A computer network is a group or groups of computers and other computing devices that are linked together through some communication channel for exchanging data, resource sharing and general purpose communication. The internal functions of this communication system are elaborated, characterized and standardized using the Open Systems Interconnection model (OSI) reference model. OSI, a networking abstract model, groups these internal functions into seven layers where each layer interacts with only its immediate layers. Each layer processes data passed by one of its adjoining layer and prepares new data for other layers. The following table contains network layers referenced by an OSI model.
|Data Transport Set||OSI Layer||Function|
|Physical layer||Provides a physical direct point to point data connection.|
|Data link layer||Creates a reliable direct point to point data connection by handling the errors in the physical layer.|
|Network layer||Specifies the routing, addressing and means of data transfer in form of packets.|
|Transport layer||Maintains the data flow with error checking and recovery of data.|
|Application Set||Session layer||Manages the connections between points in a network.|
|Presentation layer||Converts a data into a standard format and also provides encryption and decryption of data.|
|Application layer||Manages the data in human recognizable format and provide interface for application to access network services.|
The Physical layer
The physical layer defines the physical specification and characteristics of a computer network. It is concerned with sending and receiving bit streams over a physical communication channel including cables (e.g., a copper cable or optical fiber) or wireless radio waves. It describes the electrical or optical and mechanical interfaces for the medium to transmit these bits. Hence, it is the lowest layer that transfers the actual zeros and ones of data over the computer network.
Besides the network hardware, the physical layer defines a protocol for connected devices or nodes for establishing and terminating the connection (preventing a fast sender from overwhelming a slow receiver with data).
The Data link layer
The data link layer provides a reliable logical connection between connected nodes by handling errors in a physical layer thus transmitting error free data from one node to another in the form of frames (synchronization sequence with each container of stream and in its header a source and destination addresses to identify the sender and receiver). Hence, this layer is responsible for frames synchronization, which avoids any corrupt data. It is also used to identify and recover any collisions when multiple devices use the same medium simultaneously. This computer network layer is further divided into the following two sub-layers.
- The Media Access Control (MAC) layer is responsible for providing data access to all the computers on a network and to deliver the frame structure to accomplish frame synchronization.
- The Logical Link Control (LLC) layer is responsible for providing an addressing mechanism and controlling the data exchange.
The network layer provides a routing mechanism for a data, in the form of packets. Depending on the network conditions, it decides which physical path each packet will follow to reach the destination node. This computer network layer is assigned the following responsibilities.
- The logical-physical addresses mapping. It provides a logical address to each device on a network such as on the internet where each machine has its own unique IP (Internet Protocol) address.
- Fragmentation (divides packets into several fragments when a router’s maximum transmission unit’s (MTU) size is less than a packet size).
- Subnet traffic control and usage accounting (to keep track of the packet information that is sent to next node or returned as a result of the router’s buffer).
The transport layer of the OSI reference model ensures the error-free transfer of messages with no loss or duplication of data. These messages are delivered in a sequential manner and maintains the quality of service (overall performance of a computer network).
Depending on the degree of reliability, the transport layer functionality may vary. In general, it provides the following functions.
- Message segmentation (makes fragments of message from session layer and transfers it to a network layer).
- Reliability (is achieved by receiving an acknowledgement for each message sent).
- Message traffic control (tells a fast sender to back off when message buffer on receiver end is full).
The session layer is used to control the connections between nodes in the network by establishing, managing and terminating the sessions between local and remote applications. A communication session consists of a request and response that occurs between applications. These communications are coordinated by protocols implemented at the session layer.
It provides services of authentication, authorization and session check-pointing and recovery which allow streams from different sources to synchronize properly.
The presentation layer converts data to a standard format that can be further processed by applications. Encryption and decryption is performed in this layer that enables security checks to the data that needs to be transmitted on the network. In some scenarios, the compression and decompression of data is done in this layer to improve the throughput of data as it reduces the number of bits required to transfer data on the network.
The application layer in the OSI model is like a user interface that is responsible for displaying received data to a user and also processed data to the presentation layer, which sends data on the internet and acts as an interface to access network services.
The spplication layer specifies the resource availability and facilitates resource sharing (remote file and printers, etc.). Other important responsibilities of the application layer include Inter-process communication, network management, network virtual terminals and electronic messaging.
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