In this article, we will learn about OSI and TCP/IP model.
OSI (Open Systems Interconnection) model was created by the International Organization for Standardization (ISO), an international standard-setting body. To understand the functions of a communication system ,one can use this model as a reference model.
The OSI model provides a framework for creating and implementing networking standards and devices. It also describes how network applications on different computers can communicate through the network media.
The OSI model has seven layers, with each layer describing a different function of data traveling through a network. Here is the graphical representation of these layers:
The Physical layer the first layer as the numbering happens from the bottom. It is useful to remember these layers since you might find a couple of questions on the CCNA exam regarding these. Here’s how you can remember this with a mnemonic ”Please Do Not Throw Sausage Pizza Away“:
So, what is the purpose of these layers?
Vendors usually use these layers to implement some functionality into a networking device. This then enables easier interoperability with devices from other vendors.
OSI Model Layers
- Physical Layer – It defines how to move bits from one device to another. It also details how cables, connectors and network interface cards work and how these send and receive bits.
- Data Link Layer – It encapsulates a packet in a frame. A frame contains a header and a trailer that enable devices to communicate. A header (most commonly) contains a source and destination MAC address. A trailer contains the Frame Check Sequence field, which is useful in detecting transmission errors. The data link layer has two sub-layers:
- Logical Link Control – used for flow control and error detection.
- Media Access Control – used for hardware addressing and for controlling the access method.
- Network Layer – It defines device addressing, routing, and path determination. Device (logical) addressing is useful in identifying a host on a network (for example: using its IP address).
- Transport Layer – It segments big chunks of data received from the upper layer protocols. It also establishes and terminates connections between two computers. It is useful for flow control and data recovery.
- Session Layer – It defines how to establish and terminate a session between the two systems.
- Presentation Layer – It defines data formats. Also, at this layer, compression and encryption are defined.
- Application Layer – This layer is the closest to the user. It enables network applications to communicate with other network applications.
It is a common practice to refer to a protocol by the layer number or layer name. For example, HTTPS is referred to as an application (or Layer 7) protocol. Also, Network devices are sometimes described according to the OSI layer on which they operate. For example , a Layer 2 switch or a Layer 7 firewall.
The below table shows which protocols reside on which layer of the OSI model:
The TCP/IP model was created in the 1970s. The Defense Advance Research Project Agency (DARPA) created it as an open, vendor-neutral, public networking model. Just like the OSI model, it describes general guidelines for designing and implementing computer protocols. It consists of four layers: Network Access, Internet, Transport, and Application.
The following picture shows the comparison between the TCP/IP model and an OSI model:
In the figure above, the TCP/IP model has fewer layers than the OSI model. The Application, Presentation, and Session layers of the OSI model have been merged into a single layer in the TCP/IP model. Also, Physical and Data Link layers are known as Network Access layer in the TCP/IP model.
Here is a brief description of each layer:
- Link layer: It defines the protocols and hardware required to deliver data across a physical network.
- Internet layer : It defines the protocols for the logical transmission of packets over the network.
- Transport layer : It defines protocols for setting up the level of transmission service for applications. This layer is indeed responsible for reliable transmission of data and ensures error-free delivery of packets.
- Application layer : It defines protocols for node-to-node application communication and also provides services to the application software running on a computer.
Differences between OSI and TCP/IP model
There are some more differences between these two models. Besides the obvious difference in the number of layers, OSI model prescribes the steps needed to transfer data over a network and it is also very specific in it. It defines how and which protocol should be used at each layer. The TCP/IP model is not that specific.
In short, the OSI model prescribes and TCP/IP model describes.
That’s all about OSI and TCP/IP models.