osi layers
DESCRIPTION
This study guide is intended to provide those pursuing the CCNA certification with a framework of what concepts need to be studied. This is not a comprehensive document containing all the secrets of the CCNA, nor is it a “braindump” of questions and answers. I sincerely hope that this document provides some assistance and clarity in your studies.TRANSCRIPT
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• OSI means Open System Interconnect model.
• Developed by the International Organization for Standardization in 1974.
• It consists of seven layers.
• Each layer has a different but specific processing function.
OSI Model
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OSI Model Layers
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
Layer - 7
Layer - 6
Layer - 5
Layer - 4
Layer - 3
Layer - 2
Layer - 1
Upper Layer or
Software Layer
Lower Layer or
Hardware Layer
Heart of OSI
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Application Layer
Application Layer is responsible for providing Networking Services to the user. It is also known as Desktop Layer. Identification of Services is done using Port Numbers.
Ports are Entry and Exit Points to the Layer
Total No. Ports 0 – 65535
Reserved Ports 0 – 1023 Open Client Ports 1024 – 65535
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
Application Application
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Example of HTTP request
Client Web Server
http://www.zoomgroup.com
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Example of HTTP request
Client Web Server
HTTP Request http:// www.zoomgroup.com
Webpage
HTTP Request Listen on
Port 80Sending HTTP Reply
Webpage
Received HTTP Reply http://www.zoomgroup.com
Webpage
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Example of FTP request
Client FTP Server
ftp://ftp.microsoft.com
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Example of FTP request
FTP Request ftp://ftp.microsoft.com
FTP Request Listen on
Port 21Sending FTP Reply
Received FTP Reply ftp://ftp.microsoft.com
Client FTP Server
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Examples of Networking Services
Service Port No.
HTTP 80
FTP 21
SMTP 25
TELNET 23
TFTP 69
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Application Application
2180 25 6753 69
Data flow from Application Layer
Data
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
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Presentation Layer
Presentation Layer is responsible for converting data into standard format.
Examples : ASCII, EBCDIC, JPEG, MPEG, BMP, MIDI, WAV, MP3
Following tasks are perform at Presentation layer :
Encoding – DecodingEncryption – DecryptionCompression – Decompression
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
PresentationPresentation
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Data flow from Presentation Layer
Data
DataApplication Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
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Session Layer
Session Layer is responsible for establishing, maintaining and terminating session. Session ID works at Session Layer.
Examples :
RPC Remote Procedure Call SQL Structured Query LanguageNFS Network File System
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
SessionSession
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Data flow from Session Layer
Data
Data
Data
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
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Transport Layer
Transport Layer is responsible for end-to-end connectivity. It is also known as the heart of OSI Layers. Following tasks are performed at the Transport Layer : -
• Identifying Service
• Multiplexing & De-multiplexing
• Segmentation
• Sequencing & Reassembling
• Error Correction
• Flow Control
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
Transport Transport
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Identifying Service
• Transmission Control
Protocol
• Connection Oriented
• Acknowledgement
• Reliable
• Slower
• Port No. 6
• e.g. HTTP, FTP, SMTP
• User Datagram
Protocol
• Connection Less
• No Acknowledgement
• Unreliable
• Faster
• Port No. 17
• e.g. DNS, DHCP, TFTP
TCP UDP
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Application Application
PresentationPresentation
SessionSession
Multiplexing & De-multiplexing
2180 25 6753 69
Transport Transport
TCP - 6 UDP - 17
Network Network
Data LinkData Link
PhysicalPhysical
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Segmentation
Hello! How are you ?
Hello! How Are You ?
Hello!
How are you ?
A B
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Sequencing & Reassembling
Hello! How are you ?
Hello! How Are You ?
Hello!
How are you ?
A B
You Hello! How ? Are
Hello! How AreYou ?
you
Hello! How ?
are
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Sequencing & Reassembling
Hello! How are you ?
Hello!1/5
How2/5
Are3/5
You4/5
?5/5
Hello!
How are you ?
A B
Hello! How are you ?
Hello!1/5
How2/5
Are3/5
You4/5
?5/5
Hello!1/5
How2/5
Are3/5
You4/5
?5/5
Hello!
How are you ?
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Error Correction
Hello! How are you ?
Hello!1/5
How2/5
Are3/5
You4/5
?5/5
Hello!
How are you ? Hello!
1/5Are3/5
You4/5
?5/5
Hello!1/5
Are3/5
You4/5
?5/5
Segment Missing
Hello!1/5
Are3/5
You4/5
?5/5
How2/5
How2/5
Hello! How are you ?
Hello! How are you ?
A B
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Flow Control - Windowing
Sending 3-window
Received 3-window
Ack-Rec-3Sending
5-window
Received 4-window
A B
Ack. Received-4
PC-A can send 4 Segment
at a time to PC-B
PC-A can send 4 Segment
at a time to PC-B
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Data flow from Transport Layer
Data
Data
Data
DataTHSegment
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
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Network Layer
Network Layer is responsible for providing best path for data to reach the destination. Logical Addressing works on this layer. Router is a Network Layer device.
It is divided into two parts
• Routed Protocols
e.g. IP, IPX, Apple Talk.
• Routing Protocols
e.g. RIP, IGRP, OSPF, EIGRP
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
Network Network
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Routed Protocols
Hello!
How are you ?
192.168.1.1 192.168.1.2
A B
Segment
Segment
Source IP 192.168.1.1
Destination IP192.168.1.2
Source IP 192.168.1.1
Destination IP192.168.1.2
Segment
Hello!
How are you ?
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Routing Protocols
A
www.zoomgroup.com
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Data flow from Network Layer
Data
Data
Data
Segment
Segment
NH
e.g. Router
Packet
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
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Datalink Layer
Datalink Layer is divided into two Sub Layers :
• LLC – Logical Link ControlIt talks about Wan protocols e.g. PPP, HDLC, Frame-relay
• MAC – Media Access ControlIt talks about Physical Address. It is a 48 bit address i.e. 12 digit Hexadecimal Number.
It is also responsible for Error DetectionDevices working on Data Link Layer are Switch, Bridge, NIC.
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
Data LinkData Link
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Error Detection – CRC Check
Hello!
How are you ?
Packet
Packet
Source MAC 00-20-18-C0-07-71
Destination MAC00-20-18-C0-07-72
000003243422222896965543499323434349943446323111143333300000324342222289696554349932343434994344632311114Error Detected
192.168.1.100-20-18-C0-07-71
192.168.1.200-20-18-C0-07-72
A B
PC-A CRC No. 33333
PC-B CRC No. 11114Error Detected
For Error Correction Contact Source Transport
layer
PC-A CRC No. 33333
PC-B CRC No. 11114Error Detected
For Error Correction Contact Source Transport
layer
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Error Detection – CRC Check
Packet
Packet
Source MAC 00-20-18-C0-07-71
Destination MAC00-20-18-C0-07-72
00000324342222289696554349932343434994344632311114202020000032434222228969655434993234343499434463231111420202
Source MAC 00-20-18-C0-07-71
Destination MAC00-20-18-C0-07-72
Packet
Hello!
How are you ?
192.168.1.100-20-18-C0-07-71
192.168.1.200-20-18-C0-07-72
A B
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e.g. Switch
Data flow from Data Link Layer
Data
Data
Data
Segment
DH
Packet
DT
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
PacketPacketFrame
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Physical Layer
Physical Layer is responsible for electrical, mechanical and procedural checks. Data will be converted into Binary (i.e) 0’s & 1’s. Data will be in the form of electrical pulses if it is Coaxial or Twisted Pair cable and in the form of Light if it is Fiber Optic Cable.
Devices working at Physical Layer are Hubs, Repeaters, Cables, Modems etc.
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysicalPhysicalPhysical
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Physical Layer Example
Frame
Frame
10101010101010101010101010101010
Frame
Hello!
How are you ?
A B01010110101010101010101011010101101010101010101010010101101010101010101010110101011010101010101010100101011010101010101010101
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Data flow from Physical Layer
Data
Data
Data
Segment
Packet
Frame
Bits
e.g. Hub
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
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Segment
Packet
Frame
A B
Packet
Data
Data
Data
Bits
Data
Segment
Data Encapsulation & De-capsulation
Packet
Data
Data
Data
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysicalBits
DH DT
NH
THSegment
Packet
DT PacketDH DTDH
NH Segment NH
TH Data TH
Frame Packet
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Application Application
Transport Transport
InternetInternet
Network Access
Network Access
Comparing OSI with TCP/IP Layers
OSI Layers TCP/IP Layers
Application Application
PresentationPresentation
SessionSession
Transport Transport
Network Network
Data LinkData Link
PhysicalPhysical
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Example of HTTP request
http://www.zoomgroup.com
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ftp://ftp.microsoft.com
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telnet 192.168.1.150
================================Welcome to Hyderabad Router================================User Access Verificationpassword :
Microsoft Windows 2000 [Version 5.00.2195](C) Copyright 1985-2000 Microsoft Corp.C:\>Connecting .....
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http://www.microsoft.com
