THE OSI (OPEN SYSTEMS INTERCONNECTION) DATA MODEL: OSI MODEL Was A Product Of The Open Systems Interconnection Effort At The International Organization For Standardization.It is A ISO Standard For Computer Networks Design And Functioning. It Is Way Of Sub-Dividing A Communications System Into Smaller Parts Called Layers. Similar Communication Functions Are Grouped Into Logical Layers. The OSI architecture divides network functionality up into seven layers, where various protocols implement the functionality assigned to a given layer. During The Sending Process, Each Layer (From Top To Down) Will Add A Specific Header To The Raw Data. At The Reception, Headers Are Eliminated Conversely Until The Data Arrived To The Receiving Application. A Layer Provides Services To Its Upper Layer While Receiving Services From The Layer Below. On Each Layer, An Instance Provides Service To The Instances At The Layer Above And Requests Service From The Layer Below. NETWORK MODEL (OSI) The Open Systems Interconnect Model Is The Standard For Describing The Transmission Of Data Across Networks. The Seven Layer Model Is Particularly Useful In Comparing Different Architectures. The Following Diagram Should Help To Understand The Relationships Between Osi, Tcp/Ip And Communications Layers Used By Lan Manager. OSI MODEL (Which Is Also Known As Osi Layers) Is Very Important Aspect When You Learn About Data Communication. Osi Model Can Describe About All Kind Of Data Transfer Flow At Data Communication Base On Network Aspect. The Different Data Communication Hardware And Software Can Easily Integrate In One Network System If Each Of It Is Base On Osi Model. In General Way, Osi Model Is Important For All Aspect Of Data Communication, Especially For Person Who Work On It. THE OSI MODEL FOR BEGINNERS: I’d Post Some Introductory Material For Those New To The Industry, Or For Others Who May Want A Quick Refresher. Knowing The Inner-Workings Of Systems Is Important To Provide Interoperability And Integration Among So Many Diverse Platforms. Without A Foundational Understanding In Topics Such As The Osi Model, Administrators Will Be Less Than Effective. SO HERE’S A BRIEF OVERVIEW OF EACH LAYER OF THE OSI MODEL AND THEIR FUNCTIONS: LAYER 7 – APPLICATION: The Interface Between End-User Applications And Communications Software. Protocols Such As Telnet, Http, Ftp, And Smtp Communicate At This Layer. Applications That Need To Communicate Outside The Computer On Which The Application Resides. This Layer Specifies Rules For The Following Tasks: Providing Network Services. Advertising Network Services. Accessing Network Services. The Following Is The Examples Of Layer 7th OSI MODEL: Web Browser. Netware’s Service Advertising Protocol. (Sap) Simple Mail Transfer Protocol (Smtp). Tcp/Ip Network File System (Nfs.) LAYER 6 – PRESENTATION: Handles Encryption, Formatting, Compression, And Presentation Of Data Formats (Such As Jpeg) To Applications. Ssl And Tls Communicate At This Layer. Encryption Is Also Defined By Osi Model As A Presentation Layer Service: Data Translation. Data Encryption And Compression. The Following Are The Examples Of Protocols That Implement Presentation Layer Of The Osi Model Rules: Netware Core Protocol (Ncp). Apple Talk Filing Protocol. LAYER 5 – SESSION: Responsible For The Startup, Control, And Teardown Of Sessions For The Presentation Layer. Netbios And Tcp Sessions Are Examples Of Those Controlled At This Layer. This Allows The Presentation Layer To Have A Seamless View Of An Incoming Stream Of Data. The Presentation Layer Of The Osi Model Can Be Presented With Data If All Flows Occur In Some Cases. Summary Of The Session Layer Of The Osi Model: Controlling Communication Sessions Between Two Devices. Establishing, Managing, And Releasing Connections. The Following Are Examples Of Protocols That Implement Session-Layer Of The Osi Model Rules: Netware’s Services Advertising Protocol (Sap). Tcp/Ip Remote Procedure Call. LAYER 4 – TRANSPORT: Handles All Transport And Data Delivery Issues To Other Systems (A Focus On Error Recovery And Controlling Data Flow). Tcp And Udp Protocols Reside At This Layer. The Following Are Rules Specified By This Transport Layer Of The Osi Model: Hiding The Network Structure From The Upper Layer. Acknowledging That A Message Was Received. Ensuring Reliable, Error-Free Message Delivery. The Following Are Examples Of Protocols That Implement Transport Layer Of The Osi Model Rules: Netware’s Sequenced Packet Exchange (Spx) Protocol. Tcp/Ip’s Transmission Control Protocol (Tcp). Tcp/Ip Domain Name System (Dns). LAYER 3 – NETWORK: Responsible For Routing, Addressing, And Determining The Best Possible Route. Icmp, Ip, Arp, And Ipsec Reside At This Layer. It Also Defines How Routing Works And How Routes Are Learned So That The Packets Can Be Delivered. Summary Of Layer 3 Osi Model: Routing Data Between Multiple Networks. Fragmenting And Reassembling Data. Identifying Network Cable Segment. The Following Are Examples Of Protocols That Implement Network-Layer Rules Of The Osi Model: Netware’s Internetwork Packet Exchange (Ipx) Protocol. Tcp/Ip Internet Protocol (Ip) LAYER 2 – DATA LINK: Links The Data From One Host To Another, While Defining Protocols When Data Is Sent Over A Particular Medium. Where The Ip Address Is Found At Layer 3, Mac Addresses Are Found At This Layer.The data link layer, also sometimes just called the link layer, is where many wired and wireless local area networking (LAN) technologies primarily function. For example: Ethernet, Token Ring, FDDI and 802.11 (“wireless Ethernet” or “Wi-Fi’) are all sometimes called “data link layer technologies”. The Data Link Layer Transforms The Physical Layer, A Raw Transmission Facility, To A Link Responsible For Node-To-Node (Hop-To-Hop) Communication. Specific Responsibilities Of The Data Link Layer Include Framing, Addressing, Flow Control, Error Control, And Media Access Control. The Data Link Layer Divides The Stream Of Bits Received From The Network Layer Into Manageable Data Units Called Frames. The Data Link Layer Adds A Header To The Frame To Define The Addresses Of The Sender And Receiver Of The Frame. If The Rate At Which The Data Are Absorbed By The Receiver Is Less Than The Rate At Which Data Are Produced In The Sender, The Data Link Layer Imposes A Flow Control Mechanism To Avoid Overwhelming The Receiver. The Data Link Layer Also Adds Reliability To The Physical Layer By Adding Mechanisms To Detect And Retransmit Damaged, Duplicate, Or Lost Frames.When Two Or More Devices Are Connected To The Same Link, Data Link Layer Protocols Are Necessary To Determine Which Device Has Control Over The Link At Any Given Time. The Data Link Layer Is Divided Into Two Sublayer By The IEEE 802 Standards: The Logical Link Control (Llc) And Media Access Control (Mac) Sublayers. The Llc Sublayer Is Defined In IEEE 802.1 And IEEE 802.2. The Mac Sublayer Is Defined In The IEEE 802.1, IEEE 802.3, IEEE 802.5 And IEEE 802.12. LOGICAL LINK CONTROL (LLC): Conceptually, The Llc Sublayer Sits On Top Of The Mac Sublayer. It's Defined By The 802.2 Standard To Be Topology Independent. The Llc Functions Include: Managing Frames To Upper And Lower Layers. Error Control. Flow Control. The Llc Works With The Transport Layer By Providing Connection-Oriented And Connectionless Services. It Manages And Creates The Communication Link. THE LLC SUBLAYER TRANSFERS DATA IN TWO WAYS: Connectionless Services: Messages Are Not Acknowledged By The Receiving Device, Which Speeds Up The Processing. Although It Sounds Unreliable, This Type Of Transfer Is Commonly Used At This Level Because The Upper Osi Layers Implement Their Own Error-Checking And Control. Connection-Oriented Services: Because Each Message Is Acknowledged, This Service Is Much Slower Than Connectionless Services, But It's Much More Reliable. FLOW CONTROL: Another Communications Control Defined On The Llc Sublayer Is Flow Control. The Transport Layer Of The Osi Model Actually Manages The Mechanisms Used To Control The Flow Of Data Between Two Hosts. The Data Link Layer Defines The Data Values Used In The Flow Control Signaling Between Two Transmitting Hosts. There Are Two Types Of Flow Control Implemented In Data Communications - Software And Hardware: Software Flow Control, Common To Networking, Involves A Process Called Xon/Xoff, Whih Roughly Stands For Transmission On/Transmission Off. Hardware Flow Control, Also Called Rts/Cts (Ready To Send/Clear To Send), Uses Two Wires In A Cable, One For Rts And One For Cts. When Either Is Turned Off, The Flow Is Interrupted. ERROR DETECTION: Another Function Of The Data Link Layer Is Error Detection. Error Detection Is The Process Of Detecting Whether Errors Occurred During The Transmission Of The Bits Across The Wire. The Data Link Layer Uses A Calculated Value Called The Crc (Cyclic Redundancy Check) That's Placed Into The Data Link Trailer That's Added To The Message Frame Before It's Sent To The Physical Layer. The Receiving Computer Recalculates The Crc And Compares It To The One Sent With The Data. If The Two Alues Are Equal, It's Assumed That The Data Arrived Without Errors. Otherwise, The Message Frame May Need To Be Retransmitted Under Control Of An Upper Layer. Although The Data Link Layer Implements Error Detection, It Does Not Include A Function To Perform Error Recovery. This Is Left For The Upper Layers To Deal With, Primarily On The Transport Layer. LAYER 1 – PHYSICAL: This Layer Provides The Physical Transportation Of Data. It Focuses On Connectors, Currents, Pins, Light, And Other Specifications That Define Cabling Standards. This Layer Focuses On Binary Transmission. There Are Many Mnemonic Phrases Used To Help Memorize Each Layer. Some Include: People Don’t Need To See Paula Abdul. Please Do Not Take Sausage Pizza Away. All People Seem To Need Data Processing. All – Application Layer. People – Presentation Layer. Seems – Session Layer. To – Transport Layer. Need – Network Layer. Data – Data Link Layer. Processing – Physical Layer. Whichever Phrase You Choose, The Osi Layers Are Pretty Easy To Remember Over Time. Knowing How Various Systems Work And What Layer They Reside On Can Help Streamline Troubleshooting. The Main Goal Of Osi Model Is More Understanding Of Each Layers Function Regarding To Data Communication Flow Network And Application Designer. It Includes The Type Of Network Protocol And Transmission Method. Osi Model Is Abbreviation Of Open System Interconnection Reference Model Which Is An Abstract Description For Layered Data Communication Design Osi Model Divides Data Communication Network Architecture Into Seven Layers Which Are Application, Presentation, Session, Transport, Network, Data Link, And Physical Layer. Therefore, Osi Model Also Known As Osi Seven Layer Model. OSI MODEL ALSO DIVIDES INTO TWO BIG GROUPS WHICH ARE UPPER LAYER AND LOWER LAYER: Upper Layer Focus At User Side And Lower Layer Focus At Network Side. Each Layer Communicates Directly With Above And Below Layer. The Upper Layers Of The Osi Model Represent Software That Implements Network Services Like Encryption And Connection Management. The Lower Layers Of The Osi Model Implement More Primitive, Hardware-Oriented Functions Like Routing, Addressing, And Flow Control. OSI MODEL Work Base On Those Layers. Each Layer Is Particularly Responsible At Data Communication Process. Each Layer Provides Service For Layer Above It And Receives Service From Layer Below It. It Can Tell That Each Instance Of One Layer Provides Service For Instance At Layer Above And Request Service From Instance At Layer Below. For Example, Data Link Layer Instance Ensure Error Free Data For Network Layer Instance And Deliver It Trough Physical Layer Instance. We Can See That Data Go Trough The Entire Layer One By One From Application Layer Until Physical Layer At Sender Side. And Data Also Go Trough The Entire Layer At Recipient Site But With Opposite Flow. The Flow Is From Physical Layer Until Application Layer At Recipient Site. Each Layer Adds One Header At Data At Sender Side And Each Layer Remove One Header That Belongs To It At Recipient Side. PACKET VS FRAME - OSI MODEL: PACKET: This Term Is Consided By Many To Correctly Refer To A Message Sent By Protocols Operating At The Network Layer Of The Osi Reference Model. So You Will Commonly See People Refer To Ip Packets. However, This Termin Is Commonly. Also Used To Refer Generically To Any Type Of Message, As I Menitoned Earlier. FRAME: This Term Is Most Commonly Associated With Messages That Travel At Low Levels Of The Osi Reference Model. In Particular, It Is Most Commonly Seen Used In Reference To Data Link Layer Messages. It Is Occasionally Also Used To Refer To Physical Layer Messages, When Message Formatting Is Performed By A Layer 1 Technology. A Frame Gets Its Name From The Fact That It Is Created By Taking Higher-Level Packets Or Datagrams And "Framing" Them With Additional Header Information Needed At The Lower Level. SEE HOW IT GOES LAYER BY LAYER: Data= Application+Presentation+Session Layers. Segment= Transport Layer. Packet= Network Layer. Frame= Data Link Layer. Byets= Physical Layer. A Packet Exists At Layer 3 Of The Osi Model, Whereas A Frame Exists At Layer 2 Of The Osi Model.Layer 2 Is The Data Link Layer. The Best Known Data Link Layer Protocol Is Ethernet.Layer 3 Is The Network Layer. The Best Know Network Layer Protocol Is Ip (Internet Protocol).To Move Through A Network, A Packet Is Encapsulated Into One Or More Frames, Depending Upon The Mtu Size. CONCLUSION: The Goal Of This Article Is To Give An Easy Way To Understand OSI Layer For Every Beginours Who Are Going To Study Networking.Thank You! This Article Written Author By: Premakumar Thevathasan. CCNA, CCNP, CCIP, MCSA, MCSE, MCSA - MSG, CIW Security Analyst, CompTia Certified A+. The School Of Cisco Networking (SCN)