All Networks Can Be Described With the Osi Model.

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

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

Learning networking is a flake similar learning a linguistic communication - there are lots of standards and then some exceptions. Therefore, it'southward of import to really understand that the OSI model is not a prepare of rules. It is a tool for understanding how networks part.

Once you lot learn the OSI model, you volition be able to further understand and appreciate this glorious entity nosotros call the Net, as well as exist able to troubleshoot networking issues with greater fluency and ease.

All hail the Internet!

Prerequisites

You don't need whatsoever prior programming or networking experience to understand this article. However, you volition need:

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

Learning Objectives

Over the course of this article, y'all will learn:

1. What the OSI model is
2. The purpose of each of the seven layers
3. The problems that tin can happen at each of the 7 layers
4. The departure between TCP/IP model and the OSI model

Mutual Networking Terms

Here are some common networking terms that you lot should exist familiar with to become the near out of this article. I'll use these terms when I talk about OSI layers side by side.

Nodes

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

Nodes may be fix adjacent to ane other, wherein Node A can connect straight to Node B, or there may exist an intermediate node, similar a switch or a router, gear up between Node A and Node B.

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

Here's an example:

Source

For the nitpicky among us (yep, I encounter you), host is another term that you will come across 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. Delight Tweet angrily at me if you disagree.

Links

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

Links to can either be point-to-signal, 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 beingness transmitted, this may as well be described as a i-to-i vs. a one-to-many relationship.

Protocol

A protocol is a mutually agreed upon fix of rules that allows two nodes on a network to substitution 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 information technology tin can be communicated) of the communications procedure. Protocols tin can be implemented on hardware, software, or a combination of both. Protocols can exist created by anyone, just the well-nigh 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 past Internet organizations such as the Cyberspace Engineering science Task Force (IETF).

Networks

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, Homo, WAN, BAN, or VPN. Call back I'1000 just randomly rhyming things with the word can? I can't say I am - these are all existent network types. Learn more than here.

Topology

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

Source + larn more about network topologies here

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'south 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 seven layers of networking.

First, what'southward a layer?

Source

Ooo, lair.

No, a layer - non a lair. Here at that place are no dragons.

A layer is a style 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 concrete, to less tangible and less concrete just closer to the cease 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 cease user.

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

• Please | Physical Layer
• Do | Data Link Layer
• Non | Network Layer
• Tell (the) | Transport Layer
• Underground | Session Layer
• Password (to) | Presentation Layer
• Anyone | Application Layer

Go along in listen that while certain technologies, like protocols, may logically "vest to" one layer more than than another, non all technologies fit neatly into a single layer in the OSI model. For case, Ethernet, 802.11 (Wifi) and the Address Resolution Protocol (ARP) process operate on >1 layer.

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

OSI Layer ane

Layer 1 is the concrete layer. In that location'due south a lot of technology in Layer i - everything from physical network devices, cabling, to how the cables hook up to the devices. Plus if nosotros don't need cables, what the point type and manual methods are (for example, wireless broadband).

Instead of listing every type of engineering science in Layer one, 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 within 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 pivot is active or inactive?
• Functional and procedural logic. What is the part of each pivot 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 beneath).
• Cable types. Options include shielded or unshielded twisted pair, untwisted pair, coaxial and so on. Larn more than well-nigh cable types hither.
• Indicate type. Baseband is a single bit stream at a time, like a railway track - 1-way simply. Broadband consists of multiple bit streams at the same fourth dimension, like a bi-directional highway.
• Bespeak transmission method (may be wired or cable-free). Options include electric (Ethernet), low-cal (optical networks, fiber optics), radio waves (802.xi WiFi, a/b/chiliad/due north/air conditioning/ax variants or Bluetooth). If cablevision-free, then besides consider frequency: 2.5 GHz vs. 5 GHz. If it's cabled, consider voltage. If cabled and Ethernet, also consider networking standards similar 100BASE-T and related standards.

The data unit on Layer ane is the flake.

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, similar a letter, numeral, or symbol.

Bits are sent to and from hardware devices in accordance with the supported data charge per unit (transmission rate, in number of bits per second or millisecond) and are synchronized so the number of $.25 sent and received per unit of measurement of time remains consistent (this is called scrap synchronization). The way bits are transmitted depends on the signal manual method.

Nodes can ship, receive, or send and receive bits. If they can but exercise 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 aforementioned time, it's full-duplex – if not, it's but half-duplex.

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

How to Troubleshoot OSI Layer 1 Problems

Hither are some Layer 1 bug to watch out for:

• Defunct cables, for instance damaged wires or broken connectors
• Broken hardware network devices, for case damaged circuits
• Stuff beingness unplugged (...we've all been there)

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

TL;DR

Layer i contains the infrastructure that makes advice on networks possible.

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

Fun fact: deep-sea communications cables transmit data around the globe. This map will accident your heed: https://www.submarinecablemap.com/

And because y'all fabricated information technology this far, here's a koala:

Source

OSI Layer 2

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

In more official tech terms:

• Line discipline. Who should talk for how long? How long should nodes be able to transit information for?
• Flow command. How much data should be transmitted?
• Fault control - detection and correction. All data transmission methods have potential for errors, from electrical spikes to dirty connectors. Once Layer ii technologies tell network administrators about an event on Layer two 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 ii:

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

The data unit of measurement 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 exist discarded or the error may exist 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 mistake detection techniques here.

Source

Typically there is a maximum frame size limit, called an Maximum Manual Unit, MTU. Jumbo frames exceed the standard MTU, acquire more about jumbo frames hither.

How to Troubleshoot OSI Layer 2 Problems

Here are some Layer two problems to watch out for:

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

TL;DR

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

OSI Layer iii

Layer three is the network layer. This is where we send data between and beyond networks through the apply of routers. Instead of simply node-to-node communication, we tin can now practise network-to-network communication.

Routers are the workhorse of Layer 3 - nosotros couldn't accept Layer 3 without them. They move information packets beyond multiple networks.

Non only do they connect to Cyberspace Service Providers (ISPs) to provide access to the Net, they besides keep track of what'south on its network (call up that switches proceed track of all MAC addresses on a network), what other networks information technology's connected to, and the unlike paths for routing information packets across these networks.

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

Hither's a simple case of a routing table:

Source + acquire more than well-nigh routing tables here

The information unit of measurement 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 bundle is also sometimes called the payload. While each parcel 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 simply send the traffic where it'due south supposed to become. More on information send protocols on Layer iv.

Once a node is connected to the Internet, it is assigned an Cyberspace Protocol (IP) address, which looks either like 172.sixteen. 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'due south MAC address via the Accost Resolution Protocol (ARP), which resolves MAC addresses with the node's corresponding IP address.

ARP is conventionally considered part of Layer 2, merely since IP addresses don't exist until Layer iii, it's too office of Layer 3.

How to Troubleshoot OSI Layer 3 Issues

Here are some Layer three bug to watch out for:

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

Many answers to Layer 3 questions volition require the use of command-line tools similar ping, trace, testify ip route, or prove ip protocols. Larn more about troubleshooting on layer one-iii here.

TL;DR

The Network Layer allows nodes to connect to the Internet and transport data across different networks.

OSI Layer 4

Layer iv is the transport layer. This where we dive into the nitty gritty specifics of the connection betwixt two nodes and how data is transmitted between them. It builds on the functions of Layer ii - line subject, flow control, and error control.

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

Dissimilar the previous layer, Layer 4 also has an agreement of the whole bulletin, non just the contents of each individual data packet. With this understanding, Layer four is able to manage network congestion past non sending all the packets at once.

The data units of Layer 4 go by a few names. For TCP, the data unit is a bundle. For UDP, a packet is referred to as a datagram. I'll simply use the term data bundle 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 information is transmitted. The handshake confirms that data was received. If the destination node does not receive all of the data, TCP volition ask for a retry.

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

UDP, a connectionless protocol, prioritizes speed over data quality. UDP does non require a handshake, which is why information technology'due south called connectionless.

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

If data 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 nigh UDP here.

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

Learn more about sockets hither.

Larn more nigh the differences and similarities between these ii protocols here.

How to Troubleshoot OSI Layer 4 Problems

Here are some Layer four problems to sentry out for:

• All the problems that tin can crop up on previous layers :)
• Blocked ports - check your Access Command 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.

TL;DR

The Transport Layer provides end-to-end manual 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 accept nodes, merely Layer 5 doesn't need to retain the concept of a node because that's been abstracted out (taken care of) past previous layers.

Then a session is a connection that is established betwixt 2 specific end-user applications. There are two important concepts to consider here:

• Client and server model: the application requesting the information is called the customer, and the application that has the requested information is chosen the server.
• Request and response model: while a session is being established and during a session, there is a constant back-and-along of requests for information and responses containing that information or "hey, I don't have what y'all're requesting."

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

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

Examples of protocols on Layer five include Network Bones Input Output System (NetBIOS) and Remote Process Telephone call Protocol (RPC), and many others.

From hither on out (layer v and up), networks are focused on ways of making connections to cease-user applications and displaying information to the user.

How to Troubleshoot OSI Layer 5 Issues

Hither are some Layer 5 bug to watch out for:

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

TL;DR

The Session Layer initiates, maintains, and terminates connections between ii end-user applications. It responds to requests from the presentation layer and problems requests to the ship layer.

OSI Layer 6

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

The operating organization that hosts the stop-user application is typically involved in Layer 6 processes. This functionality is non always implemented in a network protocol.

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

In that location are three data formatting methods to be aware of:

• American Standard Code for Data Interchange (ASCII): this 7-chip encoding technique is the most widely used standard for character encoding. Ane 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 adapt every known, written alphabet.

Learn more than about grapheme encoding methods in this article, and also hither.

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

How to Troubleshoot OSI Layer six Problems

Hither are some Layer 6 problems to sentry out for:

• Non-existent or corrupted drivers
• Wrong Bone user access level

TL;DR

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 organisation, similar Cyberspace browsers (for example, Firefox) or word processing programs (for example, Microsoft Discussion).

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

Email programs, for instance, are specifically created to run over a network and employ networking functionality, such equally electronic mail protocols, which fall under Layer vii.

Applications will as well command finish-user interaction, such equally security checks (for example, MFA), identification of ii participants, initiation of an substitution of information, and then on.

Protocols that operate on this level include File Transfer Protocol (FTP), Secure Beat out (SSH), Simple Post 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 Issues

Here are some Layer 7 problems to lookout out for:

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

TL;DR

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

Determination

Our Layer i koala is all grown upwardly.

Learning check - tin can you lot apply makeup to a koala?

Don't have a koala?

Well - answer these questions instead. It's the next best thing, I hope.

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

Congratulations - y'all've taken one step farther to agreement the glorious entity nosotros phone call the Internet.

Learning Resources

Many, very smart people have written unabridged books about the OSI model or unabridged books well-nigh specific layers. I encourage readers to bank check out any O'Reilly-published books almost the subject area or about network engineering in full general.

Hither are some resources I used when writing this article:

• The Illustrated Network, 2nd Edition
• Protocol Data Unit (PDU): https://www.geeksforgeeks.org/divergence-between-segments-packets-and-frames/
• Troubleshooting Forth the OSI Model: https://www.pearsonitcertification.com/articles/commodity.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/

Near Me

Chloe Tucker is an artist and informatics enthusiast based in Portland, Oregon. Equally a one-time educator, she'due south 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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