The document details the functions and responsibilities of the network, transport, session, and presentation layers of the OSI model. It explains how the network layer focuses on routing and logical addressing, the transport layer manages data delivery and error control, the session layer establishes and maintains sessions, and the presentation layer translates data formats and handles encryption. Various protocols are mentioned for each layer, highlighting their respective roles in enabling effective communication across networks.

1. NETWORK LAYER – LAYER 3
THE NETWORK LAYER WORKS FOR THE TRANSMISSION OF DATA FROM ONE HOST TO THE OTHER LOCATED
IN DIFFERENT NETWORKS. IT ALSO TAKES CARE OF PACKET ROUTING I.E. SELECTION OF THE SHORTEST
PATH TO TRANSMIT THE PACKET, FROM THE NUMBER OF ROUTES AVAILABLE. THE SENDER & RECEIVER’S
IP ADDRESS ES ARE PLACED IN THE HEADER BY THE NETWORK LAYER.

2. FUNCTIONS OF THE NETWORK LAYER
• ROUTING: THE NETWORK LAYER PROTOCOLS DETERMINE WHICH ROUTE IS SUITABLE FROM SOURCE TO
DESTINATION. THIS FUNCTION OF THE NETWORK LAYER IS KNOWN AS ROUTING.
• LOGICAL ADDRESSING: TO IDENTIFY EACH DEVICE INTER-NETWORK UNIQUELY, THE NETWORK LAYER
DEFINES AN ADDRESSING SCHEME. THE SENDER & RECEIVER’S IP ADDRESSES ARE PLACED IN THE HEADER
BY THE NETWORK LAYER. SUCH AN ADDRESS DISTINGUISHES EACH DEVICE UNIQUELY AND UNIVERSALLY.
 It is a layer 3 that manages device addressing, tracks the location of devices on the network.
 It determines the best path to move data from source to the destination based on the network conditions, the
priority of service, and other factors.
 The Data link layer is responsible for routing and forwarding the packets.
 Routers are the layer 3 devices, they are specified in this layer and used to provide the routing services within an
internetwork.
 The protocols used to route the network traffic are known as Network layer protocols. Examples of protocols are
IP and Ipv6.
Internetworking: An internetworking is the main responsibility of the network layer. It provides a logical
connection between different devices.
Packetizing: A Network Layer receives the packets from the upper layer and converts them into packets. This
process is known as Packetizing. It is achieved by internet protocol (IP).

3. TRANSPORT LAYER – LAYER 4
• THE TRANSPORT LAYER PROVIDES SERVICES TO THE APPLICATION LAYER AND TAKES SERVICES FROM
THE NETWORK LAYER. THE DATA IN THE TRANSPORT LAYER IS REFERRED TO AS SEGMENTS . IT IS
RESPONSIBLE FOR THE END-TO-END DELIVERY OF THE COMPLETE MESSAGE. THE TRANSPORT LAYER
ALSO PROVIDES THE ACKNOWLEDGMENT OF THE SUCCESSFUL DATA TRANSMISSION AND RE-
TRANSMITS THE DATA IF AN ERROR IS FOUND.
• AT THE SENDER’S SIDE: THE TRANSPORT LAYER RECEIVES THE FORMATTED DATA FROM THE UPPER
LAYERS, PERFORMS SEGMENTATION , AND ALSO IMPLEMENTS FLOW AND ERROR CONTROL TO
ENSURE PROPER DATA TRANSMISSION. IT ALSO ADDS SOURCE AND DESTINATION PORT NUMBER S IN
ITS HEADER AND FORWARDS THE SEGMENTED DATA TO THE NETWORK LAYER.
• AT THE RECEIVER’S SIDE: TRANSPORT LAYER READS THE PORT NUMBER FROM ITS HEADER AND
FORWARDS THE DATA WHICH IT HAS RECEIVED TO THE RESPECTIVE APPLICATION. IT ALSO PERFORMS
SEQUENCING AND REASSEMBLING OF THE SEGMENTED DATA.

4. FUNCTIONS OF THE TRANSPORT LAYER
• SEGMENTATION AND REASSEMBLY: THIS LAYER ACCEPTS THE MESSAGE FROM THE (SESSION)
LAYER, AND BREAKS THE MESSAGE INTO SMALLER UNITS. EACH OF THE SEGMENTS PRODUCED HAS A
HEADER ASSOCIATED WITH IT. THE TRANSPORT LAYER AT THE DESTINATION STATION REASSEMBLES
THE MESSAGE.
• SERVICE POINT ADDRESSING: TO DELIVER THE MESSAGE TO THE CORRECT PROCESS, THE
TRANSPORT LAYER HEADER INCLUDES A TYPE OF ADDRESS CALLED SERVICE POINT ADDRESS OR
PORT ADDRESS. THUS BY SPECIFYING THIS ADDRESS, THE TRANSPORT LAYER MAKES SURE THAT THE
MESSAGE IS DELIVERED TO THE CORRECT PROCESS.
Services Provided by Transport Layer
 Connection-Oriented Service
 Connectionless Service

5. 1. CONNECTION-ORIENTED SERVICE: IT IS A THREE-PHASE PROCESS THAT INCLUDES:
 CONNECTION ESTABLISHMENT
 DATA TRANSFER
 TERMINATION/DISCONNECTION
Connection-Oriented Service is basically a technique that is typically used to transport and send data at session
layer. The data streams or packets are transferred or delivered to receiver in a similar order in which they have
seen transferred by sender. It is actually a data transfer method among two devices or computers in a different
network, that is designed and developed after telephone system. Whenever a network implements this service, it
sends or transfers data or message from sender or source to receiver or destination in correct order and manner.
This connection service is generally provided by protocols of both network layer (signifies different path for various
data packets that belongs to same message) as well as transport layer (use to exhibits independence among
packets rather than different paths that various packets belong to same message will follow).

6. • OPERATIONS :
THERE IS A SEQUENCE OF OPERATIONS THAT ARE NEEDED TO B FOLLOWED BY USERS. THESE
OPERATIONS ARE GIVEN BELOW :
1.ESTABLISHING CONNECTION –
IT GENERALLY REQUIRES A SESSION CONNECTION TO BE ESTABLISHED JUST BEFORE ANY DATA
IS TRANSPORTED OR SENT WITH A DIRECT PHYSICAL CONNECTION AMONG SESSIONS.
2.TRANSFERRING DATA OR MESSAGE –
WHEN THIS SESSION CONNECTION IS ESTABLISHED, THEN WE TRANSFER OR SEND MESSAGE
OR DATA.
3.RELEASING THE CONNECTION –
AFTER SENDING OR TRANSFERRING DATA, WE RELEASE CONNECTION.

7. • DIFFERENT WAYS :
THERE ARE TWO WAYS IN WHICH CONNECTION-ORIENTED SERVICES CAN BE DONE. THESE
WAYS ARE GIVEN BELOW :
1.CIRCUIT-SWITCHED CONNECTION –
CIRCUIT-SWITCHING NETWORKS OR CONNECTIONS ARE GENERALLY KNOWN AS
CONNECTION-ORIENTED NETWORKS. IN THIS CONNECTION, A DEDICATED ROUTE IS BEING
ESTABLISHED AMONG SENDER AND RECEIVER, AND WHOLE DATA OR MESSAGE IS SENT
THROUGH IT. A DEDICATED PHYSICAL ROUTE OR A PATH OR A CIRCUIT IS ESTABLISHED
AMONG ALL COMMUNICATION NODES, AND AFTER THAT, DATA STREAM OR MESSAGE IS SENT
OR TRANSFERRED.
2.VIRTUAL CIRCUIT-SWITCHED CONNECTION –
VIRTUAL CIRCUIT-SWITCHED CONNECTION OR VIRTUAL CIRCUIT SWITCHING IS ALSO KNOWN
AS CONNECTION-ORIENTED SWITCHING. IN THIS CONNECTION, A PREPLANNED ROUTE OR
PATH IS ESTABLISHED BEFORE DATA OR MESSAGES ARE TRANSFERRED OR SENT. THE MESSAGE
IS TRANSFERRED OVER THIS NETWORK IS SUCH A WAY THAT IT SEEMS TO USER THAT THERE IS
A DEDICATED ROUTE OR PATH FROM SOURCE OR SENDER TO DESTINATION OR RECEIVER.

8. CONNECTIONLESS SERVICE: IT IS A ONE-PHASE PROCESS AND INCLUDES DATA TRANSFER. IN THIS
TYPE OF TRANSMISSION, THE RECEIVER DOES NOT ACKNOWLEDGE RECEIPT OF A PACKET. THIS
APPROACH ALLOWS FOR MUCH FASTER COMMUNICATION BETWEEN DEVICES. CONNECTION-
ORIENTED SERVICE IS MORE RELIABLE THAN CONNECTIONLESS SERVICE
A Connectionless Service is technique that is used in data communications to send or transfer data or message at Layer 4 i.e., Transport
Layer of Open System Interconnection model. This service does not require session connection among sender or source and receiver or
destination. Sender starts transferring or sending data or messages to destination.
In other words, we can say that connectionless service simply means that node can transfer or send data packets or messages to its receiver
even without session connection to receiver. Message is sent or transferred without prior arrangement. This usually works due to error
handling protocols that allow and give permission for correction of errors just like requesting retransmission.
In this service, network sends each packet of data to sender one at a time, independently of other packets. But network does not have any
state information to determine or identify whether packet is part of stream of other packets. Even the network doesn’t have any knowledge
and information about amount of traffic that will be transferred by user. In this, each of data packets has source or destination address and is
routed independently from source to destination.
Therefore, data packets or messages might follow different paths to reach destination. Data packets are also called datagrams. It is also
similar to that of postal services, as it also carries full address of destination where message is to send. Data is also sent in one direction
from source to destination without checking that destination is still present there or not or if receiver or destination is prepared to accept
message.

9. • CONNECTIONLESS PROTOCOLS :
THESE PROTOCOLS SIMPLY ALLOW DATA TO BE TRANSFERRED WITHOUT ANY LINK AMONG
PROCESSES. SOME OF DATA PACKETS MAY ALSO BE LOST DURING TRANSMISSION. SOME OF
PROTOCOLS FOR CONNECTIONLESS SERVICES ARE GIVEN BELOW:
• INTERNET PROTOCOL (IP) –
THIS PROTOCOL IS CONNECTIONLESS. IN THIS PROTOCOL, ALL PACKETS IN IP NETWORK ARE ROUTED
INDEPENDENTLY. THEY MIGHT NOT GO THROUGH SAME ROUTE.
• USER DATAGRAM PROTOCOL (UDP) –
THIS PROTOCOL DOES NOT ESTABLISH ANY CONNECTION BEFORE TRANSFERRING DATA. IT JUST
SENDS DATA THAT’S WHY UDP IS KNOWN AS CONNECTIONLESS.
• INTERNET CONTROL MESSAGE PROTOCOL (ICMP) –
ICMP IS CALLED CONNECTIONLESS SIMPLY BECAUSE IT DOES NOT NEED ANY HOSTS TO HANDSHAKE
BEFORE ESTABLISHING ANY CONNECTION.
• INTERNETWORK PACKET EXCHANGE (IPX) –
IPX IS CALLED CONNECTIONLESS AS IT DOESN’T NEED ANY CONSISTENT CONNECTION THAT IS
REQUIRED TO BE MAINTAINED WHILE DATA PACKETS OR MESSAGES ARE BEING TRANSFERRED FROM
ONE SYSTEM TO ANOTHER.

10. SESSION LAYER – LAYER 5
HIS LAYER IS RESPONSIBLE FOR THE ESTABLISHMENT OF CONNECTION, MAINTENANCE OF SESSIONS, AND
AUTHENTICATION, AND ALSO ENSURES SECURITY.
WORKING OF SESSION LAYER :
SESSION LAYER, WHICH IS THE 5TH LAYER IN THE OSI MODEL, USES THE SERVICES PROVIDED BY THE
TRANSPORT LAYER, ENABLES APPLICATIONS TO ESTABLISH AND MAINTAIN SESSIONS AND TO
SYNCHRONIZE THE SESSIONS.
NOW, IN ORDER TO ESTABLISH A SESSION CONNECTION, SEVERAL THINGS SHOULD BE FOLLOWED.
FIRST THING IS WE SHOULD MAP THE SESSION ADDRESS TO THE SHIPPING ADDRESS. THE SECOND THING
IS THAT WE NEED TO SELECT THE REQUIRED TRANSPORT QUALITY OF SERVICE (ALSO REFERRED AS
QOS) PARAMETERS. NEXT THING IS WE NEED TO TAKE CARE OF THE NEGOTIATIONS WHICH SHOULD
HAPPEN BETWEEN SESSION PARAMETERS. THEN WE FURTHER NEED TO TRANSMIT LIMITED
TRANSPARENT USER DATA. THEN AT LAST, WE NEED TO MONITOR DATA TRANSFER PHASE PROPERLY.
THE ABILITY TO SEND LARGER AMOUNT OF DATA FILES IS EXTREMELY IMPORTANT AND A NECESSARY
THING TOO.

11. FUNCTIONS OF SESSION LAYER :
THE SESSION LAYER BEING THE FIFTH LAYER IN THE OSI MODEL PERFORMS SEVERAL DIFFERENT AS WELL AS IMPORTANT FUNCTIONS
WHICH ARE NEED FOR ESTABLISHING AS WELL AS MAINTAINING A SAFE AND SECURE CONNECTION.
• FOLLOWING ARE SOME OF THE FUNCTIONS WHICH ARE PERFORMED BY SESSION LAYER –
• SESSION LAYER WORKS AS A DIALOG CONTROLLER THROUGH WHICH IT ALLOWS SYSTEMS TO COMMUNICATE IN EITHER HALF-
DUPLEX MODE OR FULL DUPLEX MODE OF COMMUNICATION.
• THIS LAYER IS ALSO RESPONSIBLE FOR TOKEN MANAGEMENT, THROUGH WHICH IT PREVENTS TWO USERS TO SIMULTANEOUSLY
ACCESS OR ATTEMPTING THE SAME CRITICAL OPERATION.
• THIS LAYER ALLOWS SYNCHRONIZATION BY ALLOWING THE PROCESS OF ADDING CHECKPOINTS, WHICH ARE CONSIDERED AS
SYNCHRONIZATION POINTS TO THE STREAMS OF DATA.
• THIS LAYER IS ALSO RESPONSIBLE FOR SESSION CHECKPOINTING AND RECOVERY.
• THIS LAYER BASICALLY PROVIDES A MECHANISM OF OPENING, CLOSING AND MANAGING A SESSION BETWEEN THE END-USER
APPLICATION PROCESSES.
• THE SERVICES OFFERED BY SESSION LAYER ARE GENERALLY IMPLEMENTED IN APPLICATION ENVIRONMENTS USING REMOTE
PROCEDURE CALLS (RPCS).
• THE SESSION LAYER IS ALSO RESPONSIBLE FOR SYNCHRONIZING INFORMATION FROM DIFFERENT SOURCES.
• THIS LAYER ALSO CONTROLS SINGLE OR MULTIPLE CONNECTIONS FOR EACH-END USER APPLICATION AND DIRECTLY
COMMUNICATES WITH BOTH PRESENTATION AND TRANSPORT LAYERS.
• SESSION LAYER CREATES PROCEDURES FOR CHECKPOINTING FOLLOWED BY ADJOURNMENT, RESTART AND TERMINATION.
• SESSION LAYER USES CHECKPOINTS TO ENABLE COMMUNICATION SESSIONS WHICH ARE TO BE RESUMED FROM THAT PARTICULAR
CHECKPOINT AT WHICH COMMUNICATION FAILURE HAS OCCURRED.
• THE SESSION LAYER IS RESPONSIBLE FOR FETCHING OR RECEIVING DATA INFORMATION FROM ITS PREVIOUS LAYER (TRANSPORT
LAYER) AND FURTHER SENDS DATA TO THE LAYER AFTER IT (PRESENTATION LAYER).

12. SESSION LAYER PROTOCOLS :
SESSION LAYER USES SOME PROTOCOLS WHICH ARE REQUIRED FOR SAFE, SECURE AND ACCURATE COMMUNICATION WHICH EXISTS BETWEEN TWO-
ENDER USER APPLICATIONS.
FOLLOWING ARE SOME OF THE PROTOCOLS PROVIDED OR USED BY THE SESSION LAYER –
• APPLETALK DATA STREAM PROTOCOL (ADSP): ADSP IS THAT TYPE OF PROTOCOL WHICH WAS DEVELOPED BY APPLE INC. AND IT INCLUDES A
NUMBER OF FEATURES THAT ALLOW LOCALAREA NETWORKS TO BE CONNECTED WITH NO PRIOR SETUP. THIS PROTOCOL WAS RELEASED IN 1985.
THIS PROTOCOL RIGOROUSLY FOLLOWED THE OSI MODEL OF PROTOCOL LAYERING. ADSP ITSELF HAS TWO PROTOCOLS NAMED: APPLETALK
ADDRESS RESOLUTION PROTOCOL (AARP) AND NAME BINDING PROTOCOL (NBP), BOTH AIMED AT MAKING SYSTEM SELF-CONFIGURING.
• REAL-TIME TRANSPORT CONTROL PROTOCOL (RTCP): RTCP IS A PROTOCOL WHICH PROVIDES OUT-OF-BAND STATISTICS AND CONTROL
INFORMATION FOR AN RTP (REAL-TIME TRANSPORT PROTOCOL) SESSION. RTCP’S PRIMARY FUNCTION IS TO PROVIDE FEEDBACK ON THE QUALITY
OF SERVICE (QOS) IN MEDIA DISTRIBUTION BY PERIODICALLY SENDING STATISTICAL INFORMATION SUCH AS TRANSMITTED OCTET AND PACKET
COUNTS OR PACKET LOSS TO THE PARTICIPANTS IN THE STREAMING MULTIMEDIA SESSION.
• POINT-TO-POINT TUNNELING PROTOCOL (PPTP): PPTP IS A PROTOCOL WHICH PROVIDES A METHOD FOR IMPLEMENTING VIRTUAL PRIVATE
NETWORKS. PPTP USES A TCP CONTROL CHANNELAND A GENERIC ROUTING ENCAPSULATION TUNNEL TO ENCAPSULATE PPP (POINT-TO-POINT
PROTOCOL) PACKETS THIS PROTOCOL PROVIDES SECURITY LEVELS AND REMOTE ACCESS LEVELS COMPARABLE WITH TYPICAL VPN (VIRTUAL
PRIVATE NETWORK) PRODUCTS.
• PASSWORD AUTHENTICATION PROTOCOL (PAP): PASSWORD AUTHENTICATION PROTOCOL IS A PASSWORD-BASED AUTHENTICATION PROTOCOL
USED BY POINT TO POINT PROTOCOL (PPP) TO VALIDATE USERS. ALMOST ALL NETWORK OPERATING SYSTEMS, REMOTE SERVERS SUPPORT PAP. PAP
AUTHENTICATION IS DONE AT THE TIME OF THE INITIAL LINK ESTABLISHMENT AND VERIFIES THE IDENTITY OF THE CLIENT USING A TWO-WAY
HANDSHAKE (CLIENT-SENDS DATAAND SERVER IN RETURN SENDS AUTHENTICATION-ACK (ACKNOWLEDGEMENT) AFTER THE DATA SENT BY CLIENT
IS VERIFIED COMPLETELY).
• REMOTE PROCEDURE CALL PROTOCOL (RPCP): REMOTE PROCEDURE CALL PROTOCOL (RPCP) IS A PROTOCOL THAT IS USED WHEN A COMPUTER
PROGRAM CAUSES A PROCEDURE (OR A SUB-ROUTINE) TO EXECUTE IN A DIFFERENT ADDRESS SPACE WITHOUT THE PROGRAMMER EXPLICITLY
CODING THE DETAILS FOR THE REMOTE INTERACTION. THIS IS BASICALLY THE FORM OF CLIENT-SERVER INTERACTION, TYPICALLY IMPLEMENTED
VIAA REQUEST-RESPONSE MESSAGE-PASSING SYSTEM.
• SOCKETS DIRECT PROTOCOL (SDP): SOCKETS DIRECT PROTOCOL (SDP) IS A PROTOCOL THAT SUPPORTS STREAMS OF SOCKETS OVER REMOTE
DIRECT MEMORY ACCESS (RDMA) NETWORK FABRICS.
THE PURPOSE OF SDP IS TO PROVIDE AN RDMA-ACCELERATED ALTERNATIVE TO THE TCP PROTOCOL. THE PRIMARY GOAL IS TO PERFORM ONE
PARTICULAR THING IN SUCH A MANNER WHICH IS TRANSPARENT TO THE APPLICATION.

13. PRESENTATION LAYER – LAYER 6
HE PRESENTATION LAYER IS ALSO CALLED THE TRANSLATION LAYER . THE DATA FROM THE APPLICATION
LAYER IS EXTRACTED HERE AND MANIPULATED AS PER THE REQUIRED FORMAT TO TRANSMIT OVER THE
NETWORK.
PRESENTATION LAYER IS THE 6TH LAYER IN THE OPEN SYSTEM INTERCONNECTION (OSI) MODEL. THIS LAYER IS
ALSO KNOWN AS TRANSLATION LAYER, AS THIS LAYER SERVES AS A DATA TRANSLATOR FOR THE NETWORK.
THE DATA WHICH THIS LAYER RECEIVES FROM THE APPLICATION LAYER IS EXTRACTED AND MANIPULATED
HERE AS PER THE REQUIRED FORMAT TO TRANSMIT OVER THE NETWORK. THE MAIN RESPONSIBILITY OF THIS
LAYER IS TO PROVIDE OR DEFINE THE DATA FORMAT AND ENCRYPTION. THE PRESENTATION LAYER IS ALSO
CALLED AS SYNTAX LAYER SINCE IT IS RESPONSIBLE FOR MAINTAINING THE PROPER SYNTAX OF THE DATA
WHICH IT EITHER RECEIVES OR TRANSMITS TO OTHER LAYER(S).

14. • THE PRESENTATION LAYER, BEING THE 6TH LAYER IN THE OSI MODEL, PERFORMS SEVERAL TYPES OF FUNCTIONS, WHICH ARE
DESCRIBED BELOW-
• PRESENTATION LAYER FORMAT AND ENCRYPTS DATA TO BE SENT ACROSS THE NETWORK.
• THIS LAYER TAKES CARE THAT THE DATA IS SENT IN SUCH A WAY THAT THE RECEIVER WILL UNDERSTAND THE INFORMATION (DATA)
AND WILL BE ABLE TO USE THE DATA EFFICIENTLY AND EFFECTIVELY.
• THIS LAYER MANAGES THE ABSTRACT DATA STRUCTURES AND ALLOWS HIGH-LEVEL DATA STRUCTURES (EXAMPLE- BANKING
RECORDS), WHICH ARE TO BE DEFINED OR EXCHANGED.
• THIS LAYER CARRIES OUT THE ENCRYPTION AT THE TRANSMITTER AND DECRYPTION AT THE RECEIVER.
• THIS LAYER CARRIES OUT DATA COMPRESSION TO REDUCE THE BANDWIDTH OF THE DATA TO BE TRANSMITTED (THE PRIMARY GOAL
OF DATA COMPRESSION IS TO REDUCE THE NUMBER OF BITS WHICH IS TO BE TRANSMITTED).
• THIS LAYER IS RESPONSIBLE FOR INTEROPERABILITY (ABILITY OF COMPUTERS TO EXCHANGE AND MAKE USE OF INFORMATION)
BETWEEN ENCODING METHODS AS DIFFERENT COMPUTERS USE DIFFERENT ENCODING METHODS.
• THIS LAYER BASICALLY DEALS WITH THE PRESENTATION PART OF THE DATA.
• PRESENTATION LAYER, CARRIES OUT THE DATA COMPRESSION (NUMBER OF BITS REDUCTION WHILE TRANSMISSION), WHICH IN
RETURN IMPROVES THE DATA THROUGHPUT.
• THIS LAYER ALSO DEALS WITH THE ISSUES OF STRING REPRESENTATION.
• THE PRESENTATION LAYER IS ALSO RESPONSIBLE FOR INTEGRATING ALL THE FORMATS INTO A STANDARDIZED FORMAT FOR EFFICIENT
AND EFFECTIVE COMMUNICATION.
• THIS LAYER ENCODES THE MESSAGE FROM THE USER-DEPENDENT FORMAT TO THE COMMON FORMAT AND VICE-VERSA FOR
COMMUNICATION BETWEEN DISSIMILAR SYSTEMS.

15. • FEATURES OF PRESENTATION LAYER IN THE OSI MODEL: PRESENTATION LAYER, BEING THE 6TH LAYER IN THE OSI
MODEL, PLAYS A VITAL ROLE WHILE COMMUNICATION IS TAKING PLACE BETWEEN TWO DEVICES IN A NETWORK.
• LIST OF FEATURES WHICH ARE PROVIDED BY THE PRESENTATION LAYER ARE:
• PRESENTATION LAYER COULD APPLY CERTAIN SOPHISTICATED COMPRESSION TECHNIQUES, SO FEWER BYTES OF
DATA ARE REQUIRED TO REPRESENT THE INFORMATION WHEN IT IS SENT OVER THE NETWORK.
• IF TWO OR MORE DEVICES ARE COMMUNICATING OVER AN ENCRYPTED CONNECTION, THEN THIS PRESENTATION
LAYER IS RESPONSIBLE FOR ADDING ENCRYPTION ON THE SENDER’S END AS WELL AS THE DECODING THE
ENCRYPTION ON THE RECEIVER’S END SO THAT IT CAN REPRESENT THE APPLICATION LAYER WITH UNENCRYPTED,
READABLE DATA.
• THIS LAYER FORMATS AND ENCRYPTS DATA TO BE SENT OVER A NETWORK, PROVIDING FREEDOM FROM
COMPATIBILITY PROBLEMS.
• THIS PRESENTATION LAYER ALSO NEGOTIATES THE TRANSFER SYNTAX.
• THIS PRESENTATION LAYER IS ALSO RESPONSIBLE FOR COMPRESSING DATA IT RECEIVES FROM THE APPLICATION
LAYER BEFORE DELIVERING IT TO THE SESSION LAYER (WHICH IS THE 5TH LAYER IN THE OSI MODEL) AND THUS
IMPROVES THE SPEED AS WELL AS THE EFFICIENCY OF COMMUNICATION BY MINIMIZING THE AMOUNT OF THE
DATA TO BE TRANSFERRED.

16. PRESENTATION LAYER PROTOCOLS :
PRESENTATION LAYER BEING THE 6TH LAYER, BUT THE MOST IMPORTANT LAYER IN THE OSI MODEL
PERFORMS SEVERAL TYPES OF FUNCTIONALITIES, WHICH MAKES SURE THAT DATA WHICH IS BEING
TRANSFERRED OR RECEIVED SHOULD BE ACCURATE OR CLEAR TO ALL THE DEVICES WHICH ARE THERE
IN A CLOSED NETWORK.
PRESENTATION LAYER, FOR PERFORMING TRANSLATIONS OR OTHER SPECIFIED FUNCTIONS, NEEDS TO
USE CERTAIN PROTOCOLS WHICH ARE DEFINED BELOW –
Apple Filing Protocol (AFP): Apple Filing Protocol is the proprietary network protocol (communications protocol)
that offers services to macOS or the classic macOS. This is basically the network file control protocol specifically
designed for Mac-based platforms.
Lightweight Presentation Protocol (LPP): Lightweight Presentation Protocol is that protocol which is used to
provide ISO presentation services on the top of TCP/IP based protocol stacks.
NetWare Core Protocol (NCP): NetWare Core Protocol is the network protocol which is used to access file, print,
directory, clock synchronization, messaging, remote command execution and other network service functions.

17. NETWARE CORE PROTOCOL (NCP): NETWARE CORE PROTOCOL IS THE NETWORK PROTOCOL WHICH IS
USED TO ACCESS FILE, PRINT, DIRECTORY, CLOCK SYNCHRONIZATION, MESSAGING, REMOTE COMMAND
EXECUTION AND OTHER NETWORK SERVICE FUNCTIONS.
NETWORK DATA REPRESENTATION (NDR): NETWORK DATA REPRESENTATION IS BASICALLY THE
IMPLEMENTATION OF THE PRESENTATION LAYER IN THE OSI MODEL, WHICH PROVIDES OR DEFINES
VARIOUS PRIMITIVE DATA TYPES, CONSTRUCTED DATA TYPES AND ALSO SEVERAL TYPES OF DATA
REPRESENTATIONS.
EXTERNAL DATA REPRESENTATION (XDR): EXTERNAL DATA REPRESENTATION (XDR) IS THE STANDARD
FOR THE DESCRIPTION AND ENCODING OF DATA. IT IS USEFUL FOR TRANSFERRING DATA BETWEEN
COMPUTER ARCHITECTURES AND HAS BEEN USED TO COMMUNICATE DATA BETWEEN VERY DIVERSE
MACHINES. CONVERTING FROM LOCAL REPRESENTATION TO XDR IS CALLED ENCODING, WHEREAS
CONVERTING XDR INTO LOCAL REPRESENTATION IS CALLED DECODING.
SECURE SOCKET LAYER (SSL): THE SECURE SOCKET LAYER PROTOCOL PROVIDES SECURITY TO THE
DATA THAT IS BEING TRANSFERRED BETWEEN THE WEB BROWSER AND THE SERVER. SSL ENCRYPTS THE
LINK BETWEEN A WEB SERVER AND A BROWSER, WHICH ENSURES THAT ALL DATA PASSED BETWEEN
THEM REMAINS PRIVATE AND FREE FROM ATTACKS.

18. FUNCTIONS OF THE PRESENTATION LAYER
• TRANSLATION: FOR EXAMPLE, ASCII TO EBCDIC .
• ENCRYPTION/ DECRYPTION: DATA ENCRYPTION TRANSLATES THE DATA INTO ANOTHER FORM OR
CODE. THE ENCRYPTED DATA IS KNOWN AS THE CIPHERTEXT AND THE DECRYPTED DATA IS KNOWN
AS PLAIN TEXT. A KEY VALUE IS USED FOR ENCRYPTING AS WELL AS DECRYPTING DATA.
• COMPRESSION: REDUCES THE NUMBER OF BITS THAT NEED TO BE TRANSMITTED ON THE NETWORK.
• NOTE: DEVICE OR PROTOCOL USE: JPEG, MPEG, GIF.

19. APPLICATION LAYER – LAYER 7
• AT THE VERY TOP OF THE OSI REFERENCE MODEL STACK OF LAYERS, WE FIND THE APPLICATION
LAYER WHICH IS IMPLEMENTED BY THE NETWORK APPLICATIONS. THESE APPLICATIONS PRODUCE
THE DATA TO BE TRANSFERRED OVER THE NETWORK. THIS LAYER ALSO SERVES AS A WINDOW FOR
THE APPLICATION SERVICES TO ACCESS THE NETWORK AND FOR DISPLAYING THE RECEIVED
INFORMATION TO THE USER.
• EXAMPLE : APPLICATION – BROWSERS, SKYPE MESSENGER, ETC.
The Application Layer of OSI (Open System Interconnection) model, is the top layer in this model and takes
care of network communication. The application layer provides the functionality to send and receive data
from users. It acts as the interface between the user and the application. The application provides services like
file transmission, mail service, and many more.
In this article we will explore the application layer in the OSI model, application layer functions, the working
of the application layer, and services provided by the application layer.

20. WHAT IS AN APPLICATION LAYER IN OSI MODEL?
APPLICATION LAYER IS THE TOPMOST LAYER IN THE
OPEN SYSTEM INTERCONNECTION (OSI) MODEL. THIS LAYER PROVIDES SEVERAL WAYS FOR
MANIPULATING THE DATA (INFORMATION) WHICH ACTUALLY ENABLES ANY TYPE OF USER TO ACCESS
NETWORK WITH EASE. THIS LAYER ALSO MAKES A REQUEST TO ITS BOTTOM LAYER, WHICH IS
PRESENTATION LAYER FOR RECEIVING VARIOUS TYPES OF INFORMATION FROM IT.
THE APPLICATION LAYER INTERFACE DIRECTLY INTERACTS WITH APPLICATION AND PROVIDES
COMMON WEB APPLICATION SERVICES. THIS LAYER IS BASICALLY HIGHEST LEVEL OF OPEN SYSTEM,
WHICH PROVIDES SERVICES DIRECTLY FOR APPLICATION PROCESS.
Functions of Application Layer
The Application Layer, as discussed above, being topmost layer in OSI model, performs several kinds of
functions which are requirement in any kind of application or communication process. Following are list of
functions which are performed by Application Layer of OSI Model:

21.  Application Layer Provides A Facility By Which Users Can Forward Several Emails And It Also Provides A Storage Facility.
 This Layer Allows Users To Access, Retrieve And Manage Files In A Remote Computer.
 It Allows Users To Log On As A Remote Host.
 This Layer Provides Access To Global Information About Various Services.
 This Layer Provides Services Which Include: E-mail, Transferring Files, Distributing Results To The User, Directory Services, Network
Resources And So On.
 It Provides Protocols That Allow Software To Send And Receive Information And Present Meaningful Data To Users.
 It Handles Issues Such As Network Transparency, Resource Allocation And So On.
 This Layer Serves As A Window For Users And Application Processes To Access Network Services.
 Application Layer Is Basically Not A Function, But It Performs Application Layer Functions.
 The Application Layer Is Actually An Abstraction Layer That Specifies The Shared Protocols And Interface Methods Used By Hosts In A
Communication Network.
 Application Layer Helps Us To Identify Communication Partners, And Synchronizing Communication.
 This Layer Allows Users To Interact With Other Software Applications.
 In This Layer, Data Is In Visual Form, Which Makes Users Truly Understand Data Rather Than Remembering Or Visualize The Data In The
Binary Format (0’s Or 1’s).
 This Application Layer Basically Interacts With Operating System (Os) And Thus Further Preserves The Data In A Suitable Manner.
 This Layer Also Receives And Preserves Data From It’s Previous Layer, Which Is Presentation Layer (Which Carries In Itself The Syntax
And Semantics Of The Information Transmitted).