MODELS
A networking model is a set of guidelines and standards that defines how data is transmitted and received over a network. It is also known as a networking architecture or a networking blueprint. It provides a common framework for network devices and software to communicate with each other. Not only can these models help to understand how packets are sent back and forth through the network, they can assist in troubleshooting the connectivity of the devices that reside within it.
01
OSI Model
OSI stands for Open Systems Interconnection. It is a 7 layer architecture with each layer having a specific functionality to perform. All these 7 layers work collaboratively to transmit data from one person to another from across the globe.
When sending data over the internet it is divided into packets and the sender's end and sent through each layer until it arrives at the physical layer. When receiving data, this process occurs in the reverse order. Starting at the physical layer moving it's way up to the application layer, where it is presentable to the user.
Most devices function at more then one layer of the OSI. When it comes time to determine where they fit into the model, usually in a troubleshooting situation; you must first determine the highest level at which it operates. To do that, you must know what they do and how that relates to the model.
This table, describes where the process of sending and receiving data starts and ends.
| Send / Receive | Start | Finish |
|---|---|---|
| Sending Data | Application | Physical |
| Receiving Data | Physical | Application |
The 7 layers of the OSI model
| Layer | Name | Role | Protocol | PDU | Address |
|---|---|---|---|---|---|
| 7 | Application | Network contact initiated | HTTP, FTP, SMTP | Data | N/A |
| 6 | Presentation | Formats data, makes it presentable to the user | N/A | Data | N/A |
| 5 | Session | Initiates, maintains and tears down session | N/A | Data | N/A |
| 4 | Transport | Data transfer | TCP / UDP | Segment | Port address |
| 3 | Network | Addressing and routing | IP, ICMP and ARP | Packet | IP |
| 2 | Data Link | Frame formation | Ethernet | Frame | MAC |
| 1 | Physical | Data transmitted on media | N/A | Bits | No address |
02
TCP/IP Model
Similar to the OSI model, the TCP/IP model is used to understand how data is broken down and moved through the network. Layers of the OSI model are grouped together to cerate the 4 layers that make up the TCP/IP model.
The 4 layers of the TCP/IP Model
| Layer | Name | Included OSI Layers |
|---|---|---|
| 4 | Application | Application, Presentation and Session |
| 3 | Transport | Transport |
| 2 | Network | Network |
| 1 | Network Access | Data Link, Physical |
03
DOD4 Model
The DOD4 model was developed by the Department of Defense for the DARPA inter-network and has since grown into the rest of the internet. Similar to the TCP/IP model, it is a condensed version of the OSI model. Because of this, the OSI model is often preferred.
The 4 layers of the DOD4 Model
| Layer | Name | Included OSI Layers |
|---|---|---|
| 4 | Process | Application, Presentation and Session |
| 3 | Host To Host | Transport |
| 2 | Internet | Network |
| 1 | Network Access | Data Link, Physical |