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Data Link Layer in Computer Network

The Data Link layer is the second layer of the OSI model, which provides a reliable and error-free communication between adjacent network devices over the physical layer. The Data Link layer is responsible for the error-free transmission of data frames between devices on the same network segment.

The Data Link layer has two sublayers: the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer.

The MAC sublayer is responsible for controlling access to the physical layer and managing the transmission of data frames. The MAC sublayer is responsible for encoding the data into a form that can be transmitted over the physical layer and adding the necessary addressing and error-checking information.

The LLC sublayer is responsible for providing an interface between the network layer and the MAC sublayer. The LLC sublayer is responsible for managing the transmission of data between the network layer and the MAC sublayer and providing error-checking and flow control mechanisms.

The Data Link layer works by encapsulating data packets from the Network layer into frames that can be transmitted over the physical layer. The Data Link layer adds a header and a trailer to the data packet, which contains addressing and error-checking information.

The Data Link layer provides two main services: data framing and media access control.

  1. Data Framing

The Data Link layer divides the data received from the Network layer into frames. Each frame contains a header, a data field, and a trailer. The header and trailer contain addressing and error-checking information that is used by the receiving device to ensure the error-free delivery of the data.

The Data Link layer also provides flow control mechanisms to ensure that the receiving device is not overwhelmed with data. Flow control mechanisms prevent data loss due to buffer overflow.

  1. Media Access Control

The Data Link layer is responsible for managing access to the physical layer. The Media Access Control (MAC) sublayer of the Data Link layer provides a mechanism for sharing the network medium among multiple devices.

There are two main media access control mechanisms used by the Data Link layer: contention-based access and token-based access.

  • Contention-Based Access: In contention-based access, all devices on the network segment compete for access to the network medium. Devices transmit data when the network medium is free, and if two devices transmit at the same time, a collision occurs. When a collision occurs, the devices wait for a random amount of time before trying to retransmit the data.

  • Token-Based Access: In token-based access, a token is passed among the devices on the network segment. The device that holds the token has the right to transmit data, and once the data transmission is complete, the token is passed to the next device in the sequence. Token-based access ensures that only one device can transmit data at a time and eliminates collisions.

The Data Link layer is responsible for providing a reliable and error-free communication between adjacent network devices over the physical layer. The Data Link layer encapsulates data packets from the Network layer into frames that can be transmitted over the physical layer and provides two main services: data framing and media access control. The Data Link layer is a critical layer in the OSI model and plays a key role in ensuring the efficient and reliable transmission of data over a computer network.

In the Data Link layer of the OSI model, error detection and correction are essential functions that are used to ensure the reliable and accurate transmission of data over the network. This layer also provides control functions to manage the transmission of data over the network.

Error Detection: Error detection is the process of detecting whether errors have occurred during the transmission of data. The Data Link layer performs error detection by using a technique known as the cyclic redundancy check (CRC). The CRC is a mathematical algorithm that generates a checksum value based on the data that is being transmitted. The receiving device uses the same algorithm to generate its own checksum value, and then compares it with the checksum value sent by the transmitting device. If the two values do not match, an error is detected.

Error Correction: Error correction is the process of correcting errors that have been detected during the transmission of data. The Data Link layer does not provide error correction mechanisms, but instead relies on the higher layers to request the retransmission of data. When an error is detected, the receiving device sends a request to the transmitting device to resend the data.

Datalink Layer Control: The Data Link layer provides several control functions that are used to manage the transmission of data over the network. These control functions include:

  • Flow Control: Flow control is used to regulate the amount of data that is sent from the transmitting device to the receiving device. The Data Link layer provides flow control mechanisms to prevent the receiving device from being overwhelmed with data. The two main flow control mechanisms used in the Data Link layer are stop-and-wait and sliding window.

  • Error Control: Error control is used to detect and correct errors that may occur during the transmission of data. The Data Link layer provides error detection mechanisms, such as the CRC, to detect errors. However, error correction is not provided by the Data Link layer. Instead, the higher layers are responsible for requesting the retransmission of data when errors are detected.

  • Access Control: Access control is used to manage access to the network medium. The Data Link layer provides access control mechanisms to ensure that multiple devices on the same network segment can share the network medium. The two main access control mechanisms used in the Data Link layer are contention-based and token-based access.

The Data Link layer of the OSI model is responsible for error detection and providing control functions to manage the transmission of data over the network. Error detection is performed using the cyclic redundancy check (CRC) algorithm, while error correction is not provided by the Data Link layer. The Data Link layer provides several control functions, including flow control, error control, and access control, to manage the transmission of data over the network. These control functions are essential to ensure the reliable and accurate transmission of data over a computer network.