Data Link Layer: Functions, Protocols, and Error Control
The Data Link Layer Explained
The data link layer is responsible for the reliable transfer of information through a transmission circuit. It is the second layer of the OSI model. It receives requests from the network layer and uses the physical layer service. The goal is error-free information flow between two directly connected machines.
To achieve this goal, the data link layer mounts blocks of information (frames), assigns link-level addresses, manages error detection and correction, and handles flow control between devices. In practice, the media access sublayer is often part of the network card itself, while the logical link sublayer resides in the adapter card program.
Frame Structure
In the data link layer, data is organized into units called frames. Each frame has a header including an address and control information, and a trailer used for error detection. The header of a LAN frame contains the source and destination physical addresses. A WAN frame header contains a circuit identifier in its address field.
Key Functions of the Data Link Layer
The data link layer is responsible for reliable information transfer through a data transmission circuit. The transmission of data frames is performed by exchanging meaningful information units in the data link layer.
Main Features:
- Initiation, Termination, and Identification: Initiation covers the processes required to activate the link and involves exchanging control frames to establish station availability for transmitting and receiving information.
- Segmentation and Blocking: Segmentation arises from frame length considerations. If frames are too large, they must be divided into smaller plots. If frames are too short, multiple short messages from a higher level are concatenated into a single, longer link layer frame.
- Synchronization of Octet and Character: When transferring information at the link layer, it is necessary to identify the bit position corresponding to each character or octet within a received series. This bit synchronization includes the processes required to acquire, maintain, and restore character or octet synchronization. This involves phasing the emitter with the receiver-decoding frame.
- Frame Delimitation and Transparency: The link layer must handle frame delimitation and synchronization. Three methods can be used for synchronization:
- “Principle and Purpose” (specific characters identify the beginning or end of each frame).
- “Principle and Has” (a character indicates the beginning, followed by a counter indicating the length).
- “Script” (a bit cluster identifies the start and end with flags).
Transparency is achieved through bit insertion. When five consecutive 1s are encountered, a 0 is inserted.
Error Control: Provides detection and correction of errors in frame transmission between computers and the physical layer. Its control functions generally include: data frame identification, error detection and correction codes, flow control, and management and coordination of communication patterns. Techniques such as character counters, start and end characters with character stuffing, and bit sequence indicators with padding bits can be used. Flow Control: Prevents overwhelming the receiver. It is usually performed at the transport layer, but also at the link layer, using feedback mechanisms. It often goes hand in hand with error correction and should not limit channel efficiency. Flow control involves two important actions: error detection and error correction.