introduction to presentation layer

  Layer 6 Presentation Layer

De/Encryption, Encoding, String representation

The presentation layer (data presentation layer, data provision level) sets the system-dependent representation of the data (for example, ASCII, EBCDIC) into an independent form, enabling the syntactically correct data exchange between different systems. Also, functions such as data compression and encryption are guaranteed that data to be sent by the application layer of a system that can be read by the application layer of another system to the layer 6. The presentation layer. If necessary, the presentation layer acts as a translator between different data formats, by making an understandable for both systems data format, the ASN.1 (Abstract Syntax Notation One) used.

OSI Layer 6 - Presentation Layer

The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. An example of a presentation service would be the conversion of an EBCDIC-coded text computer file to an ASCII-coded file. The presentation layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in the form of datagrams or packets between hosts. This layer deals with issues of string representation - whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, e.g. "thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the presentation layer will deal with the rest. Serialization of complex data structures into flat byte-strings (using mechanisms such as TLV or XML) can be thought of as the key functionality of the presentation layer. Encryption is typically done at this level too, although it can be done on the application, session, transport, or network layers, each having its own advantages and disadvantages. Decryption is also handled at the presentation layer. For example, when logging on to bank account sites the presentation layer will decrypt the data as it is received.[1] Another example is representing structure, which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardized in this layer. Two common examples are 'objects' in object-oriented programming, and the exact way that streaming video is transmitted. In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HyperText Transfer Protocol (HTTP), generally regarded as an application-layer protocol, has presentation-layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the application layer. Within the service layering semantics of the OSI network architecture, the presentation layer responds to service requests from the application layer and issues service requests to the session layer. In the OSI model: the presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the same characters. If necessary, the presentation layer might be able to translate between multiple data formats by using a common format. Wikipedia

• Data conversion
• Character code translation
• Compression
• Encryption and Decryption

The Presentation OSI Layer is usually composed of 2 sublayers that are:

CASE common application service element

Sase specific application service element, layer 7   application layer, layer 6   presentation layer, layer 5   session layer, layer 4   transport layer, layer 3   network layer, layer 2   data link layer, layer 1   physical layer.

COMPUTER NETWORK BASICS

• Introduction To Computer Networks
• Uses of Computer Networks
• Line Configuration
• Types of Network Topology
• Transmission Modes
• Transmission Mediums
• Bounded/Guided Transmission Media
• UnBounded/UnGuided Transmission Media
• Types of Communication Networks
• Connection Oriented and Connectionless Services
• Network Layer
• Quality of Service(QoS)
• IGMP Protocol
• Reference Models

Physical Layer

• Digital Transmission
• Multiplexing
• Circuit-Switched
• Message-Switched Networks
• Packet Switching

Data link layer

• Error Correction
• Data Link Control
• Flow and Error
• Simplest Protocol
• Stop-and-Wait Protocol
• Go-Back-N Automatic Repeat
• Sliding Window Protocol
• HDLC Protocol
• Point-to-Point Protocol
• Multiple Access in DL
• Channelization Protocols
• Gigabit Ethernet
• Random Access Protocol
• Controlled Access Protocols
• Carrier Sense Multiple Access

Transport layer

• Transport Layer
• Telnet vs SSH
• UDP Protocol
• TCP - Protocol

ISO/OSI REFERENCE MODEL

• Introduction to Reference Models
• OSI Model: Physical Layer
• OSI Model: Datalink Layer
• OSI Model: Network Layer
• OSI Model: Transport Layer
• OSI Model: Session Layer
• OSI Model: Presentation Layer
• OSI Model: Application Layer

TCP/IP REFERENCE MODELCOMPUTER NETWORKS

• The TCP/IP Reference Model
• Difference between OSI and TCP/IP Model
• Key Terms - Computer Network

Session layer

• Session Layer

Computer Networks

• Components of Computer Networks
• Features of Computer Network
• Protocols and Standards
• Connection Oriented and Connectionless
• OSI Vs TCP/IP

Presentation layer

• Presentation Layer

Application layer

• HTTP Protocol
• FTP Protocol
• SMTP Protocol
• POP Protocol
• SNMP Protocol
• Electronic Mail
• MIME Protocol
• World Wide Web
• DNS Protocol

Presentation Layer - OSI Model

The primary goal of this layer is to take care of the syntax and semantics of the information exchanged between two communicating systems. Presentation 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. Languages(syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role translator.

In order to make it possible for computers with different data representations to communicate, the data structures to be exchanged can be defined in an abstract way. The presentation layer manages these abstract data structures and allows higher-level data structures(eg: banking records), to be defined and exchanged.

Functions of Presentation Layer

• Translation: Before being transmitted, information in the form of characters and numbers should be changed to bit streams. The presentation layer is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format the computer.
• Encryption: It carries out encryption at the transmitter and decryption at the receiver.
• Compression: It carries out data compression to reduce the bandwidth of the data to be transmitted. The primary role of Data compression is to reduce the number of bits to be 0transmitted. It is important in transmitting multimedia such as audio, video, text etc.

Design Issues with Presentation Layer

• To manage and maintain the Syntax and Semantics of the information transmitted.
• Encoding data in a standard agreed upon way. Eg: String, double, date, etc.
• Perform Standard Encoding on wire.
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OSI Presentation and Application Layers

Cite this chapter.

• Paul D. Bartoli 3  

Part of the book series: Applications of Communications Theory ((ACTH))

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This chapter discusses the Application and Presentation Layers of the Reference Model of Open Systems Interconnection (OSI) [1]. The Application and Presentation Layers perform functions necessary to exchange information between application processes; the Application Layer is concerned with the semantic aspects of the information exchange, while the Presentation Layer is concerned with the syntactic aspects. The ability to manage the semantic and syntactic elements of the information to be exchanged is key to ensuring that the information can be interpreted by the communicants.

• Application Layer
• Abstract Syntax
• Association Control
• Presentation Layer
• Service Element

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ISO 7498, “Information processing systems—Open Systems Interconnection—Basic Reference Model,” 1984. CCITT Recommendation X.200, “Reference model of open systems interconnection for CCITT applications,” 1984 (updated expected in 1988).

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ISO DIS 9545, “Information processing systems—Open Systems Interconnection—Application Layer structure,” September 1988.

ISO TR 9007, “Concepts and terminology for the conceptual schema and the information base,” 1985.

ISO 8649, “Information processing systems—Open systems interconnection—Service definition for the association control service element,” 1988. ISO 8650, “Information processing systems—Open systems interconnection—Protocol specification for the association control service element,” 1988. CCITT Recommendation X.217, “Association control service definition for open systems interconnection for CCITT applications,” 1988. CCITT Recommendation X.227, “Association control protocol specification for open systems interconnection for CCITT applications,” final text December, 1987.

ISO 8571, “Information processing systems—Open systems interconnection—File transfer, access, and management,” Parts 1–4, 1988.

ISO/DIS 9804, “Information processing systems”Open systems interconnection—Service definition for commitment, concurrency, and recovery,” 1988 (text in SC 21 N 2573, March, 1988). ISO DIS 9805, “Information processing systems—Open systems interconnection—Protocol specification for commitment, concurrency, and recovery,” 1988 (text in SC 21 N 2574, March, 1988). CCITT Recommendation X.237, “Commitment, concurrency, and recovery service definition,” Draft Text, 1988. CCITT Recommendation X.247, “Commitment, concurrency, and recovery protocol specification, Draft Text, 1988.

ISO DIS 9040, “Information processing systems—Open systems interconnection—Virtual terminal service—Basic class,” 1988 (text in SC 21 N 2615, March, 1988). ISO DIS 9041, “Information processing systems—Open systems interconnection—Virtual terminal protocol—Basic class,” 1988 (text in SC 21 N 2616, March, 1988).

ISO DIS 9066–1, “Reliable transfer service”, 1988 (text in SC 18 N 1408, March, 1988). ISO DIS 9066–2, “Reliable transfer protocol specification,” 1988 (text in SC 18 N 1409). CCITT Recommendation X.218, “Reliable transfer: Model and service definition,” 1988. CCITT Recommendation X.228, “Reliable transfer: Protocol specification,” 1988.

ISO DIS 9072–1, “Remote operations service,” 1988 (text in SC 18 N 1410, March, 1988). ISO DIS 9072–2, “Remote operations protocol specification,” 1988 (text in SC 18 N 1411, March, 1988). CCITT Recommendation X.219, “Remote operations: Model, notation, and service definition,” 1988. CCITT Recommendation X.229, “Remote operations: Protocol specification,” 1988.

ISO DIS 9594, “Information processing—Open systems interconnection—The directory,” parts 1–8, 1988 (text in SC 21 N 2751 through N 2758, April, 1988). CCITT X.500, “Series recommendations on directory,” November, 1987.

ISO DIS 10021, “Information processing—Text communication—Message oriented text interchange system,” 1988 (text in SC 18 N 1487 through N 1493, May, 1988). CCITT X.400, “Series recommendations for message handling systems,” 1988.

ISO 8613/1–8, “Office document architecture and interchange format,” 1988, awaiting publication. CCITT T.400, “Series recommendations for document architecture, transfer, and manipulation,” 1988.

ISO 8824, “Information processing systems—Open systems interconnection—Specification of abstract syntax notation one (ASN.1),” 1987; and ISO 8824/PDAD 1, “Information processing systems—Open systems interconnection—Specification for ASN.1: Proposed draft Addendum 1 on ASN.1 extensions,” 1988 (final text in SC 21 N 2341 Revised, April, 1988). CCITT Recommendation X.208, “Specification of abstract syntax notation one (ASN.1),” 1988.

ISO 8822, “Information processing systems—Open systems interconnection—Connection oriented presentation service definition,” 1988. CCITT Recommendation X.216, “Presentation service definition for open systems interconnection for CCITT applications,” 1988.

ISO 8825, “Information processing—Open systems interconnection—Specification of basic encoding rules for abstract syntax notation one (ASN.1),” 1987; and ISO 8825/ PDAD 1, “Information processing systems—Open systems interconnection—Specification of basic encoding rules for ASN.1: Proposed draft addendum 1 on ASN.1 extensions,” 1988 (text in SC 21 N 2342 Revised, April, 1988). CCITT Recommendation X.209, “Specification of basic encoding rules for abstract syntax notation one (ASN.1),” 1988.

ISO 8823, “Information processing systems—Open systems interconnection—Connection oriented presentation protocol specification,” 1988. CCITT Recommendation X.226, “Presentation protocol specification for open systems interconnection for CCITT applications,” 1988.

ISO 8326, “Information processing systems—Open systems interconnection—Basic connection oriented session service definition,” 1987; and ISO 8326/AD 2, “Information processing systems—Open systems interconnection—Basic connection oriented session service definition—Addendum 2: Incorporation of unlimited user data,” 1988. ISO 8327, “Information processing systems—Open systems interconnection—Basic connection oriented session protocol specification,” 1987; and ISO 8327/AD 2, “Information processing systems—Open systems interconnection—Basic connection oriented session protocol specification—Addendum 2: Unlimited session user data protocol specification,” 1988.

CCITT Recommendation X.215, “Session service definition for open systems interconnection for CCITT applications,” 1988. CCITT Recommendation X.225, “Session protocol specification for open systems interconnection for CCITT applications,” 1988.

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Bartoli, P.D. (1989). OSI Presentation and Application Layers. In: Sunshine, C.A. (eds) Computer Network Architectures and Protocols. Applications of Communications Theory. Springer, Boston, MA. https://doi.org/10.1007/978-1-4613-0809-6_13

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Session and Presentation layers in the OSI model

Alessandro Maggio

• December 29, 2016

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Session and presentation layers in the OSI stack can be considered fancy layers , as they are known only by a small part of Network Engineers. This is probably because all their features blend either in transport-layer protocols or in application-layer protocols. However, with this article, you will discover all the beauty of these two layers, learning what they do and how they are implemented.

UDP limitations

Back in the CCNA course, we found out that the only place we can see the session and presentation layers truly implemented is when they are based on UDP transport. UDP leaves some room to these two layers (if compared to TCP) because it is extremely simple and lacking features . These features can be then implemented in upper layers individually, adding modularity. Just to refresh your mind, UDP has the following two limitations.

UDP, all by itself, do not order packets, and therefore the receiver cannot guess in which order they were sent originally. Moreover, it is not reliable, what is lost is just lost, no UDP component will trigger a re-transmission. Adding these features, however, would increase the complexity of the algorithm behind UDP running on hosts and will add extra fields in the UDP header sent with every segment. All of that, and specifically the extra bytes added in the segment’s header, is called overhead : the extra amount of information that allows application data to be delivered correctly. Our goal is to obtain the delivery of application data as we want it with the least overhead possible .

In the article about advanced TCP , we already explained the ways we can reduce the TCP overhead using selective ACK and header compression, but all of these complex items cannot reduce the TCP header to the size of a UDP header. Instead, with UDP we follow a different approach: we start from almost no features (only delivery to the correct application using port numbers) and we add the features we need in upper layers.

Session layer

The session layer is the one implementing one-to-one application sessions: it defines the re-transmission of data, the segment ordering method, and control the communication in general. All these features are covered by TCP for applications using that transport protocol, but applications that leverage UDP have to implement these features autonomously (within the application) or rely on an extra protocol specifically sitting at the session layer. Many applications (such as TFTP) rely on the first option, while the second alternative is the privileged one for VoIP. For VoIP traffic, the protocol we rely on for the session is the Real-time Transport Protocol (RTP) . As we are talking about applications using UDP as their underlying transport, spending some time on RTP is certainly well-worth since Voice and Video are the king applications among all UDP-based applications .

The RTP header in the picture is inserted just after the UDP header, and add extra features such as the reordering of segments and their timing, which are extremely important for an application that must run real-time. Here’s the explanation of its fields.

• Version – Version of RTP, the up-to-date one is version 2
• P (Padding) – Flag used to indicate whether padding is present or not in the segment
• X (Extension) – Flag used to indicate whether extension header is present or not
• CC (CSRC count) – Number of CSRC identifiers contained in the header
• M (Marker) – Flag that, if set, indicates that this segment has some special relevance for the application
• PT (Payload Type) – Indicates the type of RTP payload (e.g. for VoIP/Video stream)
• Sequence number – Used from the receiver to reorder packets, incremented by one each segment sent
• Timestamp – Time the segment was created, used to allow the receiver to play the content of the segment (assuming that it is audio or video) at the proper interval
• SSCS – Synchronization Source Identifier, identifies a stream of UDP/RTP segments
• CSRC – Contributing Source IDs, indicates the source of the audio stream, multiple CSRCs can be specified if there are multiple sources (e.g. in a three-party conference)
• Header extension – extra header, optional and profile-specific

These fields are the minimum needed to allow the transmission and are specifically designed for real-time streams, audio, and video mainly. Looking at the header, you can still find the Sequence number as in TCP, but no acknowledgment number . This is because RTP allows the receiver to reorder segments , but not to arrange a re-transmission. This behavior is purposefully designed this way, as a VoIP call or a video stream needs to be delivered now. In case something is lost, there is no time to re-transmit it, the show must go on. If we re-transmit it, it would arrive too late so there is no point in the retransmission at all. Another interesting field adding a feature UDP is lacking is the timestamp, with that field you can know when the content was generated and reproduce the sound and video at the same interval it was generated. Otherwise, you would have audio and video streams increasing and decreasing speed according to the network connection available.

A question I’ve been asked several times when talking about reordering packets is “How they arrive in the wrong order in the first place?” and this is surely a good question. The answer has a lot to do with routing , so it lays at the Network Layer in the OSI Stack. Routers work independently from the upper layer protocols and applications, they run based on IP addressing and routes. It might happen that a node fails over the network and traffic takes a different path to avoid falling on the faulty link, or simply a router discovers a better path to a destination. Since all of this happens dynamically, at a given moment the traffic might go over a link and one second later over another. Not all links have the same speed, so it is possible that a segment sent later but over a fast link will arrive before a segment sent previously but on a slow link. Let’s have a look at the following picture.

Edge devices (computers, servers) do not know about the network infrastructure, so they just know that there is the possibility that segments are disordered during the transmission by taking different ways. Because of that, they have to implement mechanisms for reordering . Reordering is crucial to almost any application, to transfer a file we need to know the order of its part, and to transfer audio stream we need to know in which order to play the sounds.

Presentation for Real-Time applications

The presentation layer is probably the most mysterious one. This is because almost no application implements it, neither among the UDP applications. What this layer does, is to define how data should be presented to the application. Once again, coming to rescue us we have VoIP and Video stream, the ones leveraging this layer the most. Basically, when you make a VoIP call there are some parameters involved in the audio stream, such as the bitrate or the compression rate just to name a few. To have the smoothest audio stream possible, all the parties involved must agree on how they are going to exchange audio and video. RTP is the common ground that cannot be changed, here we are talking about how to write the payload of the RTP segment. In other words, we are defining the codec.

Some codecs may privilege the compression, using less bandwidth but more computational resources, others may use a lot of bandwidth but almost no computational resources or other ones require fewer resources sacrificing audio quality. No choice can be right on all occasions, the best codec for any situation depends on your needs.

Session and presentation layers in the shadow

We now know what these two layers are truly about, but let’s take a moment to check out why they are not famous like the other OSI layers. Even previously we had the chance to understand how there is almost no need to implement them, as their features are covered elsewhere, however it is time to give some examples, as in the following picture.

What the picture says is also listed below in a little more detail.

• HTTP (Hyper-Text Transfer Protocol) – Used to transfer web-pages, the session layer is handled by TCP while there is no need for a presentation layer as the information is sent in simple text or raw binary
• FTP (File Transfer Protocol) – Used to transfer files to and from a server, the session layer is handled by TCP while there is no need for a presentation layer as the information is sent in raw binary
• SMTP (Simple Mail Transfer Protocol) – Used to send emails from a server o another, the session layer is handled by TCP while there is no need for a presentation layer as the information is sent in simple text
• SSH (Secure Shell) – Used to connect to a remote device and control it via textual commands using encryption, the session layer is handled by TCP while there is no need for a presentation layer as the information is sent in simple text
• IMAP (Internet Message Access Protocol) – Used to connect to an email server and check emails, the session layer is handled by TCP while there is no need for a presentation layer as the information is sent in simple text
• VoIP (Voice over IP) – This is not a real protocol, but instead a type of application, RTP manages the session layer while the presentation layer exists and is managed at the application layer

This article was really lightweight, as UDP is. With this knowledge, you know all the differences between UDP and TCP and you are ready to discuss the technologies implemented in a network to support modern applications. In the following articles, we will start to see some application layer protocols before we can dive into the configuration items.

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What is a presentation layer?

The presentation layer changes the data from an application layer into the device native internal mathematical structure and encodes communicated information into a displayable output format.

It executes the code changes, document compressions, security encryption, etc. It also defines the data as per the software/hardware environment of the hub. For instance, demonstrating UNIX structured data in windows.

The link between the presentation layer and the application and session layer has been shown in the diagram below −

It is concerned with the syntax of data.

Translation

The procedure in two frameworks are generally to exchange the data in the form of character strings, numbers etc. The data must be exchanged into a bitstream before being transmitted.

Encryption 

To carry any sensitive data, the presentation layer encrypts the data at the sender's end and decrypts at the receiver's end.

Compression

Compression means the reduction of bits. It is required in the case of multiline text, audio and video.

Function of presentation layer

The functions of presentation layer are explained below −

Data Compression:  It decreases the various bits to be sent by shrinking the data.

Data Conversion:  It layouts the data on several hubs according to the software/hardware environment.

Code Conversion: The form and syntax (language) of the two connecting frameworks can be different. One framework uses the American Standard Code for Information Interchange (ASCII) code to document transfer, and the other facilitates the Extended Binary Coded Decimal Interchange Code (EBCDIC) developed by the Computer hardware company IBM. It offers the "translation" from ASCII to EBCDIC and vice versa.

Data Encryption:  It encodes information in a particular format so that each user or application cannot understand it.

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• What is Presentation Layer?
• The Presentation Layer of OSI Model
• Explain the functions of Presentation Layer.
• What is a Physical Layer?
• What is a network layer?
• What is a Data Link Layer?
• What is the transport layer?
• What is the session layer?
• What is the application layer?
• What is the Ozone Layer?
• Antigen Presentation: A Vital Immune Process
• What is Data Link Layer Switching?
• What is the data link layer?
• What is Layer 2 Forwarding (L2F)?
• What is Secure Socket Layer (SSL)?

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Introduction to the OSI Model

Lesson Contents

In the beginning, the development of networks was chaotic. Each vendor had its proprietary solution. The bad part was that one vendor’s solution was not compatible with another vendor’s solution. This is where the idea for the OSI model was born. Having a layered approach to networks, our hardware vendors would design hardware for the network, and others could develop software for the application layer. Using an open model which everyone agrees on means we can build networks that are compatible with each other.

To fix this problem, the International Organization for Standardization (ISO) researched different network models, and the result is the OSI-model which was released in 1984. Nowadays, most vendors build networks based on the OSI model, and hardware from different vendors is compatible….excellent!

The OSI model isn’t just a model to make networks compatible; it’s also one of the BEST ways to teach people about networks. Keep this in mind since when you are studying networking, you will see people refer a lot to the OSI model.

Here’s what the OSI model looks like:

  “ A ll P eople S eem T o N eed D ata P rocessing”

  This is the OSI model, which has seven layers; we work our way from the bottom to the top. Let’s start at the physical layer:

• Physical Layer: This layer describes stuff like voltage levels, timing, physical data rates, physical connectors, and so on. Everything you can “touch” since it’s physical.
• Data Link: This layer makes sure data is formatted the correct way, takes care of error detection, and makes sure data is delivered reliably. This might sound a bit vague, but for now, remember that this is where “Ethernet” lives. MAC Addresses and Ethernet frames are on the Data Link layer.
• Network: This layer takes care of connectivity and path selection (routing). This is where IPv4 and IPv6 live. Every network device needs a unique address on the network.
• TCP lives here; it’s a protocol that sends data in a reliable way.
• UDP lives here; it’s a protocol that sends data in an unreliable way.

I’m taking a short break here, these four layers that I just described are important for networking , and the upper three layers are about applications .

• Session: The session layer takes care of establishing, managing, and terminating sessions between two hosts. When you are browsing a website on the internet, you are probably not the only user of the web server hosting that website. This web server needs to keep track of all the different “sessions.”
• Presentation: This one will make sure that information is readable for the application layer by formatting and structuring the data. Most computers use the ASCII table for characters. If another computer would use another character like EBCDIC, then the presentation layer needs to “reformat” the data, so both computers agree on the same characters.
• Application: Here are your applications. E-mail, browsing the web (HTTP), FTP, and many more.

  “ P eople D o N eed T o S ee P amela A nderson”

This one normally gives me more smiles when I’m teaching CCNA in class, and it’s another way to remember the OSI-Model.

P = Physical D = Data Link N = Network T = Transport S = Session P = Presentation A = Application

Remember that you can’t skip any layers in the OSI model. It’s impossible to jump from the Application layer directly to the Network layer. You must always go through all the layers to send data over the network.

Let’s take a look at a real-life example of data transmission:

• You are sitting behind your computer and want to download some files from a local webserver. You start up your web browser and type in the URL of your favorite website. Your computer will send a message to the web server requesting a certain web page. You now use the HTTP protocol, which lives on the application layer.
• The presentation layer will structure the information of the application in a certain format.
• The session layer will make sure to separate all the different sessions.
• Depending on the application, you want a reliable (TCP) or unreliable (UDP) protocol to transfer data to the web server. In this case, it’ll choose TCP since you want to ensure the webpage makes it to your computer. We’ll discuss TCP and UDP later.
• Your computer has a unique IP address (for example, 192.168.1.1), and it will build an IP packet. This IP packet will contain all the data of the application, presentation, and session layer. It also specifies which transport protocol it’s using (TCP in this case) and the source IP address (your computer 192.168.1.1), and the destination (the web server’s IP address).
• The IP packet will be put into an Ethernet Frame. The Ethernet frame has a source MAC address (your computer) and the destination MAC address (webserver). More about Ethernet and MAC addresses later.
• Finally, everything is converted into bits and sent down the cable using electric signals.

Once again, you cannot “skip” any layers of the OSI model. You always have to work your way through ALL layers. If you want a real-life story converted to networking land, just think about the postal service:

• First, you write a letter.
• You put the letter in an envelope.
• You write your name and the name of the receiver on the envelope.
• You put the envelope in the mailbox.
• The content of the mailbox will go to the central processing office of the postal service.
• Your envelope will be delivered to the receiver.
• They open the envelope and read its contents.

If you put your letter directly in the mailbox, it won’t be delivered. Unless someone at the postal office is friendly enough to deliver it anyway, in network land it doesn’t work this way! Going from the application layer all the way down to the physical layer is what we call encapsulation . Going from the physical layer and working your way up to the application layer is called de-encapsulation .

Now you know about the OSI model, the different layers, and the function of each layer. During peer-to-peer communication, each layer has “packets of information.” We call these protocol data units (PDU). Now every unit has a different name on the different layers:

• Transport layer: Segments; For example, we talk about TCP segments .
• Network layer: Packets; For example, we talk about IP packets here.
• Data link layer: Frames; For example, we talk about Ethernet frames here.

This is just terminology, so don’t mix up talking about IP frames and Ethernet packets…

OSI Model in Action

All this talk about layers is nice and all, but what about some action? We can see the different layers of the OSI model in action if we capture our network traffic on our computer.

To do this, we will download Wireshark .

Wireshark is a network capture tool that allows us to capture all packets we receive/transmit on our computer, and we can look at them.

Once you have downloaded and installed Wireshark, select the “Options” in the Capture menu:

You will now see an overview of all your network cards:

In my case, it’s the Ethernet interface that I want to capture. Hit Start, and it will capture all packets entering and exiting this interface. It will look like this:

You will see a lot of stuff, don’t worry about what you see here. As you learn more about networking, you will also learn more about the different networking protocols and their packets/frames. We will capture one single frame and take a closer look at it. To do this, we will use a filter so that Wireshark only shows this traffic:

In the green bar on the top left, enter the following filter:

Now open your web browser and open http://cisco.com.

Once the website has loaded, take a look at Wireshark:

A single packet will show up with the request from our browser to fetch the Cisco.com website. At the bottom half of the screen, we can take a look at the contents of this frame. Let me break it down for you:

Wireshark has added the first piece of information. It tells us that we received an Ethernet frame that is 908 bytes. It also shows the arrival time. Here’s the second part:

Above we see layer two of the OSI model. This is the Ethernet frame, and it shows the source and destination MAC addresses. It also tells us the type. In this case, our Ethernet frame contains an IPv4 packet. Let’s check it out:

Above, we see the IP packet. This is layer three of the OSI model. Don’t worry about all the different fields here, we will cover it later. Two things you can recognize at the top are the source and destination IP addresses. Let’s continue:

Above we see layer four of the OSI model. We are using TCP as the transport protocol here (which we will discuss later in detail). Last but not least, the last layer of the OSI model:

Above, you see layer seven, the application layer. Note that you don’t see a separate session and/or presentation layer here. You can see some information about the HTTP protocol here. We used a GET request to fetch cisco.com; the user agent I used was Mozilla (Firefox).

Want to take a look at this yourself? You can download my capture file:

Wireshark Capture HTTP Cisco.com

You have now learned about the OSI model and its different layers. You have also seen how this applies to the real world with a packet capture in Wireshark. In other lessons, you will see that we use Wireshark quite often to look at different networking protocols and their inner workings.

Forum Replies

thank you for helpful lesson. i have a question about osi layers,are layers 5,6,7 in our browser(firefox,chrom…)? what about the rest of layers?for example is layer 2 only started to working when frame gets to switch or all these happen in pc in advance? it is a big question for me and mixed me up. thanks

thank you Rene

Its really easy and simple to understand and refer

Thanks again!

thanks for ur info

95 more replies! Ask a question or join the discussion by visiting our Community Forum

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What is OSI Model? – Layers of OSI Model

OSI stands for Open Systems Interconnection . It is a 7-layer architecture with each layer having specific functionality to perform. All these 7 layers work collaboratively to transmit the data from one person to another across the globe. OSI model was developed by ISO – ‘International Organization for Standardization ‘, in the year 1984.

Prerequisite: Basics of Computer Networking

Table of Content

What is OSI Model?

What are the 7 layers of the osi model, physical layer – layer 1, data link layer (dll) – layer 2, network layer – layer 3, transport layer – layer 4, session layer – layer 5, presentation layer – layer 6, application layer – layer 7, what is the flow of data in osi model, advantages of osi model, osi model in a nutshell, osi vs tcp/ip model.

The OSI model, created in 1984 by ISO, is a reference framework that explains the process of transmitting data between computers. It is divided into seven layers that work together to carry out specialised network functions, allowing for a more systematic approach to networking.

The OSI model consists of seven abstraction layers arranged in a top-down order:

• Physical Layer
• Session Layer
• Presentation Layer

The lowest layer of the OSI reference model is the physical layer. It is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits. It is responsible for transmitting individual bits from one node to the next. When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together.  

Functions of the Physical Layer

• Bit synchronization: The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at the bit level.
• Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.
• Physical topologies: Physical layer specifies how the different, devices/nodes are arranged in a network i.e. bus, star, or mesh topology.
• Transmission mode: Physical layer also defines how the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full-duplex.

Note: Hub, Repeater, Modem, and Cables are Physical Layer devices.  Network Layer, Data Link Layer, and Physical Layer are also known as Lower Layers or Hardware Layers .

The data link layer is responsible for the node-to-node delivery of the message. The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host using its MAC address.  The Data Link Layer is divided into two sublayers:  

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

The packet received from the Network layer is further divided into frames depending on the frame size of the NIC(Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC address in the header. 

The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the wire asking “Who has that IP address?” and the destination host will reply with its MAC address.  

Functions of the Data Link Layer

• Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
• Physical addressing: After creating frames, the Data link layer adds physical addresses (MAC addresses) of the sender and/or receiver in the header of each frame.
• Error control: The data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.
• Flow Control: The data rate must be constant on both sides else the data may get corrupted thus, flow control coordinates the amount of data that can be sent before receiving an acknowledgment.
• Access control: When a single communication channel is shared by multiple devices, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time.

Note: Packet in the Data Link layer is referred to as Frame.   Data Link layer is handled by the NIC (Network Interface Card) and device drivers of host machines.  Switch & Bridge are Data Link Layer devices.

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 addresses are placed in the header by the network layer. 

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 on Internetwork 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.

Note: Segment in the Network layer is referred to as Packet .  Network layer is implemented by networking devices such as routers and switches.  

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 numbers in its header and forwards the segmented data to the Network Layer. 

Note: The sender needs to know the port number associated with the receiver’s application.  Generally, this destination port number is configured, either by default or manually. For example, when a web application requests a web server, it typically uses port number 80, because this is the default port assigned to web applications. Many applications have default ports assigned.  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. 

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

1. Connection-Oriented Service: It is a three-phase process that includes

• Connection Establishment
• Data Transfer
• Termination/disconnection

In this type of transmission, the receiving device sends an acknowledgment, back to the source after a packet or group of packets is received. This type of transmission is reliable and secure.

2. 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.

Note:   Data in the Transport Layer is called Segments .  Transport layer is operated by the Operating System. It is a part of the OS and communicates with the Application Layer by making system calls.  The transport layer is called as Heart of the OSI model.  Device or Protocol Use : TCP, UDP  NetBIOS, PPTP

This layer is responsible for the establishment of connection, maintenance of sessions, and authentication, and also ensures security.

Functions of the Session Layer

• Session establishment, maintenance, and termination: The layer allows the two processes to establish, use, and terminate a connection.
• Synchronization: This layer allows a process to add checkpoints that are considered synchronization points in the data. These synchronization points help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.
• Dialog Controller: The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

Note: All the below 3 layers(including Session Layer) are integrated as a single layer in the TCP/IP model as the ????pplication Layer”.  Implementation of these 3 layers is done by the network application itself. These are also known as Upper Layers or Software Layers.   Device or Protocol Use :  NetBIOS, PPTP.

For example:-

Let us consider a scenario where a user wants to send a message through some Messenger application running in his browser. The “Messenger” here acts as the application layer which provides the user with an interface to create the data. This message or so-called Data is compressed, encrypted (if any secure data), and converted into bits (0’s and 1’s) so that it can be transmitted.  

Communication in Session Layer

The 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. 

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

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, which has 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. 

Note: 1. The application Layer is also called Desktop Layer.              2.  Device or Protocol Use :  SMTP

Functions of the Application Layer

The main functions of the application layer are given below.

• Network Virtual Terminal: It allows a user to log on to a remote host.
• FTAM- File transfer access and management: This application allows a user to access files in a remote host, retrieve files in a remote host, and manage or control files from a remote computer.
• Mail Services: Provide email service.
• Directory Services: This application provides distributed database sources and access for global information about various objects and services.

Note:  OSI model acts as a reference model and is not implemented on the Internet because of its late invention. The current model being used is the TCP/IP model. 

When we transfer information from one device to another, it travels through 7 layers of OSI model. First data travels down through 7 layers from the sender’s end and then climbs back 7 layers on the receiver’s end.

Let’s look at it with an Example:

Luffy sends an e-mail to his friend Zoro.

Step 1: Luffy interacts with e-mail application like Gmail, outlook, etc. Writes his email to send. (This happens in Layer 7: Application layer )

Step 2: Mail application prepares for data transmission like encrypting data and formatting it for transmission. (This happens in Layer 6: Presentation Layer )

Step 3: There is a connection established between the sender and receiver on the internet. (This happens in Layer 5: Session Layer )

Step 4: Email data is broken into smaller segments. It adds sequence number and error-checking information to maintain the reliability of the information. (This happens in Layer 4: Transport Layer )

Step 5: Addressing of packets is done in order to find the best route for transfer. (This happens in Layer 3: Network Layer )

Step 6: Data packets are encapsulated into frames, then MAC address is added for local devices and then it checks for error using error detection. (This happens in Layer 2: Data Link Layer )

Step 7: Lastly Frames are transmitted in the form of electrical/ optical signals over a physical network medium like ethernet cable or WiFi.

After the email reaches the receiver i.e. Zoro, the process will reverse and decrypt the e-mail content. At last, the email will be shown on Zoro’s email client.

OSI Model defines the communication of a computing system into 7 different layers. Advantages of OSI Model include:

• It divides network communication into 7 layers which makes it easier to understand and troubleshoot.
• It standardizes network communications, as each layer has fixed functions and protocols.
• Diagnosing network problems is easier with the OSI model.
• It is easier to improve with advancements as each layer can get updates separately.

Some key differences between the OSI model and the TCP/IP Model are:

• TCP/IP model consists of 4 layers but OSI model has 7 layers. Layers 5,6,7 of the OSI model are combined into the Application Layer of TCP/IP model and OSI layers 1 and 2 are combined into Network Access Layers of TCP/IP protocol.
• The TCP/IP model is older than the OSI model, hence it is a foundational protocol that defines how should data be transferred online.
• Compared to the OSI model, the TCP/IP model has less strict layer boundaries.
• All layers of the TCP/IP model are needed for data transmission but in the OSI model, some applications can skip certain layers. Only layers 1,2 and 3 of the OSI model are necessary for data transmission.

Did you Know? TCP/IP protocol ( Transfer Control Protocol/Internet Protocol ) was created by U.S. Department of Defense’s Advanced Research Projects Agency (ARPA) in 1970s.

We have discussed about What is OSI model?, What are layers of OSI model, How data flows in the 7 layers of OSI model, and the differences between TCP/IP protocol and OSI protocol.

OSI Model – FAQs

Is osi layer still used.

Yes, the OSI model is still used by networking professionals to understand data abstraction paths and processes better.

What is the highest layer of the OSI model?

Layer 7 or Application layer is highest layer of OSI model.

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