The document discusses the OSI model, which outlines the steps needed to send data from one computer to another. It describes the seven layers of the OSI model and what each layer is responsible for. The physical layer deals with physical components like cabling and data encoding. The data link layer places and retrieves data from the physical layer and provides error detection. The network layer provides addressing and routing between networks. The transport layer provides reliable data delivery. The session layer allows applications to maintain ongoing sessions. The presentation layer prepares data to be processed. The application layer gives access to network resources. Data moves down the layers at the source and up at the destination, with each layer adding its own header.

1. CS101CS101
BY:-GULSHAN KUMAR MAHESHWARIBY:-GULSHAN KUMAR MAHESHWARI
QAU_ISLAMABADQAU_ISLAMABAD

2. THE OSI MODELTHE OSI MODEL

3. THE OSI MODELTHE OSI MODEL
• OSI “ Open Systems Interconnection".OSI “ Open Systems Interconnection".
• OSI model was first introduced in 1984 by the InternationalOSI model was first introduced in 1984 by the International
Organization for Standardization (ISO).Organization for Standardization (ISO).
• OutlinesOutlines WHATWHAT needs to be done to send data from oneneeds to be done to send data from one
computer to another.computer to another.
• NotNot HOWHOW it should be done.it should be done.
• Protocols stacks handle how data is prepared for transmittal (toProtocols stacks handle how data is prepared for transmittal (to
be transmitted)be transmitted)
• In the OSI model, The specification neededIn the OSI model, The specification needed
• are contained in 7 different layers that interact with each other.are contained in 7 different layers that interact with each other.

4. WHAT IS “THE MODEL?”WHAT IS “THE MODEL?”
• Commonly referred to as the OSI reference model.Commonly referred to as the OSI reference model.
• The OSI modelThe OSI model
• is a theoretical blueprint that helps us understand how datais a theoretical blueprint that helps us understand how data
gets from one user’s computer to another.gets from one user’s computer to another.
• It is also a model that helps develop standards so that all of ourIt is also a model that helps develop standards so that all of our
hardware and software talks nicely to each other.hardware and software talks nicely to each other.
• It aids standardization of networking technologies byIt aids standardization of networking technologies by
providing an organized structure for hardware and softwareproviding an organized structure for hardware and software
developers to follow, to insure there products are compatibledevelopers to follow, to insure there products are compatible
with current and future technologies.with current and future technologies.

5. 7-Layers
• The Seven Layer OSI Model - Addition of Management and Security
– Standardizing notation or syntax for application messages (abstract syntax)
– Set of encoding rules (transfer syntax)
– Became the Presentation Layer
Presentation
Transport
Network
Physical
Data-Link
Session
Application
Presentation
Transport
Network
Physical
Data-Link
Session
Application
1

6. WHAT EACH LAYER DOESWHAT EACH LAYER DOES
2

7. PHYSICAL LAYERPHYSICAL LAYER
• Determines the specs forDetermines the specs for
all physical componentsall physical components
• CablingCabling
• Interconnect methodsInterconnect methods
(topology / devices)(topology / devices)
• Data encoding (bits to waves)Data encoding (bits to waves)
• Electrical propertiesElectrical properties
• Examples:Examples:
• Ethernet (IEEE 802.3)Ethernet (IEEE 802.3)
• Token Ring (IEEE 802.5)Token Ring (IEEE 802.5)
• Wireless (IEEE 802.11b)Wireless (IEEE 802.11b)
3

8. Physical layer
The physical layer is responsible
for the movement of individual bits
from one hop (node) to the next.

9. PHYSICAL LAYER (CONT’D0)PHYSICAL LAYER (CONT’D0)
• What are the Physical Layer components on myWhat are the Physical Layer components on my
computer?computer?
• NICNIC
• Network Interface CardNetwork Interface Card
• Has a unique 12 character Hexadecimal numberHas a unique 12 character Hexadecimal number
permanently burned into it at the manufacturer.permanently burned into it at the manufacturer.
• The number is the MAC Address/Physical address of aThe number is the MAC Address/Physical address of a
computercomputer
• CablingCabling
•

10. DATA LINK LAYERDATA LINK LAYER
• Places data and retrievesPlaces data and retrieves
it from the physical layerit from the physical layer
and provides errorand provides error
detection capabilitiesdetection capabilities
3

11. Data link layer
The data link layer is responsible for
moving frames from one hop (node) to
the next.

12. NETWORK LAYERNETWORK LAYER
• Provides network-wideProvides network-wide
addressing and a mechanism toaddressing and a mechanism to
move packets between networksmove packets between networks
(routing)(routing)
• Responsibilities:Responsibilities:
• Network addressingNetwork addressing
• RoutingRouting
• Example:Example:
• IP from TCP/IPIP from TCP/IP

13. Network layer
The network layer is responsible for
the delivery of individual packets from
the source host to the destination host.

14. NETWORK ADDRESSESNETWORK ADDRESSES
• IP (internet protocol):IP (internet protocol):
• Unique number that refers to a computer connected to theUnique number that refers to a computer connected to the
internetinternet
• 32bit number32bit number
• IP address by the ISPIP address by the ISP
• Example:127.80.144.30Example:127.80.144.30Connection orientedConnection oriented
• DNS:DNS: domain name system; method to used present domaindomain name system; method to used present domain
name.name.
• IP is difficult to remember for usersIP is difficult to remember for users
• Human readable namesHuman readable names
• Text instead of numberText instead of number
• Example:www.gmail.comExample:www.gmail.com

15. TRANSPORT LAYERTRANSPORT LAYER
• Provides reliable dataProvides reliable data
deliverydelivery
• It’s the TCP in TCP/IPIt’s the TCP in TCP/IP
• Receives info from upperReceives info from upper
layers and segments it intolayers and segments it into
packetspackets
• Can provide error detectionCan provide error detection
and correctionand correction 3

16. The transport layer is responsible for
the delivery of a message from one
process to another.
The transport layer

17. SESSION LAYERSESSION LAYER
• Allows applications toAllows applications to
maintain an ongoingmaintain an ongoing
sessionsession
• Where is it on myWhere is it on my
computer?computer?
• Workstation and ServerWorkstation and Server
Service (MS)Service (MS)
• Windows Client forWindows Client for
NetWare (NetWare)NetWare (NetWare)
3

18. PRESENTATION LAYERPRESENTATION LAYER
3

19. APPLICATION LAYERAPPLICATION LAYER
• Gives end-user applicationsGives end-user applications
access to network resourcesaccess to network resources
• Where is it on my computer?Where is it on my computer?
• Workstation or Server ServiceWorkstation or Server Service
in MS Windowsin MS Windows
3

20. HOW DOES IT ALL WORK TOGETHERHOW DOES IT ALL WORK TOGETHER
• Each layer contains a Protocol Data UnitEach layer contains a Protocol Data Unit
(PDU)(PDU)
• PDU’s are used for peer-to-peer contactPDU’s are used for peer-to-peer contact
between corresponding layers.between corresponding layers.
• Data is handled by the top three layers, thenData is handled by the top three layers, then
Segmented by the Transport layer.Segmented by the Transport layer.
• The Network layer places it into packets andThe Network layer places it into packets and
the Data Link frames the packets forthe Data Link frames the packets for
transmission.transmission.
• Physical layer converts it to bits and sends itPhysical layer converts it to bits and sends it
out over the media.out over the media.
• The receiving computer reverses the processThe receiving computer reverses the process
using the information contained in the PDUusing the information contained in the PDU..
2

21. Summary of layers

22. THE POSTAL ANALOGY:-_THE POSTAL ANALOGY:-_
• A-A- Write a 20 page letter to a foreign country.Write a 20 page letter to a foreign country.
• P-P- Translate the letter so the receiver can readTranslate the letter so the receiver can read
it.it.
• S-S- Insure the intended recipient can receiveInsure the intended recipient can receive
letter.letter.
• T-T- Separate and number pages. Like registeredSeparate and number pages. Like registered
mail, tracks delivery and requests anothermail, tracks delivery and requests another
package if one is “lost” or “damaged” in thepackage if one is “lost” or “damaged” in the
mail.mail.
• N-N- Postal Center sorting letters by zip code toPostal Center sorting letters by zip code to
route them closer to destination.route them closer to destination.
• D-D- Local Post Office determining which vehiclesLocal Post Office determining which vehicles
to deliver letters.to deliver letters.
• P-P- Physical Trucks, Planes, Rail, autos, etc whichPhysical Trucks, Planes, Rail, autos, etc which
carry letter between stations.carry letter between stations.
Presentation
Transport
Network
Physical
Data-Link
Session
Application
How would the OSI compare to the regular Post Office