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What is the presentation layer in the OSI model?

The presentation layer is the sixth layer in the OSI model and is responsible for converting different file formats. This allows two systems to communicate. Other tasks carried out by the sixth layer include data compression and encryption.

What is the presentation layer?

What does the presentation layer do, which format does the presentation layer use, presentation layer protocols, skipping the presentation layer.

The presentation layer is the sixth layer of the OSI model. It is primarily used to convert different file formats between the sender and the receiver . The OSI model is a reference model that is used to define communication standards between two devices within a network . The development of this standard began in the 1970s and it was first published at the beginning of the following decade. This standard enables seamless interaction between different technical systems.

The model is made up of a total of seven different layers, all having their own clearly defined tasks. While there are clear boundaries between the layers, the layers interact with each other, with each layer building off the one below it. The different layers are as follows:

  • Physical layer
  • Data link layer
  • Network layer
  • Transport layer
  • Session layer
  • Presentation layer
  • Application layer

The presentation layer interacts closely with the application layer, which is located directly above it. The presentation layer’s main task is to present data in such a way that it can be understood and interpreted from both the system sending the data and the system receiving it. After this has been accomplished, the application layer then determines how the data should be structured and what sort of data and values are permissible.

Using these entries, a command set, or an abstract transfer syntax, is then automatically created. The presentation layer now has the task of transferring the data in such a way that it is readable without changing the information contained within it.

The presentation layer is often also responsible for the encryption and decryption of data . The information is first encrypted on the sender’s side and then sent to the receiver in an encrypted state. Keys and encryption methods are then exchanged in the presentation layer. The recipient is then able to decrypt the unreadable data and convert it into a format that can be understood and interpreted.

If data is shown during a transfer, we often use the term transfer syntax. These are separated into the abstract transfer syntax , in which the transferred values are written, and the concrete syntax, which contains a definition of the value coding.

The receiver can only process and understand the data they receive if they receive all of the information from the presentation layer. The most common definition language is Abstract Syntax Notation One (ASN.1) , which is also recommended by the ISO. The ISO is an organization that is responsible for developing international standards in technology, management and manufacturing.

The presentation layer has many different formats. The most common text formats are the ASCII (American Standard Code for Information Interchange) and EBCDIC (Extended Binary-Coded Decimal Interchange Code). The most common image formats are GIF, JPEG and TIFF. Widely used video formats include MIDI, MPEG and QuickTime.

There are many different presentation layer protocols as well as transfer and encryption technologies in the presentation layer. These include:

The tasks which are carried out by the presentation layer are not always necessary for communication between two systems. In instances where both systems use the same formats, data conversion is not necessary. Additionally, encryption and compression are not required for every interaction and can also be carried out in another layer of the OSI model. If this is the case, the presentation layer can be skipped and the application layer (7) can communicate directly with the session layer (5) instead .

  • Encyclopedia

Build or host a website, launch a server, or store your data and more with our most popular products for less.

about presentation layer
  • 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.

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Presentation Layer

Last Edited

What is the Presentation Layer?

Presentation Layer is the Layer 6 of the seven-layer Open Systems Interconnection (OSI) reference model . The presentation layer structures data that is passed down from the application layer into a format suitable for network transmission. This layer is responsible for data encryption, data compression, character set conversion, interpretation of graphics commands, and so on. The network redirector also functions at this layer.

Presentation Layer

Presentation Layer functions

  • Translation:  Before being transmitted, information in the form of characters and numbers should be changed to bit streams. Layer 6 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:  Encryption at the transmitter and decryption at the receiver
  • Compression:  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 transmitted. Multimedia files, such as audio and video, are bigger than text files and compression is more important.

Role of Presentation Layer in the OSI Model

This layer is not always used in network communications because its functions are not always necessary. Translation is only needed if different types of machines need to talk with each other. Encryption is optional in communication. If the information is public there is no need to encrypt and decrypt info. Compression is also optional. If files are small there is no need for compression.

Explaining Layer 6 in video

Most real-world protocol suites, such as TCP/IP , do not use separate presentation layer protocols. This layer is mostly an abstraction in real-world networking.

An example of a program that loosely adheres to layer 6 of OSI is the tool that manages the Hypertext Transfer Protocol (HTTP) — although it’s technically considered an application-layer protocol per the TCP/IP model.

However, HTTP includes presentation layer services within it. HTTP works when the requesting device forwards user requests passed to the web browser onto a web server elsewhere in the network.

It receives a return message from the web server that includes a multipurpose internet mail extensions (MIME) header. The MIME header indicates the type of file – text, video, or audio – that has been received so that an appropriate player utility can be used to present the file to the user.

In short, the presentation layer

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.

  • ensures proper formatting and delivery to and from the application layer;
  • performs data encryption; and
  • manages serialization of data objects.

The OSI Model – The 7 Layers of Networking Explained in Plain English

This article explains the Open Systems Interconnection (OSI) model and the 7 layers of networking, in plain English.

The OSI model is a conceptual framework that is used to describe how a network functions. In plain English, the OSI model helped standardize the way computer systems send information to each other.

Learning networking is a bit like learning a language - there are lots of standards and then some exceptions. Therefore, it’s important to really understand that the OSI model is not a set of rules. It is a tool for understanding how networks function.

Once you learn the OSI model, you will be able to further understand and appreciate this glorious entity we call the Internet, as well as be able to troubleshoot networking issues with greater fluency and ease.

All hail the Internet!

Prerequisites

You don’t need any prior programming or networking experience to understand this article. However, you will need:

  • Basic familiarity with common networking terms (explained below)
  • A curiosity about how things work :)

Learning Objectives

Over the course of this article, you will learn:

  • What the OSI model is
  • The purpose of each of the 7 layers
  • The problems that can happen at each of the 7 layers
  • The difference between TCP/IP model and the OSI model

Common Networking Terms

Here are some common networking terms that you should be familiar with to get the most out of this article. I’ll use these terms when I talk about OSI layers next.

A node is a physical electronic device hooked up to a network, for example a computer, printer, router, and so on. If set up properly, a node is capable of sending and/or receiving information over a network.

Nodes may be set up adjacent to one other, wherein Node A can connect directly to Node B, or there may be an intermediate node, like a switch or a router, set up between Node A and Node B.

Typically, routers connect networks to the Internet and switches operate within a network to facilitate intra-network communication. Learn more about hub vs. switch vs. router.

Here's an example:

1-Router-Image

For the nitpicky among us (yep, I see you), host is another term that you will encounter in networking. I will define a host as a type of node that requires an IP address. All hosts are nodes, but not all nodes are hosts. Please Tweet angrily at me if you disagree.

Links connect nodes on a network. Links can be wired, like Ethernet, or cable-free, like WiFi.

Links to can either be point-to-point, where Node A is connected to Node B, or multipoint, where Node A is connected to Node B and Node C.

When we’re talking about information being transmitted, this may also be described as a one-to-one vs. a one-to-many relationship.

A protocol is a mutually agreed upon set of rules that allows two nodes on a network to exchange data.

“A protocol defines the rules governing the syntax (what can be communicated), semantics (how it can be communicated), and synchronization (when and at what speed it can be communicated) of the communications procedure. Protocols can be implemented on hardware, software, or a combination of both. Protocols can be created by anyone, but the most widely adopted protocols are based on standards.” - The Illustrated Network.

Both wired and cable-free links can have protocols.

While anyone can create a protocol, the most widely adopted protocols are often based on standards published by Internet organizations such as the Internet Engineering Task Force (IETF).

A network is a general term for a group of computers, printers, or any other device that wants to share data.

Network types include LAN, HAN, CAN, MAN, WAN, BAN, or VPN. Think I’m just randomly rhyming things with the word can ? I can ’t say I am - these are all real network types. Learn more here .

Topology describes how nodes and links fit together in a network configuration, often depicted in a diagram. Here are some common network topology types:

What is Network Topology? Best Guides to Types & Diagrams - DNSstuff

A network consists of nodes, links between nodes, and protocols that govern data transmission between nodes.

At whatever scale and complexity networks get to, you will understand what’s happening in all computer networks by learning the OSI model and 7 layers of networking.

What is the OSI Model?

The OSI model consists of 7 layers of networking.

First, what’s a layer?

Cave, Dragon's Lair, mountains

No, a layer - not a lair . Here there are no dragons.

A layer is a way of categorizing and grouping functionality and behavior on and of a network.

In the OSI model, layers are organized from the most tangible and most physical, to less tangible and less physical but closer to the end user.

Each layer abstracts lower level functionality away until by the time you get to the highest layer. All the details and inner workings of all the other layers are hidden from the end user.

How to remember all the names of the layers? Easy.

  • Please | Physical Layer
  • Do | Data Link Layer
  • Not | Network Layer
  • Tell (the) | Transport Layer
  • Secret | Session Layer
  • Password (to) | Presentation Layer
  • Anyone | Application Layer

Keep in mind that while certain technologies, like protocols, may logically “belong to” one layer more than another, not all technologies fit neatly into a single layer in the OSI model. For example, Ethernet, 802.11 (Wifi) and the Address Resolution Protocol (ARP) procedure operate on >1 layer.

The OSI is a model and a tool, not a set of rules.

OSI Layer 1

Layer 1 is the physical layer . There’s a lot of technology in Layer 1 - everything from physical network devices, cabling, to how the cables hook up to the devices. Plus if we don’t need cables, what the signal type and transmission methods are (for example, wireless broadband).

Instead of listing every type of technology in Layer 1, I’ve created broader categories for these technologies. I encourage readers to learn more about each of these categories:

  • Nodes (devices) and networking hardware components. Devices include hubs, repeaters, routers, computers, printers, and so on. Hardware components that live inside of these devices include antennas, amplifiers, Network Interface Cards (NICs), and more.
  • Device interface mechanics. How and where does a cable connect to a device (cable connector and device socket)? What is the size and shape of the connector, and how many pins does it have? What dictates when a pin is active or inactive?
  • Functional and procedural logic. What is the function of each pin in the connector - send or receive? What procedural logic dictates the sequence of events so a node can start to communicate with another node on Layer 2?
  • Cabling protocols and specifications. Ethernet (CAT), USB, Digital Subscriber Line (DSL) , and more. Specifications include maximum cable length, modulation techniques, radio specifications, line coding, and bits synchronization (more on that below).
  • Cable types. Options include shielded or unshielded twisted pair, untwisted pair, coaxial and so on. Learn more about cable types here .
  • Signal type. Baseband is a single bit stream at a time, like a railway track - one-way only. Broadband consists of multiple bit streams at the same time, like a bi-directional highway.
  • Signal transmission method (may be wired or cable-free). Options include electrical (Ethernet), light (optical networks, fiber optics), radio waves (802.11 WiFi, a/b/g/n/ac/ax variants or Bluetooth). If cable-free, then also consider frequency: 2.5 GHz vs. 5 GHz. If it’s cabled, consider voltage. If cabled and Ethernet, also consider networking standards like 100BASE-T and related standards.

The data unit on Layer 1 is the bit.

A bit the smallest unit of transmittable digital information. Bits are binary, so either a 0 or a 1. Bytes, consisting of 8 bits, are used to represent single characters, like a letter, numeral, or symbol.

Bits are sent to and from hardware devices in accordance with the supported data rate (transmission rate, in number of bits per second or millisecond) and are synchronized so the number of bits sent and received per unit of time remains consistent (this is called bit synchronization). The way bits are transmitted depends on the signal transmission method.

Nodes can send, receive, or send and receive bits. If they can only do one, then the node uses a simplex mode. If they can do both, then the node uses a duplex mode. If a node can send and receive at the same time, it’s full-duplex – if not, it’s just half-duplex.

The original Ethernet was half-duplex. Full-duplex Ethernet is an option now, given the right equipment.

How to Troubleshoot OSI Layer 1 Problems

Here are some Layer 1 problems to watch out for:

  • Defunct cables, for example damaged wires or broken connectors
  • Broken hardware network devices, for example damaged circuits
  • Stuff being unplugged (...we’ve all been there)

If there are issues in Layer 1, anything beyond Layer 1 will not function properly.

Layer 1 contains the infrastructure that makes communication on networks possible.

It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating physical links between network devices. - Source

Fun fact: deep-sea communications cables transmit data around the world. This map will blow your mind: https://www.submarinecablemap.com/

And because you made it this far, here’s a koala:

Closeup of a Koala

OSI Layer 2

Layer 2 is the data link layer . Layer 2 defines how data is formatted for transmission, how much data can flow between nodes, for how long, and what to do when errors are detected in this flow.

In more official tech terms:

  • Line discipline. Who should talk for how long? How long should nodes be able to transit information for?
  • Flow control. How much data should be transmitted?
  • Error control - detection and correction . All data transmission methods have potential for errors, from electrical spikes to dirty connectors. Once Layer 2 technologies tell network administrators about an issue on Layer 2 or Layer 1, the system administrator can correct for those errors on subsequent layers. Layer 2 is mostly concerned with error detection, not error correction. ( Source )

There are two distinct sublayers within Layer 2:

  • Media Access Control (MAC): the MAC sublayer handles the assignment of a hardware identification number, called a MAC address, that uniquely identifies each device on a network. No two devices should have the same MAC address. The MAC address is assigned at the point of manufacturing. It is automatically recognized by most networks. MAC addresses live on Network Interface Cards (NICs). Switches keep track of all MAC addresses on a network. Learn more about MAC addresses on PC Mag and in this article . Learn more about network switches here .
  • Logical Link Control (LLC): the LLC sublayer handles framing addressing and flow control. The speed depends on the link between nodes, for example Ethernet or Wifi.

The data unit on Layer 2 is a frame .

Each frame contains a frame header, body, and a frame trailer:

  • Header: typically includes MAC addresses for the source and destination nodes.
  • Body: consists of the bits being transmitted.
  • Trailer: includes error detection information. When errors are detected, and depending on the implementation or configuration of a network or protocol, frames may be discarded or the error may be reported up to higher layers for further error correction. Examples of error detection mechanisms: Cyclic Redundancy Check (CRC) and Frame Check Sequence (FCS). Learn more about error detection techniques here .

Example of frames, the network layer, and the physical layer

Typically there is a maximum frame size limit, called an Maximum Transmission Unit, MTU. Jumbo frames exceed the standard MTU, learn more about jumbo frames here .

How to Troubleshoot OSI Layer 2 Problems

Here are some Layer 2 problems to watch out for:

  • All the problems that can occur on Layer 1
  • Unsuccessful connections (sessions) between two nodes
  • Sessions that are successfully established but intermittently fail
  • Frame collisions

The Data Link Layer allows nodes to communicate with each other within a local area network. The foundations of line discipline, flow control, and error control are established in this layer.

OSI Layer 3

Layer 3 is the network layer . This is where we send information between and across networks through the use of routers. Instead of just node-to-node communication, we can now do network-to-network communication.

Routers are the workhorse of Layer 3 - we couldn’t have Layer 3 without them. They move data packets across multiple networks.

Not only do they connect to Internet Service Providers (ISPs) to provide access to the Internet, they also keep track of what’s on its network (remember that switches keep track of all MAC addresses on a network), what other networks it’s connected to, and the different paths for routing data packets across these networks.

Routers store all of this addressing and routing information in routing tables.

Here’s a simple example of a routing table:

A routing table showing the destination, subnet mask, and interface

The data unit on Layer 3 is the data packet . Typically, each data packet contains a frame plus an IP address information wrapper. In other words, frames are encapsulated by Layer 3 addressing information.

The data being transmitted in a packet is also sometimes called the payload . While each packet has everything it needs to get to its destination, whether or not it makes it there is another story.

Layer 3 transmissions are connectionless, or best effort - they don't do anything but send the traffic where it’s supposed to go. More on data transport protocols on Layer 4.

Once a node is connected to the Internet, it is assigned an Internet Protocol (IP) address, which looks either like 172.16. 254.1 (IPv4 address convention) or like 2001:0db8:85a3:0000:0000:8a2e:0370:7334 (IPv6 address convention). Routers use IP addresses in their routing tables.

IP addresses are associated with the physical node’s MAC address via the Address Resolution Protocol (ARP), which resolves MAC addresses with the node’s corresponding IP address.

ARP is conventionally considered part of Layer 2, but since IP addresses don’t exist until Layer 3, it’s also part of Layer 3.

How to Troubleshoot OSI Layer 3 Problems

Here are some Layer 3 problems to watch out for:

  • All the problems that can crop up on previous layers :)
  • Faulty or non-functional router or other node
  • IP address is incorrectly configured

Many answers to Layer 3 questions will require the use of command-line tools like ping , trace , show ip route , or show ip protocols . Learn more about troubleshooting on layer 1-3 here .

The Network Layer allows nodes to connect to the Internet and send information across different networks.

OSI Layer 4

Layer 4 is the transport layer . This where we dive into the nitty gritty specifics of the connection between two nodes and how information is transmitted between them. It builds on the functions of Layer 2 - line discipline, flow control, and error control.

This layer is also responsible for data packet segmentation, or how data packets are broken up and sent over the network.

Unlike the previous layer, Layer 4 also has an understanding of the whole message, not just the contents of each individual data packet. With this understanding, Layer 4 is able to manage network congestion by not sending all the packets at once.

The data units of Layer 4 go by a few names. For TCP, the data unit is a packet. For UDP, a packet is referred to as a datagram. I’ll just use the term data packet here for the sake of simplicity.

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are two of the most well-known protocols in Layer 4.

TCP, a connection-oriented protocol, prioritizes data quality over speed.

TCP explicitly establishes a connection with the destination node and requires a handshake between the source and destination nodes when data is transmitted. The handshake confirms that data was received. If the destination node does not receive all of the data, TCP will ask for a retry.

TCP also ensures that packets are delivered or reassembled in the correct order. Learn more about TCP here .

UDP, a connectionless protocol, prioritizes speed over data quality. UDP does not require a handshake, which is why it’s called connectionless.

Because UDP doesn’t have to wait for this acknowledgement, it can send data at a faster rate, but not all of the data may be successfully transmitted and we’d never know.

If information is split up into multiple datagrams, unless those datagrams contain a sequence number, UDP does not ensure that packets are reassembled in the correct order. Learn more about UDP here .

TCP and UDP both send data to specific ports on a network device, which has an IP address. The combination of the IP address and the port number is called a socket.

Learn more about sockets here .

Learn more about the differences and similarities between these two protocols here .

How to Troubleshoot OSI Layer 4 Problems

Here are some Layer 4 problems to watch out for:

  • Blocked ports - check your Access Control Lists (ACL) & firewalls
  • Quality of Service (QoS) settings. QoS is a feature of routers/switches that can prioritize traffic, and they can really muck things up. Learn more about QoS here .

The Transport Layer provides end-to-end transmission of a message by segmenting a message into multiple data packets; the layer supports connection-oriented and connectionless communication.

OSI Layer 5

Layer 5 is the session layer . This layer establishes, maintains, and terminates sessions.

A session is a mutually agreed upon connection that is established between two network applications. Not two nodes! Nope, we’ve moved on from nodes. They were so Layer 4.

Just kidding, we still have nodes, but Layer 5 doesn’t need to retain the concept of a node because that’s been abstracted out (taken care of) by previous layers.

So a session is a connection that is established between two specific end-user applications. There are two important concepts to consider here:

  • Client and server model: the application requesting the information is called the client, and the application that has the requested information is called the server.
  • Request and response model: while a session is being established and during a session, there is a constant back-and-forth of requests for information and responses containing that information or “hey, I don’t have what you’re requesting.”

Sessions may be open for a very short amount of time or a long amount of time. They may fail sometimes, too.

Depending on the protocol in question, various failure resolution processes may kick in. Depending on the applications/protocols/hardware in use, sessions may support simplex, half-duplex, or full-duplex modes.

Examples of protocols on Layer 5 include Network Basic Input Output System (NetBIOS) and Remote Procedure Call Protocol (RPC), and many others.

From here on out (layer 5 and up), networks are focused on ways of making connections to end-user applications and displaying data to the user.

How to Troubleshoot OSI Layer 5 Problems

Here are some Layer 5 problems to watch out for:

  • Servers are unavailable
  • Servers are incorrectly configured, for example Apache or PHP configs
  • Session failure - disconnect, timeout, and so on.

The Session Layer initiates, maintains, and terminates connections between two end-user applications. It responds to requests from the presentation layer and issues requests to the transport layer.

OSI Layer 6

Layer 6 is the presentation layer . This layer is responsible for data formatting, such as character encoding and conversions, and data encryption.

The operating system that hosts the end-user application is typically involved in Layer 6 processes. This functionality is not always implemented in a network protocol.

Layer 6 makes sure that end-user applications operating on Layer 7 can successfully consume data and, of course, eventually display it.

There are three data formatting methods to be aware of:

  • American Standard Code for Information Interchange (ASCII): this 7-bit encoding technique is the most widely used standard for character encoding. One superset is ISO-8859-1, which provides most of the characters necessary for languages spoken in Western Europe.
  • Extended Binary-Coded Decimal Interchange Code (EBDCIC): designed by IBM for mainframe usage. This encoding is incompatible with other character encoding methods.
  • Unicode: character encodings can be done with 32-, 16-, or 8-bit characters and attempts to accommodate every known, written alphabet.

Learn more about character encoding methods in this article , and also here .

Encryption: SSL or TLS encryption protocols live on Layer 6. These encryption protocols help ensure that transmitted data is less vulnerable to malicious actors by providing authentication and data encryption for nodes operating on a network. TLS is the successor to SSL.

How to Troubleshoot OSI Layer 6 Problems

Here are some Layer 6 problems to watch out for:

  • Non-existent or corrupted drivers
  • Incorrect OS user access level

The Presentation Layer formats and encrypts data.

OSI Layer 7

Layer 7 is the application layer .

True to its name, this is the layer that is ultimately responsible for supporting services used by end-user applications. Applications include software programs that are installed on the operating system, like Internet browsers (for example, Firefox) or word processing programs (for example, Microsoft Word).

Applications can perform specialized network functions under the hood and require specialized services that fall under the umbrella of Layer 7.

Electronic mail programs, for example, are specifically created to run over a network and utilize networking functionality, such as email protocols, which fall under Layer 7.

Applications will also control end-user interaction, such as security checks (for example, MFA), identification of two participants, initiation of an exchange of information, and so on.

Protocols that operate on this level include File Transfer Protocol (FTP), Secure Shell (SSH), Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), Domain Name Service (DNS), and Hypertext Transfer Protocol (HTTP).

While each of these protocols serve different functions and operate differently, on a high level they all facilitate the communication of information. ( Source )

How to Troubleshoot OSI Layer 7 Problems

Here are some Layer 7 problems to watch out for:

  • All issues on previous layers
  • Incorrectly configured software applications
  • User error (... we’ve all been there)

The Application Layer owns the services and functions that end-user applications need to work. It does not include the applications themselves.

Our Layer 1 koala is all grown up.

Koala with Photoshopped makeup

Learning check - can you apply makeup to a koala?

Don’t have a koala?

Well - answer these questions instead. It’s the next best thing, I promise.

  • What is the OSI model?
  • What are each of the layers?
  • How could I use this information to troubleshoot networking issues?

Congratulations - you’ve taken one step farther to understanding the glorious entity we call the Internet.

Learning Resources

Many, very smart people have written entire books about the OSI model or entire books about specific layers. I encourage readers to check out any O’Reilly-published books about the subject or about network engineering in general.

Here are some resources I used when writing this article:

  • The Illustrated Network, 2nd Edition
  • Protocol Data Unit (PDU): https://www.geeksforgeeks.org/difference-between-segments-packets-and-frames/
  • Troubleshooting Along the OSI Model: https://www.pearsonitcertification.com/articles/article.aspx?p=1730891
  • The OSI Model Demystified: https://www.youtube.com/watch?v=HEEnLZV2wGI
  • OSI Model for Dummies: https://www.dummies.com/programming/networking/layers-in-the-osi-model-of-a-computer-network/

Chloe Tucker is an artist and computer science enthusiast based in Portland, Oregon. As a former educator, she's continuously searching for the intersection of learning and teaching, or technology and art. Reach out to her on Twitter @_chloetucker and check out her website at chloe.dev .

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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.

Presentation Layer in ISO-OSI Model

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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How-To Geek

The 7 osi networking layers explained.

The Open Systems Interconnection (OSI) networking model defines a conceptual framework for communications between computer systems.

Quick Links

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

The Open Systems Interconnection (OSI) networking model defines a conceptual framework for communications between computer systems. The model is an ISO standard which identifies seven fundamental networking layers, from the physical hardware up to high-level software applications.

Each layer in the model handles a specific networking function. The standard helps administrators to visualize networks, isolate problems, and understand the use cases for new technologies. Many network equipment vendors advertise the OSI layer that their products are designed to slot into.

OSI was adopted as an international standard in 1984. It remains relevant today despite the changes to network implementation that have occurred since first publication. Cloud, edge, and IoT can all be accommodated within the model.

Diagram showing the 7 OSI networking layers

In this article, we'll explain each of the seven OSI layers in turn. We'll start from the lowest level, labelled as Layer 1.

1. Physical Layer

All networking begins with physical equipment. This layer encapsulates the hardware involved in the communications, such as switches and cables. Data is transferred as a stream of binary digits - 0 or 1 - that the hardware prepares from input it's been fed. The physical layer specifies the electrical signals that are used to encode the data over the wire, such as a 5-volt pulse to indicate a binary "1."

Errors in the physical layer tend to result in data not being transferred at all. There could be a break in the connection due to a missing plug or incorrect power supply. Problems can also arise when two components disagree on the physical encoding of data values. In the case of wireless connections, a weak signal can lead to bit loss during transmission.

2. Data Link Layer

The model's second layer concerns communication between two devices that are directly connected to each other in the same network. It's responsible for establishing a link that allows data to be exchanged using an agreed protocol. Many network switches operate at Layer 2.

The data link layer will eventually pass bits to the physical layer. As it sits above the hardware, the data link layer can perform basic error detection and correction in response to physical transfer issues. There are two sub-layers that define these responsibilities: Logical Link Control (LLC) that handles frame synchronization and error detection, and Media Access Control (MAC) which uses MAC addresses to constrain how devices acquire permission to transfer data.

3. Network Layer

The network layer is the first level to support data transfer between two separately maintained networks. It's redundant in situations where all your devices exist on the same network.

Data that comes to the network layer from higher levels is first broken up into packets suitable for transmission. Packets received from the remote network in response are reassembled into usable data.

The network layer is where several important protocols are first encountered. These include IP (for determining the path to a destination), ICMP, routing, and virtual LAN. Together these mechanisms facilitate inter-network communications with a familiar degree of usability. However operations at this level aren't necessarily reliable: messages aren't required to succeed and may not necessarily be retried.

4. Transport Layer

The transport layer provides higher-level abstractions for coordinating data transfers between devices. Transport controllers determine where data will be sent and the rate it should be transferred at.

Layer 4 is where TCP and UDP are implemented, providing the port numbers that allow devices to expose multiple communication channels. Load balancing is often situated at Layer 4 as a result, allowing traffic to be routed between ports on a target device.

Transport mechanisms are expected to guarantee successful communication. Stringent error controls are applied to recover from packet loss and retry failed transfers. Flow control is enforced so the sender doesn't overwhelm the remote device by sending data more quickly than the available bandwidth permits.

5. Session Layer

Layer 5 creates ongoing communication sessions between two devices. Sessions are used to negotiate new connections, agree on their duration, and gracefully close down the connection once the data exchange is complete. This layer ensures that sessions remain open long enough to transfer all the data that's being sent.

Checkpoint control is another responsibility that's held by Layer 5. Sessions can define checkpoints to facilitate progress updates and resumable transmissions. A new checkpoint could be set every few megabytes for a file upload, allowing the sender to continue from a particular point if the transfer gets interrupted.

Many significant protocols operate at Layer 5 including authentication and logon technologies such as LDAP and NetBIOS. These establish semi-permanent communication channels for managing an end user session on a specific device.

6. Presentation Layer

The presentation layer handles preparation of data for the application layer that comes next in the model. After data has made it up from the hardware, through the data link, and across the transport, it's almost ready to be consumed by high-level components. The presentation layer completes the process by performing any formatting tasks that may be required.

Decryption, decoding, and decompression are three common operations found at this level. The presentation layer processes received data into formats that can be eventually utilized by a client application. Similarly, outward-bound data is reformatted into compressed and encrypted structures that are suitable for network transmission.

TLS is one major technology that's part of the presentation layer. Certificate verification and data decryption is handled before requests reach the network client, allowing information to be consumed with confidence that it's authentic.

7. Application Layer

The application layer is the top of the stack. It represents the functionality that's perceived by network end users. Applications in the OSI model provide a convenient end-to-end interface to facilitate complete data transfers, without making you think about hardware, data links, sessions, and compression.

Despite its name, this layer doesn't relate to client-side software such as your web browser or email client. An application in OSI terms is a protocol that caters for the complete communication of complex data through layers 1-6.

HTTP, FTP, DHCP, DNS, and SSH all exist at the application layer. These are high-level mechanisms which permit direct transfers of user data between an origin device and a remote server. You only need minimal knowledge of the workings of the other layers.

The seven OSI layers describe the transfer of data through computer networks. Understanding the functions and responsibilities of each layer can help you identify the source of problems and assess the intended use case for new components.

OSI is an abstract model that doesn't directly map to the specific networking implementations commonly used today. As an example, the TCP/IP protocol works on its own simpler system of four layers: Network Access, Internet, Transport, and Application. These abstract and absorb the equivalent OSI layers: the application layer spans OSI L5 to L7, while L1 and L2 are combined in TCP/IP's concept of Network Access.

OSI remains applicable despite its lack of direct real-world application. It's been around so long that it's widely understood among administrators from all backgrounds. Its relatively high level of abstraction has also ensured it's remained relevant in the face of new networking paradigms, many of which have targeted Layer 3 and above. An awareness of the seven layers and their responsibilities can still help you appreciate the flow of data through a network while uncovering integration opportunities for new components.

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The OSI model explained and how to easily remember its 7 layers

A tutorial on the Open Systems Interconnection networking reference model and tips on and how to memorize the seven layers

OSI model

The Open Systems Interconnect (OSI) model is a conceptual framework that describes networking or telecommunications systems as seven layers, each with its own function.

The layers help network pros visualize what is going on within their networks and can help network managers narrow down problems (is it a physical issue or something with the application?), as well as computer programmers (when developing an application, which other layers does it need to work with?). Tech vendors selling new products will often refer to the OSI model to help customers understand which layer their products work with or whether it works “across the stack”.

The 7 layers of the OSI model

The layers are: Layer 1—Physical; Layer 2—Data Link; Layer 3—Network; Layer 4—Transport; Layer 5—Session; Layer 6—Presentation; Layer 7—Application.

It wasn’t always this way. Conceived in the 1970s when computer networking was taking off, two separate models were merged in 1983 and published in 1984 to create the OSI model that most people are familiar with today. Most descriptions of the OSI model go from top to bottom, with the numbers going from Layer 7 down to Layer 1. The layers, and what they represent, are as follows:

Layer 7 – Application

The Application Layer in the OSI model is the layer that is the “closest to the end user”. It receives information directly from users and displays incoming data to the user. Oddly enough, applications themselves do not reside at the application layer. Instead the layer facilitates communication through lower layers in order to establish connections with applications at the other end. Web browsers (Google Chrome, Firefox, Safari, etc.) TelNet, and FTP, are examples of communications  that rely  on Layer 7.

Layer 6 – Presentation

The Presentation Layer represents the area that is independent of data representation at the application layer. In general, it represents the preparation or translation of application format to network format, or from network formatting to application format. In other words, the layer “presents” data for the application or the network. A good example of this is encryption and decryption of data for secure transmission; this happens at Layer 6.

Layer 5 – Session

When two computers or other networked devices need to speak with one another, a session needs to be created, and this is done at the Session Layer . Functions at this layer involve setup, coordination (how long should a system wait for a response, for example) and termination between the applications at each end of the session.

Layer 4 – Transport

The Transport Layer deals with the coordination of the data transfer between end systems and hosts. How much data to send, at what rate, where it goes, etc. The best known example of the Transport Layer is the Transmission Control Protocol (TCP), which is built on top of the Internet Protocol (IP), commonly known as TCP/IP. TCP and UDP port numbers work at Layer 4, while IP addresses work at Layer 3, the Network Layer.

Layer 3 – Network

Here at the Network Layer is where you’ll find most of the router functionality that most networking professionals care about and love. In its most basic sense, this layer is responsible for packet forwarding, including routing through different routers . You might know that your Boston computer wants to connect to a server in California, but there are millions of different paths to take. Routers at this layer help do this efficiently.

Layer 2 – Data Link

The Data Link Layer provides node-to-node data transfer (between two directly connected nodes), and also handles error correction from the physical layer. Two sublayers exist here as well–the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. In the networking world, most switches operate at Layer 2. But it’s not that simple. Some switches also operate at Layer 3 in order to support virtual LANs that may span more than one switch subnet, which requires routing capabilities.

Layer 1 – Physical

At the bottom of our OSI model we have the Physical Layer, which represents the electrical and physical representation of the system. This can include everything from the cable type, radio frequency link (as in a Wi-Fi network), as well as the layout of pins, voltages, and other physical requirements. When a networking problem occurs, many networking pros go right to the physical layer to check that all of the cables are properly connected and that the power plug hasn’t been pulled from the router, switch or computer, for example.

Why you need to know the 7 OSI layers

Most people in IT will likely need to know about the different layers when they’re going for their certifications, much like a civics student needs to learn about the three branches of the US government. After that, you hear about the OSI model when vendors are making pitches about which layers their products work with.

In a Quora post  asking about the purpose of the OSI model, Vikram Kumar answered this way:

“The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools.”

While some people may argue that the OSI model is obsolete (due to its conceptual nature) and less important than the four layers of the TCP/IP model, Kumar says that “it is difficult to read about networking technology today without seeing references to the OSI model and its layers, because the model’s structure helps to frame discussions of protocols and contrast various technologies.”

If you can understand the OSI model and its layers, you can also then understand which protocols and devices can interoperate with each other when new technologies are developed and explained.

The OSI model remains relevant

In a post on GeeksforGeeks, contributor Vabhav Bilotia argues several reasons why the OSI model remains relevant, especially when it comes to security and determining where technical risks and vulnerabilities may exist.

For example, by understanding the different layers, enterprise security teams can identify and classify physical access, where the data is sitting, and provide an inventory of the applications that employees use to access data and resources.

“Knowing where the majority of your company’s data is held, whether on-premises or in cloud services, will help define your information security policy,” writes Bilotia. “You can invest in the correct solutions that provide you data visibility within the proper OSI layers once you have this knowledge.”

In addition, the OSI model can be used to understand cloud infrastructure migrations, particularly when it comes to securing data within the cloud.

And because the model has been around for so long and understood by so many, the uniform vocabulary and terms helps networking professionals understand quickly about the components of the networking system “While this paradigm is not directly implemented in today’s TCP/IP networks, it is a useful conceptual model for relating multiple technologies to one another and implementing the appropriate technology in the appropriate way,” Bilotia writes. We couldn’t agree more.

How to remember the OSI Model 7 layers – 8 mnemonic tricks

If you need to memorize the layers for a college or certification test, here are a few sentences to help remember them in order. The first letter of each word is the same as the first letter an OSI layer.

From Application to Physical (Layer 7 to Layer 1): 

  • All People Seem To Need Data Processing
  • All Pros Search Top Notch Donut Places
  • A Penguin Said That Nobody Drinks Pepsi
  • A Priest Saw Two Nuns Doing Pushups

From Physical to Application (Layer 1 to Layer 7):

  • Please Do Not Throw Sausage Pizza Away
  • Pew! Dead Ninja Turtles Smell Particularly Awful
  • People Don’t Need To See Paula Abdul
  • Pete Doesn’t Need To Sell Pickles Anymore

Keith Shaw was a Network World editor and the writer of the Cool Tools column. He is now a freelance writer and editor from Worcester, Mass.

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keith_shaw

The first gadget Keith Shaw ever wanted was the Merlin, a red plastic toy that beeped and played Tic-Tac-Toe and various other games. A child of the '70s and teenager of the '80s, Shaw has been a fan of computers, technology and video games right from the start. He won an award in 8th grade for programming a game on the school's only computer, and saved his allowance to buy an Atari 2600.

Shaw has a bachelor's degree in newspaper journalism from Syracuse University and has worked at a variety of newspapers in New York, Florida and Massachusetts, as well as Computerworld and Network World. He won an award from the American Society of Business Publication Editors for a 2003 article on anti-spam testing, and a Gold Award in their 2010 Digital Awards Competition for the "ABCs of IT" video series.

Shaw is also the co-creator of taquitos.net , the crunchiest site on the InterWeb, which has taste-tested and reviewed more than 4,000 varieties of snack foods.

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Three-tier architecture is a well-established software application architecture that organizes applications into three logical and physical computing tiers: the presentation tier, or user interface; the application tier, where data is processed; and the data tier, where the data associated with the application is stored and managed.

The chief benefit of three-tier architecture is that because each tier runs on its own infrastructure, each tier can be developed simultaneously by a separate development team, and can be updated or scaled as needed without impacting the other tiers.

For decades three-tier architecture was the prevailing architecture for client-server applications. Today, most three-tier applications are targets for modernization , using cloud-native technologies such as containers and microservices , and for migration to the cloud.

Presentation tier

The presentation tier is the user interface and communication layer of the application, where the end user interacts with the application. Its main purpose is to display information to and collect information from the user. This top-level tier can run on a web browser, as desktop application, or a graphical user interface (GUI), for example. Web presentation tiers are usually developed using HTML, CSS and JavaScript. Desktop applications can be written in a variety of languages depending on the platform.

Application tier

The application tier, also known as the logic tier or middle tier, is the heart of the application. In this tier, information collected in the presentation tier is processed - sometimes against other information in the data tier - using business logic, a specific set of business rules. The application tier can also add, delete or modify data in the data tier. 

The application tier is typically developed using Python, Java, Perl, PHP or Ruby, and communicates with the data tier using  API  calls. 

The data tier, sometimes called database tier, data access tier or back-end, is where the information processed by the application is stored and managed. This can be a  relational database management system  such as  PostgreSQL , MySQL, MariaDB, Oracle, DB2, Informix or Microsoft SQL Server, or in a  NoSQL  Database server such as Cassandra,  CouchDB  or  MongoDB . 

In a three-tier application, all communication goes through the application tier. The presentation tier and the data tier cannot communicate directly with one another.

Tier vs. layer

In discussions of three-tier architecture,  layer  is often used interchangeably – and mistakenly – for  tier , as in 'presentation layer' or 'business logic layer.' 

They aren't the same. A 'layer' refers to a functional division of the software, but a 'tier' refers to a functional division of the software that runs on infrastructure separate from the other divisions. The Contacts app on your phone, for example, is a  three - layer  application, but a  single-tier  application, because all three layers run on your phone.

The difference is important, because layers can't offer the same benefits as tiers.

Again, the chief benefit of three-tier architecture its logical and physical separation of functionality. Each tier can run on a separate operating system and server platform - e.g., web server, application server, database server - that best fits its functional requirements. And each tier runs on at least one dedicated server hardware or virtual server, so the services of each tier can be customized and optimized without impact the other tiers. 

Other benefits (compared to single- or two-tier architecture) include:

  • Faster development : Because each tier can be developed simultaneously by different teams, an organization can bring the application to market faster, and programmers can use the latest and best languages and tools for each tier.
  • Improved scalability : Any tier can be scaled independently of the others as needed.
  • Improved reliability : An outage in one tier is less likely to impact the availability or performance of the other tiers.
  • Improved security : Because the presentation tier and data tier can't communicate directly, a well-designed application tier can function as a sort of internal firewall, preventing SQL injections and other malicious exploits.

In web development, the tiers have different names but perform similar functions:

  • The web server  is the presentation tier and provides the user interface. This is usually a web page or web site, such as an ecommerce site where the user adds products to the shopping cart, adds payment details or creates an account. The content can be static or dynamic, and is usually developed using HTML, CSS and Javascript.
  • The application server  corresponds to the middle tier, housing the business logic used to process user inputs. To continue the ecommerce example, this is the tier that queries the inventory database to return product availability, or adds details to a customer's profile. This layer often developed using Python, Ruby or PHP and runs a framework such as e Django, Rails, Symphony or ASP.NET, for example.
  • The database server  is the data or backend tier of a web application. It runs on database management software, such as MySQL, Oracle, DB2 or PostgreSQL, for example.

While three-tier architecture is easily the most widely-adopted multi-tier application architecture, there are others you might encounter in your work or your research.

Two-tier architecture 

Two-tier architecture is the original client-server architecture, consisting of a presentation tier and a data tier; the business logic lives in the presentation tier, the data tier or both. In two-tier architecture the presentation tier - and consequently the end user - has direct access to the data tier, and the business logic is often limited. A simple contact management application, where users can enter and retrieve contact data, is an example of a two-tier application. 

N-tier architecture

N-tier architecture - also called or multi-tier architecture - refers to  any  application architecture with more than one tier. But applications with more than three layers are rare, because additional layers offer few benefits and can make the application slower, harder to manage and more expensive to run. As a result, n-tier architecture and multi-tier architecture are usually synonyms for three-tier architecture.

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Session and Presentation layers in the OSI model

Alessandro Maggio

  • December 29, 2016

Session and presentation layers

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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 limitations: segment order and reliable delivery

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 .

Real-time transport protocol header

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.

Segments over different path

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.

Audio codecs G.711 and iLBC

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.

Session and presentation layers

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 OSI Model? – Layers of OSI Model

OSI stands for Open Systems Interconnection . It was developed by ISO – ‘International Organization for Standardization’, in the year 1984. 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.

Prerequisite: Basics of Computer Networking

What is OSI 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.

OSI Model

What are the 7 layers of the OSI Model?

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

  • Physical Layer
  • Session Layer
  • Presentation Layer

Physical Layer – Layer 1

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 .

Data Link Layer (DLL) – Layer 2

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.

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 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.  

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 & 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

Session Layer – Layer 5

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

Communication in Session Layer

Presentation Layer – Layer 6

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

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, 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 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 file in a remote host, retrieve files in 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. 

OSI Model in a Nutshell

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COMMENTS

  1. Presentation layer

    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.

  2. Presentation Layer in OSI model

    Introduction : 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.

  3. What is presentation layer?

    The presentation layer resides at Layer 6 of the Open Systems Interconnection ( OSI) communications model and ensures that communications that pass through it are in the appropriate form for the recipient application. In other words, the presentation layer presents the data in a readable format from an application layer perspective.

  4. What is the presentation layer?

    Know-how What is the presentation layer in the OSI model? The presentation layer is the sixth layer in the OSI model and is responsible for converting different file formats. This allows two systems to communicate. Other tasks carried out by the sixth layer include data compression and encryption. Contents What is the presentation layer?

  5. Presentation Layer

    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.

  6. Presentation Layer of the OSI Model: Definition and Function

    The presentation layer is the sixth layer of the Open Systems Interconnection (OSI), model. In computer networking, the OSI model is a concept that describes the transmission of data from one computer to another.

  7. What is the Presentation Layer?

    The presentation layer follows data programming structure schemes developed for different languages and provides the real-time syntax required for communication between two objects such as layers, systems or networks. The data format should be acceptable by the next layers; otherwise, the presentation layer may not perform correctly.

  8. What is the Presentation Layer, Anyway?

    In technical speech, the presentation layer is the sixth of seven layers of OSI code that translates data stored in another layer into something a human can interpret. It's the presentation layer that knows to put the right information into the appropriate fields of a customer record, for example: Name, Date of Birth, Address, etc.

  9. Presentation Layer of the OSI Model

    What is the presentation layer? The presentation layer of the OSI model is responsible for translating, encrypting, and compressing data within the model. It is responsible for integrating all...

  10. Presentation Layer

    in Letter P What is the Presentation Layer? Presentation Layer is the Layer 6 of the seven-layer Open Systems Interconnection (OSI) reference model. The presentation layer structures data that is passed down from the application layer into a format suitable for network transmission.

  11. The OSI Model

    The Presentation Layer formats and encrypts data. OSI Layer 7. Layer 7 is the application layer. True to its name, this is the layer that is ultimately responsible for supporting services used by end-user applications. Applications include software programs that are installed on the operating system, like Internet browsers (for example, Firefox ...

  12. Presentation Layer of OSI Reference Model

    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.

  13. The 7 OSI Networking Layers Explained

    The presentation layer completes the process by performing any formatting tasks that may be required. Decryption, decoding, and decompression are three common operations found at this level. The presentation layer processes received data into formats that can be eventually utilized by a client application. Similarly, outward-bound data is ...

  14. The OSI model explained and how to easily remember its 7 layers

    The 7 layers of the OSI model. The layers are: Layer 1—Physical; Layer 2—Data Link; Layer 3—Network; Layer 4—Transport; Layer 5—Session; Layer 6—Presentation; Layer 7—Application. It ...

  15. What is Three-Tier Architecture

    In a three-tier application, all communication goes through the application tier. The presentation tier and the data tier cannot communicate directly with one another. Tier vs. layer. In discussions of three-tier architecture, layer is often used interchangeably - and mistakenly - for tier, as in 'presentation layer' or 'business logic layer.'

  16. OSI model

    7 Answers Sorted by: 18 The session layer is meant to store states between two connections, like what we use cookies for when working with web programming. The presentation layer is meant to convert between different formats. This was simpler when the only format that was worried about was character encoding, ie ASCII and EBCDIC.

  17. Presentation Layer ln OSI Model

    🔥Post Graduate Program In Cyber Security: https://www.simplilearn.com/pgp-cyber-security-certification-training-course?utm_campaign=PresentationLayerInOSIMo...

  18. Session and Presentation layers in the OSI model

    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 ...

  19. Presentation Layer in OSI Model

    The presentation layer is responsible for encrypting and decrypting data to avoid data leakage and data modification. The plaintext data at the source is encrypted into ciphertext (unreadable format), then it is sent to the receiver, where the ciphertext is decrypted into plaintext.

  20. What is OSI Model

    Presentation Layer - Layer 6. 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.

  21. Difference between presentation layer and user-interface

    Presentation layer is term in the taxonomy of code and associated resources. User Interface is the implementation of the intended User Experience in terms of page layout, page transitions and page control elements. (I am using "page" loosely here - you can replace it with "form" or "window").

  22. OSI Model: 7 Layers Explained in Computer Network

    6) Presentation Layer. A Presentation layer is mainly concerned with the syntax and semantics of the information exchanged between the two systems. It acts as a data translator for a network. This layer is a part of the operating system that converts the data from one presentation format to another format. The Presentation layer is also known ...

  23. Celularity's Tri-layer Decellularized, Dehydrated Human

    The study described in the poster presentation investigated Celularity's tri-layer decellularized, dehydrated human amniotic membrane technology product as a carrier of induced pluripotent stem ...