This post explains the open up Systems Interconnection (OSI) model and the 7 great of networking, in level English.

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The OSI version is a conceptual framework that is provided to define how a network functions. In plain English, the OSI model aided standardize the means computer equipment send information to every other.

Learning networking is a little like learning a language - over there are many standards and also then part exceptions. Therefore, it’s vital to really understand that the OSI version is no a collection of rules. It is a tool for understanding how networks function.

Once you find out the OSI model, friend will have the ability to further understand and also appreciate this glorious entity we contact the Internet, and be able to troubleshoot networking worries with better fluency and also ease.

All hail the Internet!


You nothing need any prior programming or networking endure to understand this article. However, you will need:

Basic familiarity with common networking state (explained below)A curiosity around how things work-related :)

Learning Objectives

Over the course of this article, you will learn:

What the OSI version isThe objective of every of the 7 layersThe troubles that can happen at each of the 7 layersThe difference in between TCP/IP model and also the OSI model

Common Networking Terms

Here are some usual networking terms the you should be acquainted with to acquire the many out the this article. I’ll usage these terms once I talk around OSI layers next.


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

Nodes may be set up nearby to one other, within Node A can attach directly come Node B, or there might be an intermediate node, choose a move or a router, set up between Node A and Node B.

Typically, routers attach networks come the Internet and also switches operate within a network come facilitate intra-network communication. Learn an ext about hub vs. Switch vs. Router.

Here"s one example:


For the nitpicky amongst us (yep, I watch you), host is one more term the you will encounter in networking. I will define a organize as a kind of node that requires an IP address. All hosts are nodes, yet not all nodes space hosts. Please Tweet angrily at me if friend disagree.


Links affix nodes on a network. Links deserve to be wired, like Ethernet, or cable-free, choose WiFi.

Links to have the right to either it is in point-to-point, wherein Node A is associated to Node B, or multipoint, where Node A is connected to Node B and Node C.

When we’re talking around information being transmitted, this may likewise be explained as a one-to-one vs. A one-to-many relationship.


A protocol is a support agreed upon collection of rule that permits two nodes ~ above a network to exchange data.

“A protocol defines the rules governing the syntax (what can be communicated), semantics (how it can be communicated), and also synchronization (when and also at what rate it can be communicated) the the interactions procedure. Protocols have the right to be implemented on hardware, software, or a mix of both. Protocols can be produced by anyone, yet the many widely embraced protocols are based on standards.” - The illustrated Network.

Both wired and cable-free links can have protocols.

While everyone can create a protocol, the many widely adopted protocols space often based on standards released by Internet institutions such together the Internet engineering Task pressure (IETF).


A network is a basic term because that a team of computers, printers, or any type of other machine that desires to share data.

Network types include LAN, HAN, CAN, MAN, WAN, BAN, or VPN. Think I’m simply randomly rhyming points with words can? ns can’t say i am - these space all actual network types. Learn more here.


Topology defines how nodes and also links fit together in a network configuration, often illustrated in a diagram. Below are some common network topology types:

Source + learn more about network topologies here

A network is composed of nodes, links in between nodes, and also protocols the govern data transmission in between nodes.

At whatever scale and also complexity networks obtain to, you will know what’s continue in all computer networks by discovering the OSI model and 7 class of networking.

What is the OSI Model?

The OSI model consists of 7 layers of networking.

First, what’s a layer?


Ooo, lair.

No, a layer - no a lair. Below there are no dragons.

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

In the OSI model, class are organized from the many tangible and most physical, to much less tangible and less physical however closer come the end user.

Each layer abstracts reduced level functionality away till by the time you get to the highest possible layer. Every the details and inner functions of all the other layers are hidden from the end user.

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

Please | physical LayerDo | Data attach LayerNot | Network LayerTell (the) | transport LayerSecret | conference LayerPassword (to) | Presentation LayerAnyone | application Layer

Keep in mind the while specific technologies, prefer protocols, may logically “belong to” one layer much more than another, no all technologies fit neatly right into a single layer in the OSI model. For example, Ethernet, 802.11 (Wifi) and the resolve Resolution Protocol (ARP) procedure operate on >1 layer.

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

OSI layer 1

Layer 1 is the physical layer. Yes a lot of modern technology in great 1 - everything from physics network devices, cabling, to exactly how the cables hook approximately the devices. Plus if we don’t require cables, what the signal form and transmission methods are (for example, wireless broadband).

Instead of listing every kind of an innovation in class 1, i have created more comprehensive categories for these technologies. Ns encourage readers to learn much more about each of these categories:

Nodes (devices) and also networking hardware components. devices include hubs, repeaters, routers, computers, printers, and so on. Hardware components that live within of these gadgets include antennas, amplifiers, Network interface Cards (NICs), and also more.Device user interface mechanics. How and also where does a cable affix to a an equipment (cable connector and an equipment socket)? What is the size and shape of the connector, and how plenty of pins does the have? What dictates as soon as a pen is energetic or inactive?Functional and also procedural logic. What is the function of each pen in the connector - send or receive? What procedural reasonable dictates the succession of occasions so a node have the right to start to communicate with an additional node on class 2?Signal type. Baseband is a solitary bit stream at a time, like a railway monitor - one-way only. Broadband is composed of multiple little streams at the exact same time, prefer a bi-directional highway.Signal transmission method (may it is in wired or cable-free). options include electric (Ethernet), irradiate (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 the cabled, consider voltage. If cabled and also Ethernet, likewise consider networking standards favor 100BASE-T and related standards.

The data unit on class 1 is the bit.

A little bit the the smallest unit the transmittable digital information. Bits space binary, so either a 0 or a 1. Bytes, consist of 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 also are synchronized for this reason the number of bits sent and received per unit the time remains constant (this is dubbed bit synchronization). The means bits are transmitted depends on the signal infection method.

Nodes deserve to send, receive, or send and receive bits. If they have the right to only execute one, climate the node uses a simplex mode. If they can do both, then the node uses a duplex mode. If a node can send and also receive in ~ the very same time, that full-duplex – if not, it’s simply half-duplex.

The initial Ethernet to be half-duplex. Full-duplex Ethernet is an choice now, given the appropriate equipment.

How come Troubleshoot OSI great 1 Problems

Here room some layer 1 troubles to watch the end for:

Defunct cables, for instance damaged wires or damaged connectorsBroken hardware network devices, for example damaged circuitsStuff gift unplugged (...we’ve all been there)

If there are worries in great 1, anything past Layer 1 will not duty properly.


Layer 1 consists of the framework that makes interaction on networks possible.

It defines the electrical, mechanical, procedural, and functional specifications because that activating, maintaining, and also deactivating physics links between network devices. - Source

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

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


OSI great 2

Layer 2 is the data attach layer. Layer 2 specifies how data is formatted for transmission, how much data can flow in between nodes, for exactly how long, and also what to do when errors space detected in this flow.

In much more official technology terms:

Line discipline. Who must talk for how long? how long should nodes have the ability to transit info for?Flow control. just how much data need to be sent ?

There are two distinct sublayers in ~ Layer 2:

Logical link Control (LLC): the llc sublayer handles framing addressing and also flow control. The speed depends on the link in between nodes, for example Ethernet or Wifi.

The data unit on class 2 is a frame.

Each frame consists of a structure header, body, and also a frame trailer:

Header: typically includes MAC addresses for the resource and destination nodes.Body: consists of the bits being transmitted.

Typically there is a maximum frame size limit, called an Maximum transmission Unit, MTU. Jumbo frames exceed the typical MTU, learn an ext about jumbo frames here.

How come Troubleshoot OSI class 2 Problems

Here space some class 2 problems to watch out for:

All the troubles that can happen on great 1Unsuccessful relationships (sessions) between two nodesSessions the are successfully established but intermittently failFrame collisions


The Data attach Layer allows nodes to communicate with each other within a regional area network. The structures of heat discipline, circulation control, and error manage are established in this layer.

OSI class 3

Layer 3 is the network layer. This is where we send information between and across networks with the usage of routers. Instead of just node-to-node communication, we have the right to now carry out network-to-network communication.

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

Not only do they connect to Internet business Providers (ISPs) to provide accessibility to the Internet, they also keep track of those on that network (remember that switches keep track of every MAC addresses ~ above a network), what other networks it’s associated to, and also the various paths because that routing data packets throughout these networks.

Routers store every one of this addressing and also routing details in routing tables.

Here’s a basic example the a routing table:

Image source + learn more about routing tables here.

The data unit on class 3 is the data packet. Typically, every data packet consists of a structure plus an IP deal with information wrapper. In other words, frames are encapsulated by class 3 addressing information.

The data being transmitted in a packet is also sometimes called the payload. While every packet has everything it demands to obtain to the destination, whether or not it provides it there is an additional story.

Layer 3 transmissions space connectionless, or best effort - lock don"t perform anything but send the web traffic where it’s an alleged to go. Much more on data transfer protocols on great 4.

Once a node is linked to the Internet, that is assigned an internet Protocol (IP) address, i beg your pardon looks either like 172.16. 254.1 (IPv4 attend to convention) or choose 2001:0db8:85a3:0000:0000:8a2e:0370:7334 (IPv6 deal with convention). Routers use IP addresses in your routing tables.

IP addresses are connected with the physics node’s MAC resolve via the deal with Resolution Protocol (ARP), i m sorry resolves MAC addresses through the node’s corresponding IP address.

ARP is traditionally considered part of layer 2, but because IP addresses don’t exist until Layer 3, the also component of class 3.

How to Troubleshoot OSI great 3 Problems

Here room some class 3 difficulties to watch out for:

All the problems that can crop up top top previous layers :)Faulty or non-functional router or various other nodeIP resolve is mistakenly configured

Many answers to Layer 3 concerns 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 permits nodes to connect to the Internet and also send information across different networks.

OSI class 4

Layer 4 is the transport layer. This whereby we dive right into the nitty gritty specifically, of the connection in between two nodes and how info istransfer between them. It builds on the functions of great 2 - line discipline, circulation control, and error control.

This great is likewise responsible because that data packet segmentation, or how data packets are broken up and also sent over the network.

Unlike the ahead layer, layer 4 likewise has an understanding of the whole message, not just the contents of each individual data packet. Through this understanding, class 4 is may be to manage network jam by not sending out all the packets at once.

The data devices of class 4 walk by a few names. Because that TCP, the data unit is a packet. For UDP, a packet is described as a datagram. I’ll simply use the hatchet data packet below for the benefits of simplicity.

Transmission control Protocol (TCP) and also User Datagram Protocol (UDP) are two the the most famous protocols in layer 4.

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

TCP explicitly creates a link with the destination node and requires a handshake in between the source and location nodes when data is transmitted. The handshake confirms the data was received. If the destination node does no receive all of the data, TCP will ask because that a retry.

TCP additionally ensures the packets are ceded or reassembled in the correct order. Learn more about TCP here.

UDP, a connectionless protocol, prioritizes speed over data quality. UDP go not need a handshake, i beg your pardon is why it’s referred to as connectionless.

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

If info is separation up right into multiple datagrams, unless those datagrams save a succession number, UDP does not ensure that packets are reassembled in the exactly order. Learn an ext about UDP here.

TCP and also UDP both send data to specific ports ~ above a network device, which has an IP address. The mix of the IP attend to and the port number is dubbed a socket.

Learn more about sockets here.

Learn more about the differences and similarities between these 2 protocols here.

How to Troubleshoot OSI great 4 Problems

Here room some great 4 troubles to watch out for:

All the troubles that can chop up ~ above previous layers :)Blocked ports - check your accessibility Control perform (ACL) & firewalls


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

OSI class 5

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

A conference is a support agreed upon connection that is established between two network applications. Not 2 nodes! Nope, we’ve moved on indigenous nodes. They were so layer 4.

Just kidding, we still have actually nodes, however Layer 5 doesn’t need to retain the concept of a node due to the fact that that’s to be abstracted the end (taken treatment of) by vault layers.

So a conference is a connection that is established between two specific end-user applications. There space two important principles to take into consideration here:

Client and also server model: the application requesting the info is referred to as the client, and the application that has the requested information is called the server.Request and solution model: if a session is being established and during a session, over there is a constant back-and-forth the requests because that information and responses comprise that info or “hey, i don’t have actually what you requesting.”

Sessions might be open for a an extremely short lot of time or a lengthy amount the time. They might fail sometimes, too.

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

Examples that protocols on class 5 encompass Network straightforward Input Output device (NetBIOS) and Remote Procedure speak to Protocol (RPC), and many others.

From here on the end (layer 5 and also up), networks are concentrated on methods of making relationships to end-user applications and also displaying data come the user.

How to Troubleshoot OSI great 5 Problems

Here space some class 5 difficulties to watch the end for:

Servers room unavailableServers are incorrectly configured, for example Apache or PHP configsSession failure - disconnect, timeout, and so on.


The conference Layer initiates, maintains, and terminates connections in between two end-user applications. The responds to requests native the presentation layer and also issues requests come the transport layer.

OSI layer 6

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

The operating mechanism that master the end-user applications is typically involved in class 6 processes. This usability is not always implemented in a network protocol.

Layer 6 renders sure the end-user applications operation on class 7 can properly consume data and, of course, eventually screen it.

There are three data formatting methods to be conscious of:

American standard Code for info Interchange (ASCII): this 7-bit encoding method is the most widely offered standard for character encoding. One superset is ISO-8859-1, which provides most the the characters necessary for languages talked in western Europe.Extended Binary-Coded Decimal Interchange code (EBDCIC): design by IBM for mainframe usage. This encoding is incompatible with various other character encoding methods.Unicode: character encodings deserve to be done through 32-, 16-, or 8-bit characters and also attempts come accommodate every known, composed alphabet.

Learn much more about personality encoding techniques in this article, and additionally here.

Encryption: SSL or TLS encryption protocols live on layer 6. This encryption protocols aid ensure the transmitted data is less vulnerable to malicious gibbs by giving authentication and also data encryption because that nodes operating on a network. TLS is the successor to SSL.

How to Troubleshoot OSI layer 6 Problems

Here room some layer 6 problems to watch out for:

Non-existent or corrupted driversIncorrect OS user accessibility level


The Presentation class 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 offered by end-user applications. Applications encompass software programs that are mounted on the operating system, like web browsers (for example, Firefox) or word processing programs (for example, Microsoft Word).

Applications deserve to perform committed network attributes under the hood and require specialized services that autumn under the umbrella of great 7.

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

Applications will additionally control end-user interaction, together as defense checks (for example, MFA), to know of two participants, initiation of an exchange the information, and also so on.

Protocols that operate on this level include document Transfer Protocol (FTP), Secure covering (SSH), simple Mail carry Protocol (SMTP), internet Message accessibility Protocol (IMAP), Domain Name service (DNS), and Hypertext move Protocol (HTTP).

While every of this protocols serve different functions and operate differently, ~ above a high level they all facilitate the interaction of information. (Source)

How come Troubleshoot OSI class 7 Problems

Here are some class 7 problems to watch out for:

All concerns on vault layersIncorrectly configured software application applicationsUser error (... We’ve all been there)


The applications Layer owns the services and functions the end-user applications must work. It does not incorporate the applications themselves.


Our class 1 koala is every grown up.


Learning inspect - can you use makeup come a koala?

Don’t have a koala?

Well - answer these inquiries instead. That the next best thing, ns promise.

See more: Flights To Las Vegas From Jacksonville Fl, Find Cheap Flights From Jacksonville To Las Vegas

What is the OSI model?What are each of the layers?How might I use this details to troubleshoot networking issues?

Congratulations - did you do it taken one step farther to expertise the glorious reality we contact the Internet.

Learning Resources

Many, very smart world have written whole books about the OSI design or entire publications about specific layers. I encourage readers to check out any type of O’Reilly-published books about the subject or around network engineering in general.

Here room some sources I provided when writing this article:

The depicted Network, 2nd Edition

About Me

Chloe Tucker is one artist and also computer scientific research enthusiast based in Portland, Oregon. Together a former educator, she"s continuously trying to find the intersection of learning and teaching, or modern technology and art. Reach out to she on Twitter