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05: Presentation layer - layer 6 of OSI model


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Do you ever ask yourself how the information transmitted through the network is understood by computers even if they are of different coding systems? do you ever question the safety of data when it is traveling from one device to another ? do you ever wonder how devices with multiple data formats understand each other and communicate successfully? cheer up ! in this lesson you will get your questions answered.

Presentation layer, the sixth layer of OSI reference model, is the one that is responsible for the syntax and semantics of the information exchanged between two systems. It is sometimes called the syntax layer. This layer has three main functions in networking: encryption/decryption, translation, compression/decompression.

 Encryption/decryption service:

To start with,  encryption/decryption is associated with the presentation layer. Encryption is the process of ciphering messages information in such a way that only the concerned nodes, system, or devices can read it. Decryption reverses the process; it put the encoded massages or information back into its original form. Sensitive information is most of the time transmitted between nodes through the internet and if the data is not encrypted, it may be at stake. So, to carry sensitive information, a system must be able to ensure privacy. Here is when one the roles of the presentation layer starts.

 The following figure shows how encryption looks like:
Thus, encryption means that the sender transforms the original information to another form and sends the resulting message out over the network to the receiver. Decryption reverses the original process to transform the message back to its original form. To know the real value of the presentation layer, consider the following example:

Let's suppose that the computer on the left 'Sender' is sending a message to the computer on the right 'Receiver'. The messages is 'Hi Adam, how are you ?'. As you see in the picture, the message is unencrypted, which means that it is sent in its original form. The receiver receives that message as it is; nothing is changed. But there is a problem ! the third computer, which is also part of the local network, is using a sniffer software program to spy on every thing being sent across the local network. Once the message is sniffed, the text will be easy to read. Now, imagine the text being transmitted is a password. This puts both computers in danger because their information is unsafe due to the fact that it is unencrypted and it is readable by human being.

In contrast, when the information to transmit is encrypted, it is difficult for the person using a sniffer software program to read it; however, there are strong software programs to decipher it, but it is still a difficult and daunting task. So, the presentation layer does not exit in a vacuum. It provides internet users with security. The encryption of data takes place at this layer. Take a look at the above picture again. The message now is difficult for us as a human being grasp its meaning, even if it is caught by a sniffer program, because it is transformed to another format that is comprehensible only by computers.

Translation service:

Translation is another service that the presentation layer is responsible for.  To comprehend how translation does its role in networks, let's start with an example of analogy. The following picture clarifies the intended purpose of the translation service in the networking world:

These two persons meet in the street. The American person is asking the Frensh one if he can show him the road that leads to the bank. unfortunately, the Frensh person does not speak and understand English and the American person does not speak ans understand Frensh. Regardless of non-verbal language, it is very difficult for them to communicate using verbal language. They need a third person to translate for both of them. This is analogous to the network. Two computers need a third party to make data being transmitted understandable. This third party is the presentation layer.

 As shown in the above picture, if data is to be transferred from one device to another, it has to be transformed into bit stream which is the only language (binary) that computers understand. This task is the responsibility of the presentation layer. It first converts data at the source machine from the actual form (video, picture, audio, etc.) into bit stream (zeroes and ones), then it is transmitted through media (wires or wireless) to the destination machine. When it reaches its destination, it is converted back into its original form at the presentaion layer of the receiver so that it is intelligible to the general public.

Another example of translation service is the conversion between two different coding systems e.g. from EBCIC (Extended Binary Coded Decimal Interchange Code) to ASCII (American Standard Code for Information Interchange). Thus, presentation layer is essentially a translator and provides coding and conversion functions. Data is adopted into a standard format before transmission and computers are configured to receive this genetically formatted data and convert it back into its native format for actual reading. Finally, The presentaion layer insures that data transmitted from the application layer of the sender can be read by the application layer of the receiver by means of the translation service provided at this layer.

Compression/decompression service:

Another crucial service provided by the presentation layer is compression/decompression. To firmly grasp this concept, I will give you an example which most us does when working on a computer. Let's assume that your are chatting with your friend on skype and your firend asks you to send him the movies that you have downloaded an watched so that to watch it as well. But the size of the movies is 2 Giga bytes. It is a big and huge video size to be sent on skype ! the resolution is to compress using a software program designed for such tasks e.g Winrar or Winzip. The size of the movie after compression becomes 300 Mega bytes. Now it can be sent in a short time. When it is received at the receiving machine, it can be decompressed again and get the actual movie.
 

 This example is similar to the compression which happens to the data before it is  transmitted across the network. Data is compressed at the source machine and when it arrives at the destination machine it is decompressed to be sent up to the application layer for actual reading. All that happens in a trice and it is unseen by computer users.

Finally, Presentation layer gets its name from its function; it represents data coming from another device to the application layer which is the seventh layer in OSI reference model. Presentation layer provides three chief services: encryption and decryption, translation, and compression and decompression.


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