Networking – OSI Model

The OSI ( Open System Interconnection ) model is actually a pretty overlooked topic in general. The model which is responsible for the reason why you are able to view this website and even use the internet. The model which was created mainly to standardize computing systems to be able to communicate with each other. Without the OSI model, each company would probably have their own ways of communication which would make it so that computing system bought from a company will not work for the other.

The Open Systems Interconnection model (OSI model) is a conceptual model that characterises and standardises the communication functions of a telecommunication or computing system without regard to its underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard communication protocols.

Wikipedia

The model itself is basically a universal standard for computing networks. There are a total of 7 layers in the OSI model, each layer having its own job and importance. If you ever tried any DDoS services, you should be familiar with the layers. For example, application-layer attacks target the 7th layer and the protocol layer attacks target the 3rd and 4th layers which are also known as the internet and transport model.

Offnfopt, Public domain, via Wikimedia Commons

Physical Layer

The Physical Layer is the lowest layer of the whole model and consists of physical tools/devices – LAN Cable, Optical Fiber Cable. This layer transmits data using bits, 1s, and 0s – On/Off. This part gets pretty impressive, an optical cable transfers data using light. This means data from one point to another can transfer at the speed of light and since they use lasers, this can travel thousands of miles. Optical cables are typically used to cover a huge distance, this can be across an entire ocean or through a country. The physical layer covers every physical infrastructure and is responsible for communications of raw data streams over a physical medium such as your LAN cable or other similar cables.

There are 3 media types for this layer, copper cables, fiber optic cable, and wireless

  • Copper Cables – This is the most used in an indoor environment. As copper cables is the cheaper option but do not cover that much distance, this is mainly used for setting up networks in homes or workplaces. The usual distance is up to 100 meters and the longer it gets, the more the signal deteriorates. This is not even taking into account electromagnetic interferences which jumbles it more. There are also cases where crosstalk occurs, which can happen due to cables being too close, this can be countered by twisted pairs
  • Fiber Optic Cables – This is used to transverse between countries. The reason why data using this medium can travel thousands of miles is mainly due to the fact it uses lasers – light impulses. This means data can travel at the speed of light and across a huge distance. Other than that, it also allows for larger bandwidth, up to 100Gbps, compared to 10Gbps on copper wires. This method of data transportation also removes the problems which the copper wire faced, such as the interferences and crosstalk is not possible. Other than that, it’s completely immune to safety hazards.
  • Wireless Media – This method is used for your WiFi, Bluetooth, and so on. This method uses radio waves which can only cover on average 10 meters. Other than the short distance, it also very weak. Although providing an advantage of not being restricted by pathways and such, its good enough for small houses – large houses often use more than 1 router.

Data Link Layer

This is the second layer of the OSI model. This layer is responsible for managing data that is received from the physical layer. Making sure it’s synchronized and packaged into something called data frames. You can summarize this layer as the whole packaging system. The layer also breaks down packets coming from the network layer into smaller frames and is also responsible for correcting any errors made from the physical layer. Most switches you see works in this layer, however, they can also operate at the 3rd layer

There are also two sublayers

  • Logical Link Control ( LLC ) – This sublayer is responsible for hop-to-hop flow and error control. Frame sequence numbers are also assigned by this sublayer.
  • Media Access Control ( MAC ) – This sublayer is responsible for encapsulating frames so that they are usable for transmission using physical hardware – LAN cables. It also generates frame check sequences which help against transmission errors

Network Layer

This is the third layer and is responsible for communications between two different networks. So if two devices belong to the same network, this layer is not needed. The layer itself breaks down segments that are passed down from the transport layer into smaller packets when sending and reassembles these packers when data is received. This layer is mostly used by routers.

Transport Layer

This is the fourth layer and mainly deals with coordinating data transfer between the hosts and systems. This layer is also responsible for the UDP and TCP protocol, which work above the Internet Protocol ( IP ) which works at layer 3. Making sure data is sent, where it goes, the rate of transfer is all done in this layer. Data that is sent from the session layer is broken down here into segments before sending it to layer 3. data is also reassembled from segments here. The layer also controls the speed of transmission, so that a person with fast internet doesn’t flood a person with a slow connection – flow control. Other than that, it also does error handling, by ensuring that the data which has been received is in a complete form, requesting another one if it’s not.

  • Transmission Control Protocol ( TCP ) – This protocol is used when data sent and received must all be there. For an example, when transferring a file, you need every data to be sent and received correctly and in order, if not, the file will no longer work due to missing bits or being in the wrong order. The protocol does this by having a Three-Way handshake. As good as this sounds, the trade off is the speed of data transmission, making this protocol very reliable.
  • User Datagram Protocol ( UDP ) – Usually this protocol is used for something which isn’t affected too much due to packet loss – one example is games, DNS, SNMP and so on. Since UDP does not re-order or makes sure the data is received, it’s faster than the TCP protocol. There is a checksum system which can verify the data transferred.

Session Layer

The session layer is the fifth layer and deals with the opening and closing of communications between two devices. This is responsible for making sure the session is opened long enough for the data to be exchanged and closing it after to free up resources. The layer also sets up checkpoints for transfers, which allows transfer to resume from a certain point if the connection was closed or interrupted. Without this, you will have to re-tranfer everything from the beginning.

There are a few protocols which this layer uses, here are the common ones

  • Session Control Protocol ( SCP )
  • Point-to-Point tunneling Protocol ( PPTP )
  • Remote Procedure Call Protocol ( RPC )

Presentation Layer

This is the sixth layer and is mainly responsible for data conversation and encryption/decryption. It is also responsible for data sent to the application layer.

There are a few important functions of this layer which include

  • Translating incoming data into the right syntax. For example, if two communicating devices use two different encodings, this layer would make sure it converts it to the right syntax which can be used by the application layer
  • Encryption and Decryption, if the user is using a connection that is encrypted, this is the layer responsible for adding encryption to the message when sending and decrypting the message when receiving. So that the application layer can have readable data
  • Compression of data, this layer is responsible for compressing data which then can be sent quickly, which also helps with the efficiency of the communication by reducing the amount of data which is transferred

Application Layer

The top most layer, the application layer. This is the layer which is the closest to the end user. From things such as the browser you are using right now to other software, they are depending on the application layer to send and receive data. Remember that applications do no run inside the application layer, rather it uses the application layer for requesting things such as web pages content and so on. It is also the only layer which receives data directly from the user.

Some common protocol are

  • Hypertext Transfer Protocol ( HTTP )
  • File Transfer Protocol ( FTP )
  • Simple Mail Transfer Protocol ( SMTP )

Conclusion

In order to make the internet, the internet, there was a lot of work which has gone into every small detail. In a way, the internet is indeed a series of tubes. If the OSI model didn’t exist, we wouldn’t have this flexible flow of data that we have. Let me take you through just a quick summary of what exactly happens when you send a simple message to someone.

When you send a file to someone, you software sends the file over to the application layer which than will pick the FTP protocol and pass the file over to the presentation layer, which will compress the data or encrypt the data which will then be passed through the session layer. Session layer will than open a communication session, the data will then hit the transport layer which it will be broken down into segments, then segments will further be broken down into packets in the network layer. The network layer will than pass the data to the data link layer, which will break down the data into frames, these frames will be passed to the physical layer and be converted into bitstreams of 1s and 0s to send through a physical medium such as your LAN cables. The data will then follow a path which leads to the receiver’s computer, once there, everything repeats, but in reverse.

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