THE OSI MODEL
The International Organization for Standardization (ISO) is an organization that is responsible for the standardizing of methods by which computers communicate with each other. In 1984 the ISO devised their model for network communication. This model is called the Open Systems Interconnection (OSI) reference model or more commonly the OSI model.
The OSI model divides network communications into seven layers. Each layer is responsible for carrying out specific functions when transmitting data on the network. Each layer of this 7 layer structure has a definite function.
7. Application Layer: Provides a user interface. Example: ping, internet explorer, telnet. File, print, message, database, and application services
6. Presentation Layer: This layer is responsible for the translation of data for the layered approach. Presents data. Handles processing such as encryption. Data encryption, compression, and translation services.
5. Session Layer: Keeps different applications’ data separate. Simplex, Full duplex or Half duplex work in this layer. Dialog control.
4. Transport Layer: Provides reliable or unreliable delivery. TCP (Transmission Control Protocol) & UDP (User Datagram Protocol) & Windowing occur here. Performs error correction before retransmitting. End-to-end connection.
3. Network Layer: IP, Routing work in this layer. Provides logical addressing, which routers use for path determination. Routing
2. Data Link Layer: Combines packets into bytes and bytes into frames. Provides access to media using MAC address. Performs error detection, not correction. Framing.
1.Physical Layer: Moves bits between devices. Specifies voltage, wire speed, and pinout of Cables. Physical topology.
TCP/IP Model Layers
Each layer of the TCP/IP has a particular function to perform and each layer is completely separate from the layer(s) next to it. The communication process that takes place, at its simplest between two computers, is that the data moves from layer 4 to 3 to 2 then to 1 and the information sent arrives at the second system and moves from 1 to 2 to 3 and then finally to layer 4.
The application layer is concerned with providing network services to applications. There are many application network processes and protocols that work at this layer, including HyperText Transfer Protocol (HTTP), Simple Mail Transport Protocol (SMTP) and File Transfer Protocol (FTP).
At this layer sockets and port numbers are used to differentiate the path and sessions which applications operate. Most application layer protocols, especially on the server side, have specially allocated port numbers, e.g. HTTP = 80 and SMTP = 25, and FTP = 20 (Control), 21 (Data).
This layer is concerned with the transmission of the data. The two main protocols that operate at this layer are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). TCP is regarded as being the reliable transmission protocol and it guarantees that the proper data transfer will take place. UDP is not as complex as TCP and as such is not designed to be reliable or guarantee data delivery. UDP is generally thought of as being a best effort data delivery, i.e. once the data is sent, UDP will not carry out any checks to see that it has safely arrived.
The Internet Layer
This is the layer that contains the packet construct that will be transmitted. This takes the form of the Internet Protocol (IP) which describes a packet that contains a source IP Address, destination IP Address and the actual data to be delivered.
Network Access Layer
This is the lowest level of the TCP/IP protocol stack and functions carried out here include encapsulation of IP packets into frames for transmission, mapping IP addresses to physical hardware addresses (MAC Addresses) and the use of protocols for the physical transmission of data.
Note: TCP/IP is actually a suite of protocols sometimes referred to as the Internet Protocol Suite.