OSI Layers

7 Layers of OSI

The ISO (International Organization for Standardization) decided to construct a framework of standards in which different vendors would be able to use in order to communicate over a network consisting of diverse equipment and applications.  This framework is now considered the standard for communication of networks.  The OSI is divided into 7 layers, which divides the task into smaller more manageable task groups.  Each task or group of tasks assigned to each layer can also be implemented independently.  This limits complications between layers because the solutions offered by one layer do not adversely affect the other layers.

The 7 layers can be split logically into two subgroups.  Layers 7 thru 4 focus on the end to end communication of data source and destinations.  Layers 3 thru 1 are provide consistent communication between the network devices.  An easier way of looking at the OSI model is dividing the upper layers (7, 6, 5) from the lower layers (4, 3, 2, 1).  The upper layers deal with application issues and are implemented only in software.  The highest layer, the application layer, is the closest to the end user.  The lower layers are responsible for the transportation of the data.  The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the wires, for example) and is responsible for placing data on the medium.

The following is a top-down explanation of the OSI Model. It starts with the user's PC and it follows what happens to the user's file as it passes though the different OSI Model layers. The top-down approach was selected specifically (vs. starting at the Physical Layer and working up to the Application Layer) for ease of understanding. It is used here to show how the user's files are transformed (through the layers) into a bit stream for transmission on the network.

LAYER 7 - APPLICATION

The application level provides services that directly support the user applications, such as user interface, e-mail, file transfer, database access, etc. There are many protocols at this layer that are commonly needed such as HTTP, WWW, FTP, TELNET, SMTP. It gives applications access to the network through the layers below. Another important function is file transfer between computers. Some computers store file names or represent text lines differently. The application layer takes care of the incompatibilities and allows a smooth transfer between systems.
Protocols: FTP1, HTTP2, SMTP3, DNS4, TFTP5, NFS6, TELNET7.

LAYER 6 - PRESENTATION

The presentation level is translator between the application and network format. Unlike the lower layers, its concern is with the syntax and semantics of the information transmitted. Most user programs do not exchange random binary bit strings. They exchange data such as names, addresses, dates, etc. Different computers store the data in a different way. In order to allow these computers to transmit the data to each other the presentation layer translates the data into a standard form to be used on the network. Another function is data compression which can be used to reduce the number of bits needed to send the packet of information. Security is also added at this layer by using data encryption and decryption. This prevents others from intercepting the data and being able to decipher the meaning of the bits.
Protocols: ASCII8, EBCDIC9, MIDI10, MPEG11, JPEG12.

LAYER 5 - SESSION

 This layer allows applications on connecting systems to communicate using a session. It opens, uses, and closes this communication link. It also acts as a dialog control mechanism controlling who is able to transmit. Sessions can allow data to be sent in both directions at the same time or only one direction. The session layer determines who has the ability to transfer at the current time. Another valuable ability is to insert checkpoints during data transfers. During a large file transmission if the system crashes the checkpoints allow the system to start downloading at the last known checkpoint. An example of this is during either a interactive login or file transfer connection, the session would recognize names in the session and register them into a history. It could then connect and reconnect in case of a system crash at either of the systems.
Protocols: SQL13, RPC14.

LAYER 4 - TRANSPORT

 The basic function of the transport layer is to accept data from the session layer, break up the data into smaller units if need be, and send these manageable data packets to the network layer. At the destination this layer is responsible for combining the packets into their original state. This layer also checks to see if the layers are in the right order when received and not in duplicated form. If there is an error in one of the packets there is a request for that packet's retransmission. There are two protocols that sit at this layer. First, the TCP protocol connects the sender and the receiver using a socket which is determined by the IP address and port number. TCP keeps track of the packet delivery order and which ones need to be resent. UDP on the other hand is a connectionless communication and does not guarantee packet delivery between sender and receiver. Because it is connectionless the sender sends the data into the network with an IP address of the receiver and hopes it makes it to its destination. Since there is not a way of asking the sender to retransmit because of an error there is little error protection if any.
Protocols: TCP15 or UDP16.



LAYER 3 - NETWORK
The network layer basically handles all of the addressing issues. This layer addresses packets, determines the best path or route, and manages network problems such as data congestion. There are three ways in which the packets are routed to their destinations. First, there could be a static route through the entire network that will never be changed. Second, there could be a static line only used during a particular session between the sender and receiver. Finally, the packets could be dynamically sent through the network using changing paths in order to prevent bottlenecks. The bottlenecks are formed when there are too many packets present in one subnet causing them to get in each other's way. The network level is also responsible for converting the network address and names to the MAC addresses of the machines. One of the most important functions of this layer is the ability to allow two different networks using conflicting addressing schemes to be able to send data to each other. The network layer allows the different protocols to "talk" to each other and understand where the packet's destination is.  Routers work at this level by sending the packets along the network.
Protocols: IP17, ICMP18, ARP19, PING20, Traceroute21.

LAYER 2 - DATA LINK

The data link layer defines the format of data on the network. All of the data sent through the network are made into a frame which is performed at this level. The frame is a uniform way of sending the data along with address information and error checking capabilities. CRC is used for the error detection at this level. If at the receiving end the CRC fails at this level there is a request back to the sender for retransmission of this packet.
Protocols: IEEE 802.222, 802.323, 802.524.

LAYER 1 - PHYSICAL

 The physical layer is responsible for establishing, maintaining and ending physical connections (point to point) between computers. This layer is concerned with the actual interpretation of the bit stream into an electrical signal that can be carried across a physical medium. The protocols at this layer deal with the binary transmission, voltage levels, and data rates. This layer would also specify physical medium properties such as cables and network cards.
Protocols: IEEE 802.323, 802.524.

-by Kali Linux