delivered error free, in
sequence, and with no lossesor duplications.
• This layer repackages messages, dividing long messages into
several packetsto betransmitted efficiently over thenetwork.
• At the receiving end, the transport layer unpacks the messages,
resembles the original messages and typically sends an
acknowledgement of receipt.

22. TransporT LayerTransporT Layer
• Serviceport addressing
• Segmentation and reassembly
• Connection control
• Flow control
• Error control

23. TransporT LayerTransporT Layer

24. TransporT LayerTransporT Layer

25. neTwork LayerneTwork Layer
• Responsible for the delivery of individual packets from the
source host to the destination host possibly across multiple
networks.
• Whereas the data link layer oversees the delivery of the packet
between two systemson thesamenetwork
• Responsiblefor;
• Logical addressing and translating it to physical addresses.
• Routing
• RouteDetermination
• Forwarding (packet Switching)
• Packetizing

26. neTwork LayerneTwork Layer

27. neTwork LayerneTwork Layer

28. DaTa Link LayerDaTa Link Layer
•The data link layer is responsible for moving
framesfrom onehop (node) to thenext (immediate).
•On the receiving end, it packages raw bits from the physical layer
into thedataframes.

29. DaTa Link LayerDaTa Link Layer
•Major duties;
• Responsiblefor Node-To-NodeDelivery
• Framing
• Physical addressing (MAC)
• Flow control (Stop & Wait, Sliding window)
• Error control (Stop & Wait ARQ, Sliding window ARQ)
• Accesscontrol (ENQ/ACK, POLL/SELECT)

30. DaTa Link LayerDaTa Link Layer

31. Data Link LayerData Link Layer
• DataLink Layer issub-divided into two sub-layers, i.e.
1. Logical Link Control (LLC);
• Responsible for establishing and maintaining links (i.e. either
logical or physical) between thecommunicating devices.
1. MediaAccessControl (MAC).
• Controls the way in which multiple devices share the same media
channel.

32. Data Link LayerData Link Layer
• Hop-to-Hop (node-to-node) delivery

33. PhysicaL LayerPhysicaL Layer
• Physical layer coordinates the functions required to transmit a
bit stream over aphysical medium.
• It dealswith themechanical and electrical specificationsof the
interfaceand transmission medium.
• It also defines the procedures and functions that physical
devices and interfaces have to perform for transmission to
occur.

34. PhysicaL LayerPhysicaL Layer
• Physical layer isconcerned with thefollowing:
• Physical characteristicsof interfacesand medium
• Representation of bits/ Encoding
• Datarate: transmission rate
• Synchronization of bits
• Lineconfiguration
• Physical topology
• Transmission mode
• baseband and broadband transmissions
• Multiplexing /Demultiplexing

35. PhysicaL LayerPhysicaL Layer

36. summary of Layerssummary of Layers

37. suggesteD reaDingsuggesteD reaDing
• Section
• 2.1,
• 2.2,
• 2.3,
• “DataCommunicationsand Networking” 4th
Edition by Behrouz A. Forouzan