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Lesson 2Lesson 2
OSIOSI Reference ModelReference Model
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OverviewOverview
Upon completing this lesson, you will beable to perform tasks related to:
Layers in a general model of communicationThe OSI Model
How the OSI Model compares with the
TCP/IP Model
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OSI Reference ModelOSI Reference Model
Overview
During the past two decades tremendous
increase in the numbers of users and networksDifferent implementations of hardware and
software Incompatibility
Proprietary networks and different vendorproducts could not communicate with each
other
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OSI Reference ModelOSI Reference Model
Overview (continues)
In order to address this problem ISO (International
Organization for Standardization) researched manynetwork schemes (DecNET, SNA, TCP/IP, etc.) and
decided to build network model which would help
network vendors implement networks that could
communicate Interoperability
ISO released OSI (Open Systems Interconnection) in
1984.
OSI and Networking standards So What?
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Communication Model of two hostsCommunication Model of two hosts
Source Destination
Data Packets
1010101...
How can I connect two PCs to each other?
If source and destination are on different platforms?
What is the data packet consists of?
Application data + Control Information
Control Information : Source and destination addresses, FCS, etc
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Communication Model of two hostsCommunication Model of two hosts
WAN ? /
INTERNET ? /
CORPORATE
NETWORK ?
WAN ? /
INTERNET ? /CORPORATE
NETWORK ?
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Computer ProtocolsComputer Protocols
In order for data packets to travel from a source to adestination on a network, it is important that all thedevices on the network speak the same language orprotocol. A Protocol is a set of rules that make
communication on a network more efficient.
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Back to OSI Reference ModelBack to OSI Reference Model
Why a layered Network Model?Reduces complexity
Standardizes interfacesFacilitates modular engineering
Ensures interoperable technology
Accelerates evolution
Simplifies Teaching and Learning
Divide and Conquer
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Seven Layers of the OSI ModelSeven Layers of the OSI ModelApplication Layer (Layer 7)Application Layer (Layer 7)
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Presentation Layer (Layer 6)Presentation Layer (Layer 6)
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Session Layer (Layer 5)Session Layer (Layer 5)
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Transport Layer (Layer 4)Transport Layer (Layer 4)
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Network Layer (Layer 3)Network Layer (Layer 3)
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Data Link Layer (Layer 2)Data Link Layer (Layer 2)
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Physical Layer (Layer 1)Physical Layer (Layer 1)
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Data EncapsulationData EncapsulationApplication Data
Encapsulation Process SourceData Network Communication
Destination Data
Decapsulation Process
Application Data
Application Data is extended with
the control information needed in
each layer. In each layer these
control information is added as
headers and trailers.
Network Layer adds network addresses to the header.
Data Link Layer adds physical addresses to the header and FCS to the trailer.
Physical Layer convert to bits for transmission. (Last step)
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Data EncapsulationData EncapsulationEthereal.lnk
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Peer-to-peer communicationPeer-to-peer communication
Each layer must communicate with its peerlayer.
Each layer exchanges information called PDUs(Protocol Data Units)Data (Common) Layer 5,6,7 PDU
Segment, Datagram, TPDU Layer 4 PDU
Packet Layer 3 PDU
Frame Layer 2 PDU
Bits Layer 1
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TCP/IP Reference ModelTCP/IP Reference Model
OSI Reference Model is universally recognizedbut the historical and technical open standard ofthe Internet is Transmission Control Protocol /Internet Control Protocol (TCP/IP)
TCP/IP Reference Model and TCP/IP Stack
Related Link
http://www.ietf.org http://www4.ulpgc.es/tutoriales/tcpip/pru/3376fm.htm
http://www.ietf.org/http://www4.ulpgc.es/tutoriales/tcpip/pru/3376fm.htmhttp://www4.ulpgc.es/tutoriales/tcpip/pru/3376fm.htmhttp://www.ietf.org/8/14/2019 Lesson 2 OSI Reference Model
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TCP/IPTCP/IP ModelModel
DoD (U.S. Department of Defense)created this model.
A network that can survive any conditions.A protocol stack that can run over any
network infrastructure.
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OSI &OSI & TCP/IPTCP/IP ModelModel ComparisonComparison
Application
Presentation
Session
Transport
Network
Data Link
Physical
OSI Model
Application
Transport
Internet
Host to Network
TCP Model
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Layers in theLayers in the TCP/IPTCP/IP ModelModel
Application LayerConsiders that higher level protocols should
include session and presentation layer.
Therefore all three layer functionalitypackaged in this layer for TCP/IP Model.
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Layers in theLayers in the TCP/IPTCP/IP ModelModelTransport Layer
Quality of Service
Issues of Reliability
Flow Control
Error Correction
A connection oriented protocol TCP includes all of the conceptsabove.
Circuit switching vs. Packet Switching. Put a question mark
here but forget it now.
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Layers in theLayers in the TCP/IPTCP/IP ModelModel
Internet Layer Considers underlying physical network by creating a
virtual network view.
Internet Protocol (IP) designed for this layer doesnt
give so much importance to the Network Access layerbeneath.
Its only concern is to provide unreliable, best-effort
connectionless packet delivery.
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Layers in theLayers in the TCP/IPTCP/IP ModelModel
Network Access Layer (also called host-to-network layer)
It includes the LAN and WAN technology details, and
all the details in the OSI physical and data link layers
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TCP/IP Protocol GraphTCP/IP Protocol Graph
NFS : Network File System
SMTP : Simple Mail Transfer Protocol
TELNET : Terminal Emulation over network
FTP : File Transfer Protocol
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TCP/IP Protocol GraphTCP/IP Protocol Graph
DNS : Domain Name
TFTP : Trivial File Transfer Protocol
RPC : Remote Procedure Call
SNMP : Simple Network Management Protocol
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TCP/IP Protocol GraphTCP/IP Protocol Graph
TCP: Transmission Control
UDP : User Datagram Protocol
TFTP : Trivial File Transfer Protocol
IP : Internet Protocol ICMP : Internet Message Control Protocol
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TCP/IP Protocol GraphTCP/IP Protocol Graph
ARP: Address Resolution Protocol
RARP : Reverse Address Resolution Protocol
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ComparisonComparison ofofTCP/IP with OSITCP/IP with OSI
Similarities
Both have layers
Both have application layers, though their
content includes different servicesBoth have compatible transport and network
layers
Packet-switched (not circuit-switched)technology is assumed
Networking professionals need to know both
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ComparisonComparison ofofTCP/IP with OSITCP/IP with OSI
Differences Session Layer + Presentation Layer + Application Layer (OSI) ~
Application Layer (TCP/IP)
Data Link + Physical Layer (OSI) ~ Network Access (TCP/IP)
TCP/IP has fewer layers but same functionality
OSI model is a guide and abstract model. Not an implementationguide. TCP/IP is a protocol suite consists of so many procotolsdeveloped for the Internet and they are widely used as industrystandards.
In TCP/IP a given protocol can be used by another protocol
within the same layer, whereas in the OSI model two layerswould be defined in such circumstances. For example ICMP andARP (Layer 3 Protocols) are carried in IP datagrams.
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Focus of the CCNA CurriculumFocus of the CCNA Curriculum
Application
FTP, HTTP, TFTP, SMTP, DNS, TELNET, SMTP
Presentation and Session Layer
Know definitions and named general functions Transport
TCP (the Internet)
Network
IP (the Internet) Data Link, Physical
Ethernet (common LAN Technology)
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Focus of the CCNA CurriculumFocus of the CCNA Curriculum
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Review Questions (Q1)Review Questions (Q1)
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Review Questions (A1)Review Questions (A1)
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Review Questions (Q2)Review Questions (Q2)
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Review Questions (A2)Review Questions (A2)
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Review Questions (Q3)Review Questions (Q3)
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Review Questions (A3)Review Questions (A3)
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Review Questions (Q4)Review Questions (Q4)
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Review Questions (A4)Review Questions (A4)
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Review Questions (Q5)Review Questions (Q5)
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Review Questions (A5)Review Questions (A5)
R i Q ti (Q6)Re ie Q estions (Q6)
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Review Questions (Q6)Review Questions (Q6)
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Review Questions (A6)Review Questions (A6)
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Review Questions (Q7)Review Questions (Q7)
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Review Questions (A7)Review Questions (A7)
Q (Q )
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Review Questions (Q8)Review Questions (Q8)
R i Q i (A8)
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Review Questions (A8)Review Questions (A8)
R i Q ti (Q9)
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Review Questions (Q9)Review Questions (Q9)
Q ( )R i Q ti (A9)
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Review Questions (A9)Review Questions (A9)
R i Q i (Q10)R i Q ti (Q10)
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Review Questions (Q10)Review Questions (Q10)
R i Q ti (A10)R i Q ti (A10)
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Review Questions (A10)Review Questions (A10)