The Open Source Interconnection (OSI) 7 layer model is often seen as a necessary evil for networking students or those wanting to transition into technology/networking. The different layers of the OSI Model can seem confusing and counterintuitive at times. Students and green IT professionals often find it difficult to remember all seven layers of the OSI model without going through an entire seven-layer burrito from Taco Bell. It has been around since 1984(a year before I was born and terms of endearment won the oscar for best picture DAMN I feel old), and despite the rise of new protocols such as IPv6, TCP/IP, and computer communications in general, it is still the go-to reference for understanding how computers communicate.
Please Do Not Throw Sausage Pizza Away is the Mnemonic that stuck with me in college because I love pizza. Please Do Not Touch Steves Pet Alligator or Please Do Not Teach Stupid People Ackronyms are also good ones for this lesson, but I’ll stick with pizza because…. I LOVE PIZZA! Let’s get to rockin:
Physical Layer 1 | (PLEASE)
The physical layer is the lowest layer of the OSI model, and it deals with electrical signals.
Basically, this layer defines how you physically send information from one computer to another. For example, you could use electricity to send a signal from one end of a wire to the other end of a wire or you could use light pulses to represent bits as they move along a fiber optic cable.
The physical layer is also responsible for defining what type of connector you’ll use to connect your computers. For example, if you wanted to connect your computer to your neighbor’s computer using an Ethernet cable, you’d have to make sure your computers both had Ethernet ports on their back panels. The physical layer tells these two computers how they should communicate over this Ethernet connection. It’ s also the first layer I look at when troubleshooting networking issues. Always make sure everything is plugged in first before moving on the next layer.
Data Link Layer 2 | (DO)
Layer 2, the data link layer, is defined by a series of protocols that handle the communications between network devices. This layer uses a variety of techniques to ensure reliable delivery of data across a network. Some of the protocols that operate at this layer include Ethernet, token ring, frame relay and X.25.
The Data link layer is responsible for ensuring reliable delivery of frames from one point to another in a LAN and for detecting and possibly correcting errors that may occur during transmission. At this layer, frames are received in their entirety. Once received and validated, they are passed to the Network Layer for further processing or routing. The Data Link Layer can also manage flow control between devices on the same network segment.
Network Layer 3 | (NOT)
The network layer is responsible for logical addressing and routing packets from one network to another. It is defined by RFC 791 as “the means used to provide the functional and procedural mechanisms necessary to transfer data between network entities.” This layer defines how packets are delivered from one host to another. It includes information such as the address of the destination host and the path that the packets should take to reach it.
The network layer is also responsible for providing error checking of packets, handling packet fragmentation, and generally keeping track of where each packet needs to go with regard to its source and destination.
The network layer hides the differences between various physical networks. You can think of this layer’s function as a post office: it maps an address (the destination host) onto a particular place in the world (the destination city or region).
Transport Layer 4 | (Throw)
The transport layer is responsible for end-to-end delivery of data. The main functions of the transport layer are:
Reliable or Unreliable Transfer Delivery — This refers to whether you want the data transmitted to be reliable or not.
Transport Protocols — This refers to the protocol used by the transport layer. Two common protocols used are TCP and UDP.TCP stands for Transmission Control Protocol, while UDP stands for User Datagram Protocol.Both TCP and UDP rely on IP which is a connectionless protocol.
Connection-Oriented vs Connectionless — This refers to the idea that TCP ensures a connection is established before transmission of any data, while UDP does not ensure a connection exists before transmission occurs.
Connection oriented means that there must exist a path between two hosts in order for data to be transmitted; whereas connectionless means that this is not necessarily the case.
Error Checking — This refers to whether or not error checking should be used during transmission.Error checking allows for detection of corrupted data during transfer.There is also a distinction between full and partial checksums (more on this later).
Flow Control — This refers to whether flow control should be used during transmission.
Session Layer 5 | (Sausage)
Layer 5 of the OSI model is the session layer. Layer 5 establishes, coordinates, and terminates sessions between systems on a network. The session layer provides for the identification, establishment, maintenance, and termination of communication sessions between applications running on network nodes. It establishes dialogues between the applications, keeping track of information needed to keep them synchronized. It also guarantees delivery of information that’s sent from one application to another.
Layer 5 works with Layer 4, which is Transport Layer Protocols: Layer 4 delivers a data stream from sender to receiver, but it doesn’t ensure that the data stream is delivered in sequence or free from errors. The Session Layer performs error control and sequencing. It uses acknowledgments to confirm that packets are received correctly and in order. When an error occurs during transmission, the Session Layer can request that the transmitting system resend the packet or packets containing the error.
Layer 5 is responsible for setting up sessions and teardowns: When a connection is established, Layer 5 negotiates and sets up parameters for that connection — such as maximum transmission unit (MTU), flow control mechanisms (windowing size), type of service (TOS), and so forth — so that communication can occur efficiently between two endpoints.
Presentation Layer 6 | (PIZZZZZZA)
The Presentation Layer provides conversion services between the Application Layer and the User. The Presentation Layer prepares data for transport across the network by converting it into a format that is required by the User. In many ways, the Presentation Layer represents the area where protocols from different applications or systems can be translated into a common transmission format.
The presentation layer is also responsible for data translation and code formatting (e.g., ASCII to EBCDIC). The presentation layer ensures that data is in a format that can be easily interpreted by both computers and users. For example, when a letter is sent from one person to another, there needs to be some way of translating it from the format on paper into a digital format that can be read by both people. The presentation layer’s primary concern is to convert data from its native representation (as generated and consumed by upper-layer processes) into network-level information that can be transmitted reliably in a relatively “lossless” manner across heterogeneous networks and systems. Some of the issues with preserving data during transit include converting between character sets (e.g., ASCII and EBCDIC), dealing with data compression, encryption, and other issues related to how the data will be handled over the network.
Application Layer 7 | (AWAY)
The applications layer is the one closest to the end-user. This layer contains all the software that users interact with directly. Examples of applications at this level include web browsers (such as Google Chrome, Firefox and Safari) and other types of office or collaboration suites (such as Outlook, Skype and Office).
The applications layer works closely with the presentation layer to create a seamless user experience. It also communicates with the transport layer to send data in a secure manner. The applications layer uses a protocol called TCP/IP. This protocol helps with the connection-oriented transmission of data between two devices. Examples of its use include HTTP, FTP, SMTP and Telnet.
The applications layer is most important to end users because it is what they interact with directly when they want to access information or send and receive messages through an application. The other layers in the OSI stack are important too, but their main role is to support the applications layer in some way or another. In order for end users to have a great experience using an application, all seven layers need to work well together.*
The Open Systems Interconnection (OSI) model was developed as a way to standardize network communication. This model is widely used in computer networks and is an essential component of the Internet. The OSI model defines seven layers of networking, with each layer adding a specific function to the process.
This model architecture has been a crucial part of the Internet since its inception. The OSI Model ensures that all computers on a network can communicate with one another regardless of manufacturer or network protocol. By using the OSI Model, any device can communicate with any other device regardless of the software that runs on it.
In conclusion, the OSI model is a great visual aid for understanding networking lingo. It’s not as intimidating when you can see how the different layers are tied together. From memory, you don’t need to be able to name each layer of the OSI model to know that 2 connections on a network uses Layers 3 and 4, or to ask which layer will be affected by slowing down your internet connection, or to understand that VPNs operate at Layer 3 of the OSI model. You might ask me why I mentioned my wife at all in this article, and I’ll tell you it was because she proved what I already believed: nobody remembers or cares about the 7 Layers of the OSI model.
Here’s a great visual from Cisco that I used to keep on my cube during my Verizon days.
Thanks for Rocking!! Next week I’ll talk about something that makes me curious and go over how I prepare for the new year with goals and calibration. Cheers y’all!