On Figure 1 you can see an illustration of the OSI reference model. Programs only talk to the seventh layer, Application, while the layer “below” the first layer is the network physical media (for example, cable or air, in the case of wireless networks). The network cabling is thus sometimes referred as “layer 0”.

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Figure 1: The OSI reference model.

The seven layers can be grouped into three groups, Application, Transport and Network, as you can see on Figure 1:

• Network: Layers from this group are low-level layers that deal with the transmission and reception of the data over the network.
• Transport: This layer is in charge of getting data received from the network and transforming them in a format nearer to the data format understandable by the program. When your computer is transmitting data, this layer gets the data and divides it into several packets to be transmitted over the network. When your computer is receiving data, this layer gets the received packets and put them back together.
• Application: These are high-level layers that put data in the data format used by the program.

Below we will explain each layer from the OSI reference model. On our examples we are assuming that our computer is sending data to the network – for example, you are sending out an e-mail thru your e-mail program.

• Layer 7 - Application: The application layer makes the interface between the program that is sending or is receiving data and the protocol stack. When you download or send e-mails, your e-mail program contacts this layer.
• Layer 6 - Presentation: Also called translation layer, this layer converts the data format received by the application layer to a common format used by the protocol stack. For example, if the program is using a non-ASCII code page, this layer will be in charge of translating the received data into ASCII. This layer can also be used to compress data and add encryption. Data compression increases network speed, as less data will be sent to the layer below (layer 5). If encryption is used, your data will be encrypted while in transit between layers 5 and 1 and they will only be decrypted on the layer 6 of the computer at the other end.
• Layer 5 - Session: This layer allows two programs in different computers to establish a communication session. In this session these two programs define how data transmission will be done, adding progress markers to the transmitted data. If the network fails, the two computers restart the transmission from the last received marker instead of retransmitting all data again. For example, you are downloading e-mail and your network fails. Instead of downloading all e-mails again, your program would automatically restart from the last downloaded e-mail. Note that not all protocols implement this feature.
• Layer 4 - Transport: On networks data is divided into several packets. When you are transferring a big file, this file is sliced into several small packets, and then the computer at the other end gets these packets and put the file back together. The Transport layer is in charge of getting data sent by the Session layer and dividing them into packets that will be transmitted over the network. At the receiving computer, this layer is also in charge of putting the packets in order, if they arrived out-of-order (this task is known as flow control), and also checking data integrity, usually sending a control signal to the transmitter called acknowledge or simply ack telling it that the packet arrived and data is intact. This layer separates the Application layers (layers 5 to 7) from the Network layers (layers 1 to 3). Network layers are concerned with how data is transmitted and received over the network, i.e. how data packets are transmitted, while Application layers are concerned with what is inside the packets, i.e. if the data itself. Layer 4, Transport, makes the interface between these two groups.
• Layer 3 - Network: This layer is in charge of packet addressing, converting logical addresses into physical addresses, making it possible to data packets to arrive at their destination. This layer is also in charge of setting the route the packets will use to arrive at their destination, based on factors like traffic and priorities.
• Layer 2 - Data Link: This layer gets the data packets send by the network layer and convert them into frames that will be sent out to the network media, adding the physical address of the network card of your computer, the physical address of the network card of the destination, control data and a checksum data, also known as CRC. The frame created by this layer is sent to the physical layer, where the frame will be converted into an electrical signal (or electromagnetic signal, if you are using a wireless network). The Data Link layer on the receiving computer will recalculate the checksum and see if the new calculated checksum matches the value sent. If they match, the receiving computer sends an acknowledge signal (ack) to the transmitting computer. Otherwise the transmitting computer will re-send the frame, as it didn’t arrive at destination or it arrived with its data corrupted.
• Layer 1 - Physical: This layer gets the frames sent by the Data Link layer and convert them into signals compatible with the transmission media. If a metallic cable is used, then it will convert data into electrical signals; if a fiber optical cable is used, then it will convert data into luminous signals; if a wireless network is used, then it will convert data into electromagnetic signals; and so on. When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together and check for its integrity.