OSI REFERENCE MODEL

The International Organization introduced the OSI layer for Standardization (ISO) in 1984 in order to provide a reference model to make sure products of different vendors would interoperate in networks. OSI is short for Open System Interconnection.

The OSI layer shows WHAT needs to be done to send data from an application on one computer, trough a network, to an application on another computer, not HOW it should be done.  A layer in the OSI model communicates with three other layers: the layer above it, the layer below it, and the same layer at its communication partner. Data transmitted between software programs passes all 7 OSI layers. The Application, Presentation and Session layers are also known as the Upper Layers.

The Data Link and Physical layers are often implemented together to define LAN and WAN specifications. 

Application Layer (Layer 7)

Application Layer provides network services directly to applications. Type of software programs vary a lot: from groupware and web browser to Tactical Ops (video game). Software programs itself are not part of the OSI model. It determines the identity and availability of communication partners, and determines if sufficient resources are available to start program-to-program communication. This layer is closest to the user. Gateways operate at this layer. Following are the examples of Application layer protocols:

i)                 Telnet

ii)               SMTP

iii)             FTP

iv)             SNMP

v)               NCP

vi)             SMB

Presentation Layer (Layer 6)

Presentation Layer defines coding and conversion functions. It ensures that information sent from the application layer of one system is readable by the application layer of another system. It includes common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes, common examples of these formats and schemes are:

i)          MPEG, QuickTime

ii)        ASCII, EBCDIC

iii)      GIF, TIFF, JPEG

Gateways operate at this layer. It transmits data to lower layers.

Session Layer (Layer 5)

The session layer establishes, manages, maintains and terminates communication channels between software programs on network nodes. It provides error reporting for the Application and Presentation layer. Examples of Session layer protocols are:

i)                 NFS

ii)               SQL

iii)             RPC

iv)             Zone Information Protocol (ZIP)

Gateways operate at this layer. It transmits data to lower layers.

Transport Layer (Layer 4)

The main purpose of this layers is making sure that the data is delivered error-free and in the correct sequence. It establishes, maintains and terminates virtual circuits. It provides error detection and recovery. It is concerned with reliable and unreliable transport. When using a connection-oriented, reliable transport protocol, such as TCP, acknowledgments is send back to the sender to confirm that the data has been received. It provides Flow Control and Windowing. It provides multiplexing; the support of different flows of data to different applications on the same host. Examples of Transport layer protocols are:

i)                 TCP (connection-oriented, reliable, provides guaranteed delivery.)

ii)               UDP (connectionless, unreliable, less overhead, reliability can be provided by the Application layer)

iii)             SPX

Gateways operate at this layer. It transmits data to lower layers.

Network Layer (Layer 3)

This layer defines logical addressing for nodes and networks/segments. It enables internetworking, passing data from one network to another. It defines the logical network layout so routers can determine how to forward packets trough an internet-work. Routing occurs at this layer, hence Routed and Routing protocols reside on this layer. Routed protocols are used to encapsulate data into packets. The header added by the Network layer contains a network address so it can be routed trough an internet-work. Examples of Network layer Routed protocols are:

i)        IP

ii)      IPX

iii)    AppleTalk

Routing protocols are used to create routing tables; routing tables are used to determine the best path / route. Routing protocols provide periodic communication between routers in an Internet work to maintain information on network links in a routing table. It transmits Packets. Routers operate at this layer. Examples of Network layer Routing protocols are:

i)                 OSPF

ii)               IGRP/EIGRP

iii)             RIP

iv)             BGP

v)               NLSP

Data Link Layer (Layer 2)

It defines psychical addressing, network topology, and is also concerned with error notification, sequencing of frames and flow control. Examples of network topologies are:

i)                 Star

ii)               Bus

iii)             Ring 

Physical Layer (Layer 1)

The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. It transmits and receives bits (bit stream) to transmission media. Physical layer specifications define characteristics such as:

• Voltage levels
• Timing of voltage changes
• Physical data rates
• Maximum transmission distances
• Physical connectors