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Saturday, May 8, 2010

Introduction to Network Concepts II( OSI Model )

Today I'm going to talk about concept that very important in CCNA . This is a actually model that describe all the functionality of communication between two computers. This model created by International Organization for Standardization (ISO) in 1984 and its called OSI Model (Open System Interconnection Model).Don't confuse with following words.
  • ISO - International Organization for Standardization (Remember this as International Standard Organization)
  • OSI Model - Open System Interconnection Model
  • IOS - Inter networking Operating System.
Before 1984 in networks operate as one unite and all the equipments that use to computer have to made by one vendor. Like IBM or some other companies produce all the parts of the computer (both hardware and software ) and one IBM computer could communicate with only another IBM computer. so this make big restriction for improvement of global communication. As a solution for this problem ISO introduce this ISO/OSI reference model.

This OSI model is a layered model and its have seven layers purpose of this layers are provide clearly define functions to each layer. Every layer have own functionality and standard for both inputs and outputs. This standards helps to manufacturing companies to create interoperable network devices and software.
In Cisco CCNA mainly talking about layer 2 and layer 3 functionality on OSI model but OSI model will be use throughout your networking career.Following are the layers of the OSI model and you should remember this sequence of OSI layers.


To remember this sequence of OSI layers you can use following sentences.

All People Seems To Need Data Processing.
or other way
Programmer Do Not Throw Sausage Pizza Away.

This top-down explanation of ISO model explain, from starting with user interact with application layer to entering bit stream into physical medium from physical layer in between this two layers other five layer perform their own functions see bottom video.

Later on I'm going describe more about this ISO model and functions of each layer now lets see some main functions and protocols of each layers.

1.Application Layer (L7)
This is the layer where the end users themselves interact with the network in this layer interact with operating system and applications whenever the user want to transfer files ,read massage or perform any other activity related to network.
  • L7 Protocols: Email protocols SMTP and POP3, Telnet, HTTP [Hyper Text Transfer Protocl], FTP[File Transfer Protocol], SNMP[Simple Network Management Protocol]
  • L7 Network Devices: Gateway redirecter working on this layer.
2.Presentation Layer(L6)
This layer ensures properly formatting data also converting user data to bits that coming from application layer and other end controlling how data present to application layer. other activities of this layer are

- Compatibility with the operating system
- proper encapsulation of data for network transmission.
- Data encryption, Decryption, Compression, and translation.

If you open a file in a word processing application, and you got pages of unrecognizable characters, that's a presentation layer issue.
  • L6 Protocols: ASCII, JPEG, GIF, MIDI, TIFF, EBCDIC, AVI, MPEG, MP3
  • L6 Network Devices: Gateway redirecter working on this layer
3.Session Layer (L5)
This layer is the manager of the two way communication between two remote hosts.this is the layer that handles the creation, maintennance, and teardown of communications between those two hosts. The overall communication itself is referred to a session.This offering three different modes Simplex, Half Duplex, Full Duplex.
  • L5 Protocols: The protocols that work on the session layer are NetBIOS, Mail Slots, Names Pipes, RPC
  • L5 Network Devices: Gateway
above three layers on OSI model known as Application set of OSI model and other 4 layers known as Transport set.

4.Transport Layer (L4)
This layer mainly provide reliable(TCP) or unreliable(UDP) services for data transfer and segmentation of upper layer data. other functions of this layer.

- Establish end to end connectivity between application
- Defines flow control
- Transport layer also provides the acknowledgement of the successful data transmission and retransmits the data if no error free data was transferred.
- error handling and connectionless oriented data deliver in the network.
  • L4 Protocols:TCP,UDP, SPX, NETBIOS, ATP and NWLINK.
  • L4 Network Devices:The Brouter, Gateway and Cable tester work on the transport layer.
5.Network Layer (L3)
This is the layer some time known as "Cisco layer". Defines logical addressees associated with a specific protocol using that it decide how to data transmit between network devices. Network layer routes the packets according to the unique network addresses. Router works as the post office and network layer stamps the letters (data) for the specific destinations.
  • L3 Protocols:IP, ICMP, ARP, RIP, OSI, IPX and OSPF
  • L3 Network Devices:Router, Brouter, Frame Relay device and ATM switch devices
6.Data Link Layer (L2)
Phisycal source and destination addresses are perform on this layer.Only error detection error correction is not perform.
-Identifies the higher layer protocol(Type or SAP)
-Frame sequencing
-Flow control
-Logical Link Control (LLC)performs Link establishment
-Media Access Control (MAC) Performs Access method
  • L2 Protocols:802.3 CSMA/CD (Ethernet) ,802.4 Token Bus (ARCnet),802.5 Token Ring,802.12 Demand Priority
  • L2 Network Devices:Bridge Switch, ISDN, Router, Intelligent Hub, NIC
7.Physical Layer (L1)
Physical layer defines and cables, network cards and physical aspects only understand 'ones' and 'zeros'.It also provides the interface between network and network communication devices.It defines the physical characteristics of the network such as connections, voltage levels and timing.
  • L1 Protocols:ISDN, IEEE 802 and IEEE 802.2
  • L1 Network Devices:Hubs, Repeaters
In this post I only try to give some introduction to functions of OSI layers next post I will talk more about OSI model layers , its protocols and its devices.


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