TCP/IP Model and OSI Model

Network Implementations are built on common network standards and models of networking that describe how devices and protocols interconnect. A network is two or more computer systems linked together by some form of the transmission medium that enables them to share information. It does not matter whether the network contains two or thousands of machines; the concept is essentially the same. 

If you have different pieces of hardware and software, they all have to talk to each other and interact to be able to make your network. To help the interconnectivity work, they come up with something called the OSI 7-layer model. 


  • A node is any device that can communicate on the network via one or more network interfaces. This term can be used to describe endpoint devices, such as computers, laptops, servers, IP phones, smartphones, or printers, and connecting or forwarding devices, such as switches and routers. 
  • A node on a wireless network is often called a station.
  • The term host is often used in TCP/IP networking to mean an end system device, such as a computer. 

When a message is sent from one node to another, it travels down the stack of layers (7 to 1) on the sending node, reaches the receiving node using the transmission media, and then passes up the stack (1 to 7) on that node. At each level (except the Physical layer), the sending node adds a header to the data payload, forming a "chunk" of data called a protocol data unit (PDU). This process is called encapsulation. 


The physical layer is right down to the hardware of the computer. This is where the electrical pulses that make up data transfer over a network are sent and received. It's the job of the physical layer to convert the binary data of the transmission into signals and transmit them across the network, as well as receive incoming signals and convert them back into binary data.

Devices that operate at the Physical Layer include:

  • Transceivers: That part of a network interface that sends and receives signals over the network media.
  • Repeaters: A device that amplifies an electronic signal to extend the maximum allowable distance for a media type. 
  • Hubs: A multiport repeater, deployed as the central point of connection for nodes.
  • Media Converters: A device that converts one media signalling type to another. 
  • Modems: A device that converts between digital and analogue signal transmission.


The data link layer focuses on the physical addressing of the transmission. It receives a packet from the network layer (that includes the IP address for the remote computer) and adds the physical (MAC) address of the receiving endpoint. Inside every network-enabled computer is a Network Interface Card (NIC) which comes with a unique MAC (Media Access Control) address to identify it. When information is sent across a network, it's actually the physical address that is used to identify where exactly to send the information. Additionally, it's also the job of the data link layer to present the data in a format suitable for transmission.

In short, Layer 2 allows the upper network layers to access media and controls how data is placed and received from media.

Connectivity devices found at the Data Link layer include:

  • Network Adapters or Network Interface Cards (NIC): A NIC joins a host to network media (cabling or wireless) and enables it to communicate over the network by assembling and disassembling frames.
  • Bridges: A bridge joins two network segments while minimizing the performance reduction of having more nodes on the same network. A bridge has multiple ports, each of which functions as a network interface.
  • Switches: An advanced type of bridge with many ports. A switch creates links between large numbers of nodes more efficiently.
  • Wireless access points (APs): An AP allows nodes with wireless network cards to communicate and creates a bridge between wireless networks and wired ones.
The Network Layer is responsible for moving data around a network of networks, known as an internetwork or the Internet. While the Data Link layer is capable of forwarding data by using hardware addresses within a single segment, the Network layers move information around an internetwork by using logical network and host IDs. The networks are often heterogeneous; that is, they use a variety of Physical layer media and Data Link protocols.

The main appliance working at layer 3 is the router. Other devices include layer 3 switches, which combine the function of switches and routers, and basic firewalls. 


At the Transport layer, the content of the packets starts to become significant. Any given host on a network will be communicating with many other hosts using many different types of networking data. One of the critical functions of the Transport layer is to identify each type of network application by assigning it a port number. For example, data from the HTTPS web browsing application can be identified as port 443, while data from an email server can be identified as port 25.

It receives data from Layer 5 and segments it. Each segment, or data unit, has a source and destination port number, as well as a sequence number. The port number ensures that the segment reaches the correct application. The sequence number ensures that the segments arrive in the correct order.

This layer also controls the amount of data transmitted. For example, your laptop may be able to handle 100 Mbps, whereas your friend’s phone can only process 10 Mbps. Layer 4 can dictate that the server slows down the data transmission, so nothing is lost by the time your friend receives it. But when your friend sends a message back, the server can increase the transmission rate to improve performance.

Lastly, Layer 4 performs error-checking. If a segment of data is missing, Layer 4 will re-transmit that segment.

TCP and UDP are both very well-known protocols, and they exist at Layer 4. TCP favours data quality over speed, whereas UDP favours speed over data quality.


The upper layers of the OSI model are less clearly associated with distinct real-world protocols. These layers collect various functions that provide useful interfaces between software applications and the Transport Layer. 


Most applications protocol requires the exchange of multiple messages between the client and server. This exchange of such a sequence of messages is called a session or dialog. This layer represents the dialog control functions that administer the process of establishing the dialog, managing data transfer, and then ending the session.


The presentation layer receives data from the application layer. This data tends to be in a format that the application understands, but it's not necessarily in a standardized format that could be understood by the application layer in the receiving computer. The presentation layer translates the data into a standardized format, as well as handles any encryption, compression, or other transformations to the data. With this complete, the data is passed down to the session layer.


Application Layer protocols provide an interface for software programs on network hosts that have established a communications channel through the lower-level protocols to exchange data. For example, one of the most-used services provided by the Application layer is file transfer.

TCP/IP Protocol Suite

The TCP/IP protocol suite maps to a four-layer conceptual model: Application, Transport, Internet, and Link. This model is referred to as the Internet Protocol Suite, the Department of Defense (DoD) model, or the ARPA model. Each layer in the Internet Protocol Suite corresponds to one or more layers of the OSI model. 

The above figure demonstrates how the OSI model compares with the TCP/IP protocol stack. Some of the OSI layers are performed by a single protocol, some layers are performed by several protocols, and some protocols cover several layers. This reflects the emphasis that consumes processing power and bandwidth, as each header consists of a series of bytes that must be transmitted and decoded in addition to the application data. Consequently, actual protocol stacks tend to be simpler than the OSI model. 

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