Archive for the ‘CCNA’ category

The OSI Model

The International Organization for Standardization (ISO) developed the Open Systems Interconnection (OSI) reference model in 1977. The OSI model is a conceptual framework that can be used to get a better understanding of the complex interactions taking place among the various devices in a network. The purpose of the OSI model is to demonstrate how the network entities should work together for communication.

The OSI model organizes communication services into seven groups called layers. The layers are as follows:

• Layer 1, The Physical Layer: The physical layer is responsible for packaging and transmitting data on the physical media. This layer conveys the bit stream through a network at the electrical and mechanical level.
• Layer 2, The Data-Link Layer: The data-link layer is responsible for error free transfer of data frames. This layer provides synchronization for the physical layer.
• Layer 3, The Network Layer: The network layer controls the operation of the subnet. It determines the physical path that data takes on the basis of network conditions, priority of service, and other factors. The network layer is responsible for routing and forwarding data packets.
• Layer 4, The Transport Layer: The transport layer ensures that messages are delivered in the order in which they are sent and that there is no loss or duplication. It ensures complete data transfer.
• Layer 5, The Session Layer: The session layer establishes a communication session between processes running on different communication entities in a network and can support a message-mode data transfer. It deals with session and connection coordination.
• Layer 6, The Presentation Layer: The presentation layer serves as the data translator for a network. It is usually a part of an operating system and converts incoming and outgoing data from one presentation format to another. This layer is also known as syntax layer.
• Layer 7, The Application Layer: The application layer serves as a window for users and application processes to access network services. It handles issues such as network transparency, resource allocation, etc. This layer is not an application in itself, although some applications may perform application layer functions.

Each communicating entity is equipped with these seven layers. When a message is sent between two communicating entities, data flows down through each layer in the sending entity and flows up through each layer in the receiving entity.

As the message travels down the sending entity layer stack, each layer it passes through (physical layer is an exception) adds a header. These headers contain pieces of control information that are read and processed by the corresponding layer on the receiving entity layer stack. As the message travels up the stack of the receiving entity, each layer strips the header added by its peer layer.

For example, two networked applications, based on Windows and Macintosh operating systems, are communicating with each other. At layer 7, the Windows application requests data from the Macintosh application. The request is sent to the Windows application’s layer 6. This layer receives the request as a data packet. It then adds some information, known as header, and passes the packet down to layer 5. The process continues until the request reaches the physical layer.

Note: Physical layer does not add a header.

The physical layer sends this request packet to the Macintosh application through the network transmission media. When the Macintosh application receives this packet, the header that was added at the data-link layer of Windows application is stripped at the data link layer of the Macintosh application. The Macintosh data-link layer performs the tasks requested in the header and passes the request packet to the next higher layer, i.e., the network layer. The process is repeated until the Macintosh application’s application layer receives the packet and interprets the request.

The DoD Model

The Department of Defense developed a four-layer reference model in 1970s for the DARPA Internetwork Project. DoD is a reference model that determines the way in which messages should be transmitted between any two-communication entities in a network. In DoD model, the two end points in a network are divided into layers. As data moves through the layers, header information is added (for transmission) or removed (for reception). The DoD model consists of the following four layers:

• Layer 1, Network Interface Layer: The network interface layer is the lowest layer in the DoD model. This layer is responsible for putting frames on the wire and pulling frames off the wire. It is equivalent to the data-link and physical layers of the OSI model.
• Layer 2, Internet Layer: The Internet layer provides three services – a connectionless delivery service , a mechanism to break data into individual packets or frames on the transmitting side and put them back together on the receiving side, and the routing functions necessary to interoperate with other networks. It is equivalent to the network layer of the OSI model.
• Layer 3, Transport Layer: The transport layer provides communication sessions between connected computers. This layer is responsible for error detection and correction. It is also known as host-to-host or transmission layer and is equivalent to the transport layer of the OSI model.
• Layer 4, Application Layer: The application layer serves as a window for users and application processes to access network services. Several standard TCP/IP utilities and services exist at the application layer. It is equivalent to the application, presentation, and session layers of the OSI model.

Comparison between the layers of the OSI and DoD models

The following figure compares the layers of the OSI model with the layers of the DoD model: