OSI Layers...

OSI models:

The standard model for networking protocols and distributed applications is the International Standard Organization’s Open System Interconnect (ISO/OSI) model released in 1984. It defines seven network layers.  Vendors design network products based on the specifications of the OSI model, it provides a description of how network hardware and software work together in a layered fashion to make communications possible It also helps with trouble shooting by providing a frame of reference that describes how components are supposed to function.

 OSI layers:

• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical

Application:

The Application layer provides network services directly to the user’s application such as a web browser, email software and Windows Explorer. This layer is said to be closest to the user,

Protocols that operate on this layer include: TELNET, HTTP, FTP, TFTP, SMTP, NTP, SNMP.

Presentation:

This layer represents the data in a particular format to the Application layer, this layer is responsible to code and decode data sent to the network points, and Specifications defined at this layer include: 

GIF, TIFF, JPEG, MPEG, MIME, and ASCII.

Session:

Establishes, maintains and terminates end-to-end connections between two applications on two network point, this layer helps out with the task to carry information from one node (workstation) to another node (workstation). A session layer has to be made before we can transport information to another computer. 

This layer include: RPC, SQL, and NETBIOS.

Transport:

The transport layer is the fourth layer of the OSI reference model. This layer converts the data received from the upper layers into segments. The Transport layer is responsible for end-to-end (also called source-to-destination) delivery of entire messages. It provides end-to-end connectivity, 

Protocols that operate on this layer: TCP, UDP, and NETBEUI.

• These protocols are either connectionless or connection-oriented.

Network:

This layer converts the segments from the Transport layer into packets and is responsible for path determination, routing, and the delivery of these individual packets across multiple networks without guaranteed delivery. The network layer treats these packets independently, without recognizing any relationship between those packets, it relies on upper layers for reliable delivery and sequencing and also this layer is responsible for logical addressing also known as network addressing or Layer 3 addressing for example IP addresses.

Examples of protocols defined at this layer: IP, IPX, AppleTalk, ICMP, RIP, OSPF, BGP, IGRP, and EIGRP

Devices that operate on this layer: Routers, Layer 3 Switches.

Data Link:

The Data-Link layer is the protocol layer in a program that handles the moving of data in and out across a physical link in a network. Responsible for reassembling bits taken of the wire by the Physical layer to frames, makes sure they are in the correct order and requests retransmission of frames in case an error occurs. Provides error checking by adding a CRC to the frame, and flow control.

Data Link layer has two sub layers:

• Logical Link Control(LLC)
• Media Access Control (MAC)

The Logical Link Control is the upper sub layer of the Data Link layer. LLC masks the underlying network technology by hiding their differences hence providing a single interface to the network layer. This layer is also responsible for frames sequencing and acknowledgements.

Media Access Control This refers to the procedures used by devices to control access to the network medium. Since many networks use a shared medium (such as a single network cable, or a series of cables that are electrically connected into a single virtual medium) it is necessary to have rules for managing the medium to avoid conflicts.

Physical:

Physical layer communicates directly with the physical media; it is responsible for activating, maintaining and deactivating the physical link. It supports electrical or mechanical interface to the physical medium, this layer consists of different types of metal, plastic, and glass that are assembled in a way that allows electricity, light, and other forms of energy to be transferred from one location to another. Although the Data Link Layer, Network Layer, and Transport Layer are responsible for encoding data, the Physical Layer is responsible for transferring the data to where it needs to go. Ethernet cabling, Token Ring network technology and SCSI all function at the Physical layer of the OSI model. Hubs and other repeaters are standard network devices that function at the Physical layer and WAN interfaces such as RS-232, BRI, V.24, V.35, X.25 and Frame Relay. Cables and connectors also are a part of the Physical layer. At the Physical layer, data are transmitted using the type of signaling supported by the physical medium:

• electric voltages
• radio frequencies
• pulses of infrared or ordinary light

OSI layer diagram:

Summary:

What basically happens when data passes from Host A to Host B?

• Application, Presentation and Session layer take user input and converts it  into data,
• Transport layer adds a segment header converting the data into segments,
• Network layer adds a network header and converts the segments into packets ,
• Data Link layer adds a frame header converting the packets into frames,
• MAC sub layer converts the frames into bits which the Physical layer can put on the wire.

The steps are known as the 5 steps of data encapsulation. When the bits stream arrives at the destination, the Physical layer takes it of the wire and converts it into frames, each layer will remove their corresponding header while the dataflow up the OSI model until it is converted back to data and presented to the user, this is known as de-capsulation.

OSI layers reference table:          

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