OSI (Open Systems Interconnection) Models …….!!

The OSI model 

         OSI divides telecommunication into seven layers. The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers (up to the network layer) are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host.

The seven OSI models are:-

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 the 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 Layer: 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 Layer: 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 sits at this layer:-

1. TCP Protocols: 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.

2. UDP Protocols: UDP 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 Layer: 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.

(1)There could be a static route through the entire network that will never be changed.

(2) There could be a static line only used during a particular session between the sender and receiver.

(3)The packets could be dynamically sent through the network using changing paths in order to prevent bottlenecks.

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 layer: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 Layer: 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.