OSI (Open Systems Interconnection) Model

OSI (Open Systems Interconnection) 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.

HISTORY

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.

OSI (Open Source Interconnection) 7 Layer Model


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.

Network Layer Interaction

Network Layer Interconnection


Packet Structure

OSI LAYER

1. Physical Layer

Packet Description

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.

Protocol

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

SONET/SDH

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

2. Data Link Layer

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

Media Access Control (MAC) layer- responsible for controlling how computers in the

network gain access to data and permission to transmit it.

Logical Link Control (LLC) layer- control error checking and packet synchronization.

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.

Protocol

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

3. 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.

Protocol

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

IPX/SPX

Internet Protocol Suite

Xerox Network Systems

TCP/IP


4. 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).

Transport Layer


Protocol

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

5. 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.

Protocol

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

SOCKS "SOCKetS"

6. 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.

Protocol

TLS Transport Layer Security

7. 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

Protocol

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

OSI (Open Systems Interconnection) Model

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