Open Systems Interconnection model
The Open System Interconnection (OSI) Model (or the OSI Reference Model) is a basic representation of how information moves from a software application through a network medium to an application on another computer.
Not encompassing newer protocol developments, the model is still considered to be a very useful introduction to network architecture.
There are seven layers in the model with information flowing from the highest layer (seven) to the lowest (one). The layers are:
- Layer 7 - Application
- Layer 6 - Presentation
- Layer 5 - Session
- Layer 4 - Transport
- Layer 3 - Network
- Layer 2 - Data Link
- Layer 1 - Physical
Application Layer (7)
The topmost layer is where most real traffic data is generated. All user program traffic such as that from e-mail and web browser software is on this layer. This is the layer which is easily seen by every user.
Presentation Layer (6)
This layer prepares data to be "presented" to the network. This involves and data compression or encryption immediately after it is sent out, or just before it is received by the Application Layer. This layer can also check and correct the character code set (ASCII, Unicode, EBCDIC, etc.)
Session Layer (5)
This layer is responsible for managing each connection session. It is directly in control of establishing, managing, and terminating connections requested by applications at both ends of the communication link. Communication methods and terms are also agreed upon at both ends by this layer.
Transport Layer (4)
The Transport Layer is responsible for coordination and managing the actual transportation and receipt of data. In this layer, data is broken down into packets which are sent one at a time. In most cases (when TCP is used), the transport layer on the recipient's end will acknowledge receiving each packet, so that if any are lost in transit, they can be resent. In some cases (when UDP is used), these acknowledgments are not sent, and lost packets are not resent. This is a less reliable method, but still useful when quality is not as important as quantity. This layer is also responsible for segment sequencing. This means simply that a series of packets is reassembled in order by this layer on the receiving end.
The Transport Layer is also in charge of service address (port numbers) which are used to deliver data to one service or program directly, so that a lower layer does not need to determine where each piece of information belongs.
Network Layer (3)
This layer is responsible for the logical addressing required to get packets to their intended destinations. Using routing tables, special computers (called routers) will direct packets from one jump to the next until they reach their recipient. All of this is accomplished using IP addresses, which are unique identifiers for each device on the internet. Each router will select from its routing table the best address to send each packet to. The recipient will then pass on the packet based on its own routing table. This enables packets to be sent to specific destinations unknown to the sender because they are simply sent to the device which does know where it should go next.
The Datalink layer is in charge of ensuring end-to-end validity of the data being exchanged. This layer is logically split into two sublayers, The Media Access Control (MAC) Sublayer and the Logical Link Control (LLC) Sublayer.
- The Media Access Control (MAC) Sublayer determines the actual physical address of the hosts. To do this, it uses MAC addresses for its communication with other devices on the network. Unlike the IP addresses which the Network Layer uses, MAC addresses (also known as Physical addresses, Layer 2 addresses, or Hardware addresses) are assigned to each device at the time of manufacture, and are not usually changed (although this is sometimes possible, it will never be done automatically). These are therefore reliable identifiers within a local network.
- The Logical Link Control sublayer is responsible for synchronizing data frames, flow control, and checking for errors.
Physical Layer (1)
This lowest level of the OSI model involves the physical hardware which transfers the raw data after encoding it into binary form. These pulses are carried by wire (Twisted Pair, Coaxial Cable, etc.) or fiber optic cable from one physical end to the other, as the Datalink Layer coordinates the transfer. Network hubs and wireless access points are also included in this layer.