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OSI Model (Open Systems Interconnection Model)

OSI Model

The Open Systems Interconnection Model (OSI) is a conceptual model that characterizes and standardizes the internal functions of a communication system by partitioning it into abstraction layers. The OSI Model is a conceptual, seven-layered model of how networks work. It tells us that how data is going through one computer to another computer, and also it simplifies to troubleshoot the network issues.

A reference model to make sure products of different vendors would work together.

OSI Model (Open Systems Interconnection) & its & Layer
OSI Model (Open Systems Interconnection) & its & Layer


In the late 1970s, two projects began independently, with the same goal: to define a unifying standard for the architecture of networking systems. One was administered by the International Organization for Standardization (ISO), while the other was undertaken by the International Telegraph and Telephone Consultative Committee, or CCITT (the abbreviation is from the French version of the name). These two international standards bodies each developed a document that defined similar networking models.

In 1983, these two documents were merged together to form a standard called The Basic Reference Model for Open Systems Interconnection. That’s a mouthful, so the standard is usually referred to as the Open Systems Interconnection Reference Model, the OSI Reference Model, or even just the OSI Model. It was published in 1984 by both the ISO, as standard ISO 7498, and the renamed CCITT (now called the Telecommunications Standardization Sector of the International Telecommunication Union or ITU-T) as standard X.200.

The concept of a seven-layer model was provided by the work of Charles Bachman, Honeywell Information Services.

OSI Network Interconnection

OSI Network Interconnection
OSI Network Interconnection

OSI Packet Structure

Packet Discription
Packet Discription


Physical Layer

Physical Layer
Physical Layer

Function of Layer 1

  • It defines the electrical and physical specifications of the data connection. It defines the relationship between a device and a physical transmission medium (e.g., a copper or fiber optical cable). This includes the layout of pins, voltages, line impedance, cable specifications, signal timing, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more.
  • It defines the protocol to establish and terminate a connection between two directly connected nodes over a communications medium.
  • It may define the protocol for flow control.
  • It defines transmission mode i.e. simplex, half & full duplex.
  • It defines topology.
  • It defines a protocol for the provision of a (not necessarily reliable) connection between two directly connected nodes, and the modulation or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over the physical communications channel. This channel can involve physical cabling (such as copper and optical fiber) or a wireless radio link.


  • Telephone network modems- V.92
  • IRDA physical layer
  • USB physical layer
  • EIA RS-232, EIA-422, EIA-423, RS-449, RS-485
  • Ethernet physical layer Including 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX and other varieties.
  • Varieties of 802.11 Wi-Fi physical layers
  • DSL
  • ISDN
  • T1 and other T-carrier links, and E1 and other E-carrier links
  • Optical Transport Network (OTN)
  • GSM Um air interface physical layer
  • Bluetooth physical layer
  • ITU Recommendations: see ITU-T
  • IEEE 1394 interface
  • TransferJet physical layer
  • Etherloop
  • ARINC 818 Avionics Digital Video Bus
  • G.hn/G.9960 physical layer
  • CAN bus (controller area network) physical layer
  • Mobile Industry Processor Interface physical layer

Data Link Layer

The Data-Link layer contains two sub layers that are described in the IEEE-802 LAN standards:

  1. Media Access Control (MAC) Layer: Responsible for controlling how computers in the network gain access to data and permission to transmit it.
  2. Logical Link Control (LLC) Layer: Control error checking and packet synchronization.
Data Link Layer
Data Link Layer

Function of Layer 2

  • Link Establishment and Termination: Establishes and terminates the logical link between two nodes.
  • Frame Traffic Control: tells the transmitting node to “back-off” when no frame buffers are available.
  • Frame Sequencing: Transmits/receives frames sequentially.
  • Frame Acknowledgment: Provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.
  • Frame Delimiting: Creates and recognizes frame boundaries.
  • Frame Error Checking: Checks received frames for integrity.
  • Media Access Management: Determines when the node “has the right” to use the physical medium.


  • ARCnet Attached Resource Computer NETwork
  • CDP Cisco Discovery Protocol
  • DCAP Data Link Switching Client Access Protocol
  • Distributed Multi-Link Trunking
  • Distributed Split Multi-Link Trunking
  • Dynamic Trunking Protocol
  • Econet
  • Ethernet
  • FDDI Fiber Distributed Data Interface
  • Frame Relay
  • ITU-T G.hn Data Link Layer
  • HDLC High-Level Data Link Control
  • IEEE 802.11 WiFi
  • IEEE 802.16 WiMAX
  • LACP Link Aggregation Control Protocol
  • LattisNet
  • LocalTalk
  • L2F Layer 2 Forwarding Protocol
  • L2TP Layer 2 Tunneling Protocol
  • LAPD Link Access Procedures on the D channel
  • LLDP Link Layer Discovery Protocol
  • LLDP-MED Link Layer Discovery Protocol – Media Endpoint Discovery
  • PAgP – Cisco Systems proprietary link aggregation protocol
  • PPP Point-to-Point Protocol
  • PPTP Point-to-Point Tunneling Protocol
  • Q.710 Simplified Message Transfer Part
  • Multi-link trunking Protocol
  • RPR IEEE 802.17 Resilient Packet Ring
  • SLIP Serial Line Internet Protocol (obsolete)
  • StarLAN
  • STP Spanning Tree Protocol
  • Split multi-link trunking Protocol
  • Token ring a protocol developed by IBM; the name can also be used to describe the token passing ring logical topology that it popularized.
  • VTP VLAN Trunking Protocol
  • VLAN Virtual Local Area Network

Network Layer

Network Layer
Network Layer

Function of Layer 3

  • Routing: Routes frames among networks.
  • Subnet Traffic Control: Routers (network layer intermediate systems) can instruct a sending station to “throttle back” its frame transmission when the router’s buffer fills up.
  • Frame Fragmentation: If it determines that a downstream router’s maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
  • Logical-Physical Address Mapping: Translates logical addresses, or names, into physical addresses.
  • Subnet Usage Accounting: Has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.


  • ARP Address Resolution Protocol
  • RARP Reverse Address Resolution Protocol
  • ATM Asynchronous Transfer Mode
  • Frame relay, a simplified version of X.25
  • IS-IS, Intermediate System – Intermediate System (OSI)
  • MPLS Multi-protocol label switching
  • SPB Shortest Path Bridging
  • X.25
  • MTP Message Transfer Part
  • NSP Network Service Part
  • HIP Host Identity Protocol

Protocol Layer 3+4

  • AppleTalk
  • DECnet
  • Internet Protocol Suite
  • Xerox Network Systems
  • TCP/IP

Transport Layer

Transport Layer
Transport Layer

Function of Layer 4

  • Message Segmentation: Accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
  • Message Acknowledgment: Provides reliable end-to-end message delivery with acknowledgments.
  • Message Traffic Control: Tells the transmitting station to “back-off” when no message buffers are available.
  • Session Multiplexing: Multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).


  • AH Authentication Header over IP or IPSec
  • IL Originally developed as transport layer for 9P
  • SCTP Stream Control Transmission Protocol
  • Sinec H1 for telecontrol
  • SPX Sequenced Packet Exchange
  • TCP Transmission Control Protocol
  • UDP User Datagram Protocol
  • DCCP Datagram Congestion Control Protocol

Session Layer

Session Layer
Session Layer

Function of Layer 5

  • Session Establishment, Maintenance and Termination: Allows two application processes on different machines to establish, use and terminate a connection, called a session.
  • Session Support: Performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.


  • 9P Distributed file system protocol developed originally as part of Plan 9
  • NetBIOS, File Sharing and Name Resolution protocol – the basis of file sharing with Windows.
  • NetBEUI, NetBIOS Enhanced User Interface
  • NCP NetWare Core Protocol
  • NFS Network File System
  • SMB Server Message Block

Presentation Layer

Presentation Layer
Presentation Layer

Function of Layer 6

  • Character Code Translation: For example, ASCII to EBCDIC.
  • Data Conversion: Bit order, CR-CR/LF, integer-floating point, and so on.
  • Data Compression: Reduces the number of bits that need to be transmitted on the network.
  • Data Encryption: Encrypt data for security purposes. For example, password encryption.


  • TLS Transport Layer Security

Application Layer

Application Layer
Application Layer

Function of Layer 7

  • Resource sharing and device redirection
  • Remote file access
  • Remote printer access
  • Inter-process communication
  • Network management
  • Directory services
  • Electronic messaging (such as mail)
  • Network virtual terminals


  • ADC, A peer-to-peer file sharing protocol
  • AFP, Apple Filing Protocol
  • BACnet, Building Automation and Control Network protocol
  • BitTorrent, A peer-to-peer file sharing protocol
  • BGP Border Gateway Protocol
  • BOOTP, Bootstrap Protoc;
  • CAMEL, an SS7 protocol tool for the home operator
  • Diameter, an authentication, authorization and accounting protocol
  • DICOM includes a network protocol definition
  • DICT, Dictionary protocol
  • DNS, Domain Name System
  • DSM-CC Digital Storage Media Command and Control
  • DSNP, Distributed Social Networking Protocol
  • DHCP, Dynamic Host Configuration Protocol
  • ED2K, A peer-to-peer file sharing protocol
  • FTP, File Transfer Protocol
  • Finger, which gives user profile information
  • Gnutella, a peer-to-peer file-swapping protocol
  • Gopher, a hierarchical hyperlinkable protocol
  • HTTP, Hypertext Transfer Protocol
  • HTTPS, Hypertext Transfer Protocol Secure
  • IMAP, Internet Message Access Protocol
  • IRC, Internet Relay Chat
  • ISUP, ISDN User Part
  • LDAP Lightweight Directory Access Protocol
  • MIME, Multipurpose Internet Mail Extensions
  • MSNP, Microsoft Notification Protocol (used by Windows Live Messenger)
  • MAP, Mobile Application Part
  • Mosh, Mobile Shell
  • NNTP, Network News Transfer Protocol
  • NTP, Network Time Protocol
  • NTCIP, National Transportation Communications for Intelligent Transportation System Protocol
  • POP3 Post Office Protocol Version 3
  • RADIUS, an authentication, authorization and accounting protocol
  • RDP, Remote Desktop Protocol
  • Rlogin, a UNIX remote login protocol
  • rsync, a file transfer protocol for backups, copying and mirroring
  • RTP, Real-time Transport Protocol
  • RTSP, Real-time Transport Streaming Protocol
  • SSH, Secure Shell
  • SISNAPI, Siebel Internet Session Network API
  • SIP, Session Initiation Protocol, a signaling protocol
  • SMTP, Simple Mail Transfer Protocol
  • SNMP, Simple Network Management Protocol
  • SOAP, Simple Object Access Protocol
  • SMB, Microsoft Server Message Block Protocol
  • STUN, Session Traversal Utilities for NAT
  • TUP, Telephone User Part
  • Telnet, a remote terminal access protocol
  • TCAP, Transaction Capabilities Application Part
  • TFTP, Trivial File Transfer Protocol, a simple file transfer protocol
  • WebDAV, Web Distributed Authoring and Versioning
  • XMPP, an instant-messaging protocol


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