OSI Model

Introduction:-The Open Systems Interconnection (OSI) model is a reference tool for understanding

data communications between any two networked systems. It divides the communications processes

into seven layers. Each layer both performs specific functions to support the layers above it and

offers services to the layers below it. The three lowest layers focus on passing traffic through the

network to an end system. The top four layers come into play in the end system to complete the

process.

The main benefits of the OSI model include the following:-

Helps users understand the big picture of networking.

Helps users understand how hardware and software elements function together.

Makes troubleshooting easier by separating networks into manageable pieces.

Defines terms that networking professionals can use to compare basic functional relationships

on different networks.

Helps users understand new technologies as they are developed.

Aids in interpreting vendor explanations of product functionality.

Seven Layers:-

1.) Application Layer

2.) Presentation Layer

3.) Session Layer

4.) Transport Layer

5.) Network Layer

6.) Data link Layer

7.) Physical Layer

Layer 1 - The Physical Layer:-

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Physical Layer is responsible for transmitting row bit stream over the physical cable. The physical

layer defines the hardware items such as cables, cards, voltages etc.

Components of the physical layer include:-

Cabling system components

Adapters that connect media to physical interfaces

Connector design and pin assignments

Hub, repeater, and patch panel specifications

Wireless system components

Parallel SCSI (Small Computer System Interface)

Network Interface Card (NIC)

Functions:-

The physical layer, the lowest layer of the OSI model, is concerned with the transmission and

reception of the unstructured raw bit stream over a physical medium. It describes the

electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the

signals for all of the higher layers. It provides:

Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to

better accommodate the characteristics of the physical medium, and to aid in bit and frame

synchronization. It determines:

o What signal state represents a binary 1

o How the receiving station knows when a "bit-time" starts

o How the receiving station delimits a frame

Physical medium attachment, accommodating various possibilities in the medium:

o Will an external transceiver (MAU) be used to connect to the medium?

o How many pins do the connectors have and what is each pin used for?

Transmission technique: determines whether the encoded bits will be transmitted by

baseband (digital) or broadband (analog) signaling.

Physical medium transmission: transmits bits as electrical or optical signals appropriate

for the physical medium, and determines:

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o What physical medium options can be used

o How many volts/db should be used to represent a given signal state, using a given

physical medium

Layer 2 - Data link Layer:-

Data link layer is responsible for controlling the error between adjacent nodes and transfer the frames

to other computer via physical layer. Data link layer is used by hubs and switches for their operation.

Components of the Data link layer include:-

Network interface cards

Ethernet and Token Ring switches

Bridges

Functions:-

The data link layer provides error-free transfer of data frames from one node to another over the

physical layer, allowing layers above it to assume virtually error-free transmission over the link. To

do this, the data link layer provides:

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.

Layer 3 - The Network Layer:-

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This layer is responsible for translating the logical network address and names into their physical

address. This layer is also responsible for addressing, determining routes for sending and managing

network problems such as packet switching, data congestion and routines.

Functions:-

The network layer controls the operation of the subnet, deciding which physical path the data should

take based on network conditions, priority of service, and other factors. It provides:

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

Layer 4 - The Transport Layer:-

This layer is responsible for end-to-end delivers of messages between the networked hosts. It first

divides the streams of data into chunks or packets before transmission and then the receiving

computer re-assembles the packets. It also guarantees error free data delivery without loss or

duplications.

Functions:-

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or

duplications. It relieves the higher layer protocols from any concern with the transfer of data between

them and their peers.

The size and complexity of a transport protocol depends on the type of service it can get from the

network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is

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required. If the network layer is unreliable and/or only supports data grams, the transport protocol

should include extensive error detection and recovery.

The transport layer provides:

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).

Layer 5 - The Session Layer:-

This layer is responsible for establishing the process-to-process communication between the hosts in

the network. The interactive login is an example of services provided by this layer in which the

connective is re-connected in care of any interruption.

Functions:-

The session layer allows session establishment between processes running on different stations. It

provides:

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.

Layer 6 - The Presentation Layer:-

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The Presentation layer is responsible for protocol conversion, date encryption/decryption, Expanding

graphics command and the date compression. This layer makes the communications between two

hosts possible.

Functions:-

The presentation layer formats the data to be presented to the application layer. It can be viewed as

the translator for the network. This layer may translate data from a format used by the application

layer into a common format at the sending station, and then translate the common format to a format

known to the application layer at the receiving station.

The presentation layer provides:

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.

Layer 7 - The Application Layer:-

The application layer provides different services to the application. Example of services provided by

this layer is file transfer, electronic messaging e-mail, virtual terminal access and network

management.

Functions:-

The application layer serves as the window for users and application processes to access network

services. This layer contains a variety of commonly needed functions:

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

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TCP/IP Model

Introduction:-The OSI model describes computer networking in seven layers. While there have

been implementations of networking protocol that use those seven layers, most networks today use

TCP/IP. But, networking professionals continue to describe networking functions in relation to the

OSI layer that performs those tasks. The TCP/IP model uses four layers to perform the functions of

the seven-layer OSI model.

The four layers of the TCP/IP architecture can be compared to certain levels of the OSI model.  It’s

important to know what each level of the TCP/IP protocol architecture does, and how these layers

map to the OSI model. The Application Layer of the TCP/IP model performs much the same tasks as

the Application, Presentation, and Session layers of the OSI model.   The Transport layer in the

TCP/IP architecture is similar to the Transport layer in the OSI model.  This layer can use TCP or

UDP as well. The TCP/IP model uses four layers to perform the functions of the seven-layer OSI

model.

Four Layers:-

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1.) Application Layer

2.) Transport Layer

3.) Internet Layer

4.) Network Layer

Layer 1-Application Layer:-

The Application Layer in TCP/IP groups the functions of OSI Application, Presentation Layer and

Session Layer. Therefore any process above the transport layer is called an Application in the

TCP/IP architecture. In TCP/IP socket and port are used to describe the path over which applications

communicate. Most application level protocols are associated with one or more port number.

Functions:-

Refers to standard network services like http, ftp, telnet as well as communication methods

used by various application programs

Also defines compatible representation of  all data

Layer 2-Transport Layer:-

In TCP/IP architecture, there are two Transport Layer protocols. The Transmission Control Protocol

(TCP) guarantees information transmission. The User Datagram Protocol (UDP) transports datagram

without end-to-end reliability checking. Both protocols are useful for different applications.

Functions:-

Manages the transfer of data by using connection oriented (TCP) and connectionless (UDP) 

transport protocols

Manages the connections between networked applications

Layer 3-Internet Layer:-

The Internet Protocol (IP) is the primary protocol in the TCP/IP Network Layer. All upper and lower

layer communications must travel through IP as they are passed through the TCP/IP protocol stack.

In addition, there are many supporting protocols in the Network Layer, such as ICMP, to facilitate

and manage the routing process.

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Functions:-

Manages addressing of packets and delivery of packets between networks

Fragments packets so that they can be dealt with by lower level layer (Network Interface

layer Network)

Layer 4-Network Layer:-

In the TCP/IP architecture, the Data Link Layer and Physical Layer are normally grouped together to

become the Network Access layer. TCP/IP makes use of existing Data Link and Physical Layer

standards rather than defining its own. Many RFCs describe how IP utilizes and interfaces with the

existing data link protocols such as Ethernet, Token Ring, FDDI, HSSI, and ATM. The physical

layer, which defines the hardware communication properties, is not often directly interfaced with the

TCP/IP protocols in the network layer and above.

Functions:-

Delivers data via physical link (Ethernet is the most common link level protocol )

Provides error detection and packet framing

Difference between OSI Model and TCP/IP Model

The Internet Protocol Suite also known as TCP/IP is the set of communications

protocols used for the Internet and other similar networks. It is named from two of

the most important protocols in it: the Transmission Control Protocol (TCP) and

the Internet Protocol (IP), which were the first two networking protocols defined

in this standard. IP networking represents a synthesis of several developments that

began to evolve in the 1960s and 1970s, namely the Internet and LANs (Local

Area Networks), which emerged in the mid- to late-1980s, together with the

advent of the World Wide Web in early 1990s.

The Internet Protocol Suite, like many protocol suites, may be viewed as a set of

layers. Each layer solves a set of problems involving the transmission of data, and

provides a well-defined service to the upper layer protocols based on using

services from some lower layers. Upper layers are logically closer to the user and

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deal with more abstract data, relying on lower layer protocols to translate data into

forms that can eventually be physically transmitted.

The main difference in two models is:-

The OSI model consists of 7 architectural layers whereas the TCP/IP only has 4 layers.

OSI is a reference model and TCP/IP is an implementation of OSI model.

TCP/IP Protocols are considered to be standards around which the internet has developed.

The OSI model however is a "generic, protocol-independent standard."

. TCP/IP combines the presentation and session layer issues into its application layer.

TCP/IP combines the OSI data link and physical layers into the network access layer.

TCP/IP appears to be a simpler model and this is mainly due to the fact that it has fewer

layers.

S.no OSI Model TCP/IP Model

1 OSI stand for the open system

interconnection. It is called because of it

allows any two different systems to

communicate regardless of their

architecture.

TCP/IP stands for transmission control

protocol/internet protocol. It is named

after these two protocols, being part of

this model

2 OSI model has seven layers physical,

data link, network, transport session

presentation and application layer

TCP/IP Model has four layers host-to-

network layer, network transport and

application layer

3 Session and presentation layers are

present in this model

Session and presentation layers are not

present in this model

4 This model provides clear distinction

between services, interfaces and

protocols

This model does not provides clear

distinction between services, interfaces

and protocols

5 In this model protocols does not fit well

in to the model

In this model protocols fits well in to the

model

6 OSI model supports both connection and

connectionless oriented communication

in network layer

TCP/IP model supports only

connectionless communication in network

layer

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