Post on 03-Jun-2018
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The OSI Reference Model
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OSI Open System s Interconnection".
OSI model was first introduced in 1984 by the InternationalOrganization for Standardization (ISO). Outlines WHAT needs to be done to send data from one computer
to another.
Not HOW it should be done.
Protocols stacks handle how data is prepared for transmittal (to betransmitted)
In the OSI model, The specification needed are contained in 7 different layers that interact with each other.
The OSI Model
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What is THE MODEL?
Commonly referred to as the OSI reference model.
The OSI model is a theoretical blueprint that helps us understand how data gets
from one users computer to another.
It is also a model that helps develop standards so that all of ourhardware and software talks nicely to each other.
It aids standardization of networking technologies by providingan organized structure for hardware and software developers tofollow, to insure there products are compatible with current andfuture technologies.
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What Each Layer Does
2
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Gives end-user applications access
to network resources
Where is it on my computer? Workstation or Server Service in
MS Windows
3
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Presentation Layer
3
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Session Layer
Allows applications tomaintain an ongoing session
Where is it on mycomputer? Workstation and Server
Service (MS) Windows Client for
NetWare (NetWare)
3
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Figure 2.9 Transport l ayer
The tr anspor t layer is responsible forthe delivery of a message from one
process to another.
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Network Layer
Provides network-wide addressingand a mechanism to move packets
between networks (routing)
Responsibilities: Network addressing Routing
Example: IP from TCP/IP 3
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Network layer
The network layer is responsible forthe delivery of individual packets f rom
the source host to the destination host.
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Data Link Layer
Places data and retrieves itfrom the physical layer andprovides error detectioncapabilities
3
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Data l ink layer
The data link layer is responsible formoving frames f rom one hop (node) to
the next.
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Sub-layers of the Data Link Layer
MAC (Media Access Control) Gives data to the NIC Controls access to the media through:
CSMA/CD Carrier Sense Multiple Access/CollisionDetectionToken passing
LLC (Logical Link Layer) Manages the data link interface (or Service Access Points
(SAPs)) Can detect some transmission errors using a Cyclic
Redundancy Check (CRC). If the packet is bad the LLC willrequest the sender to resend that particular packet.
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Physical Layer
Determines the specs forall physical components Cabling Interconnect methods (topology
/ devices) Data encoding (bits to waves) Electrical properties
Examples: Ethernet (IEEE 802.3) Token Ring (IEEE 802.5) Wireless (IEEE 802.11b)
3
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Physical layer
The physical layer is responsiblefor the movement of individual bits
from one hop (node) to the next.
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Physical Layer (contd)
What are the Physical Layer components on mycomputer?NIC Network Interface Card Has a unique 12 character Hexadecimal number
permanently burned into it at the manufacturer. The number is the MAC Address/Physical address of a
computer
Cabling Twister Pair
Fiber Optic Coax Cable
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How Does It All Work Together
Each layer contains a Protocol Data Unit(PDU)
PDU s are used for peer-to-peer contact
between corresponding layers.
Data is handled by the top three layers, thenSegmented by the Transport layer.
The Network layer places it into packets andthe Data Link frames the packets fortransmission.
Physical layer converts it to bits and sends itout over the media.
The receiving computer reverses the process
using the information contained in the PDU .
2
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Figure 2.2 OSI layers
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Data Encapsulation In TCP/IP At each layer in the TCP/IP protocol stack
Outgoing data is packaged and identified for delivery to the layerunderneath
PDU Packet Data Unit the envelop information attached to apacket at a particular TCP/IP protocol
e.g. header and trailer
HeaderPDUs own particular opening componentIdentifies the protocol in use, the sender and intended recipient
Trailer (or packet trailer) Provides data integrity checks for the payload
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Encapsulation example: E-mail
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Encapsulation
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Figure 2.3 An exchange using the OSI model
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Figure 2.14 Summary of l ayers
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The Postal Analogy
A- Write a 20 page letter to a foreign country.
P- Translate the letter so the receiver can read it.
S- Insure the intended recipient can receive letter.
T- Separate and number pages. Like registered mail,tracks delivery and requests another package if one islost or damaged in the mail. N- Postal Center sorting letters by zip code to routethem closer to destination.D- Local Post Office determining which vehicles todeliver letters.
P- Physical Trucks, Planes, Rail, autos, etc whichcarry letter between stations.
P resentation
T ransport
Network
Physical
Data-Link
Session
A pplication
How would the OSI compare to the regular Post Office
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Remembering the 7 Layers
7 - Application All
6 - P resentation P eople
5 - S ession S eem
4 - Transport To
3 - Network Need
2 - Data Link Data
1 - P hysical P rocessing
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4 layers of the TCP/IP model
Layer 4: Application
Layer 3: TransportLayer 2: InternetLayer 1: Network access
It i s imp or tant to n ote that so m e of thelayers in th e TCP/IP m o d el have the s am e
nam e as layers in the OSI m od el.Do no t con fuse the layers of the two m od els .
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TCP/IP protocol stack
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TCP/IP Reference Model
Application
Transport
Internet
Network Access(Host-to-network)
LayerHTTP TELNET FTP SMTP SNMP
Protocols
TCP UDP
IP ICMP
ETHERNET PACKET RADIO
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Data Formats Application data
dataTCP
header dataTCP
header dataTCP
header
dataTCPheaderIP
header
dataTCPheaderIP
headerEthernetheader
Ethernettrailer
applicationlayer
transportlayer
networklayer
data linklayer
message
segment
packet
frame
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Packet Encapsulation (TCP/IP) The data is sent down the protocol stack
Each layer adds to the data by prepending headers
22Bytes 20Bytes 20Bytes 4Bytes
64 to 1500 Bytes
Comparing TCP/IP with OSI
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Comparing TCP/IP with OSI
OSI Model TCP/IP Hierarchy Protocols
7th
Application Layer
6 thPresentation Layer
5 thSession Layer
4 thTransport Layer
3 rdNetwork Layer
2 ndLink Layer
1 st
Physical Layer
Application Layer
Transport Layer
Network Layer
Link Layer
Link Layer : includes device driver and network interface cardNetwork Layer : handles the movement of packets, i.e. RoutingTransport Layer : provides a reliable flow of data between two hosts
Application Layer : handles the details of the particular application
How the OSI and TCP/IP Models Relate in a Networking Environment
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Layer 7 A pplication All Browsers Application
Layer 6 Presentation PeopleCommon DataFormat
Application
Layer 5 Session SeemNFS, SQL,
RPC, X-Win
Dialogues and
Conversations Application
Layer 4 T ransport To Computer Segment TCP and UDPQuality ofService, andReliability
Transport
Layer 3 N etwork N eed Router
Segment Networkinto SmallerBroadcast
Domains
Packet
RoutableProtocols.(IP, IPX,
AppleTalk)
Path Selection,Routing, and
AddressingInternet
Words toRemember
TCP/IPModel
OSIModelLayer
Pneumonic EquipmentEquipmentPurpose
OSI ModelName
FrameNDIS, ODI,MAC Address,Ether Talk
Data Protocols
NetworkAccess
Layer 1 Physical Processing
Repeater,Hub (Multi-port) B it Physical
Signals and
Media
NetworkAccess
Layer 2
Regular Computeror A Special
Gateway. Used tocombine networks
using different
communication protocols
D ata Link-MAC-LLC
One Collision AND
One Broadcast
Frames andMedia AccessControl (MAC)
Computer D ata
Redirector,FTP, Telnet,SMTP,SNMP,
NetwareCore
D ata
Bridge (2Ports) orSwitch and
NIC
Segment Networkinto SmallerColli sion Domains
How the OSI and TCP/IP Models Relate in a Networking Environment